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.
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 [
Femoral
neck fracture and stress shielding problems remain a concern with resurfacing hip
prostheses [
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. [
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
[
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).
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 [
The
concept and geometry of the hybrid resurfacing component are shown in Figure
Hip resurfacing. (a) Hybrid HR and (b) Birmingham HR.
Composite (CF/PA12) stem braided (+45/−45)6.
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 (
(a) Geometry and load conditions, (b) FE model of BHR, and (c) FE model of HHR.
The material properties used for the analysis
are shown in Table
Mechanical properties of the bone-implant system.
Material | Modulus of elasticity (GPa) | Shear modulus (GPa) | Poisson’s ratio |
---|---|---|---|
CF/PA12 | 3. | .3 | |
3.5 | .3 | ||
3.2 | .3 | ||
PMMA | — | .3 | |
Cortical bone | — | .3 | |
Cancellous bone | — | .3 |
The models were meshed with 10 node quadratic
tetrahedron elements (see Figure
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 [
The
von Mises stresses in the trabecular bone with the Cr-Co and CF/PA12 resurfacing
implants are shown in Figure
Von Mises stress in the cancellous bone using (a) BHR implant and (b) HHR.
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 Figure
Von Mises stress in the implant using (a) conventional (BHR), (b) hybrid (HHR), and (c) stress distribution in the CF/PA12 shell.
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.
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.