Insights Into Extension and Flexion Strength Recovery Following Posterior-Stabilized Total Knee Arthroplasty and Fixed-Bearing Unicompartmental Knee Arthroplasty: A 369-Case Prospective Cohort Study

Abstract

Background: Recovery of muscle strength following total knee arthroplasty (TKA) and medial unicompartmental knee arthroplasty (UKA) has been scarcely evaluated, yet it remains crucial for guiding rehabilitation. This study evaluated recovery of extension and flexion leg strength and patient-reported outcome measures (PROMs) following TKA and medial UKA.

Methods: A prospective study was performed among 369 patients (mean age 68 years [range, 51 to 81], 55.3% women) undergoing a posterior-stabilized TKA or fixed-bearing medial UKA. Extension and flexion maximum strength (60°/second) and endurance (180°/second) were measured using isokinetic dynamometry on the operated and unaffected knees, and PROMs (Knee injury and Osteoarthritis Outcome Score-Physical Function Short Form, Oxford knee score, forgotten joint score, and numeric rating scale) were collected preoperatively and at six and 12 months post-surgery.

Results: Maximum extension strength of the operated knee improved most notably between six and 12 months postoperatively (TKA: baseline to six months 2.6 ± 29.4; six to 12 months 15.0 ± 17.8; UKA: baseline to six months 7.9 ± 29.4; six to 12 months 14.6 ± 17.1). Conversely, maximum flexion strength mostly increased within the first six months (TKA: baseline to six months 12.0 ± 20.1; six months to 12 months 9.7 ± 13.7; UKA: baseline to six months 7.6 ± 20.8; six months to 12 months 7.1 ± 14.5). At 12 months, operated knees demonstrated persistent deficits in extension strength compared to unaffected knees, while flexion strength had equalized. Regardless of arthroplasty type, significant improvements in PROMs were observed, mainly within the first six months postoperatively.

Conclusions: Extension and flexion strengths recovered at different intervals; flexion strength improved rapidly within six months, reaching parity with the unaffected knee by 12 months, whereas extension strength improves more gradually and remained lower after one year. These findings support targeted rehabilitation strategies and formulating clear patient expectation for strength recovery.

Keywords: knee osteoarthritis; medial unicompartmental knee arthroplasty; muscle; quadriceps; strength; total knee arthroplasty.