Abstract
Introduction
Mechanical properties of irradiated Ultra High Molecular Weight Polyethylene (UHMWPE) after aging have been well documented. However there was no sufficient data for the dimensional change due to irradiation and aging. This change may have adverse effects to the implant modular locking mechanism. The purpose of this study was to characterize the dimensional change of UHMWPE after irradiation and aging.
Materials and Method
Total (30) ø15mm × 50mm virgin GUR 1050 UHMWPE rods were cleaned, dried, inspected, vacuum packaged and stored in 20°C environment for 2 days. Among them, (20) samples were measured along the 50mm length at 20°C +/-2°C before and after two conditions: 1, (10) were submerged in 40°C DI water for 2 hours and dried in 40°C to simulate the cleaning process and 2, (10) were soaked in 37°C saline for 14 days to simulate initial in-vivo environment. Remaining (10) samples were measured in the same way after irradiation of 30KGy dosage and then measured again after soaking in 37°C saline for 14 days to simulate the actual radiation sterilization and in-vivo soaking conditions. Same samples were measured once more after accelerated aging per ASTM-1980-07 for 80 days to simulate the 3 year in vivo life. The differences in measurements between virgin and end conditions were documented as the percentage dimensional change. After the measurements, in the groups of DI water, saline soaking and radiation + aging, (3) samples were randomly selected for DSC measurements. The results were compared with dimensional measurements. Statistical analysis was performed by the student t test to compare virgin condition and the conditions after each treatment. 95% significance level was assumed.
Results
As shown in Table 1, after water wash and saline soaking, the average dimensions increased but not statistically significant. Radiation alone caused the net decrease in sample dimensions and remained in negative after a saline soaking. Further significant dimensional decrease (P<0.05) occurred after aging. Increased DSC % crystallinity was found in the aged samples.
Discussion
Study showed the net dimensional change of UHMWPE was shrinkage under simulated in-vivo enviroment. Aging caused further significant shrinkage which was likely related to the elevation of crytallinity.
Significance
With this shrinkage factor, the design of modular poly components may need to be compensated to avoid the loosening of modular components in-vivo condition.
