Investigating the mechanical shear-plane between core and sheath elements of peripheral nerves

Abstract

The mechanical architecture of rat sciatic nerve has been described as a central core surrounded by a sheath, although the way in which these structures contribute to the overall mechanical properties of the nerve is unknown. We have studied the retraction responses of the core and sheath following transection, together with their tensile properties and the interface between them. Nerves were harvested and maintained at their in situ tension and then either transected entirely, through the sheath only, or through an exposed section of the core. The retraction of each component was measured within 5 min and again after 45 min. Post mortem loss of retraction was tested 0 min or 60 min after excision. For fresh nerves, immediate retraction was 12.68% (whole nerve), 5.35% (sheath) and 4% (core), with a total retraction of 15%, 7.21% and 5.26% respectively. For stored nerves, immediate retraction was 5.33% (whole nerve) and 5.87% (sheath), with an extension of 0.78% for core, and a total retraction of 6.71% and 7.87% and an extension of 1.74%, respectively. Tensile extension and pullout force profiles were obtained for the sheath, the core and the interface between them. These showed a consistent hierarchy of break strengths that would, under increasing load, result in failure of the interface, then the core and finally the sheath. These data reflect the contributions of material tension and fluid swelling pressure to total retraction, and the involvement of an energy-dependent process that runs down rapidly post mortem. This study increases our understanding of the composite nature of peripheral nerve tissue architecture and quantifies the material properties of the distinct elements that contribute to overall mechanical function.

Finite element modeling of hyper-viscoelasticity of peripheral nerve ultrastructures

“…The coefficient of variations of parameter α were smaller than those of parameter μ 0 , which may due to the fact that stress is more sensitive to parameter α than μ 0 . The instantaneous shear modulus of perineurium, μ 02 , was found higher than those of the other two ultrastructures, which was consistent with the results of tensile tests (Georgeu et al, 2005;Sunderland, 1978). From histological point of view, the stiff perineurium was mainly composed of dense collagen fibrils and lamellar layers (Sunderland, 1978).…”

“…On the contrary, the epineurium was found to have the smallest stiffness which may due to the fact that it is composed of loose connective tissues and some aligned collagen fibrils (Sunderland, 1978). It has been found that the perineurium of rat sciatic nerve can bear higher tensile force before damaged (Georgeu et al, 2005) and similar function of perineurium was found from transverse elasticity in this current study. Together, the epineurium and the perineurium may provide protection against compression load.…”

Quadrilateral Space Syndrome

“…In simplified analyses, the perineurum or interface between the nerve sheath and the core provides a shear plane that facilitates independent movement of the core (endoneurum) and the sheath (epineurum). 58,59 We suspect that the axillary nerve sheath and interface (or shear plane) protect the core from injury that could otherwise be induced by stretch involved in the pulley system that occurs during AER.…”

“…Several authors suggest that mechanical properties of individual layers of peripheral nerves may help determine peripheral nerves' excursion and strain responses to stretch about a joint. 58,59 Excursion describes gliding of the axillary nerve relative to its nerve bed; strain is the change in axillary nerve length induced by longitudinal tensile stress. 58 Dynamic elongation of the nerve under tensile force is counteracted by transverse contraction of the nerve, which is reversible as the tensile force is relieved.…”

The mesh repair: Tension free alternative on dealing with nerve gaps-experimental results

“…Retraction of the axotomized nerve ends usually precludes direct approximation within a few days or weeks of the trauma, requiring the use of nerve grafts to repair the defect. [3][4][5] The standard approach to repair a peripheral nerve when a gap is present is to bridge the severed ends with a segment of autologous donor nerve. When the caliber of the donor autograft matches that of the recipient nerve, in both total diameter and average axon density, superior results are obtained.…”