ACL injury
Rupture of the anterior cruciate ligament (ACL) - one of the most common serious knee injuries in sport. ACL reconstruction is recommended for young active patients wishing to return to pivoting sport.
📊 ACL injuries affect approximately 1 in 3,500 people per year in the UK. Female athletes are 2-8 times more likely to sustain an ACL injury than male athletes in the same sport.
What is an ACL injury?
The anterior cruciate ligament (ACL) runs obliquely from the posterior femur to the anterior tibia and is the primary restraint to anterior translation and internal rotation of the tibia. ACL injuries typically occur through non-contact mechanisms (sudden deceleration, pivoting, landing from a jump) or direct contact to the knee. The injury is characterised by a pop, immediate swelling (haemarthrosis), and inability to continue playing.
ACL ruptures do not heal spontaneously because of the intra-articular environment and the disruption of normal anatomy. An untreated ACL-deficient knee is at risk of episodes of giving way, progressive meniscal damage, and early-onset knee osteoarthritis. ACL reconstruction using an autograft (the patient's own tissue - typically hamstring or bone-patellar tendon-bone) replaces the ACL and restores knee stability, allowing return to pivoting and cutting sport.
The KANON trial and other high-quality RCTs have demonstrated that structured rehabilitation without surgery can achieve equivalent functional outcomes to ACL reconstruction at 5-10 years in patients who modify their activity to avoid pivoting sport. However, patients wishing to return to high-level cutting and pivoting sport require reconstruction. Return to pre-injury level of sport after ACL reconstruction takes 9-12 months and should be guided by functional criteria (limb symmetry indices) rather than time alone.
Common causes
- Non-contact pivoting, deceleration, or landing mechanisms (most common)
- Direct contact to the knee (valgus stress)
- Hyperextension injury
- Contact sport (football, rugby, basketball, skiing, netball)
Who is at risk? Female athletes are disproportionately affected due to biomechanical, neuromuscular, and hormonal factors. Adolescents undergoing rapid growth, athletes with high training loads in pivoting sports, and those with a narrow femoral intercondylar notch are at increased risk.
Symptoms
Symptoms vary depending on the severity and duration of the condition. Common symptoms include:
- A pop at the time of injury followed by immediate pain
- Rapid swelling (haemarthrosis) within 1-2 hours
- Inability to continue playing
- Giving way episodes with pivoting and cutting movements after the initial injury
- A positive anterior drawer or Lachman test on clinical examination
When to seek help: Attend A&E after a significant acute knee injury with a pop and immediate swelling. MRI should be arranged to confirm the diagnosis and assess for associated meniscal and chondral injuries.
How is it diagnosed?
Your surgeon will take a detailed history and examine the joint. The following investigations may be arranged to confirm the diagnosis:
- Clinical examination - Lachman test (most sensitive, over 85%), anterior drawer test, pivot shift test
- MRI - confirms ACL rupture, grades it (partial vs complete), and identifies associated injuries (meniscal tears in 40-60% of ACL injuries, bone bruising, chondral damage)
- X-ray - to exclude bony avulsion fracture (Segond fracture associated with ACL rupture) and assess for associated fractures
The Segond fracture (a small avulsion fracture of the lateral tibial plateau) is pathognomonic of ACL injury. Associated meniscal tears are present in 40-60% of ACL injuries. These should be addressed at the time of ACL reconstruction if surgery is planned.
Treatment pathway
Treatment is tailored to the severity of the condition, your age, activity level, and overall health. Most conditions are treated in a stepwise fashion, starting with the least invasive options.
ACL reconstruction
The torn ACL is replaced with an autograft (hamstring tendons or bone-patellar tendon-bone are most common). Performed arthroscopically under general anaesthetic. The graft is fixed with screws or buttons in bone tunnels drilled in the femur and tibia. Return to pivoting sport takes 9-12 months.
Structured rehabilitation without surgery
A progressive neuromuscular rehabilitation programme (MOON, KANON protocol) focusing on quadriceps and hamstring strength, proprioception, and movement pattern correction. Appropriate for patients willing to avoid pivoting sport. Equivalent outcomes to surgery in RCTs for non-pivoting activities.
Physeal-sparing or transphyseal reconstruction
Modified techniques are used in skeletally immature patients to minimise the risk of growth disturbance. The timing and technique depend on the stage of skeletal maturity and require specialist assessment.
Recovery
Return to sport after ACL reconstruction should be guided by limb symmetry index (LSI) testing rather than time. A LSI of over 90% for quadriceps and hamstring strength and hop tests is required before return to pivoting sport. Premature return to sport significantly increases re-rupture risk.
