new methodology in accelerated testing of mechanical heart valve prostheses

Mechanical heart valve (MHV) prosthesis is used commonly for clinical replacement of a diseased or dysfunctional natural valve. It is expected to operate uninterruptedly in the human chest for at least 10 years. Classified as Class III medical devices, MHVs of a new design are required by the Food and Drug Administration (FDA) to undergo accelerated durability test for up to 600 million cycles, before a pre-market approval (PMA) can be considered. Knowledge of potential damage/failure mechanisms is of practical interest in assessing the results obtained from accelerated testing. During such tests, an MHV is subjected to higher stresses at higher cycling frequency, and to more severe hydrodynamic environment than those under normal physiological conditions. Three primary factors contributable to damage/failure of an MHV are investigated, namely, contact stresses, water hammer effect, and cavitation. Parametric study is conducted on the contact-induced damage, using the model of a ball-indentation test. Four possible modes of contact damage are identified. Schemes for the selection of appropriate coating parameters to minimize contact stresses in a valve component are discussed. Experimental results of dynamic stresses on a leaflet are presented. The water hammer effect at the instant of valve closure is studied. The destructive force of cavitation is investigated, by incorporating bubble dynamics in a damage tolerance analysis. Cavitation is found to have a greater effect on potential valve damage/failure during accelerated testing. A new measure, cavitation impulse (CI), is introduced to quantify the intensity of MHV cavitation. It is defined as the area under the trace of the high-frequency pressure bursts generated by collapses of cavitation bubbles. CI is modeled as a stationary stochastic process with a discrete parameter (beats), the probability structure of which is estimated from the experimental data. Each CI accounts for both the magnitude and the time-duration of the impinging high pressure of cavitation; therefore, it correlates more closely with cavitation damage on MHV components, thus the time-to-failure (life) of an MHV undergoing testing.