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Anterior Cruciate Ligament (ACL) - Structure and Biomechanical Properties

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Introduction

The Anterior Cruciate Ligament (ACL) is a key structure in the knee joint, as it resists anterior tibial translation and rotational loads.[1] It is one of the most frequently injured structures during high-impact or sporting activities.[2] Reconstruction of the ACL aims at restoring the kinematics and stability of the injured knee, to prevent future degenerative changes.[3][4] Therefore, an adequate understanding of the complex anatomy, function, and biomechanics of the ACL is critical to understand the mechanisms of injury, the fate of chronic ACL deficiency and to improve surgical reconstruction.

Description of the ACL

Broadly, the ACL is a structure that connects the femur and tibia. It originates on the posterior aspect of the medial surface of the lateral femoral condyle. It inserts anterolateral to the anterior tibial spine, blending with the anterior horn of the lateral meniscus. The ligament has its own synovial membrane.[5] There seems to be not just one "true" morphological description of the ACL and the research notes a large variability in the morphological structure of the ACL itself and its attachment areas.[6]

Insertion to the lateral femoral condyle

Femoral Footprint

The ACL is a band-like structure of dense connective tissue. It's proximally attached to the posterior-superior aspect of the medial facet of the lateral femoral condyle.[7][8][9] The shape of the ACL's femoral footprint has been described as oval or elliptic[10], however the more accurate description is that it is shaped in the form of a "segment of a circle", with its anterior border straight and its posterior border convex.[11] Some research shows that the ACL femoral footprint includes fan-shaped fibres extending to the articular cartilage of the femoral condyle.[12] The femoral footprint varies among individuals and for this reason, individualised ACL - ACL-reconstruction is important.[6] From its femoral attachment, the ACL runs anteriorly, medially, and distally to the tibia with its length ranging between 27 mm and 38 mm[6] and its width between 10 mm to 12 mm.[6] The anteromedial bundle's width is between 6 to 7 mm and the posterolateral bundle's width is between 5 to 6 mm.[6] The cross-sectional area of the femoral insertion site ranges from 60 to 130 mm², with approximately 50% of the footprint devoted to each bundle.[6]

Tibial Footprint

The three most common tibial footprint shapes

The ACL is attached to a fossa in front of and lateral to the anterior tibial spine. At this attachment, the ACL passes beneath the transverse meniscal ligament, and a few fascicles of the ACL may blend with the anterior horn of the lateral meniscus.[13] The cross-sectional area of the tibial footprint ranges from approximately 100 to 160 mm².[6] The mean width of the tibial footprint is approximately 12.6 mm and has a mean thickness of 3.3 mm.[14] The tibial footprint can be divided into three main shape classifications, elliptical, triangular and C-shaped, with elliptical being the most common shape.[15] The shape of the insertion site is a consideration in individualised ACL reconstruction.[6]

Bundle Structure of the ACL

The ACL is commonly described as a double-bundle structure with the two bundles classified as the anteromedial and posterolateral bundles. There is a tissue sheath between the two bundles and different insertion sites for the two bundles.[16] These two bundles are named for the location of their insertion on the tibia. There is also research that describes the ACL as a triple-bundle structure and classifying the bundles as the anteromedial, intermediate and posterolateral bundles[16], however, there are also studies unable to confirm the presence of the intermediate bundle in humans.[17]

From its femoral attachment the ACL courses anteriorly, medially, and distally across the joint towards the tibia. It follows a slight lateral spiral due to the orientation of its bony attachments. The orientation of the femoral attachment of the ACL, with regard to the joint position (flexion/extension), is also responsible for the relative tension of the ligament throughout the range of motion.[9] The attachment of ACL is a collection of individual fascicles that fan out over a broad, flattened area.[8] This continuum of fibres of the ACL varies in tension during knee movements and each bundle plays a role. The fibres of the anteromedial bundle mainly resist anterior translation of the tibia, while the fibres of the posterolateral bundle mainly resist internal rotation of the tibia.[7] The angle of knee flexion also influences the contribution made by the different fibres, with the anteromedial bundle fibres providing stability and resisting load with the knee in flexion and the posterolateral fibre bundles providing stability near extension.[7]

