Comparison of Epidural Pressure Decrease Pattern According to Different Lumbar Epidural Approaches

BACKGROUND

During lumbar epidural injection (LEI) using a midline approach, we might encounter failure of identifying the epidural space owing to an equivocal or absent loss of resistance (LOR) sensation. The reason for such absence of LOR sensation has been suggested as paucity of midline ligamentum flavum, paravertebral muscle, and cyst in the interspinous ligament of the lumbar spine. Despite its low specificity, LOR is the most commonly used method to identify the epidural space.

OBJECTIVES

The purpose of this study was to analyze lumbar epidural pressure decrease patterns and identify factors contributing to this pressure decrease.

STUDY DESIGN

Prospective randomized trial.

SETTING

An interventional pain management practice in South Korea.

METHODS

This prospective study included 104 patients receiving LEI due to lumbar radiculopathy. A midline or paramedian approach of LEI was determined with randomization. Among various factors, gender, age, body mass index (BMI), and diagnosis were analyzed using a subgroup that included 60 cases of only a paramedian approach.

RESULTS

Grades I, II (abrupt decrease), and III (gradual decrease) were found as patterns of epidural pressure decrease. Abrupt pressure decrease was more frequently observed in the paramedian group (P < 0.001). Age, gender, BMI, and diagnosis did not show any significant difference in frequencies between abrupt and gradual pressure decrease.

LIMITATIONS

We could not match LOR sensation with epidural pressure decrease shown in the monitor.

CONCLUSIONS

This study demonstrates that abrupt pressure decrease occurs more frequently with the paramedian approach. However, age, gender, BMI, or diagnosis did not affect the incidence of epidural pressure decrease.

KEY WORDS

Epidural, paramedian, midline, pressure decrease.

Air versus saline in the loss of resistance technique for identification of the epidural space

BACKGROUND

The success of epidural anaesthesia depends on correct identification of the epidural space. For several decades, the decision of whether to use air or physiological saline during the loss of resistance technique for identification of the epidural space has been governed by the personal experience of the anaesthesiologist. Epidural block remains one of the main regional anaesthesia techniques. It is used for surgical anaesthesia, obstetrical analgesia, postoperative analgesia and treatment of chronic pain and as a complement to general anaesthesia. The sensation felt by the anaesthesiologist from the syringe plunger with loss of resistance is different when air is compared with saline (fluid). Frequently fluid allows a rapid change from resistance to non-resistance and increased movement of the plunger. However, the ideal technique for identification of the epidural space remains unclear.

OBJECTIVES

• To evaluate the efficacy and safety of both air and saline in the loss of resistance technique for identification of the epidural space.• To evaluate complications related to the air or saline injected.

SEARCH METHODS

We searched the Cochrane Central Register of Controlled Trials (CENTRAL) (2013, Issue 9), MEDLINE, EMBASE and the Latin American and Caribbean Health Science Information Database (LILACS) (from inception to September 2013). We applied no language restrictions. The date of the most recent search was 7 September 2013.

SELECTION CRITERIA

We included randomized controlled trials (RCTs) and quasi-randomized controlled trials (quasi-RCTs) on air and saline in the loss of resistance technique for identification of the epidural space.

DATA COLLECTION AND ANALYSIS

Two review authors independently assessed trial quality and extracted data.

MAIN RESULTS

We included in the review seven studies with a total of 852 participants. The methodological quality of the included studies was generally ranked as showing low risk of bias in most domains, with the exception of one study, which did not mask participants. We were able to include data from 838 participants in the meta-analysis. We found no statistically significant differences between participants receiving air and those given saline in any of the outcomes evaluated: inability to locate the epidural space (three trials, 619 participants) (risk ratio (RR) 0.88, 95% confidence interval (CI) 0.33 to 2.31, low-quality evidence); accidental intravascular catheter placement (two trials, 223 participants) (RR 0.90, 95% CI 0.33 to 2.45, low-quality evidence); accidental subarachnoid catheter placement (four trials, 682 participants) (RR 2.95, 95% CI 0.12 to 71.90, low-quality evidence); combined spinal epidural failure (two trials, 400 participants) (RR 0.98, 95% CI 0.44 to 2.18, low-quality evidence); unblocked segments (five studies, 423 participants) (RR 1.66, 95% CI 0.72 to 3.85); and pain measured by VAS (two studies, 395 participants) (mean difference (MD) -0.09, 95% CI -0.37 to 0.18). With regard to adverse effects, we found no statistically significant differences between participants receiving air and those given saline in the occurrence of paraesthesias (three trials, 572 participants) (RR 0.89, 95% CI 0.69 to 1.15); difficulty in advancing the catheter (two trials, 227 participants) (RR 0.91, 95% CI 0.32 to 2.56); catheter replacement (two trials, 501 participants) (RR 0.69, 95% CI 0.26 to 1.83); and postdural puncture headache (one trial, 110 participants) (RR 0.83, 95% CI 0.12 to 5.71).

