Expanding repertoire in the oculomotor periphery: selective compartmental function in rectus extraocular muscles

Laterality Targeting in Graded Inferior Rectus Tenotomy Corrects Lateral Incomitance of Hypertropia in Sagging Eye Syndrome

PURPOSE

To determine if laterally selective graded vertical rectus tenotomy (GVRT) of the inferior rectus (IR) can correct the lateral incomitance of hypertropia (HT) commonly encountered in sagging eye syndrome (SES), comparing it with inferior oblique (IO) recession.

DESIGN

Retrospective comparative interventional clinical study.

METHODS

We reviewed 73 consecutive patients undergoing GVRT of the IR for correction of horizontally incomitant HT due to SES from July 2012 to October 2023. Confounding diagnoses were excluded. Using topical anesthesia, GVRT was initiated from the nasal versus temporal side corresponding to greater HT, with dosing adjusted intraoperatively until cover testing in central gaze indicated orthotropia. We compared 8 cases of IO recession to 4 mm posterior and 3 mm lateral to the IR insertion.

RESULTS

Nasal GVRT was performed in 41 patients, and temporal GVRT on 32 patients. Mean nasal GVRT was 69 ± 15% (standard deviation) and mean temporal GVRT was 62 ± 17%. Mean HT in central gaze was reduced by nasal GVRT from 3.9 ± 1.7Δ to 0.3 ± 1.4Δ, and from 4.0 ± 1.6Δ to 0.2 ± 1.1Δ by temporal GVRT. Nasal GVRT corresponding to the side of the tenotomy had greater effect in contralateral gaze at 3.2 ± 2.2Δ than ipsilateral gaze at 2.1 ± 2.0Δ (P = .025), whereas temporal GVRT had greater effect in ipsilateral gaze at 4.9 ± 2.7D than contralateral gaze at 2.9 ± 2.9D (P = .0002). Inferior oblique recession in 8 patients reduced lateral incomitance from 13 ± 5.0Δ to 0.5 ± 1.4Δ (P < .0001).

CONCLUSION

Nasal GVRT corrects about 1Δ and temporal GVRT 2Δ horizontal incomitance of HT, while IO recession corrects about 12.5Δ. Selection of GVRT laterality improves outcomes without additional risk or operating time.

Finite element model of ocular adduction with unconstrained globe translation

Details of the anatomy and behavior of the structures responsible for human eye movements have been extensively elaborated since the first modern biomechanical models were introduced. Based on these findings, a finite element model of human ocular adduction is developed based on connective anatomy and measured optic nerve (ON) properties, as well as active contractility of bilaminar extraocular muscles (EOMs), but incorporating the novel feature that globe translation is not otherwise constrained so that realistic kinematics can be simulated. Anatomy of the hemisymmetric model is defined by magnetic resonance imaging. The globe is modeled as suspended by anatomically realistic connective tissues, orbital fat, and contiguous ON. The model incorporates a material subroutine that implements active EOM contraction based on fiber twitch characteristics. Starting from the initial condition of 26° adduction, the medial rectus (MR) muscle was commanded to contract as the lateral rectus (LR) relaxed. We alternatively modeled absence or presence of orbital fat. During pursuit-like adduction from 26 to 32°, the globe translated 0.52 mm posteriorly and 0.1 mm medially with orbital fat present, but 1.2 mm posteriorly and 0.1 mm medially without fat. Maximum principal strains in the optic disk and peripapillary reached 0.05-0.06, and von-Mises stress 96 kPa. Tension in the MR orbital layer was ~ 24 g-force after 6° adduction, but only ~ 3 gm-f in the whole LR. This physiologically plausible simulation of EOM activation in an anatomically realistic globe suspensory system demonstrates that orbital connective tissues and fat are integral to the biomechanics of adduction, including loading by the ON.

Finite Element Model of Ocular Adduction by Active Extraocular Muscle Contraction

Purpose

In order to clarify the role of the optic nerve (ON) as a load on ocular rotation, we developed a finite element model (FEM) of incremental adduction induced by active contractility of extraocular muscles (EOMs), with and without tethering by the ON.

Methods

Three-dimensional (3-D) horizontal rectus EOM geometries were obtained from magnetic resonance imaging of five healthy adults, and measured constitutive tissue properties were used. Active and passive strain energies of EOMs were defined using ABAQUS (Dassault Systemes) software. All deformations were assumed to be caused by EOM twitch activation that rotated the eye about a fixed center. The medial rectus (MR) muscle was commanded to additionally contract starting from 26 degrees adducted position, and the lateral rectus (LR) to relax, further adducting the eye either with or without loading by the ON. Tridimensional heat maps were generated to represent the stress and strain distributions.

