ORBIT AND PARANASAL SINUSES Flashcards
Where is the orbital floor thinnest?
Medial wall; this is due to the laminal papyracea of the ethmoid making up the largest part of the medial wall
It is paper thin and separates the orbit from the ethmoid air cells.
Where is the orbital margin widest?
Why is the orbital margin the strongest part of the orbit?
The widest point of the orbit is around 15mm behind the orbital margin
The lateral orbital margin made of the frontal and zygomatic bones is the strongest margin with its weakest part being the suture between the 2 bones
The orbital margin is the strongest part of the orbit because:
- all 3 bones are unusually thick at the orbital margin (F, M, Z)
- The sutures by which the three bones interconnect are extensive. The thickness of the bones and the extensive sutures make the orbital margin quite resistant to blunt trauma due to their thickness, although they can be broken.
Which structures lie adjacent to orbital roof?
Above the orbit are the anterior cranial fossa and frontal sinus
Which structures lie adjacent to the medial wall of the orbit?
Medial to the orbit are the ethmoidal sinuses, sphenoid sinus, and nasal cavity
- Below the orbit = maxillary sinus
- Posterior to orbit = middle cranial fossa
Which structures is the periorbita directly continuous with:
anteriorly
posteriorly (include what it is thickened to form):
Anteriorly at the orbital margin, the periorbita is continuous with:
-the periosteum of the skull bones of the face (frontal, zygomatic, maxillary)
-orbital septum= a connective tissue membrane that extends from the orbital
margin towards the tarsal plates of the eyelids and separates the contents of the eyelids from the orbital contents
Posteriorly at the optic canal (by the orbital apex), the periorbita is continuous with:
- the periosteal layer of the dura mater surrounding the optic nerve (where the dura is fused to the optic canal).
- additionally it is thickened to form the common tendinous ring (annulus of Zinn)
What are the attachment sites of the orbital septum?
In the upper eyelid, orbital septum does NOT insert into tarsal plate
-it inserts into the aponeurosis of levator palpebrae superioris 2-5 mm above the superior border of the tarsal plate as the levator aponeurosis interposes itself between the septum and superior border of tarsal plate
In the lower eyelid the orbital septum inserts into the inferior border of the tarsal plate with the inferior Mueller’s muscle.
What does the orbital septum separate? Clinical relevance?
The orbital septum separates eyelid contents from the orbital contents
− It serves as an important anatomic barrier to infection, hemorrhage and edema
& prevents the spread of blood or inflammation from the eyelid to the orbit & vice versa and holds orbital fat in position in the orbit
**Its main function is to act as a strong barrier to prevent eyelid infections from entering the orbit.
Which bones form the orbital margin?
Which bones form the medial orbital margin? Superior orbital margin?
The orbital rim (margin) is the sharp edge of the orbital opening (which is the peripheral border of the base of the pyramid-shaped orbit). It is quadrangular in shape with rounded corners.
Superior orbital margin = frontal bone
Lateral orbital margin = frontal and zygomatic bones
Inferior orbital margin = zygomatic bone and maxillary bones
Medial orbital margin = maxillary and frontal bones
Which orbital bones are most likely to fracture if someone receives a blow to the eye socket and suffers a “blow out fracture”? Why are these orbital bones usually the ones fractured?
The most common sites for orbital fractures are the maxillary bone (by infraorbital groove) helping forming the floor & lamina papyracea of ethmoid helping form the
medial wall.
Usually occur when an object larger than orbital opening strikes orbital margin. A blow to the orbital margin causes compression of the orbital contents and a sudden increase in intraorbital pressure. The shock wave of this increased intraorbital pressure travels through the site of impact and: causes a fracture in one of the orbital walls, at a weak point in the bone (i.e. orbital plate of maxilla or lamina papyracea of ethmoid).
This may cause possible expulsion of soft tissue through the fracture site. The resultant decompression of the orbit (due to fracture of bone) is safe mechanism to prevent rupture of eyeball.
If a patient’s eyeball is displaced superior-laterally where is the mass located in the orbit?
Inferior and medial
What is the difference between the nasolacrimal canal and nasolacrimal duct?
The nasolacrimal canal is a bony canal within the maxilla that is continuous superiorly with fossa for the lacrimal sac. Lies below level of orbital floor and leads inferiorly into inferior meatus of nasal cavity and contains the nasolacrimal duct.
The nasolacrimal duct is “soft tissue” & is the duct of the lacrimal sac.
▪ It lies in the “bony” nasolacrimal canal formed mainly by the maxillary bone, with
contributions from the lacrimal bone & inferior turbinate.
▪ The nasolacrimal duct is composed of soft tissues and is lined by an epithelium
The lacrimal sac is “soft tissue” & it lies in the “bony” fossa for the lacrimal sac, a
bony depression on the medial wall of the orbit formed by the maxillary and lacrimal bones ▪ it contains the tears being drained from the eye
▪ the lacrimal sac is composed of soft tissues and is lined by an epithelium
(tears go down the lacrimal canal and into the nasolacrimal duct)
Which orbital bones form the fossa for the lacrimal sac?
