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Background: Understanding the femorotibial mechanical axis (FTMA) of the leg is a necessary requirement to assess the outcome of total knee replacement (TKR). Short radiographs, which only capture the knee joint and do not include the center of the femoral head and center of the ankle, only provide us the anatomical axis (FTAA), and not the FTMA. The purpose of this study was to compare the FTMA and FTAA before and after navigated TKR.
Methods: In 130 patients undergoing surgery to implant the same TKR model, an x-ray was taken including the hip, knee and ankle. Images were analysed using a computer program that calculates the mechanical and anatomical axis of the femur, tibia and femorotibial joint. An image-free navigation system with femorotibial tensioned gap technique was used for the arthroplasty implantation. After surgery, a new long x-ray was taken, where the measurements indicated above were taken again.
Results: Pre-operative x-rays showed a mean difference of 6º between FTAA and FTMA (p<0.001). No significant interaction was seen with patient height, but it was in patients with varus deformity and higher BMI (p=0.029); the greatest discrepancy between the anatomical and mechanical axis of the limb was found in individuals with excess weight. After TKR, the mean FTMA was modified to achieve a neutral axis (180° ±3°), as well as a concentration of the figures that showed great pre-procedure disparity, which was verified by the difference in the pre- and post-operative SD (11.85 versus 3.13). The maximum difference between FTAA and FTMA, which stood at 18º before decreased to 5.5º post-TKR.
Conclusions: There is a major discrepancy between the FTAA and FTMA that increases when there is pre-operative varus deformity and in patients with excess weight. After TKR, the FTMA became concentrated, and the mean axis shifted towards the neutral axis. Performing x-rays that only include the knee is not useful for assessing the limb’s alignment prior to or after TKR.
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