The Anterior Cruciate Ligament (ACL) is one of the most important of four strong ligaments connecting the bones of the knee joint. It is often injured.
Ligaments are strong, dense structures made of connective tissue that stabilize a joint. They connect bone to bone across the joint.
The function of the ACL is to provide stability to the knee and minimize stress across the knee joint:
- It restrains excessive forward movement of the lower leg bone (the tibia) in relation to the thigh bone (the femur).
- It limits rotational movements of the knee.
A tear to the anterior cruciate ligament (ACL) results from overstretching of this ligament within the knee.
- It's usually due to a sudden stop and twisting motion of the knee, or a force or "blow" to the front of the knee.
- The extent of the tear can be a partial or a complete tear.
- Individuals experiencing a tear to the ACL may or may not feel a pop at the time of the injury.
- It is often injured together with other structures inside the knee joint.
- After the initial injury, the knee may swell and become painful.
- Instability or a sensation the knee is "giving out" may be a major complaint following this injury.
Often, but not always, depending on a person's activity level, a torn ACL needs to be fixed. Unfortunately a simple repair by suturing the torn ligament together again is not effective. A successful repair involves completely replacing the torn ligaments, and there are a number ways that this can be done.
Understanding The Knee
The knee is a hinge joint made up of three bones held firmly together by ligaments that stabilize the joint. The bones that meet at the knee are the upper leg bone (the femur), the lower leg bone (the tibia), and the knee cap (the patella). A smooth protective layer called cartilage, which allows the bones to glide smoothly upon each other, lines the bones inside the joint. In arthritis, this smooth lining becomes damaged.
Ligaments
Ligaments are dense structures of connective tissue that fasten bone to bone and stabilize the knee. Inside the knee joint are two major ligaments:
- The anterior cruciate ligament (ACL)
- The posterior cruciate ligament (PCL)
These cross in the center of the knee (that's why they're called cruciate ligaments -a crucifix is a cross). They control the backward and forward motion of the knee. The ACL in particular restrains excessive forward motion of the knee as well as the inward twisting or rotation of the knee. The ACL is frequently injured in severe twisting injuries of the knee.
Two other major ligaments are actually located outside the knee joint, on the outer and inner side of the knee. They act to stabilize the knee's sideways motion. The ligament on the inner side of the knee is called the medial collateral ligament (MCL) (medial means inner side). The ligament on the outer side of the knee is the lateral collateral ligament (LCL) (lateral means outer side).
The patellar tendon (the 'ligament' of the knee cap) connects the lower part of the kneecap (patella) to the upper part of the tibia, specifically to the lump one can feel just below the knee on the lower leg bone (the tibia). Part of this tendon is commonly used in reconstructing a torn ACL.
What Causes An ACL Tear?
Movements of the knee that place a great strain on the ACL can cause damage to the ligament.
- Hyperextension of the knee, that is, if the knee is straightened more than 10 degrees beyond its normal fully straightened position, is a very common cause of an ACL tear. This position of the knee forces the lower leg excessively forward in relation to the upper leg.
- Pivoting injuries of the knee with excessive inward turning of the lower leg can also damage the ACL.
Basically any athletic or non-athletic related activity in which the knee is forced into hyperextension and/or internal rotation may result in an ACL tear.
Activities placing the knee into hyperextension and /or the tibia into excessive inward rotation can be from either an outside force or non-contact in nature.
The severity of the injury to the knee will depend on:
- The position of the knee at the time of the injury
- The direction of the blow
- The force of the blow
Do I Have To Have Surgery?
Treatment decisions for ACL tears are always individualized - tailored to each individual. The decision whether to offer surgery is based on the person's age, activity level, how unstable the knee is, and whether other structures in the knee have been injured.
It is important to keep in mind that surgery to reconstruct a torn ACL is not an emergency for most people. Many people with a torn ACL do not need surgery at all. Even though the chances for complete success from surgery are now excellent, surgery is not for everyone. This is because not everyone needs the ligament repaired to return to his or her pre-injury level of function. It is important to distinguish whether the work, recreational, and athletic activities of the person is light, moderate, or strenuous. Another important issue that needs to be understood by the individual considering ACL reconstruction is that it requires many weeks and months of hard work in rehabilitation following the reconstruction. This needs commitment and time.
