Radiographic location of the lateral intercondylar ridge - Its relationship to Blumensaat's line

Background: The lateral intercondylar ridge (resident's ridge) is considered to be an important landmark during anterior cruciate ligament reconstruction. Presently, no study exists describing the location of this vital landmark on plain radiographic images. Hypothesis: Lateral intercondylar ridge location can be estimated on lateral plain film images. Study Design: Descriptive laboratory study. Methods: Lateral radiographic images were taken of 20 distal femora with metallic markers overlying the lateral intercondylar ridge. The length of Blumensaat's line and the distance from the anterior extent of Blumensaat's line to the point where the lateral intercondylar ridge intersects Blumensaat's line were measured. The ratio of these measurements was then determined (Blumensaat's-ridge ratio). The angle of the lateral intercondylar ridge with respect to Blumensaat's line (Blumensaat's-ridge angle) was also determined. Results: The mean length of Blumensaat's line was 32.1 mm (95% confidence interval, 31.0-33.2 mm). The mean distance from the anterior extent of Blumensaat's line to the point where the lateral intercondylar ridge intersects Blumensaat's line was 25.3 mm (95% confidence interval, 24.3-26.3 mm). The mean Blumensaat's-ridge ratio was 0.79 (95% confidence interval, 0.77-0.81). The mean Blumensaat's-ridge angle was 75.5 degrees (95% confidence interval, 72.0 degrees-79.1 degrees). Conclusion: The lateral intercondylar ridge intersects Blumensaat's line at a point defined by multiplying the Blumensaat's line length by 0.79. From this point, the ridge runs at a 75.5 degrees angle with respect to Blumensaat's line. Clinical Relevance: Awareness of the radiographic location of the lateral intercondylar ridge may help confirm accurate tunnel placement when arthroscopic positioning is in doubt. Furthermore, femoral tunnel position can be quickly and accurately assessed in the outpatient setting in patients with unsatisfactory outcomes after anterior cruciate ligament reconstruction.