Year of operation as a risk factor in the late results of valve replacement.

The actuarial thromboembolic rates of aortic and mitral silicone ball valves used during the second decade of cardiac valve replacement are significantly lower than the rates for the same prostheses implanted during the first decade, as shown in the following table: (Formula: see text). The embolus-free rates are significantly different (p < 0.01) in both the mitral and aortic series. Five-year embolus-free rates for the composite-strut caged-ball, Björk-Shiley tilting disc, and porcine xenograft valves all fall in the range of from 81% to 92% for the mitral position and from 91% to 97% for the aortic. Thus the standard silicone ball-valve prosthesis, used during the current era, has a thromboembolic risk as low as that reported with other concurrently utilized valve substitutes. This striking reduction in thrombogenicity demonstrates that the time frame of implantation must be considered when evaluating the results of cardiac valve replacement.