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Human Lumbar Ligamentum Flavum Anatomy for Epidural Anesthesia: Reviewing a 3D MR-Based Interactive Model and Postmortem Samples

Reina, Miguel A. MD, PhD; Lirk, Philipp MD, PhD; Puigdellívol-Sánchez, Anna MD, PhD; Mavar, Marija MSc; Prats-Galino, Alberto MD, PhD

doi: 10.1213/ANE.0000000000001109
Regional Anesthesia: Technical Comunication

The ligamentum flavum (LF) forms the anatomic basis for the loss-of-resistance technique essential to the performance of epidural anesthesia. However, the LF presents considerable interindividual variability, including the possibility of midline gaps, which may influence the performance of epidural anesthesia. We devise a method to reconstruct the anatomy of the digitally LF based on magnetic resonance images to clarify the exact limits and edges of LF and its different thickness, depending on the area examined, while avoiding destructive methods, as well as the dissection processes. Anatomic cadaveric cross sections enabled us to visually check the definition of the edges along the entire LF and compare them using 3D image reconstruction methods. Reconstruction was performed in images obtained from 7 patients. Images from 1 patient were used as a basis for the 3D spinal anatomy tool. In parallel, axial cuts, 2 to 3 cm thick, were performed in lumbar spines of 4 frozen cadavers. This technique allowed us to identify the entire ligament and its exact limits, while avoiding alterations resulting from cutting processes or from preparation methods. The LF extended between the laminas of adjacent vertebrae at all vertebral levels of the patients examined, but midline gaps are regularly encountered. These anatomical variants were reproduced in a 3D portable document format. The major anatomical features of the LF were reproduced in the 3D model. Details of its structure and variations of thickness in successive sagittal and axial slides could be visualized. Gaps within LF previously studied in cadavers have been identified in our interactive 3D model, which may help to understand their nature, as well as possible implications for epidural techniques.

From the *Department of Anesthesiology, Madrid-Montepríncipe University Hospital, Madrid, Spain; School of Medicine, CEU San Pablo University, Madrid, Spain; Department of Anesthesiology, Academic Medical Center, University of Amsterdam, The Netherlands; §Laboratory of Surgical NeuroAnatomy, Human Anatomy and Embryology Unit, Faculty of Medicine, Universitat de Barcelona, Barcelona; and Antón Borja Primary Care Centre, Rubí, Barcelona, Spain.

Accepted for publication October 7, 2015.

Funding: None.

The authors declare no conflicts of interest.

Reprints will not be available from the authors.

Address correspondence to Miguel A. Reina, MD, PhD, Department of Anesthesiology, Madrid-Montepríncipe University Hospital, Valmojado, 95 1ºB 28047, Madrid, Spain. Address e-mail to miguelangel@perticone.e.telefonica.net.

The ligamentum flavum (LF) is an important structure involved in epidural anesthesia. Its identification is essential to the “loss-of-resistance” (LOR) technique, which is almost universally used to detect the epidural space.1

LOR relies on the distinctive resistance to needle advancement and fluid injection elicited by the LF. However, substantial interindividual variations of the LF have been suggested, which could influence the procedure. These include differences in shape, thickness, and lateral extension toward the articular processes.2 Midline gaps have been described, which may impede the quality of LOR.3–5

Possible variations in the measurements of LF thickness may be relevant to the performance of epidural punctures with regard to common approaches toward the interlaminar foramen. LF identification by means of sensing the resistance as the needle pierces the ligament may not be feasible if LF is extremely thin or if gaps are present. Techniques using different approaches have been compared, including dural tenting and catheter insertion,6 paresthesia,7 and vascular puncture.8 Finally, it may be relevant to determine the presence of anatomical variations in LF thickness as a preliminary step toward future studies regarding possible advantages in the midline versus paramedian approaches during the performance of epidural punctures.

We described recently a novel, interactive, 3D model that enables a complete view of the LF and its relationship to neighboring structures. This model, based on reconstruction of magnetic resonance images from patients, provides full view of LF from different perspectives and cross-sectional images in all orthogonal planes.9,10

Our aim was to devise a new method for analyzing the LF anatomy and, more specifically, for determining the LF’s shape and variability of thickness, combining the interactive 3D model obtained from patients and comparing it with information obtained from classic cadaveric investigations.

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METHODS

We studied magnetic resonance images from 7 patients and spinal cross-sections from 4 human cadavers at the Laboratory of Surgical NeuroAnatomy of the Human Anatomy and Embryology Unit (Faculty of Medicine, University of Barcelona). The study was approved by “Grupo Hospital Madrid Clinical Research Ethics Committee” (Code: 09.05.047 GHM and 10.03.433 GHM).

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Human 3D Reconstruction

Human magnetic resonance (MR) images (Philips Intera 1.5 team Software 1.1 Tesla, Amsterdam, The Netherlands) were obtained from 7 patients experiencing low back pain, with absence of morphological changes in neuroradiologic reports. MR images used in our study were obtained from Picture Archiving and Communication System of the Radiology Department with approval of “Grupo Hospital Madrid Clinical Research Ethics Committee” (Code: 10.03.433 GHM).

