One of the most important problems encountered when administering epidural anesthesia is failure to identify the epidural space. In the lumbar region, this occurs more often when the median rather than the paramedian approach is used in conjunction with the loss-of-resistance method (1). This technique relies on the resistance to injection of saline offered by the ligamentum flavum, which abruptly decreases once the tip of the needle has reached the epidural space. This vital structure, however, may not fuse in the midline. The exact morphology of the ligamentum flavum at different vertebral levels remains controversial (2,3). Therefore, the aim of the present study was to directly investigate lumbar ligamentum flavum midline gaps in embalmed cadavers.
Vertebral column specimens were obtained from 45 human cadavers in legal property of the Institute of Anatomy, Histology, & Embryology, University of Innsbruck, Austria. Cadavers were preserved in a mixture of formaldehyde and carbol (4). Vertebral arches were detached at the pedicles of L1-S1 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 gently probed using a blunt needle with a diameter of 1 mm. The determination of midline gaps was performed by two investigators (PL and JC) blinded to each other’s results.
Descriptive statistics were used for analysis. Interobserver reliability was analyzed using intraclass correlation coefficient (ICC). Scores for statistical measurements with the ICC ranged from 0 to 1, where the former shows no reliability and the latter perfect reliability.
The tissues within the spinal canal were easily removed, with only minimal adherence between adipose tissue and the inner surface of the ligamentum flavum. The ligamentum flavum appeared as a rigid but pliable structure, easily distinguishable by its yellowish color and smooth surface. Gaps because of lack of fusion in the midline were readily apparent by gentle probing. A number of specimens were damaged during harvesting from the cadavers or during dissection and were excluded from analysis. Of the 45 harvested lumbar vertebral columns, the numbers of intact specimens allowing for clear interpretation of ligamentum flavum anatomy at different levels were as follows: L1-2 = 45, L2-3 = 44, L3-4 = 45, L4-5 = 43, and L5/S1 = 33.
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 versus number of viable specimens at the following levels was: L1-2 = 10 of 45 (22.2%), L2-3 = 5 of 44 (11.4%), L3-4 = 5 of 45 (11.1%), L4-5 = 4 of 43 (9.3%), and L5/S1 = 0 of 33 (0%). ICC between the two investigators was 1.
The present study investigated the incidence of lumbar ligamentum flavum midline gaps. The main result is that, depending on the vertebral level, up to 22% of lumbar ligamenta flava are discontinuous in the midline. The incidence of gaps was most frequent at L1-2 and was infrequent at L2-3 and below.
The ligamentum flavum is composed of elastic fibers. It inserts on the inferior and antero-inferior aspects of the respective cranial vertebral arch and on the superior edge and postero-superior surface of the respective caudal lamina (5). Intervals for the passage of vessels have been described (6). At each level, the ligamentum flavum embryologically consists of a left and right lateral portion. These lateral parts of the ligamentum flavum may fuse in the midline, although the exact incidence at the lumbar levels has not been agreed upon (2,3). The ligamenta are thickest at the lumbar levels (6).
Zarzur (2) and Olszewski et al. (5) dissected 10 and 6 lumbar vertebral columns, respectively, and found no evidence of ligamentum flavum midline gaps. In contrast, a cryomicrotome investigation on 38 cadavers found that ligamentum flavum midline fusion could be absent “to a variable degree,” although no exact incidences were given (7). Furthermore, it was noted that even in any given specimen, some levels could feature a midline gap, whereas at other levels, the ligamenta were fused in the midline (7). Results from our investigation support and substantially enhance the latter findings in as much as the exact incidence of midline gaps was described for the first time.
This is important because the ligamentum flavum is a crucial structure in epidural anesthesia, which has been described as vital in the elicitation of a “loss-of-resistance (2).” The possible impact of a midline gap has been noted (7) and is, in principle, twofold; the superficial ligaments are too frail to offer resistance to the injection of saline. Therefore, if the needle is introduced through a gap, no resistance will be initially identified. Furthermore, a midline gap may attenuate the resistance to the advancement of the needle itself. Actual loss-of-resistance has, in these instances, been thought to be caused by the supra- and interspinous ligaments (3). However, in contrast to the ligamentum flavum, these ligaments are composed of collagenous fibers. Therefore, the distinct elastic resistance offered by the ligamentum flavum before entering the epidural space when using the loss-of-resistance technique should be expected to be attenuated. Using the median approach, the midline gaps described in the present study may account for many failures to recognize the epidural space in daily routine.
This is in agreement with a previous study reporting that the identification of the lumbar epidural space was impaired when using the median rather than the paramedian loss-of-resistance approach (1). However, these authors did not comment upon possible mechanisms responsible for the latter finding. In principle, a midline gap could impede the loss-of-resistance encountered using the median approach, whereas the paramedian approach can, in all instances, rely upon the penetration of the ligamentum flavum. Therefore, the anatomic variations encountered in the present study may offer an explanation for this discrepancy.
Finally, some potential limitations of this study should be briefly addressed. We chose direct dissection of embalmed specimens to investigate ligamentum flavum anatomy. Therefore, we cannot exclude potential artifacts resulting from the embalming or dissection processes. However, this is unlikely to be significant because we performed dissections with great care to avoid any damage to the ligamentum flavum. Moreover, when damage was noted, the specimen was excluded from evaluation.
In conclusion, in the present study, we determined the frequency of lumbar ligamentum flavum midline gaps. Gaps in the lumbar ligamentum flavum are most frequent between L1 and L2, but are more rare below this level. Using the midline approach, one cannot therefore rely on the ligamentum flavum to impede entering the epidural space in all patients.
We acknowledge the valuable help of Professor Quinn Hogan, MD, Department of Anesthesiology, Medical College of Wisconsin, in the preparation of the manuscript.
1. Blomberg RG, Jaanivald A, Walther S. Advantages of the paramedian approach for lumbar epidural analgesia with catheter technique: a clinical comparison between midline and paramedian approaches. Anaesthesia 1989; 44: 742–6.
2. Zarzur E. Anatomic studies of the human ligamentum flavum. Anesth Analg 1984; 63: 499–502.
3. Hogan QH. Epidural anatomy examined by cryomicrotome section: influence of age, vertebral level, and disease. Reg Anesth 1996; 21: 395–406.
4. Platzer W, Putz R, Poisel S. New system for the preservation and storage of anatomical matter. Acta Anat (Basel) 1978; 102: 60–7.
5. Olszewski AD, Yaszemski MJ, White AA3rd. The anatomy of the human lumbar ligamentum flavum: new observations and their surgical importance. Spine 1996; 21: 2307–12.
6. Gray H. Gray’s anatomy. 20th ed. New York: Bartleby, 2000.
7. Hogan QH. Lumbar epidural anatomy: a new look by cryomicrotome section. Anesthesiology 1991; 75: 767–75.