Effects of Acetabular Rim Trimming on Hip Joint Contact Pressures: How Much Is Too Much?

Background: In patients with femoroacetabular impingement (FAI), acetabular rim trimming removes the offending area of the acetabular deformity in patients with pincer-type and mixed-type FAI to improve hip joint kinematics. Although the rationale for arthroscopic acetabular rim trimming in patients with FAI is well established, the amount of rim resection has not been quantified, and the threshold at which excessive rim resection results in abnormal hip contact pressures has not been described. Purpose: To investigate the changes in contact areas, contact pressures, and peak forces within the hip joint with sequential acetabular rim trimming. Study Design: Controlled laboratory study. Methods: Six fresh-frozen, nondysplastic, human cadaveric hemipelvises were analyzed utilizing thin-film piezoresistive load sensors to measure the contact area, contact pressure, and peak force after anterosuperior acetabular rim trimming at depths of 0 mm (intact), 2 mm, 4 mm, 6 mm, and 8 mm. Each specimen was examined at 20 degrees of extension and 60 degrees of flexion. Analysis was performed on 2 regions of interest: the acetabular rim and the acetabular base (deep part of the acetabulum). After each experimental condition, the acetabulum was normalized with respect to the intact state to account for specimen variability. Statistical analysis was conducted through 1-way analysis of variance with post hoc Games-Howell tests. Results: At the acetabular base, there were significant increases in the contact area after 4-mm resection (60 degrees: 169.12% 30.64%; P = .0138), contact pressure after 6-mm resection (60 degrees: 292.76% 79.07%; P = .009), and peak force after 6-mm resection (60 degrees: 166.00% +/- 34.40%; P = .027). At the acetabular rim, there were significant decreases in the contact area after 6-mm resection (60 degrees: 66.32% +/- 18.80%; P = .0354) (20 degrees: 65.47% +/- 15.87%; P = .0127), contact pressure after 6-mm resection (60 degrees: 50.77% +/- 11.49%; P < .001) (20 degrees: 58.01% +/- 23.10%; P = .0335), and peak force after 6-mm resection (60 degrees: 60.67% +/- 9.29%; P < .001) (20 degrees: 74.44% +/- 9.84%; P = .007). Conclusion: Resecting more than 4 to 6 mm of the acetabular rim during hip arthroscopic surgery to address a pincer deformity may dramatically increase contact pressures by 3-fold at the acetabular base. The study suggests that excessive rim resection may lead to increased loads in the hip joint and may predispose to premature joint degeneration. Clinical Relevance: Resecting more than 4 to 6 mm of the acetabular rim may significantly alter hip joint biomechanics, increasing joint reactive forces and subsequent chondrolabral degeneration.