The Femoral Insertion of the Anterior Cruciate Ligament: Discrepancy Between Macroscopic and Histological Observations

Purpose: To observe the femoral anterior cruciate ligament (ACL) insertion macroscopically, histologically, and immunohistologically. Methods: We used 20 embalmed cadaveric knees (mean age, 69.8 +/- 5.3 years) for this study. The femoral ACL insertion was observed macroscopically, and areas were measured with digital calipers. The morphology of the ACL insertion was subsequently observed, and the areas were measured histologically and immunohistologically (stained for types I and III collagen). Finally, the macroscopic and microscopic measurements were compared. Results: Macroscopically, in 16 knees, the proximal ACL fibers spread in a fanlike manner on the medial aspect of the lateral femoral condyle and the femoral insertion was oval. The lengths of the long and short axes of the insertion were 17.7 +/- 2.7 mm and 4.6 +/- 0.7 mm, respectively. Microscopically, the insertion was located just behind the lateral intercondylar ridge (resident's ridge) and could be divided into the direct and indirect insertions. The direct insertion was 5.3 +/- 1.1 mm wide and did not continue to the posterior cartilage. The indirect insertion was located behind the direct insertion, and the posterior ACL fiber stained for type I collagen blended into the posterior cartilage on immunohistologic observations. Another bony ridge was found at the posterior margin of the direct insertion. The widths of the direct insertion were similar between microscopic and macroscopic measurements. Conclusions: The femoral ACL insertion observed macroscopically corresponded to the direct insertion observed microscopically. The posterior portion behind the lateral intercondylar posterior ridge was the indirect insertion microscopically and appeared membrane-like macroscopically. Clinical Relevance: Findings from observation of the lateral intercondylar posterior ridge during arthroscopy and consideration of the distance from the posterior cartilage border may contribute to surgeons' decisions about femoral tunnel placement during anatomic ACL reconstruction.