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High ACL fibers in femoral footprint bear more load vs low fibers
High fibers carried 83.9% of the total ACL load with an anterior tibial force at 30° of flexion.
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A proximal location in the ACL footprint in the region of high fibers remains the most isometric through passive flexion while bearing the most load during stability examinations, according to a study.
“The high ACL fibers within the femoral footprint [carry] more load during stability testing, specifically the Lachman and Pivot Shift tests, than fibers located in the low regions of the femoral footprint,” Danyal H. Nawabi, MD, FRCS, assistant professor of orthopedic surgery in the Department of Sports Medicine at the Hospital for Special Surgery, toldOrthopedics Today. “Second, when you place a virtual ACL graft in the higher location, it appears to be more isometric through range-of-motion than the lower location.”
High vs low ACL fibers
Nawabi and his colleagues applied a 134-N anterior force at 30° and 90° of flexion and combined valgus and internal rotation torques at 15° of flexion to 10 fresh-frozen cadaveric knees, which simulated tests of anterior and rotatory stability. By detaching either the higher band of fibers neighboring the lateral intercondylar ridge in the direct (high) insertion or the posterior, crescent-shaped fibers in the indirect (low) insertion, researchers replayed the kinematics of the ACL-intact knee and measured the reduction in load due to each sectioned portion of ACL fibers. Researchers also assessed isometry at the anteromedial, center and posterolateral bundle locations by placing virtual ACL grafts.
Results showed high fibers carried 83.9% of the total anterior ACL load with an anterior tibial force at 30° of flexion. Similarly, at 90° of flexion, researchers found high fibers carried more load than low fibers (95.2% vs. 4.8%). The high fibers carried 84.2% of the total ACL load under combined torques at 15° of flexion compared with the low fibers, which carried 15.8%. Researchers noted, with a maximum length change of approximately 3.9, the most isometric fibers were at the anteromedial bundle location in the high region of the femoral footprint.
“These findings suggest that if you place an ACL graft eccentrically in the direct insertion, higher in the footprint adjacent to the lateral intercondylar ridge and in the anteromedial bundle location, you may be reconstructing a portion of the native ACL that bears the most load during stability examinations, yet is also isometric,” Nawabi said.
Biomechanical analysis
According to Nawabi, a biomechanical analysis of actual grafts placed at the locations suggested in this study should be performed to validate their speculation of the ‘ideal’ tunnel position, as well as to confirm that a graft placed in a higher and more isometric location behaves similarly to a native ACL.
“We must be careful not to translate these findings for the native ACL directly to current ACL reconstruction techniques. What is needed is a careful comparison of these findings in this study to a cadaveric study looking at ACL grafts placed in similar locations. If the findings are similar, then we need to try the same locations in a clinical setting and see whether failure rates can be lowered by inserting a more load-bearing graft and isometric graft,” Nawabi said. – by Casey Tingle
- Reference:
- Nawabi DH, et al. Am J Sports Med. 2016;doi:10.1177/0363546516652876.
- For more information:
- Danyal H. Nawabi, MD, FRCS, can be reached at Sports Medicine and Shoulder Service, Department of Orthopedic Surgery, Hospital for Special Surgery, 535 East 70th St., New York, NY 10021; email: nawabid@hss.edu.
Disclosure: Nawabi reports no relevant financial disclosures.