Original Research
Variations and Implications of the Gross Anatomy in the Equine Nuchal Ligament Lamellae

https://doi.org/10.1016/j.jevs.2014.06.018Get rights and content

Highlights

  • Conventional equine anatomy knowledge has been based on outdated findings.

  • Modern equine displays anatomic variations that are unreported.

  • The equine nuchal ligament lamellae attaches from C2 to C5 and not from C2 to C7 in 35 horses.

  • Variations to equine anatomy should be reported so to better understand function.

  • As structure governs function, variations in structural integrity reflect in function.

Abstract

Early literature depicting equine anatomy began with stylized illustrations portraying anatomic structures in a congenial format. This study shows that modern day literature often reflects those early portrayals and, in particular, the description and depiction of the nuchal ligament lamellae (NLL) and its attachments from the second to seventh cervical vertebrae. In the dissection of 35 horses of no specific breed, which were unrelated and of mixed gender and age, it was noted that the NLL was not attached to the sixth and seventh cervical vertebrae. Furthermore, the attachment of the NLL on the fifth cervical vertebrae was quite thin and feeble in nine of those horses and that incomplete fibers were noted in the caudal attachments of the NLL in 15 horses. These variations do not correlate to those early depictions or to most of the existing equine anatomic text. This new information on the equine NLL has functional and clinical implications, with particular reference in the way we currently understand the postural and locomotive properties of the equine neck and cervicothoracic junction.

Introduction

Early introductions to equine anatomy notably depicted anatomic structures in a stylized format that could be easily viewed without offending one's sensibilities. Furthermore, it becomes clearly evident that artistic license was used in illustrations by equine anatomists such as Snape [1] in 1,683 (Fig. 1) and Stubbs [2] in 1,777 (Fig. 2), with the latter repeatedly mimicking the arched necked with pinned ears presentation.

The work by Stubbs [2] was regarded as being of considerable value for the equine veterinary profession and his 1,777 illustration of the nuchal ligament lamellae (NLL) in Fig. 2, portrayed the attachment points as being from C2 to C7. Since then, this representation has been duplicated in numerous equine anatomy textbooks for not only university students studying veterinary science [3], [4], [5], [6] but also the public in general [7], [8], [9], [10]. Of particular note is the textbook by Sack and Habel [5] that depicts the NLL as being attached from C2 to C7 but describes it as being attached from C2 to C5 and sometimes from C2 to C6. This same diagram was later used by Dyce et al [6] with the description replicating the diagram as being from C2 to C7. In this case, one particular diagram has three different descriptions and only one coinciding with the diagram.

The discrepancy between anatomy texts can be misleading, especially when there are so many significant variations between the standardized texts used by veterinarians, researchers, scientists, and horse people. The more commonly reported variations are C2–C7 [2], [3], [4], [6], [7], [8], [9], [10], [11], C2–C6 [5], [12], [13], [14], [15], [16], and where it may be absent in its attachment on C6 [5], [16], [17]. Ashdown and Done [17] noted that in the cadaver, the NLL has no attachment, in this specimen, to the dorsal spinous process of C6, as if to imply that this is an infrequent occurrence. However, even with the discrepancies regarding attachment points, there does seem to be a general consensus on the anatomic components of the nuchal ligament (NL) and its lamellae.

According to one scientific text [6], the NL consists of two clearly defined paired sections; the first or dorsal portion is a thick cord (funicular cord) that attaches to the external occipital protuberance and extends distally to the highest spines of the thoracic vertebrae. The second portion (lamellae) forms a sheet filling the space between the funicular cord and the cervical vertebrae; the fiber bundles are closely interwoven with those of the funicular cord, and these bundles are elastic fibers that run cranially and ventrally from the funicular cord and the spines of T2 and T3 to attach to the dorsal spines of C2–C7 as depicted in Fig. 3.

Sisson and Grossman [16] described the principal function of the NL and its two components, as a support mechanism for the head and neck. Wyche [9] concurred and further noted that the NLL is a stabilizer for the positioning of the cervical vertebrae from C2 to C7 and that this stability transfers along the spine. A study by Gellman and Bertram [15] examined the functional morphology of the equine NLL and noted that the more caudal portions from C3 to C6 have substantial tissue volume but only experience 8%–12% deformation during the range of motion as seen in normal locomotion. However, it was postulated that during locomotion, the more caudal lamellae and the more cranial funicular cord portions combined could increase the strain energy storage of the NL by as much as 20%. In addition, the caudal NLL segments may play an important role in lowering the head to the ground for grazing [15].

As it has been reported that the caudal NLL has a role in spinal stability and locomotion, this study is therefore designed to investigate the gross anatomy of the NLL with specific reference to the attachment points from C2 to C7 and correlate these findings to reported function.

Section snippets

Methods

Postmortem observations were collected from 35 horses of mixed gender with identifiable breeding aged between 2 and 28 years. All the horses were euthanized for purposes unrelated to this study in educational facilities. The integument and lateral neck muscles were carefully dissected on one side to reveal the NL funicular cord and associated lamellae within 48 hours of euthanasia. The horses were then placed in three categories correlating to the noted attachment points of the NLL C2–C7,

Results

Of the 35 horses observed, all 35 expressed attachment points of the lamellae from C2 to C5 as shown in Fig. 4. No attachment points of the lamellae were noted in the categories C2–C6 and C2–C7 (Table 1).

It was further noted that in nine horses listed in the C2–C5 category, the attachment of the NLL to C5 was thin and feeble in comparison with other horses in the same category (Fig. 5). In addition, incomplete fiber attachments of the caudal NLL were noted in 15 horses (Fig. 6).

In all 35

Discussion

A function of the NL is to passively counter the mechanical effect of gravity by supporting the head and neck, whereas epaxial cervical muscles are actively engaged [11]. In addition, the hypaxial cervical muscles, brachiocephalic and omotransversarius, rely on the traction provided by the NL (funicular cord and lamellae) as a counterbalance on the cervical vertebrae to bring the foreleg forward [9]. This reduces the muscular work that would otherwise be required to hold the head above the

Conclusions

This study showed that of the 35 horses dissected, all 35 displayed an absence of NLL attachment to C6 and C7. In addition, nine horses displayed thin and feeble attachments on C5 and 15 with incomplete fibers. As the function of the NLL is both a static and dynamic component of a horse's posture and movement, it would be relevant to report this variation in standard anatomic texts so to revaluate its functional role. Furthermore, these findings change our understanding of caudal cervical

Acknowledgments

The authors thank Catherine Walker for her photographic assistance, The Australian College of Equine Podiotherapy for the use of their facilities, and to those authors, editors, and/or publishers of those articles, journals, and books cited in this manuscript. S.M.D. and J.K. wrote and reviewed this article jointly. S.M.D. and J.K. have no conflict of interest in the preparation or presentation of this original research article.

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