The Cranium - The Ethmoid Bone

The Ethmoid Bone

• Originates from the Greek word "Ethmos", meaning sieve, reflecting its lightweight and spongy structure.
• A small, unpaired bone forming part of the orbital complex.
• Situated in the paranasal sinuses.
• Contributes to the medial wall of the orbit, forming part of the anterior cranial fossa.
• Separates the nasal cavity inferiorly and the cranial cavity superiorly.
• Innervation: Anterior and Posterior Ethmoidal nerves.

Features of the Ethmoid Bone

• Comprised of four parts:
  • Cribriform plate
  • Perpendicular plate
  • Ethmoid labyrinths

Cribriform Plate

• Forms the roof of the nasal cavity.
• Perforated by numerous olfactory fibres.
• The crista galli projects superiorly from the plate.

Perpendicular Plate

• Forms the superior two-thirds of the nasal septum, descending from the cribriform plate.

Ethmoid Labyrinths

• Large masses located on either side of the perpendicular plate.
• Contain ethmoid air cells, also known as ethmoid sinuses.

Clinical Relevance

• Fracture of the cribriform plate: May cause anosmia (loss of smell).
• Ethmoid labyrinth fracture: May allow communication between the nasal cavity and orbit, causing orbital emphysema.
• Inflammation:
  • Can cause rhinorrhoea (runny nose), where cerebrospinal fluid can enter the nasal cavity and drain out as a clear watery fluid from one side of the nose.
  • Can also cause rhinosinusitis.
• Damage to the Ethmoid Bone: May be caused by infection, inflammation, or facial trauma.

Ethmoid Bone

• The Ethmoid Bone's name comes from the Greek word "Ethmos," meaning "sieve," referring to its lightweight and spongy structure.
• This small, unpaired bone is situated in the paranasal sinuses, contributing to the medial wall of the orbit and forming part of the anterior cranial fossa.
• It separates the nasal cavity inferiorly from the cranial cavity superiorly.
• The Ethmoid bone receives innervation from the Anterior and Posterior Ethmoidal nerves.

Parts of the Ethmoid Bone

• The Ethmoid bone comprises four main parts:
  • Cribriform Plate
  • Perpendicular Plate
  • Ethmoid Labyrinths

Cribriform Plate

• Located at the roof of the Nasal Cavity.
• This plate is pierced by numerous olfactory fibers.
• The Crista Galli projects superiorly from the Cribriform Plate.

Perpendicular Plate

• This plate forms the upper two-thirds of the Nasal Septum and descends from the Cribriform Plate.

Ethmoid Labyrinths

• These are large masses found on either side of the Perpendicular Plate.
• They contain the Ethmoid Air Cells (also known as the Ethmoid Sinuses).

Clinical Relevance

• A fractured Cribriform Plate can result in Anosmia (loss of smell).
• A fracture of the Ethmoid Labyrinth can allow communication between the nasal cavity and orbit, potentially causing Orbital Emphysema.
• Inflammation of the Ethmoid bone can lead to Rhinorrhoea (clear watery discharge from the nose), where cerebrospinal fluid leaks into the nasal cavity. This can occur from one side of the nose.
• Damage to the Ethmoid bone can be caused by infection, inflammation, or facial trauma.
• Inflammation of the Ethmoid bone can also cause Rhinosinusitis.

Ethmoid Bone

• Originates from the Greek word "Ethmos" meaning "sieve", which refers to its lightweight & spongy structure.
• Small unpaired bone that contributes to the orbital complex.
• Located in the paranasal sinuses.
• Contributes to the medial wall of the orbit.
• Forms part of the anterior cranial fossa.
• Separates the nasal cavity inferiorly and the cranial cavity superiorly.
• Innervation: Anterior and Posterior Ethmoidal nerves.

Features of the Ethmoid Bone

• Composed of four parts:
  • Cribriform Plate
  • Perpendicular Plate
  • Ethmoid Labyrinths

Cribriform plate

• Forms the roof of the nasal cavity.
• Perforated by numerous olfactory fibers.
• The crista galli projects superiorly from the plate.

Perpendicular Plate

• Forms the superior two thirds of the nasal septum.
• Descends from the cribriform plate.

Ethmoid Labyrinths

• Large masses located on each side of the perpendicular plate.
• Contain Ethmoid air cells, also known as the Ethmoid sinuses.

Clinical Relevance

• A fracture of the cribriform plate may cause anosmia (loss of smell).
• Inflammation can cause rhinorrhea (runny nose).
• An Ethmoid labyrinth fracture may allow communication between the nasal cavity and orbit leading to orbital emphysema.
• Inflammation may cause rhinosinusitis.
• Damage to the Ethmoid bone may be caused by infection, inflammation, or facial trauma.
• Cerebral-spinal fluid may enter the nasal cavity and drain as a clear watery fluid through one side of the nose.

Ethmoid Bone

• Location: Positioned in the paranasal sinuses. It contributes to the medial wall of the orbit and forms part of the anterior cranial fossa.
• Function: Separates the nasal cavity (inferior) from the cranial cavity (superior).
• Structure:
  • Cribriform Plate: Forms the roof of the nasal cavity. Perforated with numerous olfactory fibres. Contains the crista galli which projects superiorly from the cribriform plate.
  • Perpendicular Plate: Forms the superior two-thirds of the nasal septum and descends from the cribriform plate.
  • Ethmoid Labyrinths: Large masses positioned on either side of the perpendicular plate. Contain Ethmoid air cells (Ethmoid sinuses).
• Clinical Relevance:
  • Fracture:
    • Cribriform Plate: Can cause anosmia (loss of sense of smell)
    • Ethmoid Labyrinth: Can allow communication between the nasal cavity and orbit, causing orbital emphysema (air trapped in the orbit)
  • Inflammation:
    • Can cause rhinorrhoea (runny nose) with cerebrospinal fluid draining out of one side of the nose.
    • Can also cause rhinosinusitis (inflammation of the nasal cavity and sinuses)
  • Damage: Can be caused by infection, inflammation or facial trauma.