Axial Skeleton1
Introduction
Axial skeleton
•Cranium
•Vertebral column
•Ribs
•Sternum
Appendicular skeleton
•Clavicle
•Scapula
•Pelvis
Osteology
CRANIUM
•The cranium is the bony encasement of the brain, which protects the brain and sensory organs(eyes, ears, nose, and vestibular system) and provides a means for ingesting food and liquid.
Osteology
The occipital bone forms much of the posterior base of the skull.
The foramen magnum is a large circular hole located at the base of the occipital bone, serving as the passageway for the spinal cord.
Each of the two temporal bones forms part of the lateral external surface of the skull, immediately surrounding and including the external auditory meatus.
Vertebra
Ribs
Sternum
Vertebral column
“trunk”
general term that describes the body of a person, including the sterunm, ribs, and pelvis but excluding the head, neck, and
limbs.
Vertebral(spinal) column describes the entire set of vertebrae, excluding the ribs, sternum, and pelvis.
The terms superior and inferior are used interchangeably with the terms cranial and caudal, respectively.
The vertebral column usually consists of 33 vertebral segments, divided into five regions.
Normally there are seven cervical, twelve thoracic, five lumbar, five sacral, and four coccygeal segments.
Normal curvatures within the vertebral column
Line-of-gravity
Ideal posture
•allows gravity to produce a torque that helps maintain the optimal shape of each spinal curvature.
Ligamentous support of the vertebral column
Spinal ligaments limit motion
Help maintain natural spinal curvatures
Stabilizing the spine
Protect the delicate spinal cord & spinal nerve roots.
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Flavum ligamentum
The ligamentum flavum originates on the anterior surface of one lamina and inserts on the posterior surface of the lamina below.
Situated immediately posterior to the spinal cord.
Histologically, the ligamentum flavum consists of about 80%
elastin and 20% collagen.
The elasticity may prevent the ligament from buckling inward during full extension.
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Stress-strain relationship of the ligamentum flavum
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ligaments that stabilize the vertebral column
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ligaments that stabilize the vertebral column
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Axial Skeleton2
Regional Osteologic features
“function follows structure”
Cervical region
•The cervical vertebrae are the smallest and most mobile of all movable vertebrae.
•The vertebral artery ascends through these foramina, coursing toward the foramen magnum to transport blood to the brain and spinal cord.
•In the neck, the vertebral artery is located immediately anterior to the exiting spinal nerve roots.
Regional Osteologic features
Thoracic region
•Each head of ribs 2 to 10 forms a costovertebral joint by
articulating at the junction of the T1-2 through T9-10 vertebral bodies.
Lumbar region
•Lumbar vertebrae have massive wide bodies, suitable for
supporting the entire superimposed weight of the head, trunk, and arms.
•The total mass of five lumbar vertebrae is approximately twice that of the seven cervical vertebrae.
•The L5-S1 apophyseal joints provide an important source of anterior-posterior stability to the lumbosacral junction.
Regional Osteologic features
Sacrum
•The sacrum is a triangular bone with its base facing superiorly and apex inferiorly.
•An important function of the sacrum is to transmit the weight of the vertebral column to the pelvis.
•The triangular sacral canal houses and protects the cauda equina.
Coccys
•The coccyx is a small triangular bone consisting of four fused vertebrae.
•The base of the coccyx joints the apex of the sacrum at the sacrococcygeal joint.
Cervical region
The cervical vertebrae are the smallest and most mobile of all movable vertebrae.
“Transverse foramina”
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Cervical region
Typical Cervical Vertebrae (C3 to C6)
“Uncovertebral joint”
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Cervical region
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Cervical region
“pillar”
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Cervical region
Atlas(C1)
•The primary function of the atlas is to support the head.
•Possessing no body, pedicle, lamina, or spinous process, the atlas is essentially two large lateral masses joined by anterior and posterior arches.
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Cervical region
•The large and concave superior articular facets of the atlas generally face cranially, in a position to accept the large, convex occipital
condyles.
