Jennifer B. Massie

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.

Spinal nerve root compression.

The pathophysiology of sciatica is not completely understood, although our understanding of its causes is increasing. Mechanical alterations combined with inflammatory changes lead to pain. Compression alters nerve root conduction and compromises the nutritional support of spinal nerve roots (through intrinsic and extrinsic vascularity and cerebral spinal fluid percolation). Mechanical forces can lead to intraneural damage and functional changes in nerve roots. Chemical and metabolic effects can create an inflammatory response. Varying causes of inflammation coupled with varying degrees of compression can occur anywhere along the cauda equina or spinal nerve root, including the dorsal root ganglia, and contribute to the pain response and neurologic deficits associated with sciatica.

Follow-up of Osteochondral Plug Transfers in a Goat Model A 6-Month Study

Background Osteochondral transfer procedures are increasingly used to resurface full-thickness articular cartilage defects. There has not been long-term assessment/description of autogenous donor and recipient sites. Hypothesis The healing process occurs at the donor/host cartilage and bone interfaces. Study Design Histologic, biochemical, and biomechanical changes were assessed 6 months after an osteochondral transfer in a goat model. Methods Eight adult goats were studied. In the 6 osteochondral transfer goats, 2 autogenous plugs were transferred from the femoral trochlea to defects in the weightbearing portion of the medial femoral condyle. The goats were allowed free range for 6 months. Randomly assigned plugs were assessed. Results Knees of the sacrificed animals had preservation of the joint space with mild chondromalacic changes in both transfer and contralateral control groups. Histologically, no evidence of cartilage (host/donor) healing was seen. Subchondral bone of the plug was contiguous with the surrounding recipient bone. Cellular viability in the autogenous osteochondral plug was seen, and 35SO4 uptake of the articular cartilage was not statistically different from the contralateral control condyle. The indentation stiffness of the transfer plug (mosaicplasty) and the contralateral donor site were similar—much stiffer than normal cartilage including surrounding condylar cartilage. Large structural stiffness of transferred cores and donor sites appeared to be related to their thinner cartilage layer. Conclusions At 6-month follow-up, a cleft between host and transferred articular regions remained, with no integration between the two. Clinical Relevance Autogenous transplantation of osteochondral plugs is possible with integration of subchondral bone and preservation of chondral viability.

Anatomic consideration for sacral screw placement.

Instrumentation of the lumbosacral spine increasingly involves screw fixation to the sacrum. Recommended locations and techniques for screw placement vary, particularly when bicortical purchase of the sacrum is performed. The purpose of this study was to describe the critical anatomy and potential injuries to neurovascular and visceral structures anterior to the sacrum. Lack of awareness can lead to life-threatening complications. The study included 22 fresh human cadavers with no prior spinal surgery. Specimens were placed in a prone position, and the lumbosacral spine was exposed. Two 6.5-mm screws were inserted using one of two techniques, respectively: Starting just inferior to the S1 facet one screw was angled 25° caudally and 30° laterally; in the second technique, lateral inclination was increased to 45°. In addition, all specimens had screws placed in the S2 pedicles. An anterior dissection was performed to allow evaluation of the neurovascular and visceral structures at risk for injury by, or adjacent to, the screw tips. All significant neurovascular structures in the area of concern were constant in position. The internal iliac vein and the lumbosacral nerve trunk were most at risk for injury by the 30 and 45° laterally directed screws. The sigmoid colon, though close to the S2 screw, was protected by its mesentery. Screws placed in the S1 pedicle were least likely to injure the neurovascular bundle. A lateral and a midline safe zone were identified.

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.

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.

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.

Assessment of pedicle screw placement utilizing conventional radiography and computed tomography: A proposed systematic approach to improve accuracy of interpretation

Study Design. This was a human cadaver study to determine the accuracy of conventional radiography and computed radiography in the evaluation of pedicle screw placement and to identify methodology for more precise reading of these examinations. Objectives. To determine the accuracy of conventional radiography and computed tomography in the evaluation of pedicle screw placement within lumbar vertebral pedicles and to develop methods to improve imaging interpretation. Summary of Background Data. Conventional radiography and computed tomography have been used in research and clinical settings to evaluate pedicle screw placement. This study evaluates the interpretative accuracy of readers blinded to the true position of screw placement using both imaging examinations. Furthermore, methodology was developed to improve accuracy of interpretation of these examinations. Methods. Three cadaver lumbar spines were instrumented bilaterally with pedicle screws from L1 to L5. Thirty pedicles had 6.0 mm AO pedicle screws inserted using standard surgical technique. Seven directions of deliberate misplacement as well as correct placement of screws were performed at random levels for a total of eight possible screw positions. Conventional radiographs and computed tomography scans were obtained. A senior musculoskeletal radiologist and senior spine surgeon interpreted the images while blinded to screw placement. Examiners initially assessed the screws as in or out, followed by assessment of the eight possible types of screw position. Consensus interpretation was obtained regarding the placement of individual screws. The spines were then dissected to visualize the screws and their position related to the pedicle. After determining the true position of the screws, a systematic method was designed and applied to the interpretation of the imaging methods to identify screw positions. Results. Using conventional radiographs, 63% of the screw placements were correctly identified as in or out of the pedicle. Computed tomography improved accuracy to 87%. Identifying the true directional component of screw position led to a decrease in accuracy (conventional radiographs 37% and computed tomography 47%). Using asystematic method to analyze imaging studies enabled detection of screw positions. Conclusion. Evaluation of pedicle screw placement is difficult even in experienced hands. A systematic approach to image interpretation should allow for an accurate assessment of pedicle screw placement.

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.

A preclinical post laminectomy rat model mimics the human post laminectomy syndrome.

Chronic low back pain with sciatica complicating post laminectomy surgery is poorly understood. It is likely that some aspects of persistent pain of the syndrome results from spinal facilitation in which there is lowering of pain excitation levels. A small animal preclinical model is needed that mimics the clinical condition to permit detailed studies of the underlying altered neurochemistry of the sensory pathways. We propose herein a rat laminectomy model containing the elements required for study of the neurobiology of the condition. The model consists of a surgical laminectomy that includes L5 spinal nerve manipulation and disc injury, elements necessarily employed in human disc herniation surgery. At 8 weeks post laminectomy the proposed model demonstrates paraspinous muscle spasm, tail contracture, behavioral pain behavior, tactile allodynia, epidural and nerve root scarring, and nerve root adherence by scar to the underlying disc and adjacent pedicle. Two underlying pain facilitation states are invoked in the clinical condition: (1) an inflammatory state required to achieve wound healing; and (2) a nerve injury state resulting from nerve manipulation and subsequent epidural scarring, spinal nerve scarring, and spinal nerve tethering to the adjacent disc and pedicle. Both pain facilitation states are active in the model.