- When replacing the thoracic aorta, gelatin-sealed triaxial knitted Dacron prostheses are used as they have zero porosity and are easy to handle. It is known that knitted prostheses (not triaxially constructed), when implanted in the abdominal aorta or in the peripheral vessels, show a dilatation of 10-20%. The initial in vivo expansion of this prosthesis in the thoracic aorta was assessed using computed tomography scanning in 18 patients. The graft sizes used were 18mm (n = 4). 20mm (n = 5). 22mm (n = 4), 26mm (n = 1), 28mm (n = 2) and 30mm (n = 2). All patients were kept normotensive postoperatively. Computed tomography scan measurement was performed within 4 weeks of operation. using standard computed tomography equipment and no contrast. Two radiologists checked and agreed upon the slice levels and measurements compared with the manufacturer's stated size referring to internal diameter of the crimped prosthesis. The mean(s.d.) 'in vivo' size was 16.2(6)% larger. This 'true size' should be taken as the reference measurement when considering further dilatation over the period of follow-up.