(USA TODAY) -- Stem cells harvested from a patient's own heart can be used to help repair muscle damaged during a heart attack, according to a preliminary study published online Monday in The Lancet. Though it's too soon to know whether the technique will help patients live longer, the study is the second small, promising study of cardiac stem cells in three months.

The latest study involved 25 patients who had suffered serious heart attacks; 24% of their heart's major pumping chamber had been replaced by scar tissue. One year later, doctors saw no improvement in those randomly assigned to get standard care. Among the 17 given stem cells, however, "we reversed about half the injury to the heart," said study author Eduardo Marban, director of the Cedars-Sinai Heart Institute in Los Angeles, in an e-mail. "We dissolved scar and replaced it with living heart muscle."

Warren Sherman, director of stem cell research and regenerative medicine at Columbia University Medical Center in New York, says the study was an important proof of the potential of stem cells -- harvested from patients, grown in the lab, then injected back into patients' hearts.

Doctors don't yet know exactly how the stem cells reduce the size of the dead zone of scar tissue, says Kenneth Margulies, director of heart failure and transplant research at the University of Pennsylvania.

The new study's encouraging results seem to confirm the findings of another small study of heart stem cells, published in The Lancet in November, which also showed an improvement in heart attack survivors who received the treatment, Margulies says. On the other hand, a third study found no benefit from stem cells created from patients' own bone marrow.

Copyright 2012 USA TODAY, a division of Gannett Co. Inc.