What structure lies in this fossa?
Fossa for the lacrimal sac
– Formed by the maxillary bone and the lacrimal bone
• bounded by the anterior and posterior lacrimal crests – anterior lacrimal crest = on maxillary bone
– posterior lacrimal crest = on lacrimal bone
− It contains the lacrimal sac
(remember: lacrimal SAC near nose is different from lacrimal gland behind the upper lateral border of the lids)
(CO) 1. Describe the general structure of the orbits and their walls. Be sure to include:
- The relationships of the medial wall of the 2 orbits (i.e. how far apart, parallel, etc.)
- Angle at orbital apex where lateral and medial walls of one orbit meet
- Location of widest part of orbit
The walls of the orbit are made by seven bones: maxillary, palatine, frontal, sphenoid, zygomatic, ethmoid and lacrimal.
– The medial walls of the orbit are roughly parallel and about 25 mm apart
– The lateral walls of the orbit are roughly 90° apart
– The angle where the lateral & medial walls meet at the orbital apex is roughly 45° so it resembles a pyramid
The orbit resembles a truncated pyramid. Its lateral wall, roof and floor are roughly triangular in shape while the medial wall is rectangular in shape.
▪ The base is the orbital margin, the orbital apex is by the optic canal
▪ the widest point of the orbit is 15 mm behind the orbital margin
(CO) 2. Name and describe the bones that make up the orbital roof, floor, lateral wall and medial wall and their associated foramina and landmarks
-Discuss important anatomical relationships of each wall of the orbit and be able to relate them to clinical situations where applicable
ORBITAL ROOF
-Triangular and formed by frontal bone (anterior) + lesser wing of sphenoid (posterior), mostly by orbital plate of frontal bone
-Frontal bone is thin where it separates orbit from anterior cranial fossa (prone to “blow-in fractures”)
-Anterior cranial fossa contains meninges of brain and frontal lobes (+ frontal sinus if sinus extends into orbital plate of frontal bone)
Clinical situations:
+ Penetrating wounds can fracture roof and injure frontal lobe
+ Parts of the roof can be absorbed in old age, bringing dura mater of frontal lobe in contact with periorbita
+ Orbital plate of frontal lobe may contain portion of frontal sinus so that an enlarging mucocele in the sinus can expand into the orbit through the roof
+ Trochlea lies superior-medially; fracture of superior orbital margin may damage/displace pulley leading to superior oblique paralysis
Contains:
*lacrimal fossa = depression in superior temporal part of frontal bone and houses lacrimal gland
*Fovea trochlearis (fossa) = “spot for the pulley”
Small medial depression on frontal bone (cartilaginous pulley for superior oblique)
*The lacrimal gland lies anterior and lateral in orbit adjacent to the roof
LATERAL WALL
-Triangular; formed by zygomatic (anterior third) and greater wing of sphenoid
-Thickest orbital wall
-Anteriorly, separates orbit from temporal region of skull (contains temporalis muscle)
-Posteriorly, greater wing separates lateral wall from middle cranial fossa (contains temporal lobe/meninges)
-Separated from roof by superior orbital fissure
Clinical situations:
+
Contains:
*lateral orbital tubercle = small elevation/bump on orbital surface of zygomatic; attachment site for several structures (check ligament of the lateral rectus muscle, lateral palpebral ligament, suspensory ligament of Lockwood, levator palpebrae superioris)
*zygomatico-orbital foramen = opening in zygomatic carrying zygomatic nerve/vessels
MEDIAL WALL
- Rectangular; formed by maxillary, lacrimal, lamina papyracea of ethmoid, body of sphenoid (A->P)
- Thinnest orbital wall
Clinical Sitatuations:
+ Infections in ethmoid air cells/sphenoid sinus spread to orbit causing orbital cellulitis
Contains:
* fossa for the lacrimal sac = bounded by anterior (maxillary)/posterior (lacrimal) lacrimal crest; contains lacrimal sac
* nasolacrimal canal = bony canal in maxilla continuous with fossa for lacrimal sac
ORBITAL FLOOR
-Triangular; formed by maxillary (largest part), zygomatic, palatine
Clinical Sitatuations:
+ Maxillary sinus infections can easily invade orbit leading to orbital cellulitis; tumor here can extend superiorly into orbit causing proptosis
+ Site of frequent fractures due to thinness and presence of infraorbital groove where the floor is thinnest (most common site for blow-out fractures)
Contains:
*infraorbital groove, running forward from inferior orbital fissure; becomes infraorbital canal anteriorly (with maxillary) -> opens onto the face below inferior orbital margin as infraorbital foramen (transmit infraorbital nerve/artery/vein)
(CO) 3. Describe which 2 walls of the orbit are the weakest (and state why/where)
The medial and inferior walls are the weakest, with the contents herniating into the ethmoid and maxillary sinuses respectively.
