Jump to content

Introduction to ACL Rehabilitation

Original Editor - Wanda van Niekerk

Top Contributors - Wanda van Niekerk and Jess Bell  

Introduction

The anterior cruciate ligament (ACL) is one of the knee’s most crucial stabilising structures, but is also susceptible to injury. The unique structure of the ligament prevents anterior translation of the tibia on the femur and provides resistance to tibial rotation and valgus angulation, while serving an important proprioceptive function. ACL injuries are among the most common and significant knee injuries in sports, with non-contact injury mechanisms accounting for approximately 70% of cases. Understanding the ACL’s anatomy, biomechanical properties, and injury patterns is important for effective diagnosis, treatment, and rehabilitation.

Anatomy and Function of the Anterior Cruciate Ligament (ACL)

Anatomical Structure and Attachments

The ACL is named based on its attachment to the tibia, where it inserts anterolateral to the anterior tibial spine. The ligament runs inferior to the transverse ligament and merges with the anterior horn of the lateral meniscus. From this tibial attachment, it runs posteriorly and laterally to the proximal attachment at the posterior aspect of the medial surface of the lateral femoral condyle.[1]

Between the tibia and the femur, the ligament undergoes a medial spiral of approximately 110 degrees. This twisting occurs because the attachment sites lie in different planes: the tibial attachment is oriented in the sagittal plane, while the femoral attachment is oriented in the coronal plane.[1]

[2]

Bundle Organisation and Function

The ACL can be divided into two bundles, the anteromedial bundle and the posterolateral bundle. The anteromedial bundle attaches to the anteromedial area of the tibial insertion site, while the posterolateral bundle attaches to the posterolateral area. During knee extension, the posterolateral bundle becomes tight, while the anteromedial bundle remains lax. The reverse occurs during knee flexion, with the anteromedial bundle tightening and the posterolateral bundle becoming lax. Functionally, the ACL should be viewed as a continuum, where, although different portions may tighten or loosen during knee movement, some parts always remain taut to maintain joint stability throughout the full range of motion.[1]

Primary Function

The ACL prevents anterior translation of the tibia on the femur and provides approximately 30% of the resistance to internal (medial) tibial rotation and valgus angulation of the knee.[3][4]

The ACL provides sensory information to the sensorimotor cortex, aiding in motor planning and execution.[4]

Fibre Structure and Biomechanics

The ACL mainly consists of collagen fibres and a small proportion (10%) of elastic fibres, providing the ligament with high tensile strength. The fibres of the ACL are arranged in two distinct fascicular patterns: one type runs directly between the tibial and femoral attachments, while another type spirals along the longitudinal axis of the ligament at a helical axis of 25 degrees.[1]

This spiral arrangement allows for a unique loading mechanism. When the ligament is placed under a light load, only a few fibres experience tension, but as the load increases, the ligament unwinds, recruiting additional fibres and progressively increasing the ligament’s overall strength.[1]

Loading Phases and Mechanical Properties

The ACL demonstrates two identifiable phases of loading. The first one is loading before the yield point (pre-yield loading) – the fibre deformation remains elastic as the ligament stretches and unwinds. If overloading happens (post-yield loading), the cross-links between the fibres are disrupted, causing permanent deformation.[1]

Several factors influence ACL strength. With age, the tensile strength decreases. Cyclical loading from activities, such as walking or running, can temporarily soften the ligament and decrease the yield point, though the ligament recovers within a few hours after such loading. Immobilisation decreases tensile strength over time. Internal rotation influences tensile strength and torsional forces may potentially cause more damage than other loading patterns. Conversely, regular exercise may slightly increase the ligament’s tensile strength.[1]

Bony Attachment Structure

At the bony attachments, there is a “complex interdigitation of collagen fibres from both the bone and the ligament.”[1] A transition zone of fibrocartilage between the collagen fibres at the ligament’s bony attachment prevents stress concentration in this area by allowing for gradual changes in stiffness.[1]

Proprioceptive Function

Near the femoral attachment of the ACL, some mechanoreceptors resemble Golgi tendon organs. These receptors are positioned around the periphery where maximum bending happens and are aligned parallel to the ligament’s long axis. The mechanoreceptors provide information about angular acceleration and are likely involved in reflexes that help prevent knee injury.[1]

If you'd like, you can review the anatomy of the ACL here.

