John M. Cavanaugh

Lumbar facet pain: Biomechanics, neuroanatomy and neurophysiology

Idiopathic low back pain has confounded health care practitioners for decades. Although there has been much advance in the understanding of the biomechanics of the lumbar spine over the past 25 years, the cellular and neural mechanisms that lead to facet pain are not well understood. An extensive series of experiments was undertaken to help elucidate these mechanisms and gain a better understanding of lumbar facet pain. Biomechanic and neuroanatomic studies were performed in human cadaveric facet joints and neurophysiologic studies were performed in New Zealand White rabbits. These studies provide the following evidence to help explain the mechanisms of lumbar facet pain: (1) The facet joint can carry a significant amount of the total compressive load on the spine when the human spine is hyperextended. (2) Extensive stretch of the human facet joint capsule occurs when the spine is in the physiologic range of extreme extension. (3) An extensive distribution of small nerve fibers and free and encapsulated nerve endings exists in the lumbar facet joint capsule, including nerves containing substance P, a putative neuromodulator of pain. (4) Low and high threshold mechanoreceptors fire when the facet joint capsule is stretched or is subject to localized compressive forces. (5) Sensitization and excitation of nerves in facet joint and surrounding muscle occur when the joint is inflamed or exposed to certain chemicals that are released during injury and inflammation. (6) Marked reduction in nerve activity occurs in facet tissue injected with hydrocortisone and lidocaine. Thus, the facet joint is a heavily innervated area that is subject to high stress and strain. The resulting tissue damage or inflammation is likely to cause release of chemicals irritating to the nerve endings in these joints, resulting in low back pain.

Mechanosensitive afferent units in the lumbar facet joint.

The purpose of this study was to characterize somatosensory units of the lumbar facet joint, which may play a central role in idiopathic low-back pain. A laminectomy was performed on the lumbar spine of adult male New Zealand White rabbits. Receptive fields of mechanosensitive afferent units were investigated in the lumbar facet joint and adjacent surrounding tissues, and electrophysiological recordings were obtained from filaments of the dorsal root. Twenty-four units were identified in the region of the facet joint: ten, in the capsule of the joint; twelve, in the border regions between capsule and muscle or tendon; and two, in the ligamentum flavum. Of these units, two had a conduction velocity that was slower than 2.5 meters per second (group IV), fifteen had a velocity ranging from 2.5 to twenty meters per second (group III), and seven had a velocity faster than twenty meters per second. Seven units had a von Frey threshold of more than 6.0 grams, thirteen had a threshold of less than 6.0 grams, and four were not examined. Seven units in the facet joint responded to movement of the joint. Fourteen other mechanosensitive units were found in the muscle, tendon, and interspinous ligament; seven had a conduction velocity of 2.5 to twenty meters per second, and seven had a velocity that was faster than twenty meters per second.

Pain Generation in Lumbar and Cervical Facet Joints

Facet joints are implicated as a major source of neck and low-back pain. Both cervical and lumbar facet syndromes have been described in the medical literature. Biomechanical studies have shown that lumbar and cervical facet-joint capsules can undergo high strains during spine-loading. Neuroanatomic studies have demonstrated free and encapsulated nerve endings in facet joints as well as nerves containing substance P and calcitonin gene-related peptide. Neurophysiologic studies have shown that facet-joint capsules contain low-threshold mechanoreceptors, mechanically sensitive nociceptors, and silent nociceptors. Inflammation leads to decreased thresholds of nerve endings in facet capsules as well as elevated baseline discharge rates. Recent biomechanical studies suggest that rear-end motor-vehicle impacts give rise to excessive deformation of the capsules of lower cervical facet joints. Still unresolved is whether this stretch is sufficient to activate nociceptors in the joint capsule. To answer this question, recent studies indicate that low stretch levels activate proprioceptors in the facet-joint capsule. Excessive capsule stretch activates nociceptors, leads to prolonged neural afterdischarges, and can cause damage to the capsule and to axons in the capsule. In instances in which a whiplash event is severe enough to injure the joint capsule, facet capsule overstretch is a possible cause of persistent neck pain.

