Does the optimal position of the acetabular fragment should be within the radiological normal range for all developmental dysplasia of the hip? A patient-specific finite element analysis

Background: The success of Bernese periacetabular osteotomy depends significantly on how extent the acetabular fragment can be corrected to its optimal position. This study was undertaken to investigate whether correcting the acetabular fragment into the so-called radiological "normal" range is the best choice for all developmental dysplasia of the hip with different severities of dysplasia from the biomechanical view? If not, is there any correlation between the biomechanically optimal position of the acetabular fragment and the severity of dysplasia? Methods: Four finite element models with different severities of dysplasia were developed. The virtual periacetabular osteotomy was performed with the acetabular fragment rotated anterolaterally to incremental center-edge angles; then, the contact area and pressure and von Mises stress in the cartilage were calculated at different correction angles. Results: The optimal position of the acetabular fragment for patients 1, 2, and 3 was when the acetabular fragment rotated 17 degrees laterally (with the lateral center-edge angle of 36 degrees and anterior center-edge angle of 58 degrees; both were slightly larger than the "normal" range), 25 degrees laterally following further 5 degrees anterior rotation (with the lateral center-edge angle of 31 degrees and anterior center-edge angle of 51 degrees; both were within the "normal" range), and 30 degrees laterally following further 10 degrees anterior rotation (with the lateral center-edge angle of 25 degrees and anterior center-edge angle of 40 degrees; both were less than the "normal" range), respectively. Conclusions: The optimal corrective position of the acetabular fragment is severity dependent rather than within the radiological "normal" range for developmental dysplasia of the hip. We prudently proposed that the optimal correction center-edge angle of mild, moderate, and severe developmental dysplasia of the hip is slightly larger than the "normal" range, within the "normal" range, and less than the lower limit of the "normal" range, respectively.