After knee reconstruction surgery, half of people who played sports both competitively and just for fun don't perform as well as they used to, according to an Australian study.

Of more than 300 men and women who had the surgery, a third stopped playing sports entirely and 68 who were still active said they didn't play as well as before, researchers reported in the American Journal of Sports Medicine.

The anterior cruciate ligament, or ACL, is the ligament inside the knee that helps keep the joint stable. About 150,000 ACL injuries occur each year in the United States.  

"Less than 50 percent of the study sample had returned to playing sport at their preinjury level or returned to participating in competitive sport when surveyed at 2 to 7 years after ACL reconstruction surgery," wrote Clare Ardern at La Trobe University in Victoria, Australia, who led the study.

Ardern and her colleagues followed more than 300 men and women for two to seven years. Participants had either played Australian-rules football, basketball, netball or soccer before their surgeries.

At 39 months post-surgery, 208 out of the 314 people who had the operation were still playing a sport, the researchers said.

Of those 208, 68 said they played at a lower level than before their injury and 140 said they played about the same as before their injury. The remaining 106 participants either were not playing sports or did not complete the entire study.

"Although almost all people returned to playing some form of sport, they did not play continuously for many years after their surgery," Ardern told Reuters Health in an email.

She also noted there may be other reasons why people stopped playing sports, such as fear of getting injured again or less confidence in performing.

Of the 196 people who played competitive sports before their injury, 91 returned to their competitive sport.

"This is a big injury," said Edward McDevitt, a spokesman for the American Academy of Orthopaedic Surgeons, who was not part of the study.

"Many athletes who choose surgery have a long and difficult road to face. If you're not willing to go through it, then you might be better off just getting a brace."

He added that while knee surgery does allow people to return to their sport, they couldn't perform as well as doctors might wish.

People who tear the ACL can either opt for physical therapy and surgery or just physical therapy alone.

"Some people find that they are able to function well without surgery, provided they have adequate leg strength to support the injured knee," Ardern said.

"Other alternatives may be to avoid sports involving direction changes, jumping and landing or activities that make the knee feel unstable, or use knee braces and supports."

McDevitt speculates that after surgery, some athletes may not have the same range of motion, preventing them from playing as before.

"I tell my patients, 'I can't make you like before, I'm not God. But I'll do the best I can to restore you back to the way you were,'" he added.