Mechanism of ACL Injuries

The nature of ACL injuries varies, ranging from non-contact to indirect and direct contact injuries. Non-contact ACL injuries occur when there is no direct contact with the injured knee or lower extremity. This is the most common mechanism of ACL injury and occurs in approximately 70% of cases. Movements such as rapid deceleration moments, when the injured leg is planted to cut and change direction, as well as landing from a jump, twisting, and pivoting, are all associated with non-contact ACL injuries.[5]

Contact ACL injuries can be divided into indirect and direct injuries. Indirect injuries occur when there is contact or direct trauma to other body parts, which causes an ACL injury. In contrast, direct injuries are a result of direct contact or trauma to the knee.[6]

Risk Factors for ACL Injuries

Risk factors for non-contact ACL injuries can be intrinsic or extrinsic. Extrinsic factors are external elements influencing the athlete, such as the playing surface, footwear, weather conditions, competition versus training situations and the type of sport played. Intrinsic factors are inherent to the athlete, including hormonal influences, anatomical variations, generalised joint hypermobility, landing and pivoting biomechanics, and neuromuscular control.[7] Female athletes face a 2 to 6-fold higher risk of ACL injuries compared to male athletes.[8]

Read more about ACL risk factors here.

Extent of ACL Injuries

Normal ACL versus Injured ACL

Anterior cruciate ligament injuries are classified based on the severity of the ACL damage.[9]

Grade I – the ACL is mildly damaged but still able to stabilise the knee joint – as per MRI diagnostic criteria, there is a partial tear or damage of the ACL, with less than 50% of the ligament disrupted.[9]

Grade II – a partial tear of the ACL, where the ligament is stretched – according to MRI criteria, a Grade II injury is characterised by a partial tear with more than 50% of the ligament torn.[9]

Grade III – a complete tear, leading to an unstable knee joint.[9]

If you'd like, you can read more about the different grades of ACL injuries here.

[10]

Clinical Presentation

The patient typically reports an injury mechanism that involves deceleration/acceleration in combination with a knee valgus load, such as a cutting manoeuvre, changes in direction, jumping, landing and pivoting movements. At the time of injury, the patient may have heard or felt a “pop” sound or sensation. Following the injury, joint effusion (swelling) and/or haemarthrosis are common, as well as immediate pain when the injury occurred.[11]

Patients frequently report a feeling of their knee “giving way”[12] and experience an ongoing feeling of instability.[13] Additionally, there is typically a decreased range of motion following the injury.[14]

In cases of chronic ACL injury, patients experience periodic instability and generalised knee pain, which can impact their daily activities and sporting performance.[15]

[16]

Diagnostic Procedures

Physical Examination

The physical examination includes several key diagnostic tests to assess anterior cruciate ligament injury, including the Lachman test, anterior drawer test, pivot shift test and Lever's sign, which collectively provide a comprehensive evaluation of knee stability and ACL integrity.

[17]
[18]
[19]
[20]


Imaging

MRI is the technique of choice to detect ACL lesions.[21] Radiographs may show indirect signs of injury, such as tibial eminence avulsion, anterior tibial translation or a Segond fracture.[22]

Objective Measurement of Knee Laxity after ACL Injury

There are methods to objectively measure knee laxity after an ACL injury, but currently these methods lack standardisation and consensus. Most clinical assessments remain subjective rather than applying reliable objective measures. To improve patient care and diagnostic accuracy, clinicians, researchers, device designers and patients need to collaborate to develop standardised protocols, measurements and the use of these technologies in routine clinical practice. Some methods to measure knee laxity include[23]:

  • Arthrometry devices: can measure anterior tibial translation (ATT) relative to the femur, for example, the KT-1000 arthrometer.
  • Wearable devices, such as KiRA: use inertial measurement units (IMU,) which can assess acceleration and angular velocity during clinical tests, such as the Lachman’s or pivot shift.
  • Electromagnetic Measurement Systems (EMS): measure tibial translation or acceleration with an electromagnetic field.
  • Optical motion caption systems: can monitor changes in gait after an ACL injury.
  • Dynamic MRI: can also quantify joint laxity.

