New research casts doubt on increasingly popular blood-based injections reportedly used by Tiger Woods and other athletes to speed recovery after orthopedic surgery.

In a small study at a hospital in The Netherlands, the treatment worked no better than salt water injections in patients with overuse Achilles tendon injuries.

The Achilles is the body's largest tendon, connecting calf muscles in the lower leg to the heel bone and it's prone to overuse injury from sports including running.

The treatment studied involves injections of platelet-rich plasma, or PRP, extracted from patients' own blood. Platelets help blood clot and they release growth-promoting substances that help repair damaged tissue.

Tiger Woods is reported to have had these injections after June 2008 knee surgery.

In the study, 27 adult patients got PRP injections and 27 got injections of salt water. A week after the injections, both groups gradually began three months of exercise therapy including stretching.

Six months after treatment, both groups reported the same amount of improvement in pain and activity levels.

The results appear in Wednesday's Journal of the American Medical Association.

Patients were treated at The Hague Medical Center. The study authors called the results disappointing but also called for more research and said it's possible the platelet treatment works for other types of injuries.

Previous, less rigorous studies had more promising results.

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