ABSTRACT. Six patients with a clinical diagnosis of chronic posterior cruciate ligament (PCL) rupture, based on a positive posterior drawer test, had a normal appearance of the PCL on an MRI scan. It is postulated that the PCL had been ruptured but healed in a lengthened state. 12 volunteers with no history of knee trauma underwent an MRI scan of the knee. In this control group (n512), there was a close correlation between the lateral femoral condylar width in the sagittal plane and the PCL length, with a ratio of 2:1 (95% confidence interval (CI) 5 1.817-2.095). In the clinically abnormal group (n56), the ratio was 1.49:1 (95% CI 5 1.206-1.782) (p, 0.0005). In conclusion, the ratio of the lateral femoral condylar width in the sagittal plane to the PCL length is a useful index for diagnosing PCL attenuation and lengthening in the presence of a normal morphological MR appearance. The posterior cruciate ligament (PCL) is the prime stabiliser of the knee joint and acts as a central axis about which normal rotation occurs [12]. Biomechanical studies have shown that the PCL is the primary restraint to posterior translation [3][4][5], contributing 95% restraint to tibial displacement [6].Rupture of the PCL is an uncommon injury occurring after high-energy trauma to the knee. The reported incidence of PCL rupture has varied, with PCL injuries representing between 1% and 44% of all knee ligament injuries in various studies [7][8][9][10]. Clinically, this is detected by a positive posterior drawer test or tibial sag. The accuracy of clinical examination for detecting PCL tear is reported to be 96% (sensitivity of 90% and specificity of 99%) [11]. The sensitivity and specificity of MRI in diagnosing acute PCL rupture reaches 100% [12][13][14][15][16][17][18], but is only 23% for chronic PCL injuries [19].We identified six patients with an obvious clinical PCL rupture who had a normal MRI scan. We postulate that, as the ruptured PCL heals in a lengthened state, measurement of absolute PCL length will not give information about the functional status of the PCL. An osseous parameter would give a more reproducible measurement and a ratio may be derived by comparing it to the length of the chronically ruptured PCL. This study was designed to identify an osseous parameter in the knee on the MRI scan that would correlate with PCL length.
Methods and materialsSix patients (Group 1) with a clinical Grade III PCL rupture were identified retrospectively. These patients had sustained PCL injury over a period of 5 years. The clinical findings were unequivocally positive and were recorded on cine film. The cine films obtained were merely for demonstration purposes and were not used for the purpose of publication. In addition, all patients had stress radiographs of both knees showing that the tibia subluxed posteriorly on the femur. The scans were performed with the patient in the supine position, with the knee in 10˚of flexion and 20˚of external rotation. Images were obtained in the sagittal plane with 3 mm slices. All of the scans wer...
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