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83-2736

Studies of Interface Bondings on

Implant Alloys

August 1 983

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Prepepared for: A Contract 22 4795023

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NBSIR 83-2736

STUDIES OF INTERFACE BONDINGS ON

IMPLANT ALLOYS

Anna C. Fraker, Arthur W. Ruff and Kirk J. Bundy

with

Jacqueline D. Smith, Robert W. Penn, Ann C. Van Orden

August 1 983

Prepared for: FDA Contract 22 4795023

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Studies of Interface Bondings on Implant Alloys

Introduction

This work was initiated for the purpose of investigating mechanisms

involved in strengthening or weakening the interface between the metal

prosthesis and the bone cement used for prosthesis fixation. This report

covers the second year of this investigation. Progress has been made

during this study in measurement methods of metal /bone cement interface

stength and of assessing the current state of knowledge of cemented

prosthesis fixation. This is indicated in the technical report which

follows.

The need for increased studies of various aspects of total hip

replacement was highlighted by the National Institutes of Health Consensus

Development Conference which was held March 1-3, 1982 at the Masur

Auditorium on the NIH campus. * There are approximately 100,000 total hip

replacements made in the United States each year. Each total hip

replacement costs around $9,500.2 /\ny complications or revisions can

result in multiple increases in this figure. The increased pain and

problems for the patient are beyond quantification. Nevertheless, the

prognosis for total hip surgery is good, and a conservative figure for

the success rate is 90 percent. The longer term success rate going

beyond 10 years is less well known.

Studies of groups of patients have given figures of 12 percent for

prosthesis loosening over a period of 8 years.3 These data were reported

from The Hip Society by Richard J. Johnston, M.D., who emphasized the

success of the current mode of total hip surgery and predicted a 75 percent
success rate at the end of 20 years. He cautioned against abandoning

this successful procedure in favor of other procedures. Incidences of

loosening after 10 years as indicated radi ographi cal ly have been reported

to be as high as 11.3 percent for the acetabular component and 29.9

percent for the femoral component. 4 This radiographic evidence did not

indicate prosthesis failure or the need for revision but it did show that

the prosthesis fixation was somewhat weakened for its future use.

Indications for success in the total hip surgery are at such a high level

that it seems feasible to consider that many more of the current failures

can be eliminated. Those failures associated with prosthesis loosening

originate with the bone/bone cement interface and with the metal/bone

cement interface or a combination of both. This study has dealt with the

metal /bone cement interface.

Approach

The overall assessment of the prosthesis fixation problem and the

state of current scientific research on the subject was made after

collecting an extensive literature reference base on the subject. This

literature spans a time period of at least 10 years. Expertise of NBS

scientists relating to surface films, oxides and other types of films,

was used in the initial planning of the experiments. The initial literature

review indicated that the torsional loading aspect of the femoral component

device had a significant role in the motion of the hip but had received

little attention. Based on this, it was decided to pursue this mode of

testing. The NBS implant materials torsion fatigue machine, for a period

of 3 months, was devoted to the development of a test for the metal /bone

cement interface. This fatigue machine was constructed for the purpose

of testing implant metals at low frequencies and in appropriate environments

2
related to surgical implants. After developing and proving the validity

of the test, a computer controlled Instron machine capable of torsional

loading was used. This machine is located in the Polymers Division and

its use involved the cooperation of Dr. Robert W. Penn and other NBS

scientists. Dr. Kirk J. Bundy of the Johns Hopkins University has been

associated with the Metallurgy Division, National Bureau of Standards

during the conduct of these studies. Dr. Bundy has worked with various

NBS scientists and student trainees in the conduct of the experimental

studies, and he has analyzed the results and prepared two technical

reports

Research Findings

The work performed during the contract year 1982 dealt primarily

with testing metal/bone cement interface strength using the torsion test

which was developed earlier in this project and described in NBSIR 82 2563.

The test proved to be a good one and to be reproducible. The parameters

studied to determine their influence on the metal/bone cement interface

strength were material type, specimen surface roughness, sterilization

and passivation treatments, cure time prior to testing and effects of

ultra clean surfaces. Metals used were the alloys, Co-Cr-Mo, Ti-6A1-4V

and 316L stainless steel. The bone cement used was Howmedica Surgical

Simplex P which is a poly (methyl methacrylate) type.

Results relating to the various parameters indicated interfaces with

bone cement and 316L stainless steel to be the strongest followed in turn

by those with the Co-Cr-Mo alloy and the T i -6A 1 -4V alloy ELI Grade. 316L

stainless steel is not used widely as a permanent implant or with bone

cement. The influence of surface finish or the degree of surface roughness

3
on the interface strength varied. Grit blasted or rough surfaces and

smooth surfaces with 1 urn finishes gave stronger bonds than did

intermediate surface finishes. There was more scatter in the data from

the finer surface finished materials even though the overall bond strength

was good. This would indicate that the presently used grit blasted

surface is the one most advisable to use at the present time. These

findings motivated additional research at NBS to investigate mechanisms

involved in bonding with a grit blasted (irregularly roughened) surface

and with a surface with a 1 urn metallographic polish. This is a joint

study involving scientists in the National Measurement Laboratory and the

National Engineering Laboratory, two major laboratories at NBS.

The effects of sterilization and passivation treatments on the

metal/bone cement interface did not appear to be large in preliminary

studies. This is an area where further study would be useful. Specimens

subjected to rigorous industrial cleaning showed an average of 25 percent

increased interface strength. More testing of procedures utilizing the

effects of cleaning would be useful. Cure times of the bone cement after

applying to the metal affect the test results for the metal/bone cement

interface strength. This work showed that cure times of 2 days or less

should not be used and that times ranging from 5 to 7 days would be

preferred. The application of increased pressure to the bone cement

during the forming of the metal/bone cement interface resulted in increased

interface strength. Seventy-seven tests were conducted. The description

of these tests, analysis of the results and a discussion of related

studies in the technical literature are given in the technical report

that follows entitled "An Experimental Investigation Of the Torsional

Strength of Metal /Bone Cement Interfaces".

4
Future Work

Over the past two years, this task has reviewed and established a

comprehensive literature base on bone cement interfaces, developed a

reproducible test for measuring interface strength ana carried out

experiments to obtain data on materials currently used in prosthetic

devices. It is clear from our work and research results from other

institutions that further advances in improving prosthesis fixation are

possible. Clinical data indicate the need for improving cemented prosthesis

fixation. Failure requires the patient to undergo further surgery. This

surgery and associated rehabilitation are difficult and costly for the

patient.

The data obtained on this project during the past contract year

indicate that the work described below would be important and beneficial

in clarifying mechanisms involved in strengthening the interface. The

test which was developed and proved reliable could be used to measure

interface strengths of new procedures of implant fixation which now are

being used on a clinical trial basis. The proposed work is as follows:

1. Complete the analysis of effects of surface roughness and surface

cleanliness on bond strength and explain the role of 2 mechanisms

(mechanical and chemical) in strengthening the metal/bone

cement bond. Interface strength data will be correlated with

refined surface roughness/structure measurements now in progress

at NBS

2. Prostheses which have been precoated with bone cement are in

limited use at the present time. Further study is needed to

assure that techniques employed in this procedure are an improve5
merit in prosthesis fixation and are beneficial to the patient.

NBS proposes to test materials which have been precoated with

bone cement where there would be a metal /bone cement and a bone

cement/bone cement bond to be considered.

3. Porous coated prostheses are being implanted on a clinical trial

basis in an effort to improve fixation. One aspect of the porous

coated implants is to achieve fixation by bony ingrowth attachment

Another approach is simply to apply the porous coating in areas

where increased interface strength is needed and then to use it

with cement. This would be useful for patients who could not

get sufficient bony ingrowth. NBS would investigate the porous

substrate to determine whether significant bond improvements are

obtainable in this way.

Acknowledgments

We wish to acknowledge and express our gratitude for the helpful

assistance of a number of individuals and companies. Sidney Weisman,

Howmedica, Inc. provided many useful discussions and arranged for Howmedica

to supply all of the bone cement used in this project. Ajit K. Kesh of

Howmedica provided Co-Cr-Mo specimens and had the satin finish (grit blast)

applied to the surface. Charles D. Griffin while at Zimmer, USA arranged

for this company to supply some of the T i -6A 1 -4V ELI material and conducted

grit blasted surface finishing for the Ti-6A1-4V ELI and the 316L stainless

steel specimens. 316L stainless steel material was supplied by John Disegi

of Carpenter Steel Corp.

Johns Hopkins University students, Michael Vogelbaum, Rob Kelly and

Greg Bartlett assisted with specimen polishing and preparation of specimens

6
Lewis K. Ives and Clayton Teague of NBS conducted prof i 1 ometri c measurements

of the surface roughness. We wish to thank all of these individuals for their

help and also Daniel J. Chwirut, Food and Drug Administration, for reviewing

and giving constructive comments on the rough draft of the report.

References

1. NIH Consensus Development Conference, Total Hip Replacement, March 1-3,

1982, Gordon, Stephen L., coordinator. National Institute of Arthritis,

Diabetes and Digestive and Kidney Diseases.

2. Kelsey, Jennifer L., Yale University School of Medicine, "Epidemiology

and Impact," ibid.

3. Johnston, Richard C., The Hip Society, "Long Term Outcomes - Hip Society

Collective U.S. Experience," ibid.

4. Galante, Jorge 0., Rush Presbyteri an-St. Lukes Medical Center,

"Total Hip Joint Replacements," ibid.

Pi scl ai mer

Certain commercial materials are identified in this report in order

to adequately specify the experimental procedure. In no instance does

such identification imply recommendation or endorsement by the National

Bureau of Standards nor does it imply that the material is necessarily

the best available for the purpose.

7

AN EXPERIMENTAL INVESTIGATION OF THE TORSIONAL STRENGTH

OF METAL/BONE CEMENT INTERFACES

Kirk J. Bundy
\
TABLE OF CONTENTS

Page

List of tables iii

List of figures iv

Chapter I INTRODUCTION AND BACKGROUND 1

Chapter II LITERATURE SEARCH UPDATE 3

Wolff's Law and Bone Remodeling 3

Frequency of Total Hip Replacement Surgery 5

Pre-Coating of Implants with Bone Cement 6

Properties of Bone Cement and Metal/Bone Cement

Interfaces 9

Total Hip Replacement Stress Analyses 10

Chapter III EXPERIMENTAL PROCEDURES 13

M/BC Interface Formation 13

Materials and Surface Finish 17

Mechanical Testing Conditions 17

Surface Cleaning Procedures 18

Surface Roughness Measurements 19

Chapter IV MATHEMATICAL ANALYSIS 22

Chapter V INFLUENCE OF SURFACE FINISH AND MATERIALS ON THE

TORSIONAL STRENGTH OF THE M/BC INTERFACE 35

Surface Finish 35

Comparison with Other Research 41

Differences Between Materials 48

Chapter VI OTHER FACTORS WHICH AFFECT INTERFACIAL STRENGTH

PROPERTIES 50

Influence of Surface Pretreatment 50

Influence of Strain Rate 54

Influence of Rigorous Cleaning 56

Influence of Cure Time 57

Chapter VII DISCUSSION, SUMMARY, AND CONCLUSIONS 59

Discussion 59

Summary and Conclusions 67

References 70
TABLE OF CONTENTS (Cont.

)

Appendix 1

Appendix 2

Appendix 3

Materials, Surface Finish and Pre-Treatment Procedures,

and Interface Preparation and Testing Characteristics. .

Mechanical Properties of Metal/Bone Cement Interfaces. . .

Mechanical Properties of Metal/Bone Cement Interfaces

Classified according to Materials and Surface Finish. . .

i i a

Page

74

76

78
LIST OF TABLES

Page

Table I I

I - 1 Roughness of the Surfaces of the Metals Employed to Form M/BC

Interfaces 20

Table V-l Normalized Maximum Torque Tm/L for M/BC Interfaces Formed

from Different Materials With Various Surface Finishes . . 36

Table V-2 Interfacial Shear Strength x. Determined from Torsion Tests on M/BC Interfaces Formed i^rom Different Materials With

Various Surface Finishes 38

Table V- 3 Scatter Observed in Interfacial Strength for Different Materials

With Various Surface Finishes 42

Table V-4 Interfacial Strength Measurements of Ti-6A1-4V Determined by

Other Investigators 43

Table V-5 Interfacial Strength Measurements of Co-Cr-Mo Alloys Determined

by Other Investigators 44

Table V-6 Interfacial Strength Measurements of Stainless Steels Determined

by Other Investigators 45

Table VI-1 Comparison of the Normalized Maximum Torque for Passivated and

Sterilized Specimens with that for Untreated Specimens. . 51

Table VI-2 Comparison of Mean Interfacial Shear Strength for Passivated and

Sterilized Specimens with that for Untreated Specimens. . 52

Table VI-3 Confidence Levels of t-Test Comparisons for Differences in

Interfacial Shear Strength Due to Passivation and

Sterilization 53

Table VI-4 Influence of Strain Rate on the Mechanical Properties of 316L/BC

Interfaces 55

Table VI-5 Influence of Cure Time t on the Normalized Maximum Torque

Observed During Torsion Testing of M/BC Interfaces. ... 58

Table VII-1 Confidence Levels of t-Test Comparisons for Differences in

Interfacial Strength and Normalized Maximum Torque Due

to Surface Finish 65
LIST OF FIGURES

Page

Figure I I

I - 1 Specimen Geometry 14

Figure I I

I - 2 Mold Employed to Apply Pressure to Metal/Bone Cement

Interface During Specimen Preparation 15

Figure I I

I - 3 Typical Prof i 1 ometric Trace for Measuring Surface

Roughness (Ti-6A1-4V ELI Sample With Grit-Blasted

Finish) 21

Figure IV-1 Typical Measured Applied Torque T vs. Twist Angle

y Curve (Interface 29) f*

3 ? 23

Figure IV-2 Cross-Sectional Representation of Specimen Geometry With

Parameters for the Mathematical Analysis 24

Figure IV-3 Torque T and Surface Shear Stress x.

^ Acting on Volume

Element v at Position x 26

Figure IV-4 Undeformed and Deformed Geometry for Interfacial

Element ABCD 27

Figure IV

Figure IV-6

Figure IV-7

Figure V-l

Figure V-2

Figure VII-1

Length-Normal ized* Torque T/L vs. Twist Angle y for

Interfaces A, B, and C. a

f 31

Maximum Interfacial Shear Stress xT^ vs. Twist Angle y . . 32

Average Interfacial Stress x. f at Maximum Torque vs. Acrylic

Length L ac

1

! 33

The Influence of Surface Finish on the Mean Values of Normalized

Maximum Torque Tm/L for M/BC Interfaces 39

clC The Influence of Surface Finish on the Mean Values of Interfacial

Shear Strength x^. of M/BC Interfaces 40

Hypothetical Components of the Bonding Force Between

Metal and Bone Cement Surfaces 61

Figure VII-2 The Influence of Surface Roughness on the Mean and

Extreme Values of Normalized Maximum Torque T /L for

M/BC Interfaces

a

5 . . 63
CHAPTER I

INTRODUCTION AND BACKGROUND

This report describes activities, mainly experimental, pursued in 1982 in

the Metallurgy Division of the National Bureau of Standards to study the

strength of metal/bone cement (M/BC) interfaces. Although this question is of

broader interest, most attention here is focused upon the interface as it

affects the total hip replacement (THR). The work described in this report is

an outgrowth of that performed in 1981 which was mainly a literature search.

