Examination of the Annular Tendon (Annulus of Zinn) as a Common Origin of the Extraocular Rectus Muscles: 2. Embryological Basis of Extraocular Muscles Anomalies

Numerical Aberrations of the Extraocular Muscles and the Levator Palpebrae Superioris: An Anatomical and Clinical Insight

PURPOSE

To comprehensively review the literature about numerical aberrations of the orbital muscles of ocular motility (here referred to as extraocular muscles [EOMs]) and the levator palpebrae superioris (LPS).

METHODS

The authors summarize the embryologic bases and the possible etiopathogenetic causes of numerical aberrations of the EOMs and the LPS and organize these lesions into several broad categories. The clinical and radiologic diagnostic challenges are discussed.

RESULTS

Numerical aberrations of the EOMs include: 1) the complete absence of EOMs, 2) duplication of an entire EOM, 3) the presence of muscle bands that connect 2 EOMs, and 4) minor morphological variations such as bifid muscles (partial splitting of the muscle). Some cases may defy categorization into any of the above or may resemble atavistic remnants of the retractor bulbi muscle. Broadly speaking, numerical aberrations of the LPS generally fall into the same categories although the LPS has several peculiar numerical anomalies of its own.

CONCLUSIONS

Although numerical EOM and LPS variations are relatively rare and of little clinical significance, raising awareness about their presence is a fundamental clinical keystone not just for the strabismus surgeon but for the orbital surgeon as well. During orbital surgery, this may spare the surgeon from pursuing an orbital witch hunt for these benign innocuous accessory orbital structures that were accidentally discovered by the radiologist and misinterpreted as sinister etiologies. For the strabismus surgeon, the failure to identify them may result in an unfavorable surgical outcome if these structures are missed because of a lack of awareness despite being responsible for generating complex strabismus patterns or having a restrictive potential of their own.

Clinical anatomy of the spina musculi recti lateralis: A frequently overlooked variation of the greater wing of the sphenoid

BACKGROUND

The spina musculi recti lateralis (SMRL) is often visible along the lateral rim of the superior orbital fissure (SOF). Aim of this study is to characterize SMRL morphology and topography relative to known bony landmarks.

METHODS

Orbits from 291 adult dry skulls and from 60 CT scans were analyzed to measure the distance between the SMRL and the SOF or the inferior orbital fissures (IOF) as well as its height, width and orientation. Processes other than SMRLs were also recorded. Fetal skulls were observed for comparison with adult samples.

RESULTS

Forty-one per cent of orbits on dry skulls and 43.3% by CT showed an SMRL. Additional 32.9% of orbits on dry skulls had processes with a different shape. On average, SMRL were orientated almost along the transverse plane and showed implant bases as wide as 141.9° or as narrow as 36.8°. SMRLs were close to the infero-posterior angle of the orbital plate of the sphenoid, 1.21 ± 0.84 mm in front of the SOF, 5.8 ± 1.9 mm above the IOF and 12 ± 2.3 mm from the anterior end of the SOF. They were 1.58 ± 0.64 mm high and did not show any age or sex-related prevalence. By CT, the SMRL appeared as the insertion site for the lateral rectus, tendinous ring and, sometimes, inferior rectus.

CONCLUSIONS

The SMRL is a process of the sphenoidal orbital plate rather than of the SOF. It is also a reliable landmark for the insertion of the tendinous ring and lateral rectus. Orbital surgeons should be aware of this common variant of the orbital apex.

Changing the topographical anatomy among the maxilla, palatine bone, and greater palatine nerve: a histological study using human fetuses

PURPOSE

The palatine bone (PAL) rides over the maxilla (MX) without an end-to-end suture in the bony palate of fetuses. However, changes in the topographical relationship among bones was unknown at and along the pterygopalatomaxillary suture, including the palatine canals.

METHODS

Using sagittal, frontal, and horizontal histological sections of the head from 15 midterm fetuses to 12 near-term fetuses, we depicted the changes in the topographical anatomy of the MX, PAL, and greater palatine nerve (GPN).

