Best bone of acetabulum for cup component placement in Crowe types I to III dysplastic hips: a computer simulation study

BACKGROUND: During cup implantation, vertical height of the cup center (V-HCC) should be precisely controlled to achieve sufficient bone-cup coverage (BCC). Our study aimed to investigate the acetabular bone stock and the quantitative relationship between V-HCC and BCC in Crowe types I to III hips. METHODS: From November 2013 to March 2016, pelvic models of 51 patients (61 hips) with hip dysplasia were retrospectively reconstructed using a computer software. Acetabular height and doom thickness were measured on the mid-acetabular coronal cross section. V-HCC was defined as the vertical distance of cup rotational center to the interteardrop line (ITL). In the cup implantation simulation, the cup was placed at the initial preset position, with a V-HCC of 15 mm, and moved proximally by 3-mm increments. At each level, the BCC was automatically calculated by computer. Analysis of variance and Kruskal-Wallis test were used to compare the differences between groups. RESULTS: There were no significant between-group differences in maximum thickness of the acetabular doom; however, peak bone stock values were obtained at heights of 41.63 ± 5.14 mm (Crowe type I), 47.58 ± 4.10 mm (Crowe type II), and 55.78 ± 3.64 mm (Crowe type III) above the ITL. At 15 mm of V-HCC, median BCC was 78% (75–83%) (Crowe type I), 74% (66–71%) (Crowe type II), and 61% (57–68%) (Crowe type III). To achieve 80% of the BCC, the median V-HCC was 16.27 (15.00–16.93) mm, 18.19 (15.01–21.53) mm, and 24.13 (21.02–28.70) mm for Crowe types I, II, and III hips, respectively. CONCLUSION: During acetabular reconstruction, slightly superior placement with V-HCC <25 mm retained sufficient bone coverage in Crowe I to III hips.