The most important finding of this study was that stability at 0° modestly correlated with stability in the narrow range up to 20°. However, stability at 90° correlated with stabilities in the comparatively wide range from 20° to 80°. This suggests the importance of acquiring stability at 90° flexion to achieve midflexion stability in PS-TKA.
Accurate intraoperative soft tissue evaluation is made possible by recent technological advances, such as the tension meter and navigation system [10, 15, 16, 17, 18]. However, clear indicators of soft tissue balance remain elusive. Traditionally, the aim of TKA is to achieve equal medial and lateral gaps as well as equal flexion and extension gaps. Yet, the feasibility of this concept for all cases remains controversial. One reason is that normal knees do not always have equal medial and lateral gaps or equal flexion and extension gaps [19, 20]. Nowakowski et al. demonstrated that extension and flexion gaps are asymmetric and unequal. They also showed that ACL and posterior cruciate ligament (PCL) resections produce varying gap changes using a prototypical force determining ligament balancer without the need for bony resection. The sequential cruciate ligament resections tended to reflect varying TKA designs [21]. The cruciate ligaments, functioning as secondary stabilizers against varus–valgus torque, bear approximately one-fourth of the varus–valgus load of the collateral ligaments [22, 23]. Therefore, in PS-TKA, instability in the midflexion range is suspected to occur. Medial release is a classic method for adjusting soft tissue balance. Complete release of the medial collateral ligament (MCL) increases medial stability to 6.9° in full extension and 13.4° at 90° flexion [24]. Releasing the MCL enlarges the medial flexion gap more than the extension gaps [25]. These results illustrate the difficulty in managing extension imbalance using only medial release.
To avoid instability caused by inappropriate soft tissue balance, a widely recognized goal in PS-TKA is to create symmetric, rectangular gaps (equal medial and lateral gaps as well as equal flexion and extension gaps); this has been named the gap technique. This concept can reliably achieve a more stable knee in 0° extension and 90° flexion. However, it is unclear if this technique can achieve stability at the midflexion range. Minoda et al. reported that the center size of the joint gap was loose, especially at 30° flexion in PS-TKA, even when using the gap technique. The authors postulate that this is probably because in PS-TKA, both the ACL and PCL are sacrificed and a cam-post mechanism regulates the anterior–posterior translation. There is no stabilizer of the anterior–posterior translation in the midflexion range in which the cam-post mechanism does not engage. Incavo et al. [11] examined the relationship between the difference of alignment axes and the implication of medial–lateral soft tissue balance in TKA. Their result indicated that measurements of medial and lateral joint spacing are statistically significantly different at all flexion angles between the two alignments. The anatomic alignment axes’ pattern demonstrates midflexion lateral opening and late-flexion medial joint space opening. On the other hand, mechanical axes have a consistent 2–3 mm larger lateral space than a medial joint space. In addition to these factors, the insert thickness, changes of posterior condylar offset, and design of the prosthesis has been also reported affect midflexion stability [26, 27, 28].
The present study showed that varus–valgus stability in the initial flexion range did not strongly correlate with stability at 0° or 90°. We expected a similar correlation at the wide range in midflexion and 0° in extension positions. However, modest correlation was found with 10° and 20°. Moreover, the present study showed that the values dividing the amount of varus–valgus stability at 90° by the amount of varus–valgus stability at 0° were significantly correlated only with the amount of varus–valgus stability at 10° and 50°–80°. No significant correlation was observed with the amount of varus–valgus stability at 20°–40°. These findings indicate that it is not possible to reliably acquire stability at the initial flexion range, even when achieving a stable knee at 0° extension and 90° flexion, and even when achieving equal flexion and extension stability. This suggests the possibility that another element is involved in stabilities at the initial flexion range. In some cases, removing the ACL/PCL in PS-TKA would lead to large stability at the initial flexion range, despite the recovery of proper soft tissue balance in 0° knee extension and 90° knee flexion. Careful attention would be necessary to prevent varus–valgus instability in the initial flexion range in PS-TKA. To adjust the stability of the initial flexion range in PS-TKA, it has been suggested that precise surgical measures, further improvement in the implant design, joint line control, matching implant sizes, and other factors are needed.
One limitation of this study is that it reported on the LPS-flex model only. The measured resection technique was used for bone cutting, and surgery was performed using tourniquet vascularization and non-load-bearing conditions under general anesthesia. Additionally, only patients with varus-type arthritis were treated, and the procedure involved manual passive stress by the operator. Furthermore, stress force was not standardized. These limitations restrict the generalization of our results. Ghosh et al. [10] confirmed that computer navigation can be used to analyze soft tissue balance during TKA beyond the coronal plane and throughout range of motion, and stress test results showed reproducible patterns of soft tissue balance. The strengths of the current study include the use of a navigation system that enabled precise evaluation of varus–valgus position throughout the range of motion. Additionally, evaluating the postoperative outcome at a final state of healing with a strong suture to the capsule and skin was clinically meaningful. Future research is required to determine the degree of varus–valgus stability adequate for TKA procedures. Moreover, further study is required to improve the surgical theory and implant design to reliably achieve full range stability, and to investigate the relationship between midflexion stability and patients’ clinical outcome.
