NATIONAL BUREAU OF STANDARDS REPORT

10 009

FAILURES IN WIRE BASKETS OF URETERAL STONE DISLODGERS

To

Bureau of Medicine/OM R Food and Drug Administration

Dept, of Health, Education, & Welfare

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards1 was established by an act of Congress March 3, 1901. Today, in addition to serving as the Nation’s central measurement laboratory,

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

3 120625 (G-40238)

10 009

FAILURES IN WIRE BASKETS OF URETERAL STONE DISLODGERS

By

B. W. Christ

and

C. H . Brady

Engineering Metallurgy Section

Metallurgy Division

To

Bureau of Medicine/OM

R

Food and Drug Administration

Dept, of Health, Education, & Welfare

NATIONAL BUREAU OF S for use within the Government

and review. For this reason, I whole or in part, is not auth Bureau of Standards, Washinf

the Report has been specitical

IMPORTANT NOTICE

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 201 5.

ss accounting documents intended

subjected to additional evaluation

listing of this Report, either in

2 Office of the Director, National

ly the Government agency for which

opies for its own use. <NBS>

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

Failures in Wire Baskets of Ureteral Stone Dislodgers

B. W. Christ and C. H. Brady

Keferences: 1. Bureau of Medicine Work Order Number CPF 2624-69(E).

2. Establishment inspection Report ( E I R ) on American

Cystoscope Makers, Inc., Central File No. 15,982*

A.F. Number 11-366, August 15, 1968.

3. Establishment Inspection Report ( E I R ) on

C. R. Bard, Inc., Central File No. 12,75^-* A.F.

Number 23-53^-* July 8 , 1968 .

Descr i pt i on of 1 tern:

Twelve Ureteral Stone Dislodgers were submitted under Reference 1 for identifying cause(s) of failure. This item is used by the medical

profession for removing kidney stones. The features of a stone dislodger

relevant to the purpose of this report appear in Figure la. This portion

of a stone dislodger consists of:

1) a flexible cable, 20.5 inches long and 0 . 080-inches diameter (only

about 1 1/2 inches of the flexible cable appears in Figure la).

2 ) a connector attached to the leading end of the flexible cable.

3) a "basket" of four fine wires, each about 1 1/2 inches long by 0 . 016 - inches diameter, at the leading end of the flexible cable.

4) a connector which consolidates the four basket wires and provides a threaded joint for a plastic "filiform" tip.

5 ) a connector at the trailing end of the flexible cable (not shown in

this report) for attaching a handle.

Purpose of Investigation:

It is reported* that the majority of repair items are returned because

of basket failure. Typical stone dislodgers which have failed appear in

Figures lb and lc. One can see that failure in these cases is a matter of

the wire basket having come apart near the connectors. Other failures

related to the plastic filiform tip have also been reported^, but these

will not be discussed in this report. The purpose of this report is to

identify possible metallurgical causes of the type of failure appearing in

Figures lb and lc, and to recommend procedures for minimizing the frequency

of such ;f a i 1 ures .

2 .

Ref. 2, page 4 under heading "GENERAL INFORMATION.

Ref . 3* pages 7

- 8, I terns 1 -5 under heading "COMPLAINTS".

2

Materia]

Engineering drawings describing ureteral stone dislodgers were available from two different manufacturers (Refs. 2 and 3 ). These drawings

are useful for determining how the item is put together and the materials

of which each component is made. The materials used for components near

the failed region of the stone dislodger are listed in Table 1. Each

manufacturer uses solder to fasten together the flexible cable, basket wires

and filiform tip connector. The solder used by each manufacturer is not

specified clearly in References 2 and 3-

The materials (and design) used by each manufacturer differ slightly.

For example: l) Manufacturer B specifies a nickel plate finish, presumably

for corrosion resistance, whereas Manufacturer A does not, 2) Manufacturer

B specifies that the basket wires be type 302 stainless steel with a spring temper*, whereas Manufacturer A specifies type 303 stainless, but no temper condition, 3 ) Manufacturer B specifies a connector between the

flexible cable and the basket wires, whereas Manufacturer A does not.

Detailed comparisons of each manufacturer's engineering drawings included

in Refs. 2 and 3 will show further slight variations in design and

materials. None of these variations would lead one to conclude that one manuf act urer 1 s product is better than the other.