•The inferior articular facets are generally flat to slightly concave.
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Cervical region
Axis(C2)
•The axis has a large, tall body that serves as a base for the upwardly projecting dens.
•The dens provides a rigid vertical axis of rotation for the atlas and head.
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Cervical region
Vertebra Prominens(C7)
•C7 is the largest of all cervical vertebrae, having many characteristics of thoracic vertebrae.
•C7 also has a large spinous process, characteristic of other thoracic vertebrae.
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Thoracic region
•Pedicles are directed posteriorly from the body, making the vertebral canal narrower than in the cervical region.
•The superior and inferior articular facets in the thoracic region are oriented vertically with a slight forward pitch.
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Thoracic region
•T1 has a full costal facet superiorly that accepts the entire head of the first rib, and a demifacet inferiorly that accepts part of the head of the second rib.
•The spinous process of T1 is especially elongated and often as prominent as the spinous process of C7.
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Thoracic region
•Although variable, the bodies of T10 through T12 may have a single, full costal facet for articulation with the heads of the tenth, eleventh, and twelfth ribs, respectively.
•T10 to T12 usually lack costotransverse joints.
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Lumbar region
•Lumbar vertebrae have massive wide bodies, suitable for supporting the entire superimposed weight of the head, trunk, and arms.
•The total mass of the five lumbar vertebrae is approximately twice that of all seven cervical vertebrae.
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Lumbar region
•Spinous processes are broad and rectangular, projecting horizontally from the junction of each lamina.
•The inferior articular facets of L5 articulate with the superior articular facets of the sacrum.
•The L5-S1 apophyseal joints provide an important source of anterior- posterior stability to the lumbosacral junction.
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Sacrum
•The sacrum is a triangular bone with its base facing superiorly and apex inferiorly.
•The anterior(pelvic) surface of the sacrum is smooth and concave, forming part of the posterior wall of the pelvic cavity.
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Sacrum
•The dorsal surface of the sacrum is convex and rough due to the attachments of muscle and ligaments.
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Coccyx
•The coccyx is a small triangular bone consisting of four fused vertebrae.
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Arthrology
The typical intervertebral junction has three functional components.
1.Transverse & spinous processes provide mechanical outriggers, or levers, that increase the mechanical leverage of muscles and
ligaments.
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Arthrology
2.Apophyseal joints are primarily responsible for guiding intervertebral motion.
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Arthrology
3.Interbody joints connect an intervertebral disc with a pair of vertebral bodies.
The primary function of these joints is to absorb and distribute loads across the vertbral column.
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Osteokinematics
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Structure of the apophyseal joints
The vertebral column contains twenty-four pairs of apophyseal joints.
Mechanically classified as plane joints, apophyseal joints are lined with articular cartilage and enclosed by a synovial-lined, well
innervated capsule.
The word apophysis means bony “outgrowth”
Function of the apophyseal joints
In general, the near-vertically oriented apophyseal joints within the lower thoracic, lumbar, and lumbosacral regions block excessive anterior translation of one vertebra on another.
Functionjally this is important because excessive anterior
translation significantly compromiises the volume of the vertebral canal-the space occupied by the spinal cord or passing spinal nerve roots.
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Structure and function of the interbody joints
The interbody joint is formed by the connections between intervertebral discs, vertebral endplates, and adjacent
vertebral bodies.
Anatomically, this joint complex is classified as an amphiarthrosis.
Nucleus pulposus and annulus fibrosus
The intervertebral disc consists of a central nucleus pulposus surrounded by an annulus fibrosus.
The nucleus pulposus is a pulplike gel located in the mid-to- posterior part of the disc.
“modified hydraulic shock absorber”
The annulus fibrosus in the lumbar discs consists of 15 to 20 concentric layers, or rings, of collagen fibers.
The intervertebral discs add considerable stability to the vertebral column.
Vertebral endplate
The vertebral endplates are thin caps of hyaline and
fibrocartilage located on the superior and inferior surfaces of each vertebral body.
