ACL Injury (Anterior Cruciate Ligament Tear)
An ACL (anterior cruciate ligament) injury is one of the most common serious knee injuries, typically caused by a sudden change of direction or direct blow to the knee. Mr Shakir Hussain, Consultant Orthopaedic Surgeon at the Royal Orthopaedic Hospital Birmingham, assesses ACL injuries and advises on treatment including long-term management to prevent post-traumatic knee arthritis.
What is an ACL injury?
An ACL injury is a tear or rupture of the anterior cruciate ligament, one of the two cruciate ligaments inside the knee joint. The ACL runs diagonally across the centre of the knee and is the primary restraint against the shin bone (tibia) sliding forward on the thigh bone (femur). It also controls rotational stability. ACL tears cause immediate pain, swelling, and instability, and significantly increase the long-term risk of knee osteoarthritis.
The knee contains four main ligaments: the medial collateral ligament (MCL), lateral collateral ligament (LCL), anterior cruciate ligament (ACL), and posterior cruciate ligament (PCL). The two cruciate ligaments cross inside the joint, forming an X-shape when viewed from the side. The ACL specifically prevents the tibia from translating forward relative to the femur and controls internal rotation of the tibia, which is essential for pivoting and cutting movements.
ACL tears occur most commonly through two mechanisms. The more frequent is a non-contact injury where the foot is planted and the body rotates suddenly, as when changing direction in football or landing awkwardly from a jump. The second mechanism is direct contact, such as a tackle that forces the knee into valgus (knock-kneed) position.
Women are two to eight times more likely to sustain an ACL injury than men in equivalent sports, due to differences in anatomy, hormonal influences on ligament laxity, and neuromuscular activation patterns. Older patients may sustain ACL injuries from lower-energy falls, and the knee is more likely to develop early post-traumatic arthritis in this age group.
What does an ACL injury feel like?
A pop or crack at the time of injury is the hallmark feature, followed by immediate pain and rapid swelling of the knee (haemarthrosis) within 2 to 4 hours. The knee often feels unstable or gives way when pivoting or changing direction. Many patients can still bear weight but feel that the knee is not reliable.
Symptoms can vary depending on the severity of the tear and whether other structures are injured at the same time. An isolated ACL tear has a characteristic pattern, but associated meniscal or cartilage damage can worsen both the immediate symptoms and the long-term outlook.
Pop or crack at injury
A sudden audible or felt pop or crack inside the knee at the moment of injury is reported by approximately 70 per cent of patients with a complete ACL tear. It represents the ligament failing under tension.
Rapid swelling (haemarthrosis)
The knee fills with blood quickly after an ACL tear, typically becoming visibly swollen within 2 to 4 hours. A tense, painful haemarthrosis (blood in the joint) is a strong indicator of significant ligament or bone injury.
Instability or giving way
A sense that the knee is unreliable or will give way, particularly when pivoting, cutting, or descending stairs. Chronic instability without surgery leads to repeated episodes that damage the menisci and cartilage over time.
Loss of range of motion
Acute swelling and pain restrict how far the knee can bend or straighten in the first days after injury. Some patients cannot fully straighten the knee if a bucket-handle meniscal tear has occurred at the same time.
Difficulty weight-bearing
Most patients can bear some weight after an ACL injury, but walking is painful and the knee feels unreliable. Complete inability to weight-bear suggests an associated fracture or severe multi-ligament injury.
Joint-line tenderness
Tenderness along the medial or lateral joint line indicates an associated meniscal injury, which occurs alongside an ACL tear in approximately 50 per cent of cases and significantly affects long-term prognosis.
What causes an ACL injury?
ACL injuries are most commonly caused by non-contact pivoting or deceleration movements in sport, such as a sudden change of direction, landing from a jump, or stopping abruptly. Contact mechanisms, such as a direct blow to the outer knee forcing it inward, account for fewer cases. Women sustain ACL injuries at significantly higher rates than men in equivalent sports.
The activities most commonly associated with ACL injury include football, rugby, skiing, basketball, and netball. Skiing causes a high proportion of contact ACL injuries through edge-catches and falls. In older adults, low-energy falls or missteps on uneven ground can produce similar injuries.
