Sten Holm

Interaction between the porcine lumbar intervertebral disc, zygapophysial joints, and paraspinal muscles

Study Design. A porcine model was used to study whether muscular activation in the paraspinal muscles caused by nerve stimulation in the anulus fibrosus of a lumbar intervertebral disc could be altered by saline injection into the zygapophysial (facet) joint. Objectives. To elucidate possible mechanisms regarding the nerve pathways and interactions between the intervertebral disc, zygapophysial joints, and the paraspinal musculature. Summary of Background Data. The physiologic basis for chronic low back pain, including muscular spasm, is uncertain. Although extensive research involving the lumbar motion segments and the surrounding tissues has been performed, the neuromuscular connection has not been sufficiently investigated. Materials and Methods. Twenty‐three adolescent pigs were used to measure the electromyographic response in the paraspinal musculature to electrical stimulation of the posterolateral L3‐L4 anulus fibrosus, before and after introduction of physiologic saline into the zygapophysial joint. Motor unit action potentials were recorded using three sets of needle electrodes placed into the deepest fascicles of the multifidus, bilateral to the L4 and L5 spinous processes, and into the central longissimus musculature, bilateral to the L4 spinous process. Results. Stimulation of the nerves within the posterolateral anulus of the disc elicited reactions in the paraspinal muscles, namely the lumbar multifidus and longissimus. Introduction of physiologic saline into the zygapophysial joint resulted in a reduction in the motor unit action potential amplitude. This reduction was manifested as an immediate and constant reduction, a graded reduction, or a delayed reaction, during which the reduction occurred an average of 5 minutes after the saline injection. Conclusions. Introduction of physiologic saline into the zygapophysial joint reduced the stimulation pathway from the intervertebral disc to the paraspinal musculature. The zygapophysial joints may therefore have a regulating function, controlling the intricate neuromuscular balance in the lumbar motion segment.

Edema formation in spinal nerve roots induced by experimental, graded compression. An experimental study on the pig cauda equina with special reference to differences in effects between rapid and slow onset of compression.

Edema formation in spinal nerve roots of the pig cauda equina was studied following experimental compression at various pressure levels, durations, and rates of onset, using a fluorescence microscopic technique. The time–pressure thresholds for the occurrence of edema in the nerve roots were: following rapid onset of compression (0.05–0.1 seconds), 2 minutes at both 50 mm Hg and 200 mm Hg, and following slow onset of compression (the pressure was slowly increased during 15–20 seconds), 2 hours at 50 mm Hg and 2 minutes at 200 mm Hg. Generally, the edema formation was more pronounced after rapid than after slow onset of compression. The data in this study also indicate that intraneural edema might be more easily formed in nerve roots than in peripheral nerves after compression injury.

Rapid atrophy of the lumbar multifidus follows experimental disc or nerve root injury

Study Design. Experimental study of muscle changes after lumbar spinal injury. Objectives. To investigate effects of intervertebral disc and nerve root lesions on cross-sectional area, histology and chemistry of porcine lumbar multifidus. Summary of Background Data. The multifidus cross-sectional area is reduced in acute and chronic low back pain. Although chronic changes are widespread, acute changes at 1 segment are identified within days of injury. It is uncertain whether changes precede or follow injury, or what is the mechanism. Methods. The multifidus cross-sectional area was measured in 21 pigs from L1 to S1 with ultrasound before and 3 or 6 days after lesions: incision into L3–L4 disc, medial branch transection of the L3 dorsal ramus, and a sham procedure. Samples from L3 to L5 were studied histologically and chemically. Results. The multifidus cross-sectional area was reduced at L4 ipsilateral to disc lesion but at L4–L6 after nerve lesion. There was no change after sham or on the opposite side. Water and lactate were reduced bilaterally after disc lesion and ipsilateral to nerve lesion. Histology revealed enlargement of adipocytes and clustering of myofibers at multiple levels after disc and nerve lesions. Conclusions. These data resolve the controversy that the multifidus cross-sectional area reduces rapidly after lumbar injury. Changes after disc lesion affect 1 level with a different distribution to denervation. Such changes may be due to disuse following reflex inhibitory mechanisms.

Experimental instability in the lumbar spine.

