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Anterior Cruciate Ligament (ACL)

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Introduction

The anterior cruciate ligament (ACL) is a band of dense connective tissue which courses from the femur to the tibia. It consists of type I (90%) and type III collagen.[1] The length of the ACL ranges from 27 to 38 mm and the width from 10 to 12 mm. The midsubstance cross-section area measures approximately 44 mm².[1] It represents an hourglass or bowtie shape. The ACL is a key structure in the knee joint, as it resists anterior tibial translation and internal rotational loads as well as valgus angulation.



Attachments

Femoral Attachment

Attachment site of the ACL on the femur
Attachment site of the ACL on the femur

The femoral attachment of the ACL is on the posterior part of the medial surface of the lateral femoral condyle.[2] The attachment is actually an interdigitation of collagen fibres and rigid bone through the transitional zone of fibrocartilage and mineralised fibrocartilage.[3]

Orientation

It runs inferiorly, medially and anteriorly from the femur to the tibia.

Tibial Attachment

The tibial attachment of the ACL is anterolateral to the anterior tibial spine and blends with the anterior horn of the lateral meniscus. Both the medial and lateral intercondylar tubercles of the tibia serve as the attachment points for the anteromedial and posterolateral bundles of the ACL.[4]

Bundles of the ACL

The ACL is typically divided into two bundles, the anteromedial (AMB) and posterolateral bundle (PLB). Some studies suggest that there are three components of the ACL, the smaller anteromedial bundle (AMB), the intermediate bundle and the larger posterolateral bundle (PLB), but further research is ongoing. [5] The AMB and the PMB are distinguished by a tissue sheath in between them and their differing insertion sites.[4]The bundles have distinct footprints on the femur and tibia and their names are derived from their attachment to the tibial plateau. Although there is controversy in the literature about the number of bundles, the focus should rather be that the "ACL is actually comprised of a continuum of fibres, each varying in tension during threedimensional (3D) knee rotations."[6]

Anteromedial bundle fascicles run from the most anterior and proximal aspect of the femoral attachment to the anteromedial area of the tibial attachment, while posterolateral bundle fascicles extend from the posterolateral aspect of the femoral attachment to the posterolateral aspect of the tibial attachment.[7]

The two bundles have different functions. The AMB mainly resists anterior translation of the tibia in knee flexion and the PLB resists rotation, hyperextension and anterior tibial translation with the knee in extension. The AMB is tight in flexion and the PLB is tight in extension.[7][8]

With the knee extended, resistance to anterior translation of the tibia, Lachmans Test, is by the bulky posterolateral bundle. With the knee flexed, resistance to anterior translation of the tibia, the Anterior Drawer Test, is by the anterior medial bundle.

Rupture of the posterolateral bundle causes an increase in hyperextension, anterior translation (extended knee), increase in external and internal rotation (knee extended), and increases in external rotation with the knee in mid-flexion; Rupture of the anteromedial bundle causes anterolateral instability with an increase in anterior translation in flexion, minimal increase in hyperextension, and minimal rotational instability.

For more detail on the ACL bundles: Anterior Cruciate Ligament (ACL) - Structure and Biomechanical Properties

[9]

Nerve Supply

The nerve supply of the ACL is from the posterior articular branches of the tibial nerve.[10][11] The nerve fibres enter the posterior joint capsule and courses along with the synovial and periligamentous vessels to the front of the infrapatellar fat pad.[10] These fibres are associated with the endoligamentous vasculature plays a vasomotor role. The mechanoreceptors in the nerve fibres are[11][12]:

  • Ruffini receptors which are sensitive to tensile forces (stretching) and are positioned near the surface of the ligament
  • Vater–Pacini receptors which are responsive to sudden movements and are located in the proximal and distal components
  • Golgi-like tension receptors are found near the femoral and tibial attachment site of the ACL and are sensitive to tensile forces near these attachment sites
  • Free-nerve endings which function as nociceptors and modulates effectors of vasoactivity and tissue remodelling

These mechanoreceptors play a key role in proprioception.[13]

Vascular Supply

The primary blood supply to the ACL comes from the middle genicular artery. Additional blood supply comes from the inferior genicular arteries and diffusion through the ACL's synovial sheath.[1]

The anteromedial bundle (AMB) and posterolateral bundle (PLB) are separated by a connective tissue septum that contains vascular stem cells. This membrane has periligamentous vessels that penetrate the ligament horisontally and connect with a longitudinal network of endoligamentous vessels that supply blood to the ACL.[14] The density of blood vessels within the ligament is not homogeneous[15] with the proximal part having a richer supply of blood than the distal part of the ligament.[16] The ACL has an avascular zone within the fibrocartilage at its anterior aspect, where it faces the anterior rim of the intercondylar fossa.[15] This combination of poor vascularisation and fibrocartilage is also found in compressed areas of gliding tendons and this may contribute to the ACL's traditionally poor healing capacity.[17]

[18]

Function

The anteromedial bundle of the ACL provides approximately 85% of the total restraining force of anterior translation. The posterolateral bundle has a primary role in maintaining medial-lateral and rotational stability.[8]

Clinical Relevance

Orthopaedic surgeons require good knowledge of ACL anatomy as the aim of surgery is to provide the knee with the necessary stability and allowing the athlete or patient to return to their activities or sport. Understanding the anatomical attachments, the bundle structure and the status of the ligament during different degrees of flexion is key to having correct surgical procedures such as graft placement and tensioning.[19]

