Goal of the Study?
In this comprehensive scoping review from the Oxford Journal of Pain Medication 1 the authors’ goal was to provide a systematic overview of studies that documented the topography, morphology and immunoreactivity of the neural elements within the human intervertebral disc. A scoping study differs from a systematic review in that a scoping study is broader and designed to identify knowledge gaps by scoping out a body of literature. Systematic reviews are designed to answer detailed questions about a specific topic.
Why are they doing this study?
Back pain has a lifetime relevance of 60-80% in the general population. Recent estimates of the medical costs of back pain in the US alone range between $30 and $50 billion per year. It is also estimated that more than 85% of US patients experience back pain for which an exact biological cause can not be reliably identified. This scoping review provides a comprehensive, systematic evaluation of currently available literature investigating the presence of neural elements within the human InterVertebral Disc (IVD) across genders and lifespan.
What was done?
Articles were retrieved from six primary medical online databases. A total of 9,415 articles were examined, and after removing duplicates, 5,957 citations were reviewed by two independent reviewers and screened for English, peer-reviewed, full text and use of human tissue study in IVD. This resulted in only 33 applicable research studies being selected for a full scoping review. Results from these 33 articles were tabulated and organized by characteristics of the study sample and the neural elements of the IVD. Data were further categorized by age and health status where possible.
What did they find?
Generally, studies focused on the neural elements of the IVD were limited to a relatively small sample size (median n = 16.5.) Most articles did not include information on gender. Age was usually indicated and ranged from fetal to 80 years of age, but data concerning the neural elements within the IVD were never stratified by age. This was likely due to the small sample size of the studies.
The studies suggested the IVD is modestly innervated until middle age, with the neural element predominantly localized in the outer ⅓ of the annulus fibrosus.
In the absence of pathology, this innervation is detectable during the fetal period and increases into adolescence. In adults, age-related degenerative changes likely lead to increased infiltration of the nerve fibres into the inner annulus fibrosus and nucleus pulposus of the IVD, which continue to proliferate with increasing degeneration of disease severity. However, since IVD degeneration results from the complex interplay between mechanical factors, genetic influences, and natural physiological changes associated with aging, it is not possible to associate increased innervation solely with age.
Why do these findings matter?
While the pattern of innervation within the IVD is clear, the specific topographic arrangements and functions of the neural elements in the context of back pain remain unclear. It is critical that future research undertake the study of neural elements within the IVD to discern the precise roles of neurons found in healthy and degenerative IVDs. It seems histology and immunohistochemistry is the most effective method to study these neural elements. The authors also recommend that further studies be grouped according to age, gender and degree of degeneration in order to investigate the influence of these variables on neural in-growth.
The human intervertebral disc plays a huge structural role in both mobility and stability. It undergoes intense tensile and compressive loads over the lifetime of an individual and experiences biological changes that can lead to pain and disability. Featured models include the innervation to the intervertebral disc demonstrating the anatomical topography of the sinuvertebral nerve. If education of this anatomy is important to you, explore how our dynamic disc models can deliver your message.