Background: To perform total knee arthroplasty, some surgeons prefer to use a fixed valgus angle, for example 5° or 6°,for the distal femoral cut with every patient. This angle may be appropriate for some patients, but may not be for all.

Objective: To determine the proper angle of the distal femoral cut during total knee arthroplasty (TKA) in Thai patients with osteoarthritis of the knee.

Material and Method: A cross-sectional study was conducted of 80 osteoarthritic knees of 50 Thai patients with a mean age of 69 years (range 52-82, SD 7.64) who had received total knee arthroplasty at Chiang Mai University Hospital between January 2011 and March 2013. The femoral mechanical-anatomical (FMA) angle, femorotibial (FT) angle, mechanical femorotibial (MFT) angle,femoral bowing angle, femoral length, hip offset, gender, and age of the patients were analysed using multivariate regression analysis.

Results: Average FMA angle was 6.46° (range 4°-10°, SD 1.26°). The FMA angle was 6.85° and 5.28° in patients with varus and valgus deformity, respectively. Fifteen percent of all patients showed a FMA angle either less than 5° or greater than 7°.

Conclusion: The average FMA angle of patients with gonarthrosis was 6.46°. If it is not possible to measure the FMA angle in Thai patients, 7° valgus cut angle is recommended for patients with varus deformity. Variation in this angle was high. Pre-operative measurement of FMA angle is beneficial for precise TKA.

Keywords: Knee arthroplasty, Osteoarthritis, Femoral mechanical-anatomical angle, Femoral valgus cut, Thai