Image-matching technique can detect rotational and AP instabilities in chronic ACL-deficient knees

The purpose of this study was to quantify rotational and antero-posterior instabilities using biplanar image-matching technique.

Biplanar radiographs of both chronic ACL-deficient knees and contralateral uninjured knees were taken in the pivot shift test and in the "giving way position" and lateral radiographs in stress arthrometer at 30° and 90°. Three-dimensional knee models were constructed using computed tomography. Using biplanar image-matching technique, the external rotational angle and the translation of the center of the both condyles of the femur were analyzed.

The external rotation angle of geometric center axis in the pivot shift test was 16.9° ± 5.6° and 10.9° ± 7.3° (P = 0.004), and in the "giving way position" was 16.1° ± 5.7° and 10.7° ± 6.6° (P = 0.004) in ACL-deficient knees and intact knees, respectively. In the pivot shift test, the medial and the lateral femoral condylar centers of ACL-deficient knees were translated 1.2 ± 5.1 mm anteriorly and 3.9 ± 3.4 mm posteriorly, respectively, and in the "giving way position," 2.0 ± 3.7 mm anteriorly and 2.9 ± 2.6 mm posteriorly, respectively. In stress arthrometer at 30°, the medial and the lateral femoral condylar center translated 7.1 ± 6.0 and 6.6 ± 4.8 mm posteriorly (n.s.), respectively, and at 90° translated 2.7 ± 3.4 and 2.6 ± 3.5 mm posteriorly (n.s.), respectively.

Rotational instability was evaluable in the pivot shift test and in the "giving way position." Translation of both condylar centers was similar in stress arthrometry. The image-matching technique is able to quantify dynamic rotational and antero-posterior instabilities with static parameters in ACL-deficient knees.