Clinical Outcomes in Revision Total Knee Arthroplasty for Flexion Instability at a Mean of 3.2 Years

Flexion instability is challenging to diagnose and treat yet remains an important cause of revision total knee arthroplasty (rTKA). Previous studies report modest improvements in early patient-reported outcome measures (PROMs) following revision for flexion instability compared with other etiologies. This study evaluated outcomes after rTKA for isolated flexion instability at a mean of 3.2 years of follow-up (range, one to 12).

Between 2011 and 2021, there were 987 consecutive rTKAs performed by five surgeons that were retrospectively reviewed. A total of 224 were revised for flexion instability, of which 170 were without concomitant diagnoses. Consistent clinical and radiographic diagnostic criteria for flexion instability were used, as previously described. The PROMs at the latest follow-up and the improvement from the pre-revision baseline were evaluated. Covariates, minimal clinically important difference (MCID), substantial clinical benefit (SCB), and patient-acceptable symptom state (PASS) thresholds were evaluated. Statistical significance was defined as P ≤ 0.05. The cohort was 66% women who had a mean age and body mass index (BMI) of 65 years and 33, respectively.

Post-revision improvement in all PROMs exceeded established MCIDs (P ≤ 0.001). Minimal clinically important difference, SCB, and PASS thresholds for the Knee Injury and Osteoarthritis Outcome Score for Joint Replacement were achieved in 70, 55, and 49% of cases, respectively. Furthermore, 59% of patients reported their knee "sometimes" or "always" felt normal, and 52% of patients reported being "satisfied or very satisfied" at the latest follow-up. Aseptic revision-free survivorship was 90.6% (95% confidence interval, 83 to 98) out to 11.7 years. Only 1.2% of cases required a re-revision for flexion instability.

Although frequently a challenging diagnosis, patients and surgeons can expect clinically meaningful improvement in PROMs and low re-revision rates when undergoing revision TKA for flexion instability when using consistent and established diagnostic criteria and surgical correction techniques.

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