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Retired Army Sgt. Max Ramsey, 42, recently traded his X2 microprocessor knee for the latest innovation in prosthetics at Walter Reed National Military Medical Center (WRNMMC).

The X3 waterproof and corrosion-free limb will enable Ramsey, the first amputee on the Screaming Eagles Parachute Demonstration Team - an Army sky-diving team at Fort Campbell, Ky. - to swim with the computerized knee and make a smoother transition from walking to running.

"I'm thrilled. It's great; it doesn't feel different," Ramsey said after his first steps on his new knee May 22. "I wasn't a big swimmer with two legs, but [thought] I might give it a shot."

It's a huge step forward in technology, explained Zach Harvey, the chief prosthetist at WRNMMC who switched out Ramsey's old knee, a first generation X2, for the new X3. "Whenever I put something on someone and it takes them just minutes to get used to, I know there's promise to it," he said.

With a laptop, Harvey programmed Ramsey's new knee using Bluetooth technology. Up to five customized modes can be preset to pick the best angle or amount of resistance to meet individual, specialized needs for activities such as cycling, flex-standing, driving or scuba diving.

Amputees with the X3 knee can change between modes using a small, hand-held remote control or switch modes by bouncing on the toe of the prosthesis a pre-determined number of times.

"It will look at every step you're doing. If it sees that it's a running step, it'll be in running mode for that particular step," explained Greg Schneider, a research and development prosthetist for a company involved with the evolution of the microprocessor knee.

Harvey said he believes the behavior of the new knee is close to intuitive. The chief prosthetist continued, amputees wearing the latest generation of prosthetics walk better; those with low back pain say that the standing feature has helped eliminate that.

"Just the way the knee releases and the swing - it performs well, just to keep up with their gait: slopes, steps, uneven terrain - it’s a big step forward with the addition of multiple sensors. The way that the microprocessor controls the hydraulics essentially anticipates the person's next step," Harvey explained.

"It actually felt natural," Ramsey said. "As close to natural as natural can get when you are missing a leg."

Three months into his first deployment to Ramadi, Iraq, Ramsey lost his left leg after an improvised explosive devise detonated under his humvee on March 1, 2006. That same year, prosthetists at the former Walter Reed Army Medical Center in Washington, D.C., fitted him with a C-Leg, which offered computerized knee technology using two sensors, and Ramsey took his first jump as an amputee six months after his injury.

Later, the skydiver received both a hydraulic Mercury knee, which offered refined movement, as well as a running knee. In 2010, Ramsey and 35 other wounded warriors with above-knee amputations were selected to test the X2 prototype. The X2's five sensors detected when the wearer wanted to climb stairs or switch from walk to run without a preset device, according to Harvey, a certified prosthetist orthotist.

Working with his California-based heavy engineering company, Ramsey said he walked rugged terrain with his X2 prosthesis each day: in snow, dirt, rocks, in remote areas of mountains and national parks. He reported that he climbed lots of stairs, hills and construction sites wearing the knee without problem.

"No issues with altitude either, in freefall or up there in the mountains. We were about 7,000 feet [mean sea level] (MSL) and jumping 13,000-14,000 feet," Ramsey said. An experienced sky-diver before joining the Army at age 33, the former Soldier estimated he's made more than 641 jumps with his prosthesis - nearly twice the amount he did with two legs - and plans to soon return to sky diving after a year-long break due to work obligations.

According to Chuck Scoville, chief of amputee services in the orthopedics and rehabilitation department at WRNMMC, Ramsey and 35 others received X2 knees from the very first production runs. "There were modifications that were made with them over time. The first 36 people wearing the X2 are the first 36 people to go into the X3 so they're getting the newest technology. And we're going to take the first 36 and look at wear and tear patterns to work on projecting how to improve the length of the life of the devices."

Scoville said his department plans to hold a meeting with the wounded warriors who helped evaluate the X2 and X3, along with WRNMMC rehab professionals and representatives from the company that produced the knees for a review of findings later this month.

"The biggest difference really between [the X2] knee and this knee is [the X3] is able to be submerged in water. It's more ruggedized than the first one," Harvey explained.

The Department of Defense commissioned the development of the X2 to help service members with severe limb loss return to duty, according to Schneider, who explained wading across a river is a likely situation for someone in combat.

"That part of the development really took the final two years between the first X2 proto-type to fit until now," Schneider said. "We've had people jump into the ocean with the X3 on, go swimming for 20 minutes, come back out and just walk away so that's pretty interesting for a microprocessor knee, since it's got a computer in there that's doing all those things."

The researcher said after a swim, a spray of fresh water to remove the salt water is all that's needed.