Functional alignment achieves a more balanced total knee arthroplasty than either mechanical alignment or kinematic alignment prior to soft tissue releases

Abstract

Purpose: Total knee arthroplasty with functional alignment uses pre-resection balancing to determine component position within the soft tissue envelope to achieve balance and restoration of native joint obliquity. The purpose of this study was to assess the balance achievable with a mechanical axis alignment and kinematic axis alignment plan, and the subsequent balance achievable after adjustment of the component position to functional alignment.

Methods: A prospective cohort of 300 knees undergoing cruciate retaining total knee arthroplasty were included in this study. Of these, 130 were initially planned with mechanical alignment (MA) and 170 with kinematic alignment (KA). Maximal stressed virtual gaps were collected using an optical tracking software system. The gaps were measured medially and laterally in flexion and extension. Following assessment of balance, implant position was adjusted to balance the soft tissues in functional alignment (FA) and the maximal gaps reassessed. Gaps were considered to be balanced when within 2 mm of equality. Incidence of balance within each cohort was compared to independent samples proportions test.

Results: Functional alignment obtained significantly better balance in extension, medially and overall than both MA and KA alignment without soft tissue release (p < 0.001). Overall balance was observed in 97% of FA knees, 73% of KA knees and in 55% of MA knees. The difference between KA and MA was also significant (p = 0.002). Whilst there was no difference observed in balanced achieved or limb alignment when FA was planned with either MA or KA, the joint line obliquity was maintained with an initial KA plan.

Conclusion: Functional alignment more consistently achieves a balanced total knee arthroplasty than either mechanical alignment or kinematic alignment prior to undertaking soft tissue release. Utilising an individualised KA plan allows FA to best achieve the stated goals of maintaining joint line plane and obliquity.

Level of evidence: Level III: retrospective cohort study.

Keywords: Balance; Functional alignment; Kinematic alignment; Mechanical alignment; Robotic assisted total knee arthroplasty; Total knee arthroplasty.

Fig. 1

Scatter plot demonstrating the balance of knee with different alignment plans. The x-axis shows extension balance [difference between medial extension gap and lateral extension gap measured at 10° of flexion (mm)] and the y-axis shows medial balance [difference between medial extension gap and medial flexion gap (mm)]. The red box indicates accepted 2 mm of balance. a Mechanical alignment plan (MA). b Kinematic alignment plan (KA). c Functional alignment with mechanical starting point plan [FA(m)]. d Functional alignment with kinematic starting point plan [FA(k)]