Normal Anatomy and Anomalies of the Rectus Extraocular Muscles in Human: A Review of the Recent Data and Findings

Abstract

Development of modern surgical techniques is associated with the need for a thorough knowledge of surgical anatomy and, in the case of ophthalmologic surgery, also functional aspects of extraocular muscles. Thus, the leading idea of this review was to summarize the most recent findings regarding the normal anatomy and anomalies of the extraocular rectus muscles (ERMs). Particular attention was paid to the presentation of detailed and structured data on the gross anatomy of the ERMs, including their attachments, anatomical relationships, vascularization, and innervation. This issue of ERMs innervation was presented in detail, considering the research that has recently been carried out on human material using advanced anatomical techniques such as Sihler's technique of the nerves staining. The text was supplemented with a carefully selected graphic material (including anatomical specimens prepared specially for the purpose of this review) and discussion of the clinical cases and practical significance of the presented issues.

Figure 1

Arrangement of the extraocular rectus muscles (ERMs) around the equator of the eyeball. 1: the distance of the medial rectus muscle (mr) from the limbus; 2: the distance of the inferior rectus muscle (ir) from the limbus, 3: the distance of the lateral rectus muscle (lr) from the limbus; 4: the distance of the superior rectus muscle (sr) from the limbus. The results of measurements made by various authors on individual ERMs insertions slightly vary. However, it may be observed that the distance from the lateral rectus muscle insertion to the limbus is greater than the distance from the medial rectus muscle to the limbus, as well as the distance from the lateral rectus muscle insertion to the limbus is greater than the distance from the medial rectus muscle to the limbus. According to the classical data provided by Fuchs [14] the distances of the individual rectus muscles from the limbus increase counterclockwise starting from the medial rectus muscle.

Figure 2

Anatomical relationships within the orbit. The origin of the medial rectus muscle (mr) is fixed to the medial aspect of the common tendinous ring. On its further course, the muscle runs parallel to the medial orbital wall. The inferior rectus muscle (ir) runs perpendicularly to the inferior wall of the orbit. The anterior part of the muscle is separated from the orbit by the inferior oblique muscle (io). All three muscles (mr, ir, and io) are innervated by the inferior division of the oculomotor nerve (id). The nerve to the inferior oblique muscle (nio) branches off from the inferior division of the oculomotor nerve and runs parallel to the lateral border of the inferior rectus muscle. A characteristic feature of the lateral rectus muscle (lr) is the so-called “dual headed origin,” as this muscle begins from the common tendon ring with two tendinous bands. The muscle runs along the lateral wall of the orbit. It is innervated by the abducens nerve (an). The ciliary ganglion (cg) is located medially to the internal surface of the muscle. The superior rectus muscle (sr) is located under the levator palpebrae superioris muscle (lps). Both muscles (sr and lps) are innervated by the superior division of the oculomotor nerve (sd). fn: frontal nerve; ln: lacrimal nerve; ncn: nasociliary nerve; son: supraorbital nerve; stn: supratrochlear nerve, th: troch; ear nerve.

Figure 3

Specimen, in which the nerve to the inferior oblique muscle (nio) pierces the inferior rectus muscle (ir). Lateral view to the left eyeball. Lateral rectus muscle was removed, only the insertion of the lateral rectus (lr) is visible. When the nerve to the inferior oblique pierces the inferior rectus muscle, its susceptibility to injury associated with stretching of the muscle during surgical procedures seems to be greater. sr: superior rectus muscle; io: inferior oblique muscle.

Figure 4

Superior (sr) and inferior (ir) rectus muscles connected to each other by muscular bands (mb). Such muscular bands, taking origin (marked by black arrowhead) from the common tendinous ring and attached to rectus muscles are referred to as accessory rectus muscles. The characteristic feature of accessory rectus muscles is that they are located laterally to the optic nerve (on). Sagittal section of the specimen taken from the right orbit. id: inferior division of the oculomotor nerve; mr: medial rectus muscle. This figure is a modification of the drawing taken from Haładaj et al. [43] under the terms of the Creative Commons Attribution 4.0 International License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium.

Figure 5

General arrangement of vascularization and innervation of extraocular rectus muscles. Arteries have been injected by red resin to expose muscular branches reaching individual muscles. Specimen taken from the right orbit. Scale bar shows 10 mm. an: abducens nerve; id: inferior division of the oculomotor nerve; ir: inferior rectus muscle; lr: lateral rectus muscle; mr: medial rectus muscle; nio: nerve to the inferior oblique muscle; sd: superior division of the oculomotor nerve; sr: superior rectus muscle.

Figure 6

The course and arrangements of the intramuscular nerves within the extraocular rectus muscles based on Sihler's stain. The terminal plexus (tp) has been marked. Scale bar shows 10 mm. (a) Specimen of the right lateral rectus muscle; (b) specimen of the right medial rectus muscle; (c) specimen of the right superior rectus muscle; (d) specimen of the right inferior rectus muscle; lps: muscular branches to the levator palpebrae superioris muscle wrapping around the medial border of the superior rectus muscle; nio: nerve to the inferior oblique muscle travelling along the lateral border of the inferior rectus muscle; In: inferior direction; La: lateral direction; Me: medial direction; Su: superior direction.