Last year's research results (Bundy, 1982a) have been published in an NBS

Internal Report and form the basis for an article submitted to the Journal of

Biomedical Material s Research (Bundy, 1982b). Very briefly summarized, that

work indicated that failure of the mechanical integrity of the metal /bone

cement interface, although not receiving as much attention as failure of the

bone/bone cement interface, can lead to failures of the stem component.

Hence, it is important both to further understand the biomechanical basis of

the failure of the interface and to find means of improving its strength. It

was suggested that torsional stresses induced by anterior-posterior forces

could be involved in interface failures. A torsion test for the metal/bone

cement interface was proposed (because no standard methods of testing exist

for this interfacial loading mode) and preliminary experimental data was

acqui red.

This year's activity has mainly involved an intensive investigation of

this torsion test concept and measurements on many samples have been performed

to determine what surface is best for improving interfacial strength. This

report is divided into seven chapters. Chapter II provides an update of the

literature search conducted last year in areas of importance to the performance

1
of M/BC interfaces. Chapter III describes the materials used and the experimental procedures employed to prepare and test the M/BC interfaces. Chapter IV

describes the mathematical analysis for the extraction of the interfacial

strength from the measured data. Chapters V and VI contain the experimental

results and describe the influence of materials and surface finish, type of

surface pretreatment, and other effects on the torsional strength of M/BC

interfaces. Finally, Chapter VII provides a discussion and summary of the

results, conclusions for this work, and a set of recommendations for future

research.

2
CHAPTER II

LITERATURE SEARCH UPDATE

Although review of the literature was not an area of emphasis for this

year's research activities, a certain amount of literature searching was

pursued to update and extend the work of last year, and several relevant lines

of research were found. These are described briefly below.

Wolff's Law and Bone Remodeling

In last year's report it was pointed out that if Wolff's law (i.e. the

idea that bone remodels in response to applied stress) were available in a

quantitative form, then mathematical stress analyses and stress measurements

of prostheses would be more meaningful in predicting the clinical course of

actual or proposed orthopedic procedures. In a series of papers, Cowin and

his coworkers have developed a mathematical theory for adaptive elastic

materials which they have applied to bone growth, rei nforcement , and resorption.

(Hegedus and Cowin, 1976a and 1976b), (Cowin and Nachlinger, 1978), (Cowin

and Van Buskirk, 1979), (Cowin, 1981), (Cowin and Fi roozbakhsh , 1981). The

details of the theory are complex, but basically what is involved is solution

of the governing differential equations:

3U . 8U

£ = - ( —- + — i j 2 ^ax. dX. J J

J

T

3ct . . -a*

1 + P • [V

0 + = 0

ct. . = [V + e] C. .. , (e) e. , ij L o

J ljkl v ' kl

de/dt = a(e) + A. . (e) s.

ij tj

( 1 )

( 2 )

(3)

(4)

for the problem of interest.

3
In the equations above, IK are displacement components, a^. and £^. (or

£,,) denote the stress and strain tensors, X. (or X.) denotes the coordinate

K I 1 J directions, p is the bone material density, V is a reference volume fraction

o of bone material in the porous bone structure, e is the change in volume

fraction from the reference due to the adaptive nature of bone, and b. is the

body force.

Equation (1) is the strain-displacement relation for linear elasticity;

eq. (2) is the mechanical equilibrium equation; and eq. (3) is a generalized

form of Hooke's law. As e approaches zero, and V

q

approaches unity the

equations reduce to those of linear elasticity theory. Equations (3) and (4)

are essentially a mathematical statement of the Wolff's law principle.

Although this theory represents a potentially important advance in understanding of bone remodeling, practical applications will probably have to wait

for the future. This is because there have been no experimental determinations so far of the quantities a(e) and A^.j(e) whose values would have to be

known to make quantitative predictions. The theory up to this point has been

used to consider internal remodeling, change in external bone shape, and has

indicated that a pure torsion about the long axis of a bone does not induce

remodel i ng.

A paper by Carter et al . contained preliminary steps toward an experimental

approach for Wolff's law quantification (Carter et al .

,

1981). In adult dogs

i n vivo strains were determined with strain gauges attached to the femur.

Stresses were calculated using bending theory and finite element calculations.

Normal femurs and those in which an orthopedic plate had been implanted were

measured. Metabolic activity was monitored using tetracycline labeling techniques. It was observed that alterations in stress field do appear to induce

» bone remodeling. Perhaps, with development and refinement, techniques such as

this could become useful for expressing Wolff's law in a quantitative form.

4
Frequency of Total Hip Replacement Surgery

Several papers have come to our attention which discuss the numbers of

total hip replacements performed every year. This is an important subject

with respect to implant failures because the number of failures must in some

manner be proportional to the number of operations performed.

Hori et al. (1978) studied Medicare and Veterans Administration data,

sent a questionnaire to all orthopedic surgeons in Illinois and to selected

prominent surgeons throughout the U.S., consulted sales figures of orthopedic

equipment companies, and investigated admissions data of a large number of

hospitals. The time period spanned in this study was 1972-6. Estimates of

the numbers of artificial joints implanted in 1976 derived from these sources

were: total hip replacement - 80,000; total knee replacement - 40,000; other

joints - 10,000. From the questionnaire it was estimated that, were perfect

designs available, the numbers of procedures performed on the population would

be 12%, 60%, and 240% greater, respectively for these three categories.

At the March, 1982 NIH Consensus Development Conference a paper was given

which contained statistics regarding total hip replacements (Kelsey, 1982).

The 1979 estimate of the number of total hip replacements performed was 70,000

artificial joints in 61,000 patients. The age distribution of the patients

was 57% - greater than 65 years old, 25% - 55 to 65 years, and 18% under age

55. With respect to the reasons for surgery, the most commonly mentioned

factors and their frequencies were 60% osteoarthrosi s , 11% hip fractures, 7%

rheumatoid arthritis, 7% aseptic necrosis, and 6% replacement of previously

inserted devices. This amounted to a $700,000,000 annual expenditure including

treatment for late infections. Ninety percent of hip operations are considered

successful after a 10 year follow up period.

5
In a Workshop on Internal Joint Replacement held at Northwestern University

in 1977 it was estimated that in the U.S. in 1976, there were 120,000 total

joint replacements which comprised 80,000 hips; 30,000 knees; 8,000 fingers;

and 2,000 others (ankles, shoulders, elbows, wrists) (Compere and Lewis,

1977). It was estimated that due to increasing reliability of prostheses and

an aging population, the number of artificial joints will increase to over

180,000 in the future.

Another estimate of the number of joint replacement operations was contained

in a study by the Office of Technology Assessment. (Bentkover and Drew,

1981). According to this report, U.S. annual figures are: 100,000 hip replacements, 50,000 knee replacements, and 12,000 replacements of other joints.

Annual sales of joint implants are estimated at $114,000,000 of which the two

leading companies account for two-thirds of the total.

Pre-Coating of Implants with Bone Cement

A number of papers have appeared recently (Park et al., 1979), (Barb et

a!., 1982), (Park et al .

,

1982), (Raab et al .

,

1982) which describe a concept

which could have an important relationship to the present work. This is the

idea that the M/BC interface could be pre-formed before implantation. The

beneficial aspects of this type of fabrication would be, 1) a smaller amount of

the toxic monomer released during the operation, 2) lessened heat production

and volumetric shrinkage during polymerization, 3) better positioning of the

implant in the medullary cavity, and 4) increased interfacial strength.

With respect to the latter point, in one study (Barb et al .

,

1982)

push-out tests were performed on retrieved canine hip prostheses with circular

cross-sections and sand blasted surfaces after various service times in vivo.

6
Both pre-coated and non pre-coated samples were used. The coating consisted

of a layer of surgical bone cement about 2 mm thick and was applied by

injection into a mold with a syringe. The interfacial shear strength was 17.2

MPa after one month for pre-coated implants and leveled off at about 12 MPa at

three months. This was about 2-3 times stronger than conventionally formed

interfaces. According to these investigators, the reasons that the interfacial

shear strength decreases with time is possibly "due to stress relaxation

caused by the body fluids diffusing through bone cement, i.e., plasticizing

the bone cement or relaxation of locked-in hoop stress caused by the shrinkage

of bone cement."

The old bone cement/new bone cement interface increased in strength with

time and reached a value of 18.7 MPa (as expected, a value higher than the

M/BC interface since direct chemical bonding is involved). The bone cement/bone

interfacial strength declined from proximal to distal locations due to lessened

trabecular i nterdi gi tati on. Interestingly, even the bone cement/bone interfaces

were stronger for the pre-coated samples (1.6-2. 3 MPa) as compared to conventional

interfaces (0.8-1. 6 MPa).

The latter finding indicates that production of a strong M/BC interface

will apparently lead to a retention of strength of the bone/bone cement interface. Since loosening of the latter interface is an important clinical problem,

any treatment which increases M/BC interfacial strength could potentially

improve the clinical success rate of total hip replacements. In a further

study, this group demonstrated that the mass density of pre-coated bone cement

(1.202 g/ml) was about 2% higher than nonpre-coated cement and that the precoated cement elicited milder histological reactions (Park et al .

,

1982). The

reason that pre-coated bone cement has a higher density is that there are no

large voids or inclusions of blood and marrow fat within it.

7
Raab et al . (1982) refined the pre-coating concept. Instead of using

surgical cement for the coating, an industrial type of procedure was followed

to enhance wetting. The metal surface was rigorously cleaned and then dipped

in a lacquer- type solution of PMMA in MMA which contained no catalysts or

radio-opaque agents. A silane coupling agent (gamma methacryl oxypropyl trimethoxy silane, A-174 Union Carbide Can. Ltd.) was added to provide resistance

to environmental degradation in saline solutions. The interface was annealed

by curing above the glass transition temperature of PMMA to relax residual

stresses.

Raab et al. found similar degrees of interfacial shear strength increase

as described above when comparing pre-coated to uncoated samples: 316LVM-19.1

and 11.2 MPa respectively, Co-Cr-Mo-15. 3 and 6.9 MPa, and Ti-6A1-4V-19. 2 and

12.5 MPa. Among the influential variables related to interface strength were

the PMMA/MMA ratio in the dip (which is related to contact angle) and use of

ultrasonic cleaning (which was shown to adversely affect Ti-6A1-4V interfacial

strength). Improvements due to pre-coating were more observable when interfaces

were tested dry than for 37 °C saline conditions. Other important observations

were that; 1) silane additions had a markedly beneficial effect, 2) for pre-coated

metals in saline the fracture toughness of the interface is comparable to that

of the bone cement itself and the fatigue life is much greater than for nonpre-coated samples tested dry, and 3) these coatings can be sterilized in a

practical manner.

This pre-coating concept is an exciting one for the reliability of total

hip replacement. It offers a way to improve markedly interfacial strength and

hence to diminish the incidence of stem fracture. The high pressure interface

formation technique described later in this report could also be practically

applied to form coatings remotely. Since the evidence described later implies

8
that this method produces stronger interfaces than conventional techniques,

there may be direct clinical relevance for the research work pursued here.

There is good ground for optimism, based on these studies cited above and

the work presented later in this report, that using a pre-formed interface

(for which surface cleaning, surface finish, interfacial pre- forming pressure,

and bone cement composition is optimized) a substantial level of interface

strength improvement could be obtained. An interesting concept along these

lines is the application of an electrical potential across the interface as

cement is curing to possibly influence bonding character! sties.

Properties of Bone Cement and Metal/Bone Cement Interfaces

The determination of properties of bone cement is still a very active

field of research and a number of papers of interest have recently appeared.

Krause et al . (1982) studied rheological properties of several cements in

detail and concluded that they are non-Newtonian, pseudoplastic materials with

constitutive laws of the form:

i = K(^)

n

(where t is shear stress, y is shear strain rate, and K and n are constants)

for which n < 1. This means they have a rate dependent viscosity:

u r

' \n-l

n = k(y)

which decreases with increasing shear rate.

Creep and stress relaxation of bone cement have been studied by Holm

(1980) and Pal and Saha (1982). Bone cement possibly can develop large residual

stresses during curing as the volumetric shrinkage is constrained by the

surrounding bone and femoral component. Bone cement will creep under applied

load and will relax stresses at a constant deformation. It is thought that

these characteristics can lead to cement fracture and loosening and failure of

total hip replacements. Reduction in the degree of bone cement porosity and

use of graphite fibers for reinforcement can reduce time dependent behavior.

9
With respect to the M/BC interface, Welch et al. (1971) performed push-out

tests on interfaces in which the metals and finishes used were porous vitallium,

"satin-surfaced" vitallium (presumably a finish comparable to what has been

termed "grit blasting" later in this report, and "buff polished" stainless

steel. Five samples per material were tested and interfacial shear strength

ranges were respectively determined for these three cases to be: 122-190,

2 37.3-42.9, and 40.8-46.4 kg/cm . Use of a porous metal/bone cement interface

is an interesting concept for the total hip replacement and apparently could

increase interfacial strength considerably. However, lack of interface mechanical

integrity would expose much higher surface areas of metal to body fluids and

consequently perhaps to increased possibilities of adverse biological systemic

effects resulting from injection of metallic ions into solution by dissolution

if these effects are proportional to the amount of ion release.

Mathematical modeling of the bone/bone cement interface was examined by

Sih, Moyer, and Berman (1981) using a strain energy density (i.e. fracture

toughness) criterion for failure instead of the usual stress criteria. The

same ideas could be applied to M/BC interfaces.

Total Hip Replacement Stress Analyses

A number of interesting papers related to stress analyses of total hip

replacements were reviewed. Crowni nshi el d et al. (1981) performed threedimensional finite element analysis (FEA) with forces applied at various

angles in the medial-lateral plane and considered the stresses in the bone

cement and bone for different combinations of collarless and collared prostheses

made from Ti alloys or stainless steel. It was shown that, for collarless

prostheses, the more flexible Ti stem produces higher cement stresses in the

proximal region and lower stresses in the distal region. With collar-femur

contact (a situation these authors believe would be very difficult to achieve

clinically) increased femoral stresses and decreased cement stresses result.

10
These researchers feel that cement failure proximal ly rather than distal ly is

a more major clinical cause of loosening and thus that the implant should be

made of stiffer materials. They feel that proximal bone resorption, although

it occurs, is not causally related to loosening.

Huiskes (1980) also performed three-dimensional finite element analysis

of THR's. His results demonstrated that except in the proximal region, the

femur and the stem could be analysed with beam theory (which is a two-dimensional

analysis) but that accurate determination of stresses in bone cement required

a three-dimensional analysis. This investigator is sensitive to the variation

of results of stress analyses with respect to the orientation of joint force

components as the angle of the joint force in the medial-lateral plane deviates

from the vertical and cautions against deriving general conclusions regarding

a device based upon one specific physiological load. This latter point is an

important one. Hopefully at some point in the future three-dimensional FEA

will be performed for a realistic series of hip joint force components which

will include anterior-posterior forces which produce torsional loading.

