To the best of our knowledge, there have been no reports dealing with the postoperative change in the soft tissue balance in TKA for valgus-deformed knees in the English literature. We now present our original study written in Japanese [22]. We evaluated the changes in coronal laxity after TKA in 37 valgus-deformed knees (4 osteoarthritis, 33 rheumatoid arthritis, all females). Posterior-stabilized TKA (Scorpio NRG) was used in all cases. Lateral soft tissues including the iliotibial band were properly released to adjust the soft tissue balance using a Tensor device. Postoperative alignment of the knee was 1.4 ± 1.6° varus. We measured coronal valgus or varus ligamentous laxity by stress X-ray using the same method as the previous study [18] immediately after surgery and 3, 6, and 12 months thereafter. Lateral laxity decreased significantly during these periods, and medial laxity tended to decrease during these periods (Table 15.1 , Fig. 15.2 ). Furthermore, we divided the cases into three groups according to the value of coronal laxity immediately after TKA: 3° or less, over 3° and less than 7°, and 7° or greater. The coronal laxity in the “3° or less” group had a tendency to increase over follow-up, and that in the “7° or greater” group had decreased (Figs. 15.3 and 15.4). Regardless of the size of the laxity immediately after surgery, medial and lateral laxity had converged to approximately 4° at 12 months after TKA. In the case of a valgus deformity with a narrow joint gap at the medial side, we released the soft tissues until the medial and lateral gaps were equal at least in extension. However, in the case of valgus deformity with a wide medial gap preoperatively, to equalize the gap at the medial and lateral sides, we had to release extensively at the lateral side. This extensive lateral release may result in failure with elevation of the joint line and being too loose at the lateral side in flexion. To prevent such a failure, we aimed for 1–2° of varus alignment from neutral at TKA, allowing a small amount of medial laxity.

Although flexion instability after TKA is a major reason for revision surgery [27], only a few studies reported flexion laxity objectively after TKA. Oh et al. [28] first reported flexion laxity at 90° after TKA. Patients were asked to sit on a radiolucent wooden chair, and radiographic examinations were performed with the knee joint flexed to 90°. Under varus and valgus loads with a force of 50 N, varus and valgus stress X-rays were taken. Mean lateral laxity, mean medial laxity, and total laxity were 4.7° ± 2.4°, 4.1 ± 2.1°, and 8.8° ± 3.5° in 61 TKAs at a mean 26.1 months after surgery. Yoshihara et al. [29] measured varus and valgus laxities at 90° under a 1.5-kg external force in 49 TKAs at a minimum of 10 years after surgery. They reported laxity of 6° ± 4° (0–20°) in varus, 4° ± 3° (0–9°) in valgus, and 10° ± 5° (0–21°) in total laxity. To the best of our knowledge, there have been no reports about the postoperative change in flexion laxity in TKA. If flexion laxity during TKA was lax laterally and tight medially, as has been reported [20, 30], flexion laxity would be spontaneously adjusted in the postoperative period, similar to the phenomenon found in extension [18]. Further studies are needed to clarify the postoperative change in flexion after TKA.