Functional anatomy of the orbit in strabismus surgery: Connective tissues, pulleys, and the modern surgical implications of the "arc of contact" paradigm.

Oculomotricity is a multidimensional domain characterised by a delicate interplay of anatomical structures and physiological processes. This manuscript meticulously dissects the nuances of this interplay, bringing to the fore the integral role of the extraocular muscles (EOMs) and their intricate relationship with the myriad orbital connective tissues as it harmoniously orchestrates binocular movements, ensuring synchronised and fluid visual tracking. Historically, the peripheral oculomotor apparatus was conceptualised as a rudimentary system predominantly driven by neural directives. While widely accepted, this perspective offered a limited view of the complexities inherent in ocular movement mechanics. The twentieth century heralded a paradigm shift in this understanding. With advances in anatomical research and imaging techniques, a much clearer picture of the gross anatomy of the EOMs emerged. This clarity challenged traditional viewpoints, suggesting that the inherent biomechanical properties of the EOMs, coupled with their associated tissue pulleys, play a pivotal role in dictating eye movement dynamics. Central to this revised understanding is the "arc of contact" paradigm. This concept delves deep into the mechanics of eye rotation, elucidating the significance of the point of contact between the EOMs and the eyeball. The arc of contact is not just a static anatomical feature; its length and orientation play a crucial role in determining the effective torque generated by a muscle, thereby influencing the amplitude and direction of eye rotation. The dynamic nature of this arc, influenced by the position and tension of the muscle pulleys, offers a more comprehensive model for understanding ocular kinematics. Previously overlooked in traditional models, muscle pulleys have now emerged as central players in the biomechanics of eye movement. These anatomical structures, formed by dense connective tissues, guide the paths of the EOMs, ensuring that their pulling angles remain optimal across a range of gaze directions. The non-linear paths resulting from these pulleys provide a more dynamic and intricate understanding of eye movement, challenging two-dimensional, linear models of orbital anatomy. The implications of these revelations extend beyond mere theoretical knowledge. The insights garnered from this research promise transformative potential in the realm of strabismus surgery. Recognising the pivotal role of muscle pulleys and the "arc of contact" paradigm allows for more precise surgical interventions, ensuring better post-operative outcomes and minimising the risk of complications. Surgical procedures that previously relied on basic mechanical principles now stand to benefit from a more nuanced understanding of the underlying anatomical and physiological dynamics. In conclusion, this manuscript serves as a testament to the ever-evolving nature of scientific knowledge. Challenging established norms and introducing fresh perspectives pave the way for more effective and informed clinical interventions in strabismus surgery.

Normative measurements of orbital structures on magnetic resonance images; a cross-sectional study with mini review of the literature.

PURPOSE: Awareness of normative values of extra orbital structures would provide useful information to interpret the radiological images better and use them for diagnostic purposes. This study aimed to reveal the average values of major extraocular structures measured on magnetic resonance images. METHODS: In this retrospective cross-sectional study, magnetic resonance (MR) images of 256 orbits of 128 patients were re-interpreted regarding the measurements of major orbital structures. Extraocular muscles, superior ophthalmic vein, and optic nerve-sheath complex were measured on orbital MR images of these patients. The data distributions were presented by box-plot analyses for each parameter, and the measurement results were analyzed regarding gender and age groups. RESULTS: Lateral rectus muscle thickness (LR), inferior rectus muscle thickness (IR), globe position (GP), and interzygomatic line (IZL) values were higher in the male group than in the female group (p values were < 0.001, 0.003, 0.020, and < 0.001 respectively). LR, the thickness of the superior group muscles (SUP GR), IR, superior oblique muscle thickness (SOBL), and the thickness of optic nerve-sheath complex (ON) values indicated a significant relationship between age groups. There was a significant, positive, and low-level correlation between age and LR, SUP GR, and IR values (p values were < 0.001, 0.001, and < 0.001, respectively). CONCLUSION: This study provides quantitative data on normative values of orbital structures with gender and age group comparisons. Clinicians or surgeons can easily use the measured values to gather diagnostic information from the orbital region.

Normal Diameters of Extraocular Muscles: A Nigerian Retrospective Study.