AUTHORS' CONCLUSIONS

Low-quality evidence shows that results do not differ between air and saline in terms of the loss of resistance technique for identification of the epidural space and reduction of complications. Applicability might be compromised, as most of the results described in this review were obtained from parturient patients. This review underlines the need to conduct well-designed trials in this field. 

Loading rate effect on mechanical properties of cervical spine ligaments

Mechanical properties of cervical spine ligaments are of great importance for an accurate finite element model when analyzing the injury mechanism. However, there is still little experimental data in literature regarding fresh human cervical spine ligaments under physiological conditions. The focus of the present study is placed on three cervical spine ligaments that stabilize the spine and protect the spinal cord: the anterior longitudinal ligament, the posterior longitudinal ligament and the ligamentum flavum. The ligaments were tested within 24-48 hours after death, under two different loading rates. An increase trend in failure load, failure stress, stiffness and modulus was observed, but proved not to be significant for all ligament types. The loading rate had the highest impact on failure forces for all three ligaments (a 39.1% average increase was found). The observed increase trend, compared to the existing increase trends reported in literature, indicates the importance of carefully applying the existing experimental data, especially when creating scaling factors. A better understanding of the loading rate effect on ligaments properties would enable better case-specific human modelling.

Do design variations in the artificial disc influence cervical spine biomechanics? A finite element investigation

Various ball and socket-type designs of cervical artificial discs are in use or under investigation. Many artificial disc designs claim to restore the normal kinematics of the cervical spine. What differentiates one type of design from another design is currently not well understood. In this study, authors examined various clinically relevant parameters using a finite element model of C3-C7 cervical spine to study the effects of variations of ball and socket disc designs. Four variations of ball and socket-type artificial disc were placed at the C5-C6 level in an experimentally validated finite element model. Biomechanical effects of the shape (oval vs. spherical ball) and location (inferior vs. superior ball) were studied in detail. Range of motion, facet loading, implant stresses and capsule ligament strains were computed to investigate the influence of disc designs on resulting biomechanics. Motions at the implant level tended to increase following disc replacement. No major kinematic differences were observed among the disc designs tested. However, implant stresses were substantially higher in the spherical designs when compared to the oval designs. For both spherical and oval designs, the facet loads were lower for the designs with an inferior ball component. The capsule ligament strains were lower for the oval design with an inferior ball component. Overall, the oval design with an inferior ball component, produced motion, facet loads, implant stresses and capsule ligament strains closest to the intact spine, which may be key to long-term implant survival.

Osterix is a key target for mechanical signals in human thoracic ligament flavum cells

Mechanical stress is considered to be an important factor in the progression of thoracic ossification of the ligament flavum (TOLF). To elucidate the mechanism underlying mechanical stress-induced TOLF, we investigated the effect of stretching on cultured flavum ligament cells derived from TOLF and non-TOLF patients. We found that the mRNA expression of alkaline phosphatase (ALP), osteocalcin, Runx2, and osterix, but not that of Dlx5 and Msx2, was significantly increased by stretching in TOLF cells. In addition, the effect seems to be finely tuned by stretching-triggered activation of distinct mitogen-activated protein kinase cascades. Specifically, a p38 specific inhibitor, SB203580, significantly inhibited stretching-induced osterix expression as well as ALP activity, whereas a specific inhibitor of ERK1/2, U0126, prevented stretching-induced Runx2 expression. We showed that overexpression of osterix resulted in a significant increase of ALP activity in TOLF cells, and osterix-specific RNAi completely abrogated the stretching-induced ALP activity, indicating that osterix plays a key role in stretching-stimulated osteogenic effect in TOLF cells. These results suggest that mechanical stress plays important roles in the progression of TOLF through induction of osteogenic differentiation of TOLF cells, and our findings support that osterix functions as a molecular link between mechanostressing and osteogenic differentiation.

