Biomechanics of the native hip from normal to instability

Abstract

Developmental dysplasia of the hip (DDH) is characterized as an insufficient acetabular coverage of the femoral head and is commonly associated with hip subluxation, hypermobility, and frank dislocation. Instability associated with DDH can be attributed to bony (i.e. insufficient osseous coverage or morphology) and soft tissue abnormalities (i.e. ineffective capsular function). A better understanding of capsular properties can tailor more suitable diagnostic criteria as well as surgical solutions to restore joint stability. The iliofemoral ligament is responsible for reinforcing much of the anterolateral capsule, restraining hip external rotation and anterior translations. It is also important not to disrupt the zona orbicularis during surgical management to maintain stability. In our exploratory in vitro study, we examined dysplastic hips and the effects of surgical periacetabular osteotomy (PAO) on capsular function and joint stability. Although PAO provided osseous structural stability and decreased hypermobility, it slackened the anterolateral capsule and may have increased adduction and extension instability. This may suggest that certain instability can be attributed to ineffective capsular function, secondary to acetabular undercoverage. In efforts to preserve the native hip and delay joint degeneration for DDH, it would be crucial to elucidate the amount of acetabular reorientation and capsular alterations needed without compromising hip mobility and function and causing iatrogenic instability. As such, we should always approach the joint where there is the best opportunity to preserve femoral head size and neck length, not disrupt the zona orbicularis, and only alter the capsule when/as necessary without comprising joint loading.