Summary

In RA patients, hindfoot alignment improves after TKA in Varus-Valgus and Valgus-Varus positions, where hindfoot compensatory mechanisms may be active. However, improvement may not occur if the Larsen grade of the hindfoot is 3 or higher. Additionally, hindfoot alignment may not improve after TKA in Varus-Varus and Valgus-Valgus positions, where the hindfoot compensatory mechanism is thought to be

Abstract

Background

In knee osteoarthritis (OA) patients, a positive correlation exists between knee and hindfoot alignments due to compensatory changes in the subtalar joint. Hindfoot alignment is a compensatory valgus in patients with varus knee OA. Total knee arthroplasty (TKA) often improves hindfoot alignment. Rheumatoid arthritis (RA) patients may develop concomitant destruction of the knee and subtalar joints and often experience uncommon deformities, such as a combination of knee varus and hindfoot varus, knee valgus and hindfoot valgus, potentially worsening hindfoot alignment post-TKA. This study aimed to evaluate changes in hindfoot alignment after TKA in RA patients.

Methods

A prospective study was conducted on consecutive RA patients undergoing TKA between 2013 and 2018. Knee and hindfoot alignments were simultaneously assessed by measuring the hip-to-calcaneus radiograph. The femorotibial angle (FTA) and tibiocalcaneal angle (TCA) were measured before and six months after TKA. According to previous literature, hindfoot alignment was defined as normal (TCA -2.1° to 4.5°), valgus (≥ 4.5°), or varus (≤ -2.1°). Lateral radiographs of the feet were taken to assess the Larsen grade of the subtalar joint. Patients were divided into 6 groups based on knee and hindfoot alignments: 1) Knee Varus (FTA > 174°) and Hindfoot Valgus (TCA > 4.5°); 2) Knee Varus (FTA > 174°) and Hindfoot Normal (-2.1° ≤ TCA ≤ 4.5°); 3) Knee Varus (FTA > 174°) and Hindfoot Varus (TCA < -2.1°); 4) Knee Valgus (FTA < 174°) and Hindfoot Valgus (TCA > 4.5°); 5) Knee Valgus (FTA < 174°) and Hindfoot Normal (-2.1° ≤ TCA ≤ 4.5°); 6) Knee Valgus (FTA < 174°) and Hindfoot Varus (TCA < -2.1°). The groups are denoted in the order of the knee-hindfoot. Changes in FTA and TCA were assessed using the Wilcoxon signed-rank test.

Results

Sixty-nine patients with RA were included in this study. The mean age was 66.2 years. Varus knee in 37 patients and valgus knee in 32 patients, and the mean FTA was significantly improved both pre- and postoperatively (179.4 to 173.1, 165.9 to 173.3, <0.001, respectively). In both the Varus-Varus and Valgus-Varus groups, in which the knee deformity is considered to be compensated by the hindfoot, TCA improved significantly and was within the normal range after TKA. In the Varus-Normal and Valgus-Normal groups, TCA did not change significantly after TKA. In Varus–Varus and Valgus-Varus, where no hindfoot compensation was at work, the TCA did not improve after TKA. The Varus-Varus group also showed no improvement in TCA at Larsen 3 and above, but TCA improved at Larsen 2 and below; the same was seen in the Varus-Varus group.

Conclusion

In RA patients, hindfoot alignment improves after TKA in Varus-Valgus and Valgus-Varus positions, where the hindfoot compensatory mechanism may be active. However, improvement may not occur if the Larsen grade of the hindfoot is 3 or higher. In addition, hindfoot alignment may not improve after TKA in Varus–Varus and Valgus–Valgus, in which the hindfoot compensatory mechanism is thought to be inoperative.