ACL injury

The ligaments around the knee are critical for its function. They guide, control and limit the motion of the bones in relation to each other. The anterior cruciate ligament (ACL) limits the forward movement of the tibia (shin bone) underneath the femur (thigh bone). Even more importantly, the ACL controls the rotation, or pivoting, of the tibia.

If the ACL is torn, the knee loses stability when subjected to twisting or “pivoting” motions. A person with an ACL deficiency will feel the knee slide out or give way when attempting pivoting activities.

Anatomy of the knee. The anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) are vital for the stability of the knee.

Intact ACL and PCL, seen during arthroscopy of the knee.

How is the ACL torn?

The ACL tears when it is stretched beyond its ability to resist. The mechanism of injury may be a direct blow to the leg (eg a rugby tackle), a sudden twisting movement either during a fall or while trying to pivot on a planted foot (a classic netball injury), or by hyperextending (over-straightening) the knee.

Depending on the force and direction of the injury, the ACL can either be partially or completely torn. Other structures within the knee can also be injured. The menisci within the knee, other ligaments, such as the collateral ligaments and the articular cartilage are all at risk.

The ACL is torn when the force applied to it exceeds its structural strength. This allows the tibia to rotate abnormally under the femur.

How is an ACL tear diagnosed?

The history of the injury usually points towards an ACL injury with immediate pain and giving way, often associated with a tearing or popping sensation. Swelling usually occurs within a few hours. Some patients who recover from their initial injury discover that their knee feels unstable or gives way when they attempt jumping or twisting motions.

Physical examination confirms the laxity in the knee in the front to back direction (Lachman test) and also with pivoting or rotating movements (Pivot-Shift test).

X-rays are usually normal, but may occasionally show a small piece of bone pulled off the tibia when the capsule of the knee tears with the ACL. An MRI scan is not usually required to diagnose an ACL tear, but may be used to determine the presence of other injured structures.

Interestingly, only about 10% of ACL injuries are diagnosed by the first doctor to see the patient.

What are the implications of a torn ACL?

During the event that results in a torn ACL the knee moves in a way it is not designed to. This abnormal movement can result in tears to the menisci and injuries to the articular cartilage or other ligaments. These may ultimately have a greater effect on the knee than the torn ACL per se. If you have one of these injuries, your surgeon will discuss that with you.

In the short term, the major implication of a torn ACL is ongoing knee instability. How much this affects a patient is highly variable depending on the patients other ligaments and muscles, the sporting and work demands placed on the knee. Some people are able to return to high-level sport without needing surgery, whereas other people struggle with even minor activity and need ACL reconstruction.

In the long term, it is recognized that rupturing the ACL is associated with increased rates of meniscus damage and joint degeneration. It is not clear whether the joint degeneration is related to ongoing knee instability or whether it reflects the damage inflicted on the cartilage in the initial injury.

Image: Right – Ruptured ACL stump, seen during arthroscopic ACL reconstruction surgery.

How is an ACL tear treated?

Management of an ACL injury is individualized. All patients must undertake a fairly intensive rehabilitation program to regain function and strength in their knee following ACL injury to get the best results, regardless of whether or not they have reconstruction surgery. Research into ACL treatment has shown that up to 50% of patients may do just as well, whether or not they undergo reconstruction of their ACL.

In general terms, the main reasons for having a surgical ACL reconstruction include:

  • Ongoing knee instability, or giving way episodes while trying to perform usual activities or sports
  • Desire to continue in high-demand pivoting sports
  • Significant meniscal damage requiring repair
  • Combined ligament injuries
  • Younger, active patients

Image right: ACL graft in place ACL reconstruction surgery.

What does the rehabilitation process involve?

The rehab process is similar regardless of whether or not you have surgery. It is divided into phases:

Phase I – Control of pain and inflammation

  • Rest, ice, compression and elevation
  • Pain relief medications
  • Use of crutches until able to walk without a significant limp

Phase II – Regain strength and function

  • Regaining strength in the muscles controlling knee function (hamstrings, quadriceps, calf muscles)
  • Retraining the knee joint with balance and agility exercises

Phase III – Return to exercise and sport

  • Gradual return to sport-specific exercises
  • Non-contact then contact-training
  • Return to sport

ACL reconstruction surgery

When the anterior cruciate ligament (ACL) is torn, it lacks the ability to heal itself. If you want to participate in high-demand sports, or are experiencing ongoing episodes of instability, the usual treatment is ACL reconstruction. ACL reconstruction involves using a portion of another tendon, usually the hamstring or patella tendon, as a substitute for the torn ACL.

The primary aim of surgery is to restore the stability to your knee and enable you to return to your chosen level of activity and sports. A secondary aim of surgery is to limit the ongoing damage to the meniscus and joint surfaces. This will hopefully decrease the chance of having significant arthritis in the long term.

Surgical technique

The surgery is performed under an anaesthetic. You will usually stay in hospital overnight following the procedure.

Small incisions are made on the front of the knee for arthroscopy. The arthroscope is introduced and a thorough internal examination of the knee is performed. At this stage any tears to the meniscus or cartilage injuries in the knee are dealt with.

