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The CryoLife O'Brien (CLOB) stentless porcine aortic prosthesis has shown limited long-term durability, but its deterioration mechanisms have not yet been identified. The aim of this study was to review our experience with CLOB reoperations and to investigate the pathobiology of structural valve deterioration (SVD).Between March 1996 and December 2010, 35 patients underwent reoperation 82 ± 43 months after a CLOB implant. Patient records were retrospectively reviewed and follow-up was 100% complete. Nine valves explanted for SVD underwent pathological study.Nineteen patients (54%) were operated on urgently for prosthetic valve replacement. No intraoperative technical complications occurred, and only one patient underwent aortic root replacement. One patient (3%) died in-hospital. Prosthesis dysfunction was due to SVD in 27 patients, caused by cusp commissural tearing in 22. All the nine explanted valves submitted to pathological examination showed calcifications and lipid insudation. At histology, eight prostheses disclosed cholesterol clefts and all showed a chronic inflammatory infiltrate. Transmission electron microscopy disclosed the presence of calcific deposits on cell debris and collagen fibres and confirmed the presence of cholesterol clefts. After a mean follow-up of 50 ± 33 months, survivals at 1, 5 and 7 years were 91 ± 5, 56 ± 9 and 38 ± 13%, respectively.The CLOB reoperation is a feasible procedure, with satisfactory postoperative and mid-term survival results. Cusp tears, due to lipid and calcium deposits mostly targeting the commissures, seem to be the most important mechanism of SVD, resulting even in acute prosthetic incompetence and urgent reoperation.