Instrumentation optimisation for unicompartmental knee replacement

The cementless medial Oxford Unicompartmental Knee Replacement (OUKR) has improved outcomes and almost halved revision rates compared to the cemented OUKR at 10 years. However, there is limited understanding of long-term complications of the cementless OUKR beyond 10 years, such as aseptic loosening and bearing fracture, and short-term complications, such as tibial component subsidence (TCS) and tibial periprosthetic fracture (TPF). The aims of this thesis were to investigate poorly understood OUKR complications, understand instrumentation limitations that may contribute to the risk of complications, and to optimise the instruments to minimise the risk of complications. 10-year results of a randomised controlled trial with radiostereometric and radiographic analysis found that cementless fixation is likely to be as good, if not better than cemented fixation in the second decade. Two cemented tibial components with substantial early migration had ongoing migration at 10 years. Cementless OUKRs (1/15) had fewer tibial radiolucent lines compared to cemented (6/14) at 10 years. Bearing fractures, because of excessive polyethylene wear, have been reported with the Phase III OUKR. There is a concern that cementless fixation might be associated with increased wear due to third body debris. The mean linear polyethylene wear of Phase III OUKR (0.06 mm/year) was found to be approximately three times higher than the Phase II wear rate, with no difference in wear rate between cemented and cementless implants. To minimise the risk of bearing fracture in the very long-term, surgeons should position the bearing close to the tibial wall, neutrally align the femoral component, and avoid using the thinnest Size 3 bearings in young patients. A radiographic study of 94 cementless OUKR patients identified five cases (5%) of TCS. No patient, implant, or surgical factors were found to be associated with TCS, likely due to the small number of TCS cases identified. All TCS patients had an acceptable five-year clinical and radiographic outcome, therefore it is recommended that TCS should be treated conservatively. The first surgeon survey to identify limitations of the OUKR instrumentation was conducted. The 106 surgeons who responded were satisfied with most of the OUKR instrumentation. Instruments for patient positioning (36%), tibial resection (51%), femoral preparation (41%), and tibial keel slot preparation (29%) were identified to have limitations that required improvement. Most surgeons (63%) believed that computer assisted surgery would improve the OUKR procedure. The risk of OUKR TPF is higher for cementless OUKRs. Mechanical experiments found that preventing vertical overcut using a prototype resection guide can decrease the risk of TPF. Positioning the component more medially, distally, and valgus was found to increase the risk of TPF in a small tibial model. Preparing a shorter keel slot with a buffer sawblade and rasping the slot ends increased LTF by 11% compared to the standard sawblade without compromising primary fixation. Comparatively, use of a component with a shorter and shallower keel increased LTF by 38%, also without compromising fixation. To minimise the risk of TPF with cementless OUKR it was recommended that the buffer saw and rasp should be adopted clinically until a component with a shorter and shallower keel is available. This thesis improves the understanding of the risk of aseptic loosening, polyethylene wear, bearing fracture, TCS, TPF, and instrumentation limitations from the surgeon’s perspective for the OUKR. It has proposed optimised instruments to minimise the risk of TPF for cementless OUKR and concludes by outlining how these instruments can be introduced clinically to reduce the incidence of TPF.