View source for Intervertebral disc

{{Short description|Fibrocartilaginous structure of spine}}
{{Infobox anatomy
| Name        = Intervertebral disc
| Latin       = discus intervertebralis
| Image       = 716 Intervertebral Disk.svg
| Caption     = Intervertebral disc
| Width       = 
| Image2      = 
| Caption2    = 
| Synonyms    = Spinal disc, intervertebral fibrocartilage, intervertebral cartilage, vertebral disc
| Precursor   = 
| Function = [[Fibrocartilaginous]] [[joint]] between [[spinal vertebrae]]
| System      = [[Musculoskeletal system]]
| PartOf      = [[Vertebral column]]
| Artery      = 
| Vein        = 
| Nerve       = 
| Lymph       = 
}}

An '''intervertebral disc''' ([[British English]]), also spelled '''intervertebral disk''' ([[American English]]), lies between adjacent [[vertebra]]e in the [[vertebral column]]. Each disc forms a [[fibrocartilaginous]] [[joint]] (a [[symphysis]]), to allow slight movement of the vertebrae, to act as a [[ligament]] to hold the vertebrae together, and to function as a shock absorber for the spine.

==Structure==
[[Image:Cervical vertebra english.png|thumb|250px|{{center|Cervical vertebra with intervertebral disc }}]]
Intervertebral discs consist of an outer fibrous ring, the ''anulus (or annulus) fibrosus disci intervertebralis'', which surrounds an inner gel-like center, the ''nucleus pulposus''.<ref>{{Cite journal|last1=Urban|first1=Jill PG|last2=Roberts|first2=Sally|date=2003-03-11|title=Degeneration of the intervertebral disc|journal=Arthritis Res Ther|volume=5|issue=3|pages=120–30|doi=10.1186/ar629|pmc=165040|issn=1478-6354|pmid=12723977 |doi-access=free }}</ref> 

The ''anulus fibrosus'' consists of several layers (laminae) of [[fibrocartilage]] made up of both [[type I collagen|type I]] and [[type II collagen]]. Type I is concentrated toward the edge of the ring, where it provides greater strength. The stiff laminae can withstand compressive forces. 

The fibrous intervertebral disc contains the ''nucleus pulposus'' and this helps to distribute pressure evenly across the disc. This prevents the development of stress concentrations which could cause damage to the underlying vertebrae or to their [[endplates]]. The nucleus pulposus contains loose fibers suspended in a mucoprotein gel. The nucleus of the disc acts as a shock absorber, absorbing the impact of the body's activities and keeping the two vertebrae separated. It is the remnant of the [[notochord]].<ref>{{cite journal|last=McCann|first=Matthew |author2=Owen J. Tamplin |author3=Janet Rossant |author4=Cheryle A. Séguin|title=Tracing notochord-derived cells using a Noto-cre mouse: implications for intervertebral disc development|journal=Disease Models & Mechanisms|volume=5 |pages=73–82 |date=25 October 2011|issue=1 |doi=10.1242/dmm.008128|pmc=3255545 |pmid=22028328}}</ref>

There is one disc between each pair of vertebrae, except for the first cervical segment, the ''[[Atlas (anatomy)|atlas]]''. The ''atlas'' is a ring around the roughly cone-shaped extension of the ''[[Axis (anatomy)|axis]]'' (second cervical segment). The axis acts as a post around which the atlas can rotate, allowing the neck to swivel. There are 23 discs in the human spine: 6 in the neck ([[Cervical vertebrae|cervical]]) region, 12 in the middle back ([[thoracic]]) region, and 5 in the lower back ([[lumbar]]) region.<ref>{{Cite web | url=https://www.physio-pedia.com/Intervertebral_disc | title=Intervertebral disc|website=Physio-pedia.com}}</ref> 

Discs are named by the vertebral body above and below. For example, the disc between the fifth and sixth cervical vertebrae is designated "C5-6".<ref>{{cite journal |last1=Fardon |first1=David F. |last2=Williams |first2=Alan L. |last3=Dohring |first3=Edward J. |last4=Murtagh |first4=F. Reed |last5=Gabriel Rothman |first5=Stephen L. |last6=Sze |first6=Gordon K. |title=Lumbar Disc Nomenclature: Version 2.0 |journal=The Spine Journal |date=November 2014 |volume=14 |issue=11 |pages=2538–2539 |doi=10.1016/j.spinee.2014.04.022|pmid=24768732 |doi-access=free }}</ref>