McNeice et al . (1976) performed an FEA study of THR's and concluded that

M/BC interface loosening is a strong possibility due to the large shear stresses

generated in the stem due to joint forces which act in the medial-lateral

plane. 17.5 MPa maximum shear stresses occur on the inferior aspect near the

stem neck and at the distal lateral surface. Assuming that impact loading

would double these values, the shear stress in the stem would approach the

shear strength of acrylic. They also analysed several modes of THR loosening.

The most devastating one, although not the clinically most common mode, is the

cantilever fatigue mode in which M/BC interface integrity is lost proximal ly

(thus losing the support and stiffness of the bone which resists bending in an

11
intact structure). This produces high bending stresses and fatigue failure.

An interesting metallurgical analysis of such a case is found in a recent

paper (White et al., 1979).

In a very careful clinically oriented statistical study of femoral stem

fractures this mode of failure was observed to be the most common (Chao et

al., 1981). Among the most frequently implicated variables in stem fracture

are: cancellous bone removal from the calcar, poor bone cement quality on the

proximal medial part of the stem, stress concentrations due to nick marks from

the drill bit during trochanter reattachment, and cement build up at the base

of the neck. This paper by clinicians does mention that examination of fracture

surfaces shows evidence that, in addition to loads which cause lateral bending,

loads which cause anterior-posterior bending and torsion are also important.

As was pointed out in last years report, although clinicians have made this

observation (Wroblewski, 1979), (Charnley, 1975), (Chao et al., 1981) stress

analysts do not seem to have placed sufficient importance on the threedimensional nature of the joint loads at the hip.

In an experimental study, Lanyon and his coworkers (1981) implanted

strain gauges on the surface of sheep femora and determined the strain pattern

during walking for intact bones and for femora which had undergone a total hip

replacement operation. Strains on the lateral part of the proximal portion of

the femur shaft indicated that torsion about the long axis of the femur was

present at this location. Although there obviously are differences between

the animal model and the human hip, these results are perhaps one additional

indication that torsion induced by anterior-posterior forces should be examined

with respect to its influence on the total hip replacement, particularly the

integrity of the M/BC interface.

12
CHAPTER III

EXPERIMENTAL PROCEDURES

M/BC Interface Formation

Seventy-seven samples were prepared for testing according to the procedures

described in this chapter. The geometry of the specimen is shown in Figure

III-l. The manufacturer ' s directions were followed in mixing of the bone

cement (Howmedica Surgical Simplex P). The cement consists of two components:

a methyl -methacrylate monomer and a powder which consists of poly (methyl methacrylate) and a methyl -methacryl ate styrene copolymer. 10 g of powder was mixed

with 5 ml of monomer. This is the same ratio as recommended by the manufacturer;

for clinical purposes 40 g of powder are mixed with 20 ml of monomer.

The interface formation time (i.f.t.), i.e., the time between the onset

of mixing and the first contact between bone cement and metal, was standardized to four minutes (two minutes for mixing and two minutes for kneading).

Standardization of the i.f.t. was thought to be advisable because of monomer

evaporation with time and cement viscosity increase with time. Studies where

interface strengths of metal/bone cement bonds were measured (for bonds

pressurized to a degree similar to clinical conditions) have shown that strength

changes with i.f.t. occur (Keller et al . , 1980).

The interface was formed through use of the Teflon mold and plunger

arrangement shown in cut away view in Figure III-2. The bone cement was

packed around the metal rod in about one minute or less. Following this, as

much pressure, as was possible to apply by hand, was applied until 10 minutes

had elapsed from the onset of mixing. After one hour had elapsed the sample

was removed from the mold. Due to losses of material as the procedure above

was followed, the length L of the acrylic block was somewhat different for

dC

each sample prepared.

13
14
Ill - 2 Mold Employed to Apply Pressure to Metal/Bone Cement Interface

During Specimen Preparation

15
Although the high pressure technique described above does not closely

approximate clinical conditions, the rationale behind its use was the following.

First of all, many other variables besides pressure also affect interface

formation i_n vi vo in the operating theater and would have to be controlled to

duplicate the clinical situation. Presumably, when there is great fidelity of

the interface formation technique to clinical conditions, there is less control

over interfacial strength measurements. The technique used here, on the other

hand, seems to offer the advantage of producing a higher quality interface.

Better contact of bone cement and metal is achieved so that there is a greater

true surface contact area and reduced porosity. Better control over producing

the same test conditions for repeated measurements might be expected and is

important for the aim of this i nvesti gati on--to initiate development of methodology

to sensitively assess interfacial bond strength in torsion for different

surface preparation techniques.

Even though the original idea behind the use of the high pressure technique

was to improve control over interface formation, it is possible that the

results here could have direct clinical relevance. As previously discussed,

to diminish problems associated with thermal necrosis and acute toxicity due

to monomer release as polymerization proceeds, there has been interest recently

in the concept of pre-forming the metal/bone cement bond so that less cement

is polymerized i_n situ . Improvements in mechanical performance could result

from this concept. If the use of this technique becomes widespread, then the

data from the relatively high pressure technique employed here could be used

to compare the relative performance of different materials and surface treatments

in torsion if pre- formed interfaces used clinically were fabricated using high

pressure.

16
Materials and Surface Finish

The metal rods used in these studies were the common surgical implant

metals: 316L stainless steel, Ti-6A1-4V ELI, and a Co-Cr-Mo alloy. The

surface finishes employed were either grit blasted, or polished with diamond

pastes for which the finest step was 15pm, 6pm, or 1pm. Specimens were held

in a drill press during polishing. The specimen rotated and the polishing

paper or cloth was alternately either held in place or moved vertically for

the sequential polishing steps. Specimens were polished through a series of

silicon carbide polishing papers ranging from a coarse (paper no. 320) to a

fine grit (paper no. 600). This was followed by polishing with the diamond

pastes to the desired surface finish. The diamond paste polish was applied

with polishing clothes which were moistened with mineral spirits. In some

cases the surfaces were sterilized and passivated. For these samples steam

sterilization (20 minutes at 125 °C) was used. The passivation treatment

consisted of 40 minutes immersion in 35.5 % HNO^ at room temperature. This

passivation treatment is in accordance with ASTM Standard F86-76, "Standard

Recommended Practice for Surface Preparation and Marking of Metallic Surgical

Implants." To obtain the grit blasted surface, we sent samples to the surgical

implant companies which supplied the materials used in this investigation.

The specimens were glass bead blasted (dry) and cleaned and passivated according

to the procedures used to prepare actual implants.

Mechanical Testing Conditions

The M/BC interfaces were tested with a computer controlled biaxial Instron

machine capable of simultaneous application of a torsional and a tensile or

compressive load. Either the load versus time or displacement versus time are

programmable. A holder containing Wood's metal was used to obtain accurate

axial alignment.

17
For the measurements made in this study, except as otherwise indicated,

the load consisted of a pure torque applied at a twist rate of 11.25°/sec.

This rate was chosen because it corresponded approximately to a rate which

would cause yielding in a time interval characteristic of 1/4 of a walking

cycle. The yield "strain" estimation was based upon data obtained last year

(Bundy, 1982a). The torque versus twist angle data was used to calculate an

interfacial shear strength according to the procedure described in the next

chapter. The results of the tests are described in Chapters V and VI. The

sample set tested and the correspondi ng mechanical properties are given in

Appendices 1-3 which are described later.

Surface Cleaning Procedures

To clean the polished or grit blasted samples the following cleaning

procedure was used:

1. One minute of swirling in distilled containing detergent

(Sparkleen, Fisher Scientific Company).

2. One minute running warm tap 1^0 rinse.

3. Three minutes ultrasonic cleaning in ethanol.

4. One minute final rinse by swirling in distilled h^O.

5. Hot air drying.

As a special case, three samples were subjected to an extremely thorough

cleaning procedure to test the effect of this variable on the subsequent

mechanical behavior of the interface. This procedure consisted of the following

steps:

1. One minute of swirling in distilled H^O containing detergent.

2. One minute running warm tap H

2

0 rinse.

3. Thirty minutes ultrasonic cleaning in ethylene dichloride (1,2

di chi oroethane)

18
4. Removal of the sample from the ultrasonic cleaner in a closed container so that the sample at no time contacted the surface of the

fluid.

5. Hot alkali treatment: cathodic cleaning at -5 volts for one minute

in a 95 °C solution containing 59.7 g of Oakite per liter. Oakite

is a trade name for a product containing 90% NaOH plus added surfactants, wetting agents, and detergents.

6. Two minute rinse in cold running tap H

2

O.

7. One minute rinse in running distilled H

2

O.

8. Testing for cleanliness by dipping in 10% HC1.

9. One minute running cold tap H^O rinse.

10. Two minutes hot tap H^O rinse.

11. Hot air drying.

Surface Roughness Measurements

The roughness of the surface is an important parameter for the determination of interfacial shear strength (as is described in the next chapter).

Profi lometric measurements were made on a number of the surfaces tested in

this investigation. The surface roughnesses, taken to be twice the arithmetic

average roughness, are given in Table III-l. As indicated in the table, some

of these values were obtained from actual measurements and some were estimated

based upon the measurements with the other materials. These values are probably

reliable to within a factor of less than 2, although this is uncertain.

Another set of roughness measurements, made by another group of investigators,

were seen to be in general agreement with the first set. A typical prof i 1 ometric

trace is shown in Figure I I I - 3 . Roughness measurements were taken both when

the profi lometer stylus was moved longitudinally along the surface parallel to

the axis of the rod and also when it was moved ci rcumferenti al ly . The similarity

of the results indicates that the polishing procedures used did not induce any

19
TABLE III-l

ROUGHNESS OF THE VARIOUS SURFACES OF THE METALS EMPLOYED

TO FORM M/BC INTERFACES

Material 1pm 6pm 15pm Gri t Blasted

316L 12 x 10

8

m* 14 x 10~ 8m* 20 x 10~ 8m** 140 x 10

8m

Co-Cr-Mo 12 x 10~ 8

m* 14 XE

1 00 o

(

—

X 20 x 10~ 8

m** 166 x 10

8m

Ti-6A1-4V ELI 12 x 10

8

m* 14 x 10

8

m* 20 x 10~ 8m** 205 x 10

8m

* based upon measurement with 316L.

** based upon average of measurements of 316L & Ti-6A1-4V ELI.

20
directionality in surface topography which would affect torsional strength

measurements. The particular measurement shown in Figure 1 1 1 - 3 was taken with

the stylus moving parallel to the axis of the rod.

250 fJLrr\

III - 3 Typical Prof ilometric trace for Measuring Surface Roughness

(Ti-6A!-4V ELI Sample with Grit Blasted Finish)
CHAPTER IV

MATHEMATICAL ANALYSIS

Figure IV-1 shows an actual measured torque vs. twist angle curve. It is

typical of the type of behavior which was observed for every sample. The data

which can directly be obtained from the curve are the maximum torque Tm which

corresponds to yielding of the interface, the twist angle at yielding and

the slope G Tm/y which is a measure of the elastic behavior of the

interface. These parameters are shown in the figure. Since these quantities

are sensitive to the length of the acrylic block, it is useful to normalize

them with respect to L . The normalized torque Tm/L is a particularly

useful quantity because it is a direct experimental measure of interfacial

strength, and unlike the calculated strengths later discussed, it is not based

upon any assumptions. Using the raw data available from the experiments,

calculation of the interfacial shear stress applied to the surface of the

rod by the bonded acrylic was performed as described below. The maximum t.^

which can be developed at any point on the surface before failure occurs is a

measure of the interfacial strength in torsion. Solution of as a function

of position on the surface of the rod can be achieved by modification, and

adaptation to torsional conditions, of the shear-lag analysis presented by

Greszczuk (1969) for pull-out tests and used by Beaumont and Plumpton (1977)

to obtain the shear strength of bone cement/metal junctions from experimental

data. The work of Beaumont and Plumpton was considered in more detail in last

year's report.

To calculate x.

f as a function of position along the rod buried in acrylic,

consider the rod buried in acrylic to depth L (as shown in cross-section in

Figure IV-2). The rod is subjected to an applied torque T , and the forces

22
E

hi,

*%

(%

t.V

*»t

oCO

oCO

O

SJ"

o

C\J

ro oo (LU-N) ddDl

••• o

o

>

23

Typical

Measured

Applied

Torque

T

app

vs.

Twist

/

Angle

Curve

(Interface

29)
o

IIx

CM

I >

24

Cross-Sectional

Representation

ot

Specimen

Geometry

w

Parameters

for

the

Mathematical

Analysis
and moments which act on a volume element v which is dx in length, are shown

in Figure IV-3. At any depth x, T is the torque present. Summation of moments

shows that:

T - (T + dT) + (x.

f

2rcRdx) R = 0

or

dT

dx

2rcR^x

if (5)

Taking an arbitrary cross-section at any position x along the length of

the rod shown in Figure IV-2, for the interfacial element ABCD shown in Figure IV-4

of "thickness" b^, the shear strain within it, S^, can be approximated by:

S. f = £x_ (for R/b.. »1) (6)

IT D

1f

IT

where y is the twist angle at position x. t..^ and S.. ^ are related by:

T if - G

if

s if (7)

where G.^ is the interfacial shear modulus. Substitution of (6) into (7) and

differentiation yields:

di.,. G.,R . rf _ i f dy

dx b . ^ dx

for the axi symmetric case where t.

and y are independent of the angular

position of element ABCD.

For volume element v shown in Figure IV-3 the twist angle gradient can be

expressed according to the standard formula (Dieter, 1976):

T_ _ dy

GJ dx ( 8 )

where G is the shear modulus of the metal and J is the polar moment of inertia

of the rod (nR^/2).

25
IV - 3 Torque T and Surface Shear Stress Acting on Volume

Element v at Position x

26
27
Here dy is the difference in twist angle between the two cross sections

of the ends of the volume element. By substitution of eq. (8) into eq. (7)

the interfacial stress gradient can be seen to be:

dT

if

dx

G . ,RT

i f

GJb. ,

i f

(9)

Differentiation of eq. (5) with respect to x and elimination of the

stress gradient between this equation and eq. (9) yields:

2 ^ = p

2 T, where p = [4G. /Gb

.

f

R] h

(10)

dx^

for which the solution is:

T = C^sinh px + C

^ cosh px

For the boundary conditions:

T = T at x = 0

app

(ID

T = 0 at x = L ac

it can be shown that:

T = -T coth pL sinh px + T cosh px (12)

app ^ ac app

Differentiation of this equation with respect to x and substitution into eq.

(5) yields as a function of position:

pT

x., = (- coth pL cosh px + sinh px) (13)

if

2jtR

2 ac

For an applied torque T , the maximum magnitude of x.^,

at x = 0 and is equal to:

denoted x^, occurs

P T

app

2txR

coth pLK ac (14)

28
From torque versus twist angle experimental data, the torsional interfacial

strength x. can be determined from eq. (14) by measurement of the maximum

applied torque T

m once the value of p is known. All of the parameters in (3

except for can be straightforwardly determined, b^ can be taken to be

the surface roughness measured by profilometry (see Table III-l). The sample

geometry and shear moduli of the components are known. An estimated value of

can be employed to find p, or the more rigorous procedure described below

can be used.