Anteromedial bundle (AMB) Posterolateral bundle (PLB)
Attachment originating at the proximal aspect of the femoral attachment and inserting at the anteromedial aspect of the tibial attachment originating at the distal aspect of the femoral attachment and inserting at the posterolateral aspect of the tibial attachment
Orientation In the frontal plane, the AMB has a more vertical orientation (approximately 70° to the knee) PLB is oriented more horizontally (approximately 55° to the knee)[9]
AMB is moderately lax at the extended knee and tight at flexion. PLB is tight at the extended knee and lax at flexion[8].
Functional at flexion Functional at extension

Internal rotation lengthens the ACL a little more than does external rotation, most noticeably at 30° of flexion. Furthermore, the ACL acts as a secondary restraint to varus-valgus angulation at full extension.[18] Twisting is resisted by a combination of capsular shearing, slanting collateral ligament action, joint surface, and meniscal geometry, while the cruciate play only a secondary role.[1]

While the two group designation provides a general idea as to the dynamics of the ACL through the range of motion, it oversimplifies it somewhat. The ACL is composed of a continuum of fibres, each varying in tension during knee movement.[7][19] This is of great clinical importance because, in any position of the knee, a portion of the ACL remains under tension and functional.

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[20]

Biomechanics

Control of Anterior Tibial Translation

The two bundles of the ACL have a dynamic nature and play a complex role in stabilising the knee joint. It is widely accepted that the ACL is the primary restraint to anterior tibial translation, regardless of the knee flexion angle. Initially, biomechanical studies of the ACL mainly considered the function of restraining anterior tibial translation and from these studies, it is clear that the forces on the ACL vary during different ranges of knee flexion. The highest in situ forces are experienced between 0 and 30 degrees of flexion, and the maximum force is at 15 degrees of flexion. The in situ forces are at their lowest between 60 and 90 degrees, with the minimum force at 90 degrees.[21]

The ACL "provides 70 to 87% of the resistance to an anterior tibial shear force applied to the knee at small flexion angles (0 - 30 degrees), and somewhat less of this resistance (62 - 85%) at larger flexion angles (60 -90 degrees)."[7]

The two bundles also have an individual role as part of the ACL in the restraint of anterior tibial translation. During knee flexion, the anteromedial bundle experiences relatively constant levels of in situ force, whereas the posterolateral bundle experiences high in situ forces between 0 and 15 degrees of knee flexion, and the forces quickly decrease as the knee is further flexed.[21][22]

Rotational Stability

The ACL lengthens more with internal rotation of the tibia than with external rotation. Studies focusing on the rotational stability quality of the ACL showed that both bundles contribute to the rotational stability of the knee. Cadaveric studies showed that when a valgus force and internal tibial torque were applied to the ACL, the in situ forces were 30N at 15 degrees of knee flexion and 35N at 30 degrees of knee flexion in the anteromedial bundle.[23] The posterolateral bundle showed greater variation in changes with knee flexion, with in situ forces of 21N at 15 degrees and 14N at 30 degrees. Both bundles help with the rotational stability of the knee, but in varying degrees.[22]

Research investigating the ACL dynamically with MRI has shown that there are several changes that happen in a normal ACL with weight-bearing activities.[24] As the flexion angle of the knee increases during weight bearing, the axial rotation (twisting) of the ACL bundles also increases. The ACL is internally twisted by 10 degrees when the knee is in full extension, and at 90 degrees knee flexion, the ACL is twisted by 40 degrees. The lateral angulation of the ligament also increases as knee flexion increases, showing that the ligament may have a lateral force component to help with restraining internal rotation of the tibia.[24][22]

Kinematic studies of the role of the ACL during functional activities such as walking, running and cutting showed the following:

  • an ACL-deficient knee is positioned differently than a knee with an intact ACL during the phases of walking. The ACL-deficient knee had an increased internal rotation during the loading phase of gait compared to an intact ACL.[25]
  • In patients with an ACL-deficient knee, normal kinematics were shown during walking, but with running and cutting exercises, abnormal anterior tibial translation and rotational movements were noted compared to participants with an intact ACL.[26]

The ACL is important for rotational stability and both bundles play an important role in providing stability by restraining anterior tibial translation and internal tibial rotation.[22]

