Anterior laxity of the knee assessed with gravity stress radiograph.

Objective: To clarify the advantage of prone position over supine position in radiographically-demonstrating anterior knee laxity measurement for anterior cruciate ligament (ACL) injury, and to optimize the radiographic technique for the ACL-deficient knees in a clinical setting.

Materials And Methods: Thirty-nine patients with unilateral ACL injury had consented to participate in this study. They were divided into two groups and subjected to the different radiographic evaluations: study 1 (20 patients); supine versus prone position with knee full-extended, and study 2 (19 patients); comparison of (1) prone position with knee full-extended (FPV), (2) prone position with knee flexed at 15° (AGV), and (3) supine position with calf put on a board at 15° of knee flexion (SGV). Lateral radiographs for both knees were taken and were measured the side-to-side difference of tibial position related to femur.

Results: In study 1, the side-to-side difference was 2.8 ± 1.0 mm in supine position and 4.3 ± 2.1 mm in prone position, showing a statistically significant difference. In study 2, the side-to-side difference was 3.7 ± 2.4 mm in FPV, 4.6 ± 2.0 mm in AGV, and 4.2 ± 2.8 mm in SGV, while the difference in the latter two positions was larger than that in FPV.

Conclusions: The anterior laxity in prone position is larger than that in supine position for ACL injury. Moreover, the gravity-assisted lateral radiograph in prone position with knee flexed at 15° could be one of the preferable radiographic techniques and could provide more information than the simple radiograph.