Head and Neck Anatomy Series: Bones of the Skull

Introduction

• The skull is a bony structure forming a protective cavity for the brain and supporting the face.
• Sutures are fibrous joints unique to the skull, allowing for brain growth during childhood and adolescence, eventually fusing in adulthood.

Skull Development

• Neural Crest: Forms the anterior part of the skull.
• Paraxial Mesoderm: Forms the posterior part of the skull (contributes to somitomeres and somites).

Divisions of the Skull

• Cranium (Neurocranium)
  • Protects the brain.
  • Subdivided into roof (calvarium) and base.
  • Calvarium: Frontal, parietal, occipital bones.
  • Cranial Base: Frontal, ethmoid, sphenoid, temporal, parietal, occipital bones.
• Face (Viscerocranium)
  • Supports soft tissues of the face.
  • Comprised of 14 bones, including zygomatic, maxilla, nasal, lacrimal, palatine, ethmoid, vomer, mandible.

Key Structures and Functions

• Fontanelles: Incompletely fused joints at birth, fuse within 18 months.
  • Frontal Fontanelle: Located at coronal and sagittal sutures.
  • Occipital Fontanelle: Located at sagittal and lambdoid sutures.
• Sutures: Immovable fibrous joints.
  • Frontal/Metopic Suture: Closes early (within 3-9 months).
  • Coronal, Sagittal, Squamous, Lambdoid Sutures: Close by age 20.
• Intersection Points: Bregma (frontal), Lambda (occipital).

Craniosynostosis

• Premature closure of fontanelles/sutures.
• Types:
  • Scaphocephaly: Early closure of sagittal suture (elongated AP direction).
  • Brachycephaly: Early closure of coronal and lambdoid sutures (elongated SI direction).
  • Plagiocephaly: Asymmetrical closure of coronal/lambdoid sutures on one side.

Sinuses

• Develop as pouches from the lateral nasal wall.
• Sinuses: Frontal, ethmoid, sphenoid, maxillary.
• Functions: Reduce skull weight, affect voice resonance, insulate sensitive structures, regulate gas pressure, provide immunological defense.

Mandible

• Structure influenced by alveolar process.
• Age-related Changes:
  • At birth: Angle ~150° (no teeth).
  • Children: Angle ~140° (deciduous teeth).
  • Adults: Angle ~120-130° (permanent teeth).
  • Edentulous: Angle reverts to ~140-150°.

Skull Lines

• Lines of Force: Strong areas adapted to mechanical stress.
• Principal Lines of Fracture (Laforte Lines): Common fracture sites.
  • Terion: Weak area where frontal, parietal, sphenoid, temporal bones meet.

High-Yield Facts

• Orbit Bones: Frontal, maxillary, palatine, lacrimal, ethmoid, sphenoid, zygomatic (Mnemonic: FM, please).
• Ethmoid Bone Features: Crista galli, uncinate process, cribriform plate, superior/middle nasal concha.
• Sphenoid Bone Features: Sella turcica, clinoid processes, lesser/greater wings.
• Nasal Septum: Composed of septal cartilage, perpendicular plate of ethmoid, vomer.
• Nasal Meatuses: Air passages located inferior to corresponding concha.

Conclusion

• Encouragement to use drawings for memory aids.
• Additional learning resources available via Patreon.