How Is The ACL Repaired?
There are a number of different techniques available to repair a torn ACL. Each surgeon has his preference for each particular situation.
In fact we don't talk about ACL "repair" but rather about ACL "reconstruction." This is because a torn ACL cannot simply be repaired by sewing it together again. This was the method tried in the early days of repairing ACL tears, but it was shown to be ineffective. Thus, newer methods were developed which involve reconstructing the ACL ligament, including substituting a new ligament for the damaged one. Using tendons from other parts of the body as a substitute for the ACL was found to be the most effective way of reconstructing the torn ACL. Currently, the two most popular methods in use are using part of the patella tendon or using a hamstring muscle tendon.
Today ACL reconstruction is essentially an arthroscopic procedure, though many surgeons throughout the world still prefer to open the knee. If this route is chosen, depending on how the ACL is repaired, then the incisions may not be very different from those done arthroscopically.
An arthroscope is a pen-shaped instrument to which a tiny video camera is attached. It contains optical fibers, a light source, and lenses that can magnify images 25 to 30 times. The camera attached to the end of the arthroscope sends images to a video screen. The surgeon looks at the screen and is able to get an exceptionally clear view of the inside of the joint. Because the procedure can be done using an arthroscope, and using small specialized instruments without big incisions and excessive trauma to the tissues, it is really well suited to be an outpatient procedure (one that does not require an overnight stay at the hospital). Most ACL reconstructions are commonly performed as outpatient procedures. However, many doctors prefer to keep patients overnight in the hospital following surgery.
Before actually reconstructing the torn ligament, the surgeon uses the arthroscope to carefully survey the whole joint, looking at and evaluating each key structure. During this portion of the procedure, any additional damage to any of the other knee structures can be identified, and where appropriate, is corrected surgically.
There are a number of choices available to the orthopedic surgeon in determining how best to reconstruct the torn ACL. They all involve a "graft" using something to substitute for the torn ACL.
Each of the available ACL graft tissue choices requires a unique harvesting technique. Furthermore, there are usually different methods used for fixing the grafts in the bone tunnels, depending on the characteristics and properties of the tissue selected. Because of these differences in graft techniques, the type of surgery chosen is frequently made by the surgeon based on his or her experience and comfort level with the chosen technique.
Typically, an ACL reconstruction takes two to two and a half hours. The anesthesia may be general anesthesia or a spinal anesthesia. General anesthesia allows the individual to be asleep through the entire procedure. Spinal anesthesia involves an injection in the back that numbs only the lower body. A medication is also administered with a spinal anesthesia to keep the individual sedated throughout the procedure.
There are several available operative procedures:
- Patellar tendon graft procedure
- Hamstring graft procedure
- Allograft procedure
Hamstring graft procedure
Hamstring reconstruction is an alternative to the bone-patellar-bone graft fixation and is growing in popularity. In this procedure, rather than using the patellar tendon, the surgeon uses the patient's own hamstring tendon, either the semitendinosus or gracilis tendons from the same leg.
There are several variations of this technique. Newer hamstring fixation techniques have been developed to match and even exceed the initial pullout strength of the patellar tendon bone procedure described above. Special screws with threads designed not to cut the hamstring tendons are able to fix the tendon within the bone tunnel, as described with the patellar tendon bone technique.
In younger patients who have torn their ACLs but still have growing bones, the hamstring tendon graft is a good choice because there is less chance of damaging the 'growth plates'- the area responsible for growth of the bone.
Advantages
- The hamstring incision is away from the patella, allowing patients to kneel comfortably.
- The patellar tendon is left intact.
Disadvantages
- Soft tissue-to-bone healing occurs at a slower rate than bone-to-bone healing.
- Unlike the patellar tendon, the hamstring tendons do not grow back after graft harvest resulting in a slight loss in hamstring strength (approximately. average of 10%) after recovery. However, most people do not notice this slight decline in strength.
Risks And Possible Complications Of Surgery
Any surgical procedure has possible risks and complications. Surgeons make every effort to minimize them.
They include:
- Deep venous thrombosis (DVT)
- Infection
- Stiffness
- Graft 'impingement'
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