Nerve-roots of the cauda equina. The effect of hypotension and acute graded compression on function.

The influence of hypotension on the function of nerve-roots of the cauda equina under acute graded compression was studied with use of an established porcine model. In twenty adult miniature pigs, the nerve-roots of the cauda equina were compressed at pressures of zero (sham), fifty, 100, or 200 millimeters of mercury, and hypotension was induced with sodium nitroprusside. Compression was maintained for two hours, and a ninety-minute period of recovery followed. The nerve-roots were monitored electrophysiologically throughout the experiment. To assess the effect of compression and hypotension on the function of the nerve-roots, values of afferent and efferent amplitude and nerve-conduction velocity were compared with values in twenty adult miniature pigs that had similar graded compression without hypotension. Hypotension significantly affected efferent and afferent amplitudes at the end of the compression period; the effect remained significant at the end of the recovery period. The most profound effect of hypotension occurred during compression at fifty millimeters of mercury--a pressure that had no effect on the function of the nerves in normotensive animals.

Anatomic and biomechanical analysis of the lower lumbar foraminal ligaments.

STUDY DESIGN: An anatomic cadaveric study to characterize the lumbar intraforaminal nerve root attachments. OBJECTIVES: To characterize the intraforaminal nerve root attachments and describe their anatomic relationships and biomechanical properties. SUMMARY OF BACKGROUND DATA: Observations during foraminotomies for lateral recess stenosis as well as lateral approaches for far lateral disc herniation have shown dense attachments between the nerve root and adjacent structures. Little or no information has appeared in the literature describing intraforaminal nerve root attachments. METHODS: Twelve fresh-frozen human cadaveric lumbar spines were used to study intraforaminal ligamentous structures. Four cadavers were cut into sagittal sections for qualitative description, and eight were used for biomechanical testing. Histologic analyses were performed on samples of the foraminal attachments to assure that they were not vascular or neural structures. Biomechanical testing of the nerve roots with ligamentous attachments was performed measuring load to failure along the anatomic axis of the root. RESULTS: The dissections showed four distinct bands extending radially from the nerve root sleeve. The most prominent nerve root attachment was to the facet capsule posteriorly. Other ligaments fanned out with attachments inferiorly and superiorly to the adjacent pedicles and anteriorly to the intervertebral disc. Biomechanical study of the L3, L4, and L5 nerve roots showed a significant increase in strength at failure with axial traction, progressing from L3 to L5. CONCLUSIONS: The results demonstrate that these foraminal ligaments are normal anatomic structures within the intervertebral foramen of the lumbar spine. In addition, they may play a role in limiting motion along the nerve root.

Straight leg raising. Anatomical effects on the spinal nerve root without and with fusion.

Straight leg raising (SLR) is a useful clinical test to demonstrate an inflammatory compressive process across a spinal nerve root. Several previous studies have attempted to evaluate the effect of SLR on nerve root motion, but the exact direction and amount of this motion is still unclear. Components of the SLR test that have not been adequately addressed include the effect of SLR on the intact dural-nerve root system, motion of the nerve tissues as distinct from the dura, and nerve root strain. Separately, spinal fusion is occasionally used as an adjunct to discectomy to decrease instability and subsequent "nerve root irritation." The effect of a one-level fusion on in situ nerve root biomechanics, however, has not been evaluated. Ten fresh human cadavers underwent posterior lumbar laminectomies. Spinal nerve root motion was studied while a SLR maneuver was performed. Data was recorded photographically and statistically analyzed. The results were as follows: 1) SLR induced both linear motion (0.5-5 mm) and strain (2-4%) in spinal nerves L4, L5, and S1; 2) The dura moved less than the intrathecal nerve root at the pedicle and experienced more strain (P < .05). 3) The nerve roots moved laterally toward the pedicle and thus would move into a posterolaterally herniated disc. 4) Rigid anterior stabilization did not decrease nerve root motion or strain.

The effects of induced hypertension and acute graded compression on impulse propagation in the spinal nerve roots of the pig.

Injuries caused by compression of spinal nerve roots are frequently encountered clinically. Experimental studies show that several different factors affect the pathophysiologic changes that occur after these injuries. However, the effect of hypertension in conjunction with graded compression of spinal nerve roots is yet unclear. A previously established porcine model was employed, in which the spinal nerve roots were exposed and compressed by an inflatable balloon. Impulse propagation across the compressed nerve segment was studied by the recording of efferent and afferent nerve action potentials, and nerve conduction velocity. The systemic blood pressure was increased by administration of Neo-Synephrine hydrochloride (phenylephrine HCl) (Winthrop Pharmaceuticals, New York, NY) and elevated 40 +/- 5 mm Hg above the normal (100 +/- 5 mm Hg), and electrophysiologic baseline values were recorded. The spinal nerve roots were then compressed for 2 hours with either 0 (control), 50, 100, or 200 mm Hg. The balloon was deflated and the nerve roots were allowed to recover for 1.5 hours. Impulse propagation was studied every 15 minutes and hypertension was maintained throughout the experiment. The results showed no significant changes of the efferent and afferent nerve action potentials at 0.50 or 100 mm Hg. At 200 mm Hg, efferent and afferent nerve action potentials decreased rapidly and were almost abolished within 20 minutes of compression. Some but not significant recovery was seen of the nerve impulse. Compared to data from normotensive pigs in an earlier experiment, the current study showed that hypertension significantly decreases the susceptibility of the spinal nerve roots to compression at and below 100 mm Hg.(ABSTRACT TRUNCATED AT 250 WORDS)

Cauda equina anatomy. I: Intrathecal nerve root organization.

