ACL Information

Anatomy & Function

(Refer to picture on the right)

An ACL, Anterior Cruciate Ligament, is one of the two ligaments (ligaments connect bone to bone) located inside the knee. Both the Femur (bone above the knee) and Tibia (Bone below the knee - shinbone) hold the ACL together, and vice versa. The 2 cruciate ligaments being the ACL and PCL, posterior cruciate ligament, cross inside your knee like an “X”. Since the ACL is anterior, that means it lies in front of the PCL. The ACL connects to the medial meniscus and to the lateral aspect base of the femur.

The ACL has several functions: protection, movement, and stability. Its attachments (femur and tibia) hold the ACL together and helps the leg flex or bend in order to walk. The ACL protects the shin bone from sliding forward, as well as provides rotational stability for the knee. Stability is important especially to athletes who participate in activities involving vigorous cutting (changing direction), jumping, and rapid deceleration. Lack of stability or protection to the ACL can cause injury to the ligament. 



The ACL can tear in several ways, through contact or non-contact (most common):

  • Changing direction too quickly → You foot is placed on the ground while your femur and knee twist in the opposite direction (non-contact)
  • Stopping suddenly → a sudden stop can cause the tibia to slide forward, irritating the ACL. (non-contact)
  • Landing from a jump incorrectly → this requires great stability of the muscles surrounding the knee. If the knee is unstable then the tibia can slide and cause a tear (non-contact)
  • Direct collision → your foot is firmly planted on the ground and someone hits your knee causing the twisting motion required to tear the ACL (Contact)

ACL tears are more common in females because of differences in anatomy (pelvis width and ACL size) , hormones, bio mechanics, (landing position and knee angle/knock-kneed).

Degrees of Injury

Injured ligaments are considered "sprains" and are graded on a severity scale. Partial tears are rare; most ACL injuries are complete or almost completely torn.

Grade: 1

The ligament is mildly damaged and has been slightly stretched, but is still able to help keep the knee joint stable.  This minor trauma or "sprain" to ligament causes some ligament fibers are stretched but none are torn. 

Grade: 2

The ligament is stretched to the point where it becomes loose. This is a “partial tear” of the ligament and some fibers are actually torn.

Grade: 3

This type of sprain is a “complete tear” of the ligament it's the most severe ACL knee injury. The ligament has been split into two pieces, and the knee joint is unstable. Severe trauma to the ligament, which completely tears all the ligaments fibers.


Factors for ACL Injuries

Extrinsic Risk Factors 

Outside (external) factors that contribute to tearing an ACL. People wearing shoes with longer, irregular cleats have an increased risk of ACL injury, because of increased friction at the foot-to-turf interface. Dry weather increases the risk of ACL injuries on natural grass

Intrinsic Risk Factors

Inherited (Internal) factors that contribute to tearing an ACL. 

  • Ligamentous laxity: people whose ligaments are too stretchy are 2.8 times more likely to injure their ACL. Ligaments are not firm and are less likely to prevent an ACL tear. 
  • Pelvis Width: Female Athletes have a higher risk of tearing their ACL; they are 4 to 6 times more likely to tear it than male athletes. Since women have wider pelvis than men, this means that women have a slight knocked knees configuration (knees point inwards), this causes the foot to overpronate, or fall inwards. 
  • Flat feet: people with flat feet tend to over pronate or cause an arch on their foot, which cause knee to turn inwards, and makes it easier to tear your  ACL. 

Hormonal Factors 

The ACL has receptors for estrogen, testosterone, and relaxin, which means that sex hormones may affect the mechanical properties of the ACL and influence the risk of ACL injury. Female’s ACL have half a millimeter more laxity during their menstrual cycle. Sex hormones influence ACL injury risk through indirect effects on neuromuscular growth and maturation during puberty. These influences can ultimately lead to an ACL tear. During puberty, male’s undergo large testosterone surge, which increases their muscle mass and strength, allowing them to better control their body during athletics. Females don’t experience large testosterone surges, only a small increase in testosterone levels.  This results in a much smaller increase in muscle mass and strength, which may not be enough to control their body during athletics.

Neuromuscular Factors

Females are more prone to ACL tears because they use their muscles differently than males, when landing from a jump or quickly changing direction. Females also tend to have one leg stronger than the other. 

Females tend to use their quadriceps (thigh muscles) much more than their hamstring muscles. Females also tend to have reduced knee flexion, or don’t bend their knees as much as males do when jump landing or quickly changing directions. Reduced knee flexión increases quadriceps activity, and decreased hamstring activity compared with male. When the quadriceps are used more than the hamstrings, this causes the knees to turn inwards leading to an ACL tear. 


