Viscoelastic properties of a synthetic meniscus implant

There are significant potential advantages for restoration of meniscal function using a bio-stable synthetic implant that combines long-term durability with a dependable biomechanical performance resembling that of the natural meniscus. A novel meniscus implant made of a compliant polycarbonate-urethane matrix reinforced with high modulus ultrahigh molecular weight polyethylene fibers was designed as a composite structure that mimics the structural elements of the natural medial meniscus. The overall success of such an implant is linked on its capability to replicate the stress distribution in the knee over the long-term. As this function of the device is directly dependent on its mechanical properties, changes to the material due to exposure to the joint environment and repeated loading could have non-trivial influences on the viscoelastic properties of the implant. Thus, the goal of this study was to measure and characterize the strain-rate response, as well as the viscoelastic properties of the implant as measured by creep, stress relaxation, and hysteresis after simulated use, by subjecting the implant to realistic joint loads up to 2 million cycles in a joint-like setting. The meniscus implant behaved as a non-linear viscoelastic material. The implant underwent minimal plastic deformation after 2 million fatigue loading cycles. Under low compressive loads, the implant was fairly flexible, and able to deform relatively easily (E=120-200 kPa). However as the compressive load applied on the implant was increased, the implant became stiffer (E=3.8-5.2 MPa), to resist deformation. The meniscus implant appears well-matched to the viscoelastic properties of the natural meniscus, and importantly, these properties were found to remain stable and minimally affected by potentially degradative and loading conditions associated with long-term use.