Key risk factors include:
- High-impact sport. Any sport involving rapid direction changes, jumping, and contact carries high ACL risk. Football, rugby, and skiing account for the largest number of ACL injuries in the UK.
- Female anatomy. Women have a narrower intercondylar notch (the tunnel the ACL passes through), a wider pelvis increasing the valgus angle at the knee, and greater ligament laxity under oestrogen influence, all of which increase ACL vulnerability.
- Neuromuscular factors. Inadequate hamstring-to-quadriceps strength ratios and poor neuromuscular control of knee position during landing significantly increase injury risk. Preventive training programmes addressing these factors reduce ACL injury rates by 50 per cent.
- Prior ACL injury. A previous ACL tear on either side approximately doubles the risk of a subsequent ACL injury, whether due to residual neuromuscular deficits, graft characteristics, or altered movement patterns.
- Ground surface and footwear. High-traction surfaces, particularly artificial turf, and aggressive stud patterns increase the rotational forces transmitted through the knee during a planted-foot pivot.
How is an ACL injury diagnosed?
Diagnosis combines clinical examination and MRI. The Lachman test and anterior drawer test directly assess ACL integrity, while the pivot shift test reproduces the instability the patient reports. MRI confirms the diagnosis, grades the tear, and identifies any associated meniscal or cartilage damage that will influence treatment planning.
Mr Hussain will take a history of the injury mechanism, immediate symptoms, and current functional limitations. Examination includes assessment of swelling, range of motion, and specific ligament tests:
- Lachman test. The most sensitive clinical test for ACL injury. The knee is held at 30 degrees of flexion and the tibia is pulled forward. Excessive anterior translation and absence of a firm endpoint indicates ACL rupture. Sensitivity exceeds 85 per cent in skilled hands.
- Anterior drawer test. With the knee at 90 degrees, the tibia is drawn forward. Less sensitive than the Lachman test in acute injury due to hamstring guarding, but useful in subacute presentations.
- Pivot shift test. Reproduces the giving-way sensation of ACL deficiency by applying a valgus and internal rotation force while extending the knee. Highly specific for ACL injury and useful for quantifying rotational instability.
- MRI. The gold standard for confirming ACL diagnosis. MRI visualises the ligament directly, grades the tear (partial or complete), identifies bone bruising patterns characteristic of ACL injury, and detects associated meniscal and cartilage damage.
- X-rays. Do not show the ACL itself but are taken to exclude a Segond fracture (a small avulsion at the lateral tibial margin that is pathognomonic of ACL injury) and to assess for associated bone injury.
Does an ACL injury always need surgery?
No. Many patients, particularly older or less active individuals, can manage well with physiotherapy and a structured rehabilitation programme. Surgery is typically considered for younger active patients, those with ongoing instability, or those with combined ligament injuries. The goal of either approach is to restore function and reduce the long-term risk of post-traumatic knee arthritis.
Non-surgical management is appropriate for patients who do not wish to return to high-demand pivoting sports, older patients with lower functional expectations, and those whose instability is controlled adequately with rehabilitation. Key components include:
- RICE protocol (initial phase). Rest, ice, compression, and elevation in the first 48 to 72 hours reduce haemarthrosis and pain. Gentle range-of-motion exercises begin as soon as tolerated.
- Physiotherapy and neuromuscular training. Strengthening the hamstrings, quadriceps, hip abductors, and calf muscles compensates for ACL deficiency. Proprioceptive and balance training reduces the risk of giving-way episodes.
- Functional knee brace. A hinged knee brace provides external rotational stability and may allow return to certain sports for patients who decline surgery, though evidence for its efficacy in high-demand sport is limited.
- Activity modification. Avoiding high-risk pivoting activities reduces the frequency of giving-way episodes and slows cumulative meniscal and cartilage damage.
The key limitation of non-surgical management is ongoing instability. Each giving-way episode risks additional meniscal or cartilage damage, accelerating the progression to post-traumatic knee arthritis. For this reason, younger patients with an active lifestyle are generally better served by ACL reconstruction.
ACL injury: surgical and non-surgical options
ACL reconstruction replaces the torn ligament with a tendon graft, restoring stability and allowing return to sport in most patients at 9 to 12 months. For patients who develop post-traumatic knee arthritis years after an ACL injury, knee replacement surgery in Birmingham by Mr Hussain offers lasting relief of pain and restored mobility.
