Investigating mechanical behaviour at a core-sheath interface in peripheral nerves

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

Increased intraoperative epidural pressure in lumbar spinal stenosis patients with a positive nerve root sedimentation sign

Purpose: The sedimentation sign (SedSign) has been shown to discriminate well between selected patients with and without lumbar spinal stenosis (LSS). The purpose of this study was to compare the pressure values associated with LSS versus non-LSS and discuss whether a positive SedSign may be related to increased epidural pressure at the level of the stenosis. Methods: We measured the intraoperative epidural pressure in five patients without LSS and a negative SedSign, and in five patients with LSS and a positive SedSign using a Codman™ catheter in prone position under radioscopy. Results: Patients with a negative SedSign had a median epidural pressure of 9 mmHg independent of the measurement location. Breath and pulse-synchronous waves accounted for 1-3 mmHg. In patients with monosegmental LSS and a positive SedSign, the epidural pressure above and below the stenosis was similar (median 8-9 mmHg). At the level of the stenosis the median epidural pressure was 22 mmHg. A breath and pulse-synchronous wave was present cranial to the stenosis, but absent below. These findings were independent of the cross-sectional area of the spinal canal at the level of the stenosis. Conclusions: Patients with LSS have an increased epidural pressure at the level of the stenosis and altered pressure wave characteristics below. We argue that the absence of sedimentation of lumbar nerve roots to the dorsal part of the dural sac in supine position may be due to tethering of affected nerve roots at the level of the stenosis. © 2013 Springer-Verlag