ACL injury

The anterior cruciate ligament helps stabilise the knee. Most tears happen without contact with another player — in a landing, a sudden stop or a change of direction — and are typically felt as a pop, followed by swelling within a few hours and a knee that feels unreliable when it turns. Assessment is a history and an examination of both knees, with imaging used to confirm the tear and to look at the meniscus and cartilage surfaces, which are often injured at the same time.

Symptoms

  • A pop or a snap felt in the knee at the moment of injury.
  • Swelling that comes on within the first few hours rather than over the following days.
  • The knee giving way, or feeling as though it is about to, when turning or changing direction.
  • Difficulty straightening the knee fully, and difficulty putting weight through it in the first days.
How ACL injuries happen

Most tears of the anterior cruciate ligament happen without contact with another person. The usual pattern is a sudden deceleration, a landing or a change of direction in which the knee rolls inwards over a planted foot while the shin rotates — the movement common to cutting and pivoting sports, to skiing falls and to ordinary missteps. Contact injuries make up a smaller share and tend to follow a blow to the outside of the knee. Because the force passes through the whole joint rather than the ligament alone, bruising within the bone is visible on MRI in the large majority of acute tears, and the meniscus and cartilage surfaces are frequently involved.

How it is assessed

Assessment begins with how the problem started and what the knee has done since, followed by an examination of both knees so the affected side can be compared with the other. Imaging is used where it would change what is done, rather than as a matter of routine.

In clinic

The examination compares how far the shin can be drawn forward on the thigh bone, and whether it shifts as the knee is straightened, against the uninjured side. The Lachman test is the most sensitive single test for a torn ligament; the pivot shift test is the most specific, though it is harder to perform in a painful, guarded knee. Swelling, range of movement and joint line tenderness are assessed at the same time, because they indicate what else may be injured.

Imaging

An X-ray is taken where a fracture is possible. MRI confirms whether the ligament is torn and, as importantly, shows the meniscus, the cartilage surfaces, the collateral ligaments and the pattern of bone bruising. Those findings often matter more to the decision about treatment, and to its timing, than the ligament tear itself.

Stage and severity

Injuries are described by the pattern of damage and by whether other structures in the knee are affected.

Partial tear
Some fibres are torn and the rest remain intact. The knee may still test close to normal. Whether the remaining ligament is controlling the joint matters more than the proportion of fibres involved.
Complete tear
The ligament is torn through. The shin moves forward and rotates further than on the uninjured side, and the knee is more likely to give way in activities that involve pivoting.
With other injuries
A meniscal tear, damage to a cartilage surface or a collateral ligament injury alongside the ACL tear. These combinations are common, and a repairable meniscal tear in particular usually carries more weight in the decision than the ligament tear on its own.
Associated injuries

The meniscus is the structure most often torn alongside the anterior cruciate ligament, including tears at the back of the medial meniscus where it meets the joint capsule, which are found in a substantial minority of cases and are easily missed. Damage to the cartilage surfaces, injury to the medial collateral ligament and bruising within the bone are also common; a small avulsion fracture at the outer edge of the shin bone, where it appears, is itself a marker of an ACL tear. Some of these injuries are repairable and some are not, and finding them changes both what is done and when — which is why the scan is read as a whole rather than for the ligament alone.

Treatment options

The two options below are set out side by side. Which applies depends on the findings, the structures involved and what you need to return to.

Non-surgical care

What it involves
A structured rehabilitation programme: restoring full movement, rebuilding quadriceps and hamstring strength to match the other leg, and retraining control of the knee in landing, turning and deceleration. Bracing is used selectively rather than as a matter of course.
Typical course
Supervised at first, then continued independently over several months. Progress is judged on what the knee can do — movement, strength compared with the uninjured side, and control under load — rather than on elapsed time.
Considerations
Suits a knee that stays stable in daily activity, and work or sport that does not demand repeated pivoting. A randomised trial comparing rehabilitation with the option of later surgery against early reconstruction found similar patient-reported outcomes at five years, with about half of the rehabilitation group never needing an operation. Reconstruction remains available if the knee continues to give way.
Follow-up
Reviewed as rehabilitation progresses, with the decision about surgery revisited rather than closed off.

Surgical reconstruction

What it involves
The torn ligament is replaced with a graft — commonly hamstring tendon, patellar tendon or quadriceps tendon — passed through tunnels in the thigh bone and shin bone. Meniscal or cartilage injuries found at the same time are dealt with in the same operation.
Typical course
Movement and weight-bearing begin early, followed by a staged strengthening and return-to-sport programme. Return to pivoting sport is not usually considered before nine months, and is judged on strength and functional testing as well as on the date.
Considerations
Usually advised where the knee gives way, where there is a repairable meniscal tear, or where work or sport requires reliable pivoting. Current guidance recommends reconstructing an acute isolated tear within three months, as the risk of further meniscal and cartilage damage rises with longer delay.
Follow-up
Reviewed through rehabilitation, with functional testing before return to sport is cleared.

What recovery involves

Recovery depends on which treatment is used. The stages below apply to reconstruction, and the timings are typical rather than fixed — progress is judged on what the knee can do at each stage.

  1. Settling the knee

    The first two weeks

    Reduce swelling, regain full straightening, restore quadriceps activation and walk without a limp.

  2. Movement and strength

    Weeks 2 to 12

    Full range of movement, progressive strengthening of the whole leg, and normal walking and stair mechanics.

  3. Running and agility

    Months 3 to 6

    Return to running, then to hopping, cutting and change of direction, with strength approaching that of the other leg.

  4. Return to sport

    From around nine months

    Sport-specific training and a testing battery — strength symmetry, hop testing and confidence in the knee — before full return.

Page information

Reviewed by
Dr Lai Kah Weng
Version
1.0
Review date
September 2026
Next review
September 2027

References

  1. American Academy of Orthopaedic Surgeons. Management of Anterior Cruciate Ligament Injuries: Evidence-Based Clinical Practice Guideline. 2nd ed. Rosemont, IL: AAOS; 2022.
  2. Benjaminse A, Gokeler A, van der Schans CP. Clinical diagnosis of an anterior cruciate ligament rupture: a meta-analysis. J Orthop Sports Phys Ther. 2006;36(5):267–288.
  3. Frobell RB, Roos HP, Roos EM, Roemer FW, Ranstam J, Lohmander LS. Treatment for acute anterior cruciate ligament tear: five year outcome of randomised trial. BMJ. 2013;346:f232.
  4. Grindem H, Snyder-Mackler L, Moksnes H, Engebretsen L, Risberg MA. Simple decision rules can reduce reinjury risk by 84% after ACL reconstruction: the Delaware-Oslo ACL cohort study. Br J Sports Med. 2016;50(13):804–808.

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