US troops withdrawn from the front line in Iraq and Afghanistan for medical reasons from 2004 to 2007 were increasingly evacuated because of psychiatric problems, a study released on Friday reveals.
Only 14 percent of troops taken out of combat operations on medical grounds during this four-year period were because of a combat injury.
The biggest single cause for a pullout were musculo-skeletal and joint problems, which accounted for 24 percent of medical evacuations.
In contrast, psychiatric grounds accounted for five percent of evacuations in Iraq and six percent in Afghanistan in 2004; these figures rose to 14 percent and 11 percent respectively in 2007.
The analysis, published by The Lancet, was led by Steven Cohen of Johns Hopkins School of Medicine in Baltimore.
It drew on data from more than 34,000 US personnel who had been evacuated to the main US military receiving hospital in Landstuhl, Germany.
The authors note that the increase in psychiatric cases occurred despite the introduction of mental health teams devoted to treating combat stress.
Repeat missions and dwindling public support for the Iraq war may partly account for the rise, they suggest.
"Possible explanations... are the cumulative psychological effect of repeated deployments, the increasing manpower burden borne by reserve and National Guard units, a decreased threshold for evacuation, and a shift in public opinion regarding the war in Iraq," says the paper.
Other significant causes for medical evacuation were disorders of the nervous system (10 percent) and spinal pain (seven percent).
The findings have implications for the Pentagon, seeking to prevent evacuations that sap frontline strength and disrupt operations, and for doctors and families helping returning troops.
"As survival rates of combat injuries increase, and the burden of non-battle-related injuries and psychiatric disorders continues to soar, society must be prepared to deal with the aftermath of these injuries," says the paper.