Immediate Anatomical Changes Following Medial Opening Wedge High Tibial Osteotomy Using Radiosterometric Analysis (SS-20)

Abstract

Medial Opening Wedge High Tibial Osteotomy (MOWHTO) is a joint preserving procedure used in patients with medial compartment osteoarthritis (OA) and varus mal-alignment of the knee. The technical aim of this procedure is to shift the weight bearing line out of the medial compartment. Currently we understand the anatomical changes occurring in the varus/valgus plane, but our understanding of other anatomical changes occurring around the knee are limited. The objective of this study is to determine immediate anatomical changes occurring about the knee following MOWHTO. Our hypothesis is that anatomical changes occur in all planes of motion. More specifically, we hypothesize that posterior tibial slope is increased and patellar height is decreased immediately following the osteotomy. This study was prospective in design. Approval by the hospital ethics committee was obtained. Patients with knee medial compartment OA were evaluated according to strict inclusion and exclusion criteria. Included patients were offered participation in the study and signed informed consent materials. Pre-operative clinical assessment, radiographic evaluation, and patient reported outcome scores were performed and recorded. Intraoperatively, patients had radiostereometric analysis (RSA) beads placed in a predetermined pattern decided upon from previous cadaver work. Intraoperative radiographs were taken. The MOWHTO was then performed. A second intraoperative radiograph was taken following the osteotomy. The radiographs were then analysed using dedicated RSA computer software to determine anatomical changes. Eleven patients were included in this study. The diagnosis in all cases was medial compartment knee OA. Average age was 50 years (41 - 58). There were 5 right knees and 6 left knees. Mean error and condition number were 0.70 and 74 respectively, indicating highly accurate and reliable results. Analysis of the proximal tibia showed an average valgus correction angle of 8.69 degrees (range). The average osteotomy size was 12.5mm (9 – 17.5). Posterior tibial slope increased on average by 2.07 degrees (+5.91 - -1.56). Analysis of the patella revealed anatomical changes occurred in all planes of motion. However, the only consistent change was a decrease in patellar height by an average of 8.16mm (1.79 – 13.17). Additionally, there was a consistent increase in patellar tendon length by 5.30mm (2.38-7.34). Using RSA in MOWHTO, we were able to demonstrate a useful and accurate measurement tool for this procedure. There were many immediate anatomical changes following MOWHTO that are inconsistent and not well understood. There were, however, several anatomical changes that remained consistent among all subjects. Patellar height decreased, patellar tendon length increased, and posterior tibial slope demonstrated an average increase. The location of the osteotomy proximal to the tibial tubercle with subsequent wedge distraction likely places a distracting force on the extensor mechanism. The patellar tendon is stretched, translating the patellar inferiorly in the process. The clinical and long term outcomes of this phenomenum are not known. Posterior tibial slope is decreased slightly. This decrease is possibly due to the anatomical differences between the proximal anteromedial tibial cortex and the lateral tibial cortex. However, this can be minimized using specific surgical techniques.