# Biomimetic Composite-Metal Hip Resurfacing Implant

Authors: Habiba Bougherara; Martin N. Bureau
Journal: Research Letters in Materials Science
 (2008)

---

## Abstract

Hip resurfacing technique is a conservative arthroplasty used in the young patient in which the femoral head is reshaped to accept metal cap with small guide stem. In the present investigation, a hybrid composite-metal resurfacing implant is proposed. The cup is made of carbon fiber/polyamide 12 (CF/PA12) covered with a thin layer of cobalt chrome (Co-Cr). Finite element (FE) method was applied to analyze and compare the biomechanical performances of the hybrid hip resurfacing (HHR) and the conventional Birmingham (BHR). Results of the finite element analysis showed that the composite implant leads to an increase in stresses in the cancellous bone by more than 15% than BHR, indicating a lower potential for stress shielding and bone fracture and higher potential for bone apposition with the HHR.


---

## Body

## 1. Introduction

The successful reintroduction of the improved
metal-on-metal (MOM) bearings has led to a resurgence of interest for hip resurfacing
procedure as a viable alternative to conventional arthroplasty for younger and more
active patients [1, 2]. The reported benefits of hip resurfacing
(HR) include reduced dislocation rate and increased function compared to total hip
arthroplasty (THA) [3, 4]. Other advantages of HR compared with THA
include minimal bone resection, easier revision, and reduction in the stress shielding
in the proximal femur [1, 5].Femoral
neck fracture and stress shielding problems remain a concern with resurfacing hip
prostheses [6, 7]. Currently, most implants for joint replacements
including HR are manufactured from stiff materials (i.e., stainless steel, Co-Cr,
etc.). These materials undergo stress shielding that may lead to implant loosening
and femoral neck fracture [8].Currently, conventional
implants still undergo problems of biomechanical mismatch of elastic modulus and
interfacial stability with host tissues. Fortunately, fiber-reinforced polymer composites
provide an interesting solution to face these problems since they have excellent
mechanical properties, such as high fatigue and creep resistance, rigidity, and
stiffness. Moreover, these materials have rapid, versatile, and inexpensive fabrication
processes.The feasibility of the proposed biomimetic polymer composite was previously used to manufacture a hip stem for THA. This hip stem described
by Campbell et al. [9] and Bougherara et al. [10] was evaluated numerically and experimentally.
Preliminary tests investigating the biocompatibility of the biomimetic composite
stem showed that the composite produced no adverse cytotoxic response in the periprosthetic
tissues [11, 12], and tests using simulated body fluid conditioning
showed that the hydroxyapatite-
(HA) coated composite has excellent potential of biocompatibility [13].This
new biomimetic composite implant has the ability to mimic the functions of the natural
bone that will allow stable fixation of prosthesis with minimal bone loss through
stress shielding. The biocompatibility of this new biomimetic composite was verified
[14, 15]. Miniature implants made of the same materials
and of HA-coated Ti as control were produced and inserted as femoral intramedullary
implants in rabbits. Hystomorphometric results for both composite and control implants
(HA-coated Ti) showed that the composite implants were highly osseoinductive, by
more than one order of magnitude with respect to control (implant coverage exceeding
100% of nominal implant perimeter). The protocol used here thus led us to conclude
that the HA-coated PA12/CF
composites have the potential to lead to very good osseoconduction that may reduce
stress shielding leading to bone resorption.A novel
hybrid composite Co-Cr hip resurfacing (HHR) implant is proposed to prevent stress
shielding and bone fracture. The aim of this article is thus to present the concept
design for the hybrid resurfacing implant and secondly, to compare its performance
to conventional Birmingham
hip resurfacing (BHR).


## 2. Material and Methods

### 2.1. Concept Design and 3D Geometries

Computerized
tomography (CT) scan sections were performed every 0.5 mm along the length of the
composite femur (sawbones 4th generation composite bone models) and used to generate
the 3D solid model of the proximal femur [16] (see Figure 2(a)). The elastic modulus
of the simulated cortical (short fiber-filled epoxy) and cancellous (rigid polyurethane foam) based
on ASTM D-638, D-695, and D-1621 tests were equal to 16.7 (GPa) and 206 (MPa).The
concept and geometry of the hybrid resurfacing component are shown in Figure1.
The femoral cup with a diameter of 46 mm (Birmingham) is composed of 2 mm-thick
shell structure made of several layers of continuous carbon fiber/polyamide 12 (CF/PA12)
composite fabrics (see Figure 2), onto which a 1 mm layer of Co-Cr is overlaid in
order to avoid wear debris formation. The guide stem is composed of the same material
as the cup (i.e., CF/PA12-Co-Cr). The cup is fixed to the femoral head with 1.5 mm cement made of PMMA.Hip resurfacing. (a) Hybrid HR and (b) Birmingham   HR.
(a)(b)Figure 2
Composite (CF/PA12) stem braided (+45/−45)6.