Results

Tensions in the EOMs were physiologically plausible during incremental adduction. Force in the MR increased from 10 gm at 26 degrees adduction to approximately 28 gm at 32 degrees adduction. Under identical MR contraction, adduction with ON loading reached 32 degrees but 36 degrees without it. Maximum and minimum principal strains within the MR were 16% and 22%, respectively, but when ON loading was included, resulting stress and strain were concentrated at the optic disc.

Conclusions

This physiologically plausible method of simulating EOM activation can provide realistic input to model biomechanical behavior of active and passive tissues in the orbit to clarify biomechanical consequences of ON traction during adduction.

Comparison of the Superior and Inferior Rectus Muscles in Humans: An Anatomical Study with Notes on Morphology, Anatomical Variations, and Intramuscular Innervation Patterns

A comparison of the superior and inferior rectus muscles was performed to determine whether they have similar structures and innervation attributable to their participation in the same type of, although antagonistic, eye movements. The study was conducted on 70 cadaveric hemiheads, and the anatomical variations in the superior and inferior rectus muscles were assessed. Sihler's whole mount nerve staining technique was used on 20 isolated superior and 20 isolated inferior rectus muscle specimens to visualize the intramuscular distribution of the oculomotor nerve subbranches. In two cases (~2.8%), variant muscular slips were found that connected the superior and inferior rectus muscles. In 80% of cases, muscular branches arising directly from the inferior branch of the oculomotor nerve innervated the inferior rectus muscle, while in 20% of cases, the nerve to the inferior oblique muscle pierced the inferior rectus muscle and provided its innervation. In 15 of 70 specimens (21.4%), a branch to the levator palpebrae superioris muscle pierced the superior rectus muscle. The distance between the specific rectus muscle's insertion and the anterior-most terminations of the nerves' subbranches with reference to the muscle's total length ranged from 26.9% to 47.2% for the inferior rectus and from 34.8% to 46.6% for the superior rectus, respectively. The superior rectus muscle is slightly longer and its insertion is farther from the limbus of the cornea than is the inferior rectus muscle. Both muscles share a common general pattern of intramuscular nerve subbranches' arborization, with characteristic Y-shaped ramifications that form the terminal nerve plexus located near half of the muscles' length. Unexpected anatomical variations of the extraocular muscles may be relevant during orbital imaging or surgical procedures.

Comparison of lateral and medial rectus muscle in human: an anatomical study with particular emphasis on morphology, intramuscular innervation pattern variations and discussion on clinical significance

This paper aims to present various aspects of the anatomy of horizontal (i.e., lateral and medial) rectus muscles. It mainly compares morphology and detailed patterns of intramuscular innervation of those muscles. It is also one of the first reports that uses the Sihler's stain to examine human extraocular muscles. The study was conducted on 80 isolated cadaveric hemi-heads. Sihler technique of nerves staining served to expose the course of intramuscular branches of the oculomotor and abducens nerves. The lateral rectus was longer (48 mm versus 46.5 mm) and more distant from the limbus (6.8 mm versus 5.7 mm) than the medial rectus muscle. Three variants of the abducens nerve primary division were observed in the lateral rectus muscle. In the medial rectus muscle, the motor branch from the oculomotor nerve was more evenly divided into sub-branches. In both examined horizontal rectus muscles, primary muscular branches split into secondary sub-branches, which undergo numerous further divisions. The most numerous terminal sub-branches formed the terminal plexus. The distance between the insertion and the anterior-most end of the terminal plexus referenced to the total length of the muscle ranged from 35.4 to 48.5% for the lateral rectus muscle and from 36.3 to 50.5% for medial rectus muscle. Both horizontal rectus muscles share similar general pattern of distribution of intramuscular nerves, with characteristic root-like arborizations of sub-branches. The terminal nerve plexus was observed near half of both muscles' length. Knowledge of variations and innervation pattern of the extraocular muscles may be relevant during ophthalmology surgeries.