Biomechanical Strength Properties

The ACL is a viscoelastic structure which undergoes creep and relaxation.[27] This leads to laxity under sustained loading, with stiffness recovering to its original state after a rest period.[27] The failure load or ultimate tensile force of the ACL varies between 600 and 2300 Newtons (N).[28] Studies on older cadaveric specimens have reported the maximum failure load as 496 ± 85 N.[28] This shows that ageing affects ACL strength properties. With walking, the ACL experiences in situ forces of 169N, climbing stairs below 100N and with descending stairs, the in situ forces are 445N. Thus, reconstructive graft failure loads should be higher than this for a suitable and functional ACL replacement. [28]

A systematic review of the biomechanical properties of the most commonly used grafts (bone-patellar tendon-bone graft, hamstring, and quadriceps) found that all grafts exhibit superior biomechanical properties compared to the native ACL. These graft types demonstrate similar biomechanical characteristics and represent viable options for ACL reconstruction when matched to the right patient characteristics.[27]

References

  1. ↑ 1.0 1.1 Matsumoto H, Suda Y, Otani T, Niki Y, Seedhom BB, Fujikawa K. Roles of the anterior cruciate ligament and the medial collateral ligament in preventing valgus instability. Journal of orthopaedic science. 2001 Jan 1;6(1):28-32.
  2. ↑ Van den Bogert AJ, McLean SG. ACL injuries: do we know the mechanisms?. Journal of Orthopaedic and Sports Physical Therapy. 2007;37(2):A8.
  3. ↑ Freedman KB, D'Amato MJ, Nedeff DD, Kaz A, Bach BR. Arthroscopic anterior cruciate ligament reconstruction: a metaanalysis comparing patellar tendon and hamstring tendon autografts. The American journal of sports medicine. 2003 Jan;31(1):2-11.
  4. ↑ Lohmander LS, Östenberg A, Englund M, Roos H. High prevalence of knee osteoarthritis, pain, and functional limitations in female soccer players twelve years after anterior cruciate ligament injury. Arthritis & Rheumatism: Official Journal of the American College of Rheumatology. 2004 Oct;50(10):3145-52.
  5. ↑ 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.
  6. ↑ 6.0 6.1 6.2 6.3 6.4 6.5 6.6 6.7 Musahl V, Nazzal EM, Lucidi GA, Serrano R, Hughes JD, Margheritini F, Zaffagnini S, Fu FH, Karlsson J. Current trends in the anterior cruciate ligament part 1: biology and biomechanics. Knee surgery, sports traumatology, arthroscopy. 2022 Jan;30(1):20-33.
  7. ↑ 7.0 7.1 7.2 7.3 7.4 Beaulieu ML, Ashton-Miller JA, Wojtys EM. Loading mechanisms of the anterior cruciate ligament. Sports biomechanics. 2023 Jan 2;22(1):1-29.
  8. ↑ 8.0 8.1 8.2 Girgis FG, Marshall JL, Monajem AR. The cruciate ligaments of the knee joint. Anatomical, functional and experimental analysis. Clinical orthopaedics and related research. 1975 Jan 1(106):216-31.
  9. ↑ 9.0 9.1 9.2 Zantop T, Petersen W, Sekiya JK, Musahl V, Fu FH. Anterior cruciate ligament anatomy and function relating to anatomical reconstruction. Knee surgery, sports traumatology, arthroscopy. 2006 Oct;14(10):982-92.
  10. ↑ Lalwani R, Srivastava R, Kotgirwar S, Athavale SA. New insights in anterior cruciate ligament morphology: implications for anterior cruciate ligament reconstruction surgeries. Anatomy & Cell Biology. 2020 Dec 31;53(4):398-404.
  11. ↑ Ferretti M, Ekdahl M, Shen W, Fu FH. Osseous landmarks of the femoral attachment of the anterior cruciate ligament: an anatomic study. Arthroscopy: The Journal of Arthroscopic & Related Surgery. 2007 Nov 1;23(11):1218-25.
