Block 2 Back Anatomy Notes
Back Anatomy and Surface Landmarks
Visualizing the back clinically: you can palpate or observe externally to assess the posterior aspect of the body.
Key surface landmarks: vertebral column (spinous processes in the midline), posterior muscles, bony protrusions that may serve as tendon anchoring points.
Focus of lecture: posterior (back) region; notes also touch on structures that extend into the pelvic girdle, but primary focus is the back.
Vertebral Column: Structure, Regions, and Variation
Total vertebrae: normally 33 to 34 vertebrae; intervertebral discs lie between most adjacent vertebrae and act as shock absorbers and facilitate motion while preventing bone-on-bone friction. The discs have a thick outer fibrocartilaginous layer (annulus fibrosus) and a gel-like center (nucleus pulposus).
Variation: coccyx (tailbone) can vary among individuals.
Regions (from top to bottom):
Cervical region: 7 vertebrae (C1–C7)
Thoracic region: 12 vertebrae (T1–T12); ribs attach here
Lumbar region: 5 vertebrae (L1–L5)
Sacrum: 5 fused vertebrae (sacral region) – sacral fusion with the coccyx
Coccyx: 3–5 fused vertebrae (tail end)
Spinal curvatures (curvatures that counterbalance): two primary curvatures present at birth (thoracic and sacral) and two secondary curvatures develop after birth (cervical and lumbar), fully developed with walking.
Definitions:
Primary curvatures: thoracic and sacral regions.
Secondary curvatures: cervical and lumbar regions (develop as infants begin to hold up the head and then to walk).
Clinical note on variations in development and pathology:
Scoliosis is a lateral bend in the thoracic region; can be C-shaped or progress to an S-shaped curvature involving lumbar and sometimes sacral regions.
Kyphosis: exaggerated thoracic (hunchback) or general forward rounding; common in elderly due to weakened core muscles and/or reduced bone density (e.g., osteoporosis).
Lordosis: exaggerated lumbar curvature; can be congenital or developed through activity (e.g., dancers, gymnasts).
Emphasis: most variations in the spine are not pathological except scoliosis; the others may be age-related or activity-related adaptations.
Detailed Vertebral Anatomy by Region
Shared characteristics across unfused vertebrae (cervical, thoracic, lumbar) and fused regions (sacrum, coccyx):
Each vertebra has a body (anterior) and posterior elements, plus several processes for articulation.
Superior and inferior articular processes with facets form facet joints (zygapophysial joints). Facet orientation changes with region and function.
Pedicle connects the vertebral body to the posterior elements.
Transverse process projects laterally; in cervical vertebrae, the transverse process includes a transverse foramen (contains the vertebral artery in cervical region).
Spinous process projects posteriorly; cervical spinous processes are typically bifid (split).
Vertebral foramen encases the spinal cord; the foramen is largest in the cervical region and becomes smaller as you descend.
Cervical vertebrae specifics (C3–C7 typical):
Body is relatively wide; transverse process contains the transverse foramen through which the vertebral artery passes (a branch of the subclavian artery).
Spinous processes are bifurcated; allows greater neck mobility and space when bending backward.
Vertebral arteries ascend through foramina in the transverse processes and enter the skull via the foramen magnum to form the basilar artery, contributing to the posterior circulation (Circle of Willis).
Superior articular facets are oriented posteriorly and superiorly.
C1 (atlas) and C2 (axis) are highly specialized:
C1 (atlas): ring-like; no body or spinous process; articulates with occipital condyles; contains large superior articular facets and lateral masses; anterior arch with a small, recessed area; posterior tubercle instead of a spinous process.
C2 (axis): contains the dens (odontoid process) projecting anteriorly; body is reduced; body articulates with C1 via the facets; has a bifid spinous process.
The atlas–axis complex forms the atlantoaxial joint (crucial pivot for head rotation) and the atlanto-occipital joint (head-on-body articulation).
C1–C2 anatomy is clinically significant because a Hangman fracture can involve the odontoid process (dens) and disrupt the pivot mechanism, potentially compressing the spinal cord posteriorly if displaced.
Transitional vertebrae: C7 is the last cervical vertebra and shows transitional features (elongated transverse processes; spinous process not bifurcated) between cervical and thoracic regions; prominent spinous process palpated as the vertebral prominence (C7).