BACKGROUND: The thickness of extraocular muscles (EOMs) is important in the management of several conditions associated with EOM enlargement. This study determined the normative values of EOM diameters in adult patients seen at a teaching hospital in Nigeria. MATERIALS AND METHODS: The study measured the thickness of the EOMs and the interzygomatic line (IZL) on brain images of 300 patients with non-orbital conditions (150 computed tomography [CT] and 150 magnetic resonance imaging [MRI]) archived in the radiological database of Delta State University Hospital, Nigeria, after ethical clearance. The Statistical Package for the Social Sciences (version 23) was used to obtain descriptive statistics and further compare the variables based on gender, age groups and laterality. The association between parameters was tested using Pearson's correlation test. A probability value of <5% was considered significant. RESULTS: The thickest muscles were the medial rectus (0.42 ± 0.08 cm) and superior muscle group (0.42 ± 0.33 cm) on CT and the inferior rectus (0.40 ± 0.08 cm) on MRI. The diameters were symmetrical with sexual dimorphism in the superior muscle group on CT, medial and lateral recti on MRI and sum of all EOMs on both imaging groups (P < 0.05). The superior muscle group and the sum of all EOMs showed significant age group variations and a positive correlation with age. We noted a positive correlation between each EOM diameter and the sum of all EOMs besides the IZL (P < 0.05). CONCLUSION: This study offers normative data regarding EOMs that radiologists and ophthalmologists can use to diagnose disease conditions that cause EOM enlargement and further assess their response to treatment.

Accessory Muscles Associated With the Levator Palpebrae Superioris Muscle in 100 Cadavers.

PURPOSE: This study aimed to determine the variations of the levator palpebrae superioris muscle and to reveal its morphological features. METHODS: This study conducted on 100 adult orbit cadavers in the Department of Anatomy, Istanbul University, used an exploratory, descriptive research design. The anatomical and morphological variations of the levator palpebrae superioris muscle and its relationship with the superior ophthalmic vein were evaluated. RESULTS: Variations of levator palpebrae superioris muscle were discovered in 11 of 100 orbits. Single (9%), double (1%), and triple (1%) accessory muscle slips were observed. The origin of accessory muscle slips showed variation as the accessory muscle slips originated either from the proximal or distal half of the levator palpebrae superioris muscle. Also, the insertions of accessory muscle slips were variable, as they were inserted into levator aponeurosis, trochlea, lacrimal gland, lateral orbital wall, or the fascia of the superior ophthalmic vein. CONCLUSIONS: Accessory muscles associated with levator aponeurosis were found in a significant proportion of cadavers. These muscles may cause confusion in orbital surgery and should be taken into account during surgical planning and orientation in the superior orbit.

"Hammock" suspending the superior ophthalmic vein: a magnetic resonance imaging study.

PURPOSE: The present study aimed to explore the hammock-like structure suspending the superior ophthalmic vein (SOV) using magnetic resonance imaging (MRI). METHODS: Following conventional MRI examination, 93 outpatients underwent thin-sliced, coronal T2-weighted and contrast imaging of the orbit. RESULTS: SOVs were consistently detected in all 93 patients. In 90.3% of patients, a hammock-like structure suspending the SOV was identified, which was present on both sides in 64.5% of patients. The structure was frequently located in the anterior and middle thirds of the retrobulbar orbit, suspended from the superolateral corner of the orbital walls. The medial edge of the hammocks did not reach the orbital walls; therefore, they partially encased the SOV. The morphology of the hammock was highly variable between patients, although none were tethered to the extraocular muscles. In addition, a septal band connecting the hammock and optic sheath was identified in 36.6% of patients, most frequently located in the posterior third of the retrobulbar orbit. CONCLUSIONS: The hammock suspending the SOV and the septal band connecting the hammock and optic sheath may be structures that loosely anchor the SOV to the orbital fat to maintain a constant SOV flow, in addition to preventing excessive bends and obstructions.

The oblique extraocular muscles in cetaceans: Overall architecture and accessory insertions.

