ACL injury

Background:

Anterior cruciate ligament (ACL) injuries can occur in contact or non-contact situations.

Non-contact injuries occur when a person is running or jumping and then suddenly slows and changes direction (cutting) or pivots in a way that involves rotating or bending the knee sideways. Common sports include downhill skiing, gymnastics, and tennis.Contact-related ACL injuries usually occur from a direct blow forcing the knee backwards (hyperextension) or forced inwards with rotation, often when the foot is immobile, for example in rugby, football and martial arts.

The ACL provides stability to the knee joint, particularly on rotational movements. It acts both as a fixed restraint  to prevent abnormal pivoting of the knee, but is also full of nerve endings that provide feedback about the position of the knee joint (proprioception) and hence is vital to balance. 

Injury prevention:

Women have a higher risk of ACL injuries than men, although the exact reason for this is not clear. This risk can be significantly reduced in both men and women by 'prehab' ie neuromuscular training building balance and strength both around the knee, but also around the hip, gluteal muscles and core. 

Balance exercises can be done cheaply and easily at home either with single leg balance, a 'wobbleboard' or dynamically with side hopping on either leg. Muscle strength can be achieved with lunges and squats. Core stability and gluteal exercises can also be done at home without gym access.

Acute injury:

The classic signs of an ACL injury are a pain, and a feeling (or even hearing) a 'pop' from the knee. It is not uncommon for it not to be too painful and the injured may even try to 'run it off' but often the feeling of instability and collapse will supervene. The ACL has a rich blood supply (unlike the meniscal cartilages for example) so it will bleed causing knee swelling within a few hours, whereas meniscal tears will cause a more gradual fluid build up over 24 hours for example. The only other major cause of rapid onset knee swelling would be a fracture, so an acute bloody knee swelling (haemarthrosis) should always be considered an emergency.

In addition to the ACL injury, the medial collateral ligament (MCL) on the inner side of the knee is also commonly injured, and the medial meniscus can also be torn, so a careful examination of the knee is mandatory.

Conservative management:

If a conservative course is followed,  a large number of patients, probably the majority, will regain normal use of the knee. This does however rely on keeping up muscle strength and balance exercises. If however despite this the knee is unstable, then repeated episodes of collapse may damage the knee, and hence lead to further injury, or scuffing the surface leading to arthritis. If the knee is not unstable however, there is no evidence that arthritis is greater than if surgery is performed.

Obviously, the younger you are, the more lifetime risk of reinjury and later problems there are. With natural ageing the knee tends to become a little stiffer so can cope with ACL injury potentially better, so ACL reconstruction in the over 50s is performed far less than in the 20s. If a conservative programme fails (ie symptomatic instability after rehab) or in high demand individuals (doing high level sports for example) then ACL reconstruction can be beneficial. 

ACL reconstruction:

Over 15,000 ACL reconstructions are performed annually in the UK, which is about 10 times higher than it was 15 years ago. We use the term 'ACL reconstruction' rather than repair since the ends once separated can't heal, and so an alternative tissue is needed as a scaffold. In the US using donor graft tissue is popular, but there is a much higher re-rupture rate due possibly to a low level grumbling inflammation. In the UK, the use of a graft such as part of the hamstring tendons, or part of patellar tendon (which is taken with a sliver of bone at each end, and so commonly referred to as 'bone-tendon-bone, or 'BTB'). The choice is largely surgeon and patient preference, but over the last 20 years it has become vastly more common to do an initial surgery with hamstring tendon, as there are cases of chronic knee pain and reduced ability to kneel and jump with BTB graft.

The graft tissue, once remover from the body is effectively dead, and needs reincorporating by substitution of the tissue by living fibrous tissue, a process that takes about 9 months to complete. The initial stages of this process are accompanied by an actual reduction in strength of the graft, so contact sports are not advised before 9 months. By the time sport-specific skills and confidence is regained, it is often a year from surgery for peak recovery.

Overall however, ACL reconstruction surgery is very successful for about 90 to 95% of patients enabling them to get back to sport, but that means somewhere between 5 and 10% of people go through surgery and remain with instability. The reasons for this are several, but may be related to a failure of maturation of the graft tissue, reinjury, infection, or not working hard enough on the necessary rehabilitation. As pointed out above, the ACL is not only a fixed restraint, but also has an important role in balance and proprioception, which need a lot of balance training. Some patients therefore may have a successful surgery obliterating laxity, but still have a loss of confidence in the knee due to insufficient neuromuscular retraining. 

 

The surgery:

The operation involves a general anaesthetic. It takes between 60 and 90 minute depending if additional procedures are needed such as trimming meniscal tears, dealing with damaged articular surfaces etc. It can be done as a day case, but commonly a single night stay in hospital is booked.

 

The hamstring graft is actually 2 lengths of hamstring tendons (semitendinosus and gracilis) folded over to create a 'four strand graft'. These are harvested from a 4cm incision on the inner side of the leg below the knee, and are actually stripped out 'closed' which can cause bleeding and bruising that may track down the leg post operatively as far as the foot. The graft ends have tough sutures fixed to them, and the graft tensioned to ensure it will not stretch too much once inserted.

The knee is inspected using an arthroscope via two small 5mm incisions at the front of the knee. Holes are drilled in the femur via another small incision, and through the tibia using the same incision as the hamstring harvest. The femoral tunnel can be drilled using an instrument called a 'flipcutter' that deploys a tomahawk-like blade to reverse cut a tunnel, which allows more accurate positioning than the classical route through the tibial tunnel or through the front of the knee.

The graft is pulled into the femur and held there with a tough cord attached to a small metal button, then the graft tensioned appropriately and fixed in the tibia using a screw (I use a non-metallic screw and sheath that dissolve over a few years) Full range of movement is checked, and importantly making sure the graft does not impinge against the femur in extension (if it does, the notch in the middle of the knee may need opening up slightly, a 'notchplasty') Finally the knee is washed out of any debris, and the wounds stitched up.

Postoperative care:

Immediately postoperatively full weight-bearing is permitted, but the quads muscles 'switch off' and so crutches are needed for balance for anything from one to three weeks, and beyond this with one crutch for commuting on public transport. There is a bulky bandage around the knee which can be removed at 48 hours, leaving the stitch covering dressings unless bloodstained until suture removal at 10 days post op. Physio exercises are basically knee bending and straightening (without forcing it passively) with leg lifts for the first two weeks, then closed chain exercises. 

Physio rehab progresses in a goal oriented fashion, ie reaching specific goals before progressing to the next stage rather than particular time points, but as a rough guide the first 6 weeks concentrate on range of movement, balance and muscle control, 6 weeks to 4 months are intensive muscle strengthening, and impact (light jogging) introduced at 4 or 5 months. Only when the patient can hop, jump, jog and turn is cutting allowed, and it is 9 months before full contact play is allowed.

A knee brace should not be necessary unless other repairs in the knee have been performed, but if the knee is re-injured in the future the redo repair is much more complex, so for activities such as skiing a knee brace to provide additional support may be advised. Statistically however patients tend to injure the other knee more than reinjuring the reconstructed knee!