The three-dimensional organization of the human cauda equina has not been described previously. This is partly due to the difficulties of dissecting individual, unfixed nerve roots. By the use of a newly developed in situ fixation and embedding technique on 15 fresh human cadavers, the cross-sectional anatomy of the cauda equina was defined from L2-L3 to L5-S1. A highly consistent cross-sectional pattern was observed in all specimens. The lower sacral (S2-S5) and coccygeal roots were located in the dorsal aspect of the thecal sac, whereas the lumbar and first sacral roots exhibited an oblique, layered pattern as they ascended. The motor bundle was situated anteromedial to its respective sensory bundle within each layer, Invaginations of arachnoid held the nerve roots in a fixed relationship to one another. This previously undescribed three-dimensional anatomy within the thecal sac may aid in the understanding and treatment of trauma, neurocompressive syndromes, and tumors of the cauda equina.

Effects of magnitude and duration of compression on spinal nerve root conduction.

Spinal nerve root compression occurs commonly in conditions such as herniated nucleus pulposus, spinal stenosis, and trauma. However, the pathophysiology of the symptoms and signs related to spinal nerve root compression is poorly understood. The purpose of the present study was to assess and compare effects of various pressures and durations of acute compression on spinal nerve root conduction in the pig cauda equina. Efferent conduction (compound motor action potentials) and afferent conduction (compound nerve action potentials) were monitored during compression for 2 or 4 hours with compression pressures of 0 (sham), 50, 100, or 200 mm Hg. Recovery from compression was monitored for 1.5 hours. No significant deficits in spinal nerve root conduction were observed with 0 or 50 mm Hg compression, compared to significant conduction deficits induced by 100 and 200 mm Hg compression. Three-way analysis of variance demonstrated significant effects of compression pressure and duration on conduction at the end of compression and recovery, with a significant difference between efferent and afferent conduction at the end of the recovery period. These observations suggest an interaction between biomechanical and microvascular mechanisms in the production of nerve root conduction deficits. Such information may relate to the motor and sensory dysfunction in clinical conditions associated with spinal nerve root compression.

Metallic spinal artifacts in magnetic resonance imaging.

STUDY DESIGN: The magnetic resonance artifact susceptibility of traces of surgical aluminum, titanium, and stainless steel in a human spine model was investigated. Metallic filings were deposited in noncontiguous disc spaces in five human thoracic spines before magnetic resonance imaging with spin echo and gradient echo sequences. OBJECTIVES: Spin echo and gradient echo sequences were used for quantitate and compare void artifact produced by commonly used surgical metals. This was compared to a liquid paraffin control. SUMMARY OF BACKGROUND DATA: No significant susceptibility artifact was seen with any metal in all spin echo sequences, including T1 (TR 600, TE 12), T2 (TR 2000, TE 30), proton density (TR 2000, TE 80), and fast T2 scanning (TR 3800, TE 96, Ef). METHODS: Sagittal magnetic resonance imaging permitted void artifact quantification and comparison between different metallic alloys. Two neuroradiologists, working on a blinded basis, evaluated all data and rated the void susceptibility artifact on a scale of 1 (least) to 4 (greatest). RESULTS: In general, the magnitude of an imaging artifact during magnetic resonance imaging correlated with the magnetism of the metal. Nickel, found in a larger concentration in 316L than in 304 stainless steel, decreases the magnetic resonance artifact of specific metals because of its ability to stabilize iron in a non-magnetic state. Therefore, the 316L stainless steel yielded less artifact production than 304 stainless steel on gradient echo imaging. CONCLUSION: If upon gradient echo imaging in the postoperative period significant artifact production is noted, stainless steel deposition should be suspected as the causative agent. In this situation, spin echo techniques should be the first approach for attempting optimal visualization of the spinal cord and soft tissue structures.

Experimental stretch neuropathy. Changes in nerve conduction under tension.

We developed an animal model of stretch injury to nerve in order to study in vivo conduction changes as a function of nerve strain. In 24 rabbits, the tibial nerve was exposed and stretched by 0%, 6% or 12% of its length. The strain was maintained for one hour. Nerve conduction was monitored during the period of stretch and for a one-hour recovery period. At 6% strain, the amplitude of the action potential had decreased by 70% at one hour and returned to normal during the recovery period. At 12% strain, conduction was completely blocked by one hour, and showed minimal recovery. These findings have clinical implications in nerve repair, limb trauma, and limb lengthening.