The oblique extraocular muscles (EOMs) were dissected in 19 cetacean species and 10 non-cetacean mammalian species. Both superior oblique (SO) and inferior oblique (IO) muscles in cetaceans are well developed in comparison to out-groups and have unique anatomical features likely related to cetacean orbital configurations, swimming mechanics, and visual behaviors. Cetacean oblique muscles originate at skeletal locations typical for mammals: SO, from a common tendinous cone surrounding the optic nerve and from the medially adjacent bone surface at the orbital apex; IO, from the maxilla adjacent to lacrimal and frontal bones. However, because of the unusual orbital geometry in cetaceans, the paths and relations of SO and IO running toward their insertions onto the temporal ocular sclera are more elaborate than in humans and most other mammals. The proximal part of the SO extends from its origin at the apex along the dorsomedial aspect of the orbital contents to a strong fascial connection proximal to the preorbital process of the frontal bone, likely the cetacean homolog of the typical mammalian trochlea. However, the SO does not turn at this connection but continues onward, still a fleshy cylinder, until turning sharply as it passes through the external circular muscle (ECM) and parts of the palpebral belly of the superior rectus muscle. Upon departing this "functional trochlea" the SO forms a primary scleral insertion and multiple accessory insertions (AIs) onto adjacent EOM tendons and fascial structures. The primary SO scleral insertions are broad and muscular in most cetacean species examined, while in the mysticete minke whale (Balaenoptera acutorostrata) and fin whale (Balaenoptera physalus) the muscular SO bellies transition into broad fibrous tendons of insertion. The IO in cetaceans originates from an elongated fleshy attachment oriented laterally on the maxilla and continues laterally as a tubular belly before turning caudally at a sharp bend where it is constrained by the ECM and parts of the inferior rectus which form a functional trochlea as with the SO. The IO continues to a fleshy primary insertion on the temporal sclera but, as with SO, also has multiple AIs onto adjacent rectus tendons and connective tissue. The multiple IO insertions were particularly well developed in pygmy sperm whale (Kogia breviceps), minke whale and fin whale. AIs of both SO and IO muscles onto multiple structures as seen in cetaceans have been described in humans and domesticated mammals. The AIs of oblique EOMs seen in all these groups, as well as the unique "functional trochleae" of cetacean SO and IO seem likely to function in constraining the lines of action at the primary scleral insertions of the oblique muscles. The gimble-like sling formed by SO and IO in cetaceans suggest that the "primary" actions of the cetacean oblique EOMs are not only to produce ocular counter-rotations during up-down pitch movements of the head during swimming but also to rotate the plane containing the functional origins of the rectus muscles during other gaze changes.

Regional Anesthesia for Pediatric Ophthalmic Surgery: A Review of the Literature.

Ophthalmic pediatric regional anesthesia has been widely described, but infrequently used. This review summarizes the available evidence supporting the use of conduction anesthesia in pediatric ophthalmic surgery. Key anatomic differences in axial length, intraocular pressure, and available orbital space between young children and adults impact conduct of ophthalmic regional anesthesia. The eye is near adult size at birth and completes its growth rapidly while the orbit does not. This results in significantly diminished extraocular orbital volumes for local anesthetic deposition. Needle-based blocks are categorized by relation of the needle to the extraocular muscle cone (ie, intraconal or extraconal) and in the cannula-based block, by description of the potential space deep to the Tenon capsule. In children, blocks are placed after induction of anesthesia by a pediatric anesthesiologist or ophthalmologist, via anatomic landmarks or under ultrasonography. Ocular conduction anesthesia confers several advantages for eye surgery including analgesia, akinesia, ablation of the oculocardiac reflex, and reduction of postoperative nausea and vomiting. Short (16 mm), blunt-tip needles are preferred because of altered globe-to-orbit ratios in children. Soft-tip cannulae of varying length have been demonstrated as safe in sub-Tenon blockade. Ultrasound technology facilitates direct, real-time visualization of needle position and local anesthetic spread and reduces inadvertent intraconal needle placement. The developing eye is vulnerable to thermal and mechanical insults, so ocular-rated transducers are mandated. The adjuvant hyaluronidase improves ocular akinesia, decreases local anesthetic dosage requirements, and improves initial block success; meanwhile, dexmedetomidine increases local anesthetic potency and prolongs duration of analgesia without an increase in adverse events. Intraconal blockade is a relative contraindication in neonates and infants, retinoblastoma surgery, and in the presence of posterior staphylomas and buphthalmos. Specific considerations include pertinent pediatric ophthalmologic topics, block placement in the syndromic child, and potential adverse effects associated with each technique. Recommendations based on our experience at a busy academic ophthalmologic tertiary referral center are provided.

