Departments of Anesthesiology, and Biomedical Engineering, Veterans Affairs Medical Center/Jackson Memorial Medical Center, University of Miami, Miami, FL, USA
Accepted for publication: February 15, 2000
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Abstract |
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Br J Anaesth 2000; 85: 23841
Keywords: dural leak, dural penetration, dural tenting; epidural needle orientation; position simulation
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Introduction |
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Methods |
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A neoprene-O-ring placed on a metal platform (Fig. 1) was used to hold dura tightly, and pressure was applied to the subdural surface. Two pressures were used: 65 cm H2O (which simulates the sitting position) and 5 cm H2O (which simulates the supine or lateral position).1 3 The sitting simulation pressure was slightly greater than that demonstrated in vivo in the literature, so that we could see bulging of the dural sample. A water manometer was used to measure these pressures, which were held constant throughout each experiment.
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A materials testing machine (Instron Model 1000 Canton, MA, USA) was used to manipulate the Tuohy needle on all dural samples. This device is capable of advancing the needle at 20 mm min1, while measuring travel distance (i.e. dural tenting), resistance and the peak force required for dural penetration.
Four conditions were tested in each of the dural strips, measuring force and total travel before penetration in each of these simulations.
(1) Bevel parallel to the longitudinal fibres, supine position simulation.
(2) Bevel parallel to the longitudinal fibres, sitting position simulation.
(3) Bevel perpendicular to the longitudinal fibres, supine position simulation.
(4) Bevel perpendicular to the longitudinal fibres, sitting position simulation.
Data were analysed using the pooled t-test with P<0.05 being considered significant.
A total of 10 dural samples was examined, under the four conditions outlined above; the total number was 40 simulations. Samples with artefacts in the tracings, or accidental puncture of the dura before measurement were excluded. Work required for dural penetration in each test was calculated using the data the force needed for dural penetration, and measurement of needle travel distance (work=forcexdistance).
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Results |
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Discussion |
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The rationale for using the parallel bevel orientation to minimize post-spinal headache came from studies using spinal needles,4 where the aim is to minimize leakage of CSF and the development of post-spinal headache. In epidural anaesthesia, the aim is to avoid a dural tear and therefore prevent CSF leakage. Our data show that when the needle bevel is orientated parallel to the dural fibre direction less force is required to produce perforation. If an epidural needle is introduced into the peridural space with the longitudinal orientation of the bevel, then it must be rotated 90° before introducing the catheter. Such a manoeuvre may increase further the likelihood of tearing the dura.9 Therefore, the perpendicular bevel orientation of the Tuohy needle is possibly a technique to avoid such dural tears, and what could be a potentially severe headache from a 17-gauge needle.1
Clinical studies have failed to identify an increased risk of dural puncture with the parallel as compared with perpendicular needle bevel orientation.5 It may be that the differences in penetration between the in vitro and in vivo condition can be accounted for by the resting tension of the dura. Alternatively, the difference between the clinical situation and our findings may be produced by the experimental method. The needle was advanced on the dural sample at a speed of 20 mm min1. At this slow speed the force required to penetrate the dura may be exaggerated. In addition, the speed of advancement is constant in our experiment. The clinical situation may be marked by variations in the speed of advancement, and the accidental puncture may be related to uncontrolled rapid movement of the needle.
There was no difference in the force producing dural penetrations within needle orientations regardless of the position simulations. We had expected the higher pressure in the sitting position to decrease the force required to puncture the dura. The dura was observed to bulge when pressure was added. Loose lateral fixation of the dura to the bony canal may exaggerate this change in vivo, thus the lack of position effect may be related to the experimental method.
The peak travel data suggest that the mechanisms of dural puncture may differ from the presently accepted theory. In our model, when the Tuohy needle was advanced through the centre of the dural sample, it could be displaced 0.35 cm before penetration. Dura is contained within the vertebral canal. The antero-posterior (A-P) diameter is 1.3 cm, whereas the widest transverse diameter between the inter vertebral foramina is 2.2 cm.10 At the L2 level, the inter vertebral foramina are posterior to a frontal plane passing through the midpoint of the A-P diameter. This means that the dura is not tethered at the posterior aspect of the vertebral canal, but at its midpoint just 0.60.7 cm from the anterior wall of the canal. If our dural samples are displaced 0.35 cm, the average lumbar dura at its widest point (2.2 cm) would probably tent more than this. By applying proportionality it may be assumed that dura can be displaced 0.77 cm in vivo. Because the dura is tethered only 0.6 mm from the anterior canal wall, it would be impaled prior to being stretched maximally. This is supported by the work of Holloway and Telford,11 who found that following identification of the epidural space, the needle was advanced 11.5 cm prior to obtaining CSF flow in 75% of patients. This would place the needle tip at the anterior limit of the vertebral canal.
In eight experiments with the needle oriented parallel to the fibres, an early leak was noted (i.e. water was noted on the epidural surface prior to actual dural penetration). This may be due to the needle separating the fibres, allowing for some fluid leakage prior to the formation of an actual tract.
In conclusion, our data indicate that more work is required to puncture the dura with the bevel oriented perpendicular to the longitudinal axis. This supports our contention that it may be safer to enter the epidural space with a perpendicularly oriented needle. An early leak phenomenon was observed with the needle oriented parallel to the fibres. The clinical significance of this observation and whether it plays a role in the development of postdural headache in the absence of an observable penetration requires further study.
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Footnotes |
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References |
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