General Observations on Usage:

The lifetime of a stone dislodger is variable. An individual stone

dislodger may be used repeatedly during a lifetime which can last as long

as twenty years. For example, it is reported^ that the majority of stone

dislodgers returned for repair "have been in service for periods up to

20 years". On the other hand, it has been noted that the stone dislodger

"is used once or reused at the surgeons discretion".

It is reported^ that each doctor may change "the shape of the stone

basket" prior to use, presumably by bending and twisting the basket wires

between his fingers.

When in use, the flexible cable and the basket wires are subjected

to both tensile and shear stresses applied through a handle attached to

•the trailing end of the flexible cable.

When the manufacturer cited in Ref. 2 is supplying a government

contract, stone dislodgers are given an autoclaving test which consists of

"10/10 minute intervals at 250°F, with sufficient time between to permit

* "Spring temper" is a meta 1 1 urgi ca 1 term applied to metal which has been

plastically deformed in a fabrication operation, such as wire drawing,

in order to increase the hardness and strength of the metal.

3. Ref. 2, page 2 under heading "SUMMARY OF FINDINGS".

4. Ref . 3, Exhi bit 6a

.

5. Ref. 3, page 6, last paragraph.

- 3

-

cooling to room temperature" , and subsequent examination. Presumably,

instruments in repeated use are also exposed to similar temperatures in an autoclave.

The manufacturer cited in Ref. 2 gives the solder joints at each end

of the wire basket "an acceptab i 1 i ty pull test of 40 lbs".^

Observations of Fractured Basket Wires and Cross Sections of Soldered Joints

Examination at magnifications of 10-250 times provides information

which serves to identify the possible causes of the typical basket

failures appearing in Figures lb and 1c. For example, Figure 2 shows

exterior views at 12X of the soldered joints between basket wires and

connectors. Figure 2a shows an intact joint. Figures 2b and 2c show

that wires at the filiform connector have fractured. Similarly, Figure 2d

shows that wires at the flexible cable connector have fractured. These

observations suggest that the cause of wire-basket failure is fracture of

the basket wires themselves, and not rupture of the soldered joints.

Views of two fractured basket wires appear in Figures 3 a nd 4. A view

at a magnification of 27X appears in Figures 3 a and 4a. Close-up views (at

200X magnification) of the fractured tips of these wires are shown in

Figures 3b and 4b. These wires have fractured primarily by shear after a small amount of plastic flow. Evidence of a shear failure is the fact

that the fracture surface is inclined to the longitudinal axis of the

basket wire. Evidence of plastic flow are the shape distortion of the

wires themselves and the rounded features of the fracture surface itself.

The cracks penetrating the surface of the basket wire shown in Figures 4b

originate in the n i eke 1 -p 1 ated surface layer. The wire shown in Figure

3b does not exhibit such cracks because the wire was not nickel plated.

When sample surfaces such as those appearing in Figures 3b and ^b are etched with 20$> HC1-5 HNOo, features of the meta 1 1 ur g i ca 1 structure

are revealed. For example, after etching, two failed basket wires appear

as in Figures 3c and 4c. The layer of nickel plate and its associated

microcracks show up clearly in Figure 4c. It can also be seen in Figure

4c that the microcracks are not restricted to the nickel layer; the

microcracks actually propagate into the basket wire itself.

Both Figures 3c and 4c show a typical structure for "spring temper"

material, i.e., a fiberous appearance with the fiber direction parallel

to the longitudinal axis of the wire. For comparison with Figures 3c

and 4c, the fiberous appearance of a basket wire from an unused stone

dislodger appears in Figure 5. Shape distortion is also apparent in

Figures 3c and 4c. These further observations support the viewpoint that

the cause of wire-basket failure is fracture of the basket wires themselves, and not rupture of the soldered joints.

6. Ref. 3, page 6, first paragraph.

7 . Ref . 2, Exhi bi t 6*.

To verify this viewpoint, cross-sections of the basket wires

embedded in solder within the filiform tip connector were prepared from

one unused stone dislodger, Figure 6, and from two used stone dislodgers.

Figures 7 and 8. The outlines of the basket wires and the filiform tip

connector are obvious in all these figures. Because the three different

sections have not been taken at the same level in each stone dislodger,

small differences in general appearance are noticeable. These differences

are not critical for the purposes of the present analysis, however. The

apparent termination of basket wires in cone-shaped tips is also a feature

of the sectioning process. As one would suspect, this is due to the fact

that all the basket wires do not lie completely parallel to, and in, the

plane of the section.

