Recommendations for minimum cup coverage based on anteroposterior radiographs are widely used as an intraoperative guide in total hip arthroplasty for patients with developmental dysplasia of the hip. The purpose of this study was to examine the validity of two-dimensional (2D) measurement of coverage with three-dimensional (3D) coverage and to identify parameters for determining the 3D coverage during surgery.
We developed a technique to accurately reproduce the intraoperative anatomic geometry of the dysplastic acetabulum and measure the 3D cup coverage postoperatively. With this technique, we retrospectively analyzed the difference and correlation between 2D and 3D measurements of native bone coverage in 35 patients (45 hips) with Crowe II or III DDH. Linear regression analysis was performed to examine the intraoperative parameters related to coverage. The mean follow-up period was 7.64 years (range, 6.1-9.5 years).
There was a significant difference and a fair correlation between 2D and 3D measurements. The 2D measurement underestimated the 3D cup coverage by approximately 13%. An excellent linear relationship was noted between the 3D coverage/uncoverage and the height of the uncovered portion (R2 = 0.8440, P < .0001). There was no case of loosening or revision during the follow-up.
Current minimum cup coverage recommendations based on 2D radiograph measurements should not be used as a direct intraoperative guide. The height of the uncovered portion is a useful parameter to determine the 3D coverage during surgery.