Paper 95: Why 180° is Not Always the Good Target for TKA in Varus Knee

Abstract

Varus deformity can be due to (1) wear of the medial tibial plateau, (2) extra articular bone deformity of the tibia and (3) extra articular bone deformity of the femur. In (3) any orthogonal cut of the distal femur creates a lateral laxity (resection laxity). Balancing in extension requires then a medial release which creates also a medial laxity in flexion. Balancing the knee in flexion can be challenging particularly if the Trans Epicondylar axis (TEA) is externally rotated in respect with posterior condylar line (PCL). To analyse the morphology of the distal femur in the varus knees and deduce specific difficulties due to different situations. Among 208 TKA done in 2005-2006, 158 were implanted for medial OA with varus deformity (mechanical femorotibial angle FTA = 179°). The preop planning included: frontal and lateral monopodal weight bearing X-ray of the knee, a long bipodal weight bearing X-ray of both lower limbs, a skyline view of both patellae and a computed tomography (CT) scans of the lower limb, which is part of the standard preoperative planning for TKA at our centre since 2003. TEA was measured between lateral epicondyle and sulcus of the medial epicondyle. Posterior Condylar Angle (PCA), measured between TEA and PCL was 1.9°±2.15 (−7° to 7), FTA was 171.3°±4.2 (161° to 179°), mechanical Femoral Angle a was 90.8°±2 (82° to 97°). No significant relationship was observed between a and PCA (r=0.094). In 35 knees angle a was <0° and in 27 a=0°. 12 femurs had a PCA<0°: in 4 a was <90°, in 3 a =90° and in 5 a was > 90°. Four groups were then defined according to angle a and PCA, with different technical consequences: Group I: a<90° and ACP>0° (23 knees): PCA=2.6°±1, a= 87.6°±1.8, FTA= 169.4°±5. It is impossible here to both cut the distal femur at 90°/a, and align femoral component with TEA in flexion. This option would increase medial laxity. Group II: a=90° and PCA>0° (96 knees): PCA=2.9°±1.6, a=91.8°±1.5 and FTA=172.6°±4.2. It is possible to cut the femur at 90°/a and align femoral component with TEA in flexion. Even in case of tibial bone deformity, external rotation of femoral component is favorable. Group III: a=90° and PCA=0° (25 knees): PCA=−0.92°±1.7, a=91.7±1.6 and FTA=172.4±4.8. external rotation of femoral component is here defavorable. Correction of a tibia varus requires a MCL release. Group IV: a<90° and PCA =0° (14 patients): PCA=−0.35°±0.6, a= 88.2±1.1 and FTA=170.0±4. Aligning femoral component with TEA requires sometimes internal rotation (5 cases). Orthogonal cut of femur is then possible if this is accepted. The two goals: distal cut at 90° and femoral rotation aligned with TEA are easily compatible in only 61% of the cases in this serie (group II). In group I (14.5%) and group IV (8.8%) a compromise is necessary and one can consider accepting residual varus deformity in the femur. A pre-op CT Scan is usefull in daily practice to analyse different situations and establish a correct strategy.