1 THE DEVELOPMENT OF Disc Disease and Dynamic Stabilization Lumbar Degenerative THE VERTEBRA AND THE INTERVERTEBRAL DISC 1 Safiye CAVDAR M.D. fibrosus’. The nucleus pulposus’ plus the anulus fib- 1. Precartilage Stage (mesenchimal stage) rosus form the intervertebral disc. Some remnents of the notochord may remain within the interverteb- The vertebral column develops from the mesenchi- ral disc which can result as ‘chordoma’. This neop- mal cells that accumulated around the notochord lasm frequently occurs at the base of the skull and during the 4th week of the embryonic period. At the at lumbosacral region (1-3). end of the 4th week the mesenchimal cells that de- rive from the scleratom of the somits accumulates in 3 major regions (1-3). 1.b. Region surounding the neurol tube The mesenchimal cells at this region gives rise to 1.a. Region surounding the Notchord the vertebral archs. In the 4th week of the embryonic period the sclera- toms elgine around the notochord as paired mesenc- 1.c. Region surouunding the corpus hial cells. Each of the scleratoms cells are grouped lo- The mesenchimal cells at this region gives rise to the osely at cranial and compact at caudal levels. Some costal processes. The costal porocesses will give rise of the dense cells groups migrate cranially and form to the ribs at thoracic region (1-3). the intervertebral disc. The rest of the dens cell group together with the caudal loose scleratom group uni- 2. Cartilage Stage tes and forms the mesenchimal vertebral centrum. Each centrum is formed by two adjacent scleratom At the 6th week of the embryonal stage the mesenchi- and forms an intergemental structure. mal cells forms the central cartilage of the vertebrae. The nerves are closely related to the interverteb- At the end of this embryonic period the two centers ral disc and the segmental arteies which are closely of the centrum unites to form the cartilage centrum. localized to the side of the body (corpus) of the ver- At the same time the centrum of the arch of the ver- tebrae. At the thoracic levels the dorsal intersegmen- tebra unite with each other. The elongation of the car- tal arteries are converted to intercostal arteries. tilage centers of the arches forms the ‘spinous pro- The notochord around the developing verteb- cesses’ and ‘transvers processes’ (Figure 1). ral body degenerates and vanishes. The notochord in between the vertebrae expands and forms jelati- 3. Ossification Stage nous part of the intervertebral disc. This is called the ‘nucleus pulposus’. The nucleus pulposus is surro- The typical vertebra ossification begins at this embryo- unded by regular circular fibers called the ‘anulus nic period and continuous until 25 years of age (1-3). Safiye CAVDAR M.D. 2 two at the tip of the transverse process and two at 4. Prenatal Stage the epiphysis (anular epiphysis). The two epiphysis one is located on the superior and the other is lo- The centrum consists of two primary ossification cen- cated at the inferior surface of the body of the ver- ters (ventral and dorsal). The two ossification cen- tebrae (Figure 2). The superior and inferior surface of ters unite to form a single center. At the end of this embryonic period 3 basic ossification centers deve- the body of vertebrae is formed by anular epyp- lops. One of these is located at centrum and the ot- hysis and in between there is bony structure. The her two at the vertebral arch of each vertebra half. centrum present in the body of the vertebrae, part The ossification of the arch becomes more promi- of the vertebral arch and costal heads contains arti- nent at the 8th week. At birth each vertebra is com- cular facets. The secondary ossification centers ossi- posed of 3 parts and each portion is connected to fies by the end of the 25 years of age together with each other via cartilage (1-3) (Figure 1). the vertebrae. The ossification age can show varia- tions according to individuals. The secondary ossi- fication centers should not be mixed with persistent 5. Postnatal Stage epiphysis fractures (1-3) (Figure 1). The arch of each half of the vertebral fusses between 3-5 weeks. The fusion of the lumbar arches continu- 6. The variations in the number of Vertebrae ous until 6 years of age. First fusion takes place at the the arches of the lumbar levels and then the lamina Approximately 90% of the people contain 12 thora- Lumbar Degenerative Disc Disease and Dynamic Stabilization Lumbar Degenerative of the upper level vertebrae fuss. The union of the cic, 5 lumbar, 5 sacral and 3-4 coccygeal vertebrae. arches of the vertebrae with the centrum forms the In approximately 3% there may be 1 or 2 extra or neurocentral joint. These joints allows the enlarge- less number of vertebrae (Figure 2). The vertebral co- ment of the vertebral arches during development of lumn must evaluated as whole because less verteb- the spinal cord (medulla spinalis). Between 