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.

BACKGROUND

The recovery of muscle strength has been minimally evaluated following total knee arthroplasty (TKA) and medial unicompartmental knee arthroplasty (UKA), which may be important for guiding rehabilitation. This study aimed to evaluate recovery of extension and flexion leg strengths and patient-reported outcomes (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 determined using isokinetic dynamometry for the operated and unaffected knee, and PROMs (Knee injury and Osteoarthritis Outcome Score-Physical Function Short Form [KOOS-PS], Oxford Knee Score [OKS], Forgotten Joint Score [FJS], and Numeric Rating Scale [NRS]) were collected at preoperative baseline and at six and 12 months after surgery.

RESULTS

The greatest improvement in maximum extension strength of the operated knee occurred between six and 12 months postoperatively (TKA change from baseline to six months 2.6 ± 29.4, TKA change from six to 12 months 15.0 ± 17.8; UKA change from baseline to six months 7.9 ± 29.4, UKA change from six to 12 months 14.6 ± 17.1), while maximum flexion strengths mostly increased within the first six months postoperatively (TKA change from baseline to six months 12.0 ± 20.1, TKA change from six months to 12 months 9.7 ± 13.7; UKA change from baseline to six months 7.6 ± 20.8, UKA change from six months to 12 months 7.1 ± 14.5). Following both TKA and UKA, extension strengths in the operated knee remained significantly weaker than in the unaffected knee at 12-month follow up, whereas flexion strengths attained similar levels. Independent of arthroplasty type, significant improvements in PROMs were observed during the first year postoperatively, with most improvement seen within the first six months postoperatively.

CONCLUSION

The greatest improvements in extension and flexion strengths occurred at different intervals; extension strengths peaked between six and 12 months, while flexion strengths mostly improved within the first six months postoperatively. Flexion strength of the operated knee reached levels of the unaffected knee by 12 months, while extension strength took over 12 months. These findings could aid in formulating clear patient expectations regarding strength recovery and guide rehabilitation protocols following TKA and UKA.