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.
Structural Properties Testing Before and After Sterilization Cycling
We used a standard SBS testing method to determine whether any degradation of structural properties resulted from standard repeated sterilization. The material geometries of the test specimens were 18.96×6.50×3.37 mm (length × width × thickness). Standard sterilization procedures were performed with steam sterilization using an autoclave at a temperature of 132°C (270°F) for at least 5 minutes (range, 5-14 minutes). Sample interval testing ran at 0, 200, and 400 sterilization cycles for structural properties in terms of SBS and moisture retention, with the structural properties at the 0th sterilization cycle (material before sterilization was performed) used as a baseline for comparison. Materials were subjected to 400 sterilization cycles, which is representative of the number of sterilization cycles per year an instrument or device would be subjected to.
Three structural tests were performed for each sample interval: moisture retention, LTF, and CCL. Moisture retention was investigated before and after repeated sterilization by measuring the weight of the test materials, as steam sterilization is known to affect the amount of moisture that is absorbed by a material. Twelve specimens of each proposed material were weighed at each sample interval, with the structural weight at the 0th sterilization cycle (material before sterilization is performed) serving as a baseline for comparison.
SBS testing was based on the ASTM (American Society for Testing and Materials) D2344 standard16 for LTF and CCL tests (Figure 3). Six samples of material were used for each test at every sample interval, yielding 180 samples. Seven servohydraulic material testing system instruments (1 MTS 810 and 6 MTS 858 Mini Bionix) were used to test the SBS of each material. For LTF testing, each specimen was loaded in compression from 30 N to complete structural failure at a constant displacement rate of 1.0 mm/min (0.05 in/min). Testing was initiated with 5 preconditioning loading cycles from 30 to 100 N at 1 Hz. The load was then applied continuously until failure occurred; force and displacement data were collected every 0.02 second. This procedure was performed for 6 replicates for each sample interval for each test material.
The calculation for SBS, Fsbs (MPa), for the constant loading rate until structural failure is:
Fsbs = 0.75 × Pm
b × h
where Pm (N) is the maximum applied load observed during the test, b is the measured specimen width (mm), and h is the measured specimen thickness (mm).
CCL testing consisted of each test material axially loaded with 100 to 500 N at a frequency of 1 Hz for 100,000 cycles. The maximum load of 500 N was chosen as a standard based on 80% of the minimum ultimate failure load from previous LTF tests. Displacement and force data were collected every 5 cycles at the maximum compressive load. Degradation of the material was calculated using the difference between the deflection of the initial cycle and the deflection of the final cycle (50th cycle and 100,000th cycle). This procedure was performed for 6 replicates for each sample interval for each test material.
Statistical Analysis
LTF and CCL testing data were analyzed for any differences among the test materials using 1-way analysis of variance with the least significant difference multiple comparisons post hoc test method using SPSS Version 16.0, with P < .05 denoting significance. These analyses were used to determine the statistical relevance of the difference between the SBS (LTF and CCL) of each test material. Means and standard deviations were calculated for all tests.
Results
Radiographic Density Evaluation
Overall, all the tested composite materials were significantly more radiolucent than either SS-316L or Al-7075-T6. Figure 4 shows the 6 different radiographic images 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). SS-316L can be considered radiopaque, and Al-7075-T6 has been used as a relatively radiolucent alternative. Tepex was statistically more radiolucent than the other 2 tested composite materials (Table 1). Even with 2 pieces placed anterior to the subject and 2 placed posterior, the radiodensity compared to the cortical bone was still lower than 1 piece of Al-7075-T6 either anterior or posterior to the subject.
