The most important finding of this study was that varus knee deformity was associated with morphological changes in the foot and ankle, including the ankle (increased valgus of the distal tibial plafond, decreased medial ankle joint space, and increased incidence of AOA), hindfoot (increased hindfoot valgus), and forefoot (increased hallux valgus). Furthermore, patients with severe knee varus were more prone to medial ankle pain, hindfoot pain, and multiple pain locations than those with mild knee varus. Pain in the medial ankle and hindfoot was associated with corresponding morphological changes.

To date, no consensus has been reached on the relationship between varus knee deformity and ankle morphological changes. Gao et al. [30] found that knee malalignment can induce an increased tilt angle of the ankle (TAA). Xie et al. [4] suggested that varus knee deformity was associated with an increased TAS. However, no bulk-sample studies have demonstrated an association between varus knee deformity and ankle alignment. In this study, varus knee deformity was associated with increased TAS, increased TT, and decreased MTJS. This finding suggests that knee varus is associated with increased valgus of the distal tibial plafond and decreased medial ankle joint space. Although the reported incidence of AOA in patients with end-stage KOA ranged from 24.2 to 36.8% [18], these patients notably remained significantly under-aware of AOA [3]. In the present study, the severity and incidence of AOA were significantly higher in patients with severe varus knee deformity. Our findings on ankle alignment and AOA suggest that varus knee deformity may promote AOA progression through morphological changes in the ankle, and further studies are necessary to identify the specific mechanism by which a varus knee deformity causes ankle degeneration.

Hindfoot alignment is not included in the traditional MA of the lower extremity [8]. Although hindfoot alignment and traditional MA of the lower extremities are both important components of lower extremity alignment, no consensus has been reached regarding their association between the two components [8]. In the present study, knee varus was associated with increased hindfoot valgus. This finding suggests the existence of a potential biomechanical link between the knee and hindfoot alignments. Several studies have found that varus knee deformity was correlated with hindfoot valgus [14, 31, 32], which was consistent with the results of our study. However, the mechanism by which knee varus leads to hindfoot valgus is complex, and the mechanisms reported in the abovementioned relevant studies are inconsistent. Further studies are required to address this issue.

Patients with varus KOA are more prone to concomitant foot deformities, particularly a collapsed medial longitudinal arch [33, 34]. However, few studies have analyzed the relationship between knee varus and the medial longitudinal arch morphology. In the present study, both comparison of the parameters between the two groups and results of the Pearson’s correlation test showed no association between varus knee deformity and medial longitudinal arch morphology. This finding suggests that although the incidence of collapsed medial longitudinal arch is relatively higher among patients with varus KOA [2, 34], the degree of varus knee deformity is not associated with the foot arch morphology. Ohi et al. [2] also demonstrated that the degree of knee varus was not associated with the navicular height, which supports our findings.

Hallux valgus is another prevalent forefoot deformity in patients with KOA and is characterized by lateral deviation (abduction) of the hallux with a corresponding medial deviation (adduction) of the first metatarsal [35]. In the present study, the incidence and severity of hallux valgus were significantly higher in patients with severe varus KOA. This finding suggested that patients with severe knee varus were more prone to hallux valgus. Ohi et al. [2] suggested that an increased varus knee deformity was associated with an increase in the hallux valgus angle, which was consistent with our findings. However, the mechanism by which knee varus leads to hallux valgus is complex, and no consensus has been reached. Further studies are required to determine the biomechanical link between knee varus and hallux valgus progression.

In patients with end-stage KOA, morphological changes commonly occur in the foot and ankle, which usually indicate arthritis and chronic pain [10]. However, few studies have analyzed the relationship between varus knee deformity and foot and ankle pain. In the present study, we demonstrated for the first time that concomitant foot and ankle pain in patients with varus KOA occurred mostly in the medial ankle (19.72%) and that the incidence of pain in the medial ankle was significantly higher in the severe varus group. Thus, medial ankle pain may be the most common type of foot and ankle pain in patients with KOA, and it was more common in patients with severe knee varus. Furthermore, we found that patients with severe varus KOA were more prone to hindfoot pain and multiple pain locations. However, there were no significant differences in the incidences of lateral ankle, midfoot, and forefoot pain between the two groups. Further studies are required to investigate why knee varus only causes pain in specific regions of the foot and ankle.

In this study, the incidence of pain in the medial ankle and hindfoot was significantly higher in the severe varus group. Additionally, significant morphological differences were observed in the medial ankle and hindfoot between the severe and mild varus group. Therefore, we can speculate that pain in the medial ankle and hindfoot is associated with morphological changes in the corresponding regions. In this study, we analyzed the morphological risk factors for pain and found significantly higher TT, lower MTJS, and lower DMCS in the medial ankle pain group, suggesting that a narrow medial ankle joint space is the cause of medial ankle pain in patients with KOA. We speculate that there are two possible reasons that could lead to a narrowing of the medial ankle space: one is that the lateral collateral ligament may be relaxed, causing an imbalance in muscle strength between the medial and lateral ankle; the other is the degeneration of the medial cartilage, leading to medial AOA. In addition, HA was significantly higher in the hindfoot pain group, suggesting that severe hindfoot valgus is a contributing factor to hindfoot pain. The hindfoot pain described in this study was a diffuse pain located around the hindfoot, which might include the sinus tarsi and subtalar joint. However, no study has yet analyzed the specific mechanisms of medial ankle and hindfoot pain in patients with varus KOA, which should be the focus of future studies.

The present study has several limitations. First, this was a retrospective study, which may affect the reliability of our conclusions. Second, only Asians with end-stage varus KOA were enrolled in this study; thus, the findings may not be generalizable to patients with valgus knee deformity or other races. Third, this study has a fundamental limitation in describing the 3-dimensional morphology of the foot and ankle using 2-dimensional radiographic parameters. Fourth, the pain assessment in this study was based solely on the patient’s chief complaint and did not have a relevant score as a basis. Furthermore, a non-precise description of foot and ankle pain was used in this study, which is inadequate for understanding the nature of pain. Finally, this study lacked a control group, which may affect the reliability of our conclusions. Nevertheless, we believe that this study is necessary as a preliminary study for further long-term follow-up studies.