Dynamic Canal Encroachment of Ligamentum Flavum

OBJECTIVE

In extension of the cervical spine, the ligamentum flavum (LF) may bulge and intrude into the vertebral canal and cause symptoms of myelopathy and/or radiculopathy in patients with canal stenosis. Knowledge of the relationship between the extension angle and the physiologic changes of the LF is important for a better understanding of the clinical symptoms. The current study was designed to demonstrate the dynamic correlativity of canal intrusion of the LF bulge with the extension angle of the cervical spine.

METHODS

With a novel method, the probe of the electrical resistance strain gauge was put into the cervical canals of 14 cadaveric specimens. The LF bulge distance of six segments (C2-C3 to C7-T1) and corresponding extension angles of every 5 degrees from 0 degrees to 45 degrees were collected. Angle-bulge curves were drawn.

RESULTS

In overextension (45 degrees ), C5-C6 had the biggest canal intrusion depth (3.478+/-0.527 mm), whereas the upper and lower segments declined gradually (C5-C6>C4-C5>C6-C7>C3-C4>C7-T1>C2-C3). The curves in all segments were sigmoidal, which demonstrated the dynamic change of LF, that is, during the process of extension, LF shortened and contracted first, after the original length was reached, it began to bulge and burst into the canal.

CONCLUSION

The current study has provided a method to measure inner canal kinematic changes in intact spinal specimens.

The nuchal ligament restrains cervical spine flexion

STUDY DESIGN

A biomechanical study using a cadaver model was conducted to define the function of the nuchal ligament in the restraint of flexion of the cervical spine.

OBJECTIVE

To test the hypothesis that surgical resection of the nuchal ligament significantly reduces the structural restraints to cervical flexion.

SUMMARY OF BACKGROUND DATA

Although previous studies have examined the role of the posterior ligaments and capsules on cervical stability, no prior study has quantified the biomechanical significance of the nuchal ligament. The clinical significance may include progressive loss of lordosis or even kyphosis after trauma or posterior surgical procedures such as laminectomy, laminoplasty, or tumor resection.

METHODS

Cervical spines from the occiput to the first thoracic vertebra were harvested from 12 human cadavers. Specimens were tested under 3 conditions: all ligaments intact, after resection of the nuchal ligament, and then after additional resection of the supraspinous, interspinous, and yellow ligaments. Flexion moments were applied; load and displacement were measured. Changes in flexion range of motion and tangent stiffness between treatment conditions were statistically compared.

RESULTS

The flexion range increased 28% after removing the nuchal ligament. After subsequent resections, the flexion range increased 52% compared with intact (P <0.005). Tangent stiffness decreased 27% after nuchal ligament resection; after all resections, stiffness was 35% lower than intact (P <0.05).

CONCLUSION

Resection of the nuchal ligament increased the flexion range of motion and decreased stiffness in flexion. Injury to the nuchal ligament may increase the risk of cervical spine instability and malalignment.

Development of a finite element model of the upper cervical spine and a parameter study of ligament characteristics

STUDY DESIGN

Numeric techniques were used to study the upper cervical spine.

OBJECTIVES

To develop and validate an anatomic detailed finite element model of the ligamentous upper cervical spine and to analyze the effect of material properties of the ligaments on spinal kinematics.

SUMMARY OF BACKGROUND DATA

Cervical spinal injuries may be prevented with an increased knowledge of spinal behavior and injury mechanisms. The finite element method is tempting to use because stresses and strains in the different tissues can be studied during the course of loading. The authors know of no published results so far of validated finite element models that implement the complex geometry of the upper cervical spine.

METHODS

The finite element model was developed with anatomic detail from computed tomographic images of the occiput to the C3. The ligaments were modeled with nonlinear spring elements. The model was validated for axial rotation, flexion, extension, lateral bending, and tension for 1.5 Nm, 10 Nm, and 1500 N. A material property sensitivity study was conducted for the ligaments.

RESULTS

The model correlated with experimental data for all load cases. Moments of 1.5 Nm produced joint rotations of 3 degrees to 23 degrees depending on loading direction. The parameter study confirmed that the mechanical properties of the upper cervical ligaments play an important role in spinal kinematics. The capsular ligaments had the largest impact on spinal kinematics (40% change).

CONCLUSIONS

The anatomic detailed finite element model of the upper cervical spine realistically simulates the complex kinematics of the craniocervical region. An injury that changes the material characteristics of any spinal ligament will influence the structural behavior of the upper cervical spine.

