The objectives of the current retrospective study were to estimate the change in ankle alignment to compensate for varus knee deformities and evaluate differences in sex regarding these compensatory changes. In women, the ankle alignment was significantly influenced by varus knee deformities, and as the knee mechanical axis became more varus, the distal tibia plafond secondarily became more valgus. However, a correlation between changes in the ankle alignment and knee mechanical axis was not found in male patients. Moreover, we found that abnormal biomechanics in the ankle were significantly associated with varus knee deformities in both men and women.
Although the effect of varus knee deformities on degenerative changes in the ankle has not been fully established, the relationship between varus knee deformities and ankle OA has been proven [14]. The most prevalent type of ankle OA is posttraumatic OA, while the prevalence of primary and secondary ankle OA is rarely reported. However, as previously described, the prevalence of ankle OA ranges from 28.8 to 35.2% in TKA patients without a history of ankle fracture [7, 15]. In a study on cadaveric donors, severe ankle degeneration always existed along with knee degeneration [16].
Degeneration of the lower limb joints is highly related to the joint alignment. In the current study, the LDTA decreased as the knee alignment became more varus, indicating lateral metaphyseal collapse and valgus change in the distal tibia plafond. Our results show that compensatory changes in the ankle to varus knee deformities mainly occur at the distal tibial side, which could influence the ankle tilt and cause an abnormal biomechanical state in the tibiotalar joint. Therefore, we believe that valgus change in the distal tibial plafond and increased ankle tilt could be the main causes of ankle degeneration due to varus knee deformities.
Another interesting finding of the current study is that compensatory change in the ankle tilt to varus knee deformities could increase abnormal forces on the ankle. In women, the TT decreased as the knee alignment became more varus, indicating lateral tilt of the tibiotalar joint. In addition, the relative tilt of the talus and distal tibia plafond to the ground also increased as varus knee deformities progressed. Increased tilt of the talus and distal tibia plafond could change the biomechanical state of the ankle, which could lead to narrowing of the medial ankle joint space and collapse of the lateral metaphysis of the distal tibia plafond. It is still uncertain whether the compensatory change in ankle morphology could recover after TKA. Therefore, it is necessary to evaluate the ankle alignment in knee OA patients preoperatively and postoperatively, which could further confirm the effect of varus knee deformity on the ankle alignment.
A more perplexing area of uncertainty is whether the compensatory change in ankle morphology negatively affects the outcome of TKA. TKA is the most cost-effective method to treat end-stage knee degeneration [17]. In past decades, the surgical techniques, perioperative management, and prosthesis survivorship of TKA have improved dramatically, but the patient-reported dissatisfaction rate of TKA ranges from 11 to 25% [18, 19]. Many factors could affect the patient’s satisfaction with TKA, such as age, preoperative mental health, and postoperative knee stability [20, 21]. In recent years, the effects of ankle and foot stability on TKA outcomes have received more attention. Gursu et al. [9] evaluated the ankle morphology and alignment in 80 knees with at least 10° of varus deformities and concluded that overcorrecting the tilt of the distal tibia plafond and ankle alignment could be the main reason for postoperative ankle pain. As mentioned before, in women, lateral tilt of the distal tibia plafond and valgus changes to it would increase when the knee alignment becomes more varus. In such patients, leaving a residual varus knee deformity may lead to a better clinical outcome, which corresponds to our clinical experience. For most arthroplasty surgeons, it is uncommon to estimate the alignment and abnormal biomechanical state of the ankle before TKA. Based on the results of the current study and previous articles, we suggest performing a careful preoperative examination for ankle deformities in patients with knee OA, especially women with large varus knee deformities.
The current study has some limitations. First, the number of men was not large due to the relatively low prevalence of TKA in the male population. To our knowledge, we are the first to report sex differences in terms of compensatory change in the ankle alignment due to varus knee deformities. Moreover, the findings of the current study could provide a new perspective for further research. Second, the anatomic data that were used were from the Asian population, and the results of this study may not be generalized to people of other ethnicities. Third, we did not investigate the influence of hand dominance and patient complaints (such as joint pain and range of motion) on compensatory change in ankle alignment due to knee malalignment.