Using a robotics path planning algorithm to assess the risk of mobile bearing dislocation in lateral unicompartmental knee replacement.

Dislocation of the bearing occurs in 1 - 6% of Oxford Domed Lateral (ODL) mobile bearing unicompartmental knee replacements (UKRs). Dislocations occur in flexion as the lateral ligaments are lax in this position allowing the knee to distract. Anterior and posterior dislocations are rare: clinically, their dislocation rates are acceptable. Most dislocations tend to occur medially, with the bearing sitting on top of the tibial wall. Using robotics path planning algorithms and a modified Open Motion Planning Library (OMPL) Graphical User Interface (GUI), a dislocation analysis tool was developed to assess the minimum amount of vertical distraction of the femoral component relative to the tibial component required for the mobile bearing to dislocate. In the tool, the Rapidly- exploring Random Trees (RRT) algorithm was applied to the mobile bearing, which enabled autonomous movement of the bearing from a non-dislocated to a dislocated position. Testing increased the relative distance between the femoral component and the tibial component: vertically (2-6 mm) and mediolaterally (0-4 mm) in 0.25 mm increments resulting in a total of 289 configurations. For each configuration, the tool assessed whether mobile bearing dislocation was possible (either medially, laterally, anteriorly or posteriorly). For each mediolateral translation distance, the minimum vertical distraction required for dislocation was recorded. To validate the tool, dislocation results were compared to measurements taken using a custom-built mechanical rig. The minimum amount of distraction required for medial dislocation was similar for the dislocation analysis tool (3.75 to 4.75 mm) as compared to a custom-built mechanical rig (2.5 to 4 mm). The amount of distraction for a medial dislocation was much smaller than that for an anterior or posterior dislocation (6 to 6.25 mm). This explains why medial dislocations are more common. Future work will use this tool to inform implant design, with the aim to reduce the risk of medial dislocation to match that of anterior/posterior dislocation, which is clinically acceptable.