Skeletal System Notes: Radius/Ulna to Sacrum, with Thorax, Pelvis, and Lab Practical Tips

Radius and Ulna (Antebrachium)

  • Equivalences to the lower limb:

    • Radius ↔ Tibia

    • Ulna ↔ Fibula

    • How to pair bones: line up your thumb and big toe to figure out medial vs. lateral equivalents (not in standard anatomical position).

  • Ulna anatomy and joints

    • Ulna is relatively slender and U-shaped; forms a hinge at the elbow via the trochlear notch.

    • Key processes/notches:

    • Olecranon process (top point of the U)

    • Coronoid process (bottom point; not to be confused with coracoid)

    • Radial notch: depression on the ulna where the radius sits proximally

    • Hinge joint at the elbow: humeroulnar joint (flexion/extension) via the trochlear notch wrapping around the trochlea of the humerus.

  • Radius anatomy and joints

    • Radius is thin and ends with a circular head; neck and radial tuberosity distal to the neck (in the proximal forearm).

    • Radial tuberosity is the attachment site for the biceps brachii; this muscle helps flex the elbow with both bones moving.

    • The head of the radius articulates with the humerus (capitulum) to form the humeroradial joint and with the ulna at the proximal radioulnar joint.

  • Proximal vs. distal radioulnar joints

    • Proximal radioulnar joint: head of radius articulates with the radial notch of the ulna; this is essential for pronation/supination (radius rotates around the ulna).

    • Distal radioulnar joint: the distal ends of the radius and ulna articulate; the ulna remains stationary during rotation, while the radius rotates around it.

  • Interosseous membrane

    • Dense connective tissue (interosseous membrane) between radius and ulna: a syndesmosis.

    • Allows slight movement during pronation/supination; prevents separation of the two bones.

    • Note: in tibia/fibula, the interosseous membrane holds bones together with far less rotation.

  • Movement during forearm actions

    • During flexion, both radius and ulna move.

    • During rotation (pronation/supination), the ulna remains stationary while the radius rotates around it.

  • Radial fossa

    • A depression on the distal humerus (radial fossa) accommodates the head of the radius during flexion.

The Foot, Hand, and Their Proximal Features

The Foot: ankle and arches

  • Foot division: three groups of bones

    • Tarsals (7): calcaneus, talus, navicular, cuboid, and three cuneiforms (medial, intermediate, lateral)

    • Metatarsals (5): one for each toe, numbered 1–5

    • Phalanges: toes (digits)

  • Tarsals and mnemonic

    • Calcaneus (heel bone)

    • Talus (articulates with tibia/fibula at the ankle)

    • Navicular (in front of talus)

    • Medial, Intermediate, Lateral Cuneiforms (in front of navicular)

    • Cuboid (lateral side)

    • Mnemonic to recall order from heel to toes: "Cute Tigers Need Milk" (Calcaneus, Talus, Navicular, Medial Cuneiform, Intermediate Cuneiform, Lateral Cuneiform, Cuboid).

  • Phalanges of the toes

    • Digit 1 (hallux): two phalanges (proximal and distal)

    • Digits 2–5: three phalanges each (proximal, middle, distal)

    • Naming convention for practicals: write digit number, then phalanx type, then position (proximal/middle/distal), e.g., Proximal Phalanx I, Distal Phalanx I. The hallux can also be called the pollex for the thumb, but for toes it remains hallux.

  • Metatarsals

    • Metatarsal I–V corresponding to digits 1–5; must specify which metatarsal when asked in practicals.

  • Joints of the foot

    • Proximal row: talus sits atop calcaneus; joints with tibia/fibula at the ankle (talocrural joint) and with the navicular medially.

    • Distal row: carpools of tarsal bones articulate with metatarsals via tarsometatarsal joints.

  • The ankle (talocrural) joint

    • Formed by the distal tibia and fibula with the talus.

    • The lateral malleolus (fibula) and the medial malleolus (tibia) form a boundary that stabilizes the ankle.

    • Primary movements: dorsiflexion and plantar flexion; rotation is limited.

