Validation of finite element predictions of cartilage contact pressure in the human hip joint

Abstract

Methods to predict contact stresses in the hip can provide an improved understanding of load distribution in the normal and pathologic joint. The objectives of this study were to develop and validate a three-dimensional finite element (FE) model for predicting cartilage contact stresses in the human hip using subject-specific geometry from computed tomography image data, and to assess the sensitivity of model predictions to boundary conditions, cartilage geometry, and cartilage material properties. Loads based on in vivo data were applied to a cadaveric hip joint to simulate walking, descending stairs, and stair-climbing. Contact pressures and areas were measured using pressure sensitive film. CT image data were segmented and discretized into FE meshes of bone and cartilage. FE boundary and loading conditions mimicked the experimental testing. Fair to good qualitative correspondence was obtained between FE predictions and experimental measurements for simulated walking and descending stairs, while excellent agreement was obtained for stair-climbing. Experimental peak pressures, average pressures, and contact areas were 10.0 MPa (limit of film detection), 4.4-5.0 MPa, and 321.9-425.1 mm(2), respectively, while FE-predicted peak pressures, average pressures, and contact areas were 10.8-12.7 MPa, 5.1-6.2 MPa, and 304.2-366.1 mm(2), respectively. Misalignment errors, determined as the difference in root mean squared error before and after alignment of FE results, were less than 10%. Magnitude errors, determined as the residual error following alignment, were approximately 30% but decreased to 10-15% when the regions of highest pressure were compared. Alterations to the cartilage shear modulus, bulk modulus, or thickness resulted in +/-25% change in peak pressures, while changes in average pressures and contact areas were minor (+/-10%). When the pelvis and proximal femur were represented as rigid, there were large changes, but the effect depended on the particular loading scenario. Overall, the subject-specific FE predictions compared favorably with pressure film measurements and were in good agreement with published experimental data. The validated modeling framework provides a foundation for development of patient-specific FE models to investigate the mechanics of normal and pathological hips.

Figure 1

Experimental setup for loading of hip joint. Left - schematic of lockable rotation frame and cement pan used to constrain and orient the pelvis relative to the actuator plane. Middle - femur pot attached to a lockable ball and socket joint. Right - pressure sensitive film, cut into a rosette pattern, on the surface of the femoral cartilage. Polyethylene sheets were used to keep the pressure film dry.

Figure 2

Left - finite element mesh of the entire hip joint in the walking kinematic position. Right - close-up at the acetabulum. Triangular shell elements indicate cortical bone. Cartilage was represented with a hexahedral element mesh, with three elements through the thickness.

Figure 3

Contours of cartilage thickness. Femoral and acetabular cartilage was thickest in the anterormedial and superior regions, respectively.

Figure 4

FE predicted and experimentally measured average pressure (left y-axis) and contact area (right y-axis). FE models tended to overestimate average pressure and to underestimate contact area during simulated walking and descending stairs. There was excellent agreement between FE predictions and experimental measurements for stair-climbing. Error bars indicate standard deviation.

Figure 5

Top row - experimental film contact pressures (representative results are shown). Bi-centric patterns of contact were observed during simulated walking and descending stairs, while a mono-centric pattern was observed during stair-climbing. Middle row - FE synthetic films. Models predicted mono-centric, irregularly shaped patterns of contact. Bottom row - difference images, indicating locations where contact was not predicted by the models. The best qualitative correspondence was during stair-climbing. Note: FE synthetic films and difference images are shown prior to manual alignment with experimental results.

Figure 6

FE predicted contact pressures on the femur (top) and acetabulum (bottom). Acetabular cartilage contact pressures moved from anterior to posterior as the equivalent joint reaction force vector changed from shallow extension during descending stairs to deep flexion during stair-climbing. The highest contact pressures primarily occurred near the lateral region of the acetabulum.

Figure 7

Percent changes in peak pressure, average pressure and contact area due to alterations in assumed and measured model inputs. Left column - effect of cartilage material properties and thickness. Top left - effects of changes to the shear modulus by ±1 SD. Middle left - effects of changes to cartilage compressibility (100:1, 10:1 bulk to shear ratios). Bottom left - effects of altering the cartilage thickness. Error bars indicate standard deviations over the three loading activities analyzed. Right column - effect of boundary conditions. Top right - effects of a rigid bone material assumption. Middle right- effects of removing the pubis joint constraint. Bottom right - effects of removing the trabecular bone from the FE analysis. W, DS, SC indicate walking, descending stairs, and stair-climbing, respectively.

Figure 8

Contours of cartilage contact pressure predicted by the baseline (top row) and rigid bone FE models (bottom row) for the three loading activities. The largest effect of the rigid bone assumption occurred for simulated walking and descending stairs.