Mid-Flexion Instability After Total Knee Arthroplasty: Diagnosis, Implant Design, and Outcomes

Amid the growing use of total knee arthroplasty (TKA) worldwide, instability remains one of the most common mechanisms of failure in modern prostheses, accounting for 11% to 26% of failures.^1–3 Conventionally, flexion and extension stability are routinely evaluated intraoperatively at full extension and 90º of flexion, which can potentially miss unrecognized instability at angles along the flexion-extension arc of motion. As understanding of biomechanics of ligament balancing in TKA grows, instability in mid-ranges of flexion has arisen as a possible etiology of patient dissatisfaction. Although this concept, referred to as mid-flexion instability (MFI), was first described by Martin and Whiteside⁴ in 1990, it remains a challenging and elusive diagnosis due to the ambiguous nature of its presentation and clinical findings.^5,6 Furthermore, there remains a lack of high-quality literature examining this phenomenon in vivo, and thus there is no established objective measurements of MFI, let alone diagnostic or treatment algorithm.⁷ The incidence of MFI is currently unknown, but TKA is associated with a dissatisfaction rate of up to 20%, and MFI may likely be a causative factor. Therefore, it is imperative that providers, patients, and implant manufacturers understand MFI and take measures to attenuate risk.⁸ The purpose of this review is to properly define the condition, describe diagnostic criteria and risk factors contributing to MFI, review current implant design, and present outcomes of revision surgery performed for MFI.

Definition of Mid-Flexion Instability

Much of the effort to characterize instability has been dedicated to defining laxity at full extension and 90° flexion. MFI, as opposed to other etiologies of instability (eg, global instability, flexion instability, and hyperextension instability), refers to a separate clinical entity in which the TKA is stable at standardly assessed intervals (ie, full extension and 90° of flexion), but unstable during flexion somewhere greater than 0°, but less than 90° of flexion.^6,9–15 Classically, MFI is described as laxity in the coronal plane (varus/valgus) between 30° and 60° of knee flexion; however, laxity in the sagittal (anteroposterior [AP]) and axial (rotational) planes can also occur in midflexion.⁴ Range of motion defined as “mid-flexion” following a TKA has been described variably in the literature from (1) between 30° and 45° flexion⁴; (2) “the early stage of flexion”