Title : Anatomy, biomechanics, trigonometry & psychology of prosthetic hip instability
Abstract:
In contrast to native hip dislocations (NHD), prosthetic hip dislocations (PHD) can be multi-directional & recurrent, often following minimal trauma or occurring spontaneously. Also, PHD prevention advice following anterior approaches (to avoid flexion, adduction & internal rotation) is counter-intuitive, being opposite of the advice following anterior NHD. Known causes for PHD (versus NHD) include shallower acetabular cup, smaller femoral head, unrepaired or excised anterior capsule and partial abductor paralysis.
There are other less-known factors though. Firstly, anterior approaches disrupt the posterior capsule as it attaches half-way along the femoral neck, resulting in a posterior capsular defect. Still, the postero-superior myo-capsular tether (ischeo-femoral ligament, adjacent small muscles), is intact, opposing posterior or supero-lateral PHD tendency. If this is sacrificed for the ease of femoral reaming, femur becomes hyper-mobile multi-directionally, predisposing for PHD over a wide arc supero-laterally & posteriorly.
Also, femur & acetabulum are both anteverted naturally, allowing a good range of flexion & external rotation. By inadequately reproducing anteversion of these components in anterior approaches, (because of anterior PHD concerns), the propensity towards posterior PHD increases. Attempts towards neutral position implantation often results in slight retroversion of components, further increasing the posterior PHD tendency on flexion &/or internal rotation.
These factors – anatomy of the posterior capsule, version issues, approach choice logistics and surgeon’s psychology – are not well-understood and appreciated; if these re well-publicised, then surgeons are likely to take appropriate preventive steps such as using the posterior approach where possible, preserving the postero-superior tether (if using anterior approaches) and implanting components in adequate anteversion.
Keywords: ‘Anteversion’, ‘posterior capsule’, ‘prosthetic hip dislocation’, ‘anterolateral approach’, ‘acetabular cup’, ‘femoral stem
