Mechanical contribution of endocardium during finite extension and torsionexperiments on papillary muscles

Finite extension and torsion tests on cardiac papillary muscles are present ly the best way to directly measure the response to shear along myocardial fibers. Quantifying this response is necessary for determining the complete three-dimensional constitutive behavior of myocardium as a transversely is otropic material. Analysis of such tests is complicated, however, since pap illary muscles are materially inhomogeneous, consisting of a myocardial cor e surrounded by an endocardial sheath that is rich in collagen. In this art icle, we show that the papillary muscle response to extension and torsion a dditively decouples into the response of the bare myocardial core plus the response of an endocardial sheath filled with fluid (assuming the muscle is a radially inhomogeneous and incompressible continuum with cylindrical sym metry). This result allows the endocardial response to be subtracted from t he intact papillary muscle response to obtain the response of the bare myoc ardial core. An initial estimate suggests that the endocardial sheath affec ts the axial moment significantly (50% of torque for all twists at low stre tch) but affects the axial force only slightly (<10% at moderate twists). ( C) 1999 Biomedical Engineering Society. [S0090-6964(99)01302-8].