Anatomy of Extraocular Muscles and Their Fascia 1

There are six extraocular muscles of the eyeball: the four recti muscles and the two oblique muscles. The extraocular muscles develop from three different masses of primordial cells. In the presomite embryo, cells proliferate laterally from a central mass located at the anterior end of notochord. In the 25 day-old embryo a pair of premandibular condensations can be seen, which give rise to the group of eye muscles (superior, inferior and medial recti and inferior oblique) later innervated by the oculomotor nerve. The lateral rectus and superior oblique muscles arise from its own adjacent tissue mass in the maxillo-mandibular mesoderm. At 1 month the ocular motor nerves reach their respective muscles. The muscle striations appear early in the 2nd month. The trochlea forms at 6 weeks.

Horizontal rectusmuscles: The horizontal rectus muscles are the medial rectus and the lateral rectus muscles. They arise from the annulus of Zinn, courses anteriorly along the medial and lateral orbital walls, and insert 5.5mm and 6.9mm from the limbus, respectively. The medial and lateral recti have only horizontal actions, with the medial rectus being an adductor and the lateral rectus being an abductor.

Vertical rectus muscles: The vertical rectus muscles are the superior rectus and inferior rectus muscles. The superior rectus muscle originates from the annulus of 2Zinn, courses anteriorly, upwards over the eyeball and laterally forming an angle of 23° with the visual axis of the eye in the primary position. It inserts 7.7mm from the limbus. The inferior rectus muscle arises from the annulus of Zinn, courses anteriorly, downwards, and laterally along the floor of orbit, forming an angle of 23° with the visual axis of the eye in the primary position. It inserts 6.5mm from the limbus.

Oblique muscles: The superior oblique muscle originates from the annulus of Zinn and passes anteriorly and upwards along the superomedial wall of the orbit, becoming tendinous before passing through the trochlea located on the nasal side of the superior orbital rim. The tendon is then reflected inferiorly, posteriorly and laterally, forming an angle of 51° with the visual axis of the eye in the primary position. It inserts in the posterosuperior quadrant of the eyeball, almost or entirely lateral to the mid-vertical plane or center of rotation, passing inferior to the superior rectus muscle. The primary action of the superior oblique muscle is incycloduction or intorsion and the secondary actions are depression and abduction.

Blood supply ofthe extraocular muscles: The most important blood supply is from the medial and lateral branches of the ophthalmic artery. The lateral muscular branch supplies the lateral rectus, superior rectus and superior oblique muscles; the medial muscular branch (the larger one) supplies the inferior rectus, medial rectus and inferior oblique muscles.

Horizontal recti are purely horizontal movers around the vertical axis and they have a primary action only.

Spiral of Tillaux: The rectus muscle tendons insert progressively further from the limbus in the order of medial rectus, inferior rectus, lateral rectus and superior rectus. By drawing a continuous curve which passes through these insertions, a spiral is formed. This is known as the Spiral of Tillaux.

Extraocular muscle is a specialized form of skeletal muscle.They have certain differences from typical skeletal muscles. For example, not all fiber types propagate action potentials and highly fatigue-restricted fiber types also may have fast twitch characteristics. Thus, the traditional fiber type classifications can not be applied to them. The rectus and oblique muscles exhibit two distinct regions, each with different fiber content:

The extraocular muscle fiber classification system resolves six fiber types:

Tenon’s capsule: This is composed of elastic connective tissue and is attached to the optic nerve posteriorly and becomes fused with the intermuscular membrane 3mm from the limbus to which it is attached anteriorly. Posterior to the equator, it is a fibrous condensation that separates the orbital fat inside the muscle cone from the sclera, thereby keeping that compartment free from fat. Anterior to the equator, it extends forwards over the muscles and separates them from the 0rbital fat and structures lying outside the muscle cone. All the extraocular muscles penetrate the Tenon’s capsule.

Muscle cone: This lies posterior to the equator. It is composed of the extraocular muscle, the extraocular muscle sheaths, and the intermuscular membrane. The muscle cone extends posteriorly up to the annulus of Zinn.

Muscle sheath: The rectus muscles have a surrounding fascial sheath that extends with the muscle from its origin to its insertion.7

Intermuscular septum: Each of the rectus muscles sends an extension to the adjacent rectus muscle sheath. This is a thin elastic tissue. This is called the intermuscular membrane and it joins the rectus muscles at all times in the muscle cone except where they penetrate Tenon’s capsule. It becomes fused with Teton’s capsule 3mm from the limbus.

Check ligaments: They extend from the muscle sheath, anterior to the equator, pass through the Tenon’s capsule and insert on the corresponding orbital walls. They function as support structures for the eyeball and surrounding tissue. They limit eye movements only when they are scarred. They are slightly more developed for the horizontal rectus muscles.

