Orbit Flashcards
what are the 4 areas of the orbit
roof, lateral wall, floor and medial wall
what is the purpose of the orbit
to support and provide protection to the orbital soft tissues (globe and adnexa)
how many bones make up the surrounding walls
7
name the 7 bones that make up the 4 walls of the orbit
sphenoid, frontal, ethmoid, maxillary, zygomatic, palatine, and lacrimal bones
which bones are common to both orbits
ethmoid, sphenoid, and frontal
what is the purpose of the orbital rim
protection from trauma
what is the strongest portion of the orbit
the lateral portion of the orbital rim (zygomatic bone and zygomatic process of frontal bone)
why does the lateral rim have a concavity posteriorly directed
the increase the visual field, but the eye is prone to injury
what shape is the orbital roof
a triangular shape
what bones form the orbital roof
frontal bone and lesser wing of the sphenoid bone
where is the lacrimal fossa found
in the antero-lateral angle of the orbital roof
what is the thinnest portion of the orbit
the floor
what makes up the orbital floor
orbital plate of the maxillary bone
what is the purpose of the orbital floor
the support the eye and adnexal tissues, separate them from the maxillary sinus
how long is the orbital floor
35-40mm (doesn’t extend to the apex)
during a blow out fracture, where does the eye blow out to
the maxillary sinus
why does the orbital floor break in a blow out fracture
it acts as a release valve to decrease orbital pressure and blows out to provide protection to the eye and adnexa
what symptoms might a person have with a blow out fracture
loss of vision, diplopia, enophthalmos, or a numb cheek
what nerve damage would cause the cheek to go numb in an orbital blow out fracture
damage to the infraorbital nerve (CN 7 facial)
why might someone have diplopia following a blow out fracture
it may cause restrictive strabismus (recti muscles get trapped)
what is enophthalmos
where the eye is sunken in and sometimes downward
why might a doctor purposely break the orbital floor
if they have thyroid eye disease (graves disease) to allow for the exophthalmos eye to have more room
what bones form the lateral wall and what shape do they make
zygomatic (anteriorly) and greater wing of sphenoid bone (posteriorly)- triangular shape
which wall of the orbit is the smallest
the medial wall
what bones make up the medial wall
Lamina Papyracea of the ethmoid bone, supported by the honeycomb structure of ethmoid sinus bony lamina
what protects the medial wall from being fractured
because it is being supported by the honeycomb structure of the ethmoid sinus bony lamina
what is the anterior-posterior diameter of the globe at birth
about 16mm
when does the globe reach 90% of its adult size
at 20 months (or about 2 years old)
what is the anterior-posterior diameter of an adult globe
about 24mm (varies from 20-30mm)
what may cause the anterior-posterior diameter of the adult globe to vary
refractive error (most common), gender, ethnicity
which ethnicity origin has the smallest anterior-posterior diameter of the globe
chinese origin
what is the volume of the globe
7 cm^3
what is the volume of the orbit
30cm^3
why does the eye get displaced forward (exophthalmos)
the bony orbit is incapable of acute expansion, except anteriorly
what are 2 causes of enophthalmos
blow out fracture and as we age the fat content decreases
what is the most common cause of exophthalmos
thyroid eye disease
what causes exophthalmos in thyroid eye disease
an inflammatory reaction of an unknown cause, glycosaminoglycans accumulate in the EOMs and/or connective tissue and fat, leads to swelling
what is a hemangioma
an abnormal buildup of blood vessels (can cause exophthalmos)
what is craniostenosis
fault in fusing bones in an infant skull (fusing too early), not enough room for eyeball and can cause exophthalmos
what is craniofacial dysostosis
defect in normal ossification of the fetal cartilages (embryo)
what is buphthalmos
“ox eye” a condition where the eye is stretched (in early years of development the IOP can increase and collagen isn’t fully formed)
how might severe myopia cause exophthalmos
the large refractive error will elongate the globe and stimulate exophthalmos
what instrument is the most accurate in measuring exophthalmos
Hertel exophthalmometer
where is the protrusion measured from with an exophthalmometer
from the deepest part of the lateral orbital rim to the corneal apex
what does the orbital connective tissue consist of
fibroblasts, endothelial cells, collagen types 1, 3, 4
what is collagen type 1
protein that provides structural integrity
what is collagen type 3
a cellular adhesion molecule between the connective tissue and adipocytes
what is collagen type 4
associated with the basement membranes of the vessels found in connective tissue
what artery provides the primary blood supply to the orbit
the ophthalmic artery
what is the blood supply to the orbit in about 3% of the population
the middle meningeal artery contributes equally with the opthalmic artery
which veins do parallel the orbital arteries
the lacrimal and ethmoidal veins
