Anterior Cruciate Ligament (ACL) Tear

What is an ACL tear?

The anterior cruciate ligament is a strong band in the middle of the knee that connects the thighbone to the shinbone. It stops the shinbone from sliding forward and limits excessive twisting of the knee. When it tears, the knee may feel unreliable, especially during turning and sudden changes of direction ^[1].

A tear can be partial or complete. The injury mostly affects young, physically active people. The injury can lead to a sense of instability, reduced activity and, over time, lower knee-related quality of life ^[4].

Symptoms

Symptoms usually start at the moment of injury. Many people describe a tearing feeling or a 'pop' inside the knee ^[1]. Common symptoms include:

• A popping sound and sudden pain at the time of injury ^[1]
• Swelling that develops within a few hours ^[1]
• The knee giving way or feeling as if it shifts out of place ^[1]⁠^[3]
• Difficulty fully bending or straightening the knee ^[1]
• Tenderness along the joint line and discomfort when walking ^[1]

Causes and risk factors

Most ACL tears happen without contact with another player. Stopping suddenly while running, twisting the body while the foot is planted, cutting sharply or landing from a jump with the knee collapsing inwards are typical movements that overload the ligament ^[1].

Sports that involve pivoting, cutting and jumping, such as football, basketball and volleyball, carry a higher risk. Less often, a direct blow to the knee can also tear the ligament ^[1].

An ACL tear often comes with other damage. A meniscus tear, injury to the joint cartilage or a tear of the collateral ligaments on the sides of the knee may occur at the same time, and these injuries directly affect the treatment plan ^[1].

Diagnosis

Diagnosis starts with a careful examination. The doctor compares the injured knee with the healthy one and uses specific tests to check how loose the ligament is. In many cases the diagnosis can be made from the examination alone ^[1].

X-rays do not show ligaments, but they can reveal a small piece of bone pulled off by the ligament or an associated fracture. Magnetic resonance imaging (MRI) is the best test for showing the ligament as well as soft tissues such as the meniscus and cartilage ^[1].

Treatment without surgery

Not every ACL tear needs surgery. Treatment without surgery may suit people with a partial tear and no giving way, people with a complete tear who have no symptoms in everyday life, and people with a less active lifestyle. An initial period of about 12 weeks of non-surgical care is recommended ^[2].

The core of non-surgical care is a structured exercise programme guided by a physiotherapist. The aims are to strengthen the front (quadriceps) and back thigh muscles, restore knee movement, and improve balance and muscle control. Medicines advised by the doctor may be used for pain and swelling ^[1].

Systematic reviews of randomised trials suggest that starting with rehabilitation, with the option of surgery later if needed, can lead to knee function scores similar to early surgery, although the certainty of this evidence is low to very low ^[4]⁠^[5].

In these trials, a considerable share of people who started with rehabilitation later chose surgery because their knee kept giving way. So even when surgery is not chosen at first, regular follow-up and attention to how the knee behaves remain important ^[4].

Surgical treatment

The most common operation is reconstruction. Because a torn ACL usually does not heal when simply stitched, a new ligament is created using a graft taken from the patient's own tendons or from donor tissue. In some selected tears, particularly when the ligament has come away from its attachment to bone, repair may be considered ^[1].

Surgery is especially relevant for people who want to return to sports involving pivoting and sudden direction changes, people whose knee gives way in daily life, and those who also have meniscus, cartilage or other ligament injuries. In growing children and adolescents, surgery is often preferred to lower the later risk of meniscus and cartilage damage ^[2].

The decision is made together with the patient, taking into account age, sporting goals, work demands and examination findings. No single approach is best for everyone ^[2]⁠^[5].

Recovery

Physiotherapy starts straight after surgery. Early goals are to reduce swelling, regain full straightening of the knee and activate the thigh muscles. Over the following months the focus moves to strength, balance, and jump-and-landing control. Commitment to the exercise programme is a key factor in a good result ^[2].

Most people return fully to sport between 6 and 12 months. The timing depends less on the calendar and more on strength, range of motion and control of the knee; pain should have settled, movement should be full and strength adequate ^[2].

When to see a doctor urgently

Seek emergency care or see an orthopaedic specialist without delay if ^[3]:

• You cannot put any weight on the leg or cannot move the knee at all ^[3]
• The knee looks deformed or swells severely and quickly ^[3]
• The knee is locked in one position or keeps giving way with painful clicking ^[3]
• The knee is red and hot and you have a high temperature (this may signal infection) ^[3]