Borders and anatomical relations of the orbit

• The orbit can be thought of as a pyramidal structure, with the apex pointing posteriorly and the base situated anteriorly

  • Axis of the eye is directed anteriorly while axis of the orbit is directed anteriolaterally

    

• The boundaries of the orbit are formed by seven bones

  • Roof (superior wall) - frontal bone and sphenoid (lesser wing)
  • Floor (inferior wall) - maxilla, palatine and zygomatic bones
  • Medial wall - ethmoid, maxilla, lacrimal and sphenoid bones
  • Lateral wall - zygomatic and sphenoid (greater wing)

Pathways into the orbit

• Optic canal: transmits the optic nerve and ophthalmic artery
• Superior orbital fissure: transmits the lacrimal, frontal, trochlear, oculomotor, nasociliary and abducens nerves, also carries the superior opthalmic vein
• Inferior orbital fissure: transmits the zygomatic branch of the maxillary nerve, the inferior opthalmic vein, and sympathetic nerves

Clinical notes: fractures of the bony orbit

• The eye is generally well protected
• Orbital rim fracture: fracture of the bones forming the outer rim of the bony orbit, usually occurs at the sutures joining the three bones of the orbital rim (maxilla, zygomatic and frontal)
• ‘Blowout’ fracture: refers to partial herniation of the orbital contents through one of its walls, usually occurs via blunt force trauma to the eye
  • The medial and inferior walls are the weakest, with the contents herniating into the ethmoid and maxillary sinuses respectively

The eyelids

Orbicularis oculi muscle

• Deep to skin and connective tissue
• Has three distinct parts - palpebral, lacrimal and orbital
  • Palpebral part - gently closes the eyelids
  • Lacrimal part - involved in the drainage of tears
  • Orbital part - tightly closes the eyelids
• Innervated by CN VII (facial nerve)

Tarsal plates