The following procedure can be employed to find an appropriate p value.

An average interfacial shear stress present when Tm

is applied, x.^, can be

defined as:

X if

rm

2ttR

2

L ac

(15)

Equation (14) can then be expressed as:

x

i s

X if

= pL coth pL *ac 'ac ( 15 )

where x^. , the interfacial shear strength for a sample loaded in torsion, is

the magnitude of the value of x

. ^ at x = 0 when T

m

is applied.

For a given metal surface and interface bond preparation procedure, p is

a constant, so that the right hand side of eq. (16) is a function of length

only. Hence, as L -> 0 , x . /x . , J

-» 1. Thus x. can be evaluated by measuring * ac i s i f is 3

Tm

in identically prepared specimens buried in acrylic to different depths and

extrapolating x.^ vs. L^

c data to zero length. Knowing x^, p can be determined

from eq. (16) from one of the x.. (L ) measurements.

1 T 3C

29
I

To illustrate these procedures the data obtained in last year's report is

3 shown in Figure IV-5. Using = 10 MPa (the modulus of the acrylic), the

value used by Beaumont and Plumpton (1977) to analyze the results of push-out

tests, the vs y curves derived for interfaces A and B are shown in Figure

IV-6. The values observed at yielding are about 230 MPa and 30 MPa respectively.

The i. value determined by Beaumont and Plumpton for a stainless steel rod

with a rougher surface finish than that used here was 18 ± 2 MPa. On the one

hand, the differences between the two studies point out that the relatively

high pressure technique does produce a higher quality interface than when

techniques closer to the standard clinical methods are used. On the other

hand, the extremely high x^ value indicates that the G.^. value used above is

probably a substantial overestimate.

Accordingly, the more vigorous procedure described above for obtaining

the correct p value was pursued. In this work 11 specimens of Ti-6A1-4V ELI

polished to a 15pm diamond finish were prepared with a range of L values.

SC

Previous experimental work (Beaumont and Plumpton, 1977) with push-out shear

tests indicated that one would expect a horizontal plateau on the i . , vs. L

i f ac curve at the higher values of L and a rather steeply increasing line as L SC sc gets smaller. As is shown in Figure IV-7 this type of behavior was observed

(with wide scatter) in the experimental data measured here. Below 2.5 cm

elevated x.^ values were seen. The regression line shown in the figure for

the six data points with the lowest L values (i.e. below 2.5 cm) is:

L = 2.549 - 0.2047 x.-

ac l f

which corresponds K to a x. value (i.e., x., at L =0) of 12.45 MPa. The

is ’ i f ac value that was determined by Beaumont and Plumpton for a stainless steel/PMMA

interface was 18 ± 2 MPa. Due to the wide data point scatter, if the dependent

and independent variables are interchanged in the regression, the x. value is

4.45 MPa.

30
T/L

(N-m/m)

IV - 5 Length-normalized Torque T/L

ac

vs. Twist Angle y for

Interfaces, A, B, and C

31
0 1.0 2.0 3.0 4.0

Y (°)

IV - 6 Maximum Interfacial Shear Stress T

m

if vs. Twist Angle y

32
33
Solving equation (16) for p using the value which falls closest to

the regression line given in Figure IV-7 showed that:

-1

p = 1. 549 cm

-6

Using the following values: R = 0.3175 cm, = 2.2 x 10 ' cm (the measured

4

roughness of 15pm T.. -6A1 -4VELI ) , and G-j.. = 3.85 x 10 MPa, interfacial shear

modulus G., was found to be:

i f

G.- = 0.161 MPa

l f

Although strictly speaking, one cannot exclude the possibility that G^ will

be different for different materials and surface finishes, it was not possible

to measure x._ vs L data for each combination individually (due to time

i f ac J K

constraints) so that this value of G. x was used in all calculation for the

i f

data in Chapters V and VI.

It should be emphasized that due to the uncertainties which are present

in the calculation of interfacial strength, the x. values given in Chapters V

and VI should be considered as approximate at least in comparison with the

normalized torque. These uncertainties are due to imprecision in the measurement

of surface roughness, data point scatter in the x^ vs. I_ ac regression, and

the assumption made above that the G.^. value determined for 15pm Ti-TA1-4V ELI

can be generally applied. At this point the x.

g values given in Chapters V

and VI are not known precisely enough to be used for design purposes.

It is interesting to note that the G.^ value above obtained from a torsion

test is much lower than the value obtained for the interfacial shear modulus

in the push-out test—1000 MPa (Beaumont and Plumpton, 1977). The significance,

if any, of this discrepancy is uncertain.

34
CHAPTER V

INFLUENCE OF SURFACE FINISH AND MATERIALS ON THE

TORSIONAL STRENGTH OF THE M/BC INTERFACE

The data which was obtained in this study is shown in tabular form in

three appendices. Appendix 1 provides a complete list of the samples tested

and describes, by sample number: the material, the surface finish, length of

the acrylic block L , whether or not sterilization or passivation was used,

3.C and the cure time. Special conditions for the test, such as whether an altered

value of strain rate or the rigorous surface cleaning procedure was used, are

given in the remarks column. Appendix 2 provides a listing by sample number

of the mechanical properties of the interfaces which includes the normalized

torque, Tm/L , Tm itself, the calculated interfacial shear strength t. , p, 3C 1 S

Y

y

(the twist angle at the yield point), an effective shear modulus =

T

m/Yy, and G^^/L^. Appendix 3 provides a listing of the interfacial mechanical

properties classified according to the material and surface condition which

includes Tm

, T

m/L , t. , and G . ’ ac’ is’ eff ac The sample set which is discussed in this chapter consists of those

samples which were tested at the standard twist rate of 11.25°/sec, cleaned

according to the ordinary procedures, and for which the cure time was four

days or greater. Cure times less than four days are excluded here since this

has an effect on mechanical properties as discussed in the next chapter. Only

those samples which were not sterilized and passivated are considered here,

the influence of these effects are considered in the next chapter. Thus only

the influence of materials and surface finish are discussed in Chapter V.

Surface Finish

Table V-l shows the mean value of Tm/L , the number of samples measured

ac

N, and the observed range for the materials and finishes investigated.

35
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36

observed

range

of

values

in

parentheses
Information for 316L, Ti-6A1-4V ELI, and the Co-Cr-Mo alloy with l(jm, 6(jm,

15|jm, and grit blasted finishes are given. Table V-2 shows the mean x.j s

,

number of specimens tested, and range. For visual clarity the mean values of

Tm/L and x. are shown in Figures V-l and V-2. ac i s a

There are several points to be noted from these figures. The most important

one is that, contrary to what would be intuitively expected, these measures of

the interfacial strength do not monotoni cal ly increase with a rougher surface

finish. The Tm/L curve shows a value at the finest surface finish (lpm)

ac which is comparable to or exceeds that for the grit blasted finish and has a

dip in the middle at the two intermediate finishes. The x. versus surface

finish curve is fairly flat for the three coarser finishes and increases for

the 1pm finish.

If this non-monotonic behavior were only observable in the x. curve, it

• "IS might be suspected that it was caused by an artifact in the calculation due to

the neglect, in the analysis described in Chapter IV, of some crucial factor

which influences the strength. However, the normalized torque is a purely

experimental quantity, whose value is not in any way based upon any assumptions

made in a theoretical calculation. Thus, the higher value at 1pm is a true

indication of a higher degree of "holding power" for a highly polished metal/

acrylic interface. The fundamental reasons for this is not certain. Some

discussion of possible mechanisms is found later in the report.

Another trend which was observed, at least for 316L and the Co-Cr-Mo

alloy was that, for the two finest finishes (1pm and 6pm), greater scatter and

less reproducibility was observed. Although, accurate statistical comparisons

can not be made due to small sample sizes and differing numbers of samples

tested, a rough measure would be the observed data range Ax^ normalized to

37
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05 p

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rH lt> r^. oo

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CM CM

rH r—i O

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rH

CO . r>. cd

r^> CD

H

CO 00 . CO

00 <D

CM

** •> Lf> m <X> CD CD CO •

I''.

r—

• • o UD • CO CD rH rH CD

ID CD •» •> * ** 'T m • o H" CO tD 1 co CD CD CO • • cn

^r •» • . o LO • CM o CO <x> rH rH CO oo

CD 00 rH •t « * CO rH * • C'- CO <£> rH • • o CD i—

CO • 00 cn CM • • • CO #< • CD cn cn rH o

CD V-/ CO

lo

oo

00CM

O CO • 'tf- n-rH CO00

CM

CO*3" rH

co

d o

rH iH

CM

CO

GO*3" rH

CT5 CD

r-i

d

(H rH

CO co t£> rH CO • • • <u tD 10 'w' 0) z CO CO CO .c

-pc

0)ScaQ.

CO CO co • • • C

CO LO •i

rH CO * •* •* c E CO o co co «3- CO 01 n • • • • • • >

rH] CO ro rH LO • r~ CO rH rH CM rH 05

#> •* o LO (0 • • • 05 co CsJ CSJ 3

rH rH rH i—

'w' s—

'W' CO> z CO co CO M-O

0)05

M

C

—

00 fC LU a. s- s: > •O

n— o 05 <0 1 s: • • >

•r~ r— i to sS- < S- p 05

<u _! ^Q

C_> •f— cn -p CD 1 1 c

f0 rH •r- o o

s: CO h- o *

38

mean

value

is

not

in

parentheses
/

Lqc

(N

)

700

100

0

\

316 L

AyIp 6pm 15pm

SURFACE FINISH

GRIT

BLASTED

V-1 The influence of Surface Finish on the Mean Values of

Normalized Maximum Torque T m/L ac for M/BC Interfaces

39
316 L o

Ld

ocCD <- CD ^

CO

m

—

LU

CJ<Ll. | 3

co

CDE

CMI >

(odtAi)

Sl

a

40

The

Influence

of

Surface

Finish

on

the

Mean

Values

of

Interfacial

Shear

Strength

T|s

of

M/BC

Interfaces
the mean value of x. and number of samples tested. Table V-3 shows the

values of Ax. /Nx . for the different materials and finishes. For the two

is is

coarsest finishes, this parameter is usually under 10%, whereas for the 1pm

and 6pm finish it was typically above 20%. This is what intuitively would be

expected due to the increased difficulty of specimen preparation and the

greater care which must be taken to obtain a finer surface polish.

Comparison With Other Research

It is of interest to compare the data obtained here with that from other

investigators. A precise comparison is not possible because each investigator

or group of investigators has employed different experimental procedures.

Tables V-4 through V-6 allow an approximate comparison to be made, however.

They contain interfacial strength results from push-out, pull-out, and tension

tests. Standard deviations are given for those studies in which they were

obtained. Samples made from passivated metal surfaces are included in the

tables, since as is indicated in the next chapter, passivation probably has

little influence on interfacial strength. Information regarding the surface

roughness, interface formation procedure, cure time, and interface formation

time is given in Tables V-4 through V-6 to the extent that it is specified in

the references. Some of the studies cited in these tables also include other

test results acquired under conditions less comparable to the data determined

in the present study. For brevity, these results are omitted from the tables.

There are several points to be noted when comparing the data obtained in

the torsion tests reported here in Table V-2 with that in Tables V-4 through

V-6. The first is with regard to reproducibility. For only three of the

combinations given in Table V-2 were four or more specimens measured. Calculating standard deviations for these conditions it is seen that for 15pm 316L:

41
TABLE V-3

SCATTER OBSERVED IN INTERFACIAL STRENGTH FOR DIFFERENT MATERIALS

WITH VARIOUS SURFACE FINISHES

FINISH

Materi al 1pm 6pm 15pm Grit Blasted

316L 30.9% 56.8% 2.23% 0.83%

Ti-6A1-4V ELI 7.40% 20.6% 7.51% 15.5%

Co-Cr-Mo 37.1% 24.5% 8.34% 2.04%

I

1

42
INTERFACIAL

STRENGTH

MEASUREMENTS

OF

T1-6A1-4V

DETERMINED

BY

OTHER

INVESTIGATORS

<0

4-» 01u

01

01

<0

4-> 41u

Q1

01

fO

4-> 01

i-01

01

cO

4-> 01 X>

<T3

<T3 cr:

01

01 E

E —

in -XU

caE

OJ

cr:

0) h- L- U_ 3

»-« U

C >> f- (T3 E *0

rn iH

01u

h- 3

ll u

-X C 0)

•*- CD E 5

rn «—

0)

0) E

E •*- t- 4->

4-» 01

01 s- h- t- 1—3

U_ 3 U_ Ll L4 cj »—

-X c >» C 01

*r- rg *r- 01 E *D E 5

CO H CO H

4->

01

i-3

U

>3

CO T3

4->

01u

co

j01

4-> c

LH

C OJ o i- T- 3

*-> *o

co oi E u

t- o O L. U_ Q.

“O

01 01 t- T- 4-> 01 r- 3 cn

Q. C C

CL*»- 3

CO £ rCL

• rH

XI X •— U 4-> O •*-

*3

CJ 01 u

f- 4-> 01 f— 3 cn

CL C C

O. *r- 3

CO E rCL X XT

r- L 4-> O f- H H- 5

TJ a oi

01 01 fc. 01 0) u E

1- 4-> 0) 4-> 01 *o 01 «— 3 cn 3 cn 01 u

CL C c CL C c x:

CL t- 3 CL *^- 3 in 01 <0 E r— c0 E r— 3 c

CL Q. CL o

• rH • rH X

jQ JZ XI

»— U L> »— t- 4-> <0 o W) O T~ O -p- 4-> 4~> in

r-i •«- S rH V- 5 01 c O

E E

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m ct cr oj cn cr oi cn cn >> m i- w O 01 JZ O 01 jz o o r— in

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cr: x: u > X t. > w x: w x: w O CO E m 4-> t- E <n 4-> t- in m p—

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. O r- Ifl * O »— <0 LO O m o 03 O H O CL 01 Q. O CL 01 CL rH Q. rH CL . t- -t- rH Q..—

co CM o o CO JZ • • • 4-1 o csi CM CM rH

cn co C CL •H H H •H •H

ai 2: : L- •o- on on un

4-1 • • LO iri 00 in rn eg

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<0

Ll m

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CO CO — <o <0 c

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r—

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C c C C

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o o o o O

1 +-M1 W in m <n X in c c C c m

01 01 01 01 01 3

H*|1 — h- K- H- a.

