Three-dimensional mechanical evaluation of joint contact pressure in 12 periacetabular osteotomy patients with 10-year follow-up

Abstract

Background and purpose: Because of the varying structure of dysplastic hips, the optimal realignment of the joint during periacetabular osteotomy (PAO) may differ between patients. Three-dimensional (3D) mechanical and radiological analysis possibly accounts better for patient-specific morphology, and may improve and automate optimal joint realignment.

Patients and methods: We evaluated the 10-year outcomes of 12 patients following PAO. We compared 3D mechanical analysis results to both radiological and clinical measurements. A 3D discrete-element analysis algorithm was used to calculate the pre- and postoperative contact pressure profile within the hip. Radiological angles describing the coverage of the joint were measured using a computerized approach at actual and theoretical orientations of the acetabular cup. Quantitative results were compared using postoperative clinical evaluation scores (Harris score), and patient-completed outcome surveys (q-score) done at 2 and 10 years.

Results: The 3D mechanical analysis indicated that peak joint contact pressure was reduced by an average factor of 1.7 subsequent to PAO. Lateral coverage of the femoral head increased in all patients; however, it did not proportionally reduce the maximum contact pressure and, in 1 case, the pressure increased. This patient had the lowest 10-year q-score (70 out of 100) of the cohort. Another hip was converted to hip arthroplasty after 3 years because of increasing osteoarthritis.

Interpretation: The 3D analysis showed that a reduction in contact pressure was theoretically possible for all patients in this cohort, but this could not be achieved in every case during surgery. While intraoperative factors may affect the actual surgical outcome, the results show that 3D contact pressure analysis is consistent with traditional PAO planning techniques (more so than 2D analysis) and may be a valuable addition to preoperative planning and intraoperative assessment of joint realignment.

Figure 1.

Angles describing the orientation of the acetabulum and femoral coverage are calculated from a 3D segmented model of the pelvis in frontal, sagittal, and transverse planes emulating those traditionally measured from radiographs or reformatted CT scans. In the frontal plane (top left), the AC angle measures the obliqueness of the acetabular roof between the medial aspect of the sourcil, the lateral edge of the acetabular rim, and a horizontal line. The CE angle measures the coverage of the lateral edge of the acetabulum with respect to the center of the femoral head (not shown), and may have a negative value in severely dysplastic cases. The orientation of the acetabular cup (S-AC angle) in the sagittal plane (top right) is measured from the most superior aspect of the acetabular roof to the most anterior aspect with respect to horizontal. The anteversion (AcetAV angle) is measured on a transverse plane (bottom left) viewed inferior to superior using a line created between the posterior and anterior rim of the acetabulum. The angle is measured with respect to a line normal to the two femoral head centers (intercapital centerline). The angle measurements help to characterize the orientation of the load-bearing surface of the hip joint in 3 dimensions (bottom right) during realignment (axis scale in mm; viewed isometric from anterio-lateral-superior).

Figure 2.

Mechanical results in which the postoperative outcome matched closely with the optimum contact pressure profile (patient 11). The blue line shown passing through the joint contact surface represents the force applied through the hip, while the green line represents the displacement direction of the femoral head. In the optimized case, the lines are collinear. Scale units are MPa.

Figure 3.

An example PAO case in which the postoperative pressure profile indicates overcorrection of the bone fragment (patient 5). The preoperative case shows pressure concentration on the lateral aspect of the joint contact surface. Excessive adduction of the acetabular fragment resulted in medially concentrated pressure postoperatively. Units are MPa.