- Surgery: 6-12 weeks after injury (after swelling settles and movement returns)
- Return to jogging: 3-4 months
- Return to cutting and pivoting: 6-9 months
- Return to full sport: 9-12 months
What results can I expect?
ACL reconstruction is successful in 85-90% of patients who comply with the rehabilitation protocol. Re-rupture rates are 5-25% for return to sport, higher in young athletes, females, and those returning before adequate rehabilitation. Re-rupture risk is reduced by meeting LSI criteria before return and by ACL injury prevention programmes.
ACL injury - reconstruction and return to sport
In numbers
Typical activity timelines for this condition. These are approximate and vary considerably between patients. Always follow the specific guidance given by your surgeon and physiotherapist.
| Activity | Typical timeline | Notes |
|---|---|---|
| See a knee specialist | Within weeks | Early assessment helps plan rehabilitation and decide on surgery, even if early management is non-operative.[3] |
| Start rehab early | From injury | Pre-operative or first-line rehabilitation focused on range of motion and quadriceps strength is essential.[4] |
| Drive after ACL reconstruction | 2-4 weeks | When off crutches, comfortable in the car, and able to perform an emergency stop. Sooner for left knee in an automatic.[5] |
| Return to work | 2-12 weeks | Desk work: 2-4 weeks. Standing/light manual: 6-8 weeks. Heavy manual or kneeling work: 12 weeks or more.[3] |
| Cycle and swim | 6-12 weeks | Stationary bike from around 6 weeks. Swimming once the wound is healed.[3] |
| Straight-line running | 3-4 months | Once quadriceps strength, balance, and confidence have returned.[3] |
| Return to pivoting sport | 9-12 months | Return to cutting and pivoting sport at 9-12 months, guided by functional testing and strength symmetry.[3] |
Common concerns from patients with this condition or recovering from treatment, and whether they are expected or worth mentioning to your team.
Your questions, answered
Plain-English answers to the things people most often ask, grounded in UK clinical guidance. Tap a question to open it.
About thisWhat does the ACL do?
The anterior cruciate ligament, or ACL, is one of the main stabilising ligaments inside the knee. It stops the shin bone sliding forward and helps control twisting. Tears usually happen during a sudden change of direction, landing or twisting, often in sport, and frequently with a pop and rapid swelling.
SymptomsHow do I know if I have torn it?
Many people describe a pop at the time of injury, the knee swelling within a few hours, and a sense that the knee is unstable or might give way, especially on turning. A scan and an examination by a knee specialist confirm the diagnosis.
TreatmentDo I have to have a reconstruction?
Not necessarily. Some people manage well without surgery, particularly if their knee feels stable and they avoid high-pivoting sport, using a structured rehabilitation programme instead. Reconstruction is usually recommended for those who want to return to pivoting sports or whose knee keeps giving way despite rehab.
RecoveryHow long before I can play sport again?
Return to pivoting sport after reconstruction typically takes around nine to twelve months, not weeks. Rushing back raises the risk of re-injury, so progression is guided by regaining strength, control and confidence rather than the calendar alone.
Living with itWhat happens if I leave it untreated?
A knee can function well without an ACL for everyday activities, and many people do. The concern is repeated episodes of giving way, which can damage the menisci and cartilage over time. The right choice depends on your activities, symptoms and goals.
When to worryWhat needs earlier review?
A knee that locks, repeatedly gives way, or cannot bear weight should be reviewed sooner, as these can signal an associated cartilage or meniscal injury that may change the treatment plan.
- 1.NHS: Knee ligament injuries
- 2.Cochrane Review: ACL reconstruction versus non-surgical management
- 3.British Association for Surgery of the Knee: Patient information
- 4.Chartered Society of Physiotherapy: Patient information
- 5.Royal College of Surgeons of England: Recovering from surgery, driving guidance
References are to UK clinical guidance and patient information from recognised organisations. This page is for general information and does not replace personalised advice from your own clinical team.
Preparing for surgery?
Read our step-by-step guide - what to expect before, during, and after your procedure.
How is it diagnosed?
- ✓Clinical examination - Lachman test (most sensitive, over 85%), anterior drawer test, pivot shift test
- ✓MRI - confirms ACL rupture, grades it (partial vs complete), and identifies associated injuries (meniscal tears in 40-60% of ACL injuries, bone bruising, chondral damage)
- ✓X-ray - to exclude bony avulsion fracture (Segond fracture associated with ACL rupture) and assess for associated fractures
Recovery milestones
- ●Surgery: 6-12 weeks after injury (after swelling settles and movement returns)
- ●Return to jogging: 3-4 months
- ●Return to cutting and pivoting: 6-9 months
- ●Return to full sport: 9-12 months