The primary feature to be noted in Figures 6 to 8 i s the degree of

soundness of the soldered joint. it can be seen in Figure 6, obtained on

an unused stone dislodger, that although solder has bridged within the

body of the connector and created at least seven small pores, there is

still a good deal of solder surrounding the basket wires. The bead of

solder on the surface of the filiform tip connector, Figure 6, exhibits

no pores and completely surrounds the basket wires.

The used stone dislodger shown in Figure 7 shows the features pointed

out in Figure 6. In addition, it shows some separation between the basket

wire and the bead of solder on the surface of the filiform tip. The

extent of this separation is likely to increase with increased frequency

of use of the stone dislodger. The significant point to note in Figure

7, however, is that the basket wire fractured before the separation between

wire and solder increased to the point where the wire could pull free of

the soldered joint.

The used stone dislodger appearing in Figure 8 shows a filiform tip

connector with somewhat different geometry than those connectors

appearing in Figures 6 and 7* Furthermore, the central hole in the

filiform tip of Figure 8a does not penetrate the full length of the tip,

as is the case in Figures 6a and 7 a. These small differences in design

are also illustrated by comparing Figures lc and lb. The sectioned

filiform tip in Figure 8a is the same as that appearing in Figure lc,

whereas the tip shown in Figure 7a is the same as that in Figure lb.

Since the sectioning plane is sharply inclined to the longitudinal

axis of the filiform tip shown in Figure 8, the cylindrical central hole

is somewhat obscured. Nevertheless, some bridging of the solder in the

central hole can be seen. Moreover, some separation between solder and

basket wires appears. Only a small bead of solder on the surface of the

filiform tip surrounds the basket wires at this point. (Compare the

solder bead size in Figure 8a with that appearing in Figures 6a and 7a.)

As in Figure J

,

the important point to recognize in Figure 8 is that the

basket wires distorted and fractured before the soldered joint failed.

- 5

-

Some interesting aspects of the nickel-plated basket wires appear

in Figures 9 and 10. A transverse section of an unetched ni eke 1 -p 1 ated

wire shown in Figure 9a reveals a sound, reasonably uniform film of

nickel completely surrounding the wire. The average thickness of the

nickel film is 0.0004 inches. For comparison, an unplated wire appears

in Figure 9b.

Longitudinal views of a used basket wire with its nickel plate

flaking off appear in Figure 10. These views are close-ups of the

sharply bent wire appearing in Figure lc. Figure 10a reveals a circumferential crack in the nickel layer. (The threads appearing on either

side of the crack in Figure 10a are simply dust particles and have no relationship to the crack.) Views of surfaces at the top of the bend in

Figure 10a appear in Figures 10b and 10c. Figure 10b is taken at a slightly different section level than that of Figure 10c. Separation

of the nickel layer from the basket wire due to poor bonding is obvious

in Figure 10b. The soundness of the stainless steel basket wire beneath

the flaking nickel plate is attested to in both Figures 10b and 10c.

Analysis of Wire-Basket Failures:

Evidence presented in the preceding section shows that the cause of

the typical wire basket failures shown in Figures lb and lc is fracture

of individual basket wires, in contrast to the possibility that basket

wires are pulling free of the soldered joint. Failure is caused by

application of loads beyond the fracture strength of individual wires

comprising the basket.

Yield and tensile strengths of the types of stainless steels used

for basket wire, as described in Table I, are tabulated in Table II. If

a tensile load causes a stress which exceeds the yield strength of a basket wire, shape distortion such as that which appears in the case of

the left-hand basket wire in Figure 8a will occur. Loads resulting in

stresses which exceed the ultimate tensile stress will cause fracture,

as in the case of the right-hand basket wire in Figure 8a.

A tensile load of 10 pounds will produce an average stress of

50,000 lb. /in. 2

in a wire of 0.0l6-inches diameter. This would correspond

to a total load of 40 pounds on the entire stone dislodger, assuming an even distribution of load on the four basket wires. Actually, this

assumption is not always valid, since only one or two fractured basket

wires are found on any individual failed stone dislodger. Hence, the

maximum load figures cited are the most optimistic possible. At any

rate, a stress of 50,000 lb. /in. 2

is we 1 I below the tensile yield and

ultimate tensile strengths appearing in Table II. However, tensile

loads between 15 and 60 pounds per basket wire will produce stresses

which may be about the same magnitude as the yield strength, depending

upon the degree of temper. At such loads, shape distortion will result.