Intramuscular innervation of the lateral rectus muscle evaluated using sihler's staining technique: Potential application to strabismus surgery

The latest research suggests that the abducens nerve may be divided into subbranches that reach functionally distinct zones of the lateral rectus muscle. The goal of the study was to examine this muscle's innervation, including the detailed distribution of the intramuscular subbranches of the abducens nerve. Twenty-five lateral rectus muscle specimens were harvested (with the orbital segment of the abducens nerve), fixed in 10% formalin solution, and stained with Sihler's whole mount nerve staining technique. Subbranches running to the lateral rectus divided into two main groups: superior and inferior. Both groups of subbranches are distributed in a fan-shaped manner, show a characteristic "tree-like" branching pattern and form terminal plexus near the proximal half of the lateral rectus muscle. However, some smaller subbranches run as far as the muscle's insertion, and recurrent subbranches also reach its origin. With respect to their course to the muscle's origin or insertion, the smallest subbranches running within the muscle may be associated with innervation of the tendon. In the majority of cases (88%), superior and inferior subbranches of the abducens nerve overlapped in the central one-third of the lateral rectus muscle's width so that any clearly distinct anatomical segments of the muscle could be observed based on Sihler's technique. In the remaining 12% of specimens, superior and inferior groups of subbranches innervated two distinct compartments of the lateral rectus muscle with no overlapping. Dense, fan-shaped distribution of abducens nerve intramuscular subbranches can be observed within the lateral rectus muscle. Clin. Anat. 33:585-591, 2020. © 2019 Wiley Periodicals, Inc.

Vertical vergence in nonhuman primates depends on horizontal gaze position

The goal of this study was to compare vertical fusion capability at different orbital eye positions in normal nonhuman primates and attempt to use this information to isolate the extraocular muscles (EOMs) that mediate vertical vergence. Scleral search coils were used to record movements of both eyes as two normal nonhuman primates (M1, M2) performed a vertical vergence task at different horizontal eye positions. In a control experiment, M1 was also tested at different angles of horizontal vergence. To elicit vertical vergence, a 50° x 50° stimulus comprising a central fixation cross and random dots elsewhere was presented separately to each eye under dichoptic viewing conditions. Vertical disparity was introduced by slowly displacing the stimulus for one eye vertically. Vertical fusion amplitude (maximum disparity that the monkey was able to fuse) and vertical vergence (maximum difference in vertical position of the two eyes) were measured. Vertical fusion capability differed at different orbital eye positions. Monkey M1 had significantly smaller vertical fusion capabilities when the right eye (RE) was abducted than left eye (LE) while M2 had significantly smaller vertical fusion capabilities when the RE was adducted and LE abducted. M1 also showed greater vertical fusion capability for near gaze. M1 data suggested that the vertical recti mediated vertical vergence in the RE and the oblique muscles in the LE while M2 data suggested that the oblique muscles mediated vertical vergence in the RE and the vertical recti in the LE. The variable results within the same animal and across animals suggest that EOM involvement during vertical fusional vergence is idiosyncratic and likely a weighted combination of multiple cyclovertical muscles.

Innovative techniques for the treatment of adult strabismus

Adult strabismus is often characterized by surgical complexity. In recent years, several innovative techniques for the management of complex strabismus have been developed. Strabismus surgeons should understand the indications for various strabismus surgical techniques in the management of these difficult cases. This workshop describes several new surgical techniques to manage complex strabismus, including small-angle incomitant and very large-angle strabismus, torsional diplopia, and restrictive, paralytic, and secondary strabismus. Because strabismus surgery is an ever-evolving field, it is important to continue to refine our surgical armamentarium. Strabismus surgeons may wish to add these techniques to their surgical repertoire for select cases.

Intramuscular Distribution of the Abducens Nerve in the Lateral Rectus Muscle for the Management of Strabismus

AIMS

To elucidate the intramuscular distribution and branching patterns of the abducens nerve in the lateral rectus (LR) muscle so as to provide anatomical confirmation of the presence of compartmentalization, including for use in clinical applications such as botulinum toxin injections.

METHODS

Thirty whole-mount human cadaver specimens were dissected and then Sihler's stain was applied. The basic dimensions of the LR and its intramuscular nerve distribution were investigated. The distances from the muscle insertion to the point at which the abducens nerve enters the LR and to the terminal nerve plexus were also measured.

RESULTS

The LR was 46.0 mm long. The abducens nerve enters the muscle on the posterior one-third of the LR and then typically divides into a few branches (average of 1.8). This supports a segregated abducens nerve selectively innervating compartments of the LR. The intramuscular nerve distribution showed a Y-shaped ramification with root-like arborization. The intramuscular nerve course finished around the middle of the LR (24.8 mm posterior to the insertion point) to form the terminal nerve plexus. This region should be considered the optimal target site for botulinum toxin injections. We have also identified the presence of an overlapping zone and communicating nerve branches between the neighboring LR compartments.

