Long-term Wear Evaluation of an Artificial Medial Meniscus Implant

Transcription

1 Long-term Wear Evaluation of an Artificial Medial Meniscus Implant Jonathan J. Elsner, PhD, Maoz Shemesh, MSc, Adaya Shefy-Peleg, MSc, Eyal Zylberberg, BSc, Eran Linder-Ganz, PhD. Active Implants, Netanya, Israel. Disclosures: J.J. Elsner: 3A; Active Implants. 4; Active Implants. M. Shemesh: 3A; Active Implants. A. Shefy-Peleg: 3A; Active Implants. E. Zylberberg: 3A; Active Implants. E. Linder-Ganz: 3A; Active Implants. 4; Active Implants. Introduction: Long-term wear experiments are an important tool to evaluate the potential of success of joint replacement devices during the stages of research and development. A synthetic and readily functional non-anchored polycarbonateurethane (PCU) meniscus implant, reinforced with UHMWPE fibers, was developed for medial meniscus replacement (NUsurface, Active Implants Corp., Memphis TN, USA). The implant is distinctively different from most orthopaedic implants in following the concept of soft bearings being composed of a compliant material, and also in its non-fixed femur-conforming design which enables implantation through a minimal arthrotomy, without need for bone resection or attachment [1]. Due to these unique features of the implant, it is important to show that the material and design of the device are appropriate under rigorous conditions which occur in the knee joint. Previous studies showed the static behaviour of the device in terms of loadtransfer after implantation [2], and its viscoelastic characteristics after short-term use [3]. This study aimed to evaluate the longterm performance of this device by subjecting it to a full mixed-mode loading regime for a total of 5 million load cycles (Mc). A low wear rate as well as the implant s ability to maintain its structure, function, and chemical stability following repeated longterm loading in joint-like environment were considered key parameters for success. Methods: Three tests specimens of the NUsurface device (Active Implants, Memphis, TN) underwent full mixed-mode loading for the duration of 5 Mc to simulate long-term daily use. Forces and motions were exerted on the implant using a knee simulator configuration which was adjusted to the testing of the non-anchored device, while the device was immersed in heated simulated body fluid (adult bovine serum, diluted 1:4, 37 C). The loading pattern used in the simulation consisted of vertical loading, flexion-extension, anterior-posterior translation, internal-external rotation as dictated by ISO (wear testing of a total knee replacement), scaled down to the medial compartment. A fourth implant was only loaded with the vertical component to serve as a load-soak control for gravimetric wear evaluation. The wear rate was evaluated over the course of 5 Mc using gravimetric methods, by deducting the weight of the test specimens from that of the load-soak control. The integrity of the implant structure and overall volumetric wear was also investigated by performing micro-ct scans on the test specimens before and after the simulation. Three dimensional reconstructions of the scans were used to calculate volumetric changes which are associated with wear. Other characterization methods used to detect degradation of the implant after 5 Mc included readings of their mechanical properties (strain-rate response, creep, stress-relaxation, and hysteresis), functional properties (pressure distribution ability), surface properties and roughness (SEM/ AFM), oxidation (ATR-FTIR), and molecular weight (GPC). Due to the destructive nature of some of these tests, baseline measurements were conducted on additional virgin implants from the same manufacturing batch. Results: All three tested specimens showed very little wear. Gravimetric measurements demonstrate a typical implant wear pattern which consists of an initial run-in period followed by a low steady wear rate. The peak wear rate between 1 and 2 Mc (50 mg per Mc) was reduced to 20 mg per Mc after 3 Mc, and to almost zero between 4 and 5 Mc (Fig. 1). The average wear rate over the full duration of the test is less than 20 mg/mc, but with the long-term asymptotic wear rate close to zero, this value is most likely an overestimate of the long-term wear rate. Volumetric wear evaluation by Micro-CT was used in this study. The implants which underwent 5 million mixed-mode cycles presented a minor decrease in volume during the test, reaching an average total volumetric reduction of 53 ± 62 mm3 compared to the implants' initial volume. The average volumetric wear rate obtained during the test is 10.6 mm3 per Mc (12.6 mg/mc) which is slightly lower than the wear rate which was measured by direct gravimetric measurements. Micro-CT scans confirmed that the implants two components; PCU bulk and UHMWPE fibers, remained intact, and did not undergo structural changes. Importantly, neither creep of the reinforcing fibers, nor any deformation of the main bulk, were observed. The results of FTIR analysis of these implants show very little change in the chemical footprint on the surface of the implants. Average increase in oxidation compared to baseline measurements was found to be less than 6%, and not any different from control specimens placed in air with this respect. There was a slight increase in the number average (Mn) and weight average (Mw) molecular weights (92 to 96 kg/mol and 173 to 178 kg/mol, respectively) after simulation, when compared to an unused control implant. There was no difference in the polydispersity index (Mw / Mn) between the tested specimens and unused control implant (1.78). Microscopic inspection of the implants by SEM showed very minor levels of damage to their articulating surfaces. The superior articulating surfaces demonstrated small regions of negligible abrasive wear, which was located mainly in the middle-anterior portion of the implant. AFM scanning of these specific regions showed that the average roughness post simulation increased from 9.9 nm to nm, and an increase in the Peak-to-Valley roughness from 82 nm to a range of nm. There were no

2 signs of cracks or pits or any other mechanical damage on the superior surface. No mechanical damage was observed on the inferior articulating surface. Functionality test which were focused on measurement of the implants mechanical response to load, and pressure distribution ability on the tibial plateau did not show significant loss of function following the simulation. The strain-rate response, creep, relaxation and hysteresis responses remained unchanged compared to the virgin implant, and interestingly, the pressure distribution capacity of the implant only improved during the first 2 Mc (Fig. 2). It is hypothesized that due to the implant s compliant nature it is able to adapt to the articular surface during use, and occupy a somewhat larger contact area and to reduce contact pressures. Discussion: Significance: The artificial PCU-implant, designed for medial meniscus replacement, successfully underwent the simulation of 5 million load cycles under full mixed-mode conditions without dislocating or undergoing significant degradation. The implant was found to be sufficiently stable to withstand long-term loading in simulated body environment. Five million simulated load cycles were not found to affect its structural, mechanical, chemical or functional properties. Acknowledgments: This study was funded by Active Implants Corp. References: 1.Elsner et al., Design of a free-floating polycarbonate-urethane meniscal implant using finite element modeling and experimental validation. J Biomech Eng. 132(9): Linder-Ganz et al., A novel quantitative approach for evaluating contact mechanics of meniscal replacements. J Biomech Eng. 132(2): Shemesh et al., Viscoelastic properties of a synthetic meniscus implant. J Mech Behav Biomed Mat. In Press.

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4 ORS 2014 Annual Meeting Poster No: 1820

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