Background: Implantable grade silicon nitride (Si3N4), such as MC2 TM from Amedica, is an attractive alternative to the common bearing materials used today for spinal and orthopedic applications. Ceramic bearings used in total hip arthroplasty (THA) consist of a femoral head (ball) articulating inside an acetabular cup (socket); the ball and socket are made of alumina (Al2O3) or Al2O3-based composite materials such as zirconia toughened alumina. Extensive studies on the mechanical, imaging and tribological characteristics of MC2 for spinal implants have been conducted. This paper summarizes some of the results from these studies.
Methods: Si3N4 materials properties were evaluated per ASTM standards C-1161 for flexural strength and E-399 for toughness, using the single edge notched beam method. Strength was also measured after subjecting the ceramic to accelerated aging conditions. Imaging compatibility of the MC2, PEEK, titanium, cobalt-chromium (CoCr) and trabecular metal discs was determined under a variety of imaging methods, including MRI. Wear testing of Si3N4 acetabular cups articulating against Si3N4 and CoCr femoral heads was also conducted. Biocompatibility evaluation of the Si3N4 was conducted per ISO 10993 protocols.
Results: MC2 Si3N4 exhibits improved mechanical properties over modern Al2O3 and Al2O3-based composite THA bearings, with a flexural strength of 920 ± 70 MPa, a Weibull modulus of 19, and a fracture toughness of 10 ± 1 MPa.m1/2. Accelerated aging of Si3N4 did not adversely affect the flexural strength. The imaging comparison study showed that Si3N4 did not exhibit any MRI imaging artifacts similar to PEEK, but unlike the metallic materials used for spinal implants. In wear tests using a hip simulator, Si3N4 acetabular cups produced low volumetric wear when articulating against Si3N4 or CoCr femoral heads. The implantable grade Si3N4 material was also found to be biocompatible.
Conclusions: Si3N4 exhibits a superior combination of properties including strength, toughness, MRI compatibility, and wear resistance. This may allow spinal implants with an important combination of properties than hitherto possible, including a wider range of design options, improved anatomic fit and imaging compatibility. Additionally, spinal vertebral body replacement implants were tested in static and fatigue conditions per the ASTM F 2077 and have been cleared by the FDA. An MC2 Si3N4 based total cervical disc implant has been designed and is undergoing pre-clinical testing. The versatility of the MC2 Si3N4 ceramic may enable its use in total disc replacements owing to its combination of superior wear resistance and imaging compatibility.
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Acknowledgement: Some of the work was supported by a Phase 1 NIH-SBIR Program grant titled: Composite Metal –Ceramic Bearings for THA Implants, Grant # R44-AR45517-01.