Investigating mechanical behaviour at a core-sheath interface in peripheral nerve.

As peripheral nerves bend and stretch, internal elements need to move in relation to each other. However, the way in which intraneural components interact is poorly understood. Previous work identified a distinct core and sheath in the rat sciatic nerve and provides a useful model with which to investigate this interaction. Here we have focused on identifying the mechanical and anatomical characteristics of the interface between core and sheath. Nerve samples, 15 and 20 mm long, of rat sciatic nerves were harvested and placed in a purpose-built jig, and a tensile testing machine was used to pull core from sheath. Mechanical tests of specimens in which core had been previously pulled from sheath by 25% of its initial length achieved a mean pull-out force approximately six times smaller than that achieved using intact controls. These results are consistent with the proposal that core-sheath interactions involve physical connections rather than a viscous fluid interface. Anatomical features of this interface were characterised using transmission electron microscopy. It appeared that sheath was derived from epineurium and most of the perineurium, whilst core consisted of endoneurium and a small proportion of the perineurium: the plane of cleavage appeared to involve the innermost perineurial cell layer.

Reduction of neural adhesions by biodegradable autocrosslinked hyaluronic acid gel after injury of peripheral nerves: an experimental study.

OBJECT: Adhesion formation is a serious problem in peripheral nerve surgery, frequently causing dysfunction and pain. The authors aimed to develop an objective biomechanical method of quantifying nerve adhesions and to use this technique for the evaluation of the efficacy of an autocrosslinked hyaluronic acid (HA) gel as an antiadhesion therapy. METHODS: Thirty-three female Wistar rats underwent dissection, crush injury, or transection plus repair of the sciatic nerve. The nerves were or were not treated with the HA gel. Six weeks after surgery, the adhesions formed were assessed by measuring the peak force required to break the adhesions over a standardized area. Results of biomechanical measurements demonstrated that the peak force significantly increased as the severity of the injury increased. After using the HA gel to treat the nerve, the peak force was significantly reduced in rats with any of the three types of injuries; peak force decreased by 26% in the animals in the dissection group, 29% in the crush injury group, and 38% in the transection and repair group, compared with the untreated animals. CONCLUSIONS: The biomechanical method described is an objective, quantitative technique for the assessment of nerve adherence to surrounding tissue. It will be a valuable tool in future studies on antiadhesion therapies. Furthermore, HA gel significantly reduces nerve adhesions after different types of nerve injuries.

Mechanical functioning of peripheral nerves: linkage with the "mushrooming" effect.

Biomechanical properties of nerve have been studied extensively. All neural matrix tissues have been suggested to be the main load-bearing component. Based on the ultrastructure it has been proposed that the architecture of the epineurium allows some degree of extensibility of the nerve. A role of the perineurium could be to withstand the positive endoneurial pressure. The hypothesis is that the mechanical behaviour of nerves is dependent on an interaction between the core swelling pressure and restraint by the outer sheath. Loss of this balance will alter that behaviour. To test this, rat sciatic nerves were subjected to mechanical loading at in vivo and ex vivo tension. Retraction of nerve segments was measured after excision and after incubation at 37 degrees C or freezing. Swelling properties of the nerve were measured by immersion in water or PBS (phosphate buffer solution) with intact or opened epineurium. Results showed a significant decrease in strength and stiffness with an increase in strain of the nerve after excision, compared to in vivo. Retraction was on average 11%. Freezing or incubation at 37 degrees C did not alter retraction. The swelling properties of the nerve demonstrated a significant difference between intact and opened epineurium and similar results for water and PBS, indicating that epineurium is a constraint and that the nerves are underhydrated. The proposed model for the intact nerve is a continuous connective tissue tube surrounding and constraining an inner swelling pressure of the neural core. Loss of integrity of the nerve has detrimental effects on its biomechanical properties.