Back and Vertebrae

Back:

The posterior aspect of the trunk is known as back. it is inferior to the neck and superior to buttocks.

Back includes:

1. Skin and subcutaneous tissue
2. Muscles
3. Vertebral column
4. Ribs
5. Spinal cord
6. Various segmental nerves and vessels

Vertebral Column:

Consists of vertebrae + intervertebral discs.

 It extends from the cranium to the apex of the coccyx.

Functions:

1. Protecting spinal cord 
2. Support body weight superior to the level of pelvis.
3. A partly rigid and flexible axis for body
4. Helps maintain posture and locomotion

Vertebrae:

A human body has 33 vertebrae. Of these
7 are cervical vertebrae, 12 thoracic vertebrae, 5 lumbar vertebrae, 5 sacral vertebrae that fuse to form the sacrum and 4 coccygeal vertebrae that fuse to form the coccyx.

The lumbosacral angle is at the junction of the long axes of lumbar region and the sacrum.

The vertebrae become larger as they descend to sacrum and from there they decrease in size upto coccyx.

Remember that the column is flexible because of the presence of many vertebrae.

The superior 25 vertebrae also articulate at the synovial zygophyseal joints, these give the column the flexibility.

Note: remember that significant motion omly takes place between the superior 25 vertebrae.

Structure and Function

Typical:

1. Vertebral body
2. Vertebral arch
3. 7 processes
4. Vertebral Notches

Vertebral Body:

Its the massive anterior part of the bone, gives strength to the column and supports weight. Size increases as column descends. markly from T4 below.

It consists of vascular trabecular bone enclosed by compact bone. The trabecular bone contains red bone marrow, actively hematopoitic. Superior and inferiro surfaces covered by hyaline cartilage. The periphery has an epiphyseal rim (ring of smooth bone).

Vertebral Arch:

It is present posterior to the body. Contains 2 pedicles and laminae. Pedicles project posteriorly and meet the laminae, these unite in the midline.

Walls of the vertebral foramen formed by vertebral arch and posterior surface of vertebral body.

Vertebral Processes:

The spinous process and 2 transverse processes provide attachment to the deep back muscles.

The 4 articular processes kepp the vertebrae aligned.

Vertebral Notches:

Indentations present superior and inferior to the pedicles. seen between the superior and inferior articular processes and the body. These notches and the connecting IV disc from IV foramen.

Note: L5 bears weight even when posture is erect (the inferior articular processes).

Regional Characteristics:

Regional variations in the size and shape of the vertebral canal accomodates the varying thickness of spinal cord.

Cervical Vertebrae:

There are 7 cervical vertebrae  These are the smallest, they form the skeleton of the neck between the cranium and the thoracic vertebrae. Cervical vertebrae bear less weight. in this region there is greatest range and variety of movement (due to disc thickness, horizontal orientation of the articular facets, small amount of surrounding body mass). Foramen transversarium is present in the transverse processes. These carry vertebral arteries and veins. Transverse processes end in two projections: anterior and posterior tubercles. The groove for spinal nerves between tubercles contain cervical spinal nerves. The anterior tubercle of vertebrae C6 is known as carotid tubercle. 

Carotid tubercle: the common carotid arteries are compressed here in the groove between tubercles and the body to control the bleeding from these vessels.

Vertebrae C3-C7 are TYPICAL:

• Larger vertebral foramina.
• Uncus: the elevated superiolateral margin of the body
• horizontal Articular facets
• C3-C6 have short spinous processes which are bifid.
• C7 has a long spinous process (vertebrae prominens)

C1 and C2 are ATYPICAL:

C1 (atlas):

C1 has no body and no spinous process. it has lateral mass that bears the weight of the cranium. Transverse processes are more laterally placed. Superior articular facets articulate with occipital condyles. Anterior and posterior arches extend between the lateral masses forming the complete ring. Posterior arch has a groove for vertebral artery.

C2 (axis):

Strongest cervical vertebrae. C1 rotates on C2. Blunt tooth like dens (odontoid process), projects superiorly from the body. Dnes is the pivot about which head rotation occurs.Dens is held in position by the transverse ligament of the Atlas. C2 has a large bifid spinous process.

Thoracic Vertebrae:

There are 12 Thoracic vertebrae.These are present in the upper back and provide attachment for ribs. Have costal facets for articulation of ribs. T5 to T8 are typical. T1 to T4 have some features of cervical vertebrae. T1 has long, almost horizontal spinous process. It has a complete costal facet superiorly and a demifacet on inferior edge. T9 to T12 have some features of lumabr vertebrae. T12 has a superior half that is thoracic in character and inferior half thatis lumbar in character. 

Note: the most commonly fractured vertebrae is T12 because it is subject to transitional stresses.

Lumbar Vertebrae:

There are 5 lumbar vertebrae. these are present in the lower back. They have massive bodies. Causes flexion, extension and lateral flexion but rotation is prohibited. Accessory process on posterior surface of transverse process provides attachment for intertransversarri muscles. Mamillary processes on posterior surface of superior articular processes, these give attachment to multifidus and intertransversarri muscles. 

Note: L5 largest of all moveable vertebrae, carries weight of the whole body.

Sacrum:

5 fused sacral vertebrae. Located between the hip bones. Forms the roof and the posterosuperior wall of the  posterior half of the pelvic cavity. It provides strength and stability to the pelvis. Transmits body weight to pelvic girdle. Sacral canal contains bundle of spinal nerve roots arising inferior to the L1 vertebrae, these roots are called CAUDA EQUINA. Sacral foramena form the exit of posterior and anterior rami of spinal nerves. Anterior sacral foramen (pelvic foramen) are larger. base of sacrum formed by superior surface of S1 vertebrae. Sacral promontory is the anterior projecting edge of body of S1 vertebrae.Sacrum forms posterior part of bony pelvis. Dorsal surface of sacrum has prominent ridges:

1. Median sacral crest: fused spinous processes.
2. Intermediate sacral crest: fused articular processes.
3. Lateral sacral crest: fused transverse processes.

Coccyx:

4 coccygeal bodies fuse to form coccyx. It is a small triangular bone. Co1 may remain seperate from the fused group. Pelvic surface is concave and relatively smooth. Posterior surface has rudimentary articular processes. These processes form the coccygeal cornua which articulates with sacral cornua. When sitting coccyx flexes a little indicating that it receives some weight. It provides attachment to:

1. Gluteus maximus (part of it)
2. Coccygeus Muscle
3. Anococcygeal ligament (median fibrous band of pubococcygeal muscles)