Quantitative analysis of fiber disarray developing in papillary muscles unloaded after mitral valve replacement

Muscle fiber disarray which developed in the papillary muscle released from its chordal attachment after mitral valve replacement (MVR) was analysed histometrically in 23 hearts. The degree of disarray, or how far the fiber arrangement deviated from an ideal condition in which the fibers are considered to be aligned in parallel, was evaluated in terms of the "disarray index". This index corresponds to the ratio of the minor to major axes of a regression ellipse which can be obtained histometrically by plotting the reciprocal of the number of muscle fibers intersected by a test line of equal length in every direction on a polar coordinate system. The index can vary continuously in a range from 0.00 for a completely parallel fiber alignment to 1.00 for a completely random distribution. The estimated index showed a gradual increase from 0.05 to 0.68 according to the postoperative survival period after MVR with a significant positive correlation (r = 0.793, p less than 0.01). The index was inversely correlated with the mean muscle fiber diameter (r = -0.433, p less than 0.05). These results indicate that the disarray which develops in the papillary muscle after MVR is a deformation of the normally parallel arrangement of fibers toward an isotropic texture under focal disruption of the normally longitudinally oriented tensile force.