RESULTS

In the bony greater palatine canal of these fetuses, the medial and posterior walls facing the GPN were consistently made up of the PAL. At midterm, the entire course of the GPN was embedded in the PAL (six fetuses), or the MX contributed to the lateral wall of the nerve canal (nine). At near-term, the anterior and lateral walls showed individual variations: an MX in the anterior and lateral walls (three fetuses), an anterior MX and a lateral PAL (five), an anterior PAL and a lateral MX (two), and a PAL surrounding the GPN (four).

CONCLUSION

These increasing variations suggested that the pterygopalatomaxillary suture was actually growing and that the PAL transiently expanded anteriorly and/or laterally to push the MX in fetuses. The "usual" morphology in which the GPN is sandwiched by the MX and PAL is likely established after birth, possibly during adolescence. The driving force of this change may not be produced by the masticatory apparatus. Rather, it might be triggered by the growing maxillary sinus.

Supernumerary Extraocular Muscle: A Rare Cause of Atypical Restrictive Strabismus

Background and objectives: Supernumerary extraocular muscle (SEOM) is extremely rare. The purpose of this paper was to review the clinical characteristics and surgical outcomes of SEOM patients with atypical restrictive strabismus. Materials and Methods: A retrospective review was conducted on the data from 12 SEOM cases. Pre- and post-operative measurements consisted of visual acuity, cycloplegic refraction, ocular alignment, ocular motility, binocular vision, and imaging. Management strategies included either conservative or surgical treatments. Results: Of the 12 cases reviewed (seven females, five males), the mean ± SD age was 14.3 ± 10.6 years (range: 4-38 years). The right eye was affected in six cases, the left in five, and both eyes in one case. The major clinical manifestations included restrictive ocular motility (12 cases), with seven cases in no less than three directions; varying degrees of horizontal or vertical strabismus; ipsilateral amblyopia (10 cases); and unequal palpebral aperture (10 cases). Imaging results revealed muscular bands originating from the annulus of Zinn and insertion into the globe or other recti, as well as anomalous muscular bands connecting two or more recti, sometimes with optic nerve involvement. Three patients received conservative treatment, while rectus recession with or without resection (seven patients) or rectus disinsertion plus globe fixation (two patients) were performed in those receiving surgical treatments. A surgical success was achieved in four cases. Conclusions: For restrictive strabismus, imaging plays an important role in the diagnosis of SEOM. When the SEOM is difficult to resect, a personalized surgical strategy may be required to achieve a good ocular alignment.

Optic nerve-associated connective tissue structures revisited: a histological study using human fetuses and adult cadavers

Unlike the usual peripheral nerve, the optic nerve accompanies a thick "dural sheath," a thin "sheath of pia mater" (SPM), and multiple "septa," which divides the nerve fibers into fascicles. We collected specimens from 25 adult cadavers and 15 fetuses and revisited the histological architecture of the optic and oculomotor nerves. In the optic chiasma, the meningeal layer of the dura joins the pia to form a thick SPM, and the periosteum of the sphenoid is continuous with the dural sheath at the orbital exit of the bony optic canal. The septa appeared as a cluster of irregularly arrayed fibrous plates in the intracranial course near the chiasma. Thus, the septa were not derived from either the SPM or the dural sheath. In the orbit, the central artery of the retina accompanies collagenous fibers from the dural sheath and the SPM to provide the vascular sheath in the optic nerve. These connective tissue configurations were the same between adult and fetal specimens. At the optic disk, the dural sheath and SPM merged with the sclera, whereas the septa appeared to end at the lamina cribrosa. However, in fetuses without lamina cribrosa, the septa extend into the nerve fiber layer of the retina. The SPM and septa showed strong elastin immunoreactivity, in contrast to the absence of reactivity in the sheaths of the oculomotor nerve. Each S100 protein-positive Schwann sheath of the oculomotor nerve was surrounded by collagenous endoneurium. Glial fibrillary acidic protein-positive astrocytes showed a linear arrangement along the septa. This article is protected by copyright. All rights reserved.

