In total hip arthroplasty, the minimum host bone coverage required on the cup for stable fixation has been previously reported; however, the coverage was generally evaluated on a 2-dimensional (2D) image and 3-dimensional (3D) coverage has not been well described.
We used postoperative computed tomography images to retrospectively measure 3D cup coverage in 151 hips with developmental dysplasia of the hip that underwent primary total hip arthroplasty. The aims were to (1) determine the minimum requirement of the 3D coverage for stable cup fixation; (2) evaluate the relationship between 2D and 3D coverage; and (3) identify the factors associated with 2D-3D discrepancy, defined as follows: 2D-3D discrepancy = 2D coverage−3D coverage.
All cups showed stable fixation as demonstrated by bone ingrowth with an average postoperative period of 48 months. The minimum 3D coverage was 61.2%, with a mean value of 77.1% ± 6.7% and maximum value of 97.6%. We found a significant positive but poor relationship between 3D and 2D coverage (bone coverage index; r = 0.30). Consequently, the 2D-3D discrepancy varied greatly, with a mean value of −1.6% ± 12.3% (range, −36.5% to 32.2%). Multiple linear regression analyses confirmed that lower cup center and anterior tilt of the pelvis relative to the computed tomography table were independent factors associated with increasing 2D-3D discrepancy.
This study showed that 2D coverage, measured on a projected image, carries a risk of overestimation, especially in hips with the aforementioned features.