CONCLUSION

Sihler's staining is a useful technique for visualizing the entire nerve network of the LR. Improving the knowledge of the nerve distribution patterns is important not only for researchers but also clinicians to understand the functions of the LR and the diverse pathophysiology of strabismus.

A chronicle of surgical thinking and doing for exotropia: innovations and rediscoveries

PURPOSE

To review the evolution of theories and practices concerning surgery for exotropia over the last approximately 200 years.

METHODS

Major ophthalmology texts and relevant references were reviewed to discover experience and thinking concerning the causes of comitant intermittent and constant exotropia and corrective surgical approaches.

RESULTS

The concept that excessive divergence is the cause of comitant exotropia has given way to one recognizing that this disorder is a position of rest modified by convergence. Some operations discarded as unreliable or dangerous remain so today. Others continue to be well accepted, whether or not relating to advances in understanding of this condition. Several procedures practiced at former times have regained popularity.

CONCLUSIONS

Concern for the characteristics of extraocular muscle structure and function has led to surgical thinking that goes beyond consideration only of direction and magnitude of deviation. Sophisticated devices now provide heretofore unavailable information consistent with this new approach. The history of progress in surgery for exotropia is mixed. Although some procedures now employed are new, others currently in favor had been well known and formerly utilized by ophthalmologists for many decades. These should be regarded as rediscoveries or revivals rather than as novel. This review also suggests a need to address how to better report retrospective studies.

Recent advances clarifying the etiologies of strabismus

BACKGROUND

Strabismus is commonly encountered in neuro-ophthalmology practice. Adult patients may present with symptoms including disabling diplopia and decreased quality of life. Although presentation to the neuro-ophthalmologist often prompts a thorough workup for a neurologic basis of ocular misalignment, advances in orbital imaging and understanding of orbital mechanics have revealed novel mechanical causes. A goal of this review is to clarify mechanical mechanisms of strabismus that were formerly assumed be neurologic in origin.

EVIDENCE ACQUISITION

The authors combine their own research and clinical experience with a literature review using PubMed.

RESULTS

Aberrant paths of the extraocular muscles can lead to strabismus. The extraocular muscles have connective tissue pulleys that control muscle paths and are, in turn, influenced by the extraocular muscle orbital layers. Orbital connective tissues, including the pulleys, constrain extraocular muscle paths. Abnormalities of these tissues may lead to strabismus that is not due to neurologic pathology. Some extraocular muscles are divided into independent neuromuscular compartments, so that partial motor nerve lesions may manifest as selective denervation of only 1 compartment, complicating the presentation of neuropathic strabismus.

CONCLUSIONS

Strabismus in adults due to nonneurologic causes can result from recently described abnormalities of the orbital connective tissue pulley system. Advances in understanding of compartmental extraocular muscle anatomy and innervation can explain cyclovertical strabismus in partial nerve palsies. Recognition of the underlying pathogenesis of the strabismus can lead to improved treatments.

An unusual variant of the abducens nerve duplication with two nerve trunks merging within the orbit: a case report with comments on developmental background

This study reports the first case of abducens nerve duplication along its entire intracranial course, ending within the orbit. A distinct abducens nerve duplication reaching the common tendinous ring (annulus of Zinn), as well as another split within the intraconal segment of the nerve have been revealed. Additionally, two groups (superior and inferior) of abducens nerve sub-branches to the lateral rectus muscle were visualised using Sihler's stain. The analysed anatomical variation has never been reported before and it seems to be in the middle of the spectrum between the cases of duplication occurring only within the intracranial segments of the abducens nerve found in the literature and those continuing throughout the whole course of the nerve. Abducens nerve duplication may be treated as a relic of early stages of ontogenesis. Such a variant might result from alternative developmental pathways in which axons of the abducens nerve, specific for a given segment of the lateral rectus muscle, run separately at some stage, instead of forming a single stem.

Adjustable small-incision selective tenotomy and plication for correction of incomitant vertical strabismus and torsion

PURPOSE

To evaluate the effectiveness of adjustable small-incision selective tenotomy and plication of vertical rectus muscles in correcting vertical strabismus incomitant in horizontal gaze positions and cyclotorsion.

