Custom-designed orthopedic implants evaluated using finite element analysis of patient-specific computed tomography data: femoral-component case study

Abstract

Background: Conventional knee and hip implant systems have been in use for many years with good success. However, the custom design of implant components based on patient-specific anatomy has been attempted to overcome existing shortcomings of current designs. The longevity of cementless implant components is highly dependent on the initial fit between the bone surface and the implant. The bone-implant interface design has historically been limited by the surgical tools and cutting guides available; and the cost of fabricating custom-designed implant components has been prohibitive.

Methods: This paper describes an approach where the custom design is based on a Computed Tomography scan of the patient's joint. The proposed design will customize both the articulating surface and the bone-implant interface to address the most common problems found with conventional knee-implant components. Finite Element Analysis is used to evaluate and compare the proposed design of a custom femoral component with a conventional design.

Results: The proposed design shows a more even stress distribution on the bone-implant interface surface, which will reduce the uneven bone remodeling that can lead to premature loosening.

Conclusion: The proposed custom femoral component design has the following advantages compared with a conventional femoral component. (i) Since the articulating surface closely mimics the shape of the distal femur, there is no need for resurfacing of the patella or gait change. (ii) Owing to the resulting stress distribution, bone remodeling is even and the risk of premature loosening might be reduced. (iii) Because the bone-implant interface can accommodate anatomical abnormalities at the distal femur, the need for surgical interventions and fitting of filler components is reduced. (iv) Given that the bone-implant interface is customized, about 40% less bone must be removed. The primary disadvantages are the time and cost required for the design and the possible need for a surgical robot to perform the bone resection. Some of these disadvantages may be eliminated by the use of rapid prototyping technologies, especially the use of Electron Beam Melting technology for quick and economical fabrication of custom implant components.

Figure 1

Profile line is used to determine the appropriate threshold for the 3D reconstruction.

Figure 2

Initial 3D computer model of the knee joint produced using Mimics by Materialise, Belgium.

Figure 3

A 3D computer model of a distal femur cut to fit a conventional femoral component.

Figure 4

Spline curves used to create the bone-implant interface surface.

Figure 5

3D computer model of a custom designed femoral component.

Figure 6

a) 3D computer model of a custom femoral implant component and a cut distal femur b) 3D computer model of a standard femoral implant component and a cut distal femur.

Figure 7

Load and constraints used for all finite element analysis models.

Figure 8

Comparison of stress distribution on bone surface for conventional and custom implant with loading and reaction force in center location. Maximum stresses are shown in red color at a level above 5 MPa. Green contour stress levels are 2.5 MPa.

Figure 9

Bone interface of the custom implant under second condition showing even stress distribution across both condyles. Stresses are between 0.9 MPa to 3 MPa on the contact surface.

Figure 10

Bone interface of the conventional implant under third load condition showing stress concentration along the sharp edges. Stresses at the contact surfaces are from 0.3 MPa to 15 MPa. Most of the green contour stress levels are 2.5 MPa.