===Development===
During development and at birth, vertebral discs have some vascular supply to the cartilage endplates and the anulus fibrosus. These quickly deteriorate leaving almost no direct blood supply in healthy adults.<ref name=Roberts06>{{cite journal |vauthors=Roberts S, Evans H, Trivedi J, Menage J |title=Histology and pathology of the human intervertebral disc |journal=J Bone Joint Surg Am |volume=88 |issue=Suppl 2 |pages=10–4 |date=April 2006 |pmid=16595436 |doi=10.2106/JBJS.F.00019}}</ref>

===Intervertebral disc space===
The ''intervertebral disc space'' is typically defined on an [[X-ray]] photograph as the space between adjacent [[vertebrae]].  In healthy patients, this corresponds to the size of the intervertebral disc. The size of the space can be altered in pathological conditions such as [[discitis]] (infection of the intervertebral disc).{{medcn|date=February 2018}}

==Function==
The intervertebral disc functions to separate the vertebrae from each other and provides the surface for the shock-absorbing gel of the nucleus pulposus. The nucleus pulposus of the disc functions to distribute hydraulic pressure in all directions within each intervertebral disc under compressive loads. The nucleus pulposus consists of large [[Vacuole|vacuolated]] notochord cells, small chondrocyte-like cells, [[collagen]] fibrils, and [[aggrecan]], a [[proteoglycan]] that aggregates by binding to [[hyaluronan]].  Attached to each aggrecan molecule are [[glycosaminoglycan]] (GAG) chains of [[chondroitin sulfate]] and [[keratan sulfate]].  Increasing the amount of negatively charged aggrecan increases [[oncotic pressure]], resulting in a shift of extracellular fluid from the outside to the inside of the nucleus pulposus. The amount of glycosaminoglycans (and hence water) decreases with age and degeneration.<ref>{{Cite journal | last1 = Antoniou | first1 = J. | last2 = Steffen | first2 = T. | last3 = Nelson | first3 = F. | last4 = Winterbottom | first4 = N. | last5 = Hollander | first5 = A. P. | last6 = Poole | first6 = R. A. | last7 = Aebi | first7 = M. | last8 = Alini | first8 = M. | doi = 10.1172/JCI118884 | title = The human lumbar intervertebral disc: Evidence for changes in the biosynthesis and denaturation of the extracellular matrix with growth, maturation, ageing, and degeneration | journal = Journal of Clinical Investigation | volume = 98 | issue = 4 | pages = 996–1003 | year = 1996 | pmid = 8770872 | pmc =507515 }}</ref>

==Clinical significance==
Anything arising from the intervertebral disc may be termed ''discogenic'' in particular when referring to associated pain as ''discogenic pain''.<ref name="Yang">{{cite journal |last1=Yang |first1=G |last2=Liao |first2=W |last3=Shen |first3=M |last4=Mei |first4=H |title=Insight into neural mechanisms underlying discogenic back pain. |journal=The Journal of International Medical Research |date=November 2018 |volume=46 |issue=11 |pages=4427–4436 |doi=10.1177/0300060518799902 |pmid=30270809|pmc=6259376 |doi-access=free }}</ref>

===Herniation===
[[File:Disc Herniation.JPG|thumb|Stages of spinal disc herniation]]
{{main|Spinal disc herniation}}
A spinal disc herniation, commonly referred to as a slipped disc, can happen when unbalanced mechanical pressures substantially deform the anulus fibrosus, allowing part of the nucleus to obtrude. These events can occur during peak physical performance, during traumas, or as a result of chronic deterioration (typically accompanied with poor posture), and has been associated with a ''[[Cutibacterium acnes|Propionibacterium acnes]]'' infection.<ref>{{Cite journal|last1=Capoor|first1=Manu N.|last2=Ruzicka|first2=Filip|last3=Schmitz|first3=Jonathan E.|last4=James|first4=Garth A.|last5=Machackova|first5=Tana|last6=Jancalek|first6=Radim|last7=Smrcka|first7=Martin|last8=Lipina|first8=Radim|last9=Ahmed|first9=Fahad S.|date=2017-04-03|title=Propionibacterium acnes biofilm is present in intervertebral discs of patients undergoing microdiscectomy|journal=PLOS ONE|volume=12|issue=4|article-number=e0174518|doi=10.1371/journal.pone.0174518|issn=1932-6203|pmc=5378350|pmid=28369127|bibcode=2017PLoSO..1274518C|doi-access=free}}</ref> Both the deformed anulus and the gel-like material of the nucleus pulposus can be forced laterally or posteriorly, distorting local muscle function and putting pressure on the nearby nerve. This can give symptoms typical of nerve root entrapment, which can vary between [[paresthesia]], numbness, chronic and/or acute pain, either locally or along the [[Dermatome (anatomy)|dermatome]] served by the entrapped nerve, loss of [[muscle tone]] and decreased homeostatic performance. The disc is not physically slipped; it bulges, usually in just one direction.

