Anterior Cruciate Ligament (ACL) Injury Questions - Answered

Anterior Cruciate Ligament (ACL) Injury Questions - Answered

ACL tear questions answered


Anterior cruciate ligament injuries can often be life changing, both in terms of work/personal life but also within sport and exercises. We cover a range of commonly asked questions including recovery times, treatment options and if surgery is for everyone.

What Is The Anterior Cruciate Ligament?

The anterior cruciate ligament (ACL) is one of four major ligaments that stabilises the knee joint. A ligament is a tough band of fibrous tissue, similar to a rope, which connects the bones together at a joint. There are two ligaments on the sides of the knee (collateral ligaments) that give stability to sideways motions: the medial collateral ligament (MCL) on the inner side and the lateral collateral ligament (LCL) on the outer side of the knee. Two ligaments cross each other (therefore, called “cruciate”) in the centre of the knee joint: The crossed ligament toward the front (anterior) is the ACL and the one toward the back of the knee (posterior) is the posterior cruciate ligament (PCL). The ACL prevents the lower bone (tibia) from sliding forward too much and stabilises the knee to allow cutting, twisting and jumping sports. The PCL stops the tibia from moving backwards.

How Can The ACL Tear?

The most common mechanism that tears the ACL is the combination of a sudden stopping motion on the leg while quickly twisting on the knee. This can happen in a sport such as basketball, for example, when a player lands on the leg when coming down from a rebound or is running down the court and makes an abrupt stop to pivot. In football, soccer, or lacrosse, the cleats on the shoes do not allow the foot to slip when excess force is applied. In skiing, the ACL is commonly injured when the skier sits back while falling. The modem ski boot is stiff, high, and is tilted forward. The boot thus holds the tibia forward and the weight of the body quickly shifts backwards. When excessive force is suddenly applied to the knee, the ACL gets injured. Similarly, a contact injury, such as when the player is clipped in football, forces the knee into an abnormal position. This may tear the ACL, MCL and other structures.

What Are The Signs That An ACL Is Torn?

When the ACL tears, the person feels the knee go out of joint and often hears or feels a “pop”. If he or she tries to stand on the leg, the knee may feel unstable and give out. The knee usually swells a great deal immediately (within two hours). Over the next several hours, pain often increases and it becomes difficult to walk.

What Other Knee Structures Can Be Injured When The ACL Tears?

The meniscus is a crescent shaped cartilage that acts as a shock absorber between the femur and tibia. Each knee has two menisci: medial (inner) and lateral (outer). The menisci are attached to the tibia. When the tibia suddenly moves forward and the ACL tears, the meniscus can become compressed between the femur and tibia resulting in a tear. The abnormal motion of the joint can also bruise the bones. There is a second type of cartilage in the knee joint called articular cartilage. This is a smooth, white glistening surface that covers the ends of the bones. The articular cartilage provides lubrication and as a result, there is very little friction when the joint moves. This joint cartilage can get damaged when the ACL tears and the joint is compressed in an abnormal way. If this articular cartilage is injured, the joint no longer moves smoothly. Stiffness, pain, swelling and grinding can occur. Eventually, arthritis can develop. The MCL and other ligaments in the joint can also be disrupted when the ACL tears. This is more common if an external blow to the knee causes the injury (such as if the knee was clipped while playing football, or when skiing).

What Is The Initial Treatment For A Torn ACL?

The initial treatment of the injured joint is to apply ice and gentle compression to control swelling. A knee brace and crutches are used. The knee should be evaluated by a doctor to see which ligaments are torn and to be sure other structures such as tendons, arteries, nerves, etc. have not been injured. X-rays are taken to rule out a fracture. Sometimes an MRI is needed, but usually the diagnosis can be made by physical examination.

How Will The Knee Function, If The ACL Is Torn?

If no structure other than the ACL is injured, the knee usually regains it range of motion and is painless after six or eight weeks. The knee will often feel “normal”. However, it can be a “trick knee”. If a knee does not have an ACL it can give way or be unstable when the person pivots or changes direction. The athlete can usually run straight ahead without a problem but when he or she makes a quick turning motion, the knee tends to give way and collapse. This abnormal motion can damage the menisci or articular cartilage and cause further knee problems.