The tendons to be used for reconstruction are then harvested. Most commonly, the middle third of the patellar tendon, or one to two parts of the hamstrings tendons are used. The choice of which graft to use is made based on a number of factors, including the type of activities you do, other injuries around the knee and the risks of re-injury.

Patellar tendon graft.

Hamstrings tendon graft.

Steps in ACL reconstruction

The insertion points of the ruptured tendon are identified on the femur (white oval) and tibia.

After removing the torn tendon ends, a tunnel is drilled into the femur.

A second tunnel is drilled through the tibia.

A suture is passed through both tunnels. This will be used to pull the graft into the knee.

The tendon graft is drawn into the knee through the tibial bone tunnel and into the femoral bone tunnel.

The graft is tightened in place and then secured into the tunnel at both ends to recreate the ACL.

The graft is checked to ensure proper graft tension and knee movement.

Getting ready for ACL reconstruction

Preparing for ACL reconstruction or pre-habilitation is important to the success of surgery. Ideally you need to have:

  1. A full range of knee movement
    If your knee if stiff prior to ACL reconstruction, regaining a full range of knee motion (ROM) after surgery can be very difficult. Thus having a full or near-full ROM is a pre-requisite for surgery. If you have a meniscal tear or scarring that is blocking your motion, this will be treated by arthroscopy prior to having ACL reconstruction. If loss of motion is simply due to stiffness, you will be referred for physiotherapy to regain movement prior to surgery.
  2. Good knee strength
    As much as is possible, your quads and other knee muscles should be as strong as those of your uninjured knee prior to surgery. This will make rehab post-surgery much easier and more rapid. If you have significant muscle wasting and weakness at assessment, you will be referred to physiotherapy for a strengthening program prior to surgery.
  3. Willingness to undertake rehabilitation
    Surgery is only the first step in successful ACL reconstruction. Much of the outcome depends upon your participation in a rehabilitation program. Typically this will take 45-60 minutes per day for the first 3 months and then about 30 minutes per day for the next 3-6 months, depending on your sporting requirements.

What are the risks of ACL reconstruction surgery?

The likelihood of a life-threatening surgical complication or damage to major blood vessels or nerves are very rare and unusual. The most common and important risks of ACL reconstruction that have been reported are:

Nerve injury (common)
Numbness beside the inner scar and partway down the inner shin is common. This is usually not a problem, but it can take several months for the abnormal feeling to diminish. Some of the numbness will be permanent.

Stiffness (uncommon).
This occurs in ~5% of patients and is usually treated by appropriate participation in rehabilitation. In about 1% of patients, an abnormal scarring response within the knee can occur. If this happens, further surgery may be needed to help restore the movement.

Infection (rare)
This is rare (0.2-0.4%), but if it occurs, can be devastating. Usually, further surgery is required to eradicate the infection Stiffness, cartilage damage and loss of the graft, are all risks with infection.

Recurrent instability (uncommon).
85% of patients have no further instability after reconstruction. Reinjury is the most common cause for further instability. Meniscal tears, leg weakness or pain can all contribute to feelings of instability, even if the ACL reconstruction is intact

Injury to nerves or blood vessels, blood clots, abnormal pain responses to surgery have all been reported after ACL reconstruction, but are rare.

What about returning to sport?

An ACL injury is a major and potentially career-ending injury for an athlete. The best hope for returning to pivoting sports is with ACL reconstruction combined with a comprehensive rehabilitation. When we assess athletes who have had ACL reconstruction 90% of them have normal or near-normal strength, stability and movement. In spite of this, only 45% return to competitive sport and only 40-60% get back to their pre-injury level.

Those who are most likely to return to their pre-injury function are those who play seasonal sports (ie they can rehab through the off-season) and those who have good return of function on performance testing, for example single leg hop for distance being >85% of the other side. Males are also more likely to get back to their pre-injury functional level than females.

The rehabilitation program and you progress through physio and surgeon assessments will guide your rate of return to sports. In general terms, low resistance cycling may commence after 1-2 weeks, jogging after 3 months. It usually takes a minimum of 9 months before a full return to contact or pivoting sports.

When can I return to work?

Return to sport and work depends on a number of factors to do with your particular activities as well as how well your rehabilitation is progressing.

In general terms:

  • Sedentary work after 1-2 weeks
  • Light duties, involving being on your feet for a prolonged period of 6 weeks or so
  • Moderate duties involving carrying loads on uneven ground, climbing ladders etc after 3 months
  • If you have very heavy duties, or those requiring agility on uneven ground, significant pivotingetc, you may not be able to get back to full duties for 4 months or more.

What are the risks of reinjury?

A number of studies have followed up athletes returning to sport after ACL reconstruction. A systematic review of these studies has shown that the rate of rupturing the ACL graft in the 5 years following reconstruction is about 6%. Interestingly, the risk of rupturing the ACL in the opposite knee was nearly 12%

Those at increased risk of further ACL injury are:

  • Athletes <20 years old
  • Those playing pivoting or contact sports (soccer, netball, rugby, basketball, tennis)
  • Females

Because of this risk, we strongly recommend that all athletes who have been treated for ACL injury regularly perform an ACL injury prevention warm-up program as part of their training.