Stress radiography at 30° of knee flexion is a reliable evaluation tool for high-grade rotatory laxity in complete ACL-injured knees

Purpose To evaluate the diagnostic value of stress radiography and determine the cutoff values for high-grade anterolateral rotatory laxity in complete anterior cruciate ligament (ACL)-deficient knees at different positions. Methods Forty-two patients with complete ACL rupture (group 1) and 37 normal subjects (group 2) were prospectively enrolled. The amount of anterior translation in the medial (MM) and lateral (LL) distance compartments and the difference between them (LL-MM distance) were measured using stress radiography at 30 degrees, 45 degrees, 60 degrees, and 90 degrees positions. The area under the receiver operating characteristic curve (AUC) was assessed for the presence of a high-grade (grade > 2) pivot shift. Results The MM and LL distances in group 1 were significantly different at 30 degrees and 45 degrees positions (P < 0.05). The AUC of the MM (AUC, 0.903) and LL (AUC, 0.901) distances at the 30 degrees position was significantly higher than that of the other positions (P = 0.000); however, the cutoff values were different to diagnose ACL injury (MM vs. LL, 3.1 mm vs. 5.4 mm). A 2.1-mm cutoff for the LL-MM distance showed 78.4% sensitivity and 90.3% specificity for detecting the presence of a high-grade pivot shift (AUC = 0.905, P = 0.000). Conclusion The cutoff values of stress radiography differed according to anatomical references and knee flexion positions. Stress radiography of a 2.1 mm difference in LL-MM distance at 30 degrees of knee flexion can be a reliable method for high-grade rotatory laxity in complete ACL-injured knees.