Histological investigation of rabbit ligamentum flavum with special reference to differences in spinal levels

The structure of the ligamentum flavum has yet to be fully elucidated and no studies have investigated fine structural differences at different spinal levels in any animals. The aim of the present study was to clarify structural differences in the ligamentum flavum at different spinal levels (cervical: C3/4 and C5/6; upper thoracic: T2/3; lower thoracic: T9/10; lumbar: L3/4) using light and electron microscopy of rabbit specimens. Light microscopy using resorcin-fuchsin staining revealed that the distribution of elastic fibers was diffuse in the cervical and upper thoracic regions, but was generally dense in the lower thoracic and lumbar regions. Electron microscopy demonstrated that the cervical and upper thoracic regions were rich in collagen fibers. Conversely, the lower thoracic and lumbar regions were rich in elastic fibers. Quantitative image analyses displayed thick elastic fibers in the lower thoracic and lumbar regions, with high area ratios. Radiographic examinations revealed that ranges of motion were large at the cervical region, but small at the lower thoracic and lumbar regions. These findings suggest that structure of the ligamentum flavum varies at different spinal levels with respect to differences in motion.

Geometric parameters of the in vivo tissues at the lumbosacral joint of young Asian adults

STUDY DESIGN

Magnetic resonance images (MRIs) were used to determine the geometry of the tissues studied.

OBJECTIVE

To investigate the geometric parameters of tissues around the lumbosacral joint.

SUMMARY OF BACKGROUND DATA

Cross-sectional area (CSA), moment arms, and line of action of the tissues located at lumbosacral joint (L5-S1), which are important in the study of spinal mechanics and models for the investigation of low back pain. It was insufficient information to describe the relative dimension of spinal tissues except muscles around the lumbar spine.

METHODS

MRIs from eight asymptomatic young Taiwanese male adults were collected to present CSA, moment arms, and line of action of the tissues located at lumbosacral joint (L5-S1). Four pairs of trunk muscles, erector spinae, rectus abdominis, abdominal oblique externus, and psoas muscles, ligamentum flavum, and facet joints at the bilateral sides were studied.

RESULTS

Spinal tissues at the level of lumbosacral joint were determined. The CSAs of the spinal elements at the L5-S1 normalized by the CSA of trunk ranged from 5.42% (the erector spinae) to 0.14% (the ligamentum flavum). The moment arm of the spinal elements relative to the trunk width and depth ranged from 40.91% (rectus abdominis at the y direction) to 0.38% (ligamentum flavum at the x direction). The profile of geometric elements of lumbosacral joint in the Asian male subjects was similar to the data collected from the white population. The data also showed that right-left symmetry in the aforementioned dimension.

CONCLUSION

Normalized data of the CSA, moment arm, and line of action of the spinal tissues at the L5-S1 joint were reported in vivo through the MRI techniques. The profile of geometric elements of lumbosacral joint in the Asian male subjects did not differ from the white population. In addition to muscles, the geometry of facet joint and ligament was determined, which would be important to the calculation of force distribution on the lumbosacral joint.

Geometric and mechanical properties of human cervical spine ligaments

This study characterized the geometry and mechanical properties of the cervical ligaments from C2-T1 levels. The lengths and cross-sectional areas of the anterior longitudinal ligament, posterior longitudinal ligament, joint capsules, ligamentum flavum, and interspinous ligament were determined from eight human cadavers using cryomicrotomy images. The geometry was defined based on spinal anatomy and its potential use in complex mathematical models. The biomechanical force-deflection, stiffness, energy, stress, and strain data were obtained from 25 cadavers using in situ axial tensile tests. Data were grouped into middle (C2-C5) and lower (C5-T1) cervical levels. Both the geometric length and area of cross section, and the biomechanical properties including the stiffness, stress, strain, energy, and Young's modulus, were presented for each of the five ligaments. In both groups, joint capsules and ligamentum flavum exhibited the highest cross-sectional area (p < 0.005), while the longitudinal ligaments had the highest length measurements. Although not reaching statistical significance, for all ligaments, cross-sectional areas were higher in the C5-T1 than in the C2-C5 group; and lengths were higher in the C2-C5 than in the C5-T1 group with the exception of the flavum (Table 1 in the main text). Force-deflection characteristics (plots) are provided for all ligaments in both groups. Failure strains were higher for the ligaments of the posterior (interspinous ligament, joint capsules, and ligamentum flavum) than the anterior complex (anterior and posterior longitudinal ligaments) in both groups. In contrast, the failure stress and Young's modulus were higher for the anterior and posterior longitudinal ligaments compared to the ligaments of the posterior complex in the two groups. However, similar tendencies in the structural responses (stiffness, energy) were not found in both groups. Researchers attempting to incorporate these data into stress-analysis models can choose the specific parameter(s) based on the complexity of the model used to study the biomechanical behavior of the human cervical spine.