Thoracic vertebrae specifics (T1–T12):
Bodies are heart-shaped; vertebral foramen smaller than in cervical; transverse processes have facets for rib articulation (costal facets) and typically two costovertebral articulations:
a demi-facet on the body for rib head articulation with the superior and inferior vertebral bodies (for ribs 2–9) – this is the “demi-facet” concept.
a facet on the transverse process for rib tubercle articulation (costotransverse facet).
Ribs 2–9 articulate with two vertebral bodies via demi-facets and with the corresponding transverse process via the costotransverse joint; ribs attach anteriorly to the sternum via costal cartilage.
Spinous processes slope downward and are not bifurcated.
Movement: thoracic vertebrae primarily allow rotation; flexion/extension is more limited due to rib attachments and facet orientation.
Lumbar vertebrae specifics (L1–L5):
Large, robust bodies to bear most of the upper body weight; vertebral foramen is triangular and relatively small.
Spinous processes are short and thick; facet joints oriented to allow flexion/extension more than rotation.
Movement: predominately flexion and extension; rotation is limited compared to cervical region.
Sacrum and coccyx specifics:
Sacrum: five fused vertebrae; forms the posterior wall of the pelvis; foramina in sacrum (anterior and posterior) allow passage of nerves and vessels; sacral canal continues the vertebral canal; sacral hiatus is the opening at the lower end where the canal ends and attachments to coccyx begin.
Ala (wing-like extensions) are formed by fused ribs; sacral crest forms on the dorsal surface; sacral foramina permit nerve passage.
Coccyx: 3–5 fused vertebrae; tailbone; attaches to sacrum.
Important landmarks and terminology:
Vertebral promontory: the anterior projecting edge of the sacrum; used in obstetrics for pelvic measurements and center of gravity references.
Center of gravity varies with pelvic tilt; females generally have a more anterior pelvic tilt due to anatomical differences in the pelvis, affecting balance and center of gravity, not purely due to adiposity.
The vertebral column is designed to balance flexibility and stability through regional adaptations.
Intervertebral Discs: Structure, Function, and Pathology
Composition and function:
Outer ring: annulus fibrosus (fibrocartilage); inner gel-like nucleus pulposus.
Discs account for roughly 20% of total vertebral column length.
In daily life, discs compress during the day and decompress slightly during sleep; thus people are tallest in the morning.
Biomechanics:
The annulus fibrosus resists tension; the nucleus pulposus provides compressive resistance and redistributes load.
Clinical relevance of discs:
Because humans are bipedal, the posterior-lateral regions of the annulus fibrosus are relatively thinner, making them more prone to herniation under stress.
Herniation mechanism follows the path of least resistance; posterior-lateral herniations are most common due to the thin posterior-lateral annulus.
Disc herniation progression (stages) and neural involvement:
Prolapse (bulge): initial abnormal displacement of nucleus pulposus through a weakened annulus fibrosus; can pinch nearby nerve roots.
Extrusion: nucleus pulposus breaks through the annulus fibrosus and may extend into the spinal canal.
Sequestration: detached nucleus material enters surrounding tissues; often requires surgical intervention.
Symptoms and escalation: disc herniation can cause radicular pain via nerve root compression; extruded material may provoke inflammatory nerve root irritation and rootlet ingrowth, increasing pain.
Notable anatomy for herniation risk:
The nucleus pulposus tends to protrude into the posterolateral region of the disc, where the annulus fibrosus is thinned.
The disc centers interact with adjacent vertebral bodies and the spinal nerve roots as they exit via the intervertebral foramina.
Ligaments, Joints, and Functional Connections of the Spine
Ligaments supporting the spine (high-level):
Anterior longitudinal ligament: runs along the anterior surface of the vertebral bodies from cervical to sacral regions; broad and strong.
Posterior longitudinal ligament: runs within the vertebral canal along the posterior aspect of the vertebral bodies; more narrow and interrupted by spinous processes.
Ligamentum flavum: connects adjacent laminae with elastic fibers; lies near the posterior aspect of the vertebral canal.
Interspinous ligaments: connect adjacent spinous processes.
Supraspinous ligament: runs along the tips of the spinous processes to protect the posterior surface and limit excessive flexion.
