Pressure distribution near the occluders and impact forces on the outlet struts of Björk-Shiley convexo-concave valves during closing.

BACKGROUND AND AIMS OF THE STUDY

An in vitro study of the mechanics of closure of Björk-Shiley convexo-concave (BSCC) valves is presented in order to investigate the mechanics of outlet strut fracture reported in a small fraction of the implanted valves.

MATERIALS AND METHODS

Four BSCC 29 mm valves instrumented with strain gages on the outlet strut legs were mounted in the mitral position of an axisymmetric flow chamber of a mock pulsatile flow loop. Measurements of the pressure field in the vicinity of the occluder, closing velocity of the occluder tip in the major orifice, and the impact force between the occluder and outlet strut at the instant of valve closure were obtained at a range of physiologic flow rates.

RESULTS

The results indicated an uneven pressure distribution on the occluder associated with a tendency for the occluder to over-rotate and induce loads on the outlet struts. The impact loads on the outlet struts were asymmetric with load on one leg being larger than the other by up to 25%. These results are consistent with single leg separation preceding outlet strut fracture in most of the valve failures reported. Orientation of the valve with respect to the mitral orifice (major orifice towards the top or bottom) did not significantly affect the loads on the outlet strut. A significant variation in the impact loads of the four valves was measured for identical experimental conditions suggesting that valve specific factors influence outlet strut loads.

CONCLUSIONS

This study provided an understanding of the cause-effect relationship between valve dynamics and outlet strut fracture.