T2 mapping of the extraocular muscles in healthy volunteers: preliminary research on scan-rescan and observer-observer reproducibility.

BACKGROUND: T2-mapping technique and derived T2 relaxation time allows quantitative assessment of extraocular muscles; however, the reproducibility of T2 mapping-derived parameters was seldom studied till now. PURPOSE: To evaluate the scan-rescan and observer-observer reproducibility of T2 relaxation time measurements of extraocular muscles in young healthy volunteers. MATERIAL AND METHODS: Fourteen volunteers underwent T2-mapping examinations of the extraocular muscles three times within one month on a 3.0-T MR system. Scan-rescan and observer-observer reproducibility of T2 relaxation time measurements of the extraocular muscles were assessed using intraclass correlation coefficient and coefficient of variation. RESULTS: Both scan-rescan (short-term and long-term) and observer-observer could achieve good to excellent reproducibility, while better short-term than long-term scan-rescan reproducibility was obtained. The coefficient of variation of the T2 relaxation time of each extraocular muscles during both scan-rescan and observer-observer reproducibility assessment were <6%. CONCLUSION: T2 relaxation time measurement of the extraocular muscles is proven to be highly reproducible at 3.0 T. T2 mapping may be a potential imaging technique in the diagnosis and follow-up of orbital diseases involved extraocular muscles in further studies.

A New Anatomy Finding of Levator Aponeurosis and Müller Muscle at Pretarsal Plate in Asian Males.

PURPOSE: To examine the fine anatomic structures between levator aponeurosis and Müller muscle in front of the tarsus. MATERIALS AND METHODS: Postmortem specimens of 6 Chinese males (5 elderly men, aging from 68 to 86 years; 1 child, 10 years old) were used. A 3-µm thickness sagittal section of the central part of the upper eyelid was prepared, and the samples were examined microscopically by using hematoxylin-eosin, Masson trichrome, and anti-smooth muscle actin antibodies staining. RESULTS: There are 2 new findings in this study, one is the posterior layer of the levator aponeurosis and the other is the extensions of Müller muscle. The posterior levator aponeurosis had different insertion patterns that approximately paralleled the extension line of the levator aponeurosis at the confluence of attachment site of the orbital septum on the levator superioris. Below the confluence, it took the form of a layered insertion, and then extended to the orbicularis oculi muscle and subcutaneous tissue rather than inserting directly into tarsus. The Müller muscle was a multilayered structure at the upper border of tarsus. The superior Müller muscle extended above the peripheral arcade, and the inferior Müller muscle tendon was attached to the surface of tarsus with an Umbrella-shaped fiber. CONCLUSIONS: The authors discovered that the levator aponeurosis had different insertion patterns of the posterior layers. The Müller muscle gave branches at the peripheral arcade: the anterior one crossed the peripheral arcade and extended to the tarsus, and the posterior one attached and extended to the tarsus.

In vivo experimental study on the resistance and stiffness of orbital suspension tissues with/without the extraocular muscles.

