U.S. researchers said Wednesday that it is possible to rebuild large amounts of muscles severely damaged in traumatic accidents or military wounds by implanted tissues of pig bladders.

Researchers from the University of Pittsburgh tested the experimental technique on five male patients, who had lost between 58 and 90 percent of limb muscle, and found that three of them had a 25 percent improvement in function.

"This new study is the first to show replacement of new functional muscle tissue in humans, and we're very excited by its potential," Stephen Badylak, senior investigator at the University of Pittsburgh.

"These are patients who can't walk anymore, can't get out of a car, can't get up and down from a chair, can't take steps without falling. Now we might have a way of helping them get better," Badylak said.

Large amounts of muscle can be lost as a result of trauma, such as a car accident, a sports injury, or even an explosive device. There is no treatment for such volumetric muscle loss and native muscle is unable to regenerate to that extent, to fully recovery function and tissue volume. Instead, scar tissue can form to fill the gap.

Stem cells have been shown to help damaged tissue regenerate, but in most cases, stem cells should be taken out, isolated and grown into the cell of choice, and then injected back into patients.

Badylak's team, however, used the extracellular matrix (ECM) from pig bladders to attract stem cells to the site of injury to promote regrowth of muscle.

According to the researchers, pig bladder ECM has been used for many years as the basis for medical products for hernia repair and treatment of skin ulcers. It is the biologic scaffold that remains left behind after cells have been removed.

After first showing the ECM method worked in mice with severely injured hind limbs, the researchers started a clinical trial involving five men, all at least six months removed from the time of injury, and for whom surgery and physical therapy had proved unsuccessful.

Three of the patients were in the military; two had thigh injuries from bombs while one had an exercise-induced calf injury. The other two patients were civilians with calf injuries from skiing accidents.

Before surgery, all patients completed a 12- to 16-week physical therapy program that focused specifically on their individual functional deficits. The program continued until their function and strength plateaued for a minimum of two weeks, with no signs of further improvement.

Then, the researchers surgically implanted a "quilt" of compressed ECM sheets designed to fill into their injury sites, and within 48 hours of the operation, the participants resumed physical therapy for up to 26 additional weeks.

The researchers found that three participants, including the two who had thigh injuries and one who had a calf injury, were stronger by 25 percent six months after the surgery and that biopsies and scans all indicated that muscle growth had occurred.

One thigh-injured patient improved on the "single hop test" by 1,820 percent, and the other had a 352 percent improvement in a chair lift test and a 417 percent improvement in the single-leg squat test.

Two other participants with calf injuries did not have such dramatic results, but both improved on at least one functional measure and said they felt better.

"This work represents an important step forward in our ability to repair tissues and improve function with materials derived from natural proteins. There will be more options to help our patients, " said study co-author Peter Rubin, chair of the department of plastic surgery at the University of Pittsburgh.

"We think it's remarkable that this approach was able to improve function among patients who were all well past the acute injury response phase and were not helped by the standard surgical procedures they had already had," Rubin added.

The findings were published in the U.S. journal Science Translational Medicine.