MRI findings of acute anterior instability of the knee in the absence of recent trauma.

Background: Anterior knee instability is usually encountered in the context of trauma, with the clinical examination and imaging focusing on anterior cruciate ligament (ACL) disruption. Limited data exist on magnetic resonance imaging (MRI) of acute anterior knee instability in the absence of recent trauma.

Purpose: To provide the first comprehensive account of MRI findings in acute anterior knee laxity in the absence of acute trauma and to evaluate predictors of ACL integrity and pain.

Material And Methods: A total of 84 consecutive patients with non-traumatic knee instability were prospectively studied. Instability was assessed with Lachman's, pivot shift, and Lelli's tests. MRI findings were recorded, and ACL integrity was surgically confirmed in all 24 cases of MRI suggesting tear and in 21/60 cases of MRI suggesting no tear. Binary logistic regression models were used to identify predictors of ACL tears and pain, and Mann-Whitney U test served for comparisons between continuous variables. The study was approved by the institutional review board.

Results: Osteoarthritis and notch bony outgrowth (NBO) were present in 44% and 42.9% of all knees, respectively. NBO did not correlate with osteoarthritis ( = 0.606). NBO (odds ratio [OR] = 4.157;  = 0.016) and ACL grafts (OR = 9.277;  = 0.01) predisposed to non-traumatic ACL tears (torn in 28.6% of total cases). Presence of osteoarthritis was predictive of pain (OR = 17.671;  < 0.001).

Conclusion: We present a comprehensive analysis of MRI findings in clinically significant non-traumatic anterior instability, showing that NBO and ACL grafts predispose in non-traumatic ACL tears, whereas osteoarthritis is the only predictor of pain.