BACKGROUND: The accuracy of the surgical amount of extraocular muscle (EOM) is key to the success of strabismus surgery. To establish an accurate eye movement model, it is of great theoretical value and clinical significance to determine the surgical amount of EOM. At present, only resistance and stiffness data of orbital suspension tissues with EOMs exist, while those of orbital suspension tissues without EOMs, which is critical information for eye movement modeling, have not been reported. The aim of this research is to study the resistance and stiffness of orbital suspension tissues with/without EOMs. METHODS: Fifteen healthy New Zealand white rabbits with body weights of 2.41 ± 0.13 kg were used in the study. Two recti (two horizontal recti of the left eye or two vertical recti of the right eye) or all EOMs were detached from each eye under general anesthesia. Then, a 5-0 silk suture was attached to the stump of the detached rectus insertion (two horizontal recti insertions of the left eye and two vertical recti insertions of the right eye) on the isolated eyeball. The 5-0 silk suture was connected to the INSTRON 5544 tester to facilitate the horizontal rotations of the left eyes and the vertical rotations of the right eyes, respectively. RESULTS: The resistance and stiffness of orbital suspension tissues with superior rectus, inferior rectus, superior oblique, and inferior oblique EOMs were obtained during horizontal eye movement. Similarly, the resistance and stiffness of orbital suspension tissues with lateral rectus, medial rectus, superior oblique, and inferior oblique EOMs were obtained during vertical eye movement. Then, the resistance and stiffness of orbital suspension tissues without EOMs were obtained during horizontal and vertical eye movements. The resistance and stiffness data of orbital suspension tissues with EOMs were compared with those of orbital suspension tissues without EOMs. The comparison results showed no significant difference in the resistance values between these two cases. In addition, the stiffness values of these two cases statistically differed. CONCLUSIONS: The two horizontal recti play a major role in passive horizontal eye movement. In addition, when the eye is passively moved vertically, the two vertical recti play major roles. The stiffness of orbital suspension tissues with EOMs, which has been used in eye movement modeling, is not accurate. The results of this work may serve as a reference for improving the accuracy in eye movement modeling, and then it will be beneficial for determining the surgical amount of EOMs in clinical surgery.

An accurate interactive segmentation and volume calculation of orbital soft tissue for orbital reconstruction after enucleation.

BACKGROUND: Accurate measurement and reconstruction of orbital soft tissue is important to diagnosis and treatment of orbital diseases. This study applied an interactive graph cut method to orbital soft tissue precise segmentation and calculation in computerized tomography (CT) images, and to estimate its application in orbital reconstruction. METHODS: The interactive graph cut method was introduced to segment extraocular muscle and intraorbital fat in CT images. Intra- and inter-observer variability of tissue volume measured by graph cut segmentation was validated. Accuracy and reliability of the method was accessed by comparing with manual delineation and commercial medical image software. Intraorbital structure of 10 patients after enucleation surgery was reconstructed based on graph cut segmentation and soft tissue volume were compared within two different surgical techniques. RESULTS: Both muscle and fat tissue segmentation results of graph cut method showed good consistency with ground truth in phantom data. There were no significant differences in muscle calculations between observers or segmental methods (p > 0.05). Graph cut results of fat tissue had coincidental variable trend with ground truth which could identify 0.1cm3 variation. The mean performance time of graph cut segmentation was significantly shorter than manual delineation and commercial software (p < 0.001). Jaccard similarity and Dice coefficient of graph cut method were 0.767 ± 0.045 and 0.836 ± 0.032 for human normal extraocular muscle segmentation. The measurements of fat tissue were significantly better in graph cut than those in commercial software (p < 0.05). Orbital soft tissue volume was decreased in post-enucleation orbit than that in normal orbit (p < 0.05). CONCLUSION: The graph cut method was validated to have good accuracy, reliability and efficiency in orbit soft tissue segmentation. It could discern minor volume changes of soft tissue. The interactive segmenting technique would be a valuable tool for dynamic analysis and prediction of therapeutic effect and orbital reconstruction.

Dimensions and Morphologic Variability of the Retro-Orbicularis Oculi and Frontalis Muscle Fat Pad.

