What is the ACL?

Mechanics of ACL bundles AMB and PLB: As pictured above, when the knee is extended, the AMB is slack and the PLB is taut. When the knee is flexed (shown at a 90-degree angle here), the AMB is taut while the PLB is slack. The AMB is more vertical in orientation, while the PLB is more vertically oriented.[1]
It’s a term we hear thrown around in sports a lot, but what is my ACL?Your ACL, which stands for Anterior Cruciate Ligament, is made up of two dense “bundles” of tough collagen fibers connecting the femur (thigh bone) to the tibia (shin bone) within the knee joint. These two bundles are referred to as the 1. Anteromedial bundle (AMB) and 2. Posterolateral bundles (PLB). Understanding the two-bundle model is essential for understanding how the ACL functions within your body:The different orientations and points of tension of this ligament mean that the ACL is functional throughout the knee’s entire range of motion, providing essential structure for nearly all movement of the lowering extremities. The ACL has many implications for balance, position sense, and neuromuscular response. More specifically, it is the ACL’s primary job to 1. Prevent tibial internal/medial rotation and 2. Prevent tibial anterior translation.Let’s break down some terminology before going any further:

  1. Internal rotation, also known as medial rotation: rotation towards your body’s midline. Tibial internal rotation will cause shin and foot to rotate inward

Anterior translation: simply put, tibial anterior translation is the displacement of the tibia forward in relation to the femur. Imagine the tibia being pulled forward, which you might have guessed correctly will cause instability and stress in the knee.


Where is your ACL?

Your ACL is located in the knee and it crosses from one side of the interior of the joint to the other connecting your shin bone to the thigh bone. Your ACL crosses in front of your PCL or your posterior cruciate ligament. This criss-cross orientation helps to prevent you from twisting and shearing forward and back during movement and muscle action on the joint.


The MCL and LCL attach the thigh bone to the shin bone on the inside and the outside of the knee joint respectively. The MCL stands for medial collateral ligament and this prevents the shin bone from swinging out to the side under the femur. The ACL and MCL often work together to prevent the most common kind of stress to the knee. That is a motion we call dynamic valgus where the knee falls into the inside during bending motions like going down the stairs. They are commonly injured together because this dynamic valgus motion is the most common way people injure their knee.

ACL Injury Symptoms

If you are experiencing an ACL tear (rupture), you will likely hear a loud pop or snap indicating that the ligament tore. The severity of pain can vary greatly depending on severity of injury but note that an ACL rupture will likely result in extreme pain around the knee. All ACL injuries should swell within 1-2 hours after the injury occurs, often within a matter of minutes.

If you think you may have an ACL injury, you should seek out a primary care provider or physical therapist for evaluation as soon as possible.


ACL tears are diagnosed by a combination of clinical examination techniques that can occur in an office visit with a PT or physician. Clinicians use special manual tests to assess the stability of the ligaments holding the thigh bone to the shin bone. In some cases these clinical examination findings can be inaccurate. This is where an MRI scan is the gold standard to diagnosis of an ACL tear. In some cases the full extent of an injury is not fully clear until the surgeon views the area with an arthroscope, but this is a rare event and most cases are diagnosed with the combination of clinical exam and MRI.

ACL Surgery

Most people assume that surgery is necessary for every ACL injury. While surgery is needed for many people, it is not appropriate in every case. Multiple studies have grouped people with ACL tears into three groups: copers, adaptors, and noncopers.

Copers are the people that resume their same level of activity and sport as they were pre-injury with conservative rehabilitation therapy alone.

Adaptors are the patients that manage their injury with conservative rehabilitation therapy alone, but do not return to the same activity level they performed at pre-injury. Rather, they “adapt” by lowering and modifying their activity level post-injury.

Lastly, noncopers are those who are not able to resume their same level of activity and/or sport without a surgical procedure AND report consistent instances of their knee “giving out”.

Lifestyle, demographics, severity of injury, and risk factors all play into whether a person will cope with an ACL injury without surgery. Surgery is typically recommended to:

  • Young adults and teenagers- though keep in mind performing surgery on someone who is still growing presents some unique difficulties. For more information visit https://www.hss.edu/condition-list_acl-surgery.asp#procedure “Can a teenager have ACL surgery?”
  • Athletes
  • People with an active occupation and/or lifestyle
  • People with other injuries in the knee region or elsewhere in the lower extremities
  • People who are unable to regain knee stability with conservative rehabilitation treatment

Multiple studies find that people within these categories are found to be less likely to “cope” with their ACL injuries with conservative rehabilitation alone. Nonetheless, it is still important to discuss your options with an orthopedic surgeon and/or physical therapist before deciding on whether surgery is right for you. There is no hard and fast rule on who should have surgery and who shouldn’t, but rather multiple factors and preferences to assess for every individual.


