Anterior Tibial Subluxation in Anterior Cruciate Ligament-Deficient Knees: Quantification Using Magnetic Resonance Imaging

Purpose: The purpose of this study was to quantify anterior tibial subluxation (ATS) at full extension in patients with anterior cruciate ligament (ACL) deficiency using magnetic resonance imaging (MRI), and to elucidate the relation of ATS to the duration of ACL deficiency and the extent of anterior instability. Type of Study: Retrospective case series study. Methods: Ninety-six patients with isolated ACL deficiency and no history of injury in the contralateral knee were studied. We used thick-sliced T1-weighted sagittal MRI to quantify ATS at full extension. To measure ATS, MRIs were loaded into a personal computer, and the contour of the unaffected knee was compared with the affected knee using Photoshop software (Adobe, San Jose, CA). Anterior instability was measured using a KT-1000 arthrometer (MedMetric, Inc, San Diego, CA). Results: The extent of ATS in 96 ACL-deficient knees was 2.1 ± 1.3 mm (mean ± SD) and the range was 0 to 4.6 mm. ATS was correlated positively with the duration of ACL deficiency and the extent of anterior instability. Conclusions: In ACL-deficient knees, tibiofemoral kinematics differ from a normal knee without external force. The extent increases with time and is positively correlated with instability. Clinical Relevance: In ACL reconstruction for patients with a long history or gross instability, when the location of tibial bone tunnel is determined, tibial reference points are more reliable than femoral reference points because the femur shifts relative to the tibia.