Another kind of herniation, of the nucleus pulposus, can happen as a result of the formation of [[Schmorl's nodes]] on the intervertebral disc.<ref name="Williams">{{cite journal |doi=10.1002/art.22789 |pmid= 17530687 |title=Schmorl's nodes: Common, highly heritable, and related to lumbar disc disease |journal=Arthritis & Rheumatism |volume=57 |issue=5 |pages=855–60 |year=2007 |last1=Williams |first1=F. M. K. |last2=Manek |first2=N. J. |last3=Sambrook |first3=P. N. |last4=Spector |first4=T. D. |last5=MacGregor |first5=A. J.|doi-access=free }}</ref> This is referred to as ''vertical disc herniation''.

===Degeneration===
[[Image:Degenerative disc disease - high mag.jpg|thumb|right|[[Micrograph]] showing a fragment of an intervertebral disc demonstrating degenerative fibrocartilage with loss of nuclei and nests of chondrocytes, as seen in [[degenerative disc disease]]. [[HPS stain]].]]
{{Main|Degenerative disc disease}}
Before age 40, approximately 25% of people show evidence of disc degeneration at one or more levels. Beyond age 40, more than 60% of people show evidence of disc degeneration at one or more levels on [[magnetic resonance imaging]] (MRI).<ref>{{cite web |title=Intervertebral Disc Disorders |date=1 December 2012 |work=MDGuidelines |publisher=Reed Group |url=http://www.mdguidelines.com/intervertebral-disc-disorders |access-date=1 December 2012 |archive-date=21 October 2012 |archive-url=https://web.archive.org/web/20121021124049/http://www.mdguidelines.com/intervertebral-disc-disorders }}</ref> These degenerative changes are a normal part of the ageing process and do not correlate to pain.<ref>{{Cite journal | doi=10.1056/NEJM199407143310201| title=Magnetic Resonance Imaging of the Lumbar Spine in People without Back Pain| journal=New England Journal of Medicine| volume=331| issue=2| pages=69–73| year=1994| last1=Jensen| first1=Maureen C.| last2=Brant-Zawadzki| first2=Michael N.| last3=Obuchowski| first3=Nancy|author3-link=Nancy Obuchowski| last4=Modic| first4=Michael T.| last5=Malkasian| first5=Dennis| last6=Ross| first6=Jeffrey S.| pmid=8208267| s2cid=33787081| url=http://pdfs.semanticscholar.org/8bf0/ab199d651b8517e562d80efefbc0f6a617d5.pdf| archive-url=https://web.archive.org/web/20200613145604/http://pdfs.semanticscholar.org/8bf0/ab199d651b8517e562d80efefbc0f6a617d5.pdf| archive-date=2020-06-13}}</ref>
[[File:3D Medical Animation scoliosis Intervertibral Disc.jpg|alt=3D Medical Animation still shot of Scoliosis|thumb|258x258px|3D medical animation still shot of scoliosis]]
One effect of aging and disc degeneration is that the nucleus pulposus begins to dehydrate and the concentration of proteoglycans in the matrix decreases, thus limiting the ability of the disc to absorb shock. This general shrinking of disc size is partially responsible for the common decrease in height as humans age.  The anulus fibrosus also becomes weaker with age and has an increased risk of tearing.  In addition, the cartilage endplates begin thinning, fissures begin to form, and there is [[sclerosis (medicine)|sclerosis]] of the [[subchondral bone]].<ref name=Roberts06/> Since the fissures are formed in the anulus fibrosus due to osteo-arthritic bones or degeneration in general, the inner nucleus pulposus can seep out and put pressure on any number of vertebral nerves.<ref name="McGraw-Hill Connect">{{Cite web|title = McGraw-Hill Connect|url = http://connect.mheducation.com/connect/hmEBook.do?setTab=sectionTabs|website = connect.mheducation.com|access-date = 2015-11-29}}</ref> A herniated disc can cause mild to severe pain such as [[sciatica]] and treatment for herniated discs range from physical therapy to surgery.<ref>{{cite web|url=https://www.spine.org/Portals/0/assets/downloads/ResearchClinicalCare/Guidelines/LowBackPain.pdf|title=Diagnosis & Treatment of Low Back Pain|website=Spine.org|access-date=30 June 2022}}</ref> (see also: [[Intervertebral disc arthroplasty]]) Other degeneration of the vertebral column includes [[diffuse idiopathic skeletal hyperostosis ]] (DISH)<ref>{{Cite journal|title = Diffuse idiopathic skeletal hyperostosis|journal = American Journal of Roentgenology|pages = 588–589|volume = 130|issue = 3|doi = 10.2214/ajr.130.3.588|first = D|last = Resnick|year = 1978|pmid = 415581|url = http://www.ajronline.org/doi/pdf/10.2214/ajr.130.3.588|url-access = subscription}}</ref> which is the calcification or ossification of the ligaments surrounding the vertebrae. This degeneration causes stiffness and sometimes even curvature in the lumbar and thoraco-lumbar spinal region. Burgeoning evidence suggests that long-term running may mitigate age-related degeneration within lumbar intervertebral discs.<ref>{{cite journal |last1=Mitchell |first1=Ulrike H. |last2=Bowden |first2=Jennifer A. |last3=Larson |first3=Robert E. |last4=Belavy |first4=Daniel L. |last5=Owen |first5=Patrick J. |title=Long-term running in middle-aged men and intervertebral disc health, a cross-sectional pilot study |journal=PLOS ONE |date=21 February 2020 |volume=15 |issue=2 |article-number=e0229457 |doi=10.1371/journal.pone.0229457|pmid=32084224 |pmc=7034897 |bibcode=2020PLoSO..1529457M |doi-access=free }}</ref>