Key stabilizing joints and relationships:
Zygapophysial joints (facet joints): articulations between superior and inferior articular processes; important for guiding motion and stability.
Uncovertebral joints (uncinate joints): common in the cervical region; joints formed between uncinate processes and adjacent vertebral bodies; can be congenitally present but typically develop around age ~10; can contribute to pain and disc herniation risk.
Costovertebral joints: articulation between ribs and vertebral bodies (centrum) and intervertebral discs at vertebral levels two through nine (demifacets).
Costotransverse joints: articulation between the rib tubercle and the transverse process.
Special cervical ligaments: atlas–axis stabilization
Transverse ligament of the atlas: holds the dens against the anterior arch of the atlas, preventing anterior displacement of C1 on C2.
Apical ligament of the dens and alar ligaments: help secure the dens and stabilize C1–Skull articulation to the occiput and C2.
Spinal vasculature in the suboccipital region:
In the suboccipital triangle, contents include the suboccipital nerve (posterior ramus of C1), the vertebral artery (third part passes through this region), and the vertebral venous plexus.
Compression in this area can cause vertebrobasilar syndrome due to compromised blood flow to the brain.
Clinical note: vertebral artery anatomy and potential injury can occur in the suboccipital region; awareness of the third part of the vertebral artery passing through the triangle is important clinically.
The Spinal Cord, Nerves, and Meninges
Spinal cord and caudal structures:
The adult spinal cord ends at approximately the level of the conus medullaris around L1–L2; in younger individuals or those with less postural support, the cord may extend a bit lower (to L2–L3).
After the cord ends, the cauda equina (a bundle of nerve roots) continues within the lumbar cistern to the sacral region.
Filum terminale: a pia mater strand that extends from the conus medullaris to attach to the coccyx, anchoring the spinal cord within the vertebral canal.
Dorsal (posterior) and ventral (anterior) roots join to form a mixed spinal nerve at each intervertebral foramen.
The dorsal root contains sensory fibers; the ventral root contains motor fibers.
The dorsal root ganglion (DRG) is an enlargement where sensory neuron bodies reside awaiting entry into the spinal cord.
Internal organization of the spinal cord:
Gray matter contains interneurons and motor neuron bodies; white matter contains myelinated ascending (sensory) and descending (motor) tracts.
Ascending fibers carry sensory information toward the brain (afferent); descending fibers carry motor commands away (efferent).
Commissural fibers cross from one side of the spinal cord to the other to enable cross-talk between sides.
Functional concepts:
Somatic sensory: body wall, skin, musculoskeletal systems; precise and well-localized.
Visceral sensory: internal organs; less precise localization.
Myotomes and dermatomes describe motor and sensory maps corresponding to spinal cord segments; lesions produce predictable deficits.
Loss of sensation is paresthesia; loss of function is paralysis.
Quick anatomical view of contents of the vertebral canal:
Spinal cord, dorsal/ventral roots, dorsal root ganglion, meninges (dura, arachnoid, pia), cerebrospinal fluid, vessels, and fat in the epidural space.
Clinical implications of nerve organization: knowledge of dermatomes and myotomes helps localize lesions in clinical neurology.
Lumbar Puncture vs Epidural Anesthesia
Lumbar puncture (LP):
Purpose: obtain cerebrospinal fluid for diagnostic testing (e.g., meningitis) or therapeutic measures.
Safe entry point: typically at the L4–L5 interspace (between the iliac crests along the intercristal line) because the spinal cord ends at L1–L2 in adults, reducing the risk of spinal cord injury.
Patient positioning: lie on the side with knees drawn to the chest (fetal position) to open the interlaminar spaces.
Procedure: insert a needle into the subarachnoid space after passing through the dura mater to collect CSF.
Epidural anesthesia: different from LP; executed in the epidural space (outside the dura mater) to anesthetize nerve roots without entering the subarachnoid space.
Fat-filled epidural space; a catheter can be threaded to deliver anesthetic to specific levels to block neural tissue in a controlled fashion.
A special type called caudal epidural anesthesia targets the sacral hiatus (at the tailbone) to block pudendal nerve innervation, often used during childbirth for perineal pain relief.
Important clinical distinction: an epidural and a lumbar puncture are not interchangeable; incorrect technique can lead to severe complications.
