Risk Factors for Mid-Flexion Instability After Total Knee Arthroplasty: A Systematic Review

Mid-flexion instability after total knee arthroplasty (TKA) is a clinical entity that is not well studied and one that has been associated with patient dissatisfaction and inferior outcomes. We sought to provide a comprehensive review of risk factors associated with mid-flexion instability.

A comprehensive literature search of PUBMED, EMBASE, Google Scholar, and Cochrane Library was performed using keywords "mid flexion," "instability," and "knee arthroplasty" in all possible combinations. All studies published from 2010 to 2020 in English were considered for inclusion. Research design, question studied, and outcomes were recorded for each study. Quantitative and qualitative analysis was performed.

Eighteen articles meeting inclusion criteria were identified and reviewed. There were 5 computational studies, 5 cadaveric studies, and 8 clinical studies. There were 14 different risk factors investigated in relation to mid-flexion instability after TKA: 6 implant-related, 6 technique-related, and 2 patient-related factors. Of these risk factors, 5 had contradictory results published to date, resulting in an inconclusive association with mid-flexion instability. The results of this review suggest that the effects of joint line elevation and radius-of-curvature of the femoral component on mid-flexion instability are inconclusive while articular surface conformity and preoperative joint laxity may play a bigger role than previously thought.

Mid-flexion instability after TKA is a clinical entity distinct from other established forms of instability. There are patient-related, implant-related, and technique-related factors associated with mid-flexion instability. The majority of the evidence on this topic is derived from computational and cadaveric studies, underscoring the need for further clinical studies.