Resident Physician University of California Davis Health Department of Urologic Surgery
Introduction: There is evidence to suggest that bladder wall undergoes remodeling following spinal cord injury (SCI). A thorough understanding of the mechanical behavior of bladder wall tissue is a necessary step before finding biocompatible materials to replace or regenerate the bladder. However, there is limited biomechanical data evaluating the properties of bladder wall after spinal cord injury. In this study, we investigated changes in the elastic and viscoelastic mechanical properties of bladder tissue using a rat model with induced spinal cord injury.
Methods: Seventeen adult Sprague rats received moderate mid-thoracic spinal cord contusion. Behavioral testing began 7-14 days after injury. Locomotor functional recovery was measured using the Basso, Beattie and Bresnahan (BBB) locomotor rating scale. Bladder tissue samples from controls (14) and spinal contused (2 and 9 weeks post-injury) were subjected to uniaxial stress relaxation at 50% strain to determine instantaneous and relaxation modulus, and monotonic load-to failure at 1% strain/sec to determine Young’s modulus, yield stress and strain, and ultimate stress and strain.
Results: None of the rats had a normal BBB score. At 2 weeks post-injury, SCI vs control showed instantaneous and relaxation modulus were decreased 64.3% (p>0.99) and 70.6% (p = 0.07), respectively. Similarly, at 9 weeks post-injury, instantaneous and relaxation modulus were decreased by 71.9% (p = 0.04) and 71.4% (p = 0.13), respectively, compared to controls. Young’s modulus of SCI rats was decreased 50% (p >0.99) at 2 weeks and 49% (p >0.99) at 9 weeks compared to controls. Yield stress of SCI bladders was decreased 55% (p >0.99) compared to controls at 2 weeks post-injury but was increased 62% (p = 0.027) relative to controls at 9 weeks. Yield strain showed no difference at 2 weeks post-injury but increased 90% (p < 0.01) in SCI rats at 9 weeks post-injury. Ultimate stress was decreased 37% (p = 0.39) at 2 weeks post-injury in SCI rats relative to control rats, but no differences were observed at 9 weeks post-injury, and no differences in ultimate strain were observed between groups at either time point.
Conclusions: We found that the mechanical properties of rat bladder wall after a spinal cord injury at week 2 showed little differences compared to controls, but by week 9 SCI bladders had a greater yield stress and yield strain in the spinal cord injury bladders at 9 weeks compared to 2 weeks and control. This suggests that SCI rat bladders were more compliant than control counterparts.
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.jpg "Jonathan C. Hu, MD photo")