Introduction

The inferior rectus muscle is one of the seven extraocular muscles. It is primarily responsible for depressing the eye (downgaze). The inferior rectus is one of the four rectus muscles, including the superior rectus, the medial rectus, and the lateral rectus. There are also two oblique extraocular muscles, the superior and inferior obliques. The seventh extraocular muscle is the levator palpebrae superioris.

Structure and Function

With the head and eyes facing straight ahead, the eyes are said to be in primary gaze. From this position, an action from an extraocular muscle produces a secondary or tertiary action. Although the globe can be moved about 50 degrees from the primary position, usually during normal eye movement, only 15 degrees of extraocular muscle movement occurs before head movement begins.

The annulus of Zinn, which spans the superior orbital fissure, consists of superior and inferior tendons and is the common site of origin of the rectus muscles. The superior tendon is involved with the entire superior rectus muscle and portions of the medial and lateral rectus muscles. The inferior tendon is involved with the inferior rectus muscle and portions of the medial and lateral rectus muscles.

The primary action of the inferior rectus muscle is to depress the eye, causing the cornea and pupil to move inferiorly. The inferior rectus originates from the annulus of Zinn and courses anteriorly and laterally along the orbital floor, making an angle of 23 degrees with the visual axis. This angle causes the secondary and tertiary actions of the inferior rectus muscle to be adduction and extorsion (excycloduction).

Each extraocular muscle has a functional insertion point, which is at the closest point where the muscle first contacts the globe. This point forms a tangential line from the globe to the muscle origin, known as the arc of contact. The inferior rectus inserts in the vertical meridian, approximately 6.5 mm from the limbus.[1]

Embryology

The mesenchyme of the head, including the orbit and its structures, primarily arises in mesoderm and neural crest cells. The extraocular muscles originate from the mesoderm, but the satellite and connective tissue of the muscle derive from neural crest cells. Most of the remaining connective tissue of the orbit is also derived from neural crest cells.

Blood Supply and Lymphatics

The inferior rectus muscle receives blood primarily from the inferior muscular branch of the ophthalmic artery, with a secondary contribution from the infraorbital artery. The primary blood supply for all of the extraocular muscles is the muscular branches of the ophthalmic artery, the lacrimal artery, and the infraorbital artery. The two muscular branches of the ophthalmic artery are the superior and inferior muscular branches.

Venous drainage is similar to the arterial system and empties into the superior and inferior orbital veins. Usually, there are a total of four vortex veins, and these are found at the lateral and medial sides of the superior and inferior rectus muscles. These vortex veins drain into the orbital venous system.

Nerves

The lower division of cranial nerve III (oculomotor) innervates the inferior rectus muscle. Cranial nerve III is divided into upper and lower divisions, with the upper division innervating the superior rectus, levator palpebrae superioris, and the lower division innervating the medial rectus, inferior rectus, and inferior oblique muscles. The lateral rectus muscle is innervated by cranial nerve VI (abducens), and the superior oblique muscle is innervated by cranial nerve IV (trochlear).

Additionally, the parasympathetic innervation of the sphincter pupillae and ciliary muscle travels with the branch of the lower division of cranial nerve III that supplies the inferior oblique muscle, and this bundle passes near the inferior rectus muscle. This anatomy has important surgical considerations that will be explained further.[2]

Muscles

The inferior rectus and the superior rectus are vertical rectus muscles. The medial and lateral rectus muscles are the horizontal rectus muscles. Each rectus muscle originates posteriorly at the annulus of Zinn and courses anteriorly.

Each rectus muscle inserts on the globe at varying distances from the limbus, and the curved line drawn along the insertion points makes a spiral known as the Spiral of Tillaux. Starting at the medial aspect of the globe, the medial rectus inserts at 5.5 mm from the limbus, the inferior rectus inserts at 6.5 mm from the limbus, the lateral rectus inserts at 6.9 mm from the limbus, and the superior rectus at 7.7 mm from the limbus.

The inferior rectus is 9.8 mm wide at its insertion on the globe. The tendon is 7 mm in length, measured from the origin. The entire length of the muscle is 40 mm.

