How to accurately determine the distal femoral valgus cut angle in the valgus knee arthroplasty

Purpose Distal femoral resection in total knee arthroplasty (TKA) is commonly performed using intramedullary jigs with a pre-operatively planned valgus cut angle (VCA). For valgus knees with lateral femoral condyle hypoplasia, the method of determining the accurate VCA has not been clarified. The aim of the present study is to introduce a method that can accurately determine the distal femoral VCA in the valgus knee arthroplasty. Methods Twenty patients with valgus deformity caused by lateral femoral condylar hypoplasia underwent primary TKA with individually measured VCA. The VCA was defined as the acute crossing angle of the anatomical and mechanical axes of the femur on a pre-operative X-ray film, and the two axes almost always crossed at the distal femoral diaphysis, but not the centre of the knee as generally described. The entry point of the femoral intramedullary guide rod was determined on the extension of the femoral anatomical axis and was usually medial to the centre of the knee. According to the pre- and post-operative X-ray films, the mechanical lateral distal femoral angle (mLDFA), and coronal alignment of the femoral components were measured. The post-operative knee pain and function were evaluated using the Visual Analog Scale and Knee Society Score, respectively. Results The mean VCA measured according to the above method was 6.4A degrees +/- 1.0A degrees (4.7-8.2A degrees), and the femoral entry point was located at a mean distance of 7.4 +/- 2.1 mm (4.5-10.9 mm) medial to the centre of the knee joint. The mean mLDFA before and after operation was 77.4A degrees +/- 5.7A degrees (74-82A degrees) and 88.4A degrees +/- 1.7A degrees (86-90A degrees), respectively, showing a statistically significant difference (P < 0.01). Conclusions The deformity of the distal femoral diaphysis is quite various in different valgus knees. The VCA and the femoral entry point should be determined individually for each case. The application of the current method resulted in good post-operative mechanical axis alignment and clinical results after TKA.