Electronic implant measures internal knee forces in humans

Data already collected using the implant are improving computer modeling of knee kinematics.

The basic design of the E-Knee involves a titanium tibial component constructed with the tibial tray, which consists of an upper and lower plate separated by four 2.5 mm square support posts, each 0.5 mm in height. Four strain gauges (NK Biotechnical, MN), located in the four quadrants of the tibial component, then measure localized forces applied to the tibial tray.

COURTESY OF CLIFFORD W. COLWELL, JR.

WASHINGTON - An experimental knee prosthesis is allowing researchers for the first time to directly measure forces on the knee joint during common activities of daily living.

The implant, an artificial knee replacement called E-Knee, houses transducers that measure changes in strain across the knee as the patient moves. The device gets activated by a removable external coil that fits around the knee. Data are then gathered from an antenna that transmits signals to a computer system, which converts the information into actual stress loads measured in pounds.

In early testing, the E-Knee produced real-time information on the behavior and function of an artificial knee joint inside a human body. This included information on the distribution of forces and load-carrying capabilities of bone, according to a surgeon speaking at a press conference here at the American Academy of Orthopedics Surgeons meeting.

Beyond a standard knee implant

Clifford W. Colwell Jr., MD, director of the Shiley Center for Orthopaedic Research and Education (SCORE) at Scripps Clinic, La Jolla, Calif., implanted the E-Knee into volunteer Jerry Ward, an active 80-year-old man with severe arthritis. Ward had also previously undergone a total knee replacement (TKR) and a total hip replacement.

Although the implant does serve as a standard TKR implant, the data gathered do not immediately benefit Ward. Rather, the data gathered will help researchers design future implants, Colwell said.

"We want to know ... why [an implant] wears out, how it wears out, what makes it wear out," he said.

Researchers at SCORE have been collecting data from the implant for over one year. So far, they found Ward's knee generates forces during walking approximately 2.2 times greater than his body weight. Also, stair climbing generated peak stresses approximately 2.5 times his body weight, and rising from a chair generated peak forces approximately 1.5 times, according to a press release.

One surprise for the researchers: A practice golf swing, taken by right-handed Ward, generated a force equal to approximately 4 times his body weight in his right knee, noted Darryl D'Lima, MD, one of the lead researchers of the implant.

Future applications

Data collected to date from the E-Knee device have already improved on existing computer models of TKR, which had overestimated tibial forces, Colwell said.

Eventually, the data could help develop knee implants custom-designed to patients' needs, allowing for more active lifestyles than now possible, he noted.

The researchers plan to implant similar E-Knee devices in six additional patients, including women, younger patients, obese patients and patients with different activity patterns.

For more information:
Colwell CW. Patil S, Steklov N, et.al. An electronic knee prosthesis: measurement of tibial forces in vivo. SE082. Presented at the American Academy of Orthopaedic Surgeons 72nd Annual Meeting. Feb. 23-27, 2005. Washington.