[Ultrasound imaging of the anterior cruciate ligament. Possibilities and limits]

Abstract

We carried out sonography of the femoral origin of the anterior cruciate ligament in the dorsal cross section with a 7.5-MHz linear scanner in 25 test subjects with healthy knee joints. A physiological difference between the sides in the size of the hypoechoic region of the origin could be defined on the basis of the measurements obtained in both knee joints. Values outside a reference range which could be evaluated from this difference were rated as pathological in terms of an anterior cruciate ligament rupture. The reliability of this method in definitively diagnosing an anterior cruciate ligament rupture was analysed statistically as compared to the golden standard of arthroscopy on 65 patients by means of the two-by-two frequency table test. The femoral origin of the cruciate ligament in 25 test subjects had a median width of 3.1 mm (range 1.7-5.8 mm) on the right and 3.4 mm (range 1.7-4.4 mm) on the left. The median difference between the sides is 0.4 mm (range 0-1.8 mm). If the 3rd to the 97th percentile is selected as the reference range, our data show a physiological difference between the sides of 0-1.5 mm. The median age of the 65 patients was 31 years (range 14-74 years). Two thirds of all patients were men. A rupture of the anterior cruciate ligament was diagnosed sonographically in 19 patients. The result was confirmed arthroscopically in 15 patients. In 4 patients, there was a false positive sonographic finding. Forty-six patients did not fulfil the sonographic criteria of a rupture of the anterior cruciate ligament. Of these, 38 showed an intact anterior cruciate ligament on arthroscopy. Eight patients had a false-negative sonographic finding. The corresponding statistical analysis showed sensitivity of 65%, specificity of 90% and precision of 79%. Accordingly, the diagnostic reliability of knee joint sonography in diagnosing a rupture of the anterior cruciate ligament appears to be unsatisfactory, since old ruptures, above all, cannot be imaged with certainty (sensitivity 65%). On the other hand, a positive sonographic finding points with relative certainty to a rupture of the anterior cruciate ligament even before the operation (precision 79%). Histological analysis of the cruciate ligament preparations obtained intraoperatively showed that in particular the fresh anterior cruciate ligament rupture can be imaged sonographically. Retrospectively, knowledge of the history is thus crucial for interpretation of the sonographic image. After acute trauma to the knee, a side difference in the femoral region of origin of the anterior cruciate ligament of more than 1.5 mm, as revealed sonographically, is a certain indication that the anterior cruciate ligament is ruptured. When there is a positive sonographic finding, ligament strain, synovial bleeding and partial ruptures of the anterior cruciate ligament as well as plica ruptures must be considered as possible diagnoses.