43
TABLE

V-5

IO 10 10 io

a>u X 4J 4J X 10 c OS ai 0) OS o X

L. U u u u OS

1) 0) CJ 0

S

OS H- r— r— r— X a> r- r— r— 10 oc (D 01 01 OS io * X

01 OS 0) E as E OS E •f* E E X X

X

0) XOS X

0) u 0) s- OS

wo h- U >- 3 h~ L- h- 3 u

jm: Ll 3 U. U LL. 3 CL. U 3

J- H U u M u

lO .* E C > C CJ c > c OS >w

CJ — IO —. 0) — IO — as 10 QC E -a E 5 E ~0 E ) o

CO H C0 t-H CO rH CO rH rH X

c

OS *o "U o T3 E

CO 0) OS U 01 O) L 0) 0) L 01 OS L TD OS > •r *J QJ f D 01 — 4-> 0) — X os OS u o 0J c OS — 3 ct — 3 CT — 3 CT — 3 CT X

•J u o S- x c c CL C C CL C C X c c IO os

<0 T— 3 CL— 3 X — 3 X — 3 X— 3 3 C < X X “C •0 E •— io e — «0 E — iO S — X O

t- «0 0) a. a. CL X X o OS E u • rH • rH • rH • rH r— X X — o X X jO X X X X X «J O io

1 CO c o S- r- L D — S- X — t- 4J — i- X X X IO i- Q£ —

«

u_ x o — o — O — o — OS C 10 o o •H X 3 I-* *- y »H H- 5 rH X * E — E

1 K- o <

CJ o*—

1

<J >> ls >» U

U- h- — r- T3 — '

o

O 1 O CO X X C X X C X 10 —

Ul X C <0 a c io CL X — U TD CO > O 0) to 0) <0 X > os as S— Z L 3 TD 5- 3 -o io X X z »-• IO CT CT ID CT CT OS CT u >> io 10 LU wn O ID X O ID X O CT r- > X Ct: CJ r* IA W r— W0 WO f— o x —. —

UJ LU c r- X — — X — IO >> IO CH X x <0 3 r- T3 10 3r-0 AS X L- WO ZD h- CT **J W0 O ID d ia o ai X 10 O X io CO o 3 Q) X 4-> OS XX OS X u OS X

< O E • O 10 E - O 10 E ^ OS X E

UJ >- cc r l > X U > w X E ^ w o *a X CO E W0 X — E W4Jr wo w X rZL — O wo x— u wo E — WO E — -a X Q H r- a> (A H r- H lA 3.— E — xx as )— LU • O—iO .o*—<o LD O X — HON

o z O Q. CJ Cl O CL QJ CL rH X m o . u z >-» LU X oc or H* LU

CO H»

rH X rH X —

LU IT) CT —1 Q •r— jC • < u 4-> r-» rH rH rH o

ITS CT 10 O H- c CL -H •H •H •H < U OS X

u. CJ i- w rH CT CT

oc. X X

LU c CO CT CD r>. CD CO K— #—

t

Zr1

10 T- WO •a *o TD a U wo c ID 0) OS os io OS o a TD "O TD X L. — (0 — <0 — io — 10 L- X —

n

X TD > ID > 0> > OS > OS OS LO <

10 O — i- — L- S- — u X LU X o < O < Q < o < Cr-U.

3 X >- id u ID ID >» as >> OS >» OS > X 10 X U X U X u X U X E — —

<0 U u u Im 3 U > o o o o o E C w LU u. u. u. X srrj X

Z

X

C c c c 3

o o o o O

1 X IO wo IO wo X

W0 c c c c wo

ID a 0) OS OS 3 — >- b- h- s~ X

44
INTERFACIAL

STRENGTH

MEASUREMENTS

OF

STAINLESS

STEELS

DETERMINED

BY

OTHER

INVESTIGATORS

*3 SO r— •3 •3

as <0 o P u p p c as c P as C c o

as as * o O P L u c as E 03 E CL as

as JZ as *a 3 C 3 E r— P as as i— as 3 as 3 r—

as as

u •»- as o OS r- as QC a: o ^ ca > CO CL

as as as as E E E E

•r- as P P P a_ P

2l

as as as as s- u t- <0 f— u

CO 3 3 3 O U. 3

-X

U

U <J rH rnU

sfO >» >> >» II c >» E <0 as as *r- as

as *o

u "O P E *3 OC

rH rH rH 0 (O rH O so P so JZ Jd C as P P E >>*r- 50 •*- r- J <" 5 rH

*0 P >>

O P P

as c JZ P r -aw JZ "O SOU

-c as c 03 r— t- 03rL *o o c as SO E C *r- r o as f- o as as p o u 3 as O P E 03 P E 03 O

f- 3 a.

u t- as c C r- *0 P *o P 03 *a as t- “O as t- as

as as «— as

u c as H— as

p • *r- E LS as c as t- -o ^ o -X o JZ r- u o P O •n t- r— u p as u P as *— Q. O J- as jz c >> o as c c as C JZ Q.

lu a. E *r- SO E CL *r“

p a*rp rH <0 I U

*o

«C

L CLS t- as p Q- > P as 03

so > L f > -r- < 03 O

so r— - CO • P O

L

CO .O SO

a o < r- P

as as >s 03 as rU c D U Q. JZ E <_3 as as JZ O P CO ZL P r- a u as *o •r- CO E as as 3 CL E C r— 1 ZL E O w o as o rH ro w • oc CL JZ E T- TJ s O E so 3.P as 3.TCO O N fH i— . U T- . O

<M CL r—

O CL

p ro CH o SJ3 jz • • -H P /-> rH o CM

O) as CM C CL H • 03 *H

as 2: 03 rH

t- CM in

O

p ro

</3 rH • rH

rH rH rH

r— r—

to as as so f- U so LS u LS *T- SO

as so as as as > >> "O p as *4- JZ «4- JZ P W r- as L. t- t- -*-> S- P L as TD

*o as

h /> as os as 03 as so c o P S/3 < +> c P c P so as > as JZ C LU c as c as c as h- as p Hr U. HH L M U rH L 03 Q L

as as P P p as <

s: E Q. as as </) as co E S/> r— as 3 f- •*- 03 03 3 » u >» E U > as 5- as u E t- U u JZ

•»-

C

w t- as

U as f- as as o X T- as as as as x as as ll

03 S- > -c > JZ OJ JZ JZ X Q- < S/3 < s/> 21 «n so P p P P 3 3 3 3 c O O O O o 1 1 1 1 P JZ -C JZ r— SO

SO CO 10 so c

as 3 3 3 3 as P CL CL CL CL P O

OS

*r- OS e 5

03

rHC3 U r-

"O

O CL

ro P O -C

*— TJ P

a> t- • T- J

JZ i— r- a c

CLfH <13 E C >»

•r- ns E T3

*o

0)CLP

Q.*r-

<TJ 5

*0

C

ns •*- o e u

r— as rH 03

jO

C

rU 3 O <— H P Q.

JZ

CLU i *T— O O V) O S- CO r- +» r- u a as as p — *3

as as E -o • as -C j= E SO V3 3.t- -T- rH r— r— • O O O CL Q.

•H CM

cri

*a

as "O> as •*- as o u< asu >>

t. -a o

O)o

as P

asE .

co E •*- 3.*—O

CL ID

CM

-Hro

LDoas *o> as

•*- as Q U< asu >»

i- JD

as P

asu

as

as

45

Tension

Force

Divided

10.9 ±

0.7

15pm

(metal

lographlc 1

lb

load

applied 3

min

IFT

by

Area

polish)

for 1

min

with 1

week

cure

time

pi

unger
x. = 6.65 ± 0.305 MPa, for grit blasted Ti-6A1-4V ELI: x. = 7.15 ± 2.15

IS IS

MPa, and for 15|jm H-6A1-4V ELI: x. = 10.45 ± 3.25 MPa. The standard

deviations expressed as percentages of the means are respectively 4.6%, 30.1%,

and 31.5%. Although it is difficult to make a precise comparison, it appears

that all of the tests described - push-out, pull-out, tension, and torsion

have comparable reproducibility. This implies that, rather than being limited

by the mechanical testing procedures, reproducibil ity of results is mainly

governed by the ability to prepare the metal surface identically for each sample

and to follow exactly the same interface formation procedure each time. There

are probably practical and conceptual differences between these tests regarding

their accuracy, however, which gives the torsion test a significant advantage.

These aspects are discussed later.

Generally speaking, comparison of the data obtained here (in which the

interface was formed by applying relatively high pressure to the interface as

it was setting up) with that obtained by the other methods (in which relatively

low pressure was applied) supports the hypothesis that a stronger interface is

formed by the application of higher pressure. Comparison of the data in Table

V-2 with that in Tables V-4 through V-6 indicates that in the vast majority of

cases, independent of the surface roughness, the interfacial strength values

from the high pressure interfaces are greater than those formed with low

pressure techniques. Although there are a few exceptions and the comparison is

blurred due to differences in loading mode, roughness, cure time, etc., nonetheless, the highest strength was almost always observed for the high pressure

method. Also, the greatest value observed for interfaces formed using lower

pressure very often barely exceeded the lowest value in the comparable material

which had an interface formed with the high pressure technique. These

46
increased values due to high pressure probably indicate a higher true surface

area of contact for the high pressure technique.

One apparent exception is the 18 MPa value for stainless steel/bone

cement interfaces determined by Beaumont and Plumpton (1977) via shear- lag

3 analysis of push-out tests assuming G.. ^ = 10 MPa. For the 316L/BC interfaces

given in Table V-2, the G.^ value used was 0.161 MPa (determined as described

in Chapter IV). If the values from the two studies are recomputed using

comparable G.

^ values, then the lowest t. for an interface formed by applying

high pressure is more than 20 times greater than the T.

g that Beaumont and

Plumpton observed.

Two types of tests provided some support for the hypothesis that the high

pressure interface formation technique did produce a greater degree of contact

between the metal and the acrylic. With two samples, one hour prior to the

test saline solution was put on the top surface of the acrylic block. If

there were a significant degree of porosity at the interface (as is probably

the case in the clinical situation) one would expect fluid infiltration along

the M/BC interfaces of these samples due to capillarity. Since it has been

observed (Beaumont and Young, 1977) that M/BC interfaces where water is present

are weaker than dry interfaces, it would be expected that the samples in this

study exposed to saline would be weaker than comparable interfaces tested dry

if there had been fluid infiltration due to porosity. However, these samples

had mechanical properties well within the range observed' on dry samples.

Also, in two other specimens due to errors committed during the interface

formation, the acrylic was poorly placed and contained i rregul ari ti es and

gaps. These M/BC interfaces were more similar to the clinical situation.

During mechanical testing they proved to be significantly weaker than their

counterparts which had been formed with the high pressure technique without

any flaws in procedure.

47
Differences Between Materials

A final consideration is concerned with differences between materials.

There seems to be differences between materials when test results are compared

for the same surface finishes. The effect is most clear for the normalized

torque data of Table V-l. For the 1pm, 6pm, and grit blasted finishes the

metal/bone cement interfaces rank from strongest to weakest as: 316L, Co-Cr-Mo,

and Ti-6A1-4V ELI. For the 15pm finish this ranking does not hold and the

materials are rather similar (less than ±15% difference). With respect to

interfacial shear strength, the 15pm values seem anomolously ranked also as

can be seen by examination of Table V-2. x. otherwise seems to be larger for

316L than for the other materials. For Co-Cr-Mo and Ti-6A1-4V ELI the strengths

are similar except for the 1pm finish for which the Co-Cr-Mo alloy has a

higher interfacial shear strength.

Although not enough measurements were taken at all surface finishes to be

certain, the data for sterilized and passivated alloys which had 1pm and 15pm

surface finishes which is described in Chapter VI does not show clear differences

in mechanical properties between materials.

Two studies by other groups of investigators have been conducted in which

M/BC interfacial strength has been determined for different materials under

comparable conditions of roughness and testing procedure (Raab et al . , 1981),

(Keller et al . , 1980). The results are summarized in Table V-4 through V-6.

Like the present study, each of these studies indicated that differences in

t. between materials can be observed. Keller et al

.

1 s data shows that x.

is is

changes with cure time between a one day and a one week period and that relative

rankings of the materials change as well. Further work in this study described

in Chapter VI shows that for cure times of 4 days or longer little change in

T.j s occurs. Thus, comparisons of the ranking of materials determined from the

48
measurements of Raab et al. and Keller et al. (which have a one day cure time)

with the results of the present study cannot be made.

The one week data of Keller et al. is comparable in a certain sense, but

differences in ranking between materials for that study and the present one

could simply reflect the fact that the different materials behave differently

for tensile and torsional loading of the interfaces. Keller et al.'s study is

consistent with the observation made here that different surface finishes can

produce different rankings of materials.

Thus, in summary, it appears that differences do exist between the strengths

of interfacial bonds for different materials which depend upon surface roughness

and loading mode. Care must be taken when comparing results of different

researchers to be sure all data was obtained after equivalent cure times.

Although differences between materials do exist, at this point it cannot be

determined in an overall sense which type of metal is preferable for preserving

the mechanical integrity of the metal/bone cement interface in total hip

i replacements. Further analysis of interfacial stresses in actual devices

subjected to physiological loading and further series of tests on M/BC interfaces

in which only the loading mode differed would have to be conducted before this

point could be conclusively resolved.

49
CHAPTER VI

OTHER FACTORS WHICH AFFECT INTERFACIAL STRENGTH PROPERTIES

Influence of Surface Pretreatment

A number of samples with 1pm and 15pm finishes were subjected to passivation

and sterilization treatments typical of those used for surgical implants

before interface formation. Otherwise, the metal/bone cement interfaces were

prepared and tested in the same manner as were those discussed in Chapter V.

A comparison of the mean values of normalized torque and interfacial shear

strength measurements for the passivated and sterilized specimens, (termed the

"pretreated" specimens) and the non-pretreated specimens are shown in Tables

VI-1 and VI-2. For the pre-treated specimens the ranges are also given in the

tables.

For the 15pm finish there appears to be little difference for any material

in the interfacial mechanical properties caused by the pre-treatment. There

may be a counter productive effect at 1pm of sterilization and passivation at

least for 316L and Co-Cr-Mo. The statistical significance of this finding is

doubtful, however. Table VI-3 shows the confidence levels of t-tests which

tested the hypothesis that the mean value of the treated and untreated samples

are different. If the 95% confidence level is taken to be the level considered

to be significant, then no differences due to sterilization and passivation

were observed.

The data in Table VI-2 gives some indication that sterilization and

passivation treatments reduce the scatter in torsional measurements of interfacial

shear strength. This was observed where the scatter for non-pretreated samples

was large (316L and Co-Cr-Mo with a 1pm finish). Apparently the pretreatment

produces a more uniform surface in these cases. For the other cases examined,

50
TABLE VI-1

COMPARISON OF THE NORMALIZED MAXIMUM TORQUE FOR PASSIVATED AND

STERILIZED SPECIMENS WITH THAT FOR UNTREATED SPECIMENS

Material

and

Condition N

Surface Finish

1pm

Mean

(Range) N

15pm

Mean

(Range)

316L npt * 668 N 148 N pt** 2 218 3 186

(199-237) (147-263)

Ti-6A1-4V npt 193 176

ELI pt 2 166 2 173

(164-168) (139-206)

Co-Cr-Mo npt 419 185

pt 2 178 2 152

(163-193) (142-162)

* non-pretreated

**pre-treated

51
TABLE VI-2

COMPARISON OF MEAN INTERFACIAL SHEAR STRENGTH AND DATA POINT SCATTER FOR

PASSIVATED AND STERILIZED SPECIMENS WITH THAT FOR UNTREATED SPECIMENS

Surface Finish

1pm 15pm

Material

and

Condi ti on

N Mean

(Range)

Ax . /Nt . is is

(%)

N Mean

(Range)

At . /Nt

.

is is

(%)

316L npt*

pt** 2

33.3 MPa

12.6

(12.4-12.8)

30.9

1.67 3

6.65 MPa

7.18

(6.60-7.80)

2.23

5.57

Ti-6A1-4V

npt

pt

ELI

2

14.3

13.8

(13.0-14.7)

7.40

6.32 2

10.4

8.62

(7.82-9.42)

7.51

9.28

Co-Cr-Mo• npt

pt 2

21.4

10.5

(10. 5-10.5)

37.1

0.00 2

8.63

6.66

(5.57-7.75)

8.34

16.4

* non-pretreated

** pretreated

52
TABLE VI-3

CONFIDENCE LEVELS OF t-TEST COMPARISONS FOR DIFFERENCES IN INTERFACIAL

SHEAR STRENGTH DUE TO PASSIVATION AND STERILIZATION

Materi al Finish Confidence level (C.l)

315L 1pm 90% < C.l. < 95%

316L 15pm 90% < C.l. < 95%

Ti-6A1-4V ELI 1pm 70% < C.l. < 80%

Ti - 6 A

1 -4V ELI 15pm 80% < C.l. < 90%

Co-Cr-Mo 1pm 80% < C.l. < 90%

Co-Cr-Mo 15pm 80% < C.l. < 90%

53
scatter in the non-pretreated samples was not particularly large and the

passivation and sterilization produced little change. Presumably, here the

surfaces were fairly uniform to begin with and thus pre-treatment offered

little advantage with respect to reduced data point scatter.

