Toward patient-specific articular contact mechanics

Abstract

The mechanics of contacting cartilage layers is fundamentally important to understanding the development, homeostasis and pathology of diarthrodial joints. Because of the highly nonlinear nature of both the materials and the contact problem itself, numerical methods such as the finite element method are typically incorporated to obtain solutions. Over the course of five decades, we have moved from an initial qualitative understanding of articular cartilage material behavior to the ability to perform complex, three-dimensional contact analysis, including multiphasic material representations. This history includes the development of analytical and computational contact analysis methods that now provide the ability to perform highly nonlinear analyses. Numerical implementations of contact analysis based on the finite element method are rapidly advancing and will soon enable patient-specific analysis of joint contact mechanics using models based on medical image data. In addition to contact stress on the articular surfaces, these techniques can predict variations in strain and strain through the cartilage layers, providing the basis to predict damage and failure. This opens up exciting areas for future research and application to patient-specific diagnosis and treatment planning applied to a variety of pathologies that affect joint function and cartilage homeostasis.

Fig. 1

High-level flowchart of methods for generating subject-specific finite element models of joint contact mechanics from in vivo medical image data. The source for geometry is typically obtained from CT arthrography (CTA) or MR arthrography (MRA) data. The injected contrast separates the articular layers and highlights the cartilage surfaces (Anderson et al., 2008b). Because of the complex boundaries that are produced by the contrast agent, manual segmentation is often necessary to identify the cartilage surface and the cartilage–bone boundaries. Typical finite element discretization strategies involve the use of hexahedral and/or tetrahedral elements. Discretization of the articular layers is especially difficult since the cartilage is thin and multiple elements are needed through the thickness to resolve gradients in stress and strain (Henak et al., 2013c). When combined with generic or patient-specific boundary and loading conditions, and appropriate material data, these inputs provide the basis for finite element analysis of subject-specific joint contact mechanics (Harris et al., 2012; Henak et al., 2014; Henak et al., 2013d).

Fig. 2

Patient-specific FE analysis of hip contact mechanics reveals differences in loading of the labrum between normal and dysplastic hips (Henak et al., 2014). (A) Manual segmentation of osteochondral boundary from volumetric CT arthrography data allows surface reconstruction (beige). (B) FE mesh of articular layers, labrum, pelvis and proximal femur. (C) Coronal cross-sectional images of pressure in representative normal (left) and dysplastic (right) hips, with the bones rendered as transparent. Lateral loading in the dysplastic hip results in higher contact stress in the acetabular labrum, and thus larger loads. (D) Percent load supported by the labrum during simulated activities of walking heelstrike (WH), walking midstance (WM), descending stairs heelstrike (DH) and ascending stairs heelstrike (AH). Load supported by the labrum was significantly larger for dysplastic hips than normal hips during all loading scenarios. Error bars indicate upper confidence bounds (at 95%). *Indicates pr0.05 in comparison to normal hips during the same loading scenario (n=10 in each group).

Fig. 3

Transchondral stress and strain predictions offer the potential ability to predict cartilage overload and failure due to specific pathomorphologies such as dysplasia and femoroacetabular impingement (Henak et al., 2013c). (A) Finite element model for one of the five specimens, showing the cutting plane used to sample transchondral results. (B) Fringe plot of maximum shear stress through the thickness of the acetabular and femoral cartilage layers for the specimen shown in panel A. Peak values of maximum shear stress occurred at the osteochondral interfaces. (C) Average of the maximum shear stress through the thickness of the femoral articular cartilage during ascending stairs heelstrike across all samples. Error bars=standard deviation.