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What is it?

If you have played or watched field sports you may have heard about ACL injuries. ACL stands for Anterior Cruciate Ligament, which is one of the four main ligaments of the knee.

It originates from the posteromedial corner of the lateral femoral condyle of the femur and attaches anteriorly to the intercondyloid eminence of the tibia. However, in layman’s terms, it connects your thigh bone to your shin bone!

The ACL resists anterior tibial translation (your shin moving forwards from your knee) and rotational loads.

Injuring your ACL

Most commonly the ACL is injured by a non-contact mechanism, however, can also be injured in both contact (a direct blow) and indirect (e.g wrestling) contact mechanisms. Young individuals who play sports that involve pivoting, decelerating, and jumping are most at risk.

A common complaint when suffering a potential ACL injury is feeling or hearing a sudden ‘pop’ in your knee, immediate pain and swelling, a loss of stability, and moments of your knee giving way.

I can attest to these as I have experienced them all firsthand when I ruptured my ACL in a non-contact mechanism, doing my best Roger Tuivasa-Sheck impression trying to rapidly change direction while playing oz-tag.

Women are also up to 6x more likely to injure their ACL than their male counterparts, due to a bunch of biomechanical, anatomical, neuromuscular, and physiological factors (Pfeifer et al 2018).

ACL injuries are more likely to occur during competition games rather than training sessions. There is also some evidence to suggest injuries are more likely to occur earlier in the competition season, as well as earlier in the competition game (Anderson et al 2019, Della Villa et al 2020). Della Villa and his colleagues also noted two peaks of injury incidence rate during the season, with a second peak occurring later in the season.

This leads to the hypothesis that there is a relationship between both intensity/level of competition and ACL injuries, however, the relationship is unclear.

How do we fix it?

It has long been accepted that a torn ACL cannot heal and that surgical repair is required. However, more recently there has been emerging evidence that ACL reconstruction (ACLR) may not be necessary (more on this in a future blog!).

If you do choose to undergo ACLR, your surgeon will explain to you the different options for surgery. The torn ACL will need to be removed and replaced by either an autograft (tissue taken from the same individual to make the new ACL) or an allograft (donor tissue taken from one individual to another to make the new ACL).

Some of the common locations for allografts (donor grafts) are the tibialis anterior, tibialis posterior, achilles tendon, and bone-patella tendon-bone (called BPTB, as it has bony attachments. For autografts, the main two are BPTB (Patella tendon) and semitendinosus-gracilis (ST-G) (Hamstring tendon). Iliotibial band autografts have also been used, largely in the skeletally immature patient population.

All ACL graft types have their pros and cons. In allografts, the lack of donor site symptoms post-operatively can accelerate rehabilitation. However, there is a high risk of infection and body rejection, as well as lower tensile strength compared to an autograft. For these reasons, allografts may not be as preferred compared to autografts (Buerba et al 2021).

For BPTB autografts, they mimic a natural ACL functionally the best and have great longer-term outcomes. As a subsequent result of the surgical procedure, however, there may be a higher risk of anterior knee pain, as well as a reduction in quadriceps power and eccentric strength post-operatively.

For ST-G autografts, they are much stronger and less re-rupture rates compared to an allograft. When compared to a BPTB autograft, they result in less overall morbidity. It is also an easier operation technically to perform. On the flip side, ST-G autografts result in an increased risk of hamstring-related symptoms as well as ongoing loss of knee flexion.

When choosing which graft type for your ACLR, it is important to consider factors such as what sport and what level you will be wanting to return to, any prior injury history, your occupation and the surgeon will discuss their preference.

In summary, an allograft is traditionally used for revision ACLRs or in individuals who are not returning to a high pivoting sport. BPTB autografts are usually used in higher level athletes who come from a more running based sport, have a higher quadricep muscle strength and do not require a lot of kneeling for their occupation. ST-G autografts are usually used for athletes who come from lower levels of sport, from more jumping based sports or sports that aren’t as reliant upon speed and have no prior hamstring issues.

ACL Anatomy


  • Anderson, T., Wasserman, E. B. and Shultz, S. J. (2019), ‘Anterior Cruciate Ligament Injury Risk By Season Period and Competition Segment: An Analysis of National College Athletic Association Injury Surveillance Data’, Journal of Athletic Training; 54(7)
  • Buerba, R. A., Boden, S. A. and Lesniak, B. (2021), ‘Graft Selection in Contemporary Anterior Cruciate Ligament Reconstruction’, Journal of the American Academy of Orthopaedic Surgeons, 5(10)
  • Della Villa, F. et al (2020), Systematic Video Analysis of ACL Injuries in Professional Male Football (Soccer): Injury Mechanisms, Situational Patterns and Biomechanics Study on 134 Consecutive Cases,’ British Journal of Sports Medicine; 54(23): 1423-1432
  • Pfeifer, C. E. et al (2018), ‘Risk Factors Associated With Non-Contact Anterior Cruciate Ligament Injury: A Systematic Review’, International Journal of Sports and Physical Therapy; 13(4): 575-587

Written by Jack (physiotherapist)

Post Author: YS Physio

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