In the study by Keller et al. (1980), it was reported that the tensile

bond strength of M/BC interfaces prepared from metal polished to a 0.1pm

finish and then el ectropol i shed and passivated exceeded that of interfaces

prepared from metal which was polished to 15pm and not given these treatments.

This finding is consistent with what has been observed in our study. The work

described in this report allows a correct interpretation of these results to

be made - i.e., that it is the influence of the finer surface finish and not

the passivation treatment which is responsible for these differences.

Influence of Strain Rate

Although it was not at all a primary area of emphasis for this year's

activities, a very preliminary investigation was made of the effect of twist

rate on the mechanical properties of M/BC interfaces tested in torsion. Some

effect would be expected because of the viscoelastic nature of the PMMA component.

Two 316L specimens were tested at twist rates of 2.81°/sec. and 50°/sec.

which are slower and faster respectively than the rates normally used for

these tests - 11.25°/sec. The surface finish was 15pm and the samples were

otherwise prepared identically to those for which the data appeared in Tables

V-l and V-2. The test results were quite unexpected. The interfacial shear

strength, normalized torque, and normalized effective shear modulus are shown

in Table VI-4 for the three twist rates. The interfaces tested at the slow

speed and the rapid speed were both much stronger (by a factor of 3-4) than

54
TABLE VI-4

INFLUENCE OF STRAIN RATE ON

OF 316L/BC

THE MECHANICAL

INTERFACES

PROPERTIES

Twist Rate t is (MPa) Tm/L (N) ac

G eff/L

ac^ N/degree)

2. 81°/sec 25.9 630 120

11. 25°/sec 6.65 148 101

(6.20-6.94) (145-150) (64.4-122)

50°/sec 20.5 420 82.3

55
those tested at 11.25°/sec. The shear modulus behavior was anomolous and

seemed to increase with decreasing twist rate which is the opposite of what

would be expected from any linearly viscoelastic rheological model.

Since the effect of strain rate on interfacial strength would be important

for implant performance and the effect on shear modulus would be of interest

from the fundamental point of view (if the effects above can be confirmed in

further tests), a priority area for future research is a study of the influence

of strain rate on the properties of metal/bone cement interfaces.

Influence of Rigorous Cleaning

Since the ability of two surfaces to adhere can be strongly influenced by

surface contaminants, it seems possible that interfacial bond strengths could

be improved by following a rigorous cleaning procedure. Two Ti-6A1-4V ELI

samples with 15pm surface finishes were subjected to the cleaning process

described in Chapter III as a very preliminary investigation of this possibility.

Otherwise, the samples were prepared and tested in the same manner as those

discussed in Chapter V for which the mean values of i. and Tm

r /L were 10.45

i s ac MPa and 175.7 N respectively.

The mean values of these parameters for the rigorously cleaned samples

were 12.84 MPa (range: 12.19 and 13.50) and 224.5 (range: 191.4 and 257.5)

which are, respectively 23% and 28% higher than those obtained with regular

cleaning procedures. This result suggests that rigorous attention to cleaning

procedures may be a way to increase bond strengths of pre-formed implant/bone

cement interfaces although too few tests were performed this year to accurately

assess this possibility. This effect should be investigated further in the

future.

56
Influence of Cure Time

In a previous study (Keller et al., 1980) it was noted that M/BC interfaces

tested in tension showed different strength levels when tested after cure

times of one hour, one day, and one week. A portion of the results were given

in Tables V-4 through V-6.

Nine samples used in this study were tested after a cure time of two

days. Normalized torque values are shown in Table VI-5. For comparison

purposes the means and ranges for samples tested at cure times t

c which were

greater than two days are given. The t values are also listed. For 15pm

surfaces some materials showed greater interface strength at two days and some

showed less compared to the larger cure times. For Co-Cr-Mo the effect was

different for treated and untreated samples. With the 15pm surfaces the

differences with respect to cure time was substantial, and the specimens

tested at two days never fell anywhere in the range of values observed at

greater cure times. A similar effect was not apparent for 1pm finishes,

perhaps because these show a larger data scatter.

Comparison of the one day and seven day results of Keller et al. (1980)

with these two day and four day or longer t

c data do not always show consistent

trends for a given material with respect to the magnitude and even the direction

of change of interface strength caused by cure time. However, this is not

surprising since the actual times, mode of loading (tension versus torsion),

and surface finishes and treatments were different.

The point to be made from these observations, however, is that the cure

time does seem to influence test results at least for cure times which are

less than or equal to two days. Almost all of the tests conducted here were

performed after 5-7 days of curing. It seems to be a prudent course of action

to avoid M/BC interface testing at cure times which are shorter than this time

period.

57
TABLE VI-5

INFLUENCE OF CURE TIME t ON THE NORMALIZED MAXIMUM TORQUE OBSERVED DURING

TORSION TESTING OF M/BC INTERFACES

Tm/L (N) Tm/L (N) t (days)

ac ac c Materi al Fi ni sh Condi ti on (t

c=2 days) (t

c >2 days) (for tc

>2 days

316L 15pm npt* 395 148** 4, 5, 19

(145-150)*** (5 samples total

)

Co-Cr-Mo 15pm npt 444 185 5

(157-210) (3 samples total

)

Co-Cr-Mo 15pm pt* 113 152 5

(142-162) (2 samples total

)

Ti-6A1-4V ELI 15pm pt 91.4 173 5, 6

(139-206) (2 samples total

)

316L**** 15pm npt 677 420 5

316L 1pm npt 247 668 5, 6

(225-1020) (3 samples total

)

Ti-6A1-4V ELI 1pm npt 180 193 5, 6, 7

(174-205) (3 samples total

)

Co-Cr-Mo 1pm npt 384 419 5, 6

(279-488) (245-682) (3 samples total

)

(2 samples total)

* npt: non-pretreated

pt: pretreated

**mean value is not in parentheses

*** observed range of values in parentheses

**** . test was conducted at y = 50°/sec
CHAPTER VII

DISCUSSION, SUMMARY, AND CONCLUSIONS

Pi scussi on

There are a number of points which have been raised by the experiments

conducted here which should be discussed further. The first of these concerns

the accuracy of the different testing techniques. It appears that reproducibi 1 i ty

of a given M/BC interface strength measurement is determined, as previously

described, by the surface preparation and interface formation technique rather

than the test method. Distinctions between methods can, however, still be

made.

In tension measurements, for example, the fracture which is produced will

propagate both at the interface and through the acrylic (Keller et a!., 1980),

(Raab et al., 1981), which leads to difficulty in interpreting the results in

terms of the properties of the bond itself. This problem does not occur for

the torsion specimen used here. For the push-out type shear tests, significant

components of both shear and normal stresses are present at the interface

(Raab et al .

,

1981) which can make precise identification of the failure mode

difficult. It is thought that the torsion test gives a more unambiguous

measurement of interfacial shear strength because it is possibly less influenced

by normal stress components than is the push out or pull out test. Further

improvement of the test described here could perhaps be obtained by applying

torque to both ends of the rod simultaneously to generate a more even stress

distribution.

Another interesting point is concerned with a mechanism for the observed

variation of interfacial strength with surface roughness. It seems intuitively

clear that a roughened, grit blasted surface into which the acrylic infiltrates

59
will be strengthened to an extent limited by the shear strength of the bone

cement itself and the cross-sectional area of the interface which is occupied

by acrylic. As the surface becomes smoother one would expect this mechanism

to become less effective and for interfacial strength to decline. Up to a

point, this is consistent with experimental observation.

The finest surface finish (1pm), however, was also quite strong and

either comparable to or in excess of the strength for grit blasted material,

which indicates that an additional mechanism is involved which influences

interfacial strength and increases it when the metal has a very fine surface

finish. Among the possible effects involved are an increased electrostatic

interaction between the metal and bone cement (similar to the adhesion between

two highly polished optical flats placed in contact due to the enhanced proximity

of the surfaces), a difference in the amount of trapped air or surface "porosity"

present at the interface which creates contact area differences, and size

differences of interfacial flaws and voids from which the interfacial fracture

would originate. At present the mechanism cannot be determined. Further

attention should be paid to this matter because additional understanding of

the factors which influence interface bonding could lead to strength improvements of M/BC interfaces.

With respect to the mechanism of adhesion between the metal and bone

cement it is possible that the situation could be as indicated in Figure

VII-1. The figure illustrates the idea that two components comprise the

bonding forces between the surfaces. One is a force due to mechanical interlocking which is negligible for fine surface finishes and dominates for coarse

finishes. The other force is an atomic (or chemical) interaction which dominates

at fine surface finishes due to the proximity of the surfaces and which is

negligible for coarse finishes. To some extent the normalized torque and

60
CO

CO

LlI 2X

CD

ZDOX

Ll! O£

CC

ZD

CO

61

VII- 1

Hypothetical

Components

of

the

Bonding

Force

Between

Metal

and

Bone

Cement

Surfaces.
interfacial shear strength versus surface roughness data which was given in

Figures V-l and V-2 is consistent with the notion shown in Figure V I

I

1 .

Figure VI 1-2 shows this correspondence more clearly. The data points

shown by symbols in this figure are the mean values for normalized torque

which appear in Figure V-l. The error bars associated with these points do

not give the standard deviations but rather show the maximum and minimum

values which were measured. The cross-hatched band between 100 and 200 N is

where the vast majority of the measurements at the intermediate surface

finishes were located. Except for one outlying point, all of the grit blasted

samples were well above the band which indicates the influence of the

mechanical interlock and the great reliability with which this type of surface

can be used to strengthen M/BC interfaces. The dominance of the atomic (or

chemical) force is seen at the finer surface finishes for the Co-Cr-Mo alloy

and the 316L stainless steel. The titanium alloy shows little tendancy for

atomic interaction. For the other two alloys at a 1pm finish both the mean

and maximum values are well above the cross-hatched band. The atomic interaction effect can also be seen at 6pm for the maximum values. However, it

appears that even with apparently identical surface preparation techniques, we

were not able to assure that conditions were favorable in all samples to achieve

the maximum degree of atomic interaction. At least one specimen with a weak

bond (in or very near to the cross-hatched band) was observed for Co-Cr-Mo and

316L at 1pm and 6pm. Very likely there are other variables (as yet unidentified)

which effect the performance of these specimens.

There is another point which should be considered with respect to the

adhesion mechanism between the metal and bone cement. In addition to the

roughness of the surface per se, the manner in which that surface is produced

could be important. For example, subsurface effects which occur during

fabrication, such as the layer of "disturbed metal" associated with polishing

62
Ti

-6AI-4V

ELI

• <

*

o

o

o

o

o

( N ) 0D “1 /yj JL

o

o

o

UJH

CO<

CDH

o:o

63

VII-2The

Influence

of

Surface

Roughness

on

the

Mean

and

Extreme

Values

of

Normalized

Maximum

Torque

T

m

/L

ac

for

M/BC

Interfaces.
or compressive residual stresses associated with grit blasting, might be

relevent since reactivity and oxide-forming tendency can be increased or

decreased by coldwork.

The emphasis of this year's research has been to determine, within time

and effort constraints, the main variables which influence the strength of

M/BC interfaces in torsion. The thrust has been to study the effect of a

number of different factors which possibly could be important, rather than to

focus upon the statistical design of the sample set tested to achieve precise

insight into the magnitude of the differences observed.

Some statistical calculations have been done however, to examine the two

main theses which have emerged from the experiments which have been conducted,

i.e., that 1) the finest surface (1pm) has a higher interfacial shear

strength than the others which have been tested and 2) that the coarsest

surface finish (grit blasted) and the finest surface finish have higher values

of interfacial "holding power", measured as Tm/L

ac

, than the intermediate

finishes. Specifically, t-tests were performed to test the hypotheses that

the mean values of x. and Tm/L observed for the different surface finishes

l s ac were greater than the minimum measured mean value (min.). The results are

shown in Table VII-1 which gives the confidence levels for the t-test

comparisons for the different materials and surface finishes studied.

These results are for the most part consistent with the theses mentioned

above even though, a priori, the sample set was not designed to test their

validity. For example, the normalized maximum torque for the grit blasted

specimens is elevated above the minimum mean value for all materials to a

highly significant degree (at confidence levels of 97.5% or higher). The

elevation, although evident at the 1pm finish, is not as marked in the

statistical sense. This would be expected based upon the discussion above

64
Table VII-1

Confidence Levels of t-Test Comparisons for Differences in

Interfacial Strength and Normalized Maximum Torque Due to Surface Finish

Hypothesis 316L Ti-6A£-4VELI Co-Cr-Mo

1pm > min. 90-95% 99.5-99. 9% 80-90%

6pm > min. 70-80% 80-90% 80-90%

15pm > min. -- 97.5-99% 70-80%

GritBlasted > min. c. £. > 99.95%

Hypothesi

316L

Tm/L

ac Ti-6A£-4V ELI Co-Cr-Mo

1pm > min. 90-95% 80-90% 80-90%

6pm > min. 70-80% — 70-80%

15pm > min. — 70-80% —

Grit

Blasted > min. c.Z. > 99.95% 97.5-99% c.£. > 99

65
regarding the scatter and the inability to consistly prepare the surface in an

optimum fashion for the 1pm samples to maximize interfacial strength. The two

intermediate finishes, 6pm and 15pm, show little evidence of statistically

significant elevation above the minimum mean values (confidence levels were

under 80%).

For the mean values of interfacial strength, the 1pm finishes do exhibit

significant elevations above the minimum mean value (confidence levels between

80% and 99.9%). The data for the interfacial shear strength shown in Table

VII-1 is interesting in that there appear to be differences in observed mean

values which are highly statistically significant (e.g. for 15pm and 1pm

Ti-6A£-4V ELI compared to a grit blasted surface) but which actually would

represent less improvement of interfacial strength above the minimum condition

(percentage wise) than do finishes for other materials for which the degree of

statistical significance was less. This can be seen by comparing the mean

values in Table V-2 with the t-test results in Table VII-1.

