| Description |
- Background:
The artificial urinary sphincter (AUS) is considered the gold standard in the surgical management of stress urinary incontinence, particularly for male patients. The AUS is a fluid-filled hydraulic system comprised of a urethral cuff, a mechanical pump, and a pressure regulating balloon (PRB). The PRB is vital for pressurizing the urethral cuff to provide continence; however, material failure of the PRB can reduce its ability to generate pressure, leading to recurrent incontinence. This investigation seeks to assess the influence of AUS device age on PRB tensile strength, yield stress, stress softening and relaxation properties, and microscopic characteristics through mechanical testing and microscopic analysis.
Materials and Methods:
The PRBs from 32 previously explanted AUS devices and 1 new, unused device were utilized in this study. PRBs underwent a standardized preparation regimen prior to mechanical and microscopic analysis. Tensile testing was performed on all 33 PRBs, where stress-strain curves were generated and analyzed for tensile strength and yield stress. Stress softening and stress relaxation testing was conducted on separate specimens from a subset of 7 PRBs that were evenly distributed in age. This same subset of 7 PRBs was also analyzed for microstructural defects and crack formation using scanning electron microscopy (SEM). Standard statistical methods were used.
Results:
PRBs were extracted from 32 male patients with a mean age of 76.1 ± 7.9 years. All patients had a history of prostate cancer, with 94% of patients having a prior prostatectomy and 47% of patients having previously received external beam radiation therapy. The primary reason for the explantation of the AUS was sub-cuff atrophy (41%), followed by cuff leak (28%), and less frequently by urethral erosion, urinary diversion, and infection. The time the AUS devices were implanted, or device age, was 7.8 ± 4.6 years on average, with a range of 2 months to just over 17 years. The stiffness of the specimens, also known as tensile strength or the modulus of elasticity, had a mean value of 3.4x105 ± 1.3x105 Pa, and mean yield stress was 2.3x105 ± 1.2x105 Pa. There was a moderate positive linear correlation between the age of the pressure regulating balloon (PRB) and stiffness (r = 0.68, p < 0.01). The wall thickness of the PRBs displayed a weak negative correlation with PRB age (r = -0.38, p = 0.03). There was minimal variability in the stress softening behavior of the PRB specimens with no significant effect of time during the day on stress (p = 0.38) and no significant device age-time interaction (p = 0.63) on repeated measures ANOVA. After SEM, 4 of the 7 PRB specimens revealed fine surface cracks present in their microstructure, which were evident in the 4 oldest specimens in the subset.
Conclusion:
PRB wall stiffness increases with older device age, suggesting potential crystallization of the PRB over time. The PRBs do not exhibit stress softening behavior with modeled daily functional use, but PRBs of all ages demonstrate a consistent pattern of stress relaxation. Older PRBs are more susceptible to developing microstructural defects, such as cracks and fissures, which can have implications on the functionality and longevity of the PRB. These microstructural defects are likely contributors to the loss of mechanical integrity and, consequently, the effectiveness of the AUS in maintaining continence over time. Collectively, these findings underscore the importance of considering material degradation and the mechanical properties of PRBs in the maintenance of AUS devices.
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