Structural analysis of the Björk-Shiley Delrin heart valve occluder.

BACKGROUND AND AIMS OF THE STUDY

Impact wear grooves were evident in some Delrin occluder discs of explanted Björk-Shiley Delrin (BSD) heart valves. This study focuses on the finite element analysis (FEA) method to understand the maximum principal stresses experienced during the peak in vivo loading of valves in the closed position.

MATERIALS AND METHODS

The maximum pressure difference across the valve was measured to be 130 mmHg in a pulse duplicator simulating normal sinus rhythm obtained clinically by cardiac catheterization. The corresponding measured strain was 1.81 x 10(-3). The FEA model incorporated four points of contact between the disc and the orifice ring to estimate the maximum principal stresses in the disc of the BSD heart valve. A linear pressure distribution averaging 130 mmHg was applied so that the finite element results gave a strain of 1.81 x 10(-3) at the gauge location as experimentally observed.

RESULTS

The largest stress in the Delrin disc of the BSD valve occurred when the occluder made four-point contact with the orifice ring struts. The resulting localized compressive stress on the inflow side could be as high as 42 ksi, assuming the Hertzian contact theory. The magnitude of tensile stresses were less, but were highest on the outlet surface opposite the point of contact. The highest tensile stress for an ungrooved disc was found to be 8.35 ksi, which was below the ultimate tensile strength and yield stress in flexure for Delrin. Therefore, it is unlikely that yielding or tensile failure will occur at this level of stress. Maximum tensile stresses were found to be 1.442 and 1.448 ksi for discs with single and multiple grooves respectively.

CONCLUSION

The model predicts that as a wear groove is created, the area of contact between the disc and the inlet strut of the BSD valve will increase, thereby reducing average compressive contact stress and hence, the wear rate.