Functionally aligned total knee arthroplasty: A lateral flexion laxity up to 6 mm is safe!

Abstract

PurposeLoose flexion gaps are associated with poor functional outcomes and instability in total knee arthroplasty (TKA). The effect of a trapezoidal flexion gap in a functionally aligned TKA remains unknown. The aim of this study was to investigate the effect of a larger lateral flexion gap in a robotic‐assisted (RA), functionally aligned (FA) and cruciate‐retaining (CR) TKA on clinical outcomes.MethodsData from 527 TKA in 478 patients from 2018 to 2020 were collected. All patients underwent an RA (MAKO, Stryker), FA and CR TKA. Gap measurements were collected intraoperatively. Patient‐reported outcome measures (PROMs), pain Visual analogue score (VAS) and range of motion were collected postoperatively. Patients were also asked about the ease of stair ascent and descent and kneeling on a 5‐point scale. The minimum follow‐up was 2 years. Patients were stratified into three groups based on lateral flexion laxity.ResultsAt 2 years postoperatively, the group with a looser gap (3–6 mm) had higher mean PROMs when compared with the group with a gap of 2–3 mm. There were no differences detected in any other outcomes at 2 years. A total of 70.9% of patients in the group with a 3–6 mm gap reported being able to walk down a flight of stairs 'easily', compared with 56.7% in the 2–3 mm group and 54% in the <2 mm group (p = 0.04).ConclusionThe study shows that a loose lateral flexion gap in functionally aligned CR TKA does not adversely affect outcomes in the short term.Level of EvidenceLevel III, retrospective cohort study.

Maintaining the Physiological Lateral Flexion Gap in the Kinematically Aligned TKA Does Not Compromise Clinical Outcomes at One-Year Follow-Up

“…Anatomically, the knee presents more contact in the medial compartment than in the lateral, suggesting that the centre of rotation of the knee is in the medial compartment [ 28 , 29 , 30 , 32 ]. Consequently, more significant laxity in the lateral compartment [ 9 , 13 , 34 ] would enable the physiologic posterolateral femoral roll-back [ 13 , 21 ]. Thus, it seems reasonable that some authors [ 13 , 21 , 33 ] observed better outcomes when a certain degree of asymmetry was present.…”

“…Consequently, more significant laxity in the lateral compartment [ 9 , 13 , 34 ] would enable the physiologic posterolateral femoral roll-back [ 13 , 21 ]. Thus, it seems reasonable that some authors [ 13 , 21 , 33 ] observed better outcomes when a certain degree of asymmetry was present.…”

“…Achieving perfect soft-tissue balance during TKA is difficult, so a 0 to 2 mm difference between the medial and lateral compartments is still considered symmetrical [ 11 , 15 , 21 ]. It is plausible that when restoring constitutionality, there will be inequalities in the joint gaps through the medial and lateral compartments, with a more significant lateral gap than the medial during flexion.…”

“…It is plausible that when restoring constitutionality, there will be inequalities in the joint gaps through the medial and lateral compartments, with a more significant lateral gap than the medial during flexion. Therefore, some authors have postulated that a slight asymmetry in the gap balance may be acceptable [ 10 , 11 , 13 , 17 , 21 ]. Our findings support this statement, with no significant differences between the symmetric (0–2 mm) and asymmetric (2–5 mm) flexion gaps in knee function and clinical scores.…”

“…In particular, computer- and robotic-assisted technologies enable the control of the three-dimensional component position and help to objectify the femorotibial gaps [ 9 ]. However, in this context, new alignment philosophies, such as inverse kinematic alignment or functional alignment, are still based on the principle of equal and symmetric flexion and extension gaps [ 19 , 20 , 21 ]. This contrasts with the kinematic alignment (KA) idea, which aims to align the knee prosthesis components to closely restore the patient’s constitutional and prearthritic anatomy and thus achieve more natural soft tissue tension and kinematics of the patient’s knee [ 19 , 20 ].…”