Anterior laxity of the knee assessed with gravity stress radiograph

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.

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.

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.

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.