Meniscal tears are common knee injuries that subsequently lead to degenerative arthritis, attributed to changes in stress distribution in the knee. In such cases there is need to protect the articular cartilage by repairing or replacing the menisci. While traditionally, meniscal replacement involves implantation of allografts, problems related to availability, size matching, cost and risk of disease transmission limit their use. Another optional treatment is that of biodegradable scaffolds which are based principally on tissue engineering concepts. The variability in body response to biodegradable implants and the quality of the tissue formed still pose a problem in this respect, under intense knee loading conditions. Moreover, biological solutions are mostly limited to younger patients <40 years old. Therefore, the goal of this study was, to develop a synthetic meniscal implant which can replace the injured meniscus, restore its function, and relieve pain. A composite, non-fixed self-centering discoid-shaped meniscus implant (NUsurafce®, AIC, Memphis, TN), composed of polycarbonate-urethane (PCU) and reinforced circumferentially with UHMWPE fibers is proposed (Fig. 1). The implant geometry was based on an extensive MRI study of over 100 knee scans [1]. The proposed structure aims to mimic the circumferential collagen reinforcement of the natural meniscus. Biomechanical evaluation of the implant was focused on in-vitro measurements of contact pressure under the implant in cadaver knees and computational finite element (FE) analyses [2,3]. Pressure distribution on the tibial plateau (under the meniscus implant) was measured by pressure sensitive films (Tekscan, MA) and quantified with respect to the natural meniscus. FE analyses were used to evaluate internal stress and strains, and to support the selection of optimal implant configuration. The last pre-clinical step was a large-animal (sheep) study in which the cartilage condition was evaluated microscopically over six months [4].Statement of Purpose
Methods
Meniscus replacement still represents an unsolved problem in orthopedics. Allograft meniscus implantation has been suggested to restore contact pressures following meniscectomy. However, graft availability, infection, and size matching still limit its use. 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. In the current study we present an optimization method for meniscal implant design and employ in the development of artificial polycarbonate-urethane (PCU) meniscus implant in an ovine model. The construction of the gross implant structure was based on 3D interpolation of MRI scans of the native sheep meniscus in-situ. PCU-based samples based on this design were produced for testing. 35 ovine knee joints were tested. An experimental evaluation of the implants’ biomechanical performances was conducted by measuring pressure distributions on the tibial plateau (TP) during loading. Subsequently, a pressure score of 0 to 100% was calculated. The score reflects on the magnitude of peak pressure and contact area coverage with respect to the natural meniscus. Implant design was reevaluated following changes to the initial implant configuration, e.g., modification of implant geometry, adding reinforcement material, and the applying of different fixation forces during implantation. The effect of these changes on pressure distribution was assessed by additional compression tests. The initial all-PCU implant showed limited ability to distribute pressure, The pressure score of 37% calculated for this case reflects on the small contact area (151mm2) subject to relatively high contact pressures (>
1.85MPa). The implant’s ability to distribute pressure improved significantly when circumferential reinforcement fibers were added. Applying a pretension force of 20N during fixation, improved pressure distribution and increased the contact area (273mm2). A small region of focal pressure concentration still existed in this case, but the pressure score increased markedly to 77%. Finally, it was found that optimal pressure distribution (87%) can be attained when a force of 30 to 50N is applied. In this configuration, peak pressures and coverage area (1.65MPa and 310mm2) were similar to those of the natural meniscus (1.61MPa and 373 mm2, respectively). We conclude that peripheral reinforcement of the implant (similar to the natural meniscus microstructure), in addition to pretension of 30 to 50N can significantly improve TP pressure distributions. The results are in agreement with other studies, reported on pressure distribution improvement due to reinforcement and/or pretension. We believe that the current device can be used in future as a practical solution for patients suffering from severe meniscal injury.