Effect of low back posture on the morphology of the spinal canal

OBJECTIVE

To define the possible mechanism of posture-dependent symptoms of spinal stenosis by measuring the effect of low back posture on morphologic changes of the intervertebral discs and spinal canal in healthy young people.

DESIGN

Twenty healthy young volunteers underwent magnetic resonance imaging while supine with their spine in neutral, flexed, extended, and right and left rotational positions. The axial MR images at the middle of the intervertebral discs of L3-4 and L4-5 were analyzed to measure the difference in the size and shape of the intervertebral discs and spinal canal in each posture.

RESULTS

Extension or rotation decreased the sagittal diameters and cross-sectional areas of the dural sac and spinal canal and increased the thickness of the ligamentum flavum, whereas flexion had the opposite effects. The gap between the convex posterior disc margin and the anterior margin of the facet joint on each side, represented as the subarticular sagittal diameter, increased with flexion and decreased with extension or rotation. The direction of rotation did not result in asymmetry of the subarticular sagittal diameter, but right rotation caused thickening of the right ligamentum flavum, and vice versa. The shape and dimensions of the disc did not change significantly according to the positions of the low back.

CONCLUSIONS

With extension or rotation, the thickness of the ligamentum flavum increased and the posterior margin of the intervertebral disc was approximated to the facet joint without any change in shape and size of the disc. These phenomena result in a decrease in the size of the spinal canal and dural sac in extension or rotation postures in young healthy people without disc degeneration, and may explain the posture-dependent symptom of spinal stenosis.

Schemes for the identification of tissue types and boundaries at the tool point for surgical needles

Precise control of automated invasive surgical tools requires real-time identification of tissue types and their deformation. At the focus of this paper is the epidural puncture, for which it is shown that the tissue type and deformation can respectively be determined from laser-based spectroscopy and the change in force required to push the needle through the various tissues. Studies have shown that physiological variations from one patient to another are too great to allow absolute values to be reliably used to indicate the position of the needle tip. However, the pattern of force variation during penetration is shown to be similar between specimens. Interpretation of this information in conjunction with spectroscopic techniques can be used to discriminate between tissues and tissue structure at the needle tip. This paper describes results from an investigation on automatic techniques for interpreting the type and deformation of tissues under tool action.

Morphologic and histologic study of the ligamentum flavum in the thoraco-lumbar region

The ligamentum flavum, of which there are only a few studies in the literature, has several features discussed in this work. On the macroscopic level, it has a metameric arrangement: it has two layers, superficial and deep, whose fibers are opposite, and it has close relations with the tendons of attachment of some spinal erector muscles. On the microscopic level, its structure is unique for a ligament, because of a predominance of elastic fibers, because of its intrinsic innervation at each level of the spine, and because this innervation grows poorer with increasing degeneration. The ligamentum flavum constitutes an active ligament, with an essential biomechanical role. Its injury is probably not without consequences, and therefore there are many technical problems about the surgical interlaminar approach.

Morphologic and histologic study of the ligamentum flavum in the thoraco-lumbar region

The ligamentum flavum, of which there are only a few studies in the literature, has several features discussed in this work. On the macroscopic level, it has a metameric arrangement: it has two layers, superficial and deep, whose fibers are opposite, and it has close relations with the tendons of attachment of some spinal erector muscles. On the microscopic level, its structure is unique for a ligament, because of a predominance of elastic fibers, because of its intrinsic innervation at each level of the spine, and because this innervation grows poorer with increasing degeneration. The ligamentum flavum constitutes an active ligament, with an essential biomechanical role. Its injury is probably not without consequences, and therefore there are many technical problems about the surgical interlaminar approach.

