Orbit Anatomy

BONY ORBIT
The bony orbits are quadrangular truncated pyramids
situated between the anterior cranial fossa above and
the maxillary sinuses below (Fig. 16.1). Each orbit is
about 40 mm in height, width and depth and is formed
by portions of seven bones : (1) frontal, (2) maxilla,
(3) zygomatic, (4) sphenoid, (5) palatine, (6) ethmoid
and (7) lacrimal. It has four walls (medial, lateral,
superior and inferior), base and an apex.
The medial walls of two orbits are parallel to each
other and, being thinnest, are frequently fractured
during injuries as well as during orbitotomy
operations and, it also accounts for ethmoiditis being
the commonest cause of orbital cellulitis.
The inferior orbital wall (floor) is triangular in shape
and being quite thin is commonly involved in blowout
fractures and is easily invaded by tumours of the
maxillary antrum.
The lateral wall of the orbit is triangular in shape. It
covers only posterior half of the eyeball. Therefore,
palpation of the retrobulbar tumours is easier from
this side. Because of its advantageous anatomical
position, a surgical approach to the orbit by lateral
orbitotomy is popular.
The roof is triangular in shape and is formed mainly
by the orbital plate of frontal bone.
Base of the orbit is the anterior open end of the
orbit. It is bounded by thick orbital margins.
The orbital apex (Fig. 16.2). It is the posterior end of
orbit. Here the four orbital walls converge. It has two
orifices, the optic canal which transmits optic nerve
and ophthalmic artery and the superior orbital fissure
which transmits a number of nerves, arteries and veins
(Fig. 13.2).
ORBITAL FASCIA
It is a thin connective tissue membrane lining various
intraorbital structures. Though, it is one continuous
tissue, but for the descriptive convenience it has been
divided into fascia bulbi, muscular sheaths,
intermuscular septa, membranous expansions of the
extraocular muscles and ligament of Lockwood.
Fascia bulbi (Tenon’s capsule) envelops the globe
from the margins of cornea to the optic nerve. Its
lower part is thickened to form a sling or hammock on
which the globe rests; this is called ‘suspensory
ligament of Lockwood’.
CONTENTS OF THE ORBIT
The volume of each orbit is about 30 cc.
Approximately one-fifth of it is occupied by the
eyeball. Other contents of the orbit include: part of
optic nerve, extraocular muscles, lacrimal gland,
lacrimal sac, ophthalmic artery and its branches, third,
fourth and sixth cranial nerves and ophthalmic and
maxillary divisions of the fifth cranial nerve,
sympathetic nerve, orbital fat and fascia.
SURGICAL SPACES IN THE ORBIT
These are of importance as most orbital pathologies
tend to remain in the space in which they are formed.
Therefore, their knowledge helps the surgeon in
choosing the most direct surgical approach. Each orbit
is divisible into four surgical spaces (Fig. 16.3).
1. The subperiosteal space. This is a potential space
between the bone and the periorbita (periosteum).
2. The peripheral space. It is bounded peripherally
by the periorbita and internally by the four recti
with thin intermuscular septa. Tumours present
here produce eccentric proptosis and can usually
be palpated. For peribulbar anaesthesia, injection
is made in this space.
3. The central space. It is also called muscular cone
or retrobulbar space. It is bounded anteriorly by
the Tenon’s capsule lining back of the eyeball
and peripherally by the four recti muscles and
their intermuscular septa in the anterior part. In
the posterior part, it becomes continuous with
the peripheral space. Tumours lying here usually
produce axial proptosis. Retrobulbar injections
are made in this space.
4. Tenon’s space. It is a potential space around
the eyeball between the sclera and Tenon’s
capsule.