66
Summary and Conclusions

Seventy-seven M/BC interfaces were formed using a relatively high pressure

technique and tested in torsion to obtain torque versus twist angle data.

Interfacial yielding was easily observed. An interfacial shear strength was

calculated from the maximum measured torque and the sample geometry using a

mathematical analysis based on shear-lag theory. Some prof i 1 ometri c measurements of the surface roughness were also obtained.

The main variables which were examined in this study were: 1) differences

between materials (316L stainless steel, Ti-6A1-4V ELI, and a Co-Cr-Mo alloy

were examined); 2) differences due to surface finish (grit blasted surfaces

and surfaces polished with diamond paste down to 15pm, 6pm, and 1pm were

employed); 3) the influence of sterilization and passivation treatments, and

4) the influence of cure time. A few preliminary tests involving rigorous

cleaning procedures and different twist rates were also performed.

Few statistical analyses have been given in this report. The emphasis

has been to study a fair number of different effects with a few samples each

in order to isolate the main effects and variables which influence the test

method and/or could be used to improve the interfacial strength rather than to

obtain precise statistical information for a smaller number of effects.

Further studies to get good statistics for the most important findings would

be useful in the future.

The most important findings of this study are as follows:

1. Contrary to what would intuitively be expected, interfacial strength as

assessed by the measured maximum normalized torque was greatest for the

coarsest and for the finest surface finish as compared to the intermediate

finishes. The calculated shear strength was greater for the 1pm finish

than for the coarser finishes.

67
2 . Greater data point scatter occurs for the finer surface finishes. When

the results obtained here are compared with the work of other investigators

who have performed tension, push-out and pull-out tests, the test reproducibility seems comparable in all cases. This indicates that reproducibility is probably a function mainly of the interface preparation

technique rather than the type of test. The torsion test described here

may be more accurate than these other tests, however.

3. The use of the relatively high pressure technique employed here to form

the interfaces seems to clearly increase interfacial strength when compared to tests performed by other investigators whose methods more closely

parallel clinical techniques. This gives reason for optimism that the

concept of pre-forming M/BC interfaces before implantation could be used

to enhance interfacial strengths.

4. There do seem to be some differences between implant materials when the

other conditions of metal surface preparation, interface formation, and

testing are equivalent. The 316L interfaces are the strongest, although

this finding is not significant for the practical case of total hip

replacement design because very few of these are fabricated from austenitic

stainless steels. This material was included in the testing program to

determine whether or not the material used to make the interface was a

variable which affected torsional strength. Except at 15pm, normalized

yielding torque was about 1/3 higher for the Co-Cr-Mo alloy than for the

Ti-6A1-4V ELI in the non-treated condition. Interfacial shear strength

for the non-treated Co-Cr-Mo alloy at 1pm was about 50% greater than that

of the Ti alloy. However, similar values were observed for the other

three roughnesses. Investigators who have examined M/BC interfaces with

a different loading mode have ranked the materials differently.

68
5. Only weak evidence was found, if any exists at all, to indicate that

sterilization and passivation exert an influence on M/BC interface strength.

Due to the potentially great influence that such procedures could have on

the electrochemical behavior of the metals involved due to surface alteration,

this finding may seem surprising. However, electrochemical effects such

as breakdown of passivity and consequent increase of corrosion current

are very sensitive to localized inhomogeneities and defects in the passive

film. Although, for the mechanical behavior of an interface, such film

defects could possibly be stress concentrations and could result in some

decrease in strength, M/BC interface mechanical behavior would be more

affected by the overall surface contact area. Another possible explanation

is that the surfaces were already well passivated, so that the treatment

had little additional affect.

6. In a very preliminary test, interfacial strength seemed to depend upon

twist rate.

7. A rigorous cleaning procedure for the removal of both organic and inorganic

contaminants seemed, in a preliminary test, to offer the possibility of

increasing interfacial strength.

8. Some samples tested at a cure time of two days produced different strength

results than those tested at cure times of four days or more. Thus, low

cure time tests should be avoided. Changes with cure time have been

observed by other investigators as well which are consistent with this

finding. It should be pointed out that the interfacial bond strength

values which appear in the literature are often determined after a one

day cure time.

69
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1. W. Barb, J. B. Park, G. H. Kenner, and A. F. von Recum, " Intermedull ary

Fixation of Artificial Hip Joints with Bone Cement-Precoated Implants.

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2. P. W. R. Beaumont and B. Plumpton, "The Strength of Acrylic Bone Cements

and Acrylic Cement-Stainless Steel Interfaces," J. Mater. Sci., 12,

1853-6, 1977.

3. J. D. Bentkover and P. G. Drew, "Cost Benefit/Cost Effectiveness of

Medical Technologies: A Case Study of Orthopedic Joint Implants," Case

Study No. 14, Background Paper No. 2 in The Impl i cation of Cost-Effectiveness

Analysi

s

of Medical Technology , OTS, Sept. 1981.

4. P. W. R. Beaumont and R. J. Young, "Strength and Fracture of Acrylic Bone

Cement and Cement-Metal Interfaces," Plastics and Rubber: Materials and

Applications, 2-6 Feb. 1977.

5. K. J. Bundy, "Investigation of Stress Analyses and the Metal/Bone Cement

Interface in Total Hip Replacement Prostheses" in A. C. Fraker, A. W.

Ruff, and K. J. Bundy (with S. A. Demontigny, P. Sung, A. C. Van Orden,

and K. M. Speck), Metal 1 urgi cal Studi es of Interface Bondi ng on Implant

Alloys , NBSIR 82-2563 (FDA), October, 1982a.

6. K. J. Bundy, "A Torsion Test for the Metal/Bone Cement Interface" submitted

to Journal of Biomedical Materials Research, November, 1982b.

7. D. R. Carter, R. Vasu, D. M. Spengler, and R. T. Dueland, "Stress Fields

in the Unplated and Plated Femur Calculated from I_n Vivo Strain Measurements,"

J. Biomech., 14, 63-70, 1981.

8. E. Y. S. Chao and M. B. Coventry, "Fracture of the Femoral Component

After Total Hip Replacement," J. Bone Jt. Surg. , 63-A, 1078-94, 1981.

70
9.

J. Charnley, "Fracture of Femoral Prostheses in Total Hip Replacement. A

Clinical Study," Clin. Orthop. and Rel. Res., Ill, 105-20, 1975.

10. C. L. Compere and J. L. Lewis (eds. ) Proc. Workshop on Internal Joint

Replacement, Northwestern U. , March 3-5, 1577.

11. S. Cowin, "Continuum Models of the Adaptation of Bone to Stress," in

Mechanical Properti es of Bone (ed. S. Cowin), Jt. ASME-ASCE Appl. Mech.

,

Fluids Eng., and Bioeng. Conf. , Boulder, CO, June 22-24, 1981, AMD-Vol.

45, ASME, NY 193-210.

12. S. C. Cowin and K. Fi roozbakhsh , "Bone Remodeling of Diaphysial Surfaces

Under Constant Load: Theoretical Predictions," J. Biomech. , 14, 471-84,

1981.

13. S. C. Cowin and R. R. Nachlinger, "Bone Remodeling III: Uniqueness and

Stability in Adaptive Elasticity Theory," J. Elasticity, 8, 285-95, 1978.

14. S. C. Cowin and W. C. Van Buskirk, "Surface Bone Remodeling Induced by A

Medullary Pin," J. Biomech., 12, 269-76, 1979.

15. R. D. Crowni nshi el d , R. A. Brand, R. C. Johnston, and D. R. Pedersen, "An

Analysis of Collar Function and the Use of Titanium in Femoral Prostheses,'

Clin. Orthop. and Rel. Res., 158, 270-7, 1981.

16. G. E. Dieter, Mechanical Metallurgy , McGraw-Hill, N.Y., 2nd ed. , p. 381,

1976.

17. A. S. Greenwald and A. H. Wilde, "Some Observations on the Interface

Strength of Bone Cement," Biomechanics Laboratory Report 002-74, Cleveland

Clinic Foundation, 1974.

18. L. B. Greszczuk, "Theoretical Studies of the Mechanics of the Fiber-Matrix

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Phil. , 1969.

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19. D. H. Hegedus and S. C. Cowin, "Bone Remodeling I: Theory of Adaptive

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20. D. H. Hegedus and S. C. Cowin, "Bone Remodeling II: Small Strain Adaptive

Elasticity," J. Elasticity, 6, 337-52, 1976b.

21. N. J. Holm, "The Relaxation of Some Acrylic Bone Cements, "Acta Orthop.

Scand. , 51, 727-31, 1980.

22. R. Y. Hori , J. L. Lewis, J. R. Zimmerman, and C. L. Compere, "The Number

of Total Joint Replacements in the United States," Clin. Orthop. and Rel.

Res., 132, 46-52, 1978.

23. R. Huiskes, "Stress Analyses of Implanted Orthopaedic Joint Prostheses

for Optimal Design and Fixation," Acta Orthopaedica Belgica, 46, 711-27,

1980.

24. J. C. Keller, E. P. Lautenschl ager , G. W. Marshall, and P. R. Meyer,

"Factors Affecting Surgical Alloy/Bone Cement Interface Adhesion," J.

Biomed. Mater. Res. , 14, 639-51, 1980.

25. J. L. Kelsey, "Epidemiology and Impact," Total Hip Joint Replacement, NIH

Consensus Development Conf. , Bethesda, March 1-3, 1982.

26. W. R. Krause, J. Miller, and P. Ng, "The Viscosity of Acrylic Bone Cements,"

J. Biomed. Mater. Res. , 16, 219-43, 1982.

27. L. E. Lanyon, I. L. Paul, C. T. Rubin, E. L. Thrasher, R. DeLaura, R. M.

Rose, and E. L. Radin, "Ij} Vi vo Strain Measurements from Bone and Prosthesis

Following Total Hip Replacement," J. Bone Jt. Surg. , 63-A, 989-1001,

1981.

28. G. M. McNeice, P. Eng., and H. C. Amstutz, "Finite Element Studies in Hip

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72
29. S. Pal and S. Saha, "Stress Relaxation and Creep Behavior of Normal and

Carbon Fibre Reinforced Acrylic Bone Cement," Biomaterials, 3, 93-6,

1982.

30. J. B. Park, A. F. von Recum, and G. E. Gratzick, "Precoated Orthopedic

Implants with Bone Cement," Biomat. Med. Dev. Art. Org. , 7, 41-53, 1979.

31. J. B. Park, W. Barb, G. H. Kenner, and A. F. von Recum, "Intramedullary

Fixation of Artificial Hip Joints With Bone Cement-Precoated Implants II.

Density and Histological Study," J. Biomed. Mater. Res., 16, 459-70,

1982.

32. S. Raab, A. M. Ahmed, and J. W. Provan, "The Quasistatic and Fatigue

Performance of the Implant/Bone Cement Interface," J. Biomed. Mater.

Res., 15, 159-82, 1981.

33. S. Raab, A. M. Ahmed, and J. W. Provan, "Thin Film PMMA Precoating for

Improved Implant Bone-Cement Fixation," J. Biomed. Mater. Res. , 16,

679-704, 1982.

34. G. C. Sih, E. T. Moyer, and A. T. Berman, "Analytical Modeling of BoneCement Interface and Failure Prediction," Eng. Frac. Mech. , 14, 779-87,

1981.

35. R. P. Welsh, R. M. Pilliar, and I. MacNab, "Surgical Implants - the Role

of Surface Porosity in Fixation to Bone and Acrylic," J. Bone Jt. Surg.

,

53- A , 963-77, 1971.

36. W. E. White, J. H. Wedge, and C. M. Sargent, "Failure Analysis of a

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Science, 7, 27-39, 1979.

73
APPENDIX 1

MATERIALS, SURFACE FINISH AND PRE-TREATMENT PROCEDURES, AND INTERFACE

PREPARATION AND TESTING CHARACTERISTICS

L ac Steri 1 i zed

jpec. Surface and C No. Materi al Finish (cm) Passivated? (day:

11 316L 15pm 2.41 no 4

12 316L 15pm 2.25 no 4

13 Ti-6A1-4V ELI 15pm 2.41 no 4

14 T

i - 6 A1 -4V ELI 15pm 2.64 no 5

15 T1-6A1-4V ELI 15pm 2.39 no 5

16 316L 15pm 2.55 no 5

17 Ti-6A1-4V ELI 15pm 2.76 no 5 18 316L 15pm 2.60 no 5

19 Ti-6A1-4V ELI 15pm 2.23 no 19

20 316L 15pm 2.70 no 19

21 Ti-6A1-4V ELI grit blasted 2.69 no 19

22 Ti-6A1-4V ELI 15pm 2.97 no 19

23 316L 15pm 2.48 sterilized only 19

24 316L 15pm 2.50 no 19

25 Ti-6A1-4V ELI grit blasted 2.81 no 5

26 Ti-6A1-4V ELI grit blasted 1.97 no 5

27 316L 1pm 2.47 no 5

28 316L 15pm 2.64 sterilized only 5

29 316L 15pm 1.52 yes 5

30 Ti-6A1-4V ELI grit blasted 2.60 no 5 31 Co-Cr-Mo 15pm 2.57 no 5

32 Ti-6A1-4V ELI 15pm 2.63 yes 5

33 Co-Cr-Mo 15pm 2.77 no 5

34 316L 1pm 1.78 5

35 Ti-6A1-4V ELI 15pm 2.28 yes 2

36 Co-Cr-Mo 15pm 2.32 no 2

37 316L 1pm 2.90 2

38 Co-Cr-Mo 1pm 2.83 no 2

39

40

T1-6A1-4V ELI

316L

1pm 2.1

15pm

52 < L <

2. 79

aC

3.09 no

no 43

2

41 Co-Cr-Mo 1pm 2.96 no 2

42 Co-Cr-Mo 15pm 2.65 - yes 2

43 316L 15pm 2.81 no 2

44 Ti-6A1-4V ELI 1pm 2.84 no 2

45 Co-Cr-Mo 6pm 2.27 no 6

46 Co-Cr-Mo 1pm 2.27 no 6

47 Co-Cr-Mo 1pm 1.60 no 6

48 Ti-6A1-4V ELI 6pm 1.96 no 6

49 Ti-6A1-4V ELI 15pm 1.45 yes 6

50 316L 6pm 1.98 no 6

51 316L 1pm 2.47 no 6

52 Ti-6A1-4V ELI 1pm 2.33 no 6

53 Ti-6A1-4V ELI 6pm 1.87 no 6

54 316L 6pm 2.72 no 5

55 Co-Cr-Mo 15pm 2.37 no 5

Remarks

pt 12 d. before test

acryl i c poorly plact

strain rate=50°/sec

74
APPENDIX 1

Spec. Surface *"ac No. Material Finish (cm)

