Abstract
The 10 year survivorship of THR is generally over 95%. However, the incidence of revision is usually higher in year one. The most common reason being dislocation which at least in part is driven by inadequate range of motion (ROM) leading to impingement, subluxation and ultimately dislocation which is more frequently posterior. ROM is affected by patient activity, bone and component geometry, and component placement. To reduce the incidence of dislocation, supported by registry data, there has been an increase in the use of so-called ‘lipped’ liners. Whilst this increases joint stability, the theoretical ROM is reduced. The aim of this study was to investigate the effect of lip placement on impingement.
A rigid body geometric model was incorporated into a CT scan hemi-pelvis and femur, with a clinically available THR virtually implanted. Kinematic activity data associated with dislocation was applied, comprising of five posterior and two anterior dislocation risk activities, resulting from anterior and posterior impingement respectively. Cup inclination and anteversion was varied (30°-70°, 0°-50° respectively) to simulate extremes of clinical outcomes. The apex position of a ‘lipped’ liner was rotated from the superior position, anteriorly and posteriorly in steps of 45°. Incidence and location of implant and bone impingement was recorded in 5346 cases generated.
A liner with the lip placed superior increased the occurrence of implant-implant impingement compared with a neutral liner. Rotation of the lip from superior reduced this incidence. This effect was more marked with posterior rotation which after 90° reduced anterior impingement to levels similar to a neutral liner. Complete inversion of the lipped liner reduced impingement, but this and anterior rotation both negate its function – additional stability.
This study comprises one bone geometry and component design and one set of activity profiles. Nevertheless, it indicates that appropriate lip placement can minimise the likelihood of impingement for a range of daily activities whilst still providing additional joint stability.