Shape distortion usually decreases the wire diameter, which in turn

reduces the cross-sectional area (see Figure 8a, left-hand basket wire,

for an illustration of this point), and consequently, the load bearing

capacity of the wire. During repeated use at stress levels comparable

- 6 -

to the yield strength of the basket wires, the load bearing capacity of an individual stone dislodger successively decreases until even a rather

small load at one point in the re-use cycle will produce stress levels

greater than the ultimate tensile strength. At this point, fracture of

the basket wire will occur.

In addition to the considerations cited above for tensile (axial)

loading, one must also recognize that torsion (twist) loading is also

applied to the basket wires when in use. The arguments cited for tensile

loading are also applicable to torsion loading. In fact, it may be that

fracture in torsion is a more frequent mode of failure for basket wires

than fracture in tension. This viewpoint is hinted at by the frequency

with which basket wires fracture immediately adjacent to one connector

or the other. It is at these points of maximum constraint to torsion

strain that the torsion stress achieves its maximum amplitude.

The layer of nickel plate applied to one manufacturer's stone

dislodger contributes to weakening, and probably to fracture, of basket

wires. This is illustrated in Figures 4b and 4c, where it is seen that

the nickel plated wire exhibits surface cracks which penetrate the basket

wire itself. These cracks act to decrease the effective cross-sectional

area of the basket wire, thereby decreasing its load-car ry i ng capacity.

The reason that cracks originate in the nickel layer is not known with

certainty. One plausible cause is the difference in average coefficients

of thermal expansion of stainless steel and nickel, 9-3XIO"L in./in.-°F

and 7.4X10“^ in./in.-°F, respectively. At typical autoclaving temperatures, an 0.0l6-inch diameter stainless steel wire expands radially about

372X10-7 inches, whereas a 0. 0004-inch hoop of nickel expands radially

about 7*^xlO“7 inches. Another possible cause of cracks originating in

the nickel layer is that the metallurgical structure of the plated layer

is inherently brittle. This is sometimes the case for electroplated

metals. Since fractures have been reported for both unplated and plated

basket wires, cracks originating in the nickel layer cannot be the sole cause of basket wire failure. However, they represent a potential cause

of fracture which should be eliminated.

It should be recognized that loss of spring temper by recrystallization

and conconcur rent weakening of basket wires may occur during soldering or autoclaving. Whether or not this actually does occur depends upon the

temperatures reached by basket wires during these operations. Since the

solders used by each manufacturer are not clearly specified, this point

cannot be examined in specific detail. However, since most solders melt

below 400°F, and since recommended annealing temperatures to promote

recrystallization of types 302 , 303 and 3 1 6 stainless steel are around

I900°F, the probability of significant loss of temper occurring during

soldering is negligible. The same conclusion applies to the operation

of autoclaving, where temperatures apparently do not exceed 250°F.

- 7

-

In view of the abundant evidence that plastic flow occurs in basket

wires, e.g., shape distortions as in Figures 3b, 4b, 7 and 8, it is not

likely that metal fatigue is an important contributor to basket wire

fa i lure.

Recommended Procedures to Follow in Order to Minimize the Frequency of

Wire-Basket Failures:

1. Since the analysis presented in this report shows that cumulative

damage to basket wires during re-use eventually leads to fracture,

the most obvious recommendation to make is to minimize the re-use

of individual stone dislodgers. Ideally, a stone dislodger should

have its basket wires replaced after a reasonable number of cycles

of use. A definition of what represents "a reasonable number of

cycles of use" will have to be provided by individuals having a day-to-day familiarity with the use of stone dislodgers.

2. A detailed specification should be made of the material from which

basket wires are made. Stainless steels of the types listed in row 3 of Table II have the strength and corrosion resistance required

for use as basket wires. However, it is important to specify the

degree of temper, i.e., the strength of the wire, more clearly than

it is presently specified in the manuf acturers 1 engineering drawings.

Perhaps a more direct specification of the strength of a specific

type of stainless steel wire, e.g., "minimum yield strength-225,000

lb/in. 2

; minimum tensile strength-275,000 lb/in. 2

", would be a more

meaningful specification than "spring temper". Along with such a specification, a statement about the ductility of the wire, i.e.,

capacity to stretch, would also be appropriate, e.g., "minimum

elongation in 2 inches-2

.

