Sports like skiing, tennis and basketball that involve sharp turns put athletes at a higher risk for knee injuries - and if they're female, their chances of injury can be five to eight times greater than males.

"With women becoming as athletic, and playing by the same rules as men, we are seeing a significant increase in the rate of anterior cruciate ligament (ACL) injuries," says Dr Peter Simonian, associate professor of orthopaedics, chief of the Sports Medicine Clinic and director of sports medicine research at the University of Washington.

Injuries commonly occur in more than one part of the knee. The ACL is located in the centre of the knee and acts as its main stabiliser. It stretches like a taut rubber band from the bottom of the femur (thighbone) to the top of the tibia (shinbone) and keeps the bones in proper alignment. The medial collateral ligament (MCL) runs along the inside of the knee joint. An outside blow to the knee can cause the MCL to tear. However, MCL injuries are less serious and often heal on their own. The meniscus, another vulnerable part, is cartilage that absorbs shock to the joint.

Why are women more prone to ACL injuries? Dr Simonian explains several factors:

• Bony alignment. Women are a few degrees more knock-kneed than men and have a narrower space in the femoral notch (a small space in the thighbones). This makes the knees more vulnerable to ligament tears;

• Hormones. Oestrogen tends to increase the laxity of muscular tissue, and studies suggest that women may be more at risk of injury during certain phases of the menstrual cycle;

• The speed of neuromuscular reflex is slightly slower in women, so the protective muscles in the thighs or knees can't compensate as quickly as they can in men. When people fall or lose their balance, muscles tighten up to protect against injury.

Unfortunately, ACL injuries are difficult to avoid, short of being in good condition and not getting over-fatigued. Derotational braces are a possible option, but are cumbersome and expensive.

Injuries can also occur from increased traction in footwear. If the foot is being anchored when the body goes off balance, the body spins around the knee joint, tearing or stretching the ligament. This is followed immediately by swelling, severe pain and limited motion.

There is no way to tell who is predisposed to ACL injury unless the other knee has already been injured. For instance, someone who has skied for 10 years may have had the same fall 100 times before tearing a ligament. Also, the injuries are not age-related, although they most commonly occur among those between 18 to 30.

Unlike tears to other parts of the knee, the ACL does not mend by itself. Surgery is the only means to a full recovery, although some patients can manage without a fully functional ACL if they limit themselves to straight-ahead exercise, such as bicycling, swimming or weight training.

The ACL can be replaced by grafting a tendon from the patient's own knee. Surgical procedures have improved vastly over the past decade and are minimally invasive. A fiber-optic microscope, called an arthroscope, allows the surgeon to view the inside of the joint with minimal incisions. After surgery, patients can expect to be on crutches for at least two weeks and to wear a brace for four weeks. Normal walking can resume in five to six weeks but patients must refrain from high-risk sports for approximately nine months and undergo extensive physical therapy.

Patients with any knee injury need to begin range-of-motion exercises as soon as possible after seeing a doctor to prevent stiffness adds Dr Simonian.

Health Beat, University of Washington

Date reviewed: 03 March 2000