METHODS

The medical records of all patients who underwent adjustable small-incision selective tenotomy or plication of a vertical rectus muscle for correction of horizontally incomitant vertical strabismus or cyclotorsion by a single surgeon at a single eye institute from July 2013 to September 2014 were retrospectively reviewed. Selective tenotomy and plication were performed on either the nasal or temporal side of vertical rectus muscles, based on the direction of cyclotorsion and incomitance of vertical strabismus.

RESULTS

Of 9 patients identified, 8 (89%) had successful correction of horizontally incomitant vertical strabismus, with postoperative vertical alignment within 4(Δ) of orthotropia in primary position, lateral gazes, and downgaze. Of the 8 patients with preoperative cyclotorsion, 4 (50%) were successfully corrected, with <5° of cyclotorsion postoperatively. Of the 4 patients in whom cyclotorsion did not improve, 3 had undergone prior strabismus surgery, and 2 had restrictive strabismus. Eight of the 9 patients (89%) reported postoperative resolution of diplopia.

CONCLUSIONS

Adjustable small-incision selective tenotomy and plication effectively treat horizontally incomitant vertical strabismus. These surgeries may be less effective for correcting cyclotorsion in patients with restriction or prior strabismus surgery. Advantages are that they may be performed in an adjustable manner and, in some cases, under topical anesthesia.

Graded vertical rectus tenotomy for small-angle cyclovertical strabismus in sagging eye syndrome

BACKGROUND/AIMS

Graded vertical rectus tenotomy (GVRT) is postulated as effective for small-angle vertical heterotropia. We aimed to determine the dosing recommendations for GVRT in sagging eye syndrome (SES).

METHODS

This was a retrospective, observational study of surgical outcomes for GVRT from 2009 to 2014 in a single surgeon's academic practice. There were 37 (20 women) patients of average age 68±10 (SD) years with comitant or incomitant hypertropia ≤10Δ caused by SES. The main outcome measure was the dose-effect of GVRT required to correct intraoperative hypertropia.

RESULTS

Preoperative average central gaze hypertropia measured 4.7±2.2Δ. Three patients underwent repeat GVRT for residual or consecutive hypertropia, one undergoing it twice. All surgeries were analysed, increasing the total operations to 41. The inferior rectus tendon in the hypotropic eye was operated in 32 eyes, and the superior rectus tendon in the hypertropic eye in 9 eyes. Mean tenotomy was 68±19% of tendon width. Hypertropia was always eliminated intraoperatively by progressive GVRT. Mean hypertropia was 1.1±1.6Δ at average 93 days postoperatively. Linear regression demonstrated that 3-6Δ hypertropia correction requires 30%-90% graded tenotomy (R(2)=0.32, p<0.0001), but with substantial individual variability. Undercorrection necessitated reoperation in 10% of cases.

CONCLUSION

GVRT precisely corrects hypertropia of up to 10Δ, but because of variable effect, it should be performed with intraoperative monitoring under topical anaesthesia.

Independent active contraction of extraocular muscle compartments

PURPOSE

Intramuscular innervation of horizontal rectus extraocular muscle (EOMs) is segregated into superior and inferior (transverse) compartments, whereas all EOMs are also divided into global (GL) and orbital (OL) layers with scleral and pulley insertions, respectively. Mechanical independence between both types of compartments has been demonstrated during passive tensile loading. We examined coupling between EOM compartments during active, ex vivo contraction.

METHODS

Fresh bovine EOMs were removed, and one compartment of each was coated with hydrophobic petrolatum. Contraction of the uncoated compartment was induced by immersion in a solution of 50 mM CaCl2 at 38°C labeled with sodium fluorescein dye, whereas tensions in both compartments were monitored by strain gauges. Control experiments omitted petrolatum so that the entire EOM contracted. After physiological experiments, EOMs were sectioned transversely to demonstrate specificity of CaCl2 permeation by yellow fluorescence dye excited by blue light.

RESULTS

In control experiments without petrolatum, both transverse and GL and OL compartments contracted similarly. Selective compartmental omission of petrolatum caused markedly independent compartmental contraction whether measured at the GL or the OL insertions or for transverse compartments at the scleral insertion. Although some CaCl2 spread occurred, mean (±SD) tension in the coated compartments averaged only 10.5 ± 3.3% and 6.0 ± 1.5% in GL/OL and transverse compartments, respectively relative to uncoated compartments. Fluorescein penetration confirmed selective CaCl2 permeation.

CONCLUSIONS

These data confirm passive tensile findings of mechanical independence of EOM compartments and extend results to active contraction. EOMs behave actively as if composed of mechanically independent parallel fiber bundles having different insertional targets, consistent with the active pulley and transverse compartmental hypotheses.

