Biomechanical properties of the lower uterine segment above and below the reflection of the urinary bladder flap.

OBJECTIVE To test the hypothesis that differences in the matrix arrangement of the lower uterine segment (above and below the reflection of the bladder flap) translate to differences in biomechanical or structural properties of the uterine wall. METHODS We enrolled prospectively 40 women at term randomized to a bladder flap (n=21) versus no bladder flap (n=19) at the time of a cesarean delivery for dystocia. Tensile properties of tissue biopsies from the upper and lower edges of the hysterotomy incision were quantitated using a stretching regimen designed to mimic labor. Parameters such as slope, yield point, and break point were analyzed in relationship to biochemical (total collagen, sulfated glycosaminoglycans, pyridinoline-deoxypyridinoline) and histological (collagen birefringence) properties of the tissue. RESULTS The lower edge of the hysterotomy had a higher collagen content than the upper, independently of whether the hysterotomy site was above or below the reflection of the bladder (P<.001 for edge level, P=.8 for bladder flap). The pyridinoline-to-collagen ratio (a reflection of collagen cross-linking) was significantly decreased in the lower edge of the hysterotomy, independently of a bladder flap (P<.001). Neither collagen birefringence nor the amount of sulfated glycosaminoglycans differed among sites. There were no significant differences in biomechanical properties between the upper and lower edges of the hysterotomy, whether or not a bladder flap was created. CONCLUSION The lower uterine segment of women with dystocia maintains its overall biomechanical properties despite significant quantitative and qualitative differences in fibrillar collagen between the upper and lower edge of the hysterotomy incision. LEVEL OF EVIDENCE II.