Smart orthopedic implants--those incorporating sensors capable of providing information about conditions inside the body--have been created in the research world for decades. But because they incorporate large, expensive, and complex electronic and sensing components, many smart implants have never made the leap from the laboratory to the operating room. Now, however, engineers at Rensselaer Polytechnic Institute (Troy, NY) have developed a sensor technology for use in orthopedic implants with one primary virtue: simplicity.

"The reason why smart orthopedic technology has never made it into daily clinical practice is that in all previous research applications, my own included, the electronics and sensors were very expensive and generally large and cumbersome," remarks Eric Ledet, assistant professor of biomedical engineering at Rensselaer. "More importantly, however, they could only be incorporated into custom implants--be they fracture plates or knee or hip prostheses." Thus, even if the cost of such sensors were not prohibitive, the need to custom modify each implant would render the technology unsuitable for daily clinical practice, Ledet adds.

Historically, strain gauges--tiny sensors used for measuring very small amounts of deformation--have been employed in the lab to make smart orthopedic implants. However, such sensors can only be mounted on implants that have undergone extensive surface preparation. "Strain gauges actually have to be glued on, creating a permanent bond," Ledet says. "Thus, you have to modify the implant pretty substantially just to get the old type of sensors on."

In contrast, the Rensselaer researchers' sensors can be integrated with little, or no, modifications to the implants. Tiny disks measuring as little as 4 mm in diameter by 500 µm thick, they can be attached to the implant by means of mechanical attachments or simple adhesives. Moreover, their implantable surfaces do not have electrical connections. Consequently, in polyethylene-metal knee implants, for example, the sensors can be placed in between the polyethylene component and the metal tibial component. "Their small size affords us many opportunities for slipping them into nooks and crannies," Ledet comments.

Intended to serve as smart components in future orthopedic implants, the new sensors measure a range of parameters at the surgery site, including force, pressure, and temperature, Ledet explains. Transmitted wirelessly, information about the condition of the implant is detected using an external antenna and then sent to a reader.

"The parameters measured by the sensor have one thing in common," Ledet says. "They all indicate a deformation of the sensor." For example, a sensor placed on a tibial tray in a total knee replacement can be used to measure the forces operating on the implant, enabling clinicians to gather crucial postsurgical information, Ledet adds. "Is the prosthesis being loaded appropriately? Is it going to wear properly? Is there micromotion in the femoral component that could lead to osteolysis? Our sensors can measure these and other conditions."

While the engineers acknowledge that their technology is not yet ready for prime time, they believe that it does not face any insurmountable hurdles. "Because there is pent-up demand for smart-implant technology, we are confident that
we can move forward rapidly with our concept because it is a simple system with few ways to fail."