Neural mechanisms of lumbar pain

This article discusses neuroanatomic and neurophysiologic bases for low back pain. Evidence for the existence of pain generators in facet, disc, muscle, nerve roots, and dorsal root ganglia are discussed. Mechanisms that may explain the persistence of pain, including neurogenic and non-neurogenic inflammation and central sensitization, are also presented.

Effect of Nucleus Pulposus on the Neural Activity of Dorsal Root Ganglion

Study Design. This study was designed to investigate, using neurophysiologic techniques in an in vivo rat model, the effect of application of nucleus pulposus to the nerve root on the neural activity of the dorsal root ganglion and the corresponding receptive fields. Objectives. To assess a further role of the dorsal root ganglion in mechanisms of radicular pain in lumbar disc herniation. Summary of Background Data. It has been suggested that the epidural application of autologous nucleus pulposus without mechanical compression causes nerve root inflammation and related radicular pain in lumbar disc herniation. Concerning the dorsal root ganglion, its mechanical hypersensitivity and potential for generating ectopic discharges have been reported. However, the effect of autologous nucleus pulposus on the dorsal root ganglion is uncertain. Methods. In adult Sprague-Dawley rats spontaneous neural activity was recorded from the surgically exposed L5 dorsal root using electrophysiologic techniques, and the mechanosensitivity of L5 dorsal root ganglia and corresponding receptive fields on the hind paw were measured using calibrated nylon filaments. Autologous nucleus pulposus from the tail or fat was implanted at the L5 nerve root. Neural activity was monitored for 6 hours. Results. Spontaneous neural activity in the nucleus pulposus group gradually increased and showed significant differences compared with the fat group from 2.5 to 6 hours after exposure. The mechanosensitivity of the dorsal root ganglia showed significant increases compared with the fat group. Conclusions. After application of nucleus pulposus to the nerve root, the dorsal root ganglion demonstrated increased excitability and mechanical hypersensitivity. These results suggest that nucleus pulposus causes excitatory changes in the dorsal root ganglion.

Demonstration of Substance P, Calcitonin Gene-Related Peptide, and Protein Gene Product 9.5 Containing Nerve Fibers in Human Cervical Facet Joint Capsules

Study Design. Human cervical facet joint capsules were evaluated by immunohistochemistry. Objectives. To study the neuropeptide innervation of the cadaveric cervical facet joint capsules. Summary of Background Data. Various clinical and biomechanical studies indicate a role for cervical facet joint capsules in the etiology of neck pain. However, studies on innervation of these capsules are very limited. There is also a dearth of studies on the neuropeptide nature of this innervation. Methods. Facet joint capsules harvested from unembalmed cadavers were studied by the avidin biotin peroxidase method for the presence of nerve fibers. Neuropeptide innervation was investigated by using antisera to substance P and calcitonin gene-related peptide. Antisera to protein gene product 9.5 (PGP 9.5), a general neuronal marker, were also used. Results. In a study of 12 human cervical facet joint capsules, short segments of substance P were observed in 6 capsules, while fibers reactive to calcitonin gene-related peptide were demonstrated in 7 capsules. Nerve fibers immunoreactive to protein gene product 9.5 were also observed in 9 of the 14 capsules studied. Protein gene product 9.5 reactive fibers were the most extensively distributed fibers, observed as bundles and also as single fibers. Conclusions. An abundance of protein gene product 9.5 reactive nerve fibers indicates an extensive innervation of the cervical facet joint capsules. The presence of substance P and calcitonin gene-related peptide reactive nerve fibers in a population of these lends credence to cervical facet joint capsules as a key source of neck pain.

Effects of phospholipase A2 on lumbar nerve root structure and function.

