The intelligent prosthesis

A surgeon from the Scripps Clinic Division of Orthopaedic Surgery in the US has implanted a prosthesis into a patient that has the ability to directly measure forces that occur within the knee.

Medical history was made in San Diego recently when a surgeon from the Scripps Clinic Division of Orthopaedic Surgery implanted into a patient a total knee prosthesis that has the ability to directly measure forces that occur within the knee.

This type of prosthesis has never before been implanted in a living patient and is expected to generate information that will lead to major advances in knee implant design.

The custom-designed implant, known as an electronic knee, contains transducers, a microtransmitter and an antenna that allow transmission of force measurements into a computer-readable format.

This technology will allow scientists at the Scripps Clinic Center for Orthopaedic Research and Education (SCORE) to measure in real-time the distribution of forces to the implant and the load-carrying capabilities of the bone as the patient walks, climbs stairs and exercises. This knowledge will give scientists, engineers and industry the ability to design better knee replacement prostheses in the future.

“Until now engineers have mathematically estimated forces that occur across the knee joint,” explained Clifford W. Colwell, Jr., MD, Director of SCORE, who performed the surgery.

“Now we will know exactly where the forces are coming from for each type of physical activity the patient engages in. These specific measurements will not only result in better implant design, but will also give us the ability to measure the effectiveness of other devices such as orthotics and braces,” Colwell added.

Other possible applications include better design of sporting equipment and specialised footwear.

The electronic knee project is the result of a unique collaboration among clinicians, scientists and industry professionals. “We spent over 13 years in the development process,” said John Slamin, an engineer with DePuy Orthopaedics, a Johnson & Johnson company, manufacturer of the implant.

“MicroStrain is proud to be part of the team that made this technology breakthrough possible,” said Steven Arms, President of MicroStrain, which develops wireless microsensors. “Our wireless sensor microelectronics expertise has made it possible for new data to speed improvements in the field of knee replacements.”

Another contributor to this project is NK Biotechnical, a specialty engineering firm from Minneapolis.

According to Slamin, the electronic knee implant is similar to a standard implant on the top half where it has a typical femoral component and polyethylene articular surface. The lower metallic component, made from titanium, is customised so that it allows measurement of the loads that occur across the knee.

Four metal posts separate the top and bottom plates and underneath the posts are transducers that measure the strain that changes within the metal as the patient moves. The transducers are wired to a transmitter, which is connected to an antenna.

The antenna transmits the information to a receiver outside the body. A removable coil fits around the knee and externally activates the system, then transmits data into a computer system. The electronic signals are converted into actual loads in pounds at each one of the four posts as the patient moves.

With approximately 400,000 knee replacement surgeries performed every year in the US and the population of active seniors growing, the financial impact of designing better, longer-wearing knee implants is significant.