If a person does not do sports and is relatively inactive, the knee can feel quite normal even if the ACL is torn. In young athletic patients, however, the knee will tend to reinjure frequently and 

give way during activities in which the person quickly changes direction. Therefore, it is usually recommended to reconstruct the torn ACL.

When Should Surgery Be Performed For a Torn ACL?

It is best to wait for the pain and swelling to subside and to allow associated injuries to heal before performing reconstructive surgery for the ACL. If surgery is done too soon after injury, rehabilitation is more difficult. In fact; the knee may get stiff and have permanent loss of motion. The athlete will usually get back to sports much more quickly if the knee is allowed to recover from the initial injury and to regain its full painless range of motion (usually about six weeks) before performing surgery.

In addition, regaining full motion prior to surgery allows an accelerated rehabilitation program to be used after surgery. With accelerated rehabilitation, continuous passive range of motion (CPM) is used for the first 2 weeks following surgery. The knee heals feels better more quickly, resulting in better initial joint function.

The best treatment following acute ACL injury is usually to protect the joint, apply ice and use crutches for several weeks. As the swelling and pain subside, and the patient can put weight on the leg; then the brace and crutches can be discontinued. The emphasis is on regaining knee motion. Resistive exercises to build up strength should not be done during this time to prevent damaging the knee cap and causing chondromalacia patella.

If the knee also has an injured medial collateral ligament (MCL), it is best to allow the MCL to heal completely (usually six to eight weeks) before reconstructing the ACL. The torn MCL usually does not need to be repaired surgically unless its healing is incomplete.

There may be instances when immediate surgery is indicated following injury. Examples are knee dislocations in which multiple ligaments are torn. Tears of the outer knee ligaments (lateral collateral ligament) often require timely surgical repair. Individual decisions need to be made on whether or not to reconstruct the ACL soon after injury in such instances where immediate surgery may be required.

Do All ACL Injuries Need Surgery?

No — some knees function reasonably well despite having a torn ACL. Good knee function without surgery is more common in older patients who are relatively inactive in sports. Patients who are younger, regardless of activity level, tend to have problems with instability and frequent episodes of giving way. Therefore, surgical reconstruction of a torn ACL is usually recommended for these patients. 

Treatment Options for an ACL Tear

1. Non-Operative

Some patients can function well even if the ACL is torn. However, it may be necessary to modify activities and avoid high-risk sports (such as basketball, soccer and football). The key to prevent the knee that has a torn ACL from giving out is to avoid quick pivoting motions. Wearing a knee brace can help to prevent re-injury. The main effect of a knee brace is to be a constant reminder to be careful.

However, a brace will not completely stabilise a knee that has a torn ACL. Exercises that restore the muscle strength, power, coordination, and endurance will also improve knee function and help stabilise the knee. However, a fully rehabilitated knee that has a torn ACL can still give way if a quick change in direction occurs.

2. Limited Arthroscopic Surgery

Many knees in which the ACL is torn have additional injuries such as torn menisci or fragments of articular cartilage that are displaced (creating a loose body and a defect in the articular cartilage). These associated injuries can cause symptoms of pain, swelling, and locking (in addition to symptoms of giving way due to the torn ACL). Arthroscopic surgery to remove torn menisci or to remove loose bodies can improve pain and eliminate locking. However, arthroscopy alone usually does not eliminate symptoms of instability, i.e. giving way.

3. ACL Reconstruction

Surgical reconstruction of a torn ACL involves replacing the torn ACL with soft tissue (called a “graft”, usually a tendon or ligament) from another part of the knee. The graft is then placed into a position to replace the function of the torn ACL. The most commonly used graft is taken from the middle third of the patellar tendon (the tendon connecting the knee cap to the tibial bone). Hamstring tendon grafts taken from the inner thigh on the back of the knee are also used. Occasionally, tendon grafts are taken from cadavers (referred to as “allograft”).

