What is an ACL tear?
The anterior cruciate ligament (ACL) is one of the four main ligaments that stabilise your knee. It runs diagonally through the centre of the joint and stops the shin bone (tibia) from sliding forward past the thigh bone (femur), while controlling rotation.
An ACL tear happens when this ligament is overstretched or ruptured — usually during a sudden stop, a change of direction, a hard landing, or a direct blow to the knee. Tears range from partial to a complete rupture, where the ligament is split into two.
It is one of the most common injuries in pivoting sports such as football, basketball, skiing and handball. The good news: with modern diagnosis and treatment, the large majority of patients return to the activities they love.
Signs and symptoms
Many patients describe a distinct “pop” at the moment of injury, followed by the knee giving way. Common signs include:
- checkA “pop” or snap felt or heard at the moment of injury.
- checkRapid swelling within the first few hours, caused by bleeding inside the joint.
- checkA feeling of instability — the knee buckles or “gives way,” especially when turning.
- checkPain and tenderness along the joint line, often worse with weight-bearing.
- checkReduced range of motion and difficulty fully straightening or bending the knee.
Significant swelling with an inability to bear weight, or a knee that locks and cannot be straightened, should be assessed promptly to rule out an associated meniscus or cartilage injury.
Causes & risk factors
Roughly 70% of ACL tears are non-contact injuries — the ligament fails during the athlete’s own movement rather than from a tackle. Typical mechanisms and risk factors include:
- Sudden deceleration combined with a change of direction (cutting or pivoting).
- Landing awkwardly from a jump, with the knee collapsing inward.
- Direct contact to the side of the knee.
- Female athletes carry a higher relative risk, linked to anatomy and neuromuscular factors.
- Previous injury, fatigue, and inadequate movement technique.
How it is diagnosed
Diagnosis begins with a careful history and a hands-on examination. Specific tests — the Lachman and pivot-shift tests — assess how much the tibia moves forward, indicating ligament integrity.
Imaging
An MRI scan is the gold standard. It confirms the ACL tear, shows whether it is partial or complete, and reveals associated injuries to the meniscus, cartilage or other ligaments — which together shape the treatment plan. X-rays may be taken to exclude a fracture.
An ACL tear is rarely an emergency. The priority is a precise diagnosis of the whole knee — because what surrounds the ligament often matters as much as the ligament itself.
Treatment options
Not every ACL tear needs surgery. The right path depends on your age, activity level, the degree of instability, and any associated injuries.
Structured rehabilitation
For lower-demand patients or partial tears, a guided physiotherapy programme can restore strength and stability. Progress is monitored closely for any return of instability.
ACL reconstruction
For athletes and unstable knees, the ligament is rebuilt arthroscopically using a graft. This is the most reliable route back to pivoting sport.
Where surgery is chosen, it is usually preceded by a short course of “prehab” to settle swelling and restore motion — which measurably improves the final outcome.
Recovery & outlook
Whether treated with rehab or surgery, recovery is a structured, staged process. With a reconstruction, a typical timeline looks like this:
Settle the knee
Control swelling, restore full extension, and regain quadriceps activation.
Restore motion & strength
Progressive loading, full range of motion, and a return to normal walking and daily life.
Build & run
Strength training, gradual return to running, and sport-specific drills.
Return to sport
Return is criteria-based — guided by strength and movement testing, not the calendar alone.