Ligament of lockwood: The muscle sheath of the inferior oblique muscle is connected to muscle sheath of the inferior rectus muscle. This fusion is called the Ligament of Lockwood.

Adipose tissue: The eye is supported and cushioned by a large amount of fatty tissue in the orbit. External to the muscle cone, the fatty tissue comes forward with the rectus muscles, stopping about 10 mm from the limbus. The fatty tissue is also found inside the muscle cone, kept away from the sclera by the Tenon’s capsule. If Tenon’s capsule is torn or cut, fat can prolapse inside Tenon’s capsule and may form firm adhesion to sclera, muscle, intermuscular membrane, and/or conjunctiva.

Axes of Fick and Listing’s plane:

Axes of Fick-

The axes of Fick are designated as x, y and z.

X-axis – It is a transverse axis passing through the center of the eye at the equator. Voluntary vertical rotations of the eye occur about this axis.

Y-axis – It is a sagittal axis passing through the pupil. Involuntary torsional rotations occur about this axis.

Z-axis – It is a vertical axis. Voluntary horizontal rotations occur about this axis.

Listing’s Planes–

The position of each eye in the orbit without any innervation to the extraocular muscles is the position of rest. The position of each eye is slightly divergent in normal positions.

An individual motor nerve fiber and its several muscle fibers is a motor unit.

It is the process by which more and more motor units of a particular muscle are activated and brought into play by the brain to help pull the eye in the direction of action of that muscle. As the eye fixates further in the field of action of that muscle, the frequency of activity of each motor unit increases until it reaches a peak.

Saccadic movements need a sudden, strong pulse of force from the muscle in order to move the eye rapidly against the viscosity produced by the fatty tissue and fascia in which the globe lies. The velocity of the saccadic movement and the high forces that must be produced are affected by muscle paresis, and study of saccadic velocity help in determining paresis of muscles and abnormal innervation.

Ductions are monocular rotations of the eye.

Muscles used in monocular eye movement:

It states that increased innervation and contraction of a given muscle are accompanied by a reciprocal decrease in innervation and contraction of its antagonist.

Example: As the left eye adducts, the left medial rectus muscle receives increased innervation, while the left lateral rectus receives decreased innervation.

When the binocular eye movements are conjugate and the eyes move in the same direction, they are called versions.

A pair of muscles (one in each eye) that are the prime movers of their respective eyes in the cardinal positions of gaze.

It states that equal and simultaneous innervation flows to synergistic muscles concerned with the desired direction of gaze.

When the eye movements are disjugate and the eyes move in the opposite directions, such movements are called vergences.

The constant innervational tone to the extraocular muscles when a person is awake and alert. The convergence tone is necessary in the awake state to maintain straight eyes.

The accommodative convergence of the visual axes occurs as part of the synkinetic near reflex. A fairly consistent increase of accommodative convergence(AC) occurs for each diopter of accommodation(A). The ratio between AC and A i.e., AC/A ratio is important. With an abnormally high AC/A ratio, the excess convergence tends to produce esotropia during accommodation on near targets. An abnormally low AC/A ratio will tend to make the eyes exotropic when the person looks at near targets.

There are five supranuclear eye movement systems.

This is a line joining the anterior and posterior poles of the eye.

This is a line on which the main optical components (Cornea and lens) are centered. It passes through the nodal point of the eye. It coincides more or less with the geometric axis.

This is the optical center of the eye. It lies on the optical axis slightly in front of the center of the eyeball and is adjacent to the posterior surface of the lens.

This is the point around which the eye rotates. It is situated slightly behind the centers of the eye on the optical axis line. It lies behind the nodal point.

This is a line passing from the object of fixation to the fovea. It passes through the nodal point.

This line is perpendicular to the corneas and passes through the center of the pupil to reach the nodal point of the eye. It may be regarded as equivalent to the optical axis.13

This is a line passing from the object of fixation to the center of rotation of the eye.

It is angle formed between the optical axis and the visual axis at the nodal point. The optical axis sometimes coincides with the visual axis and hence angle alpha is zero. Usually, the visual axis passes on the nasal side of the optical axis and then the term used is a positive angle alpha. Rarely, the visual axis passes on the temporal side of the optical axis and then the term used is a negative angle alpha.

It is the angle formed between the optical axis and the fixation axis at the center of rotation of the eye. This angle has similar positive and negative values like angle alpha.

It is the angle formed by the pupillary axis and the visual axis at the center of the pupil. An angle kappa is caused by the failure of the optical axis of the eye and the visual axis to coincide.

It may be present thus simulating a hyperdeviation. It is caused by superior or inferior displacement of the macula from scar tissue.