THURSDAY, April 24 (HealthDay News) -- Women who play sports are prone to tearing the knee ligament that connects the thighbone to the shinbone. Beyond the immediate pain and swelling it can cause, there's now reason to suspect that this type of injury may lead to another vexing health problem.

Researchers at the Mayo Clinic in Rochester, Minn., found that young women who had surgery to repair the anterior cruciate ligament (ACL) suffered significant bone loss around the knee even two years later. And that damage was seen even though they had undergone rehabilitation.

"Despite telling them to weight-bear early and put weight on the leg, people are going to protect an operative limb," said study author Dr. Diane L. Dahm, an assistant professor of orthopedics at Mayo. "So, it's probable that the forces going across that operative limb were not as great as the opposite limb, early on especially."

The findings were first reported at a meeting of the American Academy of Orthopaedic Surgery (AAOS).

Ligaments are tough fibers that hold bones together. The ACL, located toward the front of the knee, gives you stability on your feet. But if you land the wrong way or change direction quickly, that ligament can tear. You might even hear a "popping" sound when it happens.

Each year, an estimated 38,000 women sustain ACL tears. In fact, the incidence of ACL injuries is two to eight times greater among women than men, he AAOS said.

Even in the same sport, women are more likely to injure their ACL than men. Female basketball players, for example, are twice as likely as their male counterparts to tear an ACL, while the incidence of these injuries is four times greater among women soccer players than men who play this sport, the AAOS noted.

Although the reason for this gender gap isn't clear, orthopedic experts suspect hormones and differences in the way women use their muscles may play a role.

Dahm and her colleagues studied 18 female athletes ranging in age from 16 to 40. All were fairly active and involved in some sort of exercise, although not necessarily an organized sport. The researchers chose age 40 as the cutoff to eliminate any patients who might have osteoporosis related to early menopause.

After undergoing ACL reconstruction, the women participated in a rehabilitation program that involved routine visits with physical therapists and sports psychologists.

Before surgery, there were no significant differences in bone density between the injured and uninjured legs. Three months after surgery, however, the researchers found striking differences in bone density between the injured limb and the unaffected one. While there were improvements in the women's injured limbs over two years, "they did not get back to completely normal," Dahm observed.

Dr. Mary Lloyd Ireland, president of the Kentucky Sports Medicine Clinic in Lexington, noted that the study involved a small number of patients, making it difficult to draw definitive conclusions.

Ireland does about 100 ACL surgeries a year, and sometimes, she noted, "the bone on the involved side doesn't look as healthy." Still, she hasn't taken bone-density measurements or followed these patients to document their long-term outcomes.

Another expert questioned whether this study proves that bone loss after ACL surgery is a women's issue, in particular.

"Did they do the same study in men? asked Dr. Christopher D. Harner, professor of orthopaedic surgery at the University of Pittsburgh School of Medicine. "If not, how do they know this is a gender issue?"

Given the incidence of ACL injuries among women, orthopedists urge women athletes to take steps to prevent ligament tears from occurring. The AAOS recommends year-round training and conditioning, including strength training and practicing performing proper landing and cutting maneuvers.

One other bit of advice for the weekend warrior: "Make sure you're ready to do that sport," Ireland cautioned. "I see a lot of women in their 30s and 40s, who haven't been that active, go out and ski and tear their ACL."

More information

The AAOS has more on the ACL and preventing injuries.