Chronic loading and extension increases the acute breaking strength of polypropylene sutures

Polypropylene sutures provide satisfactory strength for construction of vascular anastomoses, but occasionally they break. Experimental studies show that they break at reduced forces when they are subjected to chronic loads. Moreover, in patients, sutures are subject to acute loads superimposed on chronic loads. For example, an episode of hypertension applies acute load that is added to the baseline chronic load in a suture that has been used to close an arteriotomy. The purpose of the present study was to examine the breaking force of 6-0 polypropylene sutures subjected to acute loads after they had been loaded with chronic loads. One hundred sixty-five 6-0 polypropylene sutures were subjected to 50-175 g chronic loads in vitro. After 38 days they were subjected to additional increasing acute loads until they broke. Five hundred ninety other sutures were subjected to "injuries" of manipulation before chronic loading. A stray knot was simulated by placing a knot in the center of 90 sutures. Nurse's tugs used to straighten folded sutures in the operating room were simulated by applying brief loads of 75-275 g to 452 other sutures. Intraoperative injuries were simulated in 48 other sutures by pinching them with DeBakey forceps. Surprisingly, chronic loading of polypropylene sutures increased their acute breaking force. It is suggested that this may have resulted from increased orientation of crystals in the core of the filaments. By contrast, disturbing the outer surface of the filament by placing a stray knot, or pinching with forceps decreased acute breaking strength. These data suggest that if polypropylene sutures do not break soon after they have been placed in a patient, they may gain strength over time.