Outcome of periacetabular osteotomy: joint contact pressure calculation using standing AP radiographs, 12 patients followed for average 2 years

Abstract

Background: Due to wide variations in acetabular structure of individuals with hip dysplasia, the measurement of the acetabular orientation may not be sufficient to predict the joint loading and pressure distribution across the joint. Addition of mechanical analysis to preoperative planning, therefore, has the potential to improve the clinical outcome. We analyzed the effect of periacetabular osteotomy on hip dysplasia using computer-aided simulation of joint contact pressure on regular AP radiographs. The results were compared with the results of surgery based on realignment of acetabular angles to the normal hip.

Patients and methods: We studied 12 consecutive periacetabular osteotomies with no femoral head deformity. The median age of patients, all females, was 35 (20-50) years. The median follow-up was 2 years (1.3-2.2). Patient outcome was measured with the total score of a self-administered questionnaire (q-score) and with the Harris hip score. The pre- and postoperative orientation of the acetabulum was defined using reconstructed 3D CT-slices to measure angles in the three anatomical planes. Peak contact pressure, weight-bearing area, and the centroid of the contact pressure distribution (CP-ratio) were calculated.

Results: While 9 of 12 cases showed decreased peak pressure after surgery, the mean changes in weight-bearing area and peak contact pressure were not statistically significant. However, CP-ratio changed (p < 0.001, paired t-test) with surgery. For the optimal range of CP-ratio (within its mid-range 40-60%), the mechanical outcome improved significantly.

Interpretation: Verifying the correlation between the optimal CP-ratio and the outcome of the surgery requires additional studies on more patients. Moreover, the anatomically measured angles were not correlated with the ranges of CP-ratio, suggesting that they do not always associate with objective mechanical goals of realignment osteotomy. Mechanical analysis, therefore, can be a valuable tool in assessing two-dimensional radiographs in hip dysplasia.

Figure 1

Reformatted CT slices: A) frontal view, B) horizontal view, and C) sagittal view. The realignment angles shown in their respective views are as follows. F-AC: articular cartilage angle in the frontal plane; F-CE: center edge angle in the frontal plane; S-AC: articular cartilage angle in the sagittal plane; H-AT: anteversion angle in the horizontal plane. F-AC angle was defined as the angle between the horizontal line and a line connecting the medial edge of the sourcil line and the most lateral point on the acetabulum (points 4 and 5 in Figure 3), measured clockwise. F-CE angle was defined as the angle between a vertical line passing through the center of the femoral head and a line between the center of the femoral head and the most lateral edge of the acetabulum, measured counterclockwise. S-AC was defined as the angle between a horizontal line and a line passing through the anterior edge of the contact surface, and the upper most point of the acetabular contact surface. H-AT was defined as the angle of a line parallel to the opening of the acetabulum and a line perpendicular to the line drawn through the centers of the femoral heads.

Figure 2

The preoperative abductor angle (θ = 21°) was corrected for the postoperative AP radiographs by considering the change in the angle of pelvis tilt (α), superior/inferior movement of the hip center (β), and the lateral/medial movement of the hip center (γ).

Figure 3

An example of a case with a low pressure gradient. A) This preoperative radiograph shows that the contact pressure distribution (small arrows) does not span the entire potential contact area (defined by the area between the lateral edge (point 4) and medial edge (point 5) of the sourcil). The peak contact pressure is on the lateral edge (point 4). Also shown in the figure are the line of pull of abductor force, with its origin at the most lateral edge of the greater trochanter (point 1), and the joint reaction force passing through the centroid of contact pressure (shown by a “+” sign). The insert shows how CP-ratio and weight-bearing area are defined with respect to the sourcil line. B) The postoperative radiograph shows an even distribution of contact pressure after surgery. Also, the size of the weight-bearing area has approached that of the potential contact area (between points 4 and 5). The peak pressure is not on the lateral edge, and the centroid of contact pressure distribution has moved to the center of the contact area (CP-ratio approached 0.5). Note also that the direction of the abductor force is corrected for pelvis tilt and the medial movement of the femur joint center. C) The preoperative planning based on minimizing the joint contact pressure shows that an acetabular rotation of 10.5 degrees would result in an even distribution of joint contact pressure (mean/peak pressure = 1) over the entire potential contact area. The CP-ratio is 0.5. In this example, optimizing ace-tabular rotation according to the peak pressure resulted in a situation close to that seen in the postoperative image. However, the surgical procedure produced additional changes in acetabular orientation (e.g. medialization). This obviously further reduced the peak contact pressure. In this example, F-AC angle changed from 18° preoperatively to −2° postoperatively, and the F-CE angle changed from 30° preoperatively to 54° postoperatively.

Figure 4

An example of a case with high pressure gradient. The figures are arranged as in Figure 3. In this example, 8.5° lateral rotation of the acetabulum during biomechanical preoperative planning (C) resulted in an even pressure distribution over the whole potential contact area. However, the postoperative analysis (B) showed significant differences compared to the biomechanical planning results, indicating nonoptimal surgical outcome in terms of joint contact pressure. Postoperatively, we observed a high peak pressure and remarkably smaller weight-bearing area. The CP-ratio was 0.8, and the peak contact pressure was shifted toward the medial edge (point 5). F-AC angle changed from 19° preoperatively to −7° postoperatively, and the F-CE angle changed from 15° preoperatively to 50° postoperatively.

Figure 5

A) When the line of pull of the abductor muscles is varied from −20° to 40°, the CP-ratio increases accordingly. The peak joint contact pressure is minimized when the CP-ratio is at its mid-range, between 40% and 60% approximately. B) When the acetabular contact surface is rotated to up to 35° medially, and up to 45° laterally in the AP plane, the CP-ratio increases linearly from the most medial to the most lateral rotation. The peak joint contact pressure is minimized when the CP-ratio is at its mid-range (between 40% and 60% approximately).

Figure 6

The mean and standard deviations of peak pressures, F-CE, and F-AC angles between group A (CP-ratio between 40% and 60%) and group B (CP-ratio outside 40% and 60%). Preoperative analysis, postoperative analysis, and biomechanical preoperative planning results are shown for each group. For group A, the peak pressures of biomechanical planning and the postoperative results are close to each other, indicating optimal postoperative results when CP-ratio was between 40% and 60%. For group B, the above values are substantially different. The biomechanical planning and the postoperative F-CE angles have no correlation with the two groups and the peak pressures. The postoperative mean F-AC angle for group B was negative, indicating an undesirable situation. This angle was highly variable, indicating that, by itself, it cannot be a substitute for biomechanical parameters.