In vivo evaluation of polymer implants for cartilage regeneration and joint load monitoring

Abstract

Osteoarthritis, which affects over 21 million people and costs the US $61 billion/yr, is devastating the US population and taxing the health care system. These numbers will increase exponentially as the population ages. It is reported that previous trauma to cartilage resulting in focal chondral defects progresses to osteoarthritis if treatment is delayed or unsuccessful. Current treatment modalities for focal chondral defects have had variable success rates. As such scaffold based therapies in combination with tissue engineering are being developed as an alternative therapy for focal chondral defects. One important area of research to be addressed for these therapies to be successful is rapid integration of native tissue with the implant. An advantage of using scaffold based therapies is that scaffolds provide a stable surface for tissue to grow on and integrate with the existing tissue. In addition, there is the opportunity to use scaffolds for measuring joint loading. These measurements are crucial for a better understanding of the loading environment leading to osteoarthritis as well as for development of rehabilitation regimens when tissue engineering is used to treat defects. It is the goal of this research to determine if mimicking the native trabecular bone structure can be utilized to promote rapid bone ingrowth into implants and to determine whether these implants can be used to directly measure in vivo joint loads. To address the goals of this study, polybutylene terephthalate scaffolds were designed and then built using a fused deposition modeling system. Two different scaffold designs were utilized to determine if mimicking bone structure results in improved bone ingrowth. One scaffold was a biomimetic scaffold that replicated the trabecular bone structure and the other utilized a simple porous structure. These scaffolds were also equipped with strain gauges so that they could be used to monitor joint loading within the knee joint. The strain gauges were used in combination with implantable miniature radio transmitters to allow a fully internal measurement system to be used to determine joint loads during gait as well as other weight bearing activities. Using histology and μCT it was observed that the biomimetic scaffolds increased bone ingrowth into the scaffold over 500% compared to the simple porous scaffolds. These biomimetic scaffolds also increased bone growth in the areas adjacent to the scaffold. Additionally, it was demonstrated that these scaffolds when outfitted with strain gauges could measure axial joint loads occurring within the knee joint during various activities. It was noted that the temporal measurements were highly correlated with video analysis and that peak loads increased as a function of time post implantation. The ability of biomimetic scaffolds to increase bone ingrowth is important for anchoring the scaffold in place and allowing successful integration of tissue engineered cartilage with the native tissue. This will improve success rates of scaffold based tissue engineering therapies. The ability of implants to measure joint loads is crucial to developing a better understanding of osteoarthritis as well as improving rehabilitation protocols. Additionally, by monitoring the change in peak loads with time it will be possible to monitor the healing response at the implant site. Overall, this research demonstrates that polybutylene terephthalate scaffolds have the ability to be used in combination with tissue engineering constructs to treat focal chondral defects and are capable or providing direct in vivo loading measurements.