For most of these procedures, the operation is done arthroscopically instead of making long incisions. The knee is examined arthroscopically and associated injuries such as torn menisci, loose bodies, etc are treated. If the middle third of the patellar tendon is used, a small incision is made on the inner side of the leg just below the knee to take the graft (this results in numbness on the front of the knee). While viewing the inside of the joint through the arthroscope, guides are used to create bone tunnels in the exact positions to allow anatomic placement of the graft. The graft is then pulled into the bony tunnels. Absorbable screws are placed in the tunnels to wedge the bone graft against the wall of the tunnel to give immediate stability and allow healing of the bone graft.

Postoperatively, an accelerated rehabilitation program allows the most rapid return of function. This necessitates using a continuous passive motion (CPM) machine for at least 10 hours per day for the first week following surgery. The patient can get up whenever he or she wishes for short periods of time using crutches and a knee brace.

The CPM is typically arranged by your surgeon. It is a small device that sits on the bed and very slowly moves the knee continuously. The knee actually has less pain and regains its function much more quickly if CPM is used. If the knee is taken out of the CPM for periods of time, it becomes stiff and more painful. Therefore, it is best to devote the first week following surgery to continuous use of the CPM at home. Two weeks after surgery, the sutures are removed and the patient can walk bearing full weight on the leg. Crutches are used from 1-6 weeks, depending on the type of graft that was used for the reconstruction.

When Can I Expect To Return To Sports Following Surgery?

Within two weeks after surgery, most patients are walking on level surfaces without a limp. Typically, a stationary bike can be used on post-operative day one. Most patients can be in a swimming pool after their sutures are removed at 2 weeks. When the knee has full range of motion (usually at six to eight weeks), muscle-strengthening exercises are done. At four months, jogging is permitted in a sports brace. Full sports competition is allowed once strength has returned, usually between 6-8 months. Full recovery takes approximately one year.

When Can I Expect To Return To Activities After Surgery?

Most people can get back to desk work or sedentary activity one or two weeks after surgery. If the right knee has been reconstructed, it may be several weeks before the knee is strong enough to drive safely. For heavy work, it may take 3-6 months before the patient is medically cleared to return with no restrictions.

The following table provides a general guide of when it is safe to begin returning to work and sport dependant on its requirement.

Return to work and sport following ACL reconstruction


What Are The Risks And Benefits Of ACL Treatment?

Non-Operative Risks

  • Repetitive injuries or giving way may cause further permanent joint damage
  • Arthritis
  • Inability to participate in sports that require pivoting


  • Avoidance of potential surgical complications


Operative Risks

  • Permanent numbness in the front of the knee near the incision
  • Other nerve injury
  • Patello-femoral pain (kneecap)
  • Flexion contracture (stiffness/reduced motion of the knee)
  • Reinjury (knee becomes unstable again)
  • Swelling
  • Superficial infection
  • Deep infection
  • Deep vein thrombosis (blood clots)
  • Delay in regaining motion
  • Vascular (damage to blood vessels)
  • Death


  • Return to work and sports with a stable knee
  • Potentially prevent further injury to the knee
  • Remain physically active

Recovery of any ACL injury can be a long process and patience, persistence and correct guidance are three vital pillars to success. Conservative rehabilitation programmes are usually implemented before opting for surgery. Operative treatments should always be a last resort.

Our 52 week ACL injury rehabilitation programme is a progressive exercise plan, working through your stages of recovery. It aims to improve your strength, range of movement and joint control as your graduate through the stages of rehabilitation following both the injury itself and surgery (if applicable).


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Arun Gray sports therapist strength and conditioning coach skegness

The author

Arun Gray is a sports therapist and strength & conditioning coach with over 15 years experience in the industry. He also has a personal history with chronic shoulder and back pain along with a range of other sporting injuries.

Arun writes about common injuries and aims to help people understand and manage their pain to prevent having to rely on national healthcare.

Read more about Arun