Expression and localization of bone morphogenetic proteins (BMPs) and BMP receptors in ossification of the ligamentum flavum

To clarify the pathogenesis of ossification of the ligamentum flavum (OLF), we examined the expression and localization of bone morphogenetic proteins (BMPs) and their receptors (BMPRs) in the ligamentum flavum of the patients with OLF by immunohistochemical staining and compared them with staining patterns in control patients. The BMPRs appeared extensively in mature and immature chondrocytes around the calcified zone and in spindle-shaped cells and round cells in the remote part from ossified foci in examined tissue of OLF. The ligands for BMPRs, BMP-2/-4 and osteogenic protein-1 (OP-1)/BMP-7, colocalized in OLF patients. In the control cases, expression of BMPs and BMPRs was observed around the calcified zone at the insertion of the ligamentum flavum to the bone, and limited expression was found in the smaller range. Thus, the expression profile of BMPs and BMPRs in OLF patients was entirely different from the control patients, suggesting that BMPs may be involved in promoting endochondral ossification at ectopic ossification sites in OLF, and that ossification activity is continuous in these patients.

Form and function of the musculoskeletal system as revealed by mathematical analysis of the lumbar spine. An essay

Wolff's law describes the mechanical ultrastructure of tissue and indicates a design for minimal stress or minimal energy expenditure. Formation of this theory for the mechanism of the back reveals the critical design of members of this musculoskeletal system, which is comparable with other anthropoids. It also reveals that the pre-adaptations that enable the individual to lift heavy weights are the same adaptations that allow him or her to walk and run.

Distraction and compression loads enhance spine torsional stiffness

We have assessed the degree to which compression and distraction forces applied to the spine of the magnitude achievable through instrumentation systems increase torsional stiffness, thereby possibly enhancing the environment for arthrodesis. A functional relationship between spine torsional stiffness and axial compression and distraction forces is described. To establish this relationship whole human thoracolumbar spines as well as individual motion segments were tested in torsion under a variety of axial loading conditions consistent with forces applied clinically. These studies indicate that applying axial loads to the whole thoracolumbar spine through the action of compression or distraction increases the stiffness of the spine in torsion. Compression and distraction forces increased the torsional stiffness of thoracic segments, but only compression forces were found to significantly increase the stiffness of lumbar segments. The soft tissue structures of both the anterior and the posterior columns have the ability to increase torsional stiffness with axial loading.

Kinematics of the cervical spine canal: changes with sagittal plane loads

Spondylotic myelopathy is a result of decreased spinal canal space due to degeneration. The space also may change with physiological movements. The knowledge of the normal physiological changes is necessary for a better understanding of the clinical symptoms. Using a novel technique, we measured the changes in disk bulge, ligamentum flavum bulge, and anteroposterior canal diameter in response to tension-compression forces (up to 40 N each) and combined loading: 2 Nm of flexion or extension moment combined with 20 N compression force in five human cadaveric lower cervical spine specimens (C4-C7). From tension to compression, the average disk bulge changed 1.13 mm or 10.1% of the original canal diameter. The ligamentum flavum bulge changed 0.73 mm or 6.5% of the canal diameter. From flexion to extension the average disk bulb changed 1.16 mm or 10.8% of the canal diameter, whereas the ligamentum flavum bulge changed 2.68 mm or 24.3% of the canal diameter. Most of the changes in the bulges occurred with a small load application around the neutral position of the spine. The results of this study demonstrate that ligamentum flavum bulge can contribute significantly to canal encroachment in extension and that a flexed posture increases the sagittal diameter of the spinal canal.

Time course in thoracic epidural pressure measurement

The purpose of this study was to measure thoracic epidural pressure at the time of insertion of an epidural needle. The pressure was measured using a closed pressure measurement system after insertion of a Tuohy needle at the T7-8 intervertebral level. This system is unique because it has a pressure transducer equipped with a device which regulates flow through the transducer and prevents the heparinized saline in the system from being evacuated into the epidural space due to sudden pressure change. As reported previously, high negative epidural pressures, up to -60 mmHg, were observed at the moment of epidural puncture. However, the pressure became positive in about 30 sec in 12 of the 13 patients, and stabilized at +3.7 +/- 3.2 mmHg (mean +/- SD) within 90 sec. Similar results were observed when the procedure was repeated within a few minutes to the same patients. This suggests that negative epidural pressures at the moment of epidural puncture are artifacts induced by tenting of the dural membrane. Subsequent adaptation of the surrounding tissue results in restoration of the normal positive epidural pressure.