title: On the heterogeneity of the femoral enthesis of the human ACL: microscopic anatomy and clinical implications.
abstract: BACKGROUND
  Most ruptures of the native anterior cruciate ligament (ACL) and ACL graft occur at, or near, the femoral enthesis, with the posterolateral fibers of the native ligament being especially vulnerable during pivot landings. Characterizing the anatomy of the ACL femoral enthesis may help us explain injury patterns which, in turn, could help guide injury prevention efforts. It may also lead to improved anatomic reconstruction techniques given that the goal of such techniques is to replicate the knee's normal anatomy. Hence, the aim of this study was to investigate the microscopic anatomy of the ACL femoral enthesis and determine whether regional differences exist.
  
  
  METHODS
  Fifteen human ACL femoral entheses were histochemically processed and sectioned along the longitudinal axis of the ACL at 20, 40, 60, and 80 % of the width of the enthesis. Four thick sections (100 μm) per enthesis were prepared, stained, and digitized. From these sections, regional variations in the quantity of calcified and uncalcified fibrocartilage, the angle at which the ligament originates from the bone, and the shape profile of the tidemark were quantified.
  
  
  RESULTS
  At least 33 % more calcified fibrocartilage and 143 % more uncalcified fibrocartilage were found in the antero-inferior region, which corresponds to the inferior margin of the origin of the anteromedial ACL fibers, than all other regions (Ps < 0.05). In addition, the anteromedial fibers of the ACL originated from the femur at an angle six times greater than did its posterolateral fibers (P = 0.032). Finally, average entheseal tidemark profiles correlated bilaterally (Pearson's r = 0.79; P = 0.036), the most common profile being convex with a single re-entrant.
  
  
  CONCLUSIONS
  Systematic regional differences were found in fibrocartilage quantity and collagen fiber attachment angles. The marked differences may reflect differences in the loading history of the various regions of the ACL femoral enthesis. These differences, which could affect the potential for injury, should also be considered when developing new ACL reconstruction approaches.
authors.affiliation: School of Kinesiology, University of Michigan, 1402 Washington Heights, Ann Arbor, MI, 48109, USA. mbeaulie@umich.edu.|Biomechanics Research Laboratory, Department of Mechanical Engineering, University of Michigan, 2350 Hayward Street, GG Brown Building, Ann Arbor, MI, 48109, USA. mbeaulie@umich.edu.
authors.forename: Mélanie L
authors.initials: ML
authors.lastname: Beaulieu
authors.affiliation: School of Kinesiology, University of Michigan, 1402 Washington Heights, Ann Arbor, MI, 48109, USA.|Biomechanics Research Laboratory, Department of Mechanical Engineering, University of Michigan, 2350 Hayward Street, GG Brown Building, Ann Arbor, MI, 48109, USA.
authors.forename: Grace E
authors.initials: GE
authors.lastname: Carey
authors.affiliation: Orthopaedic Research Laboratories, Department of Orthopaedic Surgery, University of Michigan, 109 Zina Pitcher Place, Ann Arbor, MI, 48109, USA.
authors.forename: Stephen H
authors.initials: SH
authors.lastname: Schlecht
authors.affiliation: Orthopaedic Research Laboratories, Department of Orthopaedic Surgery, University of Michigan, 109 Zina Pitcher Place, Ann Arbor, MI, 48109, USA.|MedSport, University of Michigan, Domino's Farms, 24 Frank Lloyd Wright Drive, Lobby A, P.O. Box 391, Ann Arbor, MI, 48106, USA.
authors.forename: Edward M
authors.initials: EM
authors.lastname: Wojtys
authors.affiliation: School of Kinesiology, University of Michigan, 1402 Washington Heights, Ann Arbor, MI, 48109, USA.|Biomechanics Research Laboratory, Department of Mechanical Engineering, University of Michigan, 2350 Hayward Street, GG Brown Building, Ann Arbor, MI, 48109, USA.
authors.forename: James A
authors.initials: JA
authors.lastname: Ashton-Miller
keywords: Anatomy; Anterior cruciate ligament; Enthesis; Femur; Fibrocartilage; Histology; Shape; Tidemark