Long-Term Elastic Durability of Polymer Matrix Composite Materials After Repeated Steam Sterilization
We compared the durability of 3 different selected composite materials that underwent repeated steam sterilization with the durability of traditional metal materials. Composite materials Tepex, CFR-PPS (carbon-fiber–reinforced polyphenylene sulfide), and HTN-53 (Zytel HTN53G50HSLR NC010) were evaluated for durability and water retention after repeated steam sterilization. These composites were compared with stainless steel and aluminum. The structural properties of these materials were measured (short-beam load-to-failure and cyclic compression loading tests) before, during, and after repeated steam sterilization. The relative radiographic density of these materials was also compared.
There was no significant difference in the moisture retention of these composite materials before and after repeated sterilization. The composite materials were significantly more radiolucent than the metals. For all the composite materials, load to failure deteriorated after repeated sterilization. The cyclic compression loading tests showed HTN-53 had the poorest performance, with complete failure after 400 cycles of repeated sterilization. CFR-PPS performed slightly better, with 33% failure at final testing. Tepex had no failures at final testing.
Although HTN-53 has shown promise in other orthopedic applications, its performance after repeated sterilization was relatively poor. Tepex showed the most potential for durability after repeated sterilization. Further study is needed to identify specific applications for these materials in the orthopedic industry.
Numerous studies have documented the successful clinical performance of composite materials in orthopedic, trauma, and spinal surgery applications.41-45 Bagheri and colleagues41 developed a new carbon fiber–flax–epoxy composite plate and biomechanically compared it with a standard clinical metal plate. Their results confirmed that the carbon fiber–flax–epoxy material represents a potential candidate for bone fracture plate applications, as it can simultaneously provide similar mechanical stiffness and lower stress shielding (higher bone stress) compared with a standard clinical metal bone plate. Tarallo and colleagues45 evaluated the clinical results of 40 cases at 12-month follow-up using a new plate made of carbon-fiber–reinforced polyetheretherketon (DiPhos-RM, Lima Corporate) for the treatment of distal radius fractures. They reported good clinical results for this device at early follow-up, and its use allowed maintenance of reduction in complex AO (Arbeitsgemeinschaft für Osteosynthesefragen) fractures.
The main advantage in using composites for surgical instruments is their radiolucency. These materials do not obscure images or radiographs during fluoroscopic visualization. Surgery often requires fluoroscopic visualization of internal organs or bones, which may require temporary removal of radiopaque devices (eg, retractors, clamps, forceps, hooks, distractors). Aside from being inconvenient, removal and subsequent reinsertion consume valuable time and interfere with the smooth flow of an operation.
The shortcomings of using composite materials for surgical instruments involve detectability and sterilization. A significant issue in surgery is the accidental leaving behind of instruments in patients, which can cause serious problems ranging from organ perforation and blood infection to death. Although instrument counting and other safety protocols can reduce the risk of overlooking an instrument, mistakes are bound to happen. The other shortcoming is the influence of repeated sterilization on the mechanical properties of the composite materials, as sterilization is mandatory for surgical instruments used in the operation room. The structural integrity and overall performance of the polymer composite materials—especially the stability of the interface and the interphase zones—are strongly influenced by repeated sterilization.
On the other hand, composite materials have potential advantages that may support their introduction into long-term medical implant applications, as sterilization commonly is performed only once, during packaging. The effects of sterilization by radiation or steam are much less pronounced on composite implants than on composite surgical instruments. However, composite implants require careful consideration with respect to the bioactivity of wear particles that may be produced from articulation. Further, carbon-fiber–reinforced polymer implants are still substantially more difficult to manufacture and more costly than their metallic counterparts.46
Limitations
This study has some limitations. Most important, studies of this nature do not account for biological factors such as corrosion, biological wear, and the soft-tissue attachment effects on structural properties for potential in vivo use. Another limitation was that the study tested only the mechanical properties in terms of SBS and provided no information about other mechanical properties, such as tensile, compression, and torsion strengths. We think SBS testing adequately evaluates challenging scenarios like thin and narrow instruments/devices that are anticipated in application, and information regarding other modes of failure and mechanical properties (compression, tension, torsion) would be a further area of research. An additional limitation was that our model used a relatively small number of samples. A larger study with more samples and varying layout patterns and layers of the carbon fibers may more clearly demonstrate the effect of steam sterilization on composite materials.
Conclusion
This study provided new information on 3 selected composite materials and their structural properties after repeated steam sterilization. We discovered that these composites were similar in radiographic density and water retention but behaved very differently in terms of mechanical durability after repeated steam sterilization. All selected composites demonstrated deterioration of mechanical properties after repeated steam sterilization. Knowing these results could aid in making decisions about the design and manufacturing of operative instruments and orthopedic biomaterials. Although our preliminary findings are intriguing, further study is warranted to seek specific applications for these composite materials in orthopedic surgery.
