Direct versus indirect ACL femoral attachment fibres and their implications on ACL graft placement



To further elucidate the direct and indirect fibre insertion morphology within the human ACL femoral attachment using scanning electron microscopy and determine where in the footprint each fibre type predominates. The hypothesis was that direct fibre attachment would be found centrally in the insertion site, while indirect fibre attachment would be found posteriorly adjacent to the posterior articular cartilage.


Ten cadaveric knees were dissected to preserve and isolate the entirety of the femoral insertion of the ACL. Specimens were then prepared and evaluated with scanning electron microscopy to determine insertional fibre morphology and location.


The entirety of the fan-like projection of the ACL attachment site lay posterior to the lateral intercondylar ridge. In all specimens, a four-phase architecture, consistent with previous descriptions of direct fibres, was found in the centre of the femoral attachment site. The posterior margin of the ACL attachment attached directly adjacent to the posterior articular cartilage with some fibres coursing into it. The posterior portion of the ACL insertion had a two-phase insertion, consistent with previous descriptions of indirect fibres. The transition from the ligament fibres to bone had less interdigitations, and the interdigitations were significantly smaller (p < 0.001) compared to the transition in the direct fibre area. The interdigitations of the direct fibres were 387 ± 81 μm (range 282–515 μm) wide, while the interdigitations of indirect fibres measured 228 ± 75 μm (range 89–331 μm).


The centre of the ACL femoral attachment consisted of a direct fibre structure, while the posterior portion had an indirect fibre structure. These results support previous animal studies reporting that the centre of the ACL femoral insertion was comprised of the strongest reported fibre type. Clinically, the femoral ACL reconstruction tunnel should be oriented to cover the entirety of the central direct ACL fibres and may need to be customized based on graft type and the fixation device used during surgery.

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This work was sponsored by Steadman Philippon Research Institute, Vail, Colorado.

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Correspondence to Robert F. LaPrade.

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Investigation performed at the Department of Biomedical Engineering, Steadman Philippon Research Institute, Vail, Colorado, and the Soft Tissue Mechanics Laboratory, Colorado State University, Fort Collins, Colorado.

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Moulton, S.G., Steineman, B.D., Haut Donahue, T.L. et al. Direct versus indirect ACL femoral attachment fibres and their implications on ACL graft placement. Knee Surg Sports Traumatol Arthrosc 25, 165–171 (2017).

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  • Anterior cruciate ligament
  • Scanning electron microscopy
  • Lateral intercondylar ridge
  • Direct fibres
  • Indirect fibres