The Skeleton and Joints: Structure, Landmarks, and Clinical Significance

The Skeletal System: Organization and Overview

Axial and Appendicular Skeleton

The human skeleton is divided into two main branches: the axial skeleton and the appendicular skeleton. The axial skeleton forms the central framework, while the appendicular skeleton consists of the limbs and girdles.

• Axial Skeleton: 80 bones, includes skull, thoracic cage, and vertebral column.

• Appendicular Skeleton: 126 bones, includes pectoral and pelvic girdles, upper and lower limbs.

Axial Skeleton

Bones of the Skull

The skull consists of 29 bones, grouped into cranial, facial, auditory ossicles, and the hyoid bone. These bones protect the brain and support facial structures.

• 8 Cranial Bones: frontal, occipital, 2 parietal, 2 temporal, sphenoid, ethmoid

• 14 Facial Bones: mandible, vomer, 2 nasal, 2 lacrimal, 2 palatine, 2 inferior nasal conchae, 2 zygomatic, 2 maxillary

• 6 Auditory Ossicles: 2 stapes, 2 incus, 2 malleus

• 1 Hyoid Bone: supports tongue and larynx

Thoracic Cage

The thoracic cage protects vital organs and provides attachment points for muscles involved in respiration.

• 1 Sternum: manubrium, body, xiphoid process

• 24 Ribs (12 pairs):

  • 7 true ribs: directly attached to sternum

  • 3 false ribs: indirectly attached via cartilage

  • 2 floating ribs: not attached to sternum

• Houses lungs, heart, and passageways for respiratory and digestive tracts

Vertebral Column

The vertebral column supports the body, protects the spinal cord, and allows flexible movement.

• 7 Cervical Vertebrae (C1–C7): bifid spinous process

• 12 Thoracic Vertebrae (T1–T12): costal facets for rib attachment

• 5 Lumbar Vertebrae (L1–L5): largest vertebral bodies

• 1 Sacrum: 5 fused vertebrae

• 1 Coccyx: 3 fused tail bones

Appendicular Skeleton

Pectoral Girdle

The pectoral girdle connects the upper limbs to the axial skeleton and allows for a wide range of motion.

• 2 clavicles (collar bones): connect sternum to scapula at acromion process

• 2 scapulae (shoulder blades): triangular flat bones with glenoid cavity for humerus attachment

Upper Limbs

Each upper limb contains 30 bones, organized for dexterity and strength.

• 1 humerus (arm bone)

• 2 forearm bones: radius (lateral, thumb side), ulna (medial, little finger side)

• 8 carpal bones (wrist):

  • Proximal row: scaphoid, lunate, triquetrum, pisiform

  • Distal row: trapezium, trapezoid, capitate, hamate

• 5 metacarpal bones (palm)

• 14 phalangeal bones: thumb (pollex) has 2, other digits have 3 each

Mnemonic for Carpal Bones: "Some Leaders Take Positions That They Cannot Handle" (S: Scaphoid, L: Lunate, T: Triquetrum, P: Pisiform, T: Trapezium, T: Trapezoid, C: Capitate, H: Hamate)

Pelvic Girdle

The pelvic girdle supports the lower limbs and protects pelvic organs.

• 2 pelvic (coxal/os coxae) bones: each composed of ilium, ischium, pubis

• Contains obturator foramen for passage of nerves and vessels

Lower Limbs

Each lower limb contains 30 bones, adapted for weight-bearing and locomotion.

• 1 femur (thigh bone)

• 1 patella (kneecap)

• 2 leg bones: tibia (thicker, medial), fibula (lateral)

• 7 tarsal bones: talus, calcaneus, navicular, cuboid, 3 cuneiforms (medial, intermediate, lateral)

• 5 metatarsal bones

• 14 phalangeal bones (same pattern as hand)

Key Anatomical Landmarks and Clinical Significance

Bone Landmarks

• Mental foramen (mandible): passage for nerves/vessels to tongue; trauma can cause tongue paralysis

• Carotid canal: passage for carotid artery to brain

• Foramen magnum (occipital bone): passage for spinal cord

• Occipital condyles: articulate with atlas (C1)

• Glenoid cavity (scapula): socket for humerus head

• Obturator foramen (pelvic bone): passage for nerves and vessels

Specialized Vertebrae: Atlas and Axis

• Atlas (C1): no vertebral body, allows nodding (yes/no) head movement

• Axis (C2): has odontoid process (dens), held by transverse ligament, enables head rotation

Vertebral Curvatures

• Primary curves (present at birth): thoracic and sacral (concave/outward)

• Secondary curves (develop after birth): cervical (develops at 3 months when baby lifts head), lumbar (develops when baby sits up)

• Intervertebral discs: provide cushioning; herniation can cause nerve compression and leg paralysis

Clinical Conditions

• Most commonly fractured carpal bone: scaphoid (from falling on outstretched hand)

• Most commonly dislocated carpal bone: lunate (can cause carpal tunnel syndrome by compressing median nerve)

• Nerve damage: difficult to repair; muscle damage has better regeneration potential

Summary Table: Major Bones of the Skeleton

Region	Main Bones	Key Features
Skull	Frontal, Occipital, Parietal, Temporal, Sphenoid, Ethmoid, Mandible, Maxilla, Zygomatic, etc.	Protects brain, supports facial structures, contains sinuses and foramina
Thoracic Cage	Sternum, Ribs	Protects heart and lungs, supports respiration
Vertebral Column	Cervical, Thoracic, Lumbar, Sacrum, Coccyx	Supports body, protects spinal cord, allows movement
Pectoral Girdle	Clavicle, Scapula	Connects upper limb to axial skeleton, allows mobility
Upper Limb	Humerus, Radius, Ulna, Carpals, Metacarpals, Phalanges	Dexterity, manipulation, strength
Pelvic Girdle	Ilium, Ischium, Pubis (Coxal bone)	Supports lower limb, protects pelvic organs
Lower Limb	Femur, Tibia, Fibula, Tarsals, Metatarsals, Phalanges, Patella	Weight-bearing, locomotion

Joints (Articulations)

Definition and Function

Joints (articulations) are points where two or more bones meet, allowing movement and providing structural support. Types of movement include flexion, extension, rotation, and more.

• Shoulder joint: formed by glenoid cavity and humerus head; allows wide range of motion

• Elbow joint: hinge joint formed by humerus and ulna (olecranon process)

• Knee joint: formed by femur, tibia, and patella; allows flexion and extension

Additional info:

• Osteology: the science of bones

• Pollex: anatomical term for thumb

• Obturator foramen: large opening in pelvic bone for passage of nerves and vessels

• Intervertebral disc herniation: can lead to nerve compression and paralysis

• Carpal tunnel syndrome: caused by lunate dislocation compressing median nerve

Key Formulas and Concepts

Bone Count Formula

The total number of bones in the adult human skeleton:

$\text{Total bones} = \text{Axial skeleton} + \text{Appendicular skeleton} = 80 + 126 = 206$

Mnemonic for Carpal Bones

"Some Leaders Take Positions That They Cannot Handle"

Proximal row: Scaphoid, Lunate, Triquetrum, Pisiform

Distal row: Trapezium, Trapezoid, Capitate, Hamate

Exam Preparation Tips

• Review labeled pictures of bones and landmarks

• Memorize bone names, locations, and features

• Understand clinical relevance of bone injuries and nerve damage

• Practice identifying bones and joints from diagrams

• Focus on atlas and axis vertebrae for exam questions