Sensitivity of the three-step test in diagnosis of superior oblique palsy

PURPOSE

Although the Parks-Bielschowsky three-step test is the cornerstone of cyclovertical strabismus diagnosis, it has not been validated against an external benchmark. We evaluated the test's sensitivity in clinical diagnosis of superior oblique palsy in patients with unequivocal magnetic resonance imaging (MRI) evidence of superior oblique atrophy.

METHODS

A total of 73 strabismic patients were selected from a prospective MRI study because they exhibited superior oblique atrophy indicative of superior oblique denervation and thus confirmatory of superior oblique palsy. Of these, 50 patients who had no confounding factors were included for detailed study. Ocular motility data were evaluated to determine sensitivity of single and combined clinical findings in diagnosis of superior oblique palsy.

RESULTS

Maximum mean ipsilesional superior oblique cross section was reduced to 9.6 ± 0.6 mm(2) (mean ± standard error) in superior oblique palsy, representing 52% of the 18.5 ± 0.6 mm(2) contralesional superior oblique maximum cross section and 52% of the 18.4 ± 0.4 mm(2) control maximum superior oblique cross section (P < 0.001). Of the 50 patients, 35 (70%) with superior oblique atrophy fulfilled the entire three-step test. In 14 (28%) patients two steps were fulfilled; in 1 patient (2%), only one step. Affected superior oblique cross section was similar in orbits that fulfilled the three-step test (9.8 ± 0.9 mm(2)) and those that did not (9.1 ± 0.7 mm(2); P = 0.58).

CONCLUSIONS

The complete three-step test fails to detect 30% of cases of superior oblique atrophy. Often only two of three steps are positive in superior oblique palsy.

How does the structure of extraocular muscles and their nerves affect their function?

The sensory and motor control of human extraocular muscles (EOMs) have been subjected to considerable speculation in ophthalmic literature, often related to infranuclear structures such as the unique complement of muscle fibres and their associated sensory organs. The intrafusal fibres do not resemble their somatic counterparts and their peculiar morphology has raised questions about their proprioceptive capacity. No Golgi tendon organs have so far been observed and the myotendinous nerve endings, previously assumed to convey sensory information, have recently been argued to merely represent constituents of the efferent innervation serving the multiply innervated muscles fibres. These observations raise questions about the overall capacity to monitor the activity created by the generous efferent nerve supply observed in these muscles. Furthermore, the argued independent activity of muscular layers and compartments suggest that the required feedback must be highly structured and more specific than previously assumed. Yet, uncertainty about the source of such information remains. The purpose of this paper is to provide a short review of neuromuscular properties of human extraocular muscles. Their functional implications and the most reputable sources of proprioception will also be discussed. The promoted views are based on pertinent literature and previous research undertaken by the authors.

Independent passive mechanical behavior of bovine extraocular muscle compartments

PURPOSE

Intramuscular innervation of horizontal rectus extraocular muscles (EOMs) is segregated into superior and inferior (transverse) compartments, while all EOMs are also divided into global (GL) and orbital (OL) layers with scleral and pulley insertions, respectively. We sought evidence of potential independent action by examining passive mechanical coupling between EOM compartments.

METHODS

Putative compartments of each of the six whole bovine anatomical EOMs were separately clamped to a physiologically controlled, dual channel microtensile load cell (5-mN force resolution) driven by independent, high-speed, linear motors having 20-nm position resolution. One channel at a time was extended or retracted by 3 to 5 mm, with the other channel stationary. Fiducials distributed on the EOM global surface enabled optical tracking of local deformation. Loading rates of 5 to 100 mm/sec were applied to explore speeds from slow vergence to saccades. Control loadings employed transversely loaded EOM and isotropic latex.

RESULTS

All eom bellies and tendons exhibited substantial compartmental independence when loaded in the physiologic direction, both between OL and GL, and for arbitrary transverse parsings of EOM width ranging from 60%: 40% to 80%:20%. Intercompartmental force coupling in the physiologic direction was less than or equal to 10% in all six EOMS even for saccadic loading rates. Coupling was much higher for nonphysiologic transverse EOM loading and isotropic latex. Optical tracking demonstrated independent strain distribution between EOM compartments.

CONCLUSIONS

Substantial mechanical independence exists among physiologically loaded fiber bundles in bovine EOMs and tendons, providing biomechanical support for the proposal that differential compartmental function in horizontal rectus EOMs contributes to novel torsional and vertical actions.