Myofascial cone

The myofascial cone is a structure within the orbit involved in the movement and support of the globe, or more simply the eye. It also serves as the physical division between the intraconal and extraconal compartments, but the cone itself is included in the contents of the intraconal space ¹. 

Gross anatomy

The apex of the myofascial cone lies at the posterior aspect of the orbit, the orbital apex, taking origin from the annulus of Zinn from which it extends anteriorly as the four rectus muscles towards its base at the posterior section of the globe, covered in, and connected by the orbital fascia. 

The organization of myofascial cone allows for the rotational movements of the globe to occur on a fixed axis ¹. The space created by this complex arrangement of muscles and intervening connective tissue creates the intraconal space of about 30ml in volume, in contrast to the globe which is only 7ml ².

Muscles

The annulus of Zinn is a fibrocartilagenous structure encircling the optic canal and the medial aspect of the superior orbital fissure. Along its circumference the highly vascular rectus muscles arise; notably, the lateral rectus arises from the largest portion of the annulus, and therefore is the longest muscle and has a more crescentic appearance than the origin of the medial rectus ¹.

From the common origin of the ring, the rectus muscles continue as an apparently continuous muscular tube for a few millimeters before the muscle bellies can be appreciated separately ³. It is of clinical significance that the rectus muscles insert anteriorly, just beyond the coronal equator of the globe into the sclera; it is at this point the sclera is thinnest and should be inspected in suspected globe rupture.

Fascia

The periorbital fascia or periorbita is continuous with the loosely adherent periosteum of the bony orbit, reflects onto the optic nerve sheath and the extraocular muscles upon reaching the apex of the orbit. This fascia invests circumferentially into the perimysium of the extraocular muscles and extends between the lateral margins of the muscles to complete the conical structure of the myofascial cone. 

Within the confines of this intraconal space lies the orbital fat, which is extensively occupied by fibrous septa that intervene between structures passing through the intraconal space. It is this septal arrangement that is implicated in the clinical findings of upgaze abnormalities resulting from orbital floor fractures ³. It should be noted that veins pass within the septal planes, and arteries pass indiscriminately, piercing the planes to reach their destinations ⁴.