Associated Injuries

Anterior cruciate ligament injuries are often accompanied by a range of associated injuries that can impact treatment approaches, outcomes and rehabilitation. These include chondral injuries, meniscal tears and various intra-articular lesions, such as damage to secondary stabilisers like medial meniscus ramp lesions or posterior root tears of the lateral meniscus. Additionally, patients may present with medial collateral ligament injuries, lateral collateral ligament injuries and posterior oblique ligament injuries.[24]

Cartilage injuries[24] and subchondral bone bruising[25] are also often observed injuries that require careful evaluation and management.

Impaction fractures of the lateral femoral condyle, posterolateral tibia and Segond fracture may also occur as part of the ACL injury.[24]

Management of ACL Injuries

The choice of management depends on factors such as the patient's symptoms, the type of ACL injury, the patient's goals, the amount of remaining growth in the patient’s growth plates, and examination findings. Conservative management includes progressive rehabilitation, physiotherapy, patient education, and bracing, whereas surgical management involves ACL reconstruction. Different techniques can be employed using various graft types, such as bone-patellar tendon-bone autografts, quadriceps tendon autografts, hamstring tendon autografts, and allografts from tendons like the patellar tendon, Achilles tendon, semitendinosus, gracilis, or posterior tibialis tendon.[26] The choice of the graft type usually results from a combination of surgeon preference and patient factors.[27] Surgical options may also include ACL repair using newer techniques such as the Bridge-enhanced ACL repair (BEAR), although research is still limited regarding the benefits and advantages of this method over ACL reconstruction.[27]

The following pages provide more detail on non-surgical management of ACL injuries, surgical reconstruction and the rehabilitation of ACL injuries:

