By Lucie Bosler

To achieve as much mobility as possible without compromising the body’s structural integrity, the cervical spine relies on a careful balance between flexibility and stability. In order to achieve this balance, an incredible amount of spinal structures must work together as a team to ensure the body is functional and protected. A stable spine is necessary to not only protect the spine, but also preserve nervous system functions, such as walking (Izzo et al., 2013). Furthermore, spinal instability may be detrimental to a sufferer’s quality of life. Back issues risen from spinal instability may cause functional difficulties in daily life in addition to possible severe chronic pain.

At the top most end, the upper cervical spine is entirely stabilized by the transverse, alar, and facet capsular ligaments which each have a specific role and work in conjunction with one another (Steilen et al., 2014). The transverse ligament allows the first cervical vertebra to pivot while also holding it in a proper position, which is crucial in preventing compression of the spinal cord. Next, two alar ligaments limit or “check” movement when the head turns towards the left or right, thus preventing over-rotation and consequent destabilization (Cramer, 2014). Lastly, cervical facet capsular ligaments wrap around facet joints and provide stability during neck rotation (Steilen et al., 2014).

The stability of the spinal cord during movement, such as flexion and extension, is protected by the ligamentum flavum, anterior longitudinal ligament (ALL), and posterior longitudinal ligament (Steilen et al., 2014). The ligamentum flavum connects the vertebrae by attaching itself to the front of each spinal segment. The ALL is responsible for several important stabilizing functions. Running from the top of the cervical spine to the sacrum, it works to limit extension and stabilize both the vertebrae and intervertebral discs. The posterior longitudinal ligament helps maintain neck posture and join adjacent vertebrae (Steilen et al., 2014).

Though flexibility is generally seen as a mark of athletic achievement and health, an excess of this ability may cause significant threats to spinal cord stability. Though it is possible to acquire hypermobility through extreme training, such as ballerinas, it is most often a genetic issue wherein the material holding joints is too malleable (Grahame, 1999). In some cases, the shape of the ends of a person’s bones may cause the joint to become shallow and lead to hypermobility. However, the most common cause of hypermobility is a mutation in collagen, one of the main components of connective tissue. Ehlers-Danlos Syndrome is a disorder characterized by extreme joint flexibility and is known to cause many health issues in patients. It is caused by a mutation in collagen, which often results in detrimental joint and ligament laxity (Chiarelli et al., 2019). Collagen helps make up many of the ligaments that stabilize the cervical spinal cord and is crucial to maintain the elastic strength of the intervertebral disks (Wu et al., 2016).

Several conditions may be seen when a patient has cervical spinal cord instability due to hypermobility. Firstly, flexion may deform the columns of the cord whereas extension may compress them. In fact, those with Ehlers-Danlos Syndrome may rotate the joint between the first and second cranial vertebra up to 40°. This is significant as it is known that at 35°, there is significant kinking of the vertebral artery which vascularizes the anterior blood supply to the brain. Furthermore, at 45°, both vertebral arteries may become occluded, which can cause severe symptoms such as facial pain and respiratory issues (Henderson et al., 2017). One of the most apparent ways to visualize conditions of hypoermobility is through flexion and extension X-rays. When the cervical capsular ligament becomes excessively flexible, the cervical facet becomes mobile enough to encroach on cervical nerve roots, thus causing cervical radiculopathy. Furthermore, extension may cause buckling of the ligamentum flavum, which can result in myelopathic symptoms and muscle weakness (Steilen, et al., 2014). Unfortunately, there are no true treatments for this disorder (NIH, 2017). Instead, patients must rely on symptom management in order to maintain a quality of life and in some cases, survive the disorder (NIH, 2017).

Interestingly, cervical facet ligaments are highly innervated by nociceptive neurons inbedded in the collagen matrix that cause pain when the neck is excessively stretched. It is thought that the signaling of these nociceptive neurons are highly dependent on the mechanical properties of the collagen matrix surrounding them (Zhang et al., 2016). Consequently, changes in the collagen matrix may alter pain signaling and hinder an important preventative mechanism in spinal cord health. Embedded in the matrix are a vast amount of different proteins, channels, and molecular compounds. When people move, this matrix stretches, strains, and generally has force exerted on it. Some proteins embedded in the matrix actually respond to this mechanical movement of the matrix. However, those with Ehlers-Danlos Syndrome have collagen that is looser than that of the average person. Therefore, the mutated collagen matrix is strained or stressed differently, causing the proteins to react inappropriately in the context of the movement (Zhang et al., 2016). Some of these proteins, such as PERK, are implicated in pain and are increased as a result of the abnormal stretching and straining caused by changes in the collagen matrix (Zhang et al., 2016).

Spinal health and stability is crucial for proper nervous system function and is often necessary for a high quality of life. An extreme amount of flexibility in the joints the make up the spinal cord may be detrimental to spinal cord stability and cause a variety of painful conditions. When people are born with a genetic predisposition towards flexibility, it may be crucial for them to seek treatment through physical therapy or sometimes even surgery.

References

Chiarelli, N., Ritelli, M., Zoppi, N., & Colombi, M. (2019). Cellular and Molecular Mechanisms in the Pathogenesis of Classical, Vascular, and Hypermobile Ehlers‒Danlos Syndromes. Genes, 10(8), 609. https://doi.org/10.3390/genes10080609

Cramer, G. D. (2014). Clinical Anatomy of the Spine, Spinal Cord, and Ans (G. D. Cramer & S. A. B. T.-C. A. of the S. Darby Spinal Cord, and Ans (Third Edition), Eds.). https://doi.org/https://doi

Grahame, R. (1999). Joint hypermobility and genetic collagen disorders: are they related? Archives of Disease in Childhood, 80(2), 188 LP – 191. https://doi.org/10.1136/adc.80.2.188

Henderson Sr., F. C., Austin, C., Benzel, E., Bolognese, P., Ellenbogen, R., Francomano, C. A., … Voermans, N. C. (2017). Neurological and spinal manifestations of the Ehlers–Danlos syndromes. American Journal of Medical Genetics Part C: Seminars in Medical Genetics, 175(1), 195–211. https://doi.org/10.1002/ajmg.c.31549

Izzo, R., Guarnieri, G., Guglielmi, G., & Muto, M. (2013). Biomechanics of the spine. Part I: Spinal stability. European Journal of Radiology, 82(1), 118–126. https://doi.org/https://doi.org/10.1016/j.ejrad.2012.07.024

NIH. (2017). Ehlers-Danlos Syndromes. Retrieved from Genetic and Rare Diseases Information Center website: https://rarediseases.info.nih.gov/diseases/6322/ehlers-danlos-syndromes

Steilen, D., Hauser, R., Woldin, B., & Sawyer, S. (2014). Chronic neck pain: making the connection between capsular ligament laxity and cervical instability. The Open Orthopaedics Journal, 8, 326–345. https://doi.org/10.2174/1874325001408010326

Wu, B., Meng, C., Wang, H., Jia, C., & Zhao, Y. (2016). Changes of proteoglycan and collagen II of the adjacent intervertebral disc in the cervical instability models. Biomedicine & Pharmacotherapy, 84, 754–758. https://doi.org/https://doi.org/10.1016/j.biopha.2016.09.077

Zhang, S., Bassett, D., & Winkelstein, B. (2016). Stretch-induced network reconfiguration of collagen fibres in the human facet capsular ligament. Journal of The Royal Society Interface, 13, 20150883. https://doi.org/10.1098/rsif.2015.0883

Neuroscience Anthology

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