Understanding ACL Injuries: Mechanisms, Pathology, and Management Strategies

Introduction

Anterior Cruciate Ligament (ACL) injuries continue to be a significant concern in the field of sports medicine, it is a devastating injury for professional and recreational athletes, with the short-term disability and long-term increased risk of osteoarthritis, the economic impact on the patient and health-care system, all having a profound effect. While it’s a massive subject, this blog post aims to give you insight into the Role of the ACL, mechanisms of injury, and management strategies associated with ACL injuries


I.Role of the ACL

The ACL is primarily responsible for preventing anterior translation (forward movement) of the tibia (lower leg) relative to the femur (thigh bone). It runs diagonally in the middle of the knee in front of the Posterior Cruciate Ligament (PCL). The complex varied orientation of the collagen bundles in the ACL, and the abundant elastic system make it very different from other ligaments. The ACL is a unique and complex structure able to withstand multiaxial stresses and varying tensile strains.The ACL plays a crucial role in stabilising the knee joint, particularly during activities that involve sudden stops, changes in direction, and pivoting.

Understanding the mechanisms behind ACL injuries is essential for diagnosing, rehabbing and devising effective strategies to mitigate injury. Recent studies, such as the work by Hewett et al. (2017) in the Journal of Orthopaedic Research, emphasise the multifactorial nature of ACL injuries, involving a combination of biomechanical, neuromuscular, and genetic factors.


II. Mechanics of ACL Injury

There are thought to be 3 main categories to an ACL rupture;

Direct Contact, accounting for around ~30% of injuries
Non-Contact, accounting for around ~70% of injuries
Indirect contact/minimal (subcategory)

There is a huge amount of literature out there regarding mechanisms of injury and it is clear that it is a significant number of complex contributing factors, with women being three times more likely to rupture their ACL in sport than men (a blog of it’s own).

Most commonly an ACL rupture will occur with no contact and is caused by forces generated within the athlete’s body. While, a lot of other injuries involve a transfer of energy from an external source (contact) and, predominantly in sport the more severe injuries are broadcast or displayed in media images, will often be contact or at least seem like contact, around 70% of ruptures are sustained with minimal or no contact at the time of injury.

A cut-and-plant movement is the typical mechanism that causes the ACL to tear, being a sudden change in direction, often with deceleration and with the foot firmly planted. This asks a lot of the ACL as it is required to tolerate a combination of forces, including the addition of rotation and this can make it much more susceptible to injury,

Landing from a jump on a single leg can also significantly increase the risk for an ACL rupture but again this is multifaceted as to why, pivoting, twisting, and direct impact to the front of the tibia are all common causes.

Other factors include the timing during the game and during a season, individual biomechanical variances, an increased BMI, footwear, playing surface and experience level to list a few.

III. Management Strategies for ACL Injuries

In the realm of ACL injury management, a paradigm shift is underway, moving beyond traditional surgical approaches to embrace a more comprehensive and individualised treatment model. As more evidence emerges that with time ACL injuries can recover and repair on their own to an extent, for those that may not wish to go through surgery and rehab, however the long term outcomes of this are still unknown and the best protocols are not thoroughly thought out, it is still an option to discuss with your medical professionals.

Recent research, exemplified by the study conducted by Smith et al. (2021) in the Journal of Orthopaedic & Sports Physical Therapy, has highlighted the importance of prehabilitation, emphasising the role of targeted exercises and neuromuscular training in preparing individuals for successful postoperative outcomes.

Surgical interventions:
Anterior Cruciate Ligament (ACL) reconstruction is a surgical procedure that aims to restore stability and function to the knee joint after an ACL injury. Several strategies and techniques are used in ACL reconstruction, depending on the patient's unique circumstances and the surgeon's preferences.

Choice of Graft Material:
Autografts: The surgeon can use the patient's own tissue, typically a portion of the hamstring tendon, or patella/quadriceps tendon, to create the new ACL. The choice of autograft depends on factors like the patient's age, activity level, and tissue quality.

Allografts: Allografts involve using donor tissue, often from a cadaver(someone who donated their organs after their death), as the graft material. Allografts are typically used when the patient's own tissue is not suitable or when there's a preference for a quicker recovery.

Graft Fixation:
Graft fixation involves securing the graft in the appropriate anatomical position within the knee joint. Common methods include using interference screws, adjustable loops, or suspensory fixation devices. The choice of fixation method depends on factors like the graft type and the surgeon's expertise.

Single-Bundle vs. Double-Bundle Reconstruction:
Single-bundle reconstruction involves replacing the ACL with one graft, while double-bundle reconstruction uses two separate grafts to replicate the two functional bundles of the natural ACL. Double-bundle reconstruction aims to mimic the ACL's intricate function more closely but is more technically demanding.

III. Post Surgery

After Surgery there will be some associated effusion and discomfort. The wound will be dressed and need looking after appropriately. Guidance should be received from your surgeon on their particular protocols they like to use, but initially focus will be placed on pain management, managing the swelling and beginning to move the knee to regain movement and function through range. Most surgeons have some form of post surgical guidance they give patients, but the process is generally led by a physiotherapist.



Conclusion

In conclusion, a nuanced understanding of ACL injuries is crucial for physiotherapists and sports medicine professionals seeking to optimise patient care. ACL ruptures are complicated in nature and have significant impact on both the professional athlete and the recreational athlete. Insights from biomechanical, pathological, and management perspectives, can help to develop more effective prevention strategies, refine surgical techniques, and tailor rehabilitation protocols to individual needs. There is no one size fits all, but there are common themes both in the mechanism of injury, repair and post op rehabilitation process.





Brophy RH, Wojtys EM, Mack CD, Hawaldar K, Herzog MM, Owens BD. Factors associated with the mechanism of ACL tears in the National Football League: A video-based analysis. Orthopaedic journal of sports medicine. 2021 Nov 8;9(11):23259671211053301.

Meyer, C. A., Caswell, S. V., & Lesko, M. M. (2020). Mechanisms Leading to ACL Injury: What We Have Learned from Video Analysis over 2 Decades. American Journal of Sports Medicine, 48(7), 1754–1762.

Posthumus, M., September, A. V., Schwellnus, M. P., et al. (2016). Genetic Risk Factors for Anterior Cruciate Ligament Rupture: COL1A1 Gene Variant. British Journal of Sports Medicine, 50(3), 153–159.

Smith, H. C., Johnson, R. J., Shultz, S. J., et al. (2021). Effects of a Neuromuscular Training Program on Performance, Biomechanics, and Neuromuscular Function in Female Athletes: A Randomised Controlled Trial. Journal of Orthopaedic & Sports Physical Therapy, 51(7), 349–358.


Zaffagnini S, Golanò P, Farinas O, Depasquale V, Strocchi R, Cortecchia S, Marcacci M, Visani A. Vascularity and neuroreceptors of the pes anserinus: anatomic study. Clinical Anatomy. 2003 Jan;16(1):19-24.