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The Lumbar Ligamentum Flavum Does Not Have Two Layers and Is Confluent with the Interspinous Ligament

We congratulate Iwanaga and colleagues (Iwanaga et al., 2019) including Ms. Yoshino on the recent publication of their descriptive report regarding the anatomy of the lumbar ligamentum flavum. We are particularly impressed by the preservation of anatomical relationships within the embedded tissue and the resulting histological sections. Many authors have either commented on or published images with fractures or folds affecting the quality of the sections (Okuda et al., 2004; Ramsey, 1966; Rissanen, 1960; Scapinelli et al., 2006; Yahia et al., 1989). This arises from the difficulty that this tissue presents for histological processing. To have three serial sections presented in reasonable condition is laudable and highlights the quality of data at hand. Our group is currently exploring the effect of obesity upon the interspinous ligament in a combined MRI/dissection study funded by the Australia and New Zealand College of Anesthetists. We hypothesize that islands of adipose tissue may populate an anterior midline gap in the ligament, which is associated with the ligamentum flavum divergence. The results published by Iwanaga and colleagues (Iwanaga et al., 2019) are consistent with progress we have made regarding the detailed anatomy of the lumbar interspinous ligaments. Our hypothesis is at least partially supported by the comment that “The interspinous ligament split into right and left parts at the junction with ligamentum flavum”. Although Rissanen (1960) and Heylings (1978) also made this observation, we have not noted the inclusion of this feature in an anatomical textbook before 2005 (Lawrence et al., 2017). Even today, it is not part of the standard anatomical descriptions of the interspinous ligament. As part of his study, Rissanen (1960)) examined 114 individuals less than 11 years of age. He described dense deposits of adipose tissue in this anterior split of the interspinous ligament. This would be expected to leave a legacy of adipocytes. Having shared the clinical experience of difficult epidural analgesia in severely morbidly obese individuals (Saravanakumar et al., 2006), we postulate that adipose tissue in the interspinous ligament may be an additional but unexplored factor (Hameed et al., 2012). The gross dissection portion of our study included small serial sections (<0.5 mm) through the interspinous ligament and ligamentum flavum at the level of the interspinous portion of the ligamentum flavum to measure and document the nature of these structures. At all times, these small sections from embalmed body donors remained as one unit, reinforcing the understanding of the ligamentum flavum as a single layer that is confluent with the interspinous ligament. The histological techniques used by Iwanaga and colleagues would prevent direct staining for lipids. It would offer new insights though, if the authors could comment on the nature of tissue and cells in the area of interspinous ligament splitting that was described.