Residual strains in porcine and canine trachea

Abstract

Residual strains exist in canine and porcine tracheas. They are revealed by cutting the trachea first perpendicular to its axis into rings, then radially into sectors. Each sector is characterized by an opening angle which is defined as the angle subtended between two radii joining the middle point of the inner wall to the tips of the inner wall. The trachea being non-axisymmetric, the opening angle depends on the position of the radial cut. The trachea being also nonuniform in the axial direction, the opening angle varies along the length of the trachea. In the dog, the opening angle of the trachea cut at the anterior position (cartilaginous) is about 100° at the larynx; it increases fairly linearly to 180° midway down the trachea; then increases slowly to about 200° at the lower end where the trachea bifurcates into the main bronchi. Dog trachea cut in the posterior (muscular) position have an opening angle of about 50° at the larynx, which increases to about 70° three-quarters of the way down the trachea, then drops to 60° at the lower end. In the pig, the opening angle of the trachea is much smaller, the values at anterior and posterior cuts are similar (without significant difference), and their mean value decreases from about 15° at the laryngeal end to about 5° at the lower end. These species and regional differences are discussed in relation to tracheal geometry and structure. It is shown that one radial cut reduces a tracheal ring of either dog or pig into zero-stress state; and that the opening angle obtained by cutting the ring at one point can be computed from that obtained by cutting at another point if the geometry of the tracheal sector is known at a zero-stress state.