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.

The language of knee alignment

“…Several studies have found that FA approaches physiological soft-tissue tension with less need for soft-tissue releases. 74,76 A recent CT-based radiological study reported FA results in less translation of native trochlear groove than either MA or KA, with the least alteration in groove depth of the three. 85 Furthermore, use of robotic capabilities with haptic boundary control has been demonstrated to decrease blood loss and soft-tissue trauma and to preserve the PCL footprint.…”

“…Functional alignment can be achieved with either restricted KA or the narrower AMA boundaries. 72-74…”

“…Virtual gap balancing can be performed from either an adjusted mechanical or restricted KA start point with the aim to restore balance through the first 90° of knee flexion. 74 Regardless of whether an AMA or rKA starting plan is used, the constitutional HKA alignment is equally restored in extension with both techniques. 74,83 However, an AMA starting point requires more femoral component external rotation to balance the medial flexion-to-extension gap, and results in a more neutral JLO.…”

“…74 Regardless of whether an AMA or rKA starting plan is used, the constitutional HKA alignment is equally restored in extension with both techniques. 74,83 However, an AMA starting point requires more femoral component external rotation to balance the medial flexion-to-extension gap, and results in a more neutral JLO. 74 In contrast, a rKA starting point with FA is more likely to reduce lengthening of lateral joint line, maintain medial stability of the MCL to support any medial pivoting, and restore native JLO.…”

Overview of the different personalized total knee arthroplasty with robotic assistance, how choosing?

“…Functional alignment begins with 3D imaging from preoperative CT-scan or MRI to aid planning in all planes to determine and improve implant sizing and positioning. Granted, a mechanical or kinematic alignment start point can be chosen, FA with KA starting plan is more likely to achieve all stated goals of FA, especially with regards to restoring joint line plane and obliquity (37). FA philosophies have safe boundaries within which modifications are attempted to achieve a balanced knee.…”