CHRONIC LOADING AND EXTENSION INCREASES THE ACUTE BREAKING STRENGTH OF POLYPROPYLENE SUTURES

Polypropylene sutures provide satisfactory strength for construction o f vascular anastomoses, but occasionally they break. Experimental stud ies show that they break at reduced forces when they are subjected to chronic loads. Moreover, in patients, sutures are subject to acute loa ds superimposed on chronic loads. For example, an episode of hypertens ion 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 chron ic loads. One hundred sixty-five 6-0 polypropylene sutures were subjec ted to 50-175 g chronic loads in vitro. After 38 days they were subjec ted to additional increasing acute loads until they broke. Five hundre d ninety other sutures were subjected to ''injuries'' of manipulation before chronic loading. A stray knot was simulated by placing a knot i n the center of 90 sutures. Nurse's tugs used to straighten folded sut ures in the operating room were simulated by applying brief loads of 7 5-275 g to 452 other sutures. Intraoperative injuries were simulated i n 48 other sutures by pinching them with DeBakey forceps. Surprisingly , chronic loading of polypropylene sutures increased their acute break ing force. it is suggested that this may have resulted from increased orientation of crystals in the core of the filaments. By contrast, dis turbing the outer surface of the filament by placing a stray knot, or pinching with forceps decreased acute breaking strength. These data su ggest that if polypropylene sutures do not break soon after they have been placed in a patient, they may gain strength over time.