Patient demographics, including age, sex, height, weight, and body mass index,11 technical MR details,12 and the method for obtaining the 3D portable document format (3D-PDF) model of lumbar structures of anesthetic interest9,10 have been described previously.

In brief, axial sequence acquisitions were grouped into 2 aligned adjacent blocks of 130 mm, a caudal and a rostral MR block, extending from the lowest end of the dural sac to the lower thoracic vertebrae (T11 or T10), depending on the height of patients. MR images were acquired at 16 bits and exported in DICOM (Digital Imaging and Communication in Medicine) format. Files were analyzed using Amira v5.2 3D software (Mercury Co, Boston, MA) installed in a Dell Precision graphic station. Volume estimates of the 2 phantoms matched in 98.97% and 101.51%. The T2-weighted sequence was used for cerebrospinal fluid and nerve root volume estimations within that predelineated the volumes of interest (VOI). The T1 fast field echo sequence allowed a 3D reconstruction of the dural sac VOI and surrounding structures, including LF among others (vertebrae, intervertebral disks, and ligaments). The following steps were taken: manual delimitation of the VOI of the 3D reconstructed structures, surface generation by triangulation of VOIs (approximately 0.01 cm2 per triangle) and automatic smooth of the model, and revision of the correspondence between each model contours and its corresponding structure in the MR images. Models and MR planes (13 mm intervals for axial and coronal planes and 6.5 mm for sagittal planes) were exported to Virtual Reality Modeling Language file format. Virtual Reality Modeling Language files are imported to 3D Reviewer® (Tetra 4D, Seattle, WA) to generate a Universal 3D file format, which contains graphic components that are compatible with PDF documents. Acrobat XI Pro was used to define a title area (superior), JavaScript-controlled buttons (left), and links to predefined scenes of anesthetic interest (below). The accessory plug-in “3D-PDF converter” (Tetra 4D) allows the embedding of the Universal 3D files in a 3D control area. The 3D-PDF model is available for free download at http://diposit.ub.edu/dspace/handle/2445/44844?locale=en for use in desktop computers.

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Cadaveric Study

Axial cuts, 2 to 3 cm thick, were performed in lumbar spines of 4 frozen cadavers, previously fixed with intracarotid perfusion of 4% paraformaldehyde. The samples were studied under stereoscopic microscope Zeiss S 21 OPMI 111 (Carl Zeiss, Oberkochen, Germany).

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RESULTS

We obtained 3D reconstruction of right and left LF at L2-L3, L3-L4, and L4-L5 intervertebral segments levels, together with adjacent vertebrae and dural sac. The age range of the patient was 24 to 58 years, height 1.60 to 1.82 m, weight 61 to 95 kg, and body mass index 23 to 33.7.

The topographical relationship of LF regarding the spinous process, transverse process, articular process, and dural sac is reviewed in cadavers and patients (Figs. 1 to 6). Figures 5 and 6 belong to the same patient.

Figure 1

Figure 1

Figure 2

Figure 2

Figure 3

Figure 3

Figure 4

Figure 4

Figure 5

Figure 5

Figure 6

Figure 6

In the 3D reconstruction, we observe a right and left LF in an angle of 80% to 90% with a gap at the midline (Fig. 1, A and F). The right and the left LF have an irregular shape similar to a trapezoid. It spans the external facet of the superior border of the caudal vertebrae and the inner facet of the inferior border of the cranial vertebrae (Fig. 1, C and D).

MR images and cadaver sections show the contact of the inferior and lateral portion of LF with adjacent paravertebral muscles (Figs. 1C, 1D, and 2). In some samples, we observed a thin fat layer between paravertebral muscles and LF (Fig. 1C). The LF enclosed the dorsal zone of the spinal canal, with the medial border reaching the spinous process and the lateral border extending toward the intervertebral foramen and merging with the joint capsule of the articular facets (Figs. 1A, 1B, 1F, and 2). The inner surface of the LF was separated from the external surface of the dural sac by the epidural fat occupying the posterior epidural space (Fig. 3, A and B). At the most lateral parts of the epidural space, where there was no epidural fat, the LF directly contacted the dural sac (Figs. 2 and 3A, 3B).

Interestingly, right and left LF may show varying thickness in either cadaver sections (Figs. 2 and 3A, 3B). The 3D model allows us to compare the different thickness among axial slides of the Figure 4, A to C; and among sagittal slides of Figure 5, A to D (left LF) and Figure 6, A to D (right LF).

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DISCUSSION

The present study examines a method to better define the anatomy of the LF relevant to epidural anesthesia and to embed information on the LF in an interactive tool for 3D spinal anatomy.

The LF position as the posterior border of the epidural space is well appreciated. However, its lateral extent, its relationship with the vertebral lamina, and its varying thickness are less well understood, and those details must be considered when administering an epidural block.

