The primary functions of the ACL are to prevent forward movement of the lower leg (tibia) on the femur (thigh), to control outward rotation of the lower leg when the knee is in a flexed (bent) position and to send sensory feedback to the brain providing information on joint position; so called “proprioception”.
A good simplification is that the ACL allows you to pivot. If the ACL is not working you will generally find it relatively easy to walk and run in a straight line, cycle and swim. However if you attempt to twist (e.g. side stepping in football/rugby) or land on one leg (e.g. basketball, netball etc.) then the knee can ‘give way’. This is called instability. A knee that has an injured ACL and which is potentially unstable is called ACL-deficient.
Many patients with injuries to their knee that result in mild to moderate laxity can cope without an ACL reconstruction if they modify their lifestyle to avoid pivoting sports. If you wish to pursue pivoting activities in the future (e.g. football, skiing, rugby, tennis, squash, basketball, netball) then an ACL reconstruction might be recommended.
If your knee gives way, there are two main effects. Firstly the knee is painful immediately after each episode of instability, with swelling, pain, loss of motion of the knee and a limp – this is usually transient but painful at the time. Secondly, and perhaps more importantly, each episode of giving way causes internal damage within the knee. The meniscus cartilage and the articular cartilage (the lining covering the bones) can be damaged by these episodes of giving way, resulting in osteoarthritis in the knee.
An ACL can be injured by a non-contact force or by a contact injury.
In a non-contact acute ACL injury, the knee suddenly gives way on attempted twisting. The injury is very painful. A pop or snap can often be heard by the patient and often by others. Only rarely can the patient resume the sport they were playing and on attempting to do so the knee gives way and feels very unstable. There is usually early marked swelling of the knee, usually within one hour of the injury. It is usually very difficult to walk on the knee in the first few hours of the injury.
In a chronic ACL-deficient knee, the knee gives way in a less dramatic but nonetheless painful fashion. The knee can feel perfectly normal on walking and running but when an attempt is made to pivot the knee suddenly and painfully gives way. This causes swelling and pain in the knee but often in a less marked fashioned that the acute injury.
ACL tears can often go undiagnosed as, although the knee is swollen and painful immediately following the injury, this often resolves over 5-10 days. After a period of recovery (e.g. 3-4 weeks) the knee can feel remarkably normal as there is no pain, swelling, limp or instability on linear activities. It is only when an attempt is made to return to pivoting activities that there is a problem with instability.
An ACL reconstruction is a procedure in which a tendon or ligament is used to replace the damaged ACL. It is not a repair of a ligament but a replacement where an alternative tissue is used; usually a hamstring tendon or kneecap (patella) tendon graft.
There are many ways of performing an ACL reconstruction. The methods of operative reconstruction can vary, although a typical procedure will comprise of several steps.
It is vitally important that you rest for the first 1-2 weeks after your operation. Much better results from ACL reconstructions have been noted where patients thoroughly rest and elevate their leg in the initial period after their operation.
When you go home (usually the day after your operation) you should rest as much as you can, almost as if you are in hospital. Although you are resting over this period, you will be advised to move the knee as much as possible – the combination of resting the knee, elevating it and moving it often gives very pleasing results, such that you will be able to walk almost freely and without a limp by 10 days or so after your operation. If you walk around on it too much the knee can swell, the knee loses movement and you can limp for quite some time.