Extraocular muscles have a large ratio of nerve fibers to skeletal muscle fibers. The ratio is 1:3 to 1:5; other skeletal muscles have a ratio of 1:50 to 1:125. Extraocular muscles are a specialized form of skeletal muscle with various fiber types, including slow tonic types, which resist fatigue, and saccadic (rapid) type muscle fibers.[3]

Physiologic Variants

The size of the inferior rectus muscle and its insertion point on the globe from the limbus and other anatomical measurements may vary widely from one individual to the next. The numbers described in this article reflect average distances. Congenital differences in extraocular muscles can cause ocular misalignment.

Surgical Considerations

Because the parasympathetic fibers to the sphincter pupillae and ciliary muscle travel with the innervation to the inferior oblique, and the inferior oblique crosses the inferior rectus laterally, there are potential complications from surgery in this area. If the parasympathetic fibers are damaged during surgery in this area, pupillary abnormalities may result.

The inferior rectus also interacts with the lower eyelid via a fascial connection from its sheath. Weakening or recession of the inferior rectus may widen the palpebral fissure, and this can cause lower lid droop. Conversely, strengthening or resection of the inferior rectus may cause the fissure to narrow and elevate the lower lid.

The nerves to the rectus muscles and superior oblique muscles insert into the muscles at one-third the distance from the origin to the insertion. This makes damage to these nerves during anterior segment surgery difficult but not impossible. Additionally, instruments that are advanced 26 mm posterior to the rectus muscle insertions can cause injury to the nerve.

Blood vessels may be compromised during surgery of the inferior rectus muscle. The vessels which supply blood to the extraocular muscles also supply nearly all the temporal half of the anterior segment of the eye. Most of the nasal half of the anterior segment circulation also derives from blood vessels that supply the extraocular muscles. Therefore, care must be taken during surgery of the medial rectus or other extraocular muscles to avoid disrupting this blood supply.

Other complications may result from inferior rectus surgery, which also may result from other rectus muscle surgery. Unsatisfactory ocular alignment is the most common complication and may require additional surgery to correct. Refractive changes may occur when two rectus muscles of one eye are operated on, which may resolve over months. Other possible surgical complications include diplopia, perforation of the sclera, and postoperative infections. Although uncommon, serious infections may result after strabismus surgery, including pre-septal or orbital cellulitis and endophthalmitis.[4][5][6][7][8][9]

Clinical Significance

The function of the inferior rectus muscle can be assessed along with the other extraocular muscles during the clinical exam. The movement of the extraocular muscles can be assessed by having the patient look in nine directions starting with a primary gaze, followed by the secondary positions (up, down, left, and right) and the tertiary positions (up and right, up and left, down and right, down and left). The clinician can test these positions by having the patient follow the clinician's finger tracing a wide letter "H" in the air. 

Further testing of ocular alignment can be accomplished via several methods, including cover tests, corneal light reflex testing, dissimilar image tests, and dissimilar target tests. Since many patients with extraocular muscle abnormalities are young children, the clinician may need to employ various and clever means of testing, such as using toys or other objects to elicit the child's cooperation.

Strabismus, or ocular misalignment, can be caused by abnormalities in binocular vision or abnormalities of neuromuscular control. Weakness, injury, or paralysis that involves the inferior rectus muscle can be involved in strabismus.

Because the inferior rectus is near the orbital floor, orbital floor fractures may involve this muscle. Inferior rectus muscle paresis can result from trauma to the inferior rectus muscle or nerve. This can occur either at the time of initial injury or during surgical repair of the orbital floor. If inferior rectus muscle paresis is present without entrapment, the patient may display hypertropia in the primary position. If paresis is present with entrapment, the patient may have a slight deviation or hypotropia, which decreases with downgaze. The initial management of inferior rectus paresis is observation for six months. Muscle surgery may be recommended if there is no improvement during this time.[9][10]

Other Issues

The inferior rectus has a muscle capsule that binds to the muscle capsule of the inferior oblique. This fibrous connection is known as the Lockwood ligament and connects the lower eyelid retractors muscles.