Signs & Symptoms

An ACL tear is an acute injury (happens at an instant), when an ACL is torn people encounter:

  • A "pop" or a "popping" sensation in the knee
  • Severe pain and inability to continue activity
  • Rapid swelling
  • Tenderness along the joint line
  • Loss of range of motion (ROM)
  • A feeling of instability/discomfort or "giving out" while weight bearing

Do not:

  • Return to activity/walk → stress on the knee will cause more swelling and discomfort

Immediate/Treatment (Special Tests)


  • Protect the area to prevent further damage
  • Rest the knee to prevent irritation
  • Ice the knee to reduce swelling
  • Compress the knee
  • Elevate knee (direct blood flow back to heart)

Refer to a physician 

  • Make an appointment for an MRI (Magnetic Resonance Imaging) and X-ray, if needed.
  • Special Tests (only to be done by professional):
    • Anterior Drawer Test - Patient lays on their back, patients muscles must be relaxed, bend knee 90°, (professional) sit on patients feet to stabilize movement, place both hand behind the knee with thumbs in the front, bring leg forward. If tibia easily slides too much forward, then ACL is torn.
    • Lachman’s Test - Patient lays on their back, patients muscles must be relaxed, bend knee 20°-30°,  (professional)stabilize femur with one hand, place other hand on tibia, slightly shift the tibia forward. If tibia easily slides too much forward, then ACL is torn.
    • Pivot Shift - Patient lays on their back, patients muscles must be relaxed, (professional) stabilize calcaneus (heel bone), place other hand behind knee and slightly shift knee inwards, move patients leg from extension to flexion, if tibia slides backwards when performing test then ACL is torn.

  • Note: Sometimes if these special tests are performed after injury, the swollen knee will prevent looseness from appearing. Other muscles and tissue will compensate for torn ACL, showing a false negative. 


ACL Injury Prevention Exercises

Balance training: balance exercises

  • Ex. Single Leg Balance, BOSU/Disk Ball, Single Leg Ball Toss
Jump Training: landing with  flexion at the knees and hip
  • Ex. Forward jumps, Squat Jumps, Single Leg Jumps,
Strengthening:  helps build muscle and build protection around ACL ligament
  • Ex. Walking Lunges, Squats, Leg Lifts
Skill training: Controlling body motions; in deceleration and pivoting motions
  • Ex. Suicides, Sideways Suicides, falling down from step, ballerina’s
Agility training: agility exercises
  • Ex. Ladders - zigzags, in & out, Single-Leg Sideways Taps

ACL Injury Prevention Videos

Learn more about how to prevent an ACL tear through videos. Step by step instructions on how to properly perform exercises are provided.

CLick Here

Graft Options

  • Patellar Tendon Autograft

The most commonly recommended autograf in young athletes. A piece of your patella (knee cap) is removed, as well as the patellar tendon, and a also piece of your tibia (shinbone.)

  • Pros: ACL will heal quicker, knee will heal quicker, and less post-surgery pain. 
  • Cons: The patellar tendon has a low probability of fracturing, patellar tendon can rupture, and recovery can take longer because of this.
  • Hamstring Tendon Autograft

The hamstring allows the knee to bend and straighten the hip. Most commonly used for growing children and adolescents. 

  • Pros: Small incisions, less surgical complications, less pain compared to the Patellar Tendon Autograft.
  • Cons: Amount of hamstring tendon is unknown, using too much hamstring tendon to repair ACL, hamstring will decrease in strength drastically, too little hamstring used for ACL repair will increase the risk of failure.
  • Allograft

Using a donor graft, usually a cadaver’s Achilles tendon, (muscle on the back of the heel). 

  • Pros: No need to use your own tissue, decreased pain with surgery.
  • Cons: Higher risk of graft failure (when body rejects it) and has a slightly higher risk of infection.


1. Ambrose, T. Liu. “T. Liu-Ambrose, MSc, PT, PhD (C). The Anterior Cruciate Ligament 

          and Functional Stability of the Knee Joint. BCMJ, Vol. 45, No. 10, December,

          2003, Page(s) 495-499 - Clinical Articles.” British Columbia Medical Journal,

          Dec. 2003,

2. American Academy of Orthopaedic Surgeons. “Anterior Cruciate Ligament (ACL) Injuries -

          OrthoInfo - AAOS.” OrthoInfo, Mar. 2014,

3. “Anterior Cruciate Ligament (ACL) Injury.” Edited by Stephanie Geeurickx et al., Physiopedia,

          15 Jan. 2019, 6:30pm,



4. Cluett, Jonathan. “Why Are ACL Tears Worse for Women?” Verywell Health, 21 June 2018,

5. Emory Healthcare . “Orthopedics:Conditions & Treatments.” ACL Tear, 2019,

6. Kakarlapudi, T K, and D R Bickerstaff. “Knee Instability: Isolated and Complex.” The Western

          Journal of Medicine, Copyright 2001 BMJ Publishing Group, Apr. 2001,

7. Kobayashi, Hirokazu, et al. “Mechanisms of the Anterior Cruciate Ligament Injury in Sports

          Activities: a Twenty-Year Clinical Research of 1,700 Athletes.” Journal of Sports Science

          & Medicine, Asist Group, 1 Dec. 2010,

8. Mayo Foundation for Medical Education and Research (MFMER). “ACL Injury.” Mayo Clinic,

          Mayo Foundation for Medical Education and Research, 30 Mar. 2019,

9. Miller, John. “Physio Works - Physiotherapy Brisbane.” ACL Injury, 14 Feb. 2019, 10:32 AM,

10. Quinn, Elizabeth. “You Can Help Prevent an ACL Injury By Doing These Simple Exercises.”

          Verywell Health, 12 June 2018, 


11. Seattle Children’s, Seattle, Washington. “Graft Choices for ACL Surgery in Youth.” Seattle

          Children's Hospital Research Foundation, 2017,

12. Staff, Healthwise. “Anterior Cruciate Ligament (ACL) Injuries.” Anterior Cruciate Ligament

          (ACL) Injuries | Michigan Medicine, 20 Sept. 2018,