Tensile stretching of cervical facet joint capsule and related axonal changes

Abstract

This study examines axonal changes in goat cervical facet joint capsules (FJC) subjected to low rate loading. Left C5-C6 FJC was subjected to a series of tensile tests from 2 mm to failure using a computer-controlled actuator. The FJC strain on the dorsal aspect was monitored by a stereo-imaging system. Stretched (n = 10) and unstretched (n = 7) capsules were harvested and serial sections were processed by a silver impregnation method. The mean peak actuator displacement was 21.3 mm (range: 12-30 mm). The average peak strain encompassing various regions of the capsule was 72.9 +/- 7.1%. Complete failure of the capsule was observed in 70% of the stretched capsules. Silver impregnation of the sections revealed nerve fibers and bundles in all the regions of the capsule. A blinded analysis of digital photomicrographs of axons revealed a statistically significant number of swollen axons with non-uniform caliber in stretched FJCs. Axons with terminal retraction balls, with occasional beaded appearance or with vacuolations were also observed. Stretching the FJC beyond physiological range could result in altered axonal morphology that may be related to secondary or delayed axotomy changes similar to those seen in central nervous system injuries where axons are subjected to stretching and shearing. These may contribute to neuropathic pain and are potentially related to neck pain after whiplash events.

Figures

Fig. 1

Diagram showing neurophysiology recording, and biomechanical testing of the cervical facet joint capsule. Arrow indicates the direction of stretch (modified from Lu et al. 2005)

Fig. 2

A single axon with repeated swellings and terminal retraction balls (bar = 20 μm, ×100)

Fig. 3

Group of axons in a bundle with one of them showing swellings and terminal retraction ball (bar = 50 μm; ×40)

Fig. 4

Group of axons in a capsule with multiple swollen regions (bar = 20 μm; ×100)

Fig. 5

Axons with normal appearance in an unstretched capsule (bar = 50 μm; ×40)