Introduction
In the human engaged in high-velocity, cutting, twisting, and jumping activities, the knee is the weakest link. In the United States each year, approximately 50 million young athletes participate in interscholastic or extracurricular sports. Each year, some 775,000 adolescent athletes are treated in emergency departments. Fifteen percent of these injuries involve the knee. When you consider these statistics, it's no wonder that the knee draws so much interest and demands so much of the orthopaedic surgeon's time.
Why are adolescents prone to knee injuries?
The adolescent's knee is different from the younger child's, the older teenager's or the adult's. It has very active growth plates, or physes, where bone growth occurs. The growth plates are a layer of cartilage (strong connective tissue) that separates the rounded head of the bone from the shaft of the bone. As the child grows, the cartilage forms into bone (ossifies) and becomes part of the parent bone. A person stops growing when the growth plates become solid.
The growth plates in adolescents are mechanically weaker than the surrounding bones and are often weaker than the ligaments that hold the knee joint together. (Ligaments are bands of tough, flexible tissue that support bones around the joint.) As a result, some injuries that would cause a ligament tear in an adult or older teenager will instead cause a fracture through this relatively weak area of growth plate.
Fractures in the area of the growth plates can be difficult to diagnosis and frequently lead to major problems later on. As a general rule, the growth plates about the knee close and are no longer a factor in boys at around ages 15 to 17 and in girls at around ages 13 to 15.
What causes knee injuries?
There are two main causes of knee injury: acute trauma, such as from a sudden blow to the knee, and overuse or misuse phenomena, such as from repeated jumping or twisting. Acute trauma can occur in a healthy knee or in a knee that has an underlying problem.
Acute Trauma
Bruise
The most common acute trauma injury and the most simple to manage is the bruise. A bruise is diagnosed by ruling out other conditions and asking the patient if he or she has had a blow to the knee. The physician looks for focal tenderness, darkening of the skin (ecchymosis), and otherwise normal exams. An X-ray may be needed to rule out other injuries. The bruise is treated by applying ice and the patient's gradual return to motion and activity.
Dislocated knee cap
A dislocated knee cap (patella) is the second most common acute trauma knee injury. The knee cap is designed to slide up and down in a groove on the front of the thigh bone (femur) as the knee extends and flexes. When the knee is nearly fully extended, it is not engaged in the groove. With abnormal anatomy or bad luck, a blow directly to the knee cap can cause it to dislocate. The knee cap is easily returned to its proper position by getting the athlete to relax and let the knee fall out to a full, straight position.
The knee cap must be kept in its proper position so that the soft tissue on its inner side can heal. The knee is then rehabilitated and protected to lessen the likelihood of the need for future surgery. X-rays are taken at the time of the injury to be certain that there is not an associated fracture that would require surgery.
Torn ligaments and fractures
The third most common injury is a torn ligament (specifically of the medial collateral ligament) or a fracture along the growth plates in the thigh bone (distal femoral physis fracture). As was noted earlier, the cartilaginous growth plate at the end of the thigh bone is often weaker than the ligaments, which makes it more likely to fracture.
When a fracture is present, surgery is used to reposition the growth plate and fix it in its proper position. Rupture of the ligaments down the inner side of the medial collateral ligament can be treated by bracing the knee and carefully performing strengthening exercises.
Rupture of the anterior cruciate (one of the cross-shaped pair of ligaments), which is the major support ligament up the middle of the knee, was once considered extremely rare in this age group. As adolescents have gotten bigger, stronger, faster and more aggressive in play, we have seen more of this type of injury. Though surgery can be used to treat a rupture of the anterior cruciate, it must be modified because the growth plates in adolescents are still wide open.
In most cases of torn ligaments, adolescents will report having noticed something popping in the knee as they were landing from a jump or pivoting to cut and go with a cross leg maneuver. There is usually obvious swelling within the first two to four hours.
Other acute injuries of the knee, such as cuts from falling on sharp objects, torn cartilage or meniscus (curved part of the cartilage), and other fractures or ligament injuries, do occur although not as frequently.