Mr Hussain advises on the full spectrum of ACL management, from structured rehabilitation to ACL reconstruction, and for patients in whom post-traumatic arthritis has developed following an ACL injury, he offers robotic total or partial knee replacement.
ACL Reconstruction
ACL reconstruction replaces the torn ligament with a tendon graft, most commonly from the hamstrings (semitendinosus and gracilis) or the patella tendon. The graft is fixed inside tunnels drilled in the femur and tibia. Recovery to return-to-sport takes 9 to 12 months with a structured rehabilitation protocol.
- Restores rotational stability of the knee
- Reduces the risk of secondary meniscal damage from instability
- Allows return to high-demand pivoting sport in most patients
- Hamstring or patella tendon graft options, chosen for the individual
- Physiotherapy rehabilitation 9 to 12 months post-surgery
Knee Replacement
When post-traumatic osteoarthritis develops years after an ACL injury and symptoms substantially affect daily life, knee replacement surgery is the appropriate next step. Mr Hussain offers robotic total knee replacement for these patients, with meticulous attention to alignment in the complex post-ACL knee.
- Suitable when post-traumatic arthritis causes persistent pain and disability
- Robotic-assisted surgery for precise alignment in the complex knee
- Total knee replacement is most commonly indicated
- Most patients mobilising the same day as surgery
- Revision surgery expertise available if previous surgery has failed
Long-term outlook after ACL injury
An ACL injury permanently alters knee biomechanics and significantly increases the long-term risk of post-traumatic knee osteoarthritis, with or without reconstruction. When arthritis does develop and requires knee replacement, National Joint Registry data shows approximately 90 per cent of total knee replacements are still functioning at 15 years.
The long-term risk of knee osteoarthritis after ACL injury is substantial. Studies show that between 50 and 80 per cent of patients develop radiographic knee osteoarthritis within 10 to 20 years of an ACL tear. This risk is higher when meniscal damage accompanies the ACL injury, which occurs in approximately half of cases.
ACL reconstruction reduces the frequency of instability episodes and the secondary meniscal damage they cause, but does not eliminate the long-term arthritis risk entirely. This is because the original injury disrupts normal knee mechanics in ways that persist even with a functioning graft.
For patients who reach the point of needing knee replacement after a previous ACL injury, Mr Hussain has the expertise to manage the additional complexity these knees present, including bone tunnel defects from prior surgery, altered anatomy, and ligament balance challenges. His robotic certifications on MAKO, ROSA, and CORI allow precise planning tailored to each individual knee.
If you have had a previous ACL injury and your knee is becoming increasingly painful or arthritic, post-traumatic knee arthritis and knee replacement surgery in Birmingham are the relevant pages on this site for further information.
Expertise in knee treatment in Birmingham
Consultant at the Royal Orthopaedic Hospital
Mr Hussain practises at the Royal Orthopaedic Hospital Birmingham, one of the largest specialist orthopaedic hospitals in Europe, alongside Priory Hospital Edgbaston and Harborne Hospital.
3,000+ arthroplasty cases
From a total of more than 5,000 procedures performed, giving the operative volume and case complexity required for consistently excellent outcomes. Read more about Mr Hussain's training and background.
MAKO, ROSA, and CORI robotic certifications
Mr Hussain holds certifications on all three major robotic knee replacement platforms, allowing robotic-assisted surgery to be offered to all suitable patients, including those with complex post-ACL anatomy.
Complex revision surgery expertise
Mr Hussain performs revision knee replacement for patients whose primary replacement has failed, including for failed ACL reconstruction with subsequent arthritis. See revision knee surgery for more information.
Doctify Outstanding Patient Experience 2024, 2025, and 2026
Awarded in three consecutive years, recognising consistently high patient-reported outcomes and communication throughout the surgical journey.
4.98 out of 5 from verified reviews on Doctify. Outstanding Patient Experience Award 2024, 2025, and 2026.
Frequently asked questions about ACL injury
For more questions about surgery, recovery, fees, and what to expect, see the full frequently asked questions page or read recent patient testimonials.