Jonathan J. Elsner

Design of a Free-Floating Polycarbonate-Urethane Meniscal Implant Using Finite Element Modeling and Experimental Validation

The development of a synthetic meniscal implant that does not require surgical attachment but still provides the biomechanical function necessary for joint preservation would have important advantages. We present a computational-experimental approach for the design optimization of a free-floating polycarbonate-urethane (PCU) meniscal implant. Validated 3D finite element (FE) models of the knee and PCU-based implant were analyzed under physiological loads. The model was validated by comparing calculated pressures, determined from FE analysis to tibial plateau contact pressures measured in a cadaveric knee in vitro. Several models of the implant, some including embedded reinforcement fibers, were tested. An optimal implant configuration was then selected based on the ability to restore pressure distribution in the knee, manufacturability, and long-term safety. The optimal implant design entailed a PCU meniscus embedded with circumferential reinforcement made of polyethylene fibers. This selected design can be manufactured in various sizes, without risking its integrity under joint loads. Importantly, it produces an optimal pressure distribution, similar in shape and values to that of natural meniscus. We have shown that a fiber-reinforced, free-floating PCU meniscal implant can redistribute joint loads in a similar pattern to natural meniscus, without risking the integrity of the implant materials.

A Novel Quantitative Approach for Evaluating Contact Mechanics of Meniscal Replacements

One of the functions of the meniscus is to distribute contact forces over the articular surfaces by increasing the joint contact areas. It is widely accepted that total/partial loss of the meniscus increases the risk of joint degeneration. A short-term method for evaluating whether degenerative arthritis can be prevented or not would be to determine if the peak pressure and contact area coverage of the tibial plateau (TP) in the knee are restored at the time of implantation. Although several published studies already utilized TP contact pressure measurements as an indicator for biomechanical performance of allograft menisci, there is a paucity of a quantitative method for evaluation of these parameters in situ with a single effective parameter. In the present study, we developed such a method and used it to assess the load distribution ability of various meniscal implant configurations in human cadaveric knees (n=3). Contact pressures under the intact meniscus were measured under compression (1200 N, 0 deg flexion). Next, total meniscectomy was performed and the protocol was repeated with meniscal implants. Resultant pressure maps were evaluated for the peak pressure value, total contact area, and its distribution pattern, all with respect to the natural meniscus output. Two other measures--implant-dislocation and implant-impingement on the ligaments--were also considered. If any of these occurred, the score was zeroed. The total implant score was based on an adjusted calculation of the aforementioned measures, where the natural meniscus score was always 100. Laboratory experiments demonstrated a good correlation between qualitative and quantitative evaluations of the same pressure map outputs, especially in cases where there were contradicting indications between different parameters. Overall, the proposed approach provides a novel, validated method for quantitative assessment of the biomechanical performance of meniscal implants, which can be used in various applications ranging from bench testing of design (geometry and material of an implant) to correct implant sizing.

Design of a Polycarbonate-Urethane Meniscal Implant: Finite Element Approach

The medial meniscus plays an important role in the knee joint [1]. Meniscus dysfunction due to tear is a common knee injury which leads to degenerative arthritis, attributed primarily to the changes in knee load distribution [2]. Clearly, there is a substantial need to protect the articular cartilage by either repairing or replacing the menisci. A “floating” Polycarbonate-Urethane (PCU) meniscal implant (Fig. 1a) is proposed as a solution for restoring the function of the missing meniscus and for the reduction of pain, through improved tibial and femoral pressure distribution. The implant is composed of PCU embedded with polyethylene reinforcement fibers (“Dyneema®”, DSM), and its design is based on the geometry of the articulating surfaces of the femur and tibia. Our goal was to develop an optimal meniscal implant design (in terms of composition and geometry), whose contact pressure with the tibial plateau (TP) would be similar to that of the natural meniscus and be able resist mechanical failure of any of its components. We hereby present one aspect of the implant bench-tests, finite element (FE) analyses of the implant in the medial knee under physiological relevant loading conditions.Copyright

Chondroprotective Effects of a Polycarbonate-Urethane Meniscal Implant: Semi-Quantitative Results in a Sheep Model

The menisci play a critical role in load-bearing and stability of the knee joint [1]. Damage or removal of the meniscus leads to alterations in the magnitude and distribution of stresses in the knee, which have been associated with degenerative osteoarthritis [2]. Clearly, there remains a need to develop means of protecting the articular cartilage following meniscal injury by either repairing or replacing the menisci. While allograft meniscal replacements can improve joint stability and function, they often provide little benefit in preventing osteoarthritic changes [3]. The development of an artificial meniscus that is available at the time of surgery in several sizes that can accommodate most patients would provide important therapeutic potential for treatment meniscal injury.Copyright

Design Optimization of a Polycarbonate-Urethane Meniscal Implant in the Sheep Knee: In-Vitro Study

Meniscus replacement still represents an unsolved problem in orthopedics. Allograft meniscus implantation has been suggested as a means to restore contact pressures following meniscectomy. However, issues such as graft availability, disease transmission, and size matching still limit the use of allograft menisci. Furthermore, the complexities of meniscal repairs may contribute to uneven distribution of load, instability and initiation of degenerative damage. A synthetic meniscal substitute could have significant advantages for meniscal replacement, as it could be available at the time of surgery in a substantial number of sizes and shapes to accommodate most patients. There is, however, a need to establish an optimal configuration of such an implant that would result in pressure distribution ability closest to that of the natural meniscus.Copyright

A Novel Method for Magnetic Isolation and Characterization of Polycarbonate-Urethane Wear Particles

Ferrography is a method for separating wear particles onto a slide. The method is based on the interaction between an external magnetic field and the magnetic moments of the particles suspended in a flow stream. It is advantageous in providing high detectability rate for a relatively large range of particle sizes (0.5–200 μm) [1]. A newer generation of ferrography, known as Bio-Ferrography, allows particles from five fluid samples to be isolated simultaneously on one slide and analyzed in terms of their number, chemistry, shape, dimensions, surface morphology, structure, etc. Since magnetization does not naturally occur in polymeric and biological materials, wear particles of such origins must be magnetized prior to Bio-Ferrography. This can be done, for instance, by binding to a ferromagnetic element, such as Er+3, originating from erbium chloride (ErCl3) solution. Such Bio-Ferrography technology has already been applied successfully in hip wear simulations for the separation of ultrahigh molecular weight polyethylene (UHWPE) wear debris suspended in bovine serum as lubricant [2].Copyright

Meniscal Implant Biomechanical Performance: A Novel Quantitative Approach

One of the functions of the meniscus is to distribute contact forces over the articular surfaces by increasing joint contact areas [1]. It is widely accepted that total/partial loss of the meniscus increases the risk of joint degeneration. A short-term method for evaluating whether degenerative arthritis can be prevented would be to determine if peak pressure and contact area coverage of the tibialis plateau (TP) articular surface in the knee are restored at the time of implantation. Although several published studies already utilized TP contact pressure measurements as an indicator for biomechanical performance of allograft menisci [2,3], there is a paucity of a quantitative method for evaluation of these parameters in situ with a single effective parameter. In the present study, we developed such a method and employed it on sheep and human cadaveric knees.Copyright