Characterizing the structural properties and porosity of mid-urethral slings with varied manufacturing techniques

Abstract

Introduction: Implantation of mid-urethral slings (MUSs) is a safe and effective approach for the surgical repair of stress urinary incontinence. However, concerns regarding the deformability of the prototype MUS, mechanical cut Gynecare TVT prompted manufacturers to use techniques like laser cutting, heat-sealing, and the inclusion of an interwoven stabilizing suture to decrease deformation with loading. We hypothesized that a laser cut or heat-sealed MUS would be stiffer but deform less, and experience less permanent elongation as compared to a mechanical cut MUS. Additionally, the inclusion of a stabilizing suture would minimize the loss of porosity.

Methods: Uniaxial tensile testing to failure and cyclic loading was performed to analyze the structural properties and permanent elongation, respectively, of commercially available MUSs Gynecare TVT (mechanical cut), Gynecare TVT Exact (laser cut), ArcTV (laser cut, with and without the stabilizing suture), and Desara Blue (heat-sealed). A custom Mathematica code was used to quantify the porosity of the MUSs following sequential uniaxial loading from 0 N to 10 N.

Results: Desara Blue was significantly stiffer (p-values < 0.05), elongated less at failure (p = 0.002), and experienced less permanent elongation in response to cyclic loading (p-values = 0.001) relative to Gynecare TVT. Similarly, permanent elongation was significantly less (p-values = 0.004) and the stiffness was higher (p = 0.004) for Gynecare TVT Exact as compared to Gynecare TVT. Very little differences in stiffness and no differences in relative elongation at failure nor permanent elongation were observed between ArcTV (without the interwoven suture) and Gynecare TVT (p-values > 0.05). The porosity of all MUSs significantly decreased with loading (p-values < 0.001); except for ArcTV with the stabilizing suture which showed the least amount of deformation (i.e., percent change in porosity decreasing by only 14%, p < 0.001, at 10 N).

Discussion: Overall, heat-sealing decreased deformability at the cost of markedly increasing device stiffness to a point which likely outweighs benefits, and risks increased complications. Laser cutting had different effects on the behavior of TVT Exact and ArcTV suggesting manufacturer technical differences, but overall reduced deformation without a substantial impact on stiffness. An interwoven stabilizing suture minimized the loss of porosity which translates clinically to less deformation and mechanistically to reduced mesh complications.

Keywords: cyclical loading; incontinence; mid-urethral sling; permanent elongation; polypropylene mesh; porosity.

FIGURE 1

Close-up images of the mid-section of the mid-urethral slings taken with an EOS Rebel T3 camera. Scale bar represents 1 mm.

FIGURE 2

SEM images of TVT, TVT Exact, and ArcTV with complete pores shown in the top images and the edges of the respective MUSs depicted in the bottom images. Overall, the MUSs have a general pore shape that is quadrilateral. The edges of TVT (mechanical cut) are free whereas those of TVT Exact and ArcTV (both laser cut) are sealed.

FIGURE 3

SEM images of Desara Blue heat sealed and Desara Blue mechanical cut with complete pores shown in the top images and the edges of the respective MUSs depicted in the bottom images. Similar to the other MUSs analyzed in this study, the Desara Blue MUSs have a general pore shape that is quadrilateral. The edges of Desara Blue heat sealed have a thick edge resulting from the neighboring tanged edges melting together whereas the edges of Desara Blue mechanical cut were left free with the tanged edges on one side being longer than the opposing side.

FIGURE 4

Average load-relative elongation curves following load-to-failure testing of the commercially available MUSs (a) and the commercially available Desara Blue (heat sealed) versus the non-commercially available Desara Blue (mechanical cut) (b).

FIGURE 5

Average load-relative elongation curves up to 25% relative elongation of the commercially available MUSs (a) and the commercially available Desara Blue (heat sealed) versus the non-commercially available Desara Blue (mechanical cut) (b).

FIGURE 6

Representative load-relative elongation curves following cyclical loading after Cycle 1–0.5 N–5 N for 10 cycles and Cycle 2–0.5 N–15 N for 10 cycles. Permanent elongation is evidenced by a shift to the right (arrows) of the load-relative elongation curves.

FIGURE 7

Representative images of the mid-section of the MUSs after the application of 0 N, 5 N, and 10 N. All slings contracted and the pores collapsed except ArcTV with the stabilizing suture in which the sling experienced little contraction and the pores remained open.

FIGURE 8

Average porosity following the application of 0 N, 0.1 N, 5 N, and 10 N. A significant difference in porosity between 0.1 N and 5 N is represented by (*) and between 0.1 N and 10 N is represented by (**). Error bars represent standard deviation.