Vertebral Column & Thoracic Cage

Learning Objectives

• Vertebral Column & Typical Vertebra Structure: Describe the general anatomical features of the vertebral column as a whole and explain the functional components of a standard typical vertebra.

• Intervertebral Discs: Describe the anatomical structure of intervertebral discs, including their composition and their structural and mechanical relationships with adjacent vertebrae.

• Regional Variations & Functional Significance: Detail the distinctive features of vertebrae across different spinal regions (cervical, thoracic, lumbar, sacral, and coccygeal) and explain the functional significance of these variations.

• Thoracic Cage & Rib Articulations: Describe the comprehensive anatomy of the sternum and ribs, including the precise articulation mechanisms between rib structures and thoracic vertebrae.

General Features of the Vertebral Column

• Composition & Region Breakdown: The vertebral column consists of a total of $26$ adult bones divided into five major regions:

  • Cervical Region: Consists of $7$ individual vertebrae ($C_1 - C_7$) located in the neck.

  • Thoracic Region: Consists of $12$ individual vertebrae ($T_1 - T_{12}$) articulating posteriorly with the ribs.

  • Lumbar Region: Consists of $5$ individual vertebrae ($L_1 - L_5$) supporting the lower back and bearing heavy body weight.

  • Sacral Region: Consists of $1$ fused bone, the sacrum, composed of $5$ fused sacral vertebrae ($S_1 - S_5$).

  • Coccygeal Region: Consists of $1$ fused bone, the coccyx (tailbone), composed of $3$ to $5$ (typically $4$) fused coccygeal vertebrae.

• Primary vs. Secondary Spinal Curvatures:

  • Primary Curvatures (Thoracic and Sacral Curvatures):

    • Present at birth during infant development.

    • Characterized by a single anteriorly concave (C-shaped) curvature along the entire vertebral column.

    • Functions to accommodate internal thoracic and pelvic organs.

  • Secondary Curvatures (Cervical and Lumbar Curvatures):

    • Develop postnatally as motor milestones are reached.

    • Characterized by an anteriorly convex curvature.

    • Cervical Curve: Develops around $3 - 4$ months of age when the infant learns to hold up its head independently.

    • Lumbar Curve: Develops around $12$ months of age when the child begins sitting upright, standing, and walking.

Abnormal Spinal Curvatures

• Kyphosis:

  • An exaggerated posterior curvature of the thoracic region of the vertebral column.

  • Commonly referred to as "hunchback" or "humpback".

  • Can result from osteoporosis, vertebral compression fractures, osteomalacia, or structural neuromuscular disorders.

• Lordosis:

  • An exaggerated anterior curvature of the lumbar region of the spine.

  • Commonly referred to as "swayback".

  • Can result from increased abdominal weight (e.g., during pregnancy or central obesity), poor posture, or muscle imbalances in the hip flexors and back extensors.

• Scoliosis:

  • An abnormal lateral (side-to-side) deviation of the vertebral column from the midline sagittal plane.

  • Often accompanied by vertebral rotation, creating an asymmetrical spinal axis.

  • Most frequently diagnosed during adolescence; can be idiopathic or developmental.

General Structure of a Typical Vertebra

• Vertebral Body (Centrum):

  • The thick, weight-bearing anterior portion of the vertebra.

  • Discs of fibrocartilage sit superiorly and inferiorly on the flattened surfaces of adjacent bodies.

• Vertebral Arch:

  • Forms the posterior boundary of the vertebral canal.

  • Composed of two Pedicles (short, thick processes projecting posteriorly from the body) and two Laminae (broad, flat plates fusing in the posterior midline).

• Vertebral Foramen:

  • The central opening framed by the posterior body, pedicles, and laminae.

  • When vertebrae are stacked in sequence, the collective vertebral foramina form the continuous Vertebral Canal, which protects the spinal cord.

• Vertebral Processes ($7$ Total Processes):

  • Spinous Process ($1$): Projects posteriorly and downward from the median junction of the laminae; serves as an attachment site for back muscles and ligaments.

  • Transverse Processes ($2$): Extends laterally from the junction of the pedicle and lamina on each side; serves as an articulation point for ribs in the thoracic region and attachment site for muscles.

  • Superior Articular Processes ($2$): Extend superiorly from the arch; bear smooth, cartilage-covered articular facets that pair with inferior articular processes above.