PURPOSE: To quantify the complete dimensions of the retro-orbicularis oculi fat (ROOF) pad and to determine its relationship to other fat compartments of the forehead. METHODS: The entire forehead of 14 hemifaces of seven fresh frozen human cadavers (four female, three male) was dissected in the subcutaneous and submuscular planes. For each plane, a ruler was placed at the facial midline, and images of the dissection plane were taken at 90° and 45°. Images were analyzed for vertical height, horizontal length, the distance to midline from the point of maximal height, and area for each hemiface of the ROOF and for the entire fat compartment contiguous with the ROOF. A two-tailed t test was conducted between ROOF and ROOF plus the extended fat plane across all measurements. A Wilcoxon nonparametric signed rank test was performed to determine equivalent fat distribution of the extended fat plane over each cadaver's respective eye. RESULTS: The deep fat originating from the ROOF consistently extended laterally and superiorly in each specimen, distinctly separated via septae from the deep central, deep lateral, and the deep temporal fat compartments. The color, composition, and distribution of this contiguous deep fat did not differ phenotypically from the traditional ROOF. The extended deep fat plane possessed an average vertical height of 3.09 ± 0.68 cm, average distance to midline from point of maximal height of 3.56 ± 0.53 cm, an average horizontal length of 5.37 ± 0.82 cm, and an average area of 13.40 ± 2.69 cm. The extended deep fat demonstrated a statistically significant increase in maximal height, length, and total area compared with the ROOF. A Wilcoxon nonparametric signed rank test was nonsignificant (α = 0.01) across all measurements, demonstrating that the extended fat plane was similarly distributed over each eye. CONCLUSIONS: A layer of deep fat originating from the traditionally defined ROOF extends superiorly and laterally beneath the frontalis muscle, separate from the deep lateral, deep central, and deep temporal fat pads. This is the first study to clearly demonstrate a contiguous superficial musculoaponeurotic system layer of fat extending under both the orbicularis oculi and frontalis muscles. This plane of fat is more appropriately described as the retro-orbicularis oculi and frontalis fat.

Measurement of extraocular horizontal muscle insertion distance via anterior segment optical coherence tomography of healthy children and comparison with healthy adults.

PURPOSE: The aim of the study was to determine the corneal limbus-extraocular muscle insertion distance (LID), via anterior segment optical coherence tomography, in healthy children and healthy adults and to compare the results of the measurements of the two groups. METHODS: Muscle limbus distances were measured using AS-OCT in 60 healthy cases in two groups. Children aged 8-13 years were evaluated as group 1, and healthy adults aged 25-30 years were evaluated as group 2. Measurements of 120 horizontal muscles were taken by one doctor (OBO). The values were compared according to age and gender groups, and correlation between LID measurements and spherical equivalent. Statistical evaluation was performed using SPSS 16® for Windows with the Student's t test and Pearson correlation coefficient test. RESULTS: LID measurements for MR and for lateral rectus (LR) were 5.74 ± 0.75 and 6.74 ± 1.11 mm, in the pediatric age-group, and 5.73 ± 0.75 and 6.84 ± 1.15 mm, in the adult age-group, respectively. There was no statistically significant difference between the two groups in terms of MR distances. There was a slight increase in the adult values, for the LR distance. There was no significant difference in terms of gender. Correlation was found 0.62 for MR and 0.46 for LR between LID measurements and spherical equivalent in the pediatric age-group. CONCLUSIONS: In healthy individuals, different imaging modalities can be used to measure LID, but AS-OCT can be used in pediatric age-groups as a preferred imaging method because it is easy and noninvasive.

Anatomic Considerations in Thyroid Eye Disease.

PURPOSE: To review and summarize the clinical findings in thyroid eye disease (TED) related to the unique anatomical structures of the eyelids and orbit. METHODS: A PubMed search was performed searching for anatomical relationships between eyelid and orbital anatomy and the clinical findings of TED. RESULTS: The major clinical findings associated with TED are varied. They typically involve both the eyelid and the orbit. In the eyelid, usual findings include upper eyelid retraction, contour abnormalities, eyelid edema, prolapsed orbital fat, conjunctival injection and chemosis, caruncular edema, and meibomian gland dysfunction. The exact causes of these changes remain a matter of controversy, but numerous hypotheses have been proposed, most with limited experimental support. In the orbit, inflammation and congestion characterize the acute active phase, with tissue expansion and fibrosis persisting into the chronic inactive phase. All of these findings result, at least in part, from the unique anatomy of the eyelids and orbit and their interaction with the immunologic processes underlying TED. Here we review these major characteristics of TED, with special reference to their anatomic relationships. CONCLUSIONS: The major findings characterizing TED are related to unique anatomic features in the eyelid, including Müller muscle, the levator palpebrae superioris muscle, and suspensory ligaments of the conjunctiva. In the orbit, Graves-related remolding results from the special physiologic features of the extraocular muscles, the structure of the orbital walls and their relations to adjacent paranasal sinuses, and the reaction of orbital fat to immunologic challenges.

