The anterior cruciate ligament or ACL is one of the “big four” knee ligaments and stabilises the knee in complex ways. With an ACL tear a person’s 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 the movements it is allowed to do during normal activities. The cruciate ligaments, anterior and posterior, 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 restrictor of “anterior translation of the tibia” which is the tendency for the shinbone to slide forwards under the thighbone. It also contributes to control of knee rotation and sideways movement.

ACL Tear Illustration

What Causes Anterior Cruciate Ligament Injury or Rupture?

An ACL injury is often caused in low violence activities such as decelerating from speed or contact injuries where rotation is occurring in the joint at the time. Hyperextension, where the knee is forcibly over-straightened, is also liable to damage the ligament or result in a torn ACL.

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?

Although ACL injury can occur in many activities, the most risky activities are football, skiing and basketball. Women, although having much lower numbers, are significantly more likely to suffer ACL injury in such activities.

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. There may be an increased risk of degenerative changes over time, which had lead to anterior cruciate ligament reconstruction becoming a very popular operation.

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 injuries typically occur as a series of injuries such as ACL injury, MCL injury and medial meniscus injury all together due to the knee being pushed forcibly into knock-knee.

Treatment Of An ACL Tear

An acute ACL injury is managed by a physiotherapist using the PRICE protocol. A knee brace might be necessary if the knee is unstable and crutches may be required to allow limited weight bearing on a very painful knee. Ice, a compression bandage, elevation of the leg and gentle muscle contractions will be used to 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 of a knee with a torn ACL 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

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 later than three weeks after injury but sooner than six months as meniscal injury and poorer repair can result from too long a 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 having the repair, using a part of their own body. A piece of bone is taken from the patella and from the tibial tuberosity (the large lump below the knee at the front) with part of the patellar ligament connecting the two making 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 they suffer less anterior knee pain and allow faster recovery in patients. Studies have shown HT grafts have a comparable performance to BTB autografts.


Allografts are taken from someone else, typically a deceased person. While popular in the past, there have been problems with transmission of infections and potential immune problems.

Synthetic grafts have been used but problems with failure of the graft and swelling in the knee have led to their use being stopped.

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 getting or maintaining full range of motion, increasing knee muscle strength and improving joint position sense (proprioception).
  • Phase 2 – from operation to the two-week period. The goals here 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 knee flexion 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 the unhelpful sliding of the shin bone 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 also 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.

Last Review Date: 06-09-2017

Next Review Date: 06-09-2019