An anterior cruciate ligament sprain or tear is one of the commonest knee injuries. People who play fast sports such as basketball, skiing, football and rugby are more likely to injure their cruciate ligaments.

When you injure your cruciate ligament, the treatment depends on the severity of the injury and your level of activity. You may need surgery to regain the full function of your knee in the activities you wish to perform.

The anterior cruciate ligament or ACL is one of the “big four” knee ligaments and stabilises the knee. With an ACL tear the knee is usually unstable in quick movements, decelerations and twisting manoeuvres.

Anterior Cruciate Ligament Anatomy

The knee has four major ligaments to keep it stable and guide movement during normal activities. The cruciate ligaments control forward and back movements of the knee joint and are important in the joint position sense of the knee.

The anterior cruciate ligament consists of two main bands of tissue making up a dense, strong, rounded ligament. It runs from the femur above to the tibia below. The ACL is the main limiter of “anterior translation of the tibia” when the shinbone slides forwards under the thighbone. It also contributes to control of knee rotation and sideways movement.

ACL Tear Illustration

Anterior Cruciate Ligament Injuries

Around 50% of ACL injuries involve damage to other knee structures, such as the joint cartilage, other ligaments and the menisci.

A sprain is the name given to an injured ligament and ACL injuries are graded by their severity:

  • Grade 1 Sprain. The ligament has sustained mild damage such as a stretch and is still capable of its stabilising role.
  • Grade 2 Sprain. The ligament is stretched beyond its recovery point and is then lengthened and loose. This may be called a partial tear and may result in reduced knee stability.
  • Grade 3 Sprain. Complete rupture of the ligament so it cannot function to stabilise the knee in the normal way.

Most ACL injuries are complete tears or almost complete tears.

What Causes Anterior Cruciate Ligament or Rupture?

An ACL injury is often caused in low violence activities such as:

  • Decelerating from speed when running
  • Rapid change of direction at speed (cutting)
  • Sudden stopping from speed
  • Landing awkwardly after jumping
  • Contact injuries where rotation is occurring in the joint at the time

Around half of people with ACL injuries also suffer from a meniscus tear at the same time, mostly of the medial meniscus.

Who Gets Anterior Cruciate Ligament Injury?

ACL injury can occur in many activities, but the most risky activities are football, skiing and basketball. Women are more likely to suffer ACL injury in such activities. There are many proposed reasons for this but the exact reasons are not known.

What Are The Consequences Of ACL Injury?

An ACL-deficient knee may have damage to the medial meniscus and the cartilage covering of the knee joint. Pain and instability (a tendency of the knee to give way under loads) can be a continuing problem. An increased risk of degenerative changes over time has lead to ACL reconstruction becoming very frequent.

ACL Tear Symptoms

  • Non-contact injuries involve the athlete suddenly changing direction or landing from jumping on a straight leg whilst rotating.
  • The knee swells quickly over a few minutes or hours as the ruptured ligament bleeds into the knee joint.
  • The swelling, pain and lack of stability means that players cannot continue with the activity they were doing.
  • High trauma and contact causes a series of injuries such as ACL injury, MCL injury and medial meniscus injury all together
  • There may be loss of full motion of the knee, tenderness along the joint itself and pain on walking.

Clinical Examination

Assessment of the stability of the knee and the state of the ACL cannot usually be performed soon after the injury. The swelling and pain makes examination difficult and the knee first needs to settle with physiotherapy and rest.

A doctor will take the history and examine the knee. X-rays may be taken to show whether there is any bony injury around the knee. MRI scanning shows the ligament well but is not usually necessary to make the diagnosis as the examination usually does this.

Treatment Of An ACL Tear

What treatment is recommended depends on the age, activity and needs of the patient. A young sports person will likely need a reconstruction to be able to go back to what they love doing. An older person with a more sedentary lifestyle may manage with rehabilitation and strengthening.

As an ACL tear will not heal, the doctor will decide whether non-surgical rehabilitation or surgery is the best option.


A physiotherapist will manage a recent ACL injury by using the PRICE protocol. A knee brace might be necessary if the knee is unstable with crutches to limit weight bearing on a painful knee. Ice, a compression bandage, elevation of the leg and gentle muscle contractions will settle the knee down.

Once the knee is settled and the swelling has subsided, assessment of the stability of the knee is much easier. Physio treatment concentrates on developing hamstring and quadriceps strength and on restoring the joint position sense of the knee. Much of the work is done with the foot on the ground (closed kinetic chain exercises) rather than exercises such as quads bench knee extensions (open kinetic chain exercises).

ACL Surgery

The ends of the torn anterior cruciate ligament cannot be stitched together again. This would not result in a strong enough repair to give the knee its stability back.

The ACL has to be replaced with suitable tissue that will become strong in time and hold the knee stable.

ACL ruptures are not operated on in the acute stages as this increases the risk of fibrosis occurring within the joint. An avulsion fracture, where the attachment of one end of the ligament is pulled off the bone, may be operated on early to relocate the fragment

ACL Reconstruction is typically performed more than three weeks after injury but not later than six months as meniscal injury and poorer repair can result from delay. Arthroscopic knee surgery is now the preferred method of ACL reconstruction.


Surgical Techniques

ACL Repair Illustration

Bone-Patella-Bone (BTB) Autografts

An autograft means a graft from the same person who is having the repair, using a part of his or her own body. A piece of bone is taken from the patella and the tibial tuberosity (the large lump below the knee at the front) with part of the patellar ligament connecting the two. This makes up the new anterior cruciate ligament once it is installed in the knee. Anterior knee pain, pain at the front of the knee associated with the patella, is a known complication in 10-40% of subjects.

Hamstring Tendon Grafts.

HT grafts are popular as patients suffer less anterior knee pain and it allows faster recovery. Studies have shown HT grafts have a comparable performance to BTB autografts.


Allografts are taken from someone else, typically a deceased person. There have been problems with transmission of infections and potential immune problems.

Problems with failure of synthetic grafts and swelling in the knee have stopped their use.

Non-Operative Treatment

Older people or those who do not stress their knees with risky sports may be managed with physiotherapy to regain full power and range of motion.

Physiotherapy After ACL Surgery

  • Phase 1 – the preoperative period. The physiotherapist will concentrate on full range of motion, muscle strength and improving joint position sense (proprioception)
  • Phase 2 – from operation to the two-week period. The goals are to regain full knee extension, achieve 90 degrees knee bend, limit swelling and achieve normal gait
  • Phase 3 – from three to five weeks. Increase up to full bend, maintain and strengthen extension, progress to functional exercises such as static cycling and steppers
  • Phase 4 – six weeks onwards. Continue with strengthening, range of movement and functional work. Return to sports may take from six to nine months and should be supervised by the physio and surgeon.

Knee Bracing

A knee brace is commonly used, for a variety of knee injuries, to limit movement to prevent damage and to stabilise the joint while healing occurs.

Specialised anterior cruciate braces may limit sliding of the shinbone forwards under the thigh bone (called anterior translation), at least under low loads. They may also reduce the reaction time of the hamstring muscles, which may be a negative effect


  1. Knee Ligament Surgery. NHS Choices.
  2. Kiapour AM and Murray MM. Basic Science of anterior cruciate ligament injury and repair. Bone & Joint Research 2014; 3: 20-31.
  3. Anterior Cruciate Ligament (ACL) Injuries. American Academy of Orthopaedic Surgeons.