Mid-Flexion Instability in Total Knee Arthroplasty: A Systematic Review of Risk Factors, Diagnosis, and Strategies
Abstract
Objective: To critically evaluate the available evidence on mid-flexion instability (MFI) following total knee arthroplasty (TKA), its recognition as a distinct clinical entity, associated risk factors, diagnostic considerations, and preventive strategies. Materials and Methods: A systematic search of PubMed, Embase, Scopus, and the Cochrane Library was conducted through May 2025 in accordance with PRISMA guidelines. Clinical and biomechanical studies specifically evaluating MFI following TKA were included. Results: Thirteen studies met the inclusion criteria. The factors most consistently associated with MFI were anteroposterior translation ≥7 mm at 30° of flexion, changes in posterior condylar offset >2 mm, joint line elevation ≥4 mm, and preoperative ligamentous laxity. Medially stabilized implants provided greater mid-flexion stability than posterior-stabilized designs in the available randomized controlled trial. Surgical techniques aimed at optimizing ligament balance, particularly flexion-first balancing approaches, were associated with reduced mid-flexion laxity. However, considerable heterogeneity persists in the definition ofMFI. Conclusions: The available evidence supports MFI as a distinct clinical entity with a multifactorial etiology. Prevention requires preserving the joint line and femoral geometry, optimizing ligament balance, selecting an appropriate implant design, and systematically assessing intraoperative stability between 30° and 60° of flexion. Higher-quality studies are needed to establish standardized diagnostic criteria and determine the true incidence of MFI.Downloads
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