Clinical Conditions: Degenerative and Developmental Issues
Osteoporosis and vertebral fractures:
Osteoporosis is characterized by decreased bone density due to imbalance between osteoclast activity and osteoblast activity.
It is especially prevalent in postmenopausal women and can cause compression fractures of the spine, even with normal activities like walking downstairs.
Disc herniation and radiculopathy:
Common sources of back pain; symptoms arise from nucleus pulposus herniating through a weakened annulus fibrosus and compressing nerve roots.
Progression and nerve involvement:
Prolapse: disc bulge; nucleus pulposus pushes through the annulus fibrosus but remains within it.
Extrusion: nucleus pulposus breaks through annulus fibrosus toward or into the spinal canal.
Sequestration: nucleus material breaks free and can cause inflammatory and nerve root irritation; pain worsens due to nerve root ingrowth of nerve fibers.
Pain is typically due to nerve root irritation, with radicular pain radiating along dermatomes.
Location risk: posterolateral portion of the disc is thinner; this is the common site for herniation.
Spina bifida (developmental defect): spectrum from occult to open defects
Spina bifida occulta (least severe): failure of vertebral arches (lamina and/or spinous process) to fuse completely; often not visible externally; may be found incidentally via imaging.
Closed spinal dysraphism with lipoma: local fat accumulation associated with defective fusion of vertebral arches; may require monitoring.
Meningocele: meninges and CSF protrude through the defect, forming a cyst-like structure; no neural tissue protrudes.
Myelomeningocele: most severe open defect; protrusion includes meninges, CSF, and spinal cord/cauda equina; often associated with significant neurologic deficits.
Role of folic acid: prenatal supplementation reduces the incidence of spina bifida; public health guidance emphasizes early and ongoing folic acid intake during pregnancy.
Clinical relevance of anatomy in back pain and surgery:
Understanding the levels of the spinal cord, nerve roots, and vertebral structures helps in diagnosing and planning surgical interventions.
Knowledge of the sacral hiatus and caudal anatomy informs procedures like caudal epidurals.
Muscles of the Back: Extrinsic vs Intrinsic, and Their Roles
Extrinsic back muscles (outer layer, superficial and intermediate):
Superficial layer: trapezius and latissimus dorsi (latissimus dorsi also acts in respiration as an accessory muscle during deep breathing).
Intermediate layer: serratus posterior superior and serratus posterior inferior (their primary contribution is to respiration; they assist in elevating ribs; notable in COPD patients).
Deep layer includes: levator scapulae and rhomboids (major and minor) which help stabilize and retract the scapula; work with serratus anterior to hold the scapula against the thoracic wall.
Serratus anterior role: stabilizes the scapula; winged scapula can occur with nerve injury (serratus anterior or rhomboids).
Intrinsic back muscles (deep to extrinsic muscles):
Superficial layer: splenius capitis and splenius cervicis (capitis attaches to skull; cervicis to cervical vertebrae).
Erector spinae group (postural, primary deep back muscles): iliocostalis, longissimus, and spinalis; together these three columns are called the erector spinae and are organized from lateral to medial as: iliocostalis, longissimus, spinalis; each has regional subdivisions (e.g., iliocostalis lumborum/thoracis/cervicis; longissimus thoracis/cervicis/capitis; spinalis thoracis).
Deep, small intrinsic muscles near the vertebral column include semispinalis (thoracic, cervicis, capitis); rotatores; multifidus; interspinales; intertransversarii; these deeper muscles provide segmental stability and precise fine motor control of the spine.
The text highlights the existence of very small intrinsic muscles that lie directly on the vertebrae to support subtle movements and stability.
Suboccipital region:
Suboccipital triangle borders defined by deep muscles after removing trapezius and semispinalis capitis.
Contents include the suboccipital nerve (C1), suboccipital venous plexus, and the third part of the vertebral artery.
Clinically relevant: compression or pathology in this area can affect posterior brain circulation (vertebrobasilar system).
The Suboccipital Triangle and Vertebral Artery
Vertebral artery: path from subclavian artery up through the transverse foramina of cervical vertebrae, enters skull through foramen magnum, converges with the contralateral artery to form the basilar artery, contributing to posterior circulation.