Comparison of hydroxyapatite granules to autogenous bone graft in fusion cages in a goat model.

BACKGROUND: With the increased use of fusion cages to achieve lumbar intervertebral fusion, the question arises as to the potential for bone ingrowth from the host bone through the entire cage. Is it even necessary to have an autogenous graft to achieve total bone incorporation? METHODS: Nine adult male goats had fusion cages implanted into three vertebral bodies. The design was Surgical Dynamics/Ray Fusion Cage, measuring 21 mm x 14 mm. In each animal, one fusion cage was filled with autogenous graft, one with hydroxyapatite, porous granules, and the other with nonporous granules. Amount of new bone formation was determined by backscatter electron microscopy at 3 months post implantation in all animals. RESULTS: The histologic section shows that there was total incorporation in all specimens at 3 months. There was slightly more new bone (43%) with the nonporous granules compared with the porous granules (35%). The amount of residual void space was about the same in all specimens, indicating that the amount of new bone formation was similar and not statistically different in cages filled with hydroxyapatite granules versus granules of autogenous bone. CONCLUSION: This study confirms that total incorporation by ingrowth of new bone can be expected in fusion cages. The amount of ingrowth is about the same for autogenous graft versus hydroxyapatite granules. Apparently, it is not necessary to use bone graft to achieve successful bone incorporation if an acceptable biocompatable lattice, such as hydroxyapatite granules, is used.

A morphologic, biochemical, and biomechanical assessment of short-term effects of osteochondral autograft plug transfer in an animal model.

PURPOSE: The objective of this study was to assess the short-term changes that occur after an osteochondral autograft plug transfer from the femoral trochlea to the medial femoral condyle in a goat model. TYPE OF STUDY: Articular cartilage repair animal study. METHODS: Six adult male goats were used in this study. Two 4.5-mm osteochondral plugs were transferred from the superolateral femoral trochlea to 2 recipient sites in the central portion of the medial femoral condyle for a survival period of 12 weeks. Postmortem, the global effects of the procedure were assessed by gross morphologic inspection and by analyzing the synovial DNA for inflammatory response. The recipient sites were also evaluated histologically and biomechanically. Metabolic activity was determined by (35)SO(4) uptake, and viability was assessed using a live/dead stain and by confocal laser microscopy. RESULTS: There was no evidence of significant gross morphologic or histologic changes in the operative knee as a result of the osteochondral donor or recipient sites. The patella, tibial plateau, and medial meniscus did not show any increased degenerative changes as a result of articulating against the donor or recipient sites of the osteochondral autografts. Analysis of synovial DNA revealed no inflammatory response. Biomechanically, 6- to 7-fold greater stiffness was noted in the cartilage of the transferred plugs compared with the control medial femoral condyle. Furthermore, on histologic examination, the healing subchondral bone interface at the recipient site had increased density. Glycosaminoglycan synthesis as determined by (35)SO(4) uptake was upregulated in the transplanted cartilage plug relative to the contralateral control, showing a repair response at the site of implantation. And finally, confocal microscopy showed 95% viability of the transferred plugs in the medial femoral condyle region. CONCLUSIONS: Our findings demonstrate the ability to successfully transfer an osteochondral autograft plug with maintenance of chondrocyte cellular viability. The transferred cartilage is stiffer than the control medial femoral condyle cartilage, and there is concern regarding the increased trabecular mass in the healing subchondral plate, but these do not result in increased degenerative changes of the opposing articular surfaces in the short term.

Postoperative posterior spinal wound infections.

The incidence of postoperative spinal infections increases with the complexity of the procedure. Diskectomy is associated with less than a 1% risk of infection; spinal fusion without instrumentation is associated with a 1%-5% risk; and fusion with instrumentation may be associated with a risk of 6% or more. Twenty-two postoperative posterior spinal infections that occurred during a three-year period were reviewed for this report. Staphylococcus aureus was the most frequent organism cultured (more than 50% of the cases). Other recurring organisms were Staphylococcus epidermis, Peptococcus, Enterobacter cloacae, and Bacteroides. Many patients had multiple organisms. Risk factors appeared to include advanced age, prolonged hospital bed rest, obesity, diabetes, immunosuppression, and infection at remote sites. Operative factors included prolonged surgery (greater than five hours), high volume of personnel moving through the operating room, and instrumentation. Postoperative contamination may occur and may be related to prolonged postoperative bed rest, skin maceration (thoracolumbosacral orthoses), and drainage tubes exiting distally from lumbar wounds (toward the rectum). Effective treatment includes early diagnosis, surgical debridement and irrigation, and parenteral antibiotics. Superficial infections were treated successfully with wound closure over outflow tubes, and deep infections with inflow-outflow systems. Maintaining the instrumentation in place was possible in most cases. Parenteral antibiotics were maintained for six weeks in every case.