Clinical review and insights into lateral patellar instability in deep flexion

Background: Lateral patellar instability in deep knee flexion is a poorly understood and underreported condition that differs etiologically and biomechanically from the more common instability near extension. Aim: This paper presents a comprehensive review combined with clinical experience on lateral patellar instability in deep flexion, along with insights into the underlying anatomical and biomechanical characteristics. Methods: A systematic literature search was performed using the terms “patellar instability” and “knee flexion.” Inclusion criteria included original studies, book chapters, and reviews in English, whereas computational or biomechanical studies were excluded. In addition, clinical experience from cases was incorporated into the considerations and assessments. Results: Nine studies met the selection criteria consisting of three case reports, three case series, a book chapter, and two reviews. The analysis of clinical, anatomical, biomechanical, and kinematic factors in patients with lateral patellar instability in deep flexion did not reveal reliable arguments for the same etiological factors causing instability near extension. Instead, factors such as changes in the shapes of the lateral and medial condyle during knee flexion, variations in the shape of the lateral femoral condyle, terminal sulcus or false groove, short extensor muscles, contractures of soft tissues lateral to the patella, and laxity of the medial ligaments play significant roles in flexion instability. Conclusion: Lateral patellar instability in deep flexion is a rare but severely disabling condition that often begins at a younger age. The etiological factors leading to deep flexion instability differ from those causing patellar instability near extension, necessitating a clear distinction between these two types of patellofemoral instability. Accordingly, surgical treatment should address all documented etiological factors for flexion instability and involve a combination of procedures. Relevance for patients: Accurate differentiation between lateral patellar instability in deep flexion and instability near extension is essential for devising effective treatment strategies.