3-6 years rae at one segment level can be compensated with an of age the union of centrum and the vertebral arch extra vertebrae at other segment level. For example disappears. After puberty 5 ossification centers are the number of thoracic vertebrae can be 11, however defined; one at the tip of the spinous process and the number of lumbar vertebrae can be 6 (4,5) (Figure 2). Intervertebral Notochord Nucleus pulposus and disc intervertebral disc Precartilage vertebral corpus Arter Vein Anulus fibrosus Segment of scleratom a b c Figure 1: Development of vertebras and embryonel disc; a) Notochord, b) Nucleus pulposus and intervertebral disc, c) Mature intervertebral disc The Development of the Vertebra and the Intervertebral Disc 3 is located the spinal cord, meninx and the roots of Disc Disease and Dynamic Stabilization Lumbar Degenerative Wedge vertebrae Hemivertebrae the spinal nerves. Where the pedicle meets the la- mina there are 3 pairs of processes these are called ‘superior articular processes,’ inferior articular pro- cesses and ‘transverse process’. Where the two la- mina meets there is a single process which extends posteriorly called the ‘spinous processes. The supe- rior articular process extends superiorly and has an articular surface on its posterior surface. This articu- lar surface articulates with the articular surface on the inferior articular process of the vertebrae above. The inferior articular process extends inferiorly and has an articular surface on its anterior surface (5,6) (Figure 3a,b). It articulates with the articular surface on the superior articular process of the vertebrae below. The movement (facet joint or zygapohysial joint) ca- pacity of this joint is highly restricted and variati- ons according to the vertebral level. The transvers process extends horizontally and it receives the at- tachments of the muscles which produce the rota- Unilateral segmentation Bilateral segmentation tion and lateral flexion of the trunk. The transverse defect defect processes of the thoracic vertebra has and articular surface which articulates with the ribs. The trans- Figure 2: Vertebral Column Development Anomalies. verse processes are formed by the union of real transverse processes and costal elements (5,6). When we view the vertebrae from the side between the body, pedicle and the superior articular process is 7. Abnormal development of the vertebral a notch called ‘superior vertebral notch’. Similarly column (Spina Bifida Occulta) between the body, pedicle and the inferior articular process is another notch called ‘inferior vertebral In the case of spina bifida the vertebral arches do notch’ (Figure 3a,b). The inferior vertebral notch is dee- not develop and does not unite in the mid-line. This per than the superior (5,6). In an articulated vertebral congenital abnormality can occur in the form of total column the inferior vertebral notch of the vertebra absence to partial absence of the vertebral arch (2,3). above and the superior notch of the vertebrae be- The stability of the vertebral column is achieved by low forms a complete foramen called the ‘interver- the intervertebral disc, ligaments and the muscles. tebral foramen’. The roots of the spinal nerves exits During the movements of the vertebral column the from these foramens (Figure 3a,b). discs has a function of absorbing the shocks. For this reason the shape and volume of the vertebrae chan- 8. The anatomy of the lumbar vertebrae ges according to the segmental level. A typical ver- tebra anteriorly is composed of a body and posteri- The number of lumbar vertebra varies from 5 to 6. orly an arch. The projections extending posteriorly The characteristic features of the lumbar vertebra is from the body is called the ‘pedicles’. The pedicles that they are very large compared to the rest of the extending further posteriorly become flat and they vertebrae. They do not have an articular surfaces on are called ‘lamina’. The body, pedicle and the lamina their transverse processes for the articulation with encircle a foramen called the ‘vertebral foramen’. In the ribs and do not have a foramen in their trans- an articulated vertebral column the vertebral fora- vers processes. The transverse diameter of the cor- men pile one on top of the other and form a canal pus of the lumbar vertebra are larger compared to called the ‘vertebral canal’ (Figure 3a,b). Within this canal the other vertebrae. The vertebral foremen of the Safiye CAVDAR M.D. 4 Spinose process (Prosesus Spinosus) Superior Superior Articular Process Articular Vertebral (Prosesus Articularis Superior) Process Lamina Transverse Corpus (Prosesus Foramen process Articularis (Prosesus Superioris) Transverse process Transversus) (Prosesus Transversus) Pedicule Spinose process (Prosesus Spinosus) Inferior Articular Process Corpus a b (Prosesus Articularis Inferior) Figure 3: Anatomia of lumbar vertebrae.