Study Design. An in vivo animal model of lumbar segmental instability, involving both passive and active stabilizing components of the spine, was developed. Objective. The aim of this investigation was to dynamically study the alterations in segmental kinematics as a result of interventions to the passive stabilizing components and to the lumbar musculature. Summary of Background Data. Segmental instability in the lumbar spine is associated with abnormal intervertebral motion. The majority of biomechanical studies have examined the in vitro effects of transecting individual stabilizing structures (i.e., intervertebral disc, facet joints, and ligaments), and have not simultaneously considered the effects of active musculature on spinal kinematics, which exist in the in vivo environment. Also, few studies have evaluated the kinematic behavior in the neutral region, for example, the transition phase between flexion and extension. Methods. Four experimental groups comprised 33 pigs, each of which followed different surgical injury sequences to the L3-L4 motion segment. An instrumented linkage attached to the L3-L4 motion segment was used to measure the sagittal kinematics during dynamic flexion-extension after each surgical injury and after bilateral stimulation of the lumbar paraspinal musculature. Results. Injuries to the disc resulted in greater overall axial translation. Graded injuries to the facet joint mainly caused changes in sagittal rotation and shear translation. When the facet injuries were compounded by removal of the transverse processes, there was significantly greater coupled motion and increased hysteresis in the neutral region for rotation. Extensive muscular stimulation after each of the injuries caused significantly greater rotation and shear translation, along with a tendency toward reduced axial translation, when compared to the unstimulated case. Although increasing the range of motion, increased muscular activity stabilized the injured motion segment by smoothing the erratic rotation pattern of motion, particularly in the neutral region. Conclusions. Because of the direct attachment to the vertebrae, both passive and active strain from the musculature influence the spinal kinematics in normal or destabilized motion segments. Although increasing the range of motion, stimulation of the musculature surrounding the injured motion segment has a stabilizing effect by reducing abrupt kinematic behavior, particularly in the neutral region where the muscles are under reduced tension. A facetectomy produces a paradoxical kinematic behavior, which enhances the unstable condition of the motion segment. Surgical and rehabilitative treatments for patients with segmental instability need to consider the physiologic influences of the spinal musculature.

Variations in the nutrition of the canine intervertebral disc induced by motion.

In this experiment study, we have investigated some transport and metabolic parameters of the canine intervertebral disc induced by spinal motion. The movements of the spine were obtained during supervised exercise, where either moderate, violent, or specific movements were performed. The results show changes in the metabolic rates as well as in concentration profiles of metabolites of the exercised groups in comparison to the control group. An increasing aerobic metabolism was noted in the outer part of the annulus and in the central part of the nucleus pulposus, resulting in a reduction of the lactate concentration. These findings do indicate that spinal movements, over a longer period of time give rise to positive nutritional variations. Furthermore, the reported changes might be of significance also for the human lumbar disc problem, since previous studies have demonstrated nutritional similarities between the discs of dogs and men.

Electromyographic response of the porcine multifidus musculature after nerve stimulation.

Study Design In this study, a porcine model was used to study whether a nerve reaction in the anulus fibrosus of a lumbar disc or in a facet joint capsule could cause a muscular response in the multifidus musculature. Objectives To determine if there is an interrelationship between the intervertebral disc and facet joint innervation and the multifidus musculature as a possible pain mechanism. Summary of Background Data The innervation of the anulus fibrosus of the intervertebral disc and the capsule of the facet joint is well described in the literature, although the functions of these nerves are poorly understood. An interrelationship between this innervation and the paraspinal musculature has not been previously described. Methods Fifteen adult pigs were used to measure the electromyographic response in the multifidus musculature to electrical stimulation of the lateral region of the disc anulus and the facet joint capsule in the L1-L2 motion segment. Motor unit action potentials were recorded using three sets of bipolar needle electrodes placed into the deepest fascicles of the multifidus, bilateral to the L2, L3, and L4 spinous processes. The effect of lidocaine injection into the facet joint and subperiosteal muscle detachment on the electromyographic response were studied. Results Stimulation of the disc anulus fibrosus induced reactions in the multifidus on multiple levels and on the contralateral side, whereas stimulation of the facet joint capsule induced reactions predominantly on the same side and segmental level as the stimulation. Introduction of lidocaine into the facet joint resulted in a significantly reduced electromyographic response to either stimulation, with the most drastic reduction seen when stimulation, with the most drastic reduction seen when stimulating the facet joint capsule. Subperiosteal detachment of the paraspinal muscles prevented any muscular response. Conclusions Stimulation of the disc and the facet joint capsule produced contractions in multifidus fascicles. The clinical implications are that there may be interactive responses between injured or diseased structures, i.e., disc or facet joints, and the paraspinal musculature. Activation of the multifidus muscles may have a stabilizing effect, constraining the motion of the lumbar spine. Longstanding muscular contraction may produce ischemic conditions and may be a potential source of pain.

