Common Knee Injuries Explained

A knee injury can threaten any athlete’s career. After headlines have assured readers that a star athlete has a minor knee sprain, arthroscopic examination often reveals that a combination of three injuries have occurred: two torn knee ligaments and a torn cartilage. Among coaches, trainers, and orthopedic surgeons, this triple knee injury bears the ominous name “the terrible triad”. It has brought many brilliant athletic careers to an abrupt end.

Knee Ligaments

Ligaments are relatively inelastic bands of tissue. Their main function is to tie bones together by bridging joints. They also contribute to joint stability. Two pairs of ligaments hold the leg bones together at the knee, and are very important to stability in the knee joint. The two collateral ligaments on the sides of the knee give side-to-side stability. The cruciate ligaments cross within the knee joint, and provide front-to-back stability. They also keep the joint from rotating out of its normal arc. Knee ligaments may be sprained or torn by a direct blow (in sports or everyday life). However, many ligament injuries happen without contact simply due to the knee rotating improperly.

Collateral Ligament Injuries

One of the two knee ligaments torn in the event referred to as the terrible triad is the medial (inside) collateral ligament, though this by itself, medial collateral ligament injury causes sharp pain on the inside of the knee. Treatment with bandage or cast may be sufficient if a sprain or minor tear has occurred. However, permanent joint instability can result if a tear is not treated appropriately. Surgical repair, if needed, may be the only means to recovering knee joint stability. Ligament may be sewn to itself or to bone.

Cruciate Ligament Injuries

Full recovery of knee function is not as likely with anterior cruciate ligament injuries, along or as the second component of the terrible triad. Most cruciate ligament tears occur while pivoting, landing from a jump or simply stopping quickly, and are not due to heavy contact. A person rushing to catch a plane in a crowded airport concourse can tear an anterior cruciate ligament as easily as a halfback eluding would-be tacklers. The typical signs are popping sensations in the knee, followed by swelling and pain within two hours. Anyone with these symptoms should keep the injured leg elevated, apply ice and a comfortable snug elastic bandage and seek orthopedic treatment. The ligament can be critically important for knee stability, but because the anterior cruciate ligament has very little blood supply, new tissue does not readily form to mend the injury. Surgical repair is usually the best option. In some cases, the outcome of surgery may be improved by grafting healthy tissue of similar strength to the torn ligament.

The Terrible Triad

A tear in a knee cartilage called the meniscus is the third element of the terrible triad. A meniscus is a tough, rubbery pad, which cushions the knee joint. When the medial collateral ligament and the anterior cruciate ligament tear at the same time, one or both of the menisci can be caught and torn as the ends of the femur and tibia grind against each other. Some tears near the edges of the meniscus can heal, though most often surgery to trim off torn portions is required. If meniscal repair alone is needed, this surgery can be done arthroscopically, usually on an outpatient basis.

Joint Instability

Patients who have had knee ligament tears or just the terrible triad can face permanent knee joint weakness and instability. Because of these risks, postponing treatment of knee injuries is not advised. Waiting and walking on a ligament tear after swelling subsides a bit can cause further ligament tearing and additional damage to knee cartilage, and can seriously affect the chances for recovery of knee function. Fortunately, using the arthroscope, it is possible to investigate the extent of ligament or cartilage injury before performing open surgery. In some cases, magnetic evaluation, MRI uses a magnetic field to take very vivid pictures of bones and soft tissues (which X-rays cannot see).


A patient’s commitment to rehabilitation following knee injury is of the utmost importance. Exercises to gradually return the knee to full flexibility and weight bearing must be performed faithfully. Regaining flexibility and building strength in the thigh and calf muscles is particularly important in order to provide as much stability as possibly to support the ligament. Athletic activity is not usually resumed for twelve months after surgical repair of an anterior cruciate ligament, and a knee brace is frequently recommended.

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