Knee hyperextension is a sign of through-range laxity and may be effectively managed without routine joint line distalisation in a robotic functional alignment workflow

Abstract

Purpose: Traditional management of hyperextension deformity in total knee arthroplasty (TKA) involves routine under-resection of distal femoral bone to lower the joint line and close the extension gap. This study aimed to assess how a robotic functional alignment (FA) workflow with pre-resection gap assessment managed hyperextending knees to achieve balance compared to controls. Methods: A retrospective analysis of a prospective registry was performed using a single implant/robotic platform, two surgeons, between 2016 and 2023. 100 hyperextending TKA patients with hyperextension of 5° or more were compared to a control group of 1881 patients with 0°–10° of fixed flexion contracture. Results: The hyperextension group had a greater maximum flexion ROM (137° vs. 132°) compared to controls. All four mean start-of-case gaps (medial and lateral in flexion and extension) were larger for the hyperextension patients by 0.7–1.2 mm. The femoral joint line was not routinely distalised by a clinically significant amount in the hyperextension patients (0.3 mm difference in means; 0.6 mm vs. 0.3 mm. The hyperextension cohort had relative tibial under-resection (4.8 mm vs. 5.4 mm; p < 0.001) and additional insert thickness (11.2 vs. 10.3 mm; p < 0.001) resulting in an overall mean tibial joint line elevation of 1.5 mm greater than controls. At the 1-year mark, there was no difference between groups when assessing range of motion (p > 0.05) and forgotten joint score (FJS, p > 0.05). Two patients had hyperextension recurrence at 1-year review, measured at –2° and –5°. Conclusions: Hyperextending arthritic knees may not require routine distalisation of the femoral joint line to achieve balance and close the extension gap. These knees have a global laxity that can often be effectively managed with predominantly tibial sided changes. Level of Evidence: Therapeutic Level III, retrospective comparative study.