Experimental disc degeneration due to endplate injury.

The aim of this study was to create an experimental model of disc degeneration that closely mimicked human disc degeneration. In six domestic pigs, an L4 cranial endplate perforation into the nucleus pulposus was made. Three months postoperatively, compressive testing was performed on the L2–L4 motion segments, and intradiscal pressure was measured in the intervening discs. Histochemical and morphologic examinations were made on the excised degenerated and adjacent discs. A significant reduction in water content was observed in the outer anterior annulus of the degenerated disc. In the nucleus, the proteoglycan content was significantly reduced, as well as the cellularity, although not significantly. The nucleus lost its gel-like structure and was discolored, and there was delamination of annular layers. Intradiscal pressure in the nucleus was significantly lower in the degenerated disc. In conclusion, experimental degeneration of the intervertebral disc induced by endplate penetration resembled human disc degeneration, as exemplified by biochemical and structural changes.

Sensorimotor control of the spine

The spinal viscoelastic structures including disk, capsule and ligaments were reviewed with special focus on their sensory motor functions. Afferent capable of monitoring proprioceptive and kinesthetic information are abundant in the disc, capsule and ligament. Electrical stimulation of the lumbar afferents in the discs, capsules and ligaments seem to elicit reflex contraction of the multifidus and also longissimus muscles. The muscular excitation is pronounced in the level of excitation and with weaker radiation 1 to 2 levels above and below. Similarly, mechanical stimulation of the spinal viscoelastic tissues excites the muscles with higher excitation intensity when more than one tissue (ligaments and discs for example) is stimulated. Overall, it seems that spinal structures are well suited to monitor sensory information as well as to control spinal muscles and probably also provide kinesthetic perception to the sensory cortex.

Experimental nerve root compression : a model of acute, graded compression of the porcine cauda equina and an analysis of neural and vascular anatomy

Nerve root compression has been suggested as one important pathogenetic factor in low-back pain syndromes and sciatica. The underlying pathophysiologic mechanisms are, however, incompletely known, partly because of the lack of experimental data on this topic. In the present study, a model for experimental compression of the porcine sacrococcygeal cauda equina is presented. The model consists of surgical exposure of the cauda equina and compression of the cauda equina toward the ventral aspect of the spinal canal by an inflatable balloon fixed to the spine. This compression system was shown to have a high accuracy in pressure transmission from the balloon to the cauda equina. The gross and microscopic neural anatomy and the vascujar anatomy of the porcine cauda equina were analyzed with light microscopic and ink-perfusion techniques. The porcine cauda equina showed a close anatomic resemblance to the human lumbosacral cauda equina. The presented model offers unique possibilities for experimental studies on nerve root compression injury because of the easy surgical exposure and the sufficient length of the nerve roots. In separate studies, this model, along with investigations of solute transport to the nerve tissue and of impulse propagation, has been used to analyze the effects of acute, graded compression on blood flow and edema formation in the cauda equina. The porcine cauda equina would also be particularly suitable for chronic compression studies because any neurologic deficit acquired would be restricted to the tail.

Compression-induced changes of the nutritional supply to the porcine cauda equina.

The effects of compression on the transport of 3H-labeled methyl glucose to spinal nerve roots were analyzed in an experimental model of the pig cauda equina. A rapid onset of compression (0.05-0.1 s) induced more pronounced effects than a slow onset (20 s) at corresponding pressure levels. There was evidence that this observed difference may be related to the magnitude of intraneural edema formed outside the compression zone. The results also indicate that the nutritional transport might be impaired at very low pressure levels and that diffusion from adjacent tissues with a better nutritional supply, including the cerebrospinal fluid, may not fully compensate for any compression-induced impairment of the intraneural blood flow.