When a surgeon-in-training takes part in an operation, the patient's risk of serious complications appears to be no greater than normal, a U.S. study finds.

Looking at data on more than 60,000 surgeries done in the U.S. between 2005 and 2007, researchers found that when a resident was involved, just under six percent of patients had a major complication like severe bleeding or a serious post-surgery infection, such as pneumonia.

The rate was the same for surgeries where no resident took part.

Residents are medical school graduates receiving training in a specialty. Surgical residents obviously have to be trained, but some studies have raised the possibility that having a resident on board increases patients' risk of complications.

"This shows that resident participation is safe," said Dr. Ravi Kiran, the lead researcher on the new study and a colorectal surgeon at the Cleveland Clinic in Ohio.

The study, published in the Annals of Surgery, did find that the odds of minor complications, mainly skin infection at the surgery site, were slightly higher when residents took part.

But, Kiran said, "the differences were very small, and not likely to be clinically significant."

When residents were involved in the surgery, three percent of patients had a "superficial" infection at the surgery site, versus 2.2 percent of patients who had surgery with no resident on hand.

It's not fully clear why such infections were more common when residents took part in surgery. But on average, those procedures took a little longer -- 122 minutes, versus 97 minutes when no resident was involved.

Other studies, Kiran noted, have found that longer surgery times tend to carry a higher risk of superficial infection. So it's possible that could help explain the higher risk seen with operations involving residents.

Past studies have come to mixed conclusions on whether resident involvement might boost complication risks. But most, according to Kiran's team, did not control well enough for other factors.

The typical patient at a "teaching hospital" - centers connected to a medical school - may be sicker or having a more complex procedure, for example, versus the typical patient at a smaller community hospital.

For their study, Kiran and his colleagues used information from a national database on surgery outcomes. They focused on some 40,000 procedures done with a resident on the team, and about 20,000 with no resident.

The two groups of patients were "matched" for a range of factors - including their age, the complexity of their surgery and medical conditions like diabetes and heart disease.

As with serious complications, there was no significant difference in death rates between the two groups, Kiran's team found: 0.18 percent of patients in the resident group died, versus 0.2 percent in the non-resident group.

"This data is reassuring," said Dr. David Farley, a professor of surgery at the Mayo Clinic in Rochester, Minnesota, who was not involved in the study.

But he also said he thinks the bulk of the evidence on this question has been reassuring. Other studies have shown that patients' death rates are the same or lower when residents are involved, Farley noted in interview.

Kiran said he thinks the benefits of having surgery at a teaching hospital, with a resident involved, would outweigh any potential risks. Those benefits, he said, would include "better round-the-clock care," before and after the surgery itself.

"Some would argue with that," Farley said. But he agreed that some advantages of teaching hospitals are the availability of "in-house" staff at all times, and the fact that residents help keep the senior doctors informed on their patients.

But Farley said the bottom line for prospective surgery patients is to look for a surgeon experienced in the procedure they need.

A number of studies have found that surgeon experience is key in patients' outcomes and that patients at large, busy centers tend to fare better than those at smaller hospitals.

Experienced surgeons at large medical centers are often the same ones who have a resident on the team.