ACL - Anterior Cruciate Ligament
The Anterior Cruciate Ligament (ACL) is a ligament deep inside the centre of the knee. It is the main structure within the knee that allows you to turn and pivot on the foot, for example changing direction whilst playing football or making a turn whilst skiing. Unfortunately, it is often during one of these activities that the ACL is injured. If someone wants to return to sport, the ACL usually needs to be reconstructed. However, injury to the ACL is often not as simple as “torn or not torn”. There may be a partial tear and sometimes the remaining ACL is enough to allow the knee to perform as it did before the injury. At the other end of the spectrum, other structures apart from the ACL may also be injured. Unless these other injuries are addressed, reconstructing just the ACL will not be enough. One of the other ligaments in the knee may be damaged as well, ie the MCL, PCL or LCL/PLC. Damage to these ligaments is often easy to pick up, particularly with a comprehensive examination.
The traditional teaching was that only one third of people who injure their ACL need it to be reconstructed. However, long term studies have shown that most people who want to return to an activity that requires turning or pivoting are unable to do so without a reconstruction. The remaining few who are able to return to sport without a reconstruction are often using their “secondary restraints” within their knee (primarily the menisci – see below). This can lead to damage to these secondary structures that can ultimately lead to early osteoarthritis.
The ultimate goal of any ACL reconstruction is to return the knee to ‘normal’. Trying to measure what normal is has proven to be quite difficult. Traditionally the way to measure how well an ACL has been reconstructed was with a machine (a KT-1000) that measures how far forward the tibia moves on the femur. When an ACL is ruptured, extra forward movement of the tibia is one way of diagnosing the injury. However, it is the rotational movement of the tibia on the femur that happens when the ACL is ruptured that is the cause of symptoms. Measuring this objectively has proved to be very difficult.
When arthroscopic surgery was first popularised in the 1980s, the goal with ACL surgery was felt to be to reconstruct a ligament that was “isometric” – ie it had the same tension throughout the range of motion of the knee. This could be done through a very small incision and very quickly. The KT-1000 measurements appeared to show that these reconstructions were performing well. For some people they undoubtedly did help. However long term studies showed that a large proportion of people were not returning to the same level of activity that they had been at prior to their ACL rupture. We already know that an ACL rupture increased the chances of developing osteoarthritis (OA) of the knee, other studies showed that this increased chance was not being reduced by ACL reconstruction. The technique of ACL reconstruction was re-visited to look at how we could improve the outcome of ACL reconstruction, as well as reduce the risk of developing OA.
Fundamentally it was the lack of rotational control of the knee that was the problem. Whilst the KT-1000 measurements were good, a reconstructed knee could rotate around a vertical graft. This would mean that turning and pivoting would be difficult and over use of the secondary restraints (see above) would occur, leading to OA. These vertical grafts were isometric, but they weren’t where the old ACL used to be.
This has led to a greater understanding of the anatomy of the native ACL. The ‘holy grail’ of modern ACL surgery is to reconstruct a graft that mimics the original ACL as closely as possible. The ACL is formed of two different bundles of fibres, wrapped around each other. The first attempts at trying to change how we reconstruct the ACL involved using hamstring grafts to make two discreet bundles. Following this, the position of where the tunnels are drilled in the knee were moved to a more anatomical position. Whilst the benefits of a double bundle reconstruction over a single bundle one are debatable, the modern position of the tunnels are undoubtedly an improvement. The position of the native ACL is more oblique within the knee, whereas the old ‘traditional’ position resulted in a more vertical graft. The oblique ‘anatomical’ graft is far better at controlling the rotational movement which is the cause of symptoms and is so damaging to the cartilage in the knee. This new anatomical position is likely to be better at preventing arthritis. However, it can take up to twenty years to develop arthritis in the knee, so it is too early to say for sure.
The native ACL is made of collagen, a type of protein. When it ruptures, it does not naturally heal itself into the same position. Unlike, for example, bone. Attempts in the past have been made to repair the ACL, but these usually failed. What is now standard treatment is replacing the old ACL with a new graft. The graft is most commonly a piece of donor tissue from the same knee. This is usually either two hamstring tendons (semitendinosus and gracilis) or the middle third of the patella tendon. There are advantages and disadvantages to both grafts. The patella tendon graft allows earlier mobilisation as it allows bone to bone healing, but the hamstring graft involves a smaller incision with less chance of anterior (towards the front) knee pain. Artificial grafts have been tried in the past, unfortunately most of them failed early and so are not an option today. Whilst this is true of grafts within the knee, artificial grafts outside of the knee still have a role. Allograft – donor tissue from a cadaver – is also occasionally an option. However, there is a higher revision (re-do) rate after use of allograft. I tend to use it when there are multiple ligaments to reconstruct, or in difficult revision cases, when the patient’s own tissue has already been used.