===Scoliosis===
{{Main|Scoliosis}}
While this may not cause pain in some people, in others it may cause chronic pain.  Other spinal disorders can affect the morphology of intervertebral discs.  For example, patients with [[scoliosis]] commonly have calcium deposits (ectopic calcification) in the cartilage endplate and sometimes in the disc itself.<ref>{{cite journal |author=Giachelli CM |title=Ectopic calcification: gathering hard facts about soft tissue mineralization |journal=Am. J. Pathol. |volume=154 |issue=3 |pages=671–5 |date=March 1999 |pmid=10079244 |pmc=1866412 |doi=10.1016/S0002-9440(10)65313-8 }}</ref> Herniated discs are also found to have a higher degree of [[cellular senescence]] than non-herniated discs.<ref name=Roberts06/> In addition to scoliosis, which is the lateral 'S' curvature of the spine, the fused vertebrae can also experience other abnormalities such as [[kyphosis]]<ref name="McGraw-Hill Connect"/> (hunchback) which shows in old age, or [[lordosis]]<ref name="McGraw-Hill Connect"/> (swayback), which is often present in pregnancy and obesity.

== Etymology ==
The Latin word ''anulus'' means "little ring"; it is the diminutive of ''anus'' ("ring"). However, modern English also spells the word more phonetically ''annulus'', as with the term [[annular eclipse]], where the moon blocks the sun except for a bright ring around it.<ref>Federative Committee on Anatomical Terminology (FCAT) (1998). ''Terminologia Anatomica''. Stuttgart: Thieme</ref>

==Additional images==
<gallery>
File:Annulus Fibrosus.png|Anulus fibrosus
File:Vertebre-IMG 0032.jpg|Artificial intervertebral disc
File:Gray302.png|Posterior longitudinal ligament, in the thoracic region
File:Gray308.png|Median sagittal section through the occipital bone and first three cervical vertebrae
File:Gray312.png|Costovertebral articulations. Anterior view.
File:Slide1gt.JPG|Lumbar and sacral plexus. Deep dissection. Anterior view.
File:Slide2gt.JPG|Lumbar and sacral plexus. Deep dissection. Anterior view.
File:Invertebral disc polarised light microscopy image by Claudio Vergari (27177975104).jpg|Polarised light microscopy image of the annulus fibrosus, showing the concentric layers of fibrous tissue
</gallery>

== See also ==
{{Anatomy-terms}}
* [[Back pain]]
* [[Degenerative disc disease]]
* [[Spinal disc herniation]]
* [[Spinal decompression]]
* [[Lumbar spinal stenosis]]
* [[List of distinct cell types in the adult human body]]
* [[Spelling of disc]]

==References==
{{Reflist}}

==External links==
{{Commons category|Intervertebral discs}}
*[https://www.spineuniverse.com/anatomy/intervertebral-discs Intervertebral Discs]
*[https://web.archive.org/web/20050923144151/http://my.webmd.com/hw/health_guide_atoz/sts15130.asp Spinal Disc Summary]
* {{ViennaCrossSection|pembody/body12a}}

{{Joints of torso}}
{{Dorsopathies}}
{{Authority control}}
{{Use dmy dates|date=April 2017}}

[[Category:Skeletal system]]
[[Category:Soft tissue]]
[[Category:Back anatomy]]
[[Category:Bones of the vertebral column]]