  • Inferior Articular Processes ($2$): Extend inferiorly from the arch; bear smooth facets articulating with superior articular processes below.

Intervertebral Discs and Foramina

• Intervertebral Discs:

  • Fibrocartilaginous pads positioned between adjacent vertebral bodies from $C_2$ down to the sacrum.

  • Anulus Fibrosus: The tough outer ring composed of fibrocartilage arranged in concentric rings; confines the inner core and binds vertebrae together.

  • Nucleus Pulposus: The inner gelatinous, highly elastic core that absorbs vertical shock and provides compressibility during spinal movements.

  • Functional Significance: Provides shock absorption, reduces friction between bone surfaces, and permits flexibility of the vertebral column.

• Intervertebral Foramen:

  • Lateral openings formed along the spine by the superior and inferior pedicle notches of adjacent stacked vertebrae.

  • Functional Significance: Serves as the designated passageway for spinal nerves entering and exiting the spinal cord, as well as associated blood vessels.

Regional Characteristics of Vertebrae

Cervical Vertebrae ($C_1 - C_7$)

• General Features:

  • Smallest, lightest vertebrae with small vertebral bodies.

  • Vertebral foramen is relatively large and triangular to accommodate the thick cervical segment of the spinal cord.

  • Spinous processes on typical cervical vertebrae ($C_3 - C_6$) are short and bifid (forked or notched at the tip).

  • $C_7$ possesses a uniquely long, non-bifid spinous process that forms a prominent palpable ridge at the base of the neck, termed the Vertebra Prominens.

  • Transverse Foramina: Distinct openings present within each transverse process that serve as conduits for the vertebral arteries and veins serving the brain.

The Unique Atlas ($C_1$) and Axis ($C_2$)

• Atlas ($C_1$):

  • Lacks a true body and lacks a spinous process.

  • Consists of an anterior arch, a posterior arch, and robust lateral masses.

  • Superior articular facets are large, concave, and oval-shaped; they articulate with the Occipital Condyles of the skull to form the Atlanto-Occipital Joint.

  • Permits head nodding motion ("yes" movement).

• Axis ($C_2$):

  • Possesses a prominent vertical peg-like projection extending superiorly from its body called the Dens (or Odontoid Process).

  • The dens fits into the anterior arch of the atlas, held in position by the Transverse Ligament of the Atlas.

  • Forms the Atlanto-Axial Joint, acting as a pivot point around which the atlas and skull rotate.

  • Permits head rotation from side to side ("no" movement).

Thoracic Vertebrae ($T_1 - T_{12}$)

• General Features:

  • Medium-sized, heart-shaped vertebral body.

  • Vertebral foramen is roughly circular.

  • Spinous process is long, slender, and points sharply downward (inferiorly), overlapping the vertebra below.

• Costal Articulation Facets:

  • Costal Facets / Demifacets on Body: Smooth facets (superior and inferior demifacets) located on the lateral aspect of the bodies for articulation with the heads of the ribs.

  • Transverse Costal Facets: Smooth articular facets located on the transverse processes for articulation with the tubercles of the ribs (present on $T_1 - T_{10}$).

Lumbar Vertebrae ($L_1 - L_5$)

• General Features:

  • Massive, thick, block-like vertebral bodies with a kidney bean shape, designed to support heavy axial loading.

  • Pedicles and laminae are short, thick, and sturdy.

  • Vertebral foramen is triangular.

  • Spinous process is short, thick, flat, and hatchet-shaped; projects almost straight posteriorly (horizontally).

  • Articular process orientation locks the lumbar spine in place, allowing flexion and extension while significantly restricting rotation.

Sacrum and Coccyx

• Sacrum:

  • A single triangular bone formed by the fusion of $5$ sacral vertebrae ($S_1 - S_5$).

  • Forms the posterior wall of the pelvic girdle.

  • Median Sacral Crest: Remnant of fused spinous processes along the posterior midline.

  • Anterior & Posterior Sacral Foramina: Rows of openings flanking the central midline that transmit spinal nerves and blood vessels.

  • Sacral Canal: Central canal continuation of the vertebral canal.

  • Sacral Hiatus: Inferior opening of the sacral canal where laminae fail to fuse.

  • Auricular Surfaces: Rough lateral surfaces that articulate with the ilium of the coxal bones to form the Sacroiliac Joints.

