Background: It has been proposed that cervical facet joint capsules
are a major source of whiplash pain. However, there is a paucity of
neurophysiologic data to support this hypothesis. The purposes of this study
were to determine the distribution of A-d and C-fiber sensory receptors
in the facet joint capsule and to test their patterns of response to stretch
and related sensory function.
Methods: Laminectomy from C4 to C7 was performed in seventeen goats,
while they were under general anesthesia, to expose the C6 nerve roots.
Customized dual bipolar electrodes were used to record neural activity from
one of the C6 branches. An 8 or 15-V electrical stimulus was used to provoke
receptor activity in nine designated areas on the dorsal part of the C5-C6
facet joint capsule. Receptors were classified on the basis of conduction
velocities. The waveform of an identified receptor was set up as a template to
determine its neural activity in response to capsular stretch. The
characteristics of each single receptor's response to capsular stretch were
analyzed to determine its sensory function as a mechanoreceptor or
nociceptor.
Results: Two hundred and forty-eight receptors on the dorsal part of
the C5-C6 facet joint capsule were evoked by electrical stimulation in the
seventeen goats. More C-fiber receptors were found on the dorsolateral aspect
of the facet joint capsule, where tendons and muscles were attached. The
response to stretch of 120 receptors, from twelve goats, were analyzed to
classify them into one of four categories (high-threshold mechanoreceptors,
non-saturated low-threshold mechanoreceptors, saturated low-threshold
mechanoreceptors, and silent receptors) or as unclassified receptors.
Conclusions: The existence of receptors in the facet joint capsule
indicates that the capsule has pain and proprioceptive sensory functions.
Clinical Relevance: The cervical facet joint capsule may be a pain
generator in whiplash-related disorders and chronic neck pain. On the basis of
the assumption that human and goat facet-joint-capsule innervations are
similar, it appears that treatment focusing on the dorsolateral aspect of the
cervical facet joint may produce more effective outcomes by ablation of neural
activity from nociceptors and the mechanoreceptors that are involved in
pain.