6.19: Osteology of Vertebrae

Thoracic Vertebrae (143)

Costal vertebrae are distinctive

Body concave above down from side to side (vertebral foramen are rounded).

-           on back of body two foramina allow basivertebral veins

-           heart shaped compact bone at margins, enclosing central cancellous bone.

Pedicle projects back from upper half of body, level with upper body

-           superior and inferior articular processes mark boundaries of intervertebral foramina

-           thoracic nerve of same number passes through here.

Laminae unit to midline completing arch

-           sup and inf articular processes arise where pedicle and laminae meet, come together like the arc of a circle, allowing rotation.

Spinous processes show successively increasing down-slope to T7 then more horizontal to T12.

-           tips of upper 4 spines lie opposite the body of lower vertebrae

-           tips of spines 5-8 lie opposite vertebrae 2 lower

-           tips of lowest opposite their own vertebrae.

Transverse processes project back and laterally, carry costal facet (concave upper six, lower flat)

-           hence in inspiration, rotation occurs at upper 6, gliding at lower facets

Curvature caused by column leaning forwards (so transverse processes tilt up)

-           hence neck of first rib slopes back and up, so can be ‘sided’ by looking at it.

First

Broad body, large facet for C1 rib, demifacet for C2 rib.

C8 nerve root exits above, T1 through its foramen.

Lower T vertebrae

T10-12 have single facets only for ribs of same number; and T11-12 have no costal facets on transverse processes.  T12 has mamillary and accessory tubercles like a lumbar vertebrae.

Lumbar vertebrae (143)

Increase in breadth from above down.

Concave like T-vertebrae but kidney shaped / flat-backed so canal is triangular rather than round.

Transverse processes variable length, 4th longest, 5th massive, short, triangular and joins body as well as pedicle and lamina.

-           are really fused ribs

-           true transverse processes ® mamillary process (convexity on back of superior facet) & ® accessory tubercle below this.

Spinous processes roughly horizontal, lower border concave (upper straight)

Articular processes: upper face medially; L5 inferior process faces straight anteriorly to meet sacrum, stopping it sliding off the S1 slope.

-           intervertebral disc strongly unites it, making a very stable weight-bearing jt, with erector spinae supporting from behind.

-           Spondylolisthesis is slipping caused by neural arch disruption.

-           sacralisation = fusion of 5th lumbar vertebrae to the sacral vertebrae.

Sacrum (145)

5 progressively smaller vertebrae, fused.  Concave to pelvis.

-           lateral articular surface (posterior) meets ilium at sacroiliac jts.

-           Tapers below this to apex.

Sacral promontory is a ridge projected anteriorly from top of broad S1

Ala (broad wings) lie lateral to this, crossed anteriorly by (medial ® lateral): sympathetic trunk, lumbosacral trunk, obturator nerve. (see partly on Pl 383)

Pelvic surface

Bodies fused, four ridges persist marking ossification lines, four anterior sacral foramina lie anterior.

-           rounded bars between foramina are costal elements

Lateral mass is formed by fusion of costal elements, deeply grooved by anterior rami of upper four sacral nerves (pass anteriorly from sacral foramina)

-           piriformis arises from three ridges separating anterior foramina and lateral mass.

Dorsal surface

Irregular and rough, lamina have fused except at distal sacral hiatus (145), closed by fibrous tissue forming superficial sacrococcygeal ligament.

-           median sacral crest is a midline ridge formed by adjacent spinous processes

-           thickened bone laterally represents fused transverse processes.

-           fused articular processes form a line of irregular tubercles to each side

-           distally the rounded sacral cornu remains to articulate with coccyx.

Sacral canal

Triangular cross section, curves with sacrum.  Meninges ® L2; filum terminale ® coccyx.

-           around dura filled with usual fat and internal vertebral venous plexus

-           posterior root ganglia are in the sacral canal

-           posterior and ventral rami emerge separately through anterior & posterior foramina.

Sex differences

Male = body wider than ala, reversed in females.

Males have gentle curve, females flat at first, then turn more prominently.

Coccyx

Fused at sacrococcygeal jt; lateral sacrococcygeal ligaments fuse sides to inferolateral angle of sacrum

-           completes the foramen for S5 nerve anteriorly.

Cervical (12-13)

Body: kidney-shaped. Concave upwards as uncus projection which meets next vertebra as a jt.

Pedicle: attached below uncus; foramina bounded by body, uncovertebral jt and intervening disc.

-           foramen surrounded by projections ant & post called tubercles (posterior = costal elements)

-           large C6 anterior tubercle is called the carotid tubercle because can press carotid against it.

-           Tubercles joined by intertubercular lamella grooved by same number nerve root

-           Vertebral artery lies in the foramen, and the posterior root ganglion lies behind it.

Laminae enclose large foramen with usual superior and inferior facets at the junction.

Spinous processes are bifid.

C7

Vertebrae prominens as is prominent; atypically long spine, not bifid, ends in rounded tubercle

-           and its foramen does not carry the vertebral artery.

Atlas and Axis

Have been described.

Atlas has an anterior tubercle for the anterior longitudinal ligament and longus colli.

-           lateral mass carries weight-bearing facets; projected into transverse process perforated by foramen; can palpate tip of this just anteroinferior to mastoid (IJV in front of it)

Axis has massive lateral masses for WBing

-           transverse process has no tubercles but rounded tip for attachment.

-           Foramen faces up and outwards so that vertebral artery can do its lateral bend.

Development

Sclerotome of mesodermal somites

-           surround notochord and neural tube in a sheath of mesoderm.

-           A series of hyaline rings form by somite fusion in the mesodermal sheath.

-           ® ossifies in 3 centres: centrum and two halves of neural arch

-           ® failure of fusion causes hemivertebra.

By week 8 ossification commences, half arches unite in year 1 post-birth, from bottom to top.

-           centra unit first in cervical region by age 3, lumbar not complete until age 6

-           ® failure of fusion causes spina bifida: occult if skin intact over the top; meningocoele if dural sac bulges out; myelomeningocoele if part of spinal cord / nerve roots bulge out.

-           Usually lumbar / lumbosacral.

Epiphyses for bodies appear after puberty; ridged & grooved with body surfaces

-           fusion occurs in early 20s

-           secondary centres in spinous and transverse processes & mamillary fuse around 20 also

Costal elements ossify by direct contact to neural arch

-           except weight bearing centres of sacrum which have their own ossification centres.

Atlas ossifies at 7th week of fetal life by centres in lateral masses, unite around posterior arch at 4th year.

-           anterior arch centre appears in 1st year, fuses at 7th year

Axis ossifies in halves from primary centres in second month; pair of centres for centrum soon fuse, ossify in 4th month.

-           dens has a pair of centres in 6th month, join before birth.

-           All 4 parts fuse within first few years, but cartilage between dens and body may remain cartilaginous until old age.

-           secondary centre for tip of dens appears at 6, unites with rest of dens by 12.

-           secondary centre for lower surface of body appears at puberty, fuses in early 20s.