ACL stands for anterior cruciate ligament.  The anterior cruciate ligament is one of a pair of cruciate ligaments in the centre of the human knee.  These two ligaments form part of a complex ligamentous structure which provides stability to the human knee.  The ACL is a relatively short ligament with two bundles and connects the top of the tibia to the end of the femur within the knee joint.  Its main function is to provide stability to the knee mainly stopping the tibia from translating too far anteriorly.  The ACL provides around 85% of the restraining force to this anterior tibial displacement. The ACL is the most injured ligament of the four located in the knee.  

What is ACL injury?

The ACL is the most common type of knee injury and the most commonly injured knee ligament. The ACL usually tears as a result of sudden change in direction causing the knee to rotate inwards. This causes an excessive force to go through the knee joint and through the ACL and if this force exceeds the mechanical strength of the ACL the ACL will tear. This usually happens in sporting injuries and can be due to contact, for example during a tackle and sometimes and mostly in a non-contact situation when landing from a jump awkwardly. The most common sports are football for men and netball in women that result in ACL injury. ACL injuries can also occur in other sports where twisting around the knee can take place such as skiing. Injuries to the knee can also occur during accidents such as trips and falls down the stairs or road traffic accidents.

What are the symptoms of ACL injury?

Following an injury to the ACL the knee swells up immediately (this is called a haemarthrosis which is bleeding into the knee).  Immediately following the injury, it is very difficult to bear weight through the injured joint and the range of movement is usually very restricted due to the severe bleeding and swelling into the knee.  It is very important at this stage to seek immediate advice and attention from a qualified physiotherapist or GP.  The immediate treatment for this is usually icing the knee and keeping the range of movement going.  The pain and swelling usually takes two to three weeks to settle down and for some movement in the knee to return. 

How is an ACL injury assessed?

In this early stage of injury an assessment from a physiotherapist and investigations such as an MRI scan are very important.  This helps to delineate the extent of the injury.  Sometimes the ACL can be just sprained and not completely torn, in which case physiotherapy alone is required to rehabilitate the knee and bring the patient back up to pre-injury levels of function. 

The ACL is usually injured with other structures within the knee, for example the meniscus, the cartilage or other ligaments.  An MRI scan will help to pick this up and your surgeon will be able to appreciate the full extent of the injury and therefore plan appropriate treatment.

What causes ACL rupture after reconstruction?

The primary ACL reconstruction surgery can fail due to a number of reasons.  This can be due to a recurrence of trauma despite being no other factors within the knee itself.  This is very unfortunate and does however happen.  An athlete can make a full recovery from the primary surgery and go back to sport and have a further injury that can re-rupture the ACL.  This can be simply coincidental and unfortunate or sometimes can be due to a too early return to sport or lack of tone, strength and conditioning of the muscles in the leg predisposing to injury.

Other factors that can result in re-rupture of the ACL and reconstruction surgery are surgical and technical factors.  These can range from poor tunnel placement, small graft size and missing other structural injury at the time of primary surgery, for example meniscal tears, meniscal ramp lesions or other ligamentous instability.  If there are other structures that are injured in the primary injury and not addressed as part of the primary ACL reconstruction, this results in instability in other parts of the knee that can predispose the ACL reconstruction to re-rupture and failure.