Abstract
Introduction
Impingement of total hip arthroplasties (THAs) has been reported to cause rim damage of polyethylene liners, and in some instances has led to dislocation and/or mechanical failure of liner locking mechanisms in modular designs. Elevated rim liners are used to improve stability and reduce the risk of dislocation, however they restrict the possible range of motion of the joint, and retrieval studies have found impingement related damage on lipped liners.
The aim of this study was to develop a tool for assessing the occurrence of impingement under different activities, and use it to evaluate the effects a lipped liner and position of the lip has on the impingement-free range of motion.
MATERIALS & METHOD
A geometrical model incorporated a hemi-pelvis and femur geometries of one individual with a THA (DePuy Pinnacle® acetabular cup with neutral and lipped liners; size 12 Corail® stem with 32mm diameter head) was created in SOLIDWORKS (Dassault Systèmes). Joint motions were taken from kinematic data of activities of daily living that were associated with dislocation of THA, such as stooping to pick an object off the floor and rolling over. The femoral component was positioned to conform within the geometry of the femur, and the acetabular component was orientated in a clinically acceptable position (45° inclination and 20° anteversion). Variation in orientation of the apex of the lip was investigated by rotating about the acetabular axes from the superior (0°) in increments of 45° (0°−315°), and compared to a neutral liner.
Results
When a lipped liner was used, implant (neck on acetabular rim) impingement was found to occur when performing sit-to-stand from a normal seat, leg cross and pivot, whereas no impingement occurred with a neutral liner. The presence and position of the lip reduced the impingement-free range of motion, compared to the neutral liner. Impingement occurred when the lip was positioned superiorly and anteriorly, when performing most of the activities that were prone to posterior dislocation, and posteriorly, posterior-superiorly and posterior-inferiorly when performing activities prone to anterior dislocation. During sit-to-stand from a normal seat no impingement occurred when a lipped or neutral liner was used. Bone impingement was observed when the performing the roll activity with both lipped and neutral liners.
DISCUSSION
Impingement was observed more with lipped liners compared to neutral liners, this agrees with the findings of some clinical studies. The results indicate that the positioning of the lip influences the possible range of impingement-free motion. Considering this and the improved joint stability of using a lipped liner, a balance is required to achieve an optimal range of motion without increasing the risk of dislocation. This tool could potentially to be used to optimise lipped liner design and position, and could assist with the liner selection for patients based on their activities.