### 2.2. Finite Element Models

3D FE
models of a composite femur were meshed and analyzed using ANSYS 11.0 software.
The femoral head was reamed and embedded with two types of hip resurfacing implants.
The first was a model of the femoral bone in which the implanted HR was made of
a Cr-Co-base alloy (E=200 GPa, v=0.3) (see
Figure 3(b)), while the second was the hybrid composite (CF/PA12) metal (Co-Cr) implant (see Figure
3(c)).(a) Geometry and load conditions, (b) FE model of BHR, and (c) FE model of HHR.
(a)(b)(c)The material properties used for the analysis
are shown in Table1.Table 1
Mechanical properties of the bone-implant system.
MaterialModulus of elasticity (GPa)Shear modulus (GPa)Poisson’s ratioCF/PA12Ex=15.43..3Ey=15.43.5.3Ez=3.53.2.3PMMAE=2.—.3Cortical boneE=16.7—.3Cancellous boneE=.206—.3The models were meshed with 10 node quadratic
tetrahedron elements (see Figure3(b)). Bonded contact was assumed at the cup-bone
interfaces in view of simulating full bone ingrowth and perfect cement fixation.
This resulted in a rigid link between the composite cup and the PMMA contact
surfaces, allowing no relative sliding or gap opening. The bone-stem interface was
simulated using frictionless contact allowing free sliding (see Figure 4).Figure 4
Type of contact used in the FEA.For
each FE model, the load corresponded to 8 running and consisted of a 3704 kN load
applied to the femoral head and a 1085 kN great trochanter muscles load [17]. The loads were distributed over several
nodes to avoid stress concentration, and for all FE analyses, the displacement of
all nodes at the distal end of the femoral bone was rigidly constrained (Figure 3(a)).


## 2.1. Concept Design and 3D Geometries

Computerized
tomography (CT) scan sections were performed every 0.5 mm along the length of the
composite femur (sawbones 4th generation composite bone models) and used to generate
the 3D solid model of the proximal femur [16] (see Figure 2(a)). The elastic modulus
of the simulated cortical (short fiber-filled epoxy) and cancellous (rigid polyurethane foam) based
on ASTM D-638, D-695, and D-1621 tests were equal to 16.7 (GPa) and 206 (MPa).The
concept and geometry of the hybrid resurfacing component are shown in Figure1.
The femoral cup with a diameter of 46 mm (Birmingham) is composed of 2 mm-thick
shell structure made of several layers of continuous carbon fiber/polyamide 12 (CF/PA12)
composite fabrics (see Figure 2), onto which a 1 mm layer of Co-Cr is overlaid in
order to avoid wear debris formation. The guide stem is composed of the same material
as the cup (i.e., CF/PA12-Co-Cr). The cup is fixed to the femoral head with 1.5 mm cement made of PMMA.Hip resurfacing. (a) Hybrid HR and (b) Birmingham   HR.
(a)(b)Figure 2
Composite (CF/PA12) stem braided (+45/−45)6.


## 2.2. Finite Element Models

3D FE
models of a composite femur were meshed and analyzed using ANSYS 11.0 software.
The femoral head was reamed and embedded with two types of hip resurfacing implants.
The first was a model of the femoral bone in which the implanted HR was made of
a Cr-Co-base alloy (E=200 GPa, v=0.3) (see
Figure 3(b)), while the second was the hybrid composite (CF/PA12) metal (Co-Cr) implant (see Figure
3(c)).(a) Geometry and load conditions, (b) FE model of BHR, and (c) FE model of HHR.
(a)(b)(c)The material properties used for the analysis
are shown in Table1.Table 1
Mechanical properties of the bone-implant system.
MaterialModulus of elasticity (GPa)Shear modulus (GPa)Poisson’s ratioCF/PA12Ex=15.43..3Ey=15.43.5.3Ez=3.53.2.3PMMAE=2.—.3Cortical boneE=16.7—.3Cancellous boneE=.206—.3The models were meshed with 10 node quadratic
tetrahedron elements (see Figure3(b)). Bonded contact was assumed at the cup-bone
interfaces in view of simulating full bone ingrowth and perfect cement fixation.
This resulted in a rigid link between the composite cup and the PMMA contact
surfaces, allowing no relative sliding or gap opening. The bone-stem interface was
simulated using frictionless contact allowing free sliding (see Figure 4).Figure 4
Type of contact used in the FEA.For
each FE model, the load corresponded to 8 running and consisted of a 3704 kN load
applied to the femoral head and a 1085 kN great trochanter muscles load [17]. The loads were distributed over several
nodes to avoid stress concentration, and for all FE analyses, the displacement of
all nodes at the distal end of the femoral bone was rigidly constrained (Figure 3(a)).


