The Stress and Strain States of the Posterior Annulus Under Flexion

Abstract

Experimental measurement of bovine caudal annulus fibrosus (AF) biaxial stress-strain states for unloaded motion segments and those loaded in flexion combined with axial compression. To measure AF biaxial stress-strain states for motion segments free of external loads and those loaded in flexion combined with axial compression. In vitro, cyclically loaded flexion is associated with the complete rupture of the AF in acute cases. However, currently little is known about the tissue-level stress and strain states in this damaging load configuration. Surface strains of the posterior annulus were measured, during loading of intact bovine caudal motion segments in flexion combined with axial compression, using the unloaded motion segment as a reference. Planar annulus specimens were excised from the motion segments, and the measured strains from the unloaded and flexed intact discs were applied to the excised specimens in a planar biaxial stress-strain device to determine the stress state in each condition. The annulus strain state of the flexed disc was input into a previously developed nonlinear strain energy function with additive terms that directly represent the structural features of the AF. The AF posterior surface strain for motion segments in flexion combined with compression was -0.1087 (circumferential) and 0.1051 (axial) on average. This strain state corresponded to average biaxial stresses in the annulus of 0.046 MPa (circumferential) and 0.224 MPa (axial). The planar AF specimens contracted after sectioning, resulting in small negative strains. In the absence of external loads on a motion segment, the annulus was approximately in a state of equibiaxial tensile stress. The theoretical analysis indicated that collagen crosslinks store a greater portion of the strain energy than the matrix or collagen fibers. In flexion combined with compression, the posterior AF stress state is biaxial and is much larger in the axial direction in bovine caudal discs.