finer control of fixation and in smooth and finely graded eye movements, particularly vergence control.
These novel properties of eye muscles lead to differential responses to local anesthetics and pharmaceuticals such as botulinum toxin and calcium channel blockers, as well as to disease processes such as myasthenia gravis and muscular dystrophy.
Finally, there is recent evidence for compartmentalization of rectus muscle innervation. For example, studies in primates and humans have shown distinct superior and inferior zones within the horizontal rectus muscles. The clinical significance of these observations is currently being investigated.
da Silva Costa RM, Kung J, Poukens V, Yoo L, Tychsen L, Demer JL. Intramuscular innervation of primate extraocular muscles: unique compartmentalization in horizontal recti. Inv Ophthalmol Vis Sci. 2011;52(5):2830–2836.
Orbital and Fascial Relationships
Within the orbit, a complex musculofibroelastic structure suspends the globe, supports the EOMs, and compartmentalizes the fat pads (Fig 3-5). In recent years, the interconnectedness of the orbital tissues, as well as its extent and complexity, has come to light. The intense fibrous connections throughout the orbit can be illustrated clinically by the consequences of tissue entrapment in blowout fractures and of post–retrobulbar hemorrhage fibrosis of delicate fibrous septa. This topic remains under investigation.
Figure 3-5 The muscle cone contains 1 fat pad and is surrounded by another; these 2 fat pads are separated by the rectus muscles and intermuscular septum. Note that the intermuscular septum does not extend all the way back to the apex of the
orbit. (Modified with permission from Yanoff M, Duker J, eds. Ophthalmology. 2nd ed. London: Mosby; 2004:553.)
Adipose Tissue
The eye is supported and cushioned within the orbit by a large amount of fatty tissue. External to the muscle cone, fatty tissue comes forward with the rectus muscles, stopping about 10 mm from the limbus. Fatty tissue is also present inside the muscle cone, kept away from the sclera by the Tenon capsule (see Fig 3-5).
Muscle Cone
The muscle cone lies posterior to the equator. It consists of the EOMs, their sheaths, and the intermuscular septum. High-resolution magnetic resonance imaging (MRI) has shown that the muscle cone does not extend back to the orbital apex; rather, it ends in the area of the globe–optic nerve junction.
Muscle Capsule
Each rectus muscle has a surrounding fascial capsule that extends with the muscle from its origin to its insertion. These capsules are thin posteriorly, but near the equator they thicken as they pass through the sleeve of the Tenon capsule, continuing anteriorly with the muscles to their insertions. Anterior to the equator, between the undersurface of the muscle and the sclera, there is almost no fascia, only connective tissue footplates that connect the muscle to the globe. The smooth, avascular surface of the muscle capsule allows the muscles to slide easily over the globe.
The Tenon Capsule
The Tenon capsule (fascia bulbi) is the principal orbital fascia and forms the envelope within which the eyeball moves (Fig 3-6). The Tenon capsule fuses posteriorly with the optic nerve sheath and anteriorly with the intermuscular septum at a position 3 mm from the limbus. The posterior portion of the Tenon capsule is thin and flexible, allowing for free movement of the optic nerve, ciliary nerves, and ciliary vessels as the globe rotates, while separating the orbital fat inside the muscle cone from the sclera. At and just posterior to the equator, the Tenon capsule is thick and tough, suspending the globe like a trampoline by means of connections to the periorbital tissues. The global layer of the 4 rectus muscles penetrates this thick fibroelastic tissue approximately 10 mm posterior to their insertions. The oblique muscles penetrate the Tenon capsule anterior to the equator. The Tenon capsule continues forward over these 6 EOMs and separates them from the orbital fat and structures lying outside the muscle cone.
Figure 3-6 A, Anterior and posterior orifices of the Tenon capsule shown after enucleation of the globe. B, The Tenon space
shown by injection with India ink. (Modified with permission from von Noorden GK, Campos EC. Binocular Vision and Ocular Motility: Theory and Management of Strabismus. 6th ed. St Louis: Mosby; 2002:45.)
Pulley System
The 4 rectus muscles are surrounded by fibroelastic pulleys that maintain the position of all of the EOMs relative to the orbit. The pulleys consist of collagen, elastin, and smooth muscle, allowing them to contract and relax. Dynamic MRI studies show that, in some cases, the pulleys act mechanically as the rectus muscle origins. The pulleys may also serve to stabilize the muscle path, preventing sideslipping or movement perpendicular to the muscle axis (Fig 3-7). Anteriorly, the
pulleys merge with the intermuscular septum, which fuses with the conjunctiva 3 mm posterior to the limbus. The posterior section of the intermuscular septum separates the intraconal fat pads from the extraconal fat pads. Numerous extensions from all of the EOM sheaths attach to the orbit and help support the globe.
Figure 3-7 Structure of orbital connective tissues. IO, inferior oblique; IR, inferior rectus; LPS, levator palpebrae superioris; LR, lateral rectus; MR, medial rectus; SO, superior oblique; SR, superior rectus. The 3 coronal views are represented at the
levels indicated by arrows in horizontal section. (Modified with permission from Demer JL, Miller JM, Poukens V. Surgical implications of the rectus extraocular muscle pulleys. J Pediatr Ophthalmol Strabismus. 1996;33(4):208–218.)
The inferior oblique muscle originates inferonasally on the maxillary bone near the orbital rim, adjacent to the anterior lacrimal crest, and it continues laterally, entering its connective tissue pulley inferior to the inferior rectus muscle, at the site where the inferior oblique muscle also penetrates the Tenon capsule. The inferior oblique pulley and inferior rectus pulley join to form the Lockwood ligament (Fig 3-8). Attached to the conjoined inferior oblique and inferior rectus pulley complex is the dense NFVB containing the inferior oblique motor nerve.
Figure 3-8 Attachments of the upper and lower eyelids to the vertical rectus muscles. (Modified with permission from Buckley EG,
Freedman S, Shields MB, eds. Atlas of Ophthalmic Surgery, Vol III: Strabismus and Glaucoma. St Louis: Mosby-Year Book; 1995:15.)
The active pulley hypothesis proposes that the pulley positions are shifted by the contraction of the orbital layer against the elasticity of the pulley suspension. This concept remains controversial: whether there is actual innervational control of the pulleys is still debated. However, high-resolution MRI scans have shown that the pulleys are located only a short distance from the globe center; therefore, small shifts in pulley location would confer large shifts in EOM pulling direction. Normal pulleys shift only slightly in the coronal plane, even during large ductions. Heterotopy (malpositioning) of the rectus pulleys may cause some cases of incomitant strabismus and A or V patterns, and these anomalies can mimic oblique muscle dysfunction by misdirecting the forces of the rectus muscles (see Chapters 10 and 11). Bony abnormalities such as those seen with craniosynostosis can also alter the direction of pull of rectus muscles by causing malpositioning of the pulleys. Posterior fixation sutures (fadenoperation; posterior fixation of an EOM to the underlying sclera) were designed to reduce an EOM’s effect in its field of action by decreasing the arc of contact and rotational lever arm. However, MR imaging suggests that the actual effect of the fadenoperation is to hinder the posterior shift of the contracting EOM pulley, mechanically restricting EOM action.
The pulley model and its implications have been challenged by other high-resolution MRI studies, which show that, during eye movements into eccentric fields, the posterior portions of rectus muscles shift. These findings are consistent with the more traditional model of eye muscle function, which is the “restrained shortest-path model.”
Demer JL. Mechanics of the orbita. Dev Ophthalmol. 2007;40:132–157.