    • The malleoli prevent side-to-side displacement of the talus during movement.

  • Distal articulations and the interosseous membrane (lower limb)

    • Similar logic to forearm but without the pronounced rotation between tibia and fibula in the knee; the ankle joint movement is mainly hinge-like.

The Hand: carpals, metacarpals, and phalanges

  • Carpal bones (8 total)

    • Proximal row (starting at the thumb side): Scaphoid, Lunate, Triquetrum, Pisiform (pisiform is best seen from the anterior side).

    • Distal row: Trapezium, Trapezoid, Capitate, Hamate.

    • Mnemonic for order on the hand side: Scaphoid, Lunate, Triquetrum, Pisiform (proximal row) and Trapezium, Trapezoid, Capitate, Hamate (distal row).

    • Pisiform is visible from the anterior surface; if you flip the hand, you may not see it.

  • Metacarpals (5)

    • Numbered I–V from thumb to pinky; proximal bases articulate with distal carpal row; heads form the knuckles.

  • Phalanges of the hand

    • Digit I (thumb/pollux): Proximal and Distal phalanges (no middle phalanx)

    • Digits II–V: Proximal, Middle, Distal phalanges

    • Naming convention similar to the foot: Digit number + Phalanx + Position (proximal/middle/distal)

  • Spelling tips and lab accuracy

    • Watch for similar-sounding terms: coracoid (scapula) vs coronoid (ulna process)

    • Trapezium vs trapezius; spelling matters for practicals.

The Postcranial Axial Skeleton: Thorax, Vertebral Column, and Pelvic Attachments

The Thoracic Cage

  • Components: thoracic cage including sternum and ribs; posterior ribs attach to the vertebral column.

  • Ribs and their groups (by articulation with the sternum)

    • True ribs: ribs 1–7, each has its own costal cartilage attaching directly to the sternum.

    • False ribs: ribs 8–10, their cartilage attaches to the cartilage of rib 7 (indirect attachment to sternum).

    • Floating ribs: ribs 11–12, do not attach to the sternum; attach posteriorly to the vertebrae and have no anterior attachment.

  • Intercostal spaces and breathing

    • Intercostal spaces are the gaps between ribs; muscles run in these spaces to enable thoracic expansion and contraction during breathing.

  • Costal cartilage and joints

    • Costosternal joints: connections between ribs and sternum via hyaline cartilage (costal cartilage).

    • These joints are synchondroses (rigid but with slight movement) to allow breathing while providing anterior stability.

  • Sternum anatomy and CPR relevance

    • Sternum is a flat, slightly curved bone of the chest; three fused parts: Manubrium (superior), Body (middle), Xiphoid process (inferior).

    • Clavicle articulates with the manubrium at the clavicular notch (sternoclavicular joint) and rib 2 articulates partly with the manubrium and body.

    • Xiphoid process is mostly hyaline cartilage and ossifies with age (around age 6060–7070).

    • CPR hand placement: palms on the body of the sternum, avoid compressing the xiphoid process to prevent injury to the heart or lungs; correct hand placement is crucial for effective resuscitation.

  • Sternoclavicular and costosternal joints

    • Sternoclavicular joint: articulation between the sternal end of the clavicle and the manubrium’s clavicular notch; only attachment between pectoral girdle and axial skeleton.

    • Costosternal joints: connections between ribs and sternum via hyaline cartilage; allow limited movement for respiration.

  • Notable clinical tidbits

    • Open-heart surgery sometimes requires cutting through the sternum; this bone typically heals, but cartilage does not have a direct blood supply and heals more slowly.

The Vertebral Column: Structure and Regions

  • Overall organization and function

    • The vertebral column is segmented into vertebrae; provides protection and housing for the spinal cord (vertebral canal) and site for muscle attachment; supports posture and transfers weight to the pelvis.

    • The column is segmented and flexible, enabling movement with muscular attachments.

  • Embryology connection

    • Embryologically, ribs and the axial skeleton derive from the sclerotome portion of somites (a derivative of mesoderm).

    • The nucleus pulposus of intervertebral discs is derived from the notochord.