Extraocular muscle architecture in hawks and owls.

OBJECTIVE: A complete and accurate understanding of extraocular muscle function is important to the veterinary care of the avian eye. This is especially true for birds of prey, which rely heavily on vision for survival and yet are prone to ocular injury and disease. To better understand the function of extraocular muscles in birds of prey, we studied extraocular muscle architecture grossly and histologically. ANIMALS STUDIED: This sample was composed of two each of the following species: red-tailed hawk (Buteo jamaicensis), Harris's hawk (Parabuteo unicinctus), great horned owl (Bubo virginianus), and barn owl (Tyto alba). PROCEDURES: All extraocular muscles were dissected and weighed. To analyze muscle fiber architecture, the superior oblique and quadratus muscles were dissected, weighed, and sectioned at 5 µm thickness in the transverse plane. We calculated the physiologic cross-sectional area and the ratio of muscle mass to predicted effective maximum tetanic tension. RESULTS AND CONCLUSIONS: Hawk and owl extraocular muscles exhibit significant physiological differences that play roles in ocular movements and closure of the nictitating membrane. Owls, which do not exhibit extraocular movement, have muscle architecture suited to stabilize the position of a massive, tubular eye that protrudes significantly from the orbit. Hawks, which have a more globose eye that is largely contained within the orbit, do not require as much muscular stability and instead have muscle architecture that facilitates rapid eye movement.

Bilateral muscular slips between superior and inferior rectus muscles: case report with discussion on classification of accessory rectus muscles within the orbit.

Accessory rectus muscles have rarely been reported as muscular 'bands' or 'slips' originating from the common tendinous ring (annulus of Zinn) and inserting in atypical location. This group of muscles is innervated by the inferior branch of the oculomotor nerve, lies on lateral side of the optic nerve and inserts in rectus muscles. Since there are only few descriptions of such unusual findings in the medical literature, the anatomical data on accessory rectus muscles is limited. Furthermore, existing reports vary in terms of studied objects (cadavers or living subjects), medical history (absence or presence of ocular movement disorders or eye movement abnormalities) and details of anatomical description. This report complements earlier publications and provides complete anatomical description of the accessory rectus muscle observed bilaterally during the dissection of a 68-year-old male cadaver with no eye movement abnormalities reported in the medical history. The accessory rectus muscle was divided into two 'slips' or 'heads'-superior and inferior-running in the sagittal plane (laterally to the optic nerve and the main trunk of the ophthalmic artery) and attached to the superior and inferior rectus muscles. Noticeable thickening of both superior and inferior rectus muscles at the insertion point of the accessory muscle heads was observed only in the sagittal plane. On both sides, the inferior head of the accessory rectus muscle was innervated by one of sub-branches derived from the inferior branch of the oculomotor nerve. No sub-branches to the superior head were macroscopically observed during the dissection. The classification, embryological background and clinical relevance of this variation have been discussed.

Semiautomatic procedure to assess changes in the eye accommodative system.

PURPOSE: The aim of this pilot study was to evaluate a new semiautomatic procedure to assess in vivo changes in the crystalline lens and ciliary muscle during accommodation. METHODS: A total of 14 subjects were divided into two groups, young (aged between 20 and 25 years) and adult (aged between 35 and 40 years), and measured with an anterior segment optical coherence tomography. A semiautomatic procedure was implemented to measure the central lens thickness (CLT), anterior lens radius (ALR) and the ciliary muscle area (CMA) for the unaccommodated eye and for a vergence of - 3.00 D. RESULTS: The CLT increase for each population group was smaller than 5%, and the dispersion of each group was similar between them. Contrariwise, the reduction in the ALR was about 30% for both groups, although the young one showed the largest variability. The CMA increase was smaller than 30% for both groups, and the dispersion was similar between them. For each metric, differences between both groups were not statistically significant. CONCLUSIONS: The semiautomatic procedure seems to be useful for the in vivo analysis of the accommodative system. Additionally, the results obtained showed that changes in the CLT were much smaller compared to those obtained for the ALR or CMA.