## 3. Results

### 3.1. Stresses in the Trabecular Bone

The
von Mises stresses in the trabecular bone with the Cr-Co and CF/PA12 resurfacing
implants are shown in Figure5. The stress inside the trabecular bone was higher
for the conventional Co-Cr resurfacing implant (von Mises stress between 0 and 4.7 MPa)
than the one using the hybrid CF/PA12-Co-Cr (von Mises stress level ranged between
0 and 4.1 MPa). The highest stress patterns were noted in the posteroinferior region
near the rim of the implant. Peak stresses were 6.09 MPa for the Co-Cr prosthesis
and 5.26 MPa for the hybrid HR.Von Mises stress in the cancellous bone using (a) BHR implant and (b)  HHR.
(a)(b)


### 3.2. Stresses in the Implant

The
values of stresses in the conventional Co-Cr implant were ranged between 0 and 77
compared to 0 and 69 MPa when the hybrid implant is used (see Figure6(a)). The
peak stress in the conventional implant was higher (173.6 MPa) than that found in
the hybrid implant (156.2).Von Mises stress in the implant using (a) conventional (BHR), (b) hybrid (HHR), and (c) stress distribution in the CF/PA12 shell.
(a)(b)(c)


## 3.1. Stresses in the Trabecular Bone

The
von Mises stresses in the trabecular bone with the Cr-Co and CF/PA12 resurfacing
implants are shown in Figure5. The stress inside the trabecular bone was higher
for the conventional Co-Cr resurfacing implant (von Mises stress between 0 and 4.7 MPa)
than the one using the hybrid CF/PA12-Co-Cr (von Mises stress level ranged between
0 and 4.1 MPa). The highest stress patterns were noted in the posteroinferior region
near the rim of the implant. Peak stresses were 6.09 MPa for the Co-Cr prosthesis
and 5.26 MPa for the hybrid HR.Von Mises stress in the cancellous bone using (a) BHR implant and (b)  HHR.
(a)(b)


## 3.2. Stresses in the Implant

The
values of stresses in the conventional Co-Cr implant were ranged between 0 and 77
compared to 0 and 69 MPa when the hybrid implant is used (see Figure6(a)). The
peak stress in the conventional implant was higher (173.6 MPa) than that found in
the hybrid implant (156.2).Von Mises stress in the implant using (a) conventional (BHR), (b) hybrid (HHR), and (c) stress distribution in the CF/PA12 shell.
(a)(b)(c)


## 4. Discussion

The
new concept design for hip resurfacing using a hybrid composite metal was described.
Finite element analysis has been performed to evaluate and analyze the potential
of the proposed design concept.In the
case of the hybrid implant, numerical results have shown that the von Mises stresses
were more uniform and higher by more than 15% in the cancellous bone. This indicates
the potential of the hybrid composite implant to reduce stress shielding, especially
in the great trochanter region. Also higher peak stress in the cancellous bone was
found when the Co-Cr is used, which may lead to one fracture. Again the use of the
hybrid implant reduced considerably the peak bone stress in the cancellous bone
and thus prevents from bone fracture.Results
of simulation indicate also that the hybrid implant reduced the stress in the femoral
cup by approximately 10 to 20%. Therefore, more loads are transferred to the bone
with the hybrid composite Co-Cr resurfacing implant than the conventional ones.The
main objective of the current work was to present a preliminary study on the metal composite femoral
component. However, the FE model needs some improvements, such as the inclusion
of the load bearing surfaces (i.e.,
acetabular cup). The thickness of this acetabular cup would be expected to
affect the load transfer across the bearing surfaces.


## 5. Conclusions

The
outcome of this study showed that the hybrid composite metal hip resurfacing had the potential to
reduce stress shielding, preserve bone stock, and prevent from bone fracture compared
to conventional metallic hip resurfacing implants.



---

*Source: 368985-2008-07-09.xml*