Sterilized

and c Passivated (days) Remarks

56 Co-Cr-Mo 15pm 2.64 yes 5

57 Co-Cr-Mo 15pm 2.12 yes 5

58 316L 15pm 2.69 no 5 strain rate=50°/sec

59 T i — 6A1 -4V ELI 15pm 4.09 no 5

60 T i — 6A 1 -4V ELI 15pm 0.87 no 5

61 Co-Cr-Mo 6pm 2.47 no 5

62 Ti-6A1-4V ELI 15pm 1.67 no 5

63 Co-Cr-Mo grit blasted 2.57 no 5

64 316L 15pm 2.06 no 6 strain rate=2. 81°/sec

65 316L 6pm 2.10 no 6

66 Co-Cr-Mo grit blasted 2.44 no 6

67 Co-Cr-Mo grit blasted 2. 22<L <3.

2.32

aC

71 no 6 acrylic poorly placed

68 316L grit blasted no 6

69 316L grit blasted 2.57 no 6

70 316L grit blasted 2.61 no 6

71 Ti-6A1-4V ELI 15pm 2.75 no 6 exposed to saline sol

ution for 1 hr. prior

to test

72 Ti-6A1-4V ELI 15pm 2.72 no 6 same as 71

73 Ti-6A1-4V ELI 15pm 1.66 no 6

74 Co-Cr-Mo 6pm 2.68 no 6

75 316L 1pm 2.30 yes 6

76 316L 1pm 2.65 yes 6

77 Ti-6A1-4V ELI 15pm no rigorous cleaning

procedure followed

78 Ti-6A1-4V ELI 15pm 2.04 no 7 same as 77

79 Ti-6A1-4V ELI 15pm 2.48 no 7 same as 77

80 Ti-6A1-4V ELI 6pm 2.55 no 7

81 Ti-6A1-4V ELI 1pm 2.39 yes 7

82 Ti-6A1-4V ELI 1pm 2.15 no 7

83 Co-Cr-Mo 1pm 2.34 yes 5

84 Ti-6A1-4V ELI 1pm 2.20 no 5

85 Co-Cr-Mo 1pm 2.77 yes 5

86 T

i - 6A1 -4V ELI 1pm 2.65 yes 5

87 Co-Cr-Mo 1pm 2.66 no 5

L - length of the acrylic block

clC t - cure time

c

75
11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

APPENDIX 2

MECHANICAL PROPERTIES OF METAL/BONE CEMENT INTERFACES

Tm/L

ac

(N)

Tm

(N-m)

T is

(MPa)

P

, -1

(cm

148 3.56 6.46 1.15

150 3.37 6.20 1.15

252 6.08 15.6 1.62

138 3.64 9.35 1.62

152 3.64 9.33 1.62

150 3.83 6.94 1.15

151 4.15 10.65 1.62

145 3.76 6.82 1.15

151 3.37 8.66 1.62

866 23.4 42.4 1.15

354 9.55 8.72 0.507

123 3.64 9.34 1.62

147 3.64 6.60 1.15

149 3.73 6.76 1.15

324 9.08 8.16 0.507

192 3.78 3.97 0.507

758 18.7 43.8 1.48

149 3.93 7.12 1.15

263 4.00 7.80 1.15

322 8.38 7.74 0.507

157 4.03 7.30 1.15

139 3.67 9.42 1.62

188 5.21 9.46 1.15

1020 18.2 43.0 1.48

91.4 2.09 5.35 1.62

444 10.3 18.9 1.15

247 7.16 16.8 1.48

279 7.89 18.5 1.48

54.4* 1.68 5.56 2.10

395 11.0 20.0 1.15

488 14.4 33.8 1.48

113 3.01 5.45 1.15

677 19.0 34.4 1.15

180 5.11 16.9 2.10

126 2.85 6.18 1.37

682 15.5 36.3 1.48

329 5.26 12.5 1.48

171 3.36 10.3 1.94

206 2.98 7.82 1.62

114 2.25 4.88 1.37

225 5.56 13.0 1.48

205 4.78 15.8 2.10

133 2.49 7.62 1.94

143 3.89 8.43 1.37

210 4.98 9.13 1.15

162 4.27 7.75 1.15

Y

y

(degrees)

G eff~

(N-m/deg.

)

G ,./

L

eff ac (N/deg.

1.21 2.94 122

1.25 2.69 120

3.77 1.61 66.9

3.50 1.04 39.4

2.18 1.67 69.8

1.48 2.59 102

2.72 1.53 55.4

2.25 1.67 64.4

1.95 1.73 77.6

10.8 2.17 80.3

5.46 1.75 64. 5 1.72 2.12 71.5

1.23 2.97 120

1.25 3.00 120

4.88 1.86 66.3

2.12 1.78 90.6

8.13 2.30 93.3

1.37 2.87 109

1.72 2.34 153

3.98 2.11 81.0

1.56 2.58 100

2.15 1.71 64.8

1.44 3.63 131

7.09 2.57 144

1.37 1.52 66.8

3.88 2.65 114

2.69 2.66 91.7

2.33 3.39 120

0.94 1.79 57.9

5.09 2.17 77.7

6.14 2.35 79.5

1.35 2.23 84.1

9.42 2.02 71.9

3.59 1.42 50.2

1.36 2.10 92.8

4.31 3.60 158

1.44 3.67 229

1.71 1.97 100

1.85 1.61 111

0.90 2.51 127

1.71 3.26 132

2.54 1.89 80.7

1.41 1.76 94.2

1.25 3.13 115

1.43 3.48 147

1.12 3.83 145

76
APPENDIX 2 (Cont.)

MECHANICAL PROPERTIES OF METAL/BONE CEMENT INTERFACES

>pec.

No.

Tm/L

ac

(N)

T

m

(N-m)

T

i s (MPa)

(3 y*y

(cm ) (degrees)

G eff«

(N-m/deg.

)

G .,/L eff ai (N/deg.

57 142 3.01 5.57 1.15 1.01 3.00 141

58 420 11.3 20.5 1.15 5.10 2.21 82.3

59 161 6.60 16.9 1.62 2.75 2.40 58.7

60 245 2.14 6.17 1.62 1.21 1.77 203

61 272 6.72 14.6 1.37 1.71 3.93 159

62 301 5.04 13.1 1.62 3.17 1.59 95.1

63 378 9.72 7.93 0.399 2.66 3.65 142

64 680 14.0 25.9 1.15 5.65 2.48 120

65 633 13.3 28.7 1.37 5.08 2.61 125

66 403 9.85 8.26 0.399 2.49 3.96 162

67 210* 7.78 5.44 0.399 2.43 3.20 86.2

68 575 13.4 12.0 0.434 4.84 2.76 119

69 554 14.2 12.1 0.434 5.22 2.73 106

70 556 14.5 12.3 0.434 5.75 2.53 96.8

71 146 4.01 10.3 1.62 2.25 1.78 64.6

72 136 3.69 9.46 1.62 2.04 1.81 66.6

73 153 2.54 6.60 1.62 1.49 1.71 103

74 256 6.87 14.9 1.37 1.94 3.54 132

75 237 5.47 12.8 1.48 2.15 2.55 111

76 199 5.29 12.4 1.48 1.97 2.69 101

77

78 258 5.26 13.5 1.62 2.99 1.76 86.1

79 191 4.76 12.2 1.62 2.70 1.77 71.0

80 182 4.66 14.3 1.94 2. 34 1.99 77.9

81 164 3.92 13.0 2.10 2.20 1.79 74.6

82 202 4.33 14.3 2.10 2.44 1.78 82.6

83 193 4.50 10.5 1.48 1.43 3.15 135

84 174 3.82 12.7 2.10 2.20 1.74 79.0

85 163 4.50 10.5 1.48 1.07 4.21 152

86 168 4.45 14.7 2.10 2.28 1.95 73.7

87 245. 6.52 15.3 1.48 1.52 4.29 161

based on maximum L for

ar this sample.

77
MECHANICAL

PROPERTIES

OF

METAL/BONE

CEMENT

INTERFACES

CLASSIFIED

ACCORDING

TO

MATERIALS

AND

SURFACE

FINISH

*

•HC

01 T3

\Oo

in

ll<X> «-> U TO

0> 0)

i- U ^ *-» 0) \ 'TJ C </> O >

1/3 u

roe

q:

u c u o

(TJ 4-> O 'TJ

•*-> EU

<u o

D HT3 0) TO

*-> u u

(T3 •*- V. -*-> **- 1/3 •H «TJ U r—

<0 CL

'TJ O 03 u o . *-> in cm

i/»

ll II

>»

<T3 o

m a <D

(13 u

I/)

(/>

*TJ CL

«6

T3 TO TO d ii ai H N N

CM

II ICC r- rrtj ITJ t_ t_ S- u u a; a) L>-> 4-> «-> -H 4->

• mm «/)(/)«/>

u a>

(TJ 0) _J u N OJ

OJ H- TO

a> \ o z

CO *T CO 03 ID . CM O CM . . rHOLO CM CM O O' O H H 0*1 rHrHrHmCO

or cn o . . • o CM rH CM CM

rH N CO H

OCM 03 CO o in

tf» <V CD o CM ID O CM o CO co

-r- CL o rH CO CM 03 ao o in 03 *D rH co co p*^ z • s-/ CM CM CM ID ID VD id CM D o *T O in ID co co

<D rH rH rH CM CO CM CM CM

rHCO<

r“\E UD r- CO ID CO CO o in 03 e | M* CM m m CO ao O' O o CO O D 03 o CM ao CO h- z • • • CO CO CO CO CO CO CO rH 03 rH *T CO CO CM CO CO

rH rH rH CM rH rH rH |H rH

u

*0 m^fLD od o o in o

in in o' m in <d m m in hhhpIco

o> in

*T 03

rH CO O O N CM CO

ID ^ lO

03 co

*T & rH rH CM

CO CO h c ro H H ifl

C

<yu

<yCL

to

u

<w -QE

3

CO 03 O H CM ID CO O

LO lO »HrHfHHCM o co co -r cooooi o m

cm f m io cm cm cm m in lo

au

CO %-u

D

oo

c

o

TJc

o

CD

TJ

a>

<n JO

t- o

Ea

LO 1

78

'
316L

(continued)

LO

srae

D

DC

*o *o

<D 0)

ca 03 > >

tO><T3 TD

CM

II

to to

to to

IT5 rtj Q. CL

oa

T3 TD

0) D H M

S- t-

<d

+-> 4-J to to

U ID

<V <D -J S- \ 05 V- <u -a

<d \ a z

co r>.

• <3- cm ro tr m rH

cn h h cr»

to «3 T- Q_ H 21 CO O O CO CO «** cn cn cn to cm cm

TJ* ^ r-1 rH rH rH

cn

CM LO LO

o

co cm m r-* cn

in o cm ^ cn 01 H CM CM CM pH

c

<DE

•r-U

<DCL VI

u

<D n

s

o2

r>. rH r- in lo

cm co in co r>.

79
APPENDIX

3

•a

*0 T3 T) 01 01 1>->-> H <13 <13 -*- > >

U> *A

l/» iSt

<13 <0 CL CL

v- -a

Oi N O Z

0 wo ro X f- a.

1 v- X

oi u

e o>

•*- .O U E

C

o» o u •*-

13 4-» T3C

oo

>»-0 -D

<T3 a» a» ”U N N

wO V/> >» >» •O T3

0) Oi

I U

Oi Oi

u*-> •*-> -> WO wo

CM CM

II II

CM

CM CM • O rH vo vo o co it H H CO H H H

CO

• <TV CM

cm m co O H rH

LT> O • CM CT> rH P*

CO VO vf) OMN O VO CO CO rH m in n rs m

co

rH VO O

CM tO CO N CO N

CO rH CO O CM Oin cm

co co co ^r

«

CO co o

P** O rH

CO if CM N CO O in CO rH H rH CM

VO CM VO cm m rH CM CMcm in

CO CM <T VO CO CM

CO rH

co

uo

CLE

<0

80
Grit

blasted

21

354

9.55

8.72

64.5

u c

«c o

+J H 4-> r- U

*r- O

L- •*-

<0 > >

wo v>

wo wo

CT3 «3 Q. CX, wo w wO •0 -3 >>

wo fO "O TD >•> TD

a» <V N N T3 N t— *r- rH

r— »— CSi •*- •*- TD

t- U J| O

a> OJ 4->

H-> 4-> 003

WO WO 4-> >

u t- 3 3

WO wo T5 "O

<U 01 c c

03 iO

0) 4> o u

>» >» o *—

WO WO 3 3

O O

U U

O O

cs ca

X o

0 WO u

CL 0> Q. C

ro —

fc. 03

01 wo

TJ (U

o» u ^3 1) wo wo E

o> o <3

H-» CL W U U

O 0) *3 0» ro wo o ai o co wd m rs »-h co

. . . ro.ro

co o rH tococncnr^rHcoocno

wo ci co corococnr^r^mcsicnrH co

05

toCO

VO CO rH o

VO VO rH

VO CO 05

CSj <7> o . . O N rH <7*

cor^^r cn m co^j* e-* o m n in

rl Cl N cororococorocnrHrHco wr CO n

CO

ro *r vo csj CM n id n

05 m rs m ci o o co c» co wo ro wo <x» m

co co co coo-^inr^^o^***r co cn O N fO O VO CO rH CO CO CO H O VO CO CT» O

rH cn CO CO VO On CO o vo cm ro co o ro co cororo^rorovoegmeg ro cm cm

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co rH ro eg rH

eg rH rH fO rH m wn in m eg wo «r rH rH rH rH rH eg

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81
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TD MM

>> ^ ^

i- || U U O CJ oj o u •*->•*-> cx w * v»

U CJ

t) a» _J u N ca

V- QJ V- TD « N O Z

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O N (MS

co co rs y)

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. 82
NBS-114A (rev. 2 -ec) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 83-2736

2. Performing Organ. Report No. 3. Publication Date

September 1983

4. TITLE AND SUBTITLE

Studies of Interface Bondings on Implant Alloys

5. AuTHOR(sAnna C. Fraker, Arthur W. Ruff, and Kirk J. Bundy

Jacqueline D. Smith, Robert W. Penn, Ann C. Van Orden

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions)

national bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No. 22 4795023

8. Type of Report & Period Covered

Oct. ,1 981 -Oct . ,1982

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP)

Bureau of Medical Devices

Food and Drug Administration

Si lver Spring, MD

10.

SUPPLEMENTARY NOTES

J Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. I f document includes a significant

bibliography or literature survey, mention it here)

The work dealth primarily with testing metai/bone cement interface strength

using the torsion test which was developed earlier in this project and described

in NBSIR 82-2563. The test proved to be a good one and to be reasonably

reproducible. The parameters studied to determine their influence on the metal/

bone cement interface strength were material type, specimen surface roughness,

sterilization and passivation treatments, cure time prior to testing and effects

of ultra clean surfaces. Metals used were the alloys, Co-Cr-Mo, Ti-6A1-4V

and 316L stainless steel. The bone cement used was Howmedica Surgical Simplex P

which is a poly (methyl methacrylate) type. Seventy-seven tests were conducted.

The description of these tests, analysis of the results and a discussion of

related studies in the technical literature are given in the technical report

entitled "An Experimental Investigation of the Torsional Strength of Metal/Bone

Cement Interface."

12.

KEY WORDS (Six to twelve entries; alphabet! cal order; capitalize only proper names; and separate key words by semicolon s) Bone cement; interface strength; metals ; poly (methyl methacrylate) ; prosthesis

fixation; surfaces; surgical implants.

13.

AVAILABILITY

[ y| Unlimited

1 | For Official Distribution. Do Not Release to NTIS

| | Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

90

15. Price

USCOMM-OC 6043-P 80