Segmentation of the lateral femoral notch sign with MRI using a new measurement technique

Background: The goal of this present study was to precisely determine the dimension and location of the impaction fracture on the lateral femoral condyle in patients with an ACL rupture. Methods: All patients with post-injury bi-plane radiographs and MRI images after sustaining a tear to the anterior cruciate ligament were included. Lateral radiographs of the affected knee were inspected for a lateral femoral notch sign. MRIs of patients with a lateral condylopatellar sulcus ≥1.5 mm were used to segment and measure the lateral condylopatellar sulcus. The MRI examination was interpreted by an expert in musculoskeletal radiology. The study was approved by the ethics committee of the state of Salzburg. Results: A "lateral femoral notch sign"was seen in 50 patients. The average total surface area of the lateral femoral condyle was 3271.7 mm2 (SD 739.5 mm2). The defect had a mean surface area of 266.1 mm2 (SD 125.5 mm2), a mean volume of 456.5 mm3 (SD 278.5 mm3), a mean depth of 3.0 mm (SD 0.8 mm). On average 169 mm2 (SD 99.6 mm2) of the surface of the condyle were affected by the impaction fracture which corresponds to 5.2 % (SD 2.8 %) of the surface of the lateral femoral condyle. In 51 % the impaction fracture was located in the central-external area of the femoral condyle. Conclusions: In cases of a clinically suspected ACL rupture lateral radiographs of the knee should be checked for a lateral femoral notch sign further MRI for confirmation should be performed. Knowing of the precise defect on the lateral femoral condyle is an additionally valuable information, as concomitant injuries to a rupture of the anterior cruciate ligament increase the risk for early-onset osteoarthritis in the future.