Distal Femoral Valgus Resection Angle in Conventional Total Knee Arthroplasty - a CT Scanogram Study

In conventional total knee arthroplasty (TKA), the distal femur valgus resection angle (DFVA) is decided either by measuring the specific resection angle for each patient on preoperative anteroposterior hip-knee-ankle (HKA) weight-bearing radiograph or using a fixed resection angle of five to seven degrees, when such facilities are not available. This study aims to measure the DVFA in TKA patients using preoperative HKA non-weight-bearing computerized tomography (CT) scanogram scout films and determine its relation with preoperative coronal plane lower-limb deformities.

In this retrospective radiological study, various measurements were performed on bilateral, preoperative hip-knee-ankle CT scanograms of 73 knee osteoarthritis patients who had presented for total knee replacement surgery using a standard protocol. The angle between the femoral anatomical axis and femoral mechanical axis was measured as the femoral mechanical anatomical angle (FMAA), which corresponds to the surgical DFVA. The angle between the femoral and tibial mechanical axes was measured as mechanical femorotibial angle (MFTA). The correlation between FMAA and MFTA was studied.

The mean FMAA for the study group was 6.45° (range 3° to 11°, SD 1.17°). The MFTA for the study group ranged from 24° varus to 14° valgus. The alignment was valgus in 14.4% (n=21), varus in 84.2% (n=123), and "0 degrees" in 1.3% (n=2). With valgus coronal alignment taken as positive and varus as negative, the Pearson's correlation coefficient for MFTA with FMAA was r = -0.5183 (p<0.001), indicating that valgus knees tended to have a smaller FMA angle and varus knees tended to have a larger FMA angle.

In the non-availability of individualized measurements, in primary TKA, we recommend setting DFVA as five degrees for valgus deformities, six degrees for mild/moderate varus deformities (MFTA <15°) and seven degrees for severe varus deformities (MFTA > 15°).