Intervertebral foraminal fibrosis and spinal nerve tethering in a laminectomy rat model

Introduction: A rat model of the human post-laminectomy condition has been developed and published [1]. In this study, we intend to clarify the following in this model: 1) the effect of laminectomy and disc-injury in the creation of fibrosis within the intervertebral foramen; 2) the ability of hyaluronate to decrease interverte-bral foraminal fibrosis; 3) the correlation between fibrosis within the intervertebral foramen and pain behavior (tactile allodynia). Materials and Methods: Thirty-eight age-matched 400+ grams male Harlan Sprague-Dawley rats were used for the study, and divided into four groups: normal controls (twelve, " Control "), surgical bilateral L5-L6 laminectomies via posterior midline incision (eight, " Lami "), surgical bilateral L5-L6 laminectomies with right-sided L5-6 disc injury using a stereotyped insertion of 27-guage needle (ten, " Lami-Disc "), and surgical bilateral L5-L6 laminectomies with right-sided L5-6 disc injury with 0.1 cc of topical high molecular weight hyaluronate layered over the dura prior to wound closure (eight, " HA "). The VA Institutional Animal Care Use Committee approved the research protocol. Pain Behavior Testing Method Withdrawal thresholds (50% probability) were assessed using calibrated von Frey filaments (Stoelting, Wood Dale, Il) with buckling forces between 4.0-148mN (0.41-15.10g) using a previously published method [2]. As published previously , we used a deviation from normal (15.10g withdrawal threshold) at the eighth week as an indicator of tactile allodynia [3]. Biomechanics Method Intervertebral foramen fibrosis of the right L5 spinal nerve was quantified using a biomechanical methodology measuring the load-to-failure as the intact nerve is pulled free from the intervertebral foramen [4]. The nerve was displaced distally at a constant velocity of 1 cm/min along the axis of the spinal nerve and load-to-failure was measured. Statistical significance (p < 0.05) was evaluated with ANOVA (Statview 5.0, SAS Institute, Cary, NC) and we used appropriate correction for multiple comparisons (Fisher's PSLD). The mean load-to-failure for the right L5 nerve was compared between surgical groups, and also compared between animals with and without tactile allodynia. Results: All surgical animals survived eight weeks until sacrifice. One HA animal , three Lami animals, and seven Lami-Disc animals showed a withdrawal threshold of less than 15 grams, indicating tactile allodynia. The load-to-failure of the right L5 spinal nerve was significantly greater in the Lami-Disc (p<0.0002), HA (p<0.02), and Lami groups (p<0.05) compared to the Control group. The Lami-Disc group did demonstrate a trend towards a significantly higher mean load-to-failure when compared to …