A newly designed implant for direct attachment of a prosthesis to the skeletal system after transfemoral amputation can provide better bone maintenance and lower failure risk during normal walking compared with current osseointegrated transfemoral prostheses, according to a recent study.
“From literature we already knew there were patients with excessive bone loss and bone damage around implants, so we decided to analyze it,” said Pawel K. Tomaszewski PhD, of the department of biomedical engineering at the University Medical Center Groningen in Groningen, The Netherlands, and Orthopaedic Research Lab at the Radboud University Medical Center Nijmegen Medical Center, Nijmegen, The Netherlands. “Using results from the analysis, we decided to design a different fixation implant, one that could help people with shorter stumps to make use of this technology and that could generate lower stresses and be safer for patients.”
Failure risk parameter
Tomaszewski and colleagues created generic 3D models of intact femoral bones and amputated bones implanted with direct-fixation implants then loaded each model with a normal walking and a forward fall load. Each model was based on a prosthetic implant: The OPRA system, the ISP Endo/Exo prosthesis and a new design created by the researchers. Researchers used the strain adaptive bone remodeling theory to predict long-term bone changes around the implants and the von Mises stress criterion to evaluate the periprosthetic bone failure risk.
Overall, the new prosthetic implant design provided a near-physiological distribution of stresses in the bone and lower bone failure risk for normal walking vs. the OPRA and ISP implants, according to study results. For the second load configuration, researchers found the highest failure risk for the OPRA and ISP implants in the bone region in contact with the proximal end of their stems.
Study results showed, when periprosthetic bone failure risk was assessed for walking and forward fall loading of the implants directly postoperative and after 60 months, higher bone failure risk was found by falling vs. normal walking. Reduction of the failure risk parameter (FRP) for all implants and load configuration was caused by bone remodeling. The ISP implant had the highest FRP during normal walking, whereas the new design had the lowest. The FRP was also highest in the ISP implant during the forward fall.
“Whether the new implant is sensitive to adverse loading conditions, such as occurring during a falling incident, needs to be further investigated,” the researchers concluded. “This positive outcome should encourage further developments of the presented concept, which in our opinion has a potential to considerably improve safety of the rehabilitation with the direct fixation implants and allow treatment of the patients with short stumps.”
“On the one hand, we are expecting to have the implant offer safer fixation, which is especially important for patients with heavy disuse of steel prosthetics,” Tomaszewski said. “The second thing is that we are able to use shorter implants and have people with short femur remnants to use this kind of fixation implant for external prosthetic systems. Additionally, it can be adapted to fix other implants in long bones, like total hip or knee replacements.”
Tomaszewski and colleagues followed up the study by patenting their new implant design and are looking for a group of interested investors.
“We patented our new design and are looking for investors who could take the project to clinical phase,” Tomaszewski said. “We are thinking about running additional studies, but for this we really partners from the commercial side. We’re looking for someone who has more experience than us to commercialize it.”
Disclosure: Tomaszewski is an inventor/applicant of the pending patent “Osseointegration system for a long bone.” The study was supported by Fonds NutsOhra grant. Invibio Ltd. and Eurocoating spa provided materials for testing.