Trauma on top of an existing problem
Knee cap dislocation and pain
Acute trauma can also occur on top of a pre-existing problem that makes the knee more susceptible to injury. These problems are often a previous injury that was not properly treated or allowed to heal. In some case, the condition may be a deformity in the knee. By far the most common of this type of injury is a dislocation or severe increased pain in the knee cap.
Discoid lateral meniscus
The adolescent may also have a deformity in the meniscus, a piece of C-shaped cartilage in the knee. Rather than a "C" shape, the meniscus is a full crescent shape (called "discoid lateral meniscus"). Because of its abnormal shape, the meniscus constantly gets caught in the joint. At about the mid-adolescent ages, we see spontaneous tears in this meniscus.
Osteochondritis dissecans
Adolescents may also suffer from a condition called osteochondritis dissecans, in which a segment of the bone dies and then the overlying cartilage breaks loose. This condition can show up as simple knee pain with no explainable cause, knee pain that is felt to be secondary to an injury, or a catching and locking of the knee.
X-rays are the only way to make a diagnosis of osteochondritis dissecans. If studies or symptoms indicate that the fragment is loose, the cartilage must be surgically re-attached or, if it cannot be re-attached, removed. If the cartilage is still in place and not loose then conservative treatment with rest and non-weight bearing exercises can promote healing.
Overuse injuries
Runner's knee
Runner's knee (patellofemoral pain or chondromalacia patellae) is the most common type of overuse injury. This condition causes a pain under the knee cap as it slides up and down its groove. People with runner's knee will complain that the pain is worse when they go up and down stairs and that the knee feels stiff when they first stand up after sitting for prolonged periods. They may occasionally have flares of severe pain and swelling in the knees with no recognizable cause.
These aches and pains are generally caused by poor tracking of the knee cap as it slides up and down in the groove on the femur. When diagnosing this condition, the physician looks for physical changes in the knee and at the type of exercise or activity that may be causing the problem. Treatments include changing the activity, rehabilitating the knee and, occasionally, using orthotics (such as pads inside the shoe) to try and realign the knee so that the knee cap tracks better. If all of these treatments fail, then surgery may be used to realign the knee cap.
Osgood-Schlatter's disease
Another common overuse injury is Osgood-Schlatter's disease. Although this condition usually shows up in the pre-adolescent ages, its symptoms of pain and tenderness often become more severe during adolescence, a time when the athlete is larger and more active in sports. Though it has a big name, Osgood-Schlatter's disease is just a painful bump over the front of the knee where the primary tendon (band of tissue that connects muscle to bone) of the knee cap attaches to the front of the lower leg bone (tibia). This point is a fulcrum, a power point for running, kicking and jumping. All of the pull of the quadriceps muscle comes through this attachment.
Osgood-Schlatter's ends as the adolescent matures and the growth plates close. The most important point I can emphasize about this condition is that, despite its name, it is not a disease. It is not dangerous; it does not lead to fracture or arthritis; and it does not require excessively aggressive treatment; it should not be treated with a cast or any form of stiff bracing.
There is no reason to make a child stop playing sports because of Osgood-Schlatter's, and there is no reason to operate on Osgood-Schlatter's in an adolescent. Treatments are ice, nonsteroidal anti-inflammatories as tolerated, knee pads to protect the area from being bumped, quadriceps stretching exercises, and activities as tolerated.
Tendinitis
Tendinitis is inflammation of a tendon, usually as result of strain. There is a form of tendinitis known as jumper's knee that occurs at either the upper end or, more often, the lower end of the knee cap. This condition is very common in basketball players and hence the name jumper's knee. It is a form of chronic overuse tendinitis, much like tennis elbow in adults. Again, it is not dangerous. It does not lead to tendon rupture. It does not require any specific dramatic treatment. The best treatment is ice, stretching and activities as tolerated.
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This information is provided by the Cleveland Clinic and is not intended to replace the medical advice of your doctor or health care provider. Please consult your health care provider for advice about a specific medical condition. This document was last reviewed on: 1/3/2002
