Anterior cruciate ligament (ACL) injuries are a common knee joint injury, with over 200,000 cases occurring annually in the United States alone. Diagnosis is primarily based on MRI, which cannot assess the functional stability of the joint. Ultrasonographic diagnosis of musculoskeletal disorders has been shown to be highly accurate and this has expanded its applicability to a wide range of orthopedic conditions. The purpose of this study was to evaluate the accuracy, sensitivity, and specificity of ultrasound based evaluation of ACL injuries through a biomechanical cadaveric study. We hypothesized that anterior approach by dynamic ultrasound provides high reliability and diagnostic performance in diagnosing ACL injuries.
Ten fresh-frozen knee cadaveric specimens were used and dynamic ultrasound testing was performed while simulating the Lachman test (anterior translation of the tibia with 0 and 100N, with the knee in 20 degrees flexion) and the pivot shift test (internal rotation (IR) torque with 0 and 10Nm). Ultrasound examination was performed by a fellowship-trained orthopedic surgeon, with the probe placed at the antero-medial(AM) and antero-lateral (AL) joint line. Anterior translation of the tibia relative to the femur was calculated in the AM and AL views. Knee joint instability was tested under three conditions: (1) intact ACL (2) ACL deficiency and (3) ACL and Anterolateral ligament (ALL) deficiency. Intra-observer and inter-observer reliability was calculated to evaluate the reproducibility and accuracy of the ultrasound methods in this study. Receiver operating characteristic (ROC) curve analysis was performed to calculate an ideal cutoff point (mm) for detecting instability in the anterior drawer loading in ACL deficient and ACL+ALL deficient conditions.
SECTION: In the AM view, both ACL deficient and ACL+ALL deficient conditions significantly increased the anterior translation of the tibia during anterior loading in comparison to the Intact ACL condition. Anterior translation of the tibia in AM view with anterior drawer load was 0.9±0.17 mm (Intact ACL), 4.4±2.3 mm (ACL deficiency), and 5.0±3.5 mm (ACL +ALL deficiency). There was no significant difference in the anterior translation of the tibia during IR torque test, between all conditions (p=0.17). The area under ROC curve (AUC) predicting ACL deficiency was 0.89(95% CI, 0.66-0.97) in the AM view with anterior drawer loading. Using Youden's J statistic, the optimal cut off value of anterior tibial translation to distinguish ACL deficient from intact condition during anterior drawer loading test was 2.6 mm in the AM view (sensitivity=80%, specificity=90%, accuracy=85%) and 1.6 mm in the AL view (sensitivity=80%, specificity=70%, accuracy=75%). Intra-observer reliability of both AM view (ICC=0.94; 95% confidence interval, 0.90-0.97) and AL view (ICC=0.91; 95% confidence interval, 0.84-0.95) was substantial Inter-observer reliability of AM view was substantial (ICC= 0.86; 95%CI , 0.70-0.93), but that of AL view was slight (ICC= 0.49; 95%CI , 0.14-0.71).
The most important finding of this study was that ultrasound evaluation of ACL injury and instability from the anterior aspect of the knee, is an effective and novel method with high sensitivity and specificity, accuracy, and reproducibility. Another useful finding is that this method can evaluate instability quantitatively, and can thus be used for postoperative follow-up and functional evaluation. Overall, since ultrasound equipment is readily available in the outpatient and emergency departments, its use for accurate and early diagnosis of ACL injuries is encouraged to potentially reduce the development of complications and economic burden for our patients.