A good understanding of ACL anatomy is key to injury prevention, diagnosis and rehabilitation. It is important to remember that the ACL functions as a continuum of fibres. During different positions of the knee, these fibres experience varying tension. The anteromedial fibres primarily control anterior tibial translation in knee flexion and the posterolateral fibres resist peak load near full knee extension and controls internal rotation of the tibia. This knowledge helps explain injury patterns such as posterolateral bundle ruptures typically occurring with pivoting movements with rotation near full knee extension and anteromedial bundle ruptures occurring during more explosive, anterior forces. Injury prevention programmes should therefore focus on rotational control in extension. Preventative and rehabilitation exercises should address knee angle-specific loading patterns.[6]

Assesment

Please see the following pages on the assessment of ACL injuries:

Treatment

Please see the following pages on the treatment of ACL injuries:

Resources

http://www.slideshare.net/slideshow/embed_code/1609713/
 Anterior Cruciate Ligament Injury

This presentation, created by Terdsak Rojsurakitti, Doctor at Managed Care, discusses anatomy, mechanism of injury, surgical options and rehabilitation of ACL tears.

References

  1. ↑ 1.0 1.1 1.2 Morales-Avalos R, Torres-González EM, Padilla-Medina JR, Monllau JC. ACL anatomy: Is there still something to learn?. Revista espanola de cirugia ortopedica y traumatologia. 2024 Jul 1;68(4):422-7.
  2. ↑ Soames RW. Anatomy and Human Movement: Structure and function. Elsevier Health Sciences; 2024 April 16.
  3. ↑ Wheeless, C,R. Wheeless' Textbook of Orthopaedics. http://www.wheelessonline.com/ortho/anatomy_of_acl Accessed 8/1/12.
  4. ↑ 4.0 4.1 Hassebrock JD, Gulbrandsen MT, Asprey WL, Makovicka JL, Chhabra A. Knee ligament anatomy and biomechanics. Sports medicine and arthroscopy review. 2020 Sep 1;28(3):80-6.
  5. ↑ MacKay JW, Whitehead H, Toms AP. Radiological evidence for the triple bundle anterior cruciate ligament. Clinical Anatomy. 2014 Oct;27(7):1097-102.
  6. ↑ 6.0 6.1 Beaulieu ML, Ashton-Miller JA, Wojtys EM. Loading mechanisms of the anterior cruciate ligament. Sports biomechanics. 2023 Jan 2;22(1):1-29.
  7. ↑ 7.0 7.1 Stone AV, Marx S, Conley CW. Management of partial tears of the anterior cruciate ligament: a review of the anatomy, diagnosis, and treatment. JAAOS-Journal of the American Academy of Orthopaedic Surgeons. 2021 Jan 15;29(2):60-70.
  8. ↑ 8.0 8.1 Evans J, Mabrouk A. Anterior cruciate ligament knee injury. InStatPearls [Internet] 2023 Nov 17. StatPearls Publishing.
  9. ↑ Non-Contact ACL Injury, Treatment and Rehabilitation Animation. The Normal Function of the Anterior Cruciate Ligament. Available from: http://www.youtube.com/watch?v=RwwxtD-xT4Y[last accessed 04/10/14]
  10. ↑ 10.0 10.1 Kennedy JC, Alexander IJ, Hayes KC. Nerve supply of the human knee and its functional importance. Am J Sports Med. 1982 Nov-Dec;10(6):329-35.
  11. ↑ 11.0 11.1 Banovetz MT, Familiari F, Kennedy NI, Russo R, Palco M, Simonetta R, DePhillipo NN, LaPrade RF. Anatomy of the anterior cruciate ligament and the common autograft specimens for anterior cruciate ligament reconstruction. Annals of Joint. 2023 Jul 3;8:28.
  12. ↑ Duthon VB, Barea C, Abrassart S, Fasel JH, Fritschy D, Ménétrey J. Anatomy of the anterior cruciate ligament. Knee surgery, sports traumatology, arthroscopy. 2006 Mar;14(3):204-13.
  13. ↑ Vitharana TN, King E, Welch N, Devitt B, Moran K. Sensorimotor Dysfunction Following Anterior Cruciate Ligament Injury (Part 1). What Is It? How Can Clinicians Assess It?. Journal of Orthopaedic & Sports Physical Therapy. 2025 Jun;55(6):390-406.
  14. ↑ Irarrázaval S, Albers M, Chao T, Fu FH. Gross, arthroscopic, and radiographic anatomies of the anterior cruciate ligament: foundations for anterior cruciate ligament surgery. Clinics in sports medicine. 2017 Jan 1;36(1):9-23.
  15. ↑ 15.0 15.1 Petersen W, Tillmann B. Structure and vascularization of the cruciate ligaments of the human knee joint. Anat Embryol (Berl). 1999 Sep;200(3):325-34.
  16. ↑ Chandra A, Agarwal A, Azam MQ. Demystifying Partial Tears of the Anterior Cruciate Ligament: A Review of Current Diagnostic and Management Strategies. Journal of Arthroscopy and Joint Surgery. 2023 Jan 1;10(1):1-9.
  17. ↑ Giori NJ, Beaupré GS, Carter DR. Cellular shape and pressure may mediate mechanical control of tissue composition in tendons. J Orthop Res. 1993 Jul;11(4):581-91.
  18. ↑ ClinicalPhysio. ACL Blood Supply. Available from: https://www.youtube.com/shorts/_BRwJ-_94U4 [last accessed 29/09/2025]
  19. ↑ Yoo H, Marappa-Ganeshan R. Anatomy, bony pelvis and lower limb, knee anterior cruciate ligament. InStatPearls [Internet] 2023 Jul 24. StatPearls Publishing.
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