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ACL Injury – The Basics

The Anterior Cruciate Ligament (ACL) has been in the news everywhere and it’s usually for bad news. In other words, the only thing you hear about the ACL is when an athlete has torn it and is out for the season. One of the things that we get asked a lot is how the ACL is torn and why it takes so long to recover. In this article, you will learn all about the ACL. This will be perfect for anyone that has torn it and wants to learn more about their injury, or for someone that is interested in learning more about this topic.

What is the ACL?

The ACL attaches to the anterior intercondylar fossa of tibia, or front part of the lower leg bone, and to the femur at the medial aspect of lateral condyle, or the inside part of the upper leg bone. It runs at an oblique angle (superior and lateral). The ACL function is primarily to limit forward sliding of the tibia on the femur. Secondarily, it limits internal rotation of the tibia during flexion. In other words, the ACL limits hyperextension of the knee and twisting of the knee inward.

How is the ACL Injured?

It is typically injured with forceful or excessive internal or external tibial rotations, valgus stress or hyperextension at the knee joint. In other words, it is injured with excessive inward twisting of the knee on a planted leg, excessive hyperextension, excessive valgus stress, or any combination of those. 

In a contact injury, the ACL is torn when the tibia is pushed forward in relation to the femur when the foot is planted, or forced into hyperextension. For example, when a teammate or opponent accidentally falls and rolls up into your knee, forcing your knee into hyperextension. While the contact mechanism is typically out of your control and is an unfortunate part of the sport, over 70% of ACL injuries occur via a non-contact mechanism. 

Non-contact injuries occur typically with that excessive twist/pivoting in the knee on a planted leg. For example, when some is driving into the lane and performs a euro-step or landing after dunking or grabbing a rebound.  

Here is a video in which we go into further detail

Am I At Risk for Injury?

There are several risk factors that contribute to an ACL tear that can be categorized into either intrinsic or extrinsic factors. Intrinsic factors are individual-based and are broken up into modifiable and non-modifiable. Extrinsic factors are those that are outside of the control of the individual.

Extrinsic Factor Examples:

  • Weather in the Arena or at the Field
  • Court or Field Conditions
  • Footwear or Equipment; Shoe-Surface Interaction

Intrinsic Non-Modifiable Risk Factors:

  • Genetics
  • Female or Larger Q-Angle
  • Cogenital Ligament Laxity
  • ACL Size
  • Intercondylar Notch Width

Intrinsic Modifiable Risk Factors:

  • High BMI
  • Poor Landing and/or Deceleration Mechanics – Increased knee valgus, or knee abduction/internal rotation moment and angle on landing or deceleration activities
  • Poor core stability and Glute Control

How Do I Know if I Tore My ACL?

There are three clinical tests that can be performed on you that can indicate an ACL tear. The three tests are Lachman’s Stress Test (Gold Standard for Clinical Tests), Anterior Drawer Test, and Lever Sign. The anterior drawer test and lachman’s stress test both are positive for an ACL tear if there is excessive anterior glide of your tibia on your femur. Meanwhile, the lever sign indicates an ACL tear if there is a lack of terminal knee extension during the exam. 

The Gold Standard to assess if the ACL is intact or torn is an MRI.

Do I Need Surgery?

Yes and no. It is a loaded question that requires numerous considerations before answering that question. On a side note, it is estimated that 1/3 of individuals who tore their ACL may be able to return to previous activity without surgery.  We will discuss this question and topic in further detail in a future blog article.

How Long Would I Be Out For?

It depends on the type of graft you used for the reconstruction. Autograft, or using your own tissue, is usually 8-12 months recovery. Allograft, or a donor graft/not your own tissue, ranges from 10-14 months because the allograft remodeling (ligamentization) and incorporation are slower and presumably more susceptible to early failure. Also, we will go into more depth of ACL graft choices in a future blog article.

In conclusion, most athletes begin the process of returning to sport in those time frames based on graft choice; however, we always tell our athletes that those time frames are not set-in-stone because many factors can change your timetable for return. Furthermore, when you return to your sport, it is going to take time to develop the confidence, skill level, intensity, and endurance you once had. Most athletes don’t feel like themselves until 2 years after their surgery if not later. 


Bousquet, Brett A et al. Post-operative criterion based rehabilitation of ACL Repairs: A clinical commentary. 2018. Pfeifer, E Craig et al. Risk factors associated with non-contact anterior cruciate ligament injury: a systematic review. 2018 Vyas, Dharmesh et al. Allograft anterior cruciate ligament reconstruction: indications, techniques, and outcomes. 2012.

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