Hitting the Target: Natural History of the Hip Based on Achieving an Acetabular Safe Zone Following Periacetabular Osteotomy

Background: Periacetabular osteotomy (PAO) remains the gold-standard treatment for acetabular dysplasia in skeletally mature patients with preserved cartilage. The purpose of this multicenter cohort study was to delineate the long-term radiographic natural history of the dysplastic hip following PAO based on the final position of the acetabular fragment. Methods: We evaluated patients who underwent PAO performed by 4 hip preservation surgeons to treat acetabular dysplasia with or without concomitant retroversion from 1996 to 2012 at 3 academic institutions. There were 288 patients with a mean clinical and radiographic follow-up of 9 years (range, 5 to 21 years). Postoperative radiographs made at the first clinical visit were used to determine if the acetabular fragment fell into a safe zone according to the absence of retroversion, a lateral center-edge angle (LCEA) of 25 degrees to 40 degrees, an anterior center-edge angle (ACEA) of 25 degrees to 40 degrees, and a Tonnis angle of 0 degrees to 10 degrees. Every available subsequent radiograph was assessed for degenerative changes by the Tonnis classification (grades 0 to 3). The time to progression was analyzed using Cox proportional hazards regression and multistate modeling. Results: Only the absence of retroversion was independently associated with a decreased risk of progressing at least 1 Tonnis grade during follow-up: hazard ratio (HR), 0.60 (95% confidence interval [CI], 0.38 to 0.94; p = 0.025). Achieving the ACEA safe zone yielded the greatest time increase for remaining in Tonnis grade 0 or 1 (43 years for having an ACEA in the safe zone compared with 28 years for not having an ACEA in the safe zone), followed by the absence of retroversion (34 years for the absence of retroversion compared with 24 years for the presence of retroversion). However, attaining the Tonnis angle or LCEA safe zones did not delay progression. The achievement of additional safe zones generally increased the length of time that patients spent in Tonnis grade 0 or 1: 25 years for 0 safe zones, 36 years for 1 safe zone, 29 years for 2 safe zones, 37 years for 3 safe zones, and 44 years for 4 safe zones. Conclusions: This study demonstrates the importance of achieving appropriate acetabular reorientation to enhance the longevity of the native hip following PAO. Although the LCEA and the Tonnis angle are the most common metrics used to assess appropriate acetabular correction, this study shows that adequately addressing retroversion and the ACEA has a greater impact on improving the natural history.