References

  1. ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 Soames RW. Anatomy and Human Movement: Structure and function. Elsevier Health Sciences; 2024 April 16.
  2. ↑ Jeffrey, B. Whitty, MD. Anterior Cruciate Ligament (ACL) - Anatomy and Function. Available from: https://www.youtube.com/watch?v=1J1Qc_tOq40[last accessed 09/09/2025]
  3. ↑ 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.
  4. ↑ 4.0 4.1 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.
  5. ↑ Wetters N, Weber AE, Wuerz TH, Schub DL, Mandelbaum BR. Mechanism of injury and risk factors for anterior cruciate ligament injury. Operative Techniques in Sports Medicine. 2016 Mar 1;24(1):2-6.
  6. ↑ Takahashi S, Nagano Y, Ito W, Kido Y, Okuwaki T. A retrospective study of mechanisms of anterior cruciate ligament injuries in high school basketball, handball, judo, soccer, and volleyball. Medicine. 2019 Jun 1;98(26):e16030.
  7. ↑ Apseloff NA, Hughes JD, Devitt BM, Musahl V. Primary anterior cruciate ligament injury: extrinsic and intrinsic risk factors. JAAOS-Journal of the American Academy of Orthopaedic Surgeons. 2022 May 13:10-5435.
  8. ↑ Bruder AM, Culvenor AG, King MG, Haberfield M, Roughead EA, Mastwyk J, Kemp JL, Pazzinatto MF, West TJ, Coburn SL, Cowan SM. Let’s talk about sex (and gender) after ACL injury: a systematic review and meta-analysis of self-reported activity and knee-related outcomes. British Journal of Sports Medicine. 2023 May 1;57(10):602-10.
  9. ↑ 9.0 9.1 9.2 9.3 Li K, Jhonatan FY, Yu Z, Liu J, Huang L, Yang H, Du J. A new modified MR dual precision positioning of thin-slice oblique sagittal fat suppression proton density weighted imaging: its diagnostic accuracy in anterior cruciate ligament injury. Scientific Reports. 2023 Dec;13(1):23109.
  10. ↑ El Paso Manual Physical Therapy.ACL Tear Grading System Explained To Better Understand Your MRI. Available from: https://www.youtube.com/watch?v=XwMBB_HxYjM [last accessed 6/6/2009]
  11. ↑ Filbay SR, Grindem H. Evidence-based recommendations for the management of anterior cruciate ligament (ACL) rupture. Best practice & research Clinical rheumatology. 2019 Feb 1;33(1):33-47.
  12. ↑ Tanaka S, Inoue Y, Masuda Y, Tian H, Jung H, Tanaka R. Diagnostic accuracy of physical examination tests for suspected acute anterior cruciate ligament injury: a systematic review and meta-analysis. International journal of sports physical therapy. 2022 Aug 1
  13. ↑ Rodriguez K, Soni M, Joshi PK, Patel SC, Shreya D, Zamora DI, Patel GS, Grossmann I, Sange I. Anterior cruciate ligament injury: conservative versus surgical treatment. Cureus. 2021 Dec 6;13(12).;17(5):742.
  14. ↑ Korakakis V, Kotsifaki A, Korakaki A, Karanasios S, Whiteley R. Current perspectives and clinical practice of physiotherapists on assessment, rehabilitation, and return to sport criteria after anterior cruciate ligament injury and reconstruction. An online survey of 538 physiotherapists. Physical Therapy in Sport. 2021 Nov 1;52:103-14.
  15. ↑ Jenkins SM, Guzman A, Gardner BB, Bryant SA, Del Sol SR, McGahan P, Chen J. Rehabilitation after anterior cruciate ligament injury: review of current literature and recommendations. Current reviews in musculoskeletal medicine. 2022 Jun;15(3):170-9.
  16. ↑ Clinical Physio. How do you diagnose an ACL RUPTURE?! | Expert Explains Key signs for an ACL tear you need to know!. Available from: https://www.youtube.com/watch?v=mLuj2alBeH0 [last accessed 9/11/2025]
  17. ↑ Clinical Physio. Lachman's Test for ACL Rupture | Clinical Physio. Available from: https://www.youtube.com/watch?v=VbjDoMcWf-I [last accessed 10/9/2025]
  18. ↑ Clinical Physio. Knee ACL Anterior Draw Test | Clinical Physio Premium. Available from: https://www.youtube.com/watch?v=_zkV-xIJ7-k [last accessed 10/9/2025]
  19. ↑ The Physio Channel. Pivot Shift Test - How to do the Pivot Shift ACL knee ligament test. Available from: https://www.youtube.com/watch?v=a1XoVPWbFcM [last accessed 10/9/2025]
  20. ↑ Clinical Physio. Lelli's Sign (Lever Test) | Expert Physio guides you through how to complete and perform ACL testing. Available from: https://www.youtube.com/watch?v=0PHw4h3Fy68 [last accessed 10/9/2025]
  21. ↑ Petersen W, Häner M, Guenther D, Lutz P, Imhoff A, Herbort M, Stein T, Schoepp C, Akoto R, Höher J, Scheffler S. Management after acute injury of the anterior cruciate ligament (ACL), part 2: management of the ACL-injured patient. Knee Surgery, Sports Traumatology, Arthroscopy. 2023 May;31(5):1675-89.
  22. ↑ Al Mohammad B, Gharaibeh MA. Magnetic resonance imaging of anterior cruciate ligament injury. Orthopedic Research and Reviews. 2024 Dec 31:233-42.
  23. ↑ Allott NE, Oladipo FT, Cox KL, Finnerty CM, Banger MS, McGregor AH. The methods of quantifying knee laxity in the ACL-injured population: A review. The Knee. 2025 Aug 1;55:85-103.
  24. ↑ 24.0 24.1 24.2 Cristiani R, van de Bunt F, Kvist J, Stålman A. High prevalence of associated injuries in anterior cruciate ligament tears: a detailed magnetic resonance imaging analysis of 254 patients. Skeletal Radiology. 2024 Nov;53(11):2417-27.
  25. ↑ Seil R, Pioger C, Siboni R, Amendola A, Mouton C. The anterior cruciate ligament injury severity scale (ACLISS) is an effective tool to document and categorise the magnitude of associated tissue damage in knees after primary ACL injury and reconstruction. Knee surgery, sports traumatology, arthroscopy. 2023 Jul;31(7):2983-97.
  26. ↑ Papaleontiou A, Poupard AM, Mahajan UD, Tsantanis P, Mahajan U, Tsantanis P. Conservative vs surgical treatment of anterior cruciate ligament rupture: a systematic review. Cureus. 2024 Mar 20;16(3).
  27. ↑ 27.0 27.1 Rund JM, Christensen GV, Fleming JA, Wolf BR. Anterior Cruciate Ligament Tears among Football Players. Current reviews in musculoskeletal medicine. 2025 May;18(5):183-9.
Retrieved from "https://www.physio-pedia.com/w/index.php?title=Introduction_to_ACL_Rehabilitation&oldid=372631"