• Coccyx:

  • Small, triangular terminal bone formed by the fusion of $3$ to $5$ (most commonly $4$) rudimentary coccygeal vertebrae.

  • Serves as an attachment point for pelvic floor muscles and ligaments.

Anatomy of the Thoracic Cage

The Sternum

• The sternum (breastbone) is a flat, narrow bone situated in the anterior midline of the thorax.

• Anatomical Landmarks:

  • Manubrium: Superior portion; articulates laterally with the clavicles (sternoclavicular joint) and the costal cartilages of the first and second ribs. Features a central superior notch termed the Jugular Notch.

  • Gladiolus (Body): Middle, elongated portion; articulates directly or indirectly via costal cartilages with Ribs $2$ through $10$.

  • Sternal Angle: A palpable horizontal ridge formed at the junction of the manubrium and gladiolus; marks the anatomical attachment site of Rib $2$

  • Xiphoid Process: Inferior cartilaginous projection that ossifies with age; serves as an attachment site for abdominal wall muscles and the diaphragm.

Rib Structure and Vertebral Articulation

• Anatomy of a Typical Rib:

  • Head: Posterior end featuring two articular facets separated by a crest.

  • Neck: Short, constricted segment lateral to the head.

  • Tubercle: Knob-like projection at the junction of the neck and shaft; bears an articular facet for the transverse process of a thoracic vertebra.

  • Shaft (Body): Main elongated, curved portion; features a Costal Groove along its internal inferior border for the passage of intercostal nerves and blood vessels.

• Rib Articulation Mechanism:

  • The head of a rib typically articulates with demifacets on two adjacent vertebral bodies:

    • The Superior Articular Facet of the rib head articulates with the Inferior Costal Facet of the superior vertebra body.

    • The Inferior Articular Facet of the rib head articulates with the Superior Costal Facet of the corresponding vertebra body.

    • Example: The head of Rib $6$ articulates with the inferior costal facet of $T_5$ body and the superior costal facet of $T_6$ body.

  • The tubercle of the rib articulates with the Transverse Costal Facet of its corresponding thoracic vertebra (e.g., Rib $6$ tubercle articulates with the transverse costal facet of $T_6$).

Classification of Ribs: True, False, and Floating

• The thoracic cage contains $12$ pairs of ribs numbered $1 - 12$ sequentially from superior to inferior:

  • True Ribs (Vertebrosternal Ribs, Pairs $1 - 7$):

    • Attach directly to the sternum anteriorly via individual strips of hyaline cartilage known as Costal Cartilages.

  • False Ribs (Pairs $8 - 12$):

    • Do not attach directly to the sternum.

    • Vertebrochondral Ribs (Pairs $8 - 10$): The costal cartilages of these ribs fuse together, attaching indirectly to the costal cartilage of Rib $7$ above them.

    • Floating Ribs (Vertebral Ribs, Pairs $11 - 12$): Have no anterior attachment to the sternum or adjacent cartilages; their anterior ends remain unattached within the muscles of the abdominal wall.

Appendicular Skeleton Overview

Upper Extremity and Pectoral Girdle

• Pectoral Girdle:

  • Clavicle: S-shaped collarbone articulating with the manubrium of the sternum.

  • Scapula: Shoulder blade featuring the Acromion and the Glenoid Cavity (socket for head of humerus).

• Upper Extremity Bones:

  • Humerus: Long bone of the upper arm.

  • Radius: Lateral bone of the forearm (thumb side).

  • Ulna: Medial bone of the forearm (pinky side).

  • Carpal Bones: $8$ wrist bones.

  • Metacarpals: $5$ bones forming the palm.

  • Phalanges: $14$ finger bones.

Lower Extremity and Pelvic Girdle

• Pelvic Girdle:

  • Os Coxa (Coxal Bones & Sacrum): Formed by the fusion of ilium, ischium, and pubis; connected anteriorly at the fibrocartilaginous Pubic Symphysis joint.

• Lower Extremity Bones:

  • Femur: Thigh bone; largest and strongest bone in the body.

  • Patella: Kneecap; sesamoid bone.

  • Tibia: Medial, weight-bearing bone of the lower leg (shin bone).

  • Fibula: Lateral, non-weight-bearing bone of the lower leg.

  • Tarsals: $7$ ankle bones.

  • Metatarsals: $5$ foot bones.

  • Phalanges: $14$ toe bones.