Abstract
Introduction
Malalignment of lower limb is a common feature in patients with osteoarthritis (OA). This, either cause or effect of OA, is known to alter the normal anatomy of knee and affects progression of wear and tear in mechanically stressed compartment. We investigated the relationship of mechanical axis to wear and tear in varus, neutral and valgus knees.
Materials and Methods
A retrospective analysis of 136 consecutive patients, with OA, who underwent total knee replacement using computer navigation. The thickness of medial and lateral cuts of distal femur and proximal tibia were recorded. Pre-op coronal deformity was assessed using long leg radiographs and Femoral Tibial Mechanical Angle (FTMA) calculated. Patients were evaluated as one group and three subgroups based on preop varus, neutral or valgus lower limb alignment. Student t test and Pearson's correlation coefficient were used for statistical analysis.
Results
When considered as a whole group (136 patients) there was a significant difference between the medial and lateral cuts on both femoral and tibial side (p <0.001). We also found a significant negative correlation between FTMA and femoral lateral cut (r = −0.45). In varus group (103 patients) tibial medial and lateral cuts were significantly different (p<0.05) while there was no significant difference in femoral medial and lateral cuts. In valgus group (n=23) there was a significant difference between the femoral medial and lateral cuts (p<0.0001) while no significant difference was found between tibial medial and lateral cuts. Intergroup comparison showed that there was a significant difference between the varus and valgus group with regards to femoral medial cut, femoral lateral cut and tibial medial cut (all p<0.01). There was no significant difference between the tibial lateral cut between the varus and valgus group. We found a significant negative correlation between tibial lateral cut and FTMA in valgus group (r=-0.68).
Discussion
In both varus and neutral group tibial lateral cut was more as compared to medial cut and this difference was significant while there was no significant difference between femoral medial and tibial cuts. This emphasises the point that in varus and neutral knees tibial side wear is responsible for causing the deformity as compared to femoral. There was no significant difference in tibial lateral cut between varus and valgus group. In valgus knees femoral side is responsible for producing malalignment rather than tibial side and less amount of lateral femur removal is required but amount of lateral tibial removal is not significantly different. This study shows that varus deformity is mainly a tibial phenomenon while valgus deformity mainly occurs in femur. Surprisingly, approximately a mean 9 mm of lateral tibial cut was required, irrespective of whether the patient had varus, neutral or valgus preop lower limb alignment.
