Recent reports have shown that despite extensive preclinical testing, vascular grafts of biological origin undergo severe biodegradation and aneurysm formation after two or more years of implantation in man. The purpose of this study was to develop a laboratory model to quantitate and correlate the stability of crosslinked collagen grafts in vitro and in vivo. This resistance to biodegradation was assessed by measuring changes in suture pullout force and sample weight in response to controlled digestion with bacterial collagenase, in 0.5-cm-long cylindrical graft segments (chemically processed bovine carotid artery and human umbilical cord vein) that were implanted in the rat subcutis for 2 to 12 weeks. Scar tissue was removed from the explants by brief enzymatic digestion, a process that was inhibited when graft segments had become infected. Changes in dry weight were more consistent than were changes in wet weight; drying the graft segments had no effect on their degradation in vivo or in vitro. Intact cylindrical rings suffered somewhat less damage than did opened, flattened cylinders. Graft degradation increased markedly with implantation time, and was detected after only 3 weeks. We conclude that the rat subcutis model, when combined with controlled enzymatic digestion, first to remove scar tissue and then to challenge structural integrity, provides an accelerated assay by which to predict the stability of collagen vascular grafts.
IN T RODUC T ION
Chemically crosslinked biologic grafts have been used for distal vascular reconstructions, in the absence of suitable autogenous vein, for over 30 years."6 However, many of them undergo continual biodegradation and become aneurysmal after 2 or more year~,ZI-'~ despite the expectation, from extensive animal implantation and in vitro st~dies,',2~-~,'~-'~ of a more favorable result. It is apparent that previous laboratory tests did not adequately predict this structural biodegradation. The questions are (a) whether such degrada-