  • Intervertebral discs and joints

    • Intervertebral discs: fibrocartilage outer ring (annulus fibrosus) surrounding a gelatinous center (nucleus pulposus).

    • Nucleus pulposus origin: remnant of the notochord.

    • Intervertebral joints (between vertebral bodies): vertebral bodies are connected by fibrocartilaginous discs; these joints are largely immobile for bending in daily function (discs contribute flexibility).

    • Zygapophysial (facet) joints: synovial joints between superior and inferior articular processes; allow flexion, extension, and rotation.

  • Intervertebral foramina

    • Lateral openings formed when vertebrae stack together; spinal nerves pass through these foramina.

  • Curvatures and development

    • Primary curvatures: thoracic and sacral (present at birth; concave posteriorly).

    • Secondary curvatures: cervical and lumbar (develop after birth; due to head lifting and standing/walking).

Regional vertebrae: Features and Identification

General vertebrae features to know on all levels

  • Common features (present on most vertebrae)

    • Body: anterior, weight-bearing portion.

    • Vertebral foramen: circular/oval opening behind the body; houses the spinal cord; stacking forms the vertebral canal.

    • Spinous process: posterior projection; attachment point for muscles; palpable along the back.

    • Transverse processes: lateral projections; attachment sites for muscles; important for rotation and lateral flexion.

    • Articular processes: superior and inferior facets for articulation with adjacent vertebrae.

  • Intervertebral discs and foramina

    • Intervertebral discs lie between vertebral bodies; spinal nerves pass through intervertebral foramina between successive vertebrae.

Cervical vertebrae (C1–C7)

  • Number and naming

    • There are 77 cervical vertebrae; C1 and C2 have special names:

    • C1: Atlas

    • C2: Axis

  • Atlas (C1)

    • Characteristics: no body and no true spinous process; largest vertebral foramen; articulates with the skull via the occipital condyles; allows nodding (flexion/extension).

    • Visual cues: atlas is unique and can resemble a Yoda-like shape; superior surface has a large concave articulation for the occipital condyles.

  • Axis (C2)

    • Characteristics: dens (odontoid process) projects upward to fit into the Atlas; creates the pivot for rotation of the head.

    • When atlas and axis align, the dens sits within the Atlas to permit rotational movement (yes -> no -> rotation).

  • General notes for C3–C7

    • Features to identify: presence of a transverse foramen in all cervical vertebrae; bifid spinous processes are common in some, not always.

    • C7 often has a non-bifid spinous process and is used as a landmark for counting vertebrae.

    • Transverse foramen houses the vertebral artery/vein passing to the brain.

Thoracic vertebrae (T1–T12)

  • Number and naming

    • There are 1212 thoracic vertebrae.

  • Distinguishing features

    • Body: heart-shaped (roughly) with articular surfaces for ribs.

    • Vertebral foramen: circular (larger than in some regions).

    • Spinous process: long, slender, and points downward (giraffe-like).

    • Costal facets: each thoracic vertebra has superior and inferior costal facets on the body and a transverse costal facet on the transverse process for articulation with the corresponding rib.

    • These features relate to articulation with ribs; T11–T12 have limited or no transverse costal facets.

  • Practical notes

    • The rib heads articulate with the bodies of thoracic vertebrae; tubercles articulate with transverse processes via the transverse costal facets.

Lumbar vertebrae (L1–L5)

  • Number and naming

    • There are 55 lumbar vertebrae.

  • Distinguishing features

    • Body: largest and kidney-shaped.

    • Vertebral foramen: relatively large but more triangular in shape.

    • Spinous process: broad, flat, and projects posteriorly (moose-like appearance).

    • Transverse processes: long and slender; articulation surfaces differ from thoracic vertebrae.

  • Practical notes

    • The lumbar region shows the greatest motion potential in flexion/extension due to larger, more accommodating facets and absence of ribs.

The Sacrum and Coccyx

  • Sacrum

    • A single fused bone formed by the fusion of 55 sacral vertebrae.

    • Features: sacral canal (continuation of vertebral canal), sacral hiatus at the inferior end, and foramina on the lateral sides for exiting spinal nerves.