Study Design. To investigate the effects of phospholipase A2 on the neurophysiology and histology of rat lumbar spinal nerves and the corresponding behavioral changes. Objectives. To study possible mechanisms of sciatica. Summary of Background Data. The pathophysiology of sciatica is uncertain, although mechanical, chemical, and ischemic factors have been proposed. Methods. Phospholipase A2 was injected into the rat L4‐L5 epidural space, and the rats were observed for 3 or 21 days. Behavioral studies were conducted daily during the survival period. On the 3rd or 21st day, extracellular nerve recordings were made from dorsal roots, to determine discharge properties and mechanical sensitivity. The nerve roots were then sectioned for a light‐microscopic examination. Results. Motor weakness of hind limbs and altered sensation were observed. In the 3‐day phospholipase A2 groups, squeezing the dorsal roots at the L4‐L5 disc level (force = 0.8 g) evoked sustained ectopic discharge that lasted approximately 8 minutes. Squeezing the roots distal to the L4‐L5 area did not result in sustained discharges. In sham, control, and 21‐day phospholipase A2 groups, squeezing the dorsal roots elicited only a transient firing that lasted approximately 0.1 second. Loss of myelin was seen in the nerve root cross sections in the 3‐day group, and remyelination was observed in the 21‐day group. No abnormality was found in the control groups. Conclusions. Based on these studies, it is hypothesized that phospholipase A2 causes demyelination that results in hypersensitive regions where ectopic discharge may be elicited by mechanical stimulation. These ectopic discharges may be a source of sciatica. We believe that, as long as these irritating factors are present, the hypersensitive nerve root nerve will continue to fire, and sciatic pain will persist.

Distribution of A-δ and C-fiber receptors in the cervical facet joint capsule and their response to stretch

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-delta 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 receptors 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.

A morphological study of the fibrous capsule of the human lumbar facet joint

Study Design Macroscopic and microscopic investigations of the human lumbar facet joint capsule were undertaken. Objective To describe the morphologic characteristics of the fibrous capsule of the lumbar facet joints. Summary of Background Data Previous biomechanical and neurophysiologic studies by the authors have shown that the lumbar facet joint capsule may be a source of low back pain. Methods Macroscopic investigation was performed on the facet joint capsules dissected from five fresh adult cadavers. For microscopic studies, facet joint capsules obtained from cadaver dissection and spinal surgeries were stained by the hematoxylin and eosin method and the Elastica‐Van Gieson method. Results The outer layer of the fibrous capsule is a dense regular connective tissue that is composed of parallel bundles of collagenous fibers. The inner layer of the fibrous capsule consists of bundles of elastic fibers, similar to the ligamentum flavum. In the superior and middle part of the joint, the fibers run in the medial to lateral direction, crossing over the joint gap. In the inferior part of the joint, the fibers are relatively long and run in a superior‐medial to inferior‐lateral direction, covering the inferior articular recess. They are thicker than the layer in the superior and middle parts of the joint. Conclusions Anatomical and histologic features of the lumbar facet joint capsule are different between its outer layer and inner layer. This complex of morphologic factors can affect the biomechanics and neurophysiology of the lumbar facet joint.

The effects of controlled mechanical loading on group-II, III, and IV afferent units from the lumbar facet joint and surrounding tissue. An in vitro study.

An in vitro model was developed to investigate the responses of afferent units in the lumbar spine to controlled loading as measured by a load-cell. The neuronal discharge was recorded simultaneously with loading. Three types of neuronal responses were observed. The first type of response involved phasic-type mechanoreceptors, which responded to movement, regardless of direction or initial position. The response did not outlast the movement phase of loading. These units may serve as velocity detectors. The second type of response was seen in slowly adapting low-threshold mechanoreceptors, which tended to respond to loading in the 0.3 to 0.5-kilogram range with an immediate and sustained increase in the rate of firing. This type of response appears to be associated with the activation of low-threshold group-II and group-III fibers, which were located in muscles and tendons inserting into the facet joint. The third type of response involved slowly adapting high-threshold mechanoreceptors, which could not be activated until a threshold of three to five kilograms had been exceeded. It appears that this type of response is at least partially due to the activation of high-threshold group-III and group-IV capsular afferent units, which may signal noxious mechanical stimulation.