Polymer matrix composite materials have been widely promoted for orthopedic use in a variety of settings, including surgical instruments, medical devices, implants, and bone models.1-13 These types of composites are engineered from 2 or more constituent materials with significantly different physical or chemical properties; these materials remain separate and distinct on a macroscopic level within the finished composite structure. As a result of ongoing biomaterial research, polymer matrix composite materials can be engineered with a wide range of physical, mechanical, and surface properties, tailored to their application. Given their advantages (eg, high strength-to-weight ratio, radiolucency), these polymer matrix composite materials have gained popularity over traditional metallic materials.
Sterilization is an essential day-to-day procedure in the health care sector, both for single- and multiple-use devices or instruments, and thus a composite material used in medical components should remain unaffected by the process. The type of sterilization most commonly performed is steam sterilization, which achieves microbiological death by moist heat and pressure. Steam is created in an autoclave at a temperature of 132°C (270°F) in typical hospital settings. Steam sterilization cycles last 5 to 14 minutes based on specific manufacturer recommendations. Most medical-grade plastics used in health care have been designed and formulated to withstand the required sterilization cycles without sacrificing key properties. The structure integrity and overall performance of polymer matrix composites may be strongly influenced by the stability of the fiber/polymer interfacial region in terms of physical, chemical, and mechanical characteristics of the material at different scales.14 Absorption of moisture causes dilatational expansion and induces stresses, which are associated with the moisture-induced expansion resulting in degradation of structure stability.15 Thus, steam sterilization could affect the properties of the polymer matrix composite materials by excessive absorption of moisture by the polymer.
To our knowledge, no one has studied whether polymer matrix material properties degrade from long-term, repeated steam sterilization followed by mechanical loading. We conducted a study to evaluate the structural properties (short-beam strength, SBS) of several composite materials exposed to repeated sterilization as compared with traditional metal materials: SS-316L (stainless steel 316L) and Al-7075-T6 (aluminum 7075-T6).
Materials and Methods
We evaluated 3 types of composite materials: Tepex (Tepex Dynalite 201; HiPer Technology Inc.), CFR-PPS (carbon-fiber–reinforced polyphenylene sulfide, Cetex PPS; TenCate Advanced Composites USA Inc.), and HTN-53 (Zytel HTN53G50HSLR NC010; HiPer Technology Inc.) (Figure 1). Tepex is being used for orthopedic applications (knee braces, orthoses, insoles) and sporting goods applications. The performance of this material is superior to that of unreinforced thermoplastics. CFR-PPS represented the state of the art in composite materials for aerospace applications (eg, airframe structures, engine nacelles, fan casings, floorboards, interior parts). This is a high-performance material with exceptional high temperature and aggressive chemical resistance characteristics. CFR-PPS is also used to make filter fabric for coal boilers, papermaking felts, electrical insulation, specialty membranes, gaskets, and packing. It is not solubilized by any known solvents, even in long-term exposure, at temperatures up to 200°C. In addition, it exhibits exceptional resistance to organic and inorganic solutions, acids and alkali solutions, and a wide array of miscellaneous chemicals. HTN-53 is a 50% glass-reinforced, lubricated, high-performance polyamide resin with improved flow, developed for applications requiring excellent surface appearance with water-heated molds. This material has specifically shown survivability in hot, cold, chemically aggressive, and load-bearing environments. In addition, it has shown superior moisture and temperature resistance. These 3 composite materials were compared with SS-316L and Al-7075-T6. SS-316L is commonly used for implants in orthopedics, and Al-7075-T6 is a relatively radiolucent alternative for medical applications. Two different tests were performed to evaluate and validate these composite materials: (1) radiographic density evaluation and (2) structural property tests (short-beam load-to-failure [LTF] test, short-beam cyclic compression loading [CCL] test) before and after sterilization cycling.
Radiographic Density Evaluation
The radiographic density of the 5 materials was evaluated with radiographic images of a cadaveric knee specimen (Figure 2). Radiographic image intensification is the gold standard for repeated radiographic imaging in the operating room. Six different radiographic images were obtained for each material superimposed over a cadaveric knee to recreate potential instrument positioning during surgery: posterior to subject (1 piece), posterior to subject (2 pieces), anterior to subject (1 piece), anterior to subject (2 pieces), anterior and posterior to subject in alignment (1 piece), and anterior and posterior to subject in alignment (2 pieces). Image-Pro Plus software (Media Cybernetics) was used to measure the radiographic density of the materials from the grayscale of the images.