  12. ↑ Iriuchishima T, Ryu K, Aizawa S, Fu FH. The difference in centre position in the ACL femoral footprint inclusive and exclusive of the fan-like extension fibres. Knee Surgery, Sports Traumatology, Arthroscopy. 2016 Jan;24(1):254-9.
  13. ↑ Soames RW. Anatomy and Human Movement: Structure and function. Elsevier Health Sciences; 2024 April 16.
  14. ↑ Śmigielski R, Zdanowicz U, Drwięga M, Ciszek B, Williams A. The anatomy of the anterior cruciate ligament and its relevance to the technique of reconstruction. The bone & joint journal. 2016 Aug 1;98(8):1020-6.
  15. ↑ Guenther D, Irarrázaval S, Nishizawa Y, Vernacchia C, Thorhauer E, Musahl V, Irrgang JJ, Fu FH. Variation in the shape of the tibial insertion site of the anterior cruciate ligament: classification is required. Knee Surgery, Sports Traumatology, Arthroscopy. 2017 Aug;25(8):2428-32.
  16. ↑ 16.0 16.1 Cone SG, Howe D, Fisher MB. Size and shape of the human anterior cruciate ligament and the impact of sex and skeletal growth: a systematic review. JBJS reviews. 2019 Jun 1;7(6):e8.
  17. ↑ Schillhammer CK, Reid III JB, Rister J, Jani SS, Marvil SC, Chen AW, Anderson CG, D'Agostino S, Lubowitz JH. Arthroscopy up to date: anterior cruciate ligament anatomy. Arthroscopy: The Journal of Arthroscopic & Related Surgery. 2016 Jan 1;32(1):209-12.
  18. ↑ Markolf KL, Mensch JS, Amstutz HC. Stiffness and laxity of the knee--the contributions of the supporting structures. A quantitative in vitro study. JBJS. 1976 Jul 1;58(5):583-94.
  19. ↑ WELSH RP. Knee joint structure and function. Clinical Orthopaedics and Related Research (1976-2007). 1980 Mar 1;147:7-14.
  20. ↑ ReadySetMed. What is the ACL and How Does it Work? - ACL Series . Available from: https://www.youtube.com/watch?v=-3ZYNV0Py5E [last accessed 12/3/2022]
  21. ↑ 21.0 21.1 Sakane M, Fox RJ, Glen SL, Livesay A, Li G, Fu FH. In situ forces in the anterior cruciate ligament and its bundles in response to anterior tibial loads. Journal of Orthopaedic Research. 1997 Mar;15(2):285-93.
  22. ↑ 22.0 22.1 22.2 22.3 Prodromos C. The Anterior Cruciate Ligament: Reconstruction and Basic Science E-Book: Reconstruction and Basic Science. Elsevier Health Sciences; 2017 May 31.
  23. ↑ Gabriel MT, Wong EK, Woo SL, Yagi M, Debski RE. Distribution of in situ forces in the anterior cruciate ligament in response to rotatory loads. Journal of orthopaedic research. 2004 Jan;22(1):85-9.
  24. ↑ 24.0 24.1 Li G, DeFrate LE, Rubash HE, Gill TJ. In vivo kinematics of the ACL during weight‐bearing knee flexion. Journal of orthopaedic research. 2005 Mar;23(2):340-4.
  25. ↑ Andriacchi TP, Dyrby CO. Interactions between kinematics and loading during walking for the normal and ACL deficient knee. Journal of biomechanics. 2005 Feb 1;38(2):293-8.
  26. ↑ Waite J.C., Beard D.J., Dodd C.A., Murray D.W., Gill H.S.: In vivo kinematics of the ACL-deficient limb during running and cutting . Knee Surg Sports Traumatol Arthrosc 2005; 13 (5): pp. 377-384.
  27. ↑ 27.0 27.1 27.2 Malige A, Baghdadi S, Hast MW, Schmidt EC, Shea KG, Ganley TJ. Biomechanical properties of common graft choices for anterior cruciate ligament reconstruction: a systematic review. Clinical Biomechanics. 2022 May 1;95:105636.
  28. ↑ 28.0 28.1 28.2 Marieswaran M, Jain I, Garg B, Sharma V, Kalyanasundaram D. A review on biomechanics of anterior cruciate ligament and materials for reconstruction. Applied bionics and biomechanics. 2018 May 13;2018.


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