The cavity of the orbit has a truncated pyramid shape, with a posterior
apex, and a base corresponding to the anterior aperture. The orbit contains
mainly adipose tissue, and the globe is suspended in the anterior part. The
four rectus muscles of the eye insert anteriorly near the equator of the
globe (Figure 21–1). Posteriorly, they insert together at the apex
on the tendineus anulus communis of Zinn, through which the optic nerve
enters the orbit. The four rectus muscles delineate the retrobulbar cone,
which is not sealed by any intermuscular membrane.14–17
Sensory innervation is supplied by the ophthalmic nerve (first branch of the
trigeminal nerve [V]), which passes through the muscular cone
(Figure 21–2). The trochlear nerve (IV) provides motor control to
the superior oblique muscles, the abducens nerve (VI) to the lateral rectus
muscle, and the oculomotor nerve (III) to all other extraocular muscles. All
but the trochlear nerve pass through the muscular conus. Injection of LA
solution inside the cone will provide anesthesia and akinesia of the globe
and the extraocular muscles. Only the motor nerve to the orbicularis muscle
of the eyelids has an extraorbital course, coming from the superior branch
of the facial nerve (VII). Many major structures are located within the
muscular conus and are therefore at risk of needle and injection injury.
These include the optic nerve with its meningeal coverings; blood vessels of
the orbit; and the autonomic, sensory, and motor innervation of the globe.
For this reason, some authors advise that introduction of the needle into
the muscular cone be avoided and suggest that needle insertion be limited to
the extraconal space.18,19 However, posterior to the equator of the globe, the extraconal space
is only a virtual space, because the rectus muscles are in contact with the
bone walls of the orbit.