Topographical anatomy of the annulus of Zinn

The anatomy and even existence of a common tendinous origin of the extraocular eye muscles, or annulus of Zinn, has widely been debated in anatomical literature. This study explored the anatomical origins of the recti muscles, their course into the orbit and the dural connections of the common tendinous origin with the skull base. Twenty orbits of ten adult human cadavers were dissected. The orbital apex and its dural connections were photographed. Histological examination of apical specimens was performed. In all cadavers, extraocular muscles were observed to have a common tendinous origin at the orbital apex, continuous with dural connections extending into the skull base. Accessory slips of the medial rectus were observed across all cadavers. Dual heads of the lateral rectus were observed in fourteen orbits of seven cadavers. The origin of the levator palpebrae superioris appeared to be contiguous with the superior rectus at the common tendinous origin in all but one cadaver. These results support the existence of a common tendinous origin of the extraocular muscles, that is continuous with the skull base dura. In addition, they support the existence of variations in orbital anatomy including dual or accessory muscle slips of the extraocular muscles.

Association between the developing sphenoid and adult morphology: A study using sagittal sections of the skull base from human embryos and fetuses

The developing sphenoid is regarded as a median cartilage mass (basisphenoid [BS]) with three cartilaginous processes (orbitosphenoid [OS], ala temporalis [AT], and alar process [AP]). The relationships of this initial configuration with the adult morphology are difficult to determine because of extensive membranous ossification along the cartilaginous elements. The purpose of this study was therefore to evaluate the anatomical connections between each element of the fetal sphenoid and adult morphology. Sagittal sections from 25 embryos and fetuses of gestational age 6-34 weeks and crown-rump length 12-295 mm were therefore examined and compared with horizontal and frontal sections from the other 25 late-term fetuses (217-340 mm). The OS was identified as a set of three mutually attached cartilage bars in early fetuses. At all stages, the OS-post was continuous with the anterolateral part of the BS. The BS included the notochord and Rathke's pouch remnant in embryos and early fetuses. The dorsum sellae was absent from embryos, but it protruded from the BS in early fetuses before a fossa for the hypophysis became evident. Although not higher than the hypophysis at midterm, the dorsum sellae elongated superiorly after gestational age 25 weeks. In early fetuses, the AP was located on the side immediately anterior to the otic capsule. The AT developed on the side immediately posterior to the extraocular rectus muscles. At late term, the greater wing was formed by membranous bones from the AT and AP. The AT and AP formed a complex bridge between the BS and the greater wing. A small cartilage, future medial pterygoid process (PTmed) was located inferior to the AT in early fetuses. At midterm, one endochondral bone and multiple membranous bones formed the PTmed. The lateral pterygoid process (PTlat) was formed by a single membranous bone plate. Therefore, we connected fetal elements and the adult morphology as follows. (1) Derivative of the OS makes not only the lesser wing but also the anterior margin of the body of the sphenoid. (2) Derivatives of the BS are the body of the sphenoid including the sella turcica and the dorsum sellae. (3) Most of the greater wing including the foramen rotundum and the foramen oval originate from the AT and AP and multiple membranous bones. (4) The PTmed originate from endochondral bones and multiple membranous bones, while the PTlat derive from a single membranous bone.

Human orbital muscle in adult cadavers and near-term fetuses: its bony attachments and individual variation identified by immunohistochemistry

PURPOSE

To compare fetal and adult morphologies of the orbital muscle (OM) and to describe the detailed topographical anatomy in adults.

METHODS

Using unilateral orbits from 15 near-term fetuses and 21 elderly cadavers, semiserial horizontal or sagittal paraffin sections were prepared at intervals of 20-100 µm. In addition to routine histology, we performed immunohistochemistry for smooth muscle actin.

RESULTS

At near term, the OM consistently extended widely from the zygomatic bone or the greater wing of the sphenoid to the maxilla or ethmoid. Thus, it was a large sheet covering the future inferior orbital fissure. In contrast, the adult OM was a thin and small muscle bundle connecting (1) the greater wing of the sphenoid to the maxilla (11/19 cadavers), (2) the lesser wing of the sphenoid to the maxilla (5/19) or the greater wing (3/19). The small OM was likely to be restricted within the greater wing (5/19 cadavers) or the maxilla (3/19). Two of these five types of OM coexisted in eight orbits. OM attachment to the lesser wing was not seen in fetuses, whereas ethmoid attachment was absent in adults.

CONCLUSIONS

The lesser wing attachment of the OM seemed to establish after birth. A growing common origin of the three recti was likely involved in "stealing" the near-term OM attachment from the ethmoid. The strong immunoreactivity of remnant-like OM in the elderly suggests that OM contraction is still likely to occur against the increased flow through a thin vein. However, the contraction might have no clinical significance.