A new teaching model for demonstrating the movement of the extraocular muscles.

The extraocular muscles consist of the superior, inferior, lateral, and medial rectus muscles and the superior and inferior oblique muscles. This study aimed to create a new teaching model for demonstrating the function of the extraocular muscles. A coronal section of the head was prepared and sutures attached to the levator palpebral superioris muscle and six extraocular muscles. Tension was placed on each muscle from a posterior approach and movement of the eye documented from an anterior view. All movements were clearly seen less than that of the inferior rectus muscle. To our knowledge, this is the first cadaveric teaching model for demonstrating the movements of the extraocular muscles. Clin. Anat. 30:733-735, 2017. © 2017Wiley Periodicals, Inc.

Quantitative MR imaging of intra-orbital structures: Tissue-specific measurements and age dependency compared to extra-orbital structures using multispectral quantitative MR imaging.

The orbit can be affected by unique pathologic conditions and often requires MRI evaluation. The purpose of this study was to investigate the age-related changes in multiple intra-orbital structures using quantitative MRI (qMRI). Thirty-eight subjects (20 males, 18 females; ages 0.5-87 years) underwent MRI with a mixed turbo spin echo sequence. T1 and T2 measurements were obtained within ROI in 6 intra-orbital structures (medial and lateral rectus muscles, medial and lateral retrobulbar fat, lacrimal gland, and optic nerve), and compared with those of corresponding extra-orbital structures (masseter muscle, subcutaneous cheek fat, buccal fat, parotid gland, and frontal white matter). Statistical analyses were performed using Pearson's correlation coefficients. T1 and T2 values of the extra-ocular muscles increased with age, with higher T1 and T2 values compared to the masseter muscles. Retrobulbar fat showed significant age-associated increases in T1 values in the lateral side and in T2 values in both sides. T1 and T2 values in the lacrimal gland increased with age, while the parotid gland showed an age-associated increase in T2 values and decrease in T1 values. Optic nerves demonstrated age-related changes, similar to that of frontal white matter; rapid decreases with age in T1 and T2 times in early stages of life, and slight increases in T1 and T2 times later in life. Intra-orbital structures demonstrated specific qMRI measurements and aging patterns, which were different from extra-orbital structures. Location-specific age-related changes of intra-orbital structures should be considered in the qMRI assessment of the orbital pathology.

Functional morphometry demonstrates extraocular muscle compartmental contraction during vertical gaze changes.

Anatomical studies demonstrate selective compartmental innervation of most human extraocular muscles (EOMs), suggesting the potential for differential compartmental control. This was supported by magnetic resonance imaging (MRI) demonstrating differential lateral rectus (LR) compartmental contraction during ocular counterrolling, differential medial rectus (MR) compartmental contraction during asymmetric convergence, and differential LR, inferior rectus (IR), and superior oblique (SO) compartmental contraction during vertical vergence. To ascertain possible differential compartmental EOM contraction during vertical ductions, surface coil MRI was performed over a range of target-controlled vertical gaze positions in 25 orbits of 13 normal volunteers. Cross-sectional areas and partial volumes of EOMs were analyzed in contiguous, quasi-coronal 2-mm image planes spanning origins to globe equator to determine morphometric features correlating best with contractility. Confirming and extending prior findings for horizontal EOMs during horizontal ductions, the percent change in posterior partial volume (PPV) of vertical EOMs from 8 to 14 mm posterior to the globe correlated best with vertical duction. EOMs were then divided into equal transverse compartments to evaluate the effect of vertical gaze on changes in PPV. Differential contractile changes were detected in the two compartments of the same EOM during infraduction for the IR medial vs. lateral (+4.4%, P = 0.03), LR inferior vs. superior (+4.0%, P = 0.0002), MR superior vs. inferior (-6.0%, P = 0.001), and SO lateral vs. medial (+9.7%, P = 0.007) compartments, with no differential contractile changes in the superior rectus. These findings suggest that differential compartmental activity occurs during normal vertical ductions. Thus all EOMs may contribute to cyclovertical actions.