Reducing mobile bearing dislocations in the Oxford Domed Lateral unicompartmental knee replacement implant

• Main Author: Yang, Irene
• Format: Dissertation
• Language: English
• Published: University of Oxford 2022
• Subjects: Biomedical engineering; Medicine; Science

The Oxford Domed Lateral (ODL) mobile bearing Unicompartmental Knee Replacement (UKR) provides good clinical results, but the bearing dislocation rate is unacceptably high. A systematic review and meta-analysis found that changing the articulating surface on the tibial component from a flat to a domed shape reduced the overall bearing dislocation rate from 17% to 3.7%. The risk of anterior/posterior (AP) dislocations was reduced substantially. Most dislocations were found to occur medially and with the knee in a flexed position. Dislocations occur in flexion because when the knee is flexed, the lateral ligaments are lax. Therefore a mechanical rig, which allowed the relative position of components to be adjusted and the joint to be distracted, was used to determine the minimum Distraction required for a Dislocation (DD), which is related to the risk of dislocation. However, as testing with the rig was inefficient, a computational dislocation analysis tool based on the mechanical rig was developed and validated against results from the rig. The tool uses path planning algorithms from the field of robotics to allow the bearing to find a viable dislocation pathway. It was found that medial dislocations required a much lower DD than AP dislocations, which explains why medial dislocations are more common than AP dislocations. Further, changing the tibial surface from a flat to a dome increased the DD for AP dislocations substantially, which explains why this change primarily decreased AP dislocations. A series of potential implant design changes were studied to see if they would decrease the DD for medial dislocations to that of AP dislocations as the risk of AP dislocation was considered acceptable. The simplest change to achieve this was to increase the tibial wall height by 2 mm, but to avoid the femoral and tibial components hitting if there is bearing wear, the minimum bearing thickness would have to be 5 mm. Alternatively if the centre of the femoral component sphere was moved laterally, the bearing made wider laterally and the wall height increased, then an acceptable dislocation rate would be achieved with a minimum bearing thickness of 4 mm. For both these modifications to be successful, surgeons must ensure that the bearing is not more than 4 mm from the tibial wall. Cadaveric and clinical studies are required to assess the viability of the proposed changes. If the risk of dislocation can be minimised or reduced, more patients may benefit from the advantages of lateral mobile bearing UKRs.