Clinical note: in the suboccipital triangle, the third part of the vertebral artery passes; compression can cause transient vertebrobasilar symptoms (e.g., dizziness, stars) if blood flow to the brain is briefly compromised.
The Spinal Cord, Cauda Equina, and Nerve Roots: Functional and Clinical Implications
Cauda equina: a bundle of nerve roots resembling a horse’s tail; extends beyond the end of the spinal cord to innervate lower limbs and pelvic organs.
Filum terminale: a pia mater filament that extends from the conus medullaris to the coccyx, anchoring the spinal cord within the vertebral canal.
Dorsal/ventral roots meet to form spinal nerves at intervertebral foramina; dorsal root ganglion houses sensory neuron cell bodies.
Nerve pathways:
Sensory (afferent) pathways ascend toward the brain via ascending fibers.
Motor (efferent) pathways descend from the brain to the periphery via descending fibers.
Internal communication within spinal cord uses commissural fibers to cross from one side to the other.
Dermatomes and myotomes:
Dermatomes describe cutaneous (skin) sensory territories supplied by a single spinal nerve.
Myotomes describe muscular innervation by a single spinal nerve.
Clinical relevance:
Lesions at different spinal levels produce predictable deficits in sensation and motor function (paresthesia vs paralysis) according to dermatomal/myotomal maps.
The Clinically Important Region: Spinal Tap, Epidural, and Childbirth Considerations
Lumbar puncture details reiterated:
Level: L4–L5 interspace preferred for safety; patient in lateral decubitus with knees and chin drawn to chest; the iliac crests and intercristal line help identify the level.
Stylet insertion through the skin, ligaments, dura to access cerebrospinal fluid in the subarachnoid space.
Epidural anesthesia specifics reiterated:
Epidural space contains fat; needle/stylet insertion into this space allows catheter placement to deliver anesthetic with diffusion to neural tissue at targeted levels.
Caudal epidural anesthesia via the sacral hiatus (tailbone area) blocks pudendal nerve for pain control during childbirth; important in managing somatic (perineal) pain.
Spina Bifida: From Occulta to Myelomeningocele
Spina bifida occurrence and spectrum:
Occulta: least severe; lamina/spinous process fusion incomplete but externally not evident; may be discovered incidentally later in life.
Closed spinal dysraphism with lipoma: lipomatous tissue in the spinal canal due to closed fusion defects.
Meningocele: meninges and CSF protrude through a vertebral defect; neural tissue remains inside.
Myelomeningocele: protrusion includes meninges, CSF, and spinal cord/cauda equina; open spinal defect.
Prenatal prevention: folic acid supplementation reduces incidence; important counseling for prospective parents.
Management varies by case; surgical options depend on involvement and severity; prenatal and postnatal care are tailored to the individual.
Quick Reference to Key Concepts and Terms
Key regions and numbers:
Cervical: 7 vertebrae; C1 (atlas) and C2 (axis) are specialized; typical cervical vertebrae: C3–C7.
Thoracic: 12 vertebrae; costal facets for ribs; heart-shaped bodies; limited flexion/extension; rotation predominant.
Lumbar: 5 vertebrae; large bodies; flexion/extension emphasis.
Sacrum: 5 fused vertebrae; sacral foramina; sacral canal; sacral promontory.
Coccyx: 3–5 fused vertebrae.
Angles and landmarks:
Vertebral prominens: C7 vertebra; palpable in the neck.
Intercristal line: anatomical line used to identify L4–L5 level for LP; lies between the iliac crests.
Spinal levels for procedures:
Lumbar puncture: usually L4–L5 interspace.
Epidural anesthesia: can also use L4–L5 interspace or sacral hiatus for caudal epidural.
Functional anatomy reminders:
The spinal cord ends around L1–L2 in adults; cauda equina continues to descend.
The pia mater extends as filum terminale to anchor to the coccyx.
The dorsal root ganglion is the sensory neuron hub within the dorsal root before joining with the ventral root to form the spinal nerve.
Practical considerations:
Center of gravity and pelvic tilt affect balance and biomechanics; differences between genetic males and females relate to pelvic orientation, not adiposity.
The spine’s ligaments, discs, and nerves work together to provide stability, flexibility, shock absorption, and protection of neural structures; degeneration or injury can disrupt this balance.