This two-part blog post will look at the Anterior Cruciate Ligament (ACL) tear and what we can do to prevent it.

ACL tears

The right knee – you can see the meniscus, cruciates and collateral ligaments.

The Anterior Cruciate Ligament (ACL) is one of the most notorious parts of the body to sportsmen and sportsfans. When torn or ruptured, it is a devastating injury to an athlete. We hear about it regularly on TV with respect to professional athletes in multiple disciplines, and the odds are high that we know a friend or a child that has had this injury. Sadly, the cold hard fact is that many people, even professionals, do not get back to their pre-injury level.  Even more concerning, recent research¹ has shown there is an epidemic of ACL injury and surgery in youth sports. A study of US¹ data from 1994 to 2006 showed a 924% increase in ACL surgery amongst patients 15 years old and younger. During the same period, the overall increase for the general population was only 37%. That is a huge difference and one that should be of concern to any parent whose child plays sports. It should be noted this was a figure for all sports, which means there are some higher and lower risk sports amongst these figures. For parents whose children play soccer, the concern might be great and we’ll learn more about that below.

So, exactly what is the ACL, and what are the repercussions of a tear? For this post, we will briefly discuss the anatomy, biomechanics, and injury of the ACL. The second post will then discuss research on how to reduce ACL injury risk. Always respecting the evidence, PhysioWorks incorporates these methods into our practice, so that we can reduce the ACL injury rate in the Huntsville, Alabama area!

The ACL

I created the video above to help explain the anatomy of the ACL. The ACL is one of four major ligaments in the knee. It runs from the front inside portion of a shelf of bone on the top of the shin (anterior-medial part of the tibial plateau) and runs back and outwards to the inside of an outcrop of bone on the thigh (the intercondylar groove of the lateral femoral condyle). The ACL actually has two distinct bundles of fibers. You can see in the video that the position of these bundles is different depending if the knee is straight or bent/flexed. Because there are two bundles, the ligament works to not only resist shear forces, but also rotational forces. When considering the ACL anatomy and its ability to resist rotational forces, we should also note that this capability is aided by the The Posterior Cruciate Ligament (PCL), which runs in the opposite direction crossing the ACL.

The purpose of a ligament is to passively keep two bones from moving too far apart. In the case of the knee, the purpose of the ACL is to stop the tibia moving too far forward on the femur. You can see in the video that if a shear force is applied to the tibia the ACL would be placed under tension. We test for this tension in the clinic using the Lachmans test, where we want to feel an end-feel to tell us the ACL is intact. If the ACL is damaged, the tibia will move further than it should, and we will feel laxity; if ruptured, there will be an empty unconstrained feel. You can see this test in the video below:

Ligaments are passive structures. The best analogy being that they are like a rope mooring a boat; as the boat drifts, eventually tension on the rope stops it going any further – unless the force is extreme and violent and the material cannot cope with the stress. Ligaments are not alone in providing stability in a joint; they are assisted by muscles. When compared with a ligament, muscles are active and have feedback by the nervous system. Going back to our boat analogy, their equivalent would be small directional motors (muscles) controlled by a captain (the brain) that work to keep the boat in a good position.

In the context of a sporting situation, the ACL can suffer injury due to either contact or non-contact activity. In both cases, the ligament is stretched beyond its material properties. Contact injury is mostly a freak accident and cannot be affected. The only type of contact injury that could be avoided would be one that is happening consistently in a sport and could, for example, be stopped/reduced by a change in rules or equipment. Non-contact injury, however, has been found to be more predictable. Non-contact ACL injuries tend to occur during fast deceleration, hyperextension, and rotation of the knee. Research has shown us which sports have the most non-contact ACL injuries. It has also identified several risk factors. More recently, research is showing us what we can do to reduce the risk of these injuries!

What risk factors are there?

• Type of sport:
  • A recent paper² was the first to look at first-time non-contact ACL injuries in collegiate and high school athletes playing soccer, lacrosse, basketball, field hockey, football, rugby, or volleyball. The study consistently found soccer and rugby to have a higher rates of non-contact ACL injury, while basketball and field hockey were found to have significantly lower rates.
• Gender:
  • It is widely understood that females have more knee injuries as a whole than males. The above study on non-contact ACL injuries² found a relative risk ratio of females to male athletes of 2.10. This means the probability of non-contact ACL injury occurring is more than double in females than males. There are several possibilities for why this gender difference exists³, some of which can be modified and some that cannot. Examples of factors that are not modifiable include fluctuating hormone levels and biomechanical differences such as deeper femoral condyles, shallower tibial plateau in females. Recently there has been an interesting article showing genetic differences in the ligament tissue itself between males and females⁴. Factors that are modifiable include reduced hamstring and gluteal strength and poor landing biomechanics.
• Level of Sport:
  •  Injury rates have been shown to be high for varsity athletes than non-varsity athletes, and higher for college athletes than high school athletes, with a relative risk ratio of 2.38².These two sets of findings fit with the hypothesis that injury risk increases with level of competition. This also fits with the statistic from the first paragraph, of a 924% increase in ACL surgery in under 15’s over a decade. The world-renowned orthopedic surgeon, Dr. James Andrews, thinks that the epidemic of ACL injuries in youth sports is due to children being treated like professionals. Sadly, this does not mean that children are getting the same levels of treatment as professionals; instead it means that they are being asked to perform at volumes that are more expected of professionals. I can attest to his beliefs. With the possibility of college scholarships and even fame and fortune in professional sports, children, then adolescents, then college students often ramp up their participation too quickly without enough quality supervision. In my career it has not been unusual for me to treat young athletes who are accustomed to practicing their sport 6 or 7 days a week, nearly year-round. Earlier specialization in a single sport may also be a contributing factor to the rise in ACL injuries. It is my aim at PhysioWorks to equip young athletes with the underlying physical requirements to allow them to progress at their sports in a safe manner, so that they can enjoy athletics now, and as adults.

Stay tuned for part 2, where we discuss the evidence about reducing these injuries, and how I apply this evidence to my practice at PhysioWorks.

Reference List:

1) Buller LT, Best MJ, Baraga MG, Kaplan LD. Trends in Anterior Cruciate Ligament Reconstruction in the United States. The Orthopaedic Journal of Sports Medicine, 1015;3(1); 1-8.

2) Beynnon BD, Vacek PM, Newell MK, et al. The Effects of Level of Competition, Sport, and Sex on the Incidence of First-Time Noncontact Anterior Cruciate Ligament Injury. Am J Sports Med. 2014;42(8):1806-1812.

3) Myer GD, Ford KR, Di stasi SL, Foss KD, Micheli LJ, Hewett TE. High knee abduction moments are common risk factors for patellofemoral pain (PFP) and anterior cruciate ligament (ACL) injury in girls: is PFP itself a predictor for subsequent ACL injury?. Br J Sports Med. 2015;49(2):118-22.

4) Johnson JS, Morscher MA, Jones KC, et al. Gene expression differences between ruptured anterior cruciate ligaments in young male and female subjects. J Bone Joint Surg Am. 2015;97(1):71-9.