Finite element prediction of cartilage contact stresses in normal human hips

Abstract

Our objectives were to determine cartilage contact stress during walking, stair climbing, and descending stairs in a well-defined group of normal volunteers and to assess variations in contact stress and area among subjects and across loading scenarios. Ten volunteers without history of hip pain or disease with normal lateral center-edge angle and acetabular index were selected. Computed tomography imaging with contrast was performed on one hip. Bone and cartilage surfaces were segmented from volumetric image data, and subject-specific finite element models were constructed and analyzed using a validated protocol. Acetabular contact stress and area were determined for seven activities. Peak stress ranged from 7.52±2.11 MPa for heel-strike during walking (233% BW) to 8.66 ± 3.01 MPa for heel-strike during descending stairs (261% BW). Average contact area across all activities was 34% of the surface area of the acetabular cartilage. The distribution of contact stress was highly non-uniform, and more variability occurred among subjects for a given activity than among activities for a single subject. The magnitude and area of contact stress were consistent between activities, although inter-activity shifts in contact pattern were found as the direction of loading changed. Relatively small incongruencies between the femoral and acetabular cartilage had a large effect on the contact stresses. These effects tended to persist across all simulated activities. These results demonstrate the diversity and trends in cartilage contact stress in healthy hips during activities of daily living and provide a basis for future comparisons between normal and pathologic hips.

Figure 1

FE model creation and simulated loading scenarios. (a) CT section through center of proximal femur with contrast agent between opposing acetabular and femoral cartilage. Segmentation lines follow the contour of the pelvis and femur bone morphology. (b)3D reconstruction of bone (off-white) and cartilage (blue). (c)Lateral view of the model at the acetabulum shows triangular shell and hexahedral elements representing bone and cartilage, respectively (d–j). Orientation of the femur and pelvis during simulated activities captured from an identical view of the yz-plane; displacements were centered in the joint and directed vertically. From left to right: WHS, WHM, WMD, WML, WLS, AHS, and DHS.

Figure 2

Contact stresses on the acetabular cartilage for each subject during walking, ascending stairs, and descending stairs. Left is anterior. Variations in stresses were greater among subjects (columns) than among loading scenarios (rows).

Figure 3

CT image and model predictions of contact stress for one subject, illustrating effects of bony geometry. The small cavity located in the superior acetabular roof (arrow, top panel) created a depression that is identifiable in the model before (arrow, middle panel) and during loading (arrow, bottom panel). This resulted in an area of relatively low contact stress at the corresponding location of contact.

Figure 4

Contact area on the articulating cartilage surface as a function of activity. Percentages represent the portion of the total articulating surface in contact.

Figure 5

Average contact stress as a function of region and activity. Significant differences (shown with asterisk) occurred between at least two regions for every activity, except AHS.