3. The diameter of a basket wire determines its load-carrying capacity.

For a given diameter, d, tensile stress varies as l/d^, and torsion

stress varies as l/d3. Hence, the diameter of the basket wires

should be as large as is consistent with the use of the stone

di s lodger

.

4. The n i eke 1 -p I ated layer on stone dislodgers produced by one manufacturer acts as a source of cracks which contribute to the

fracture of basket wires. It may be desirable to eliminate the nickel

plating operation altogether. Stainless steel has an intrinsic

corrosion resistance which makes it unnecessary to apply a nickel

layer for corrosion resistance. Elimination of the nickel layer

would also avoid the problem of flaking associated with the nickel

layer, as shown in Figure 10.

- 8 -

5. Examination for flaking nickel layers on basket wires, and elsewhere,

is desirable for stone dislodgers presently in use. This could be

accomplished simply by looking at stone dislodgers after each use under a low-power binocular microscope. A magnification of 27X, as

in Figure 10a, would be appropriate.

6. Although soldered joints do not seem to be a primary cause of failure,

features of the soldered joints noted earlier are worth some discussion

a) Maximum mechanical strength of the basket-wi re-to-connector

joint is achieved if the basket wires pass entirely through

the connector, as in Figures 6a and 7a * Purely from the

standpoint of maximum achievable mechanical strength, the

joints in Figures 6a and 7a have an intrinsically better

design than the joint in Figure 8a.

b) Bridging of solder within the connector, as in Figures 6a, 7a and 8a, should be minimized by applying sufficient flux and

heat during the soldering operation.

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C X — C c . ID M 4- 2 0) 33 <_>

in

L.

cu03 Xo3O)

Used

stone

dislodger

which

has

failed.

Unit

produced

by

American

Cystoscope

Makers,

Inc.

1

1

l

,d) 12X

Filiform tip end of unused stone

dislodger shown in Figure la.

(b) 12X

Filiform tip end of used stone

dislodger shown in Figure lb.

(c) 12X

Filiform tip end of used stone

dislodger shown in Figure lc.

(d) 12X

Flexible cable end of used stone

dislodger shown in Figure lc.

Figure 2. Exterior Views of Soldered Joints.

Figure 3* Tip of Fractured Basket Wire Appearing in Figure lb.

Figure 4. Tip of Fractured Basket Wire Appearing in Figure Ic.
' ")
250X

Figure 5 . Fiberous appearance of basket wire in spring temper

condition. Sample taken from unused stone dislodger

appearing in Figure la. Etched in 20$ HC1, 5$> HNO^.

50X

Filiform tip and basket Solder bead appearing in Figure 7a a t point where

wires. wires enter filiform tip.

Figure 7* Cross -sect i ona 1 view of soldered joint within filiform tip. Samp 1

taken from used stone dislodger appearing in Figure lb.

(6a)

Fi 1 i form

wi res

Figure 6

tip

9.5X

and basket

(6b) 5CX

Solder bead appearing in Figure 6a at point where

basket wires enter filiform tip.

Cross-sectional view of soldered joint within filiform tip.

taken from unused stone dislodger appearing in Figure la.

Samp 1
f[

czn

cz2 g
Filiform tip and basket Solder bead appearing in Figure 8a at point where

wires. basket wires enter filiform tip.

Figure 8. Cross-sectional view of soldered joint within filiform tip. Sample

taken from used stone dislodger appearing in Figure 1c.

(9a) I60X

Nickel-plated basket wire. Sample taken from

stone dislodger shown in Figure 1c. The nickel

layer shows up as the annular ring surrounding

the basket wire. Wire diameter-0.016 inches.

Thickness of nickel layer-0.0004 inches.

(9b) 160X

Unplated basket wire. Sample taken from stone

dislodger shown in Figure lb. Wire diameter0.014 inches.

Figure 9. Transverse Sections of Basket Wires.

;<0 <u LT\ L. CU =>cn *j — 0

u_

in

C flj — _Q

U) E

a>Lm c 0 ^ • i_ 5 O 1- u-

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ro to

<T3 cn

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<4-

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a l •

4-1 O TOQ. > O O 0) <4- t/) 1/1 <D 0 — u

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— 2

<10c)

250 X

View

of

top

of

bend

in

wire

appearing i

rv

Figure

10a,

ata

different

level

than

that

shown

in

Figure

10b.

Etched

in

20

'jo

HC1,

HMOo.