People who get a steroid injection in their shoulder might be better off waiting a few weeks before returning to regular activities or starting physical therapy, according to a new study.

The reason?

"Steroid injection temporarily produces a molecular response in the tissue that is similar to that of a tendon injury, possibly making it more vulnerable to damage during this time," says senior study investigator Dr. John Callaci of Loyola University/Chicago's Stritch School of Medicine.

"This is especially important because steroids often gives patients rapid pain relief," says Callaci. "If a patient returns to rigorous activities right after a steroid injection, the weakened tissue may not be able to sustain itself."

He cautions that these findings should not stop people from using steroids when medically appropriate. "The study gives us a better understanding of what is happening on the molecular level," he said.

"We found that steroid injections cause a tendon to behave in a way where it thinks it has been acutely injured," notes Callaci. "Steroids rapidly provide anti-inflammatory and pain relief. That is why steroids are so popular, but physical therapy also can produce some of the same effects. It just takes longer."

Loyola is currently conducting a biomechanical study to determine if the steroids change the strength, stress, strain or elasticity of tendons. "We are investigating the biological basis of how tendons heal and how steroids may modulate that healing," says Callaci. "We want to see if there are actual differences in biomechanical properties right after steroid injection," he said. "If you have a tendon injury already and you're given steroids, how is that modulating the healing response? Do you get more scar formation or do other things change? These are the questions we are researching now."

These findings were presented February 12, 2007 at the 53rd annual meeting of the Orthopaedic Research Society.