Previous works developing 3D software and models13–16 and analyzing human MR images11,12 assisted us in our study. The application of those methods allowed us to produce 3D images of the LF, the spinal canal, and dural sac. We could confirm how the right and left LF contribute to the closing of the interlaminar space and of dorsal part of the spinal canal, often leaving a gap in the midline zone. Thus, any paramedian approach, even when extreme angles are needed, will involve the puncture of the LF, whereas the median approach may sometimes encounter a midline gap and, potentially, a diminished LOR.

The LF thickness may become altered under certain pathologic conditions. For example, a hypertrophic LF may contribute significantly to nerve root compression at the level of the lateral spinal recess17 because of a reduction in the diameter of the spinal canal.18 Such stenosis may significantly compress the dural sac and nerve roots, resulting in symptoms, even without a bulging annulus fibrous or herniated nucleus pulpous.18 Patients with asymmetric LF thickening showed greater LF thickness on the side with worse facet degenerative disease, as subjectively evaluated.19

The LF may undergo degeneration with age or after trauma, but no influence of sex has been described.20 Although LF thickness at all levels significantly increases with age, significant changes after 60 years of age occurred only at L3-L4. Under such circumstances, it usually increases in thickness and may calcify or become infiltrated with fat.21 Ossification of the LF is reported to occur most often in the thoracic and thoracic-lumbar regions of the spine.22 Abbas et al.23 reported that absolute and relative LF thickness are significantly greater in patients with lumbar spinal stenosis at the levels of L3-L4 and L4-L5 compared with those without spinal stenosis (P < 0.05).23

The embryonic fusion of the right and left LF in the midline leads to an apparent single ligament in each interlaminar space. Thus, during an epidural puncture, it often offers a greater resistance than the interspinous ligament traversed previously, and the resistance is also more elastic. Such finger perception of the increased and more elastic resistance would indicate that the epidural space is in immediate proximity. Some patients, however, show an altered union, and a “gap” has been described as an anatomic variant with potential clinical implications (Fig. 1, A and F; Table 1).3–5 The LF is often perforated in the midline by small blood vessels connecting the internal and external posterior vertebral venous plexus.24 Altogether, these gaps may lead to a less distinctive loss of resistance, increasing the possibility of needle malposition.

Table 1

Table 1

Finally, it must be remembered that the spine is a dynamic structure that allows movements among vertebrae. LF thickness is also modified during flexion and extension of the spine: in maximal extension, the LF can become 2 mm thicker than in flexion.25–27 This should be considered when inferring data from cadaver measures or from standard images from MR and computerized tomography taken from patients in a decubitus position, when patients will receive anesthetic epidural punctures in a flexed position.28,29

Considering the advantages offered by 3D visualization of LF and other structures through a 3D-PDF tool, which is produced by accurate processing of clinical data, it is probable that similar studies will soon expand the potential benefits of this tool for training and teaching purposes.

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CONCLUSIONS

Anatomic cadaveric cross sections enable us to check visually that the definition of the edges along the entire LF is superior when applying 3D image reconstruction methods.

LF occupies the interlaminar space but leaves a midline gap in some of the patients examined in our study. This could lead to a blunted loss of resistance providing false tactile feedback that the epidural space had been reached (premature LOR). The LF thickness varies between the different lumbar vertebral levels in our patients, and in any given ligamentum, it varies both between medial and lateral sections, as well as between the rostral and the caudal ends.

With this new method, it may become possible to obtain feasible data, supported by a large series of patients, which would provide clinically relevant information (i.e., clinical heterogeneity) and enable analysis of normal and pathologic features of the spine among different groups.

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DISCLOSURES

Name: Miguel A. Reina, MD, PhD.

Contribution: This author helped design the study, conduct the study, collect the data, analyze the data, and prepare the manuscript. This author is also the archival author.

Attestation: Miguel A. Reina approved the final manuscript.

Name: Philipp Lirk, MD, PhD.

Contribution: This author helped design the study, conduct the study, collect the data, analyze the data, and prepare the manuscript.

Attestation: Philipp Lirk approved the final manuscript.

Name: Anna Puigdellívol-Sánchez, MD, PhD.

Contribution: This author helped design the study, conduct the study, collect the data, analyze the data, and prepare the manuscript.

Attestation: Anna Puigdellívol-Sánchez approved the final manuscript.

Name: Marija Mavar, MSc.

Contribution: This author helped collect the data, analyze the data, and prepare the manuscript.

Attestation: Marija Mavar approved the final manuscript.

Name: Alberto Prats-Galino, MD, PhD.

Contribution: This author helped collect the data, analyze the data, and prepare the manuscript.

Attestation: Alberto Prats-Galino approved the final manuscript.

This manuscript was handled by: Terese T. Horlocker, MD.

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ACKNOWLEDGMENTS

The authors thank Olga Fuentes, Human Anatomy and Embryology Unit, Faculty of Medicine, University of Barcelona, for her collaboration in image reconstruction.

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