The Biomechanics of an ACL tear

Tearing of the ACL has become pretty synonymous with most knee injuries and often the return to activity time can take up to a year. For ACL injuries to occur, something traumatic has to happen to the knee involved. By traumatic, I am not just referring to a gruesome injury where the femur goes one way and the tibia goes the opposite way.  Sometimes all it takes is a turn in the wrong direction, a little too fast for the body to realize what is happening and adapt to it.

So let’s start from the beginning – what is the ACL?

ACL stands for Anterior Cruciate Ligament – the main job of this ligament is to prevent the tibia (the shin bone) from moving too far forward of the femur (thigh bone) during different movements. It also has a role to play in the rotational stability of the knee when we’re moving forward or sideways. The ACL attaches to the femur on the lateral (outside) condyle and on the tibia (shin bone) in between the medial and lateral condyle (intercondylar area) (picture below)

ACL

 

 

Because of the direction in which the ligament goes, it is involved in a lot of knee movements, hence why 70% of knee injuries involve the ACL to a certain level.

 

Even though the ACL is a strong ligament, there are various conditions that could predispose someone to either spraining or tearing their ACL and these are quad dominance over hamstring dominance, Q angle, having limited hip mobility, & lack of proprioception.

Quad Dominance

 

Your quads are a very strong group of muscles that are very active in any movement that involves knee extension. Due to how much sitting we are doing these days and our posture, these muscles end up being more dominant than the hamstrings. Your hamstrings are muscles on the posterior aspect of the femur that attach from the pelvis all the way to the tibia or the fibula. If you’re placed in a position where the ACL is under a lot of tension, the strength difference in these muscles can either protect it or aid in tearing it. Your quads are responsible for knee extension and your hamstrings for knee flexion. If tension is being placed on the ACL and the tibia is moving too far forward, your hamstrings can either protect the ACL by keeping the tibia closer to the femur but if your quads over power it, they’re going to pull that tibia even more anterior resulting in  ligament damage.

quad heavy

 

Q – Angle

The reason why it is said that females are most likely to tear their ACL is closely related to hip mobility but for the purpose of today’s blog post, we will separate them. Structurally the way females are built, their pelvis is wider than males. Why would a difference in pelvic width affect the knee? The tip of the pelvis (that bony part you feel at the front of either side of your hip) is called the ASIS (anterior superior iliac spine). If you were to draw a straight line from the ASIS to the middle of the kneecap and then from the kneecap to the tibial tuberosity (bump at the top of your shin bone, right under the knee and back up, creates an angle known as the “Q” angle (pictured below).

ASIS

A wide “Q” angle makes one more susceptible to valgus forces (forces the push the knee inwards) which would put a strain on the ligament because of how it is laid, especially if nothing is done to strengthen the glute muscles to prevent that valgus from happening. This is why exercises that promote abduction are important for female athletes, especially ones that are involved in sports that require a lot of cutting, jumping, landing and etc. For all these to happen properly, hip mobility is important.

Hip Mobility

Hip mobility is one of those concepts that we hear a lot about because it makes our life better but it also is so important when it comes to injury prevention. Our hips are a huge joint that take on a lot of force and they are surrounded by the glutes, which are the strongest muscles in our body. However, for our glutes to work properly, the joint has to be able to get in and out of the necessary ranges so that them muscle fibers can work. Since we are in a society where the hip joint is compressed – whether it is because the hip flexors shorten or because of other injuries – our hips don’t ever use their full potential.

When playing sports, the ability to use your hips is very important. This allows for a safe and efficient change in direction. Most non contact traumatic ACL injuries happen when there is a “plant and twist” motion done without the incorporation of the hips. When we decelerate in one plane of motion and try and accelerate in another plane, it is important to use the hips to absorb the initial force so it can be replicated it in the new plane of motion. For this to happen, the hip joint needs its full range so that it can adapt when and where it needs to. If the hips have limited mobility, we are not able to absorb the force safely and since our ankle is adhered to the ground, the joint that takes the brunt of it, is the knee.

Lack of Proprioception

Now, how does this relate to lack of proprioception? First of all, let us define proprioception. Proprioception is your body’s ability to know where it is in space. Without it, you would have to look at your hands when raising them above your head to see where they were or you would have to walk looking at your feet to see where they were stepping. This is because our body has little nerve receptors around different joints and muscles giving it feedback on where they are in space and also how much tension the muscles are under. Some of the proprioceptors are around the capsules of synovial joints and their main focus is to detect pressure, especially ones that happen during the acceleration and deceleration of the joint. These are called joint kinesthetic receptors and they are found around the ligaments in our body (Wiley, 2009,pg684). When these receptors detect a lot of pressure is being placed on the joint, they get the muscles around the joint to fire to protect it. This is why there are certain exercises that are recommended to do to prevent ACL injuries from happening as best as we can and also should be done during the first couple of phases of rehab – hamstring strength being one of them.

 

Rehabilitation and Return to Play/Activity

 

Most ACL injuries take at least 9 months post surgery before you’re fully able to return to your activity level before. The way you teach your body to move over this 9 month rehabilitation period is what’s going to determine how well you return to competition and how safe you are in it. As an athletic therapist and a strength coach, one of my main goals is to rebuild the body into proper alignment and firing pattern. After surgery, it is a lot of range of motion exercises, proper muscle activation, reactivating the proprioceptors, before we can start teaching movement techniques while building strength/power. Teaching the body how to move properly not only lowers the chances of the injury happening again but it also builds confidence in going back and redoing the movement that caused the initial injury.

 

References

 

Clark, N., Forshey, T., Mulligan, I., & Kindel, C. (2019). Knee mechanics during a change of direction movement in division I athletes following full return to sport from anterior cruciate ligament reconstruction. Physical Therapy in Sport,35, 75-78. doi:10.1016/j.ptsp.2018.11.008

 

Guelich, D., Xu, D., Koh, J., Nuber, G. and Zhang, L. (2019). Different roles of the medial and lateral hamstrings in unloading the anterior cruciate ligament.

 

Swanik, C. “. (2015). Brains and Sprains: The Brains Role in Noncontact Anterior Cruciate Ligament Injuries. Journal of Athletic Training, 50(10), 1100-1102. doi:10.4085/1062-6050-50.10.08

 

Davidson, S., & Mclean, S. (2016). Effects of maturation on combined female muscle strength and ACL structural factors. Journal of Science and Medicine in Sport, 19(7), 553-558. doi:10.1016/j.jsams.2015.07.016

 

PRINCIPLES OF HUMAN ANATOMY. (2009). S.l.: JOHN WILEY.