1 Department of Anesthesiology and Critical Care Medicine, Innsbruck Medical University and 2 Institute of Anatomy, Histology and Embryology, Innsbruck Medical University, Anichstrasse 35, 6020 Innsbruck, Austria
* Corresponding author. E-mail: philipp.lirk{at}uibk.ac.at
Accepted for publication February 23, 2005.
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Abstract |
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Methods. Vertebral column specimens were obtained from 47 human cadavers. Ligamentum flavum midline gaps were recorded between the vertebral levels T6 and L1.
Results. The incidence of midline gaps/number of viable specimens at the following levels was: T67: 2/45 (4.4%), T78: 1/47 (2.1%), T89: 2/45 (4.4%), T910: 7/39 (17.9%), T1011: 12/34 (35.2%), T1112: 10/35 (28.5%), T12/L1: 6/38 (15.8%).
Conclusions. In the present study we have determined the frequency of lower thoracic ligamentum flavum midline gaps. Gaps are less frequent than at cervical levels, but more frequent than at lumbar levels. Peak incidence was found in the region between T10 and T12. Using a strict midline approach, one cannot therefore rely on the ligamentum flavum to impede entering the epidural space in all patients.
Keywords: anaesthetic techniques, extradural ; anatomy ; model, cadaver ; spinal cord, extradural space
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Introduction |
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Methods |
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Vertebral arches were detached at the pedicles of Th6L1 and removed en bloc. The dural sac and epidural connective tissue were removed by blunt dissection, and the ligamentum flavum was directly examined anteriorly.
At each dissected level, the ligamentum flavum was thoroughly investigated for signs of midline gaps and then probed gently using a blunt needle with a diameter of 1 mm. Two investigators (P.L. and J.C.), blinded to each other's results, recorded the frequency of midline gaps.
Statistics
Descriptive statistics were used for analysis. Inter-observer reliability was analysed using intra-class correlation coefficient (ICC). Scores for statistical measurements with the ICC range from 0 to 1, where the former shows no reliability and the latter perfect reliability.
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Results |
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The following variations were encountered: complete fusion of the ligamentum flavum in the midline, and midline gap throughout the entire height (Fig. 1). The incidence of midline gaps/number of viable specimens at the following levels was (Fig. 2): T67: 2/45 (4.4%), T78: 1/47 (2.1%), T89: 2/45 (4.4%), T910: 7/39 (17.9%), T1011: 12/34 (35.2%), T1112: 10/35 (28.5%), T12/L1: 6/38 (15.8%). ICC between the two investigators was 1.
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Discussion |
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During epidural anaesthesia, the needle traverses three main ligamentous structures, the supraspinal and intraspinous ligaments, and the ligamentum flavum. Whereas the first two are composed of collagenous fibres, the ligamentum flavum consists of 80% elastin, and its dense homogenous texture is readily appreciated as a needle passes through it. There is considerable tension in the ligamentum flavum as it spans from the upper rim of the anterior surface of the cephalad lamina of an adjacent pair of vertebrae to the posterior aspect of the lower lamina. Ligamenta flava are thinnest at cervical levels, and increase in strength towards the lumbar spine.5 In a transverse plane, it extends from the midline to the roots of the articular processes, where it reinforces the anterior capsules of the zygapophyseal joints. Imaging studies describe ligamenta as most frail in the midline,6 the most plausible reason being that the ligamentum flavum embryologically consists of a left and right lateral portion.7 These lateral parts of the ligamentum flavum may fuse in the midline, or remain separate.2 3 8 Between the left and right half of the ligamentum flavum, frequent intervals for the passage of veins connecting the posterior external vertebral venous plexus with the posterior internal vertebral venous plexus have been described.5
The incidence of ligamentum flavum midline gaps has been controversially discussed (summarized in Table 1). In general, there seems to be agreement that gaps most regularly occur in the cervical region, and decrease in frequency at high thoracic and lumbar levels.2 3 Most investigations have been carried out in the lumbar region using diverse techniques. Here, reported midline gap incidences range from never9 to always.10 11 More congruence exists for the upper thoracic region, where the incidence is roughly between one-third2 and one-half8 of cases. In the cervical region, incidences between 50 and 74% have been described. Results from this investigation support and substantially widen the latter findings, in as much as the exact incidence of midline gaps in the lower thoracic region was described for the first time. Gaps appear to be less frequent than at cervical levels, but more frequent as compared with lumbar levels.
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In principle, a midline gap could thus impede the LOR encountered using the median approach, whereas the paramedian approach should, in all instances, rely safely upon the penetration of the ligamentum flavum. Therefore, the anatomic variations encountered in the present study may offer one possible explanation for impaired epidural space detection during midline puncture.13 However, it is not known how often strict midline punctures are actually midline, rather than actually slightly paramedian. In clinical practice, cervical and high thoracic epidural punctures are classically performed using the hanging-drop technique, while the LOR is more commonly used to determine needle position in the lower thoracic and lumbar region. The reasons for this can only be speculated upon. First, this obviously, at least in part, reflects the excellent reliability of cervical epidural subatmospheric pressure as an indicator of epidural needle placement, which is by far less pronounced in the lumbar region. Secondly, ligamenta flava are thinnest at cervical, and thickest at lumbar levels, increasing the resistance to needle advancement the more caudally puncture is performed.2 3 Thirdly, the findings from this and previous studies suggest that using a strict midline approach, one cannot always rely on the ligamentum flavum as a perceptible barrier to epidural needle placement, and, moreover, that this effect is more likely to occur at cervical and high thoracic than at lower thoracic and lumbar levels.2 3
Finally, some potential limitations of this study should be addressed briefly. We chose direct dissection of embalmed specimens to investigate ligamentum flavum anatomy. Therefore, we cannot exclude a potential artifact resulting from the embalming or dissection processes. This, however, is unlikely to be a significant factor as we performed dissections with great care to avoid any damage to the ligamentum flavum. Moreover, if such damage was noted, the specimen was excluded from evaluation. Furthermore, results from anatomic dissections2 3 are congruent with previous investigations on non-embalmed specimens.1 8 There are no reports that midline gaps of the ligamentum flavum are degenerative in nature, which is relevant to the present study, since the cadavers represented a distinct sub-population owing to their high average age. In previous investigations in patients suffering from, for example low back pain, no midline fusion defects were described as characteristic or etiologic for degenerative disease.10 11
In conclusion, in the present study we determined the frequency of lower thoracic ligamentum flavum midline gaps. The latter are most frequent in the region between T10 and T12. Using a strict midline approach, one cannot therefore rely on the ligamentum flavum to impede entering the epidural space in all patients.
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Footnotes |
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References |
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