ABSTRACT
PURPOSE OF THE STUDY The purpose of the study is to verify the correct alignment of components of the Oxford medial unicompartmental knee arthroplasty using the Zimmer Microplasty® instrumentation at the beginning of the learning curve. The implantation of prosthetic components of partial knee arthroplasty in proper alignment has an effect on long-term survival of the prosthesis and should eliminate the occurrence of frequent complications. MATERIAL AND METHODS The study group includes 20 patients, 9 men with the mean age of 68 years (range 62-78 years) and 11 women with the mean age of 69 years (range 52-81 years). 13 patients underwent surgery on the right knee and 7 patients on the left knee. The mean length of symptoms was 13 months (range 7-20 months), the mean varus knee deformity was 7° (range 4-12°). The selected patients met both the clinical and radiological criteria for enrolment in the study. In all of them medial unicompartmental knee arthroplasty was performed in 2019 by the same surgeon. After surgery, the parameters of accuracy of alignment of individual prosthetic components were measured on the full-length radiograph of the limb under load. The WOMAC score was recorded preoperatively and one year after surgery and subsequently compared. The knee joint flexion preoperatively and one year after surgery was evaluated. RESULTS The analysis of radiographs revealed that in 12 cases the femoral component was implanted in the mean valgus angle of 1.6° (range 1-3°) and in 8 cases in the mean varus angle of 3° (range 1-5°). All femoral components were implanted in the mean flexion of 7.3° (range 3-11°), no component was implanted in extension. As concerns the tibial component, 19 components were implanted in a neutral or minimum varus angle with the mean value of 1.1° (range 0.3-4°). One component only was implanted in the valgus angle of 1°. All tibial components were implanted with the mean dorsal slope of 6.5° (range 4-8°). The incision was 5 mm deep on average (range 3-6 mm). There was a slight divergence between the components, namely 2.8° on average (range 2-7°) and the distance between the components was 4mm on average (range 3-5 mm). The mean knee joint flexion achieved preoperatively by patients was 115° (range 110-123°), whereas postoperatively the mean flexion achieved was 126° (range 111-138°). The preoperative Womac score was 84.5 points on average (range 64-96 points), whereas postoperatively it was 26.4 points on average (range 7-52 points). None of the components was implanted outside the permitted range, no early complications of the partial replacement (luxation of polyethylene mobile bearing insert, early loosening of the prosthesis, tibial fracture) were observed. DISCUSSION Our radiographic measurements show that when Zimmer Microplasty® instrumentation is used correct alignment of the femoral and tibial component can be achieved and the individual components were correctly aligned within the recommended range. Every single component met the required criteria for alignment. When comparing the values obtained by us in measurements with those obtained by other authors from abroad, similar results regarding the alignment of components were achieved. CONCLUSIONS When using Zimmer Microplasty® instrumentation, excellent results can be achieved also at the beginning of the learning curve of partial knee replacement. Key words: medial gonarthrosis, unicompartmental arthroplasty, hemiarthroplasty, alignment of components.