Patellar Tendon Graft (Bone-Patellar-Bone)

  • Currently considered the gold standard for ACL graft choice
  • Central third of patellar tendon is used, about 10-11mm
  • Use of bone creates opportunity for early vascularization and improved bone healing
  • Better outcomes in youth athletes, high performance athletes (reserved for youth athletes that have finished, or to close to finishing growing due to location of growth plates in the bones)
  • Maintains hamstring/knee flexion strength and tendon integrity
  • Lower revision rate compared to hamstring (1.9-6.6% versus 4.9%-17.5%)
  • Cons- potential for increased anterior (front) of knee discomfort*, increased time for full quadricep activation, prolonged discomfort with kneeling
  • *studies have shown reduced anterior knee pain following initiation of high caliber rehab program

Hamstring Graft

  • Thicker tendon (greater cross sectional area compared to BPB)
  • Generally recommended for individuals ~35+ that would like to maintain an active lifestyle
  • Some long term studies have demonstrated reduced relative risk of knee osteoarthritis 10 years after surgery
  • Cons- Graft size may be unpredictable
  • May demonstrate increased time to maintain full knee extension and flexion
  • Reduced hamstring and quad strength post-surgery

Quads Graft (Quadriceps Tendon)

  • Better outcomes in youth/active patients
  • Can be performed in youth athletes that are still growing
  • Less anterior knee pain, discomfort with kneeling
  • Reduced retear rates in active individuals compared to hamstring tendon grafts
  • Cons- prolonged quadriceps weakness

Cadaver Graft

  • High availability and reduced cost
  • Predictable graft size
  • Generally recommended for older individuals, or return to lower intensity active lifestyle
  • May be more appropriate for multiligamentous knee injuries or inadequate autograft tissue
  • Higher failure rate
  • Increased risk for rejection and delayed incorporation
  • Reduced return to sport rate

Buerba, R. A., Boden, S. A., & Lesniak, B. (2021). Graft Selection in Contemporary Anterior Cruciate Ligament Reconstruction. JAAOS Global Research & Reviews, 5(10).Vyas, D., Rabuck, S. J., & Harner, C. D. (2012). Allograft anterior cruciate ligament reconstruction: indications, techniques, and outcomes. journal of orthopaedic & sports physical therapy, 42(3), 196-207.

ACL Non-surgical Treatment

Non-surgical management of an ACL tear occurs in Four phases

Acute Phase

During this phase we use PEACE and LOVE to manage swelling, restore ROM and minimize strength loss. What that stands for is:

P – Protect – Unload or restrict movement for 1-3 days
E – Elevate –  The limb above the heart to promote fluid flow out of the tissue.
A – Avoid –  Anti Inflammatory medication as they may impair tissue regeneration.
C – Compress – Compression garments, or wraps to reduce the swelling.
E – Educate – About active recovery, outcome and time frame.
L – Load – Load optimally with exercise to minimize loss of muscle mass without injuring
O – Optimism – Staying positive improves healing. Visualize your full recovery
V – Vascularisation – Cardio workout to boost motivation and increase blood flow
E – Exercise – Keep moving to optimize blood flow, hormones, and recovery.


Strength Development Phase

During this phase you will go through a program of progressive overload to build back strength and muscle mass to >80% of your uninjured side. We call this measurement your Limb Symmetry Index or your LSI.

We use special tools called dynamometers to measure your progress and determine when it is time to progress you to the next phase.

Sports Specific Movement Phase

Once you reach 80% of your uninjured side we progress to more complex motions that will prepare you for your full return to sport. The complexity increases by increasing the speed, decreasing the stability and adding unexpected perturbations.

We also look at what the demands of your particular sport is and tailor your exercises to help you build strength, mobility and timing where you will need it. 

Return to Sport Phase

Before you are cleared to return to sport you must pass a series of physical tests that measure your ability to perform with your injured side. Your scores must be greater than or equal to 90% of the score from your uninjured leg before we can recommend you return. This is a very rewarding part of your recovery.

ACL Recovery Timeline

It will take you between 6 mo and 12 mo before you are able to return to sport following an ACL injury. It is definitely worth mentioning that earlier return to sport is associated with a higher rate of re-tear and additional surgeries. This is one of the reasons why you want a physical therapist who is skilled and experienced with rehabilitating ACL injuries. The length of your recovery is determined by a number of factors. Your age, the activity level you plan to return to and the type of graft that is used are just a few. Your rehabilitation takes place within 5 phases. Our blog ACL Recovery Timeline blog provides information on your rehab exercises, how you can expect to feel and what you might be able to do during each of these phases.

ACL Exercises

Each phase of your rehabilitation has exercises that will help you achieve the goals for that phase. Early on in your recovery you will be focused on movements that restore ROM and reduce swelling.

Quad Set

From there the exercises are focused on restoring strength while protecting your recovering joint and your healing graft.

Hip to Wall Endurance

As your strength progresses so will the resistance, complexity and speed of the exercises you are given.

Cork Screw

We place special emphasis on restoring your neuromuscular control skills like balance, coordination, and rate of force development.

Standing Dynamic Balance

We build strength in key muscles that have been shown to reduce the rates of re-injury such as your hamstrings and lateral hip rotators.

Nordic Curls

We also spend a great deal of time cleaning up your landing mechanics and your ability to change directions quickly with cutting drills.

Single Leg Box Hop

“Want to give a shout out for the amazing physical therapy here before and after ACL surgery with Jessica! Definitely recommend going here. Front desk staff are also super nice and helpful. Thank you Matthew!”

I’ve been going to Union PT for a little over a year now for various issues, and I’ve always had the best experience. Jon has been an incredible listener, and creates well rounded plans to ensure problems don’t persist. Last year I was nearly convinced I wouldn’t be able to ski again with a knee injury from the summer, and Jon had me stronger than before and ready to go before the snow even started to stick.

I honestly can’t say enough good things as a patient of this clinic, the admin team and the PTs I’ve seen here have been nothing but kind and proficient at their jobs. If you have climbing or skiing injuries come here!

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