Best Bone of Acetabulum for Cup Component Placement in Crowe Types I to III Dysplastic Hips: A Computer Simulation Study
Total hip arthroplasty (THA) is the standard and effective treatment for advanced degenerative arthritis in patients with congenital dysplasia of the hip (CDH). However, the wide variability in dysplastic acetabulum deficiencies poses significant challenges for THA in these patients. Dysplastic hips are classified using the Crowe classification system, which also predicts surgical complexity and the likelihood of complications. In severe cases of CDH, acetabular reconstruction is technically demanding due to shallow acetabular concavity, anterolateral bone deficiencies, and osteophytes. The optimal cup position and reconstruction strategies remain a topic of debate, with many advocating for placing the acetabular components in an anatomic position to optimize the biomechanical environment. However, for a cementless cup, this may result in bone-cup coverage (BCC) less than 75% to 80%, which is against biologic fixation.
Upward placement of the cementless cup component may allow for better bone stock than an anatomic position, especially in Crowe type II or III hips. Recent reports have shown satisfactory long-term outcomes in dysplastic hips treated with a superiorly placed cementless acetabular component. However, high hip center placement remains controversial, with some studies indicating it as a risk factor for dislocation, delayed recovery of abductor muscle moment, and lower ranges of flexion and internal rotation after THA.
This study aimed to investigate the acetabular bone stock and the quantitative relationship between the vertical height of the cup center (V-HCC) and BCC in Crowe types I to III hips using computer simulation software. The study was conducted on pelvic models of 51 patients (61 hips) with hip dysplasia, retrospectively reconstructed from November 2013 to March 2016. The acetabular height and dome thickness were measured on the mid-acetabular coronal cross-section. V-HCC was defined as the vertical distance of the 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 the computer.
The results showed no significant between-group differences in the maximum thickness of the acetabular dome. 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, the median BCC was 78% (75–83%) for Crowe type I hips, 74% (66–71%) for Crowe type II hips, and 61% (57–68%) for Crowe type III hips. 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.
The study concluded that during acetabular reconstruction, slightly superior placement with V-HCC <25 mm retained sufficient bone coverage in Crowe I to III hips. The 3D computer simulation demonstrated that V-HCC <25 mm retained sufficient bone coverage, and the acetabular bone stock correlates with the degree of hip dysplasia.
The study had some limitations, including the small number of subjects in the Crowe type III group, the absence of normal hips, and the fixed cup inclination and anteversion in the simulation study. However, the results provided valuable insights into the optimal cup component position and corresponding bone coverage, which could help surgeons evaluate acetabular bone stock distribution, make intraoperative decisions, and predict the survival of the cup component.
The bone stock of the medial acetabular wall determines the amount of cup medialization, while the bone stock of the acetabular dome determines the height at which the cup component achieves sufficient bony support. The study evaluated the bone stock distribution in the medial acetabular wall, from the bottom edge to the acetabular roof, in Crowe types I, II, and III hips. There were no significant between-group differences in maximum thickness of the acetabular roof; 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 56.55 ± 3.56 mm (Crowe type III) above the ITL.
In summary, in patients undergoing THA secondary to CDH, the bone stock distribution of the acetabulum, which varies according to dysplasia severity, should be taken into account when reaming the socket during acetabular reconstruction. Slightly superior placements with V-HCC <25 mm retained sufficient bone coverage, providing a reliable guideline for surgeons in managing dysplastic hips.
doi.org/10.1097/CM9.0000000000000527
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