    • Auricular surface on the lateral side for articulation with the ilium (sacroiliac joint).

    • Median sacral crest (remnant of fused spinous processes).

  • Coccyx

    • The tailbone; fused from multiple small segments; a remnant of the embryonic tail.

    • Serves as an attachment for pelvic muscles and supports the pelvic cavity.

  • Male vs. female sacrum and pelvis

    • Female pelvis: wider sacrum and pelvic inlet; sacrum tends to be shorter with a more pronounced curvature; coccyx oriented more posteriorly.

    • Male pelvis: narrower sacrum, longer, and coccyx more anteriorly oriented.

Nerves and Clinical Correlations

  • Intervertebral foramina

    • Formed by successive pedicles and vertebrae; spinal nerves pass through these openings.

  • Herniated discs

    • Nuclear pulposus (from nucleus pulposus) can bulge and compress spinal nerves or spinal cord, causing pain and neurologic symptoms.

    • Treatment considerations: discectomy with fusion is common; complete removal of the disc may be necessary to stabilize the spine in severe cases.

  • Embryology connections to clinical anatomy

    • Notochord contributes to nucleus pulposus; understanding embryology helps explain disc structure and potential pathologies.

Regional Review and Study Tips

  • Key regional counts to memorize

    • Vertebrae: 77 cervical, 1212 thoracic, 55 lumbar; sacrum (fusion of 55), coccyx (fused, variable segments).

  • How to identify vertebral regions visually

    • Cervical: transverse foramen present in all; C1 (atlas) has no body; C2 (axis) has a dens.

    • Thoracic: heart-shaped body, circular foramen, downward-pointing spinous processes; costal facets on bodies and transverse processes.

    • Lumbar: large kidney-shaped bodies, triangular foramen, broad spinous processes.

  • Practical lab tips

    • Pull out clavicle, rib 1, rib 2, rib 11, rib 12, and a large rib for specimen study.

    • Learn the eight carpal bones and their order; memorize pisiform as an anterior-side landmark.

    • For the skull-related references, prepare to visualize the atlas/axis relationship with the occipital condyles.

    • Spelling accuracy matters (coronoid vs coracoid; trapezium vs trapezius).

  • Exam scope reminder

    • The next test covers skeletal system, joints, appendicular skeleton, and postcranial axial skeleton.

Quick Reference Facts (condensed)

  • True ribs: 1−71-7; False ribs: 8−108-10; Floating ribs: 11−1211-12.

  • Number of ribs: 1212 pairs.

  • Foot bones: 77 tarsals; 55 metatarsals; phalanges as described.

  • Hand bones: 88 carpals; 55 metacarpals; phalanges as described.

  • Intervertebral discs: 2323 in the vertebral column.

  • Notochord contribution: nucleus pulposus of discs.

  • Normal joint types observed: hinge joints (elbow, ankle), pivot joints (atlantoaxial rotation), synovial joints at facet joints, synchondroses (costosternal joints).

Final Practical Tips (from the lecture)

  • Spelling and terminology matter a lot on practicals; the instructor emphasized careful spelling of anatomical terms.

  • Lab guidance includes concrete tips like identifying pisiform from the anterior view and preparing multiple rib types (1, 2, 11, 12) for quick recognition.

  • Know the CPR implications: correct hand placement on the sternum is crucial; avoid pressing over the xiphoid process to prevent injury.

  • Visual mnemonics can help: atlas as Yoda-like shape; axis with the dens acting as a pivot; thoracic vertebrae as giraffes; lumbar as moose.

Connections to Foundational Principles

  • Embryology links: sclerotome gives rise to axial skeleton; notochord remnants become nucleus pulposus.

  • Structural-functional relationships: the thoracic cage forms a protective yet flexible cylinder, enabling respiration; intervertebral discs provide shock absorption while allowing movement; the spine’s curvatures balance stability and mobility.

  • Clinical relevance: open-heart surgery implications, disc herniation, and the biomechanics behind gait and bipedal posture are tied to these skeletal structures.