Background

Midflexion Instability (MFI) following total knee arthroplasty (TKA) has been used to describe a phenomenon whereby the knee is stable in extension and flexion >90° but exhibits instability while in midrange during functional activities. Whilst there is a growing body of literature on MFI as a concept, the underlying mechanisms that result in MFI remain poorly understood leading to different definitions of MFI being used within literature. As such, the definition of MFI and its mechanism has not been established.

Purpose

To comprehensively synthesise the clinical definitions of MFI and describe the mechanisms underlying the phenomenon.

Methods

The US National Library of Medicine (PubMed/MEDLINE), EMBASE, Web of Science, and the Cochrane Database of Systematic Reviews were systematically searched to identify eligible publications. Publications that studied, identified, or described MFI following primary TKA were eligible for inclusion. Twenty studies fulfilled our inclusion criteria; 11 in vivo studies, 5 cadaveric studies and 4 finite element model studies. The studies were categorised according to the reported cause of MFI.

Results

Implant design; Flexion-extension gap differences; Joint line (JL) position; Posterior condylar offset (PCO); Posterior tibial slope; and Preoperative coronal plane alignment (varus-valgus) were identified as potential contributing factors for MFI. Several possible mechanisms were identified as the common route for these causes: excessive AP translation; paradoxical femoral movement; coronal plane laxity; and condylar lift-off. The accumulative results of the findings were inconclusive, possibly due to the variety of definitions used for the range of midflexion.

Conclusions

Inconsistent definitions, aetiologies and causal mechanisms of MFI appear within the literature. Whilst the amount of literature surrounding MFI is growing, minimal progress has been made to clearly define and understand the phenomanon. Before aiming to understand the etiologies and causal mechanisms, future investigations should firstly agree on the arc of motion that equates to midflexion.