Orbital Pulleys Research Articles

Biomechanical modeling of actively controlled rectus extraocular muscle pulleys

The Active Pulley Hypothesis (APH) is based on modern functional anatomical descriptions of the oculomotor plant, and postulates behaviors of the orbital pulleys proposed to be positioned by the extraocular muscles (EOMs). A computational model is needed to understand this schema quantitatively. We developed and evaluated a novel biomechanical model of active horizontal rectus pulleys. The orbital (OL) and global (GL) layers of the horizontal rectus EOMs were implemented as separate musculoskeletal strands. Pulley sleeves were modeled as tube-like structures receiving the OL insertion and suspended by elastic strands. Stiffnesses and orientations of pulley suspensions were determined empirically to limit horizontal rectus EOM side-slip while allowing anteroposterior pulley travel. Independent neural drives of the OL greater than GL were assumed. The model was iteratively refined in secondary gazes to implement realistic behavior using the simplest mechanical configuration and neural control strategy. Simulated horizontal rectus EOM paths and pulley positions during secondary gazes were consistent with published MRI measurements. Estimated EOM tensions were consistent with the range of experimentally measured tensions. This model is consistent with postulated bilaminar activity of the EOMs, and the separate roles of the GL in ocular rotation, and OL in pulley positioning.

Standard coronal orbital magnetic resonance imaging is an effective technique for diagnosing sagging eye syndrome.

The aim of this study was to determine the importance and efficacy of the standard coronal magnetic resonance imaging (MRI) analysis method for the correct clinical diagnosis of the sagging eye syndrome. This retrospective study evaluated the standard coronal MRI efficacy by comparing the positions of the orbital pulleys and extraocular muscles in patients with sagging eye syndrome as well as controls. The participants included 50 patients with sagging eye syndrome (aged 73.3 ± 6.7years) and 17 age-matched control patients (aged 70.9 ± 4.3years) that presented optic neuritis. The participants were classified into groups of age-related distance esotropia and cyclo-vertical strabismus. Lateral rectus-superior rectus band condition, lateral rectus vertical angle, lateral rectus tilting angle, and superior rectus-lateral rectus displacement angle were examined. The lateral rectus-superior rectus band was more frequently disordered in the two groups than that in the control. The lateral rectus vertical and tilt angle examinations exhibited a significant difference in the age-related distance esotropia [(- 13.3 ± 10.9°, p < 0.05) and (- 24.1 ± 9.5°, p < 0.01)] and cyclo-vertical strabismus [(- 11.9 ± 6.9°, p < 0.05) and (- 20.8 ± 9.3°, p < 0.01)] groups compared with those in the control group [(- 7.5 ± 5.1°) and (- 12.2 ± 9.1°)]. The superior rectus-lateral rectus displacement angle did not present a significant difference between the patients and control groups. Patients with sagging eye syndrome exhibited disordered orbital pulley and extraocular muscle malposition, as determined by standard coronal magnetic resonance imaging, indicating the effectiveness of the procedure for diagnosing sagging eye syndrome.

Infinitely variable transmission with orbital pulleys

The purpose of this article is to propose an infinite variable transmission with orbital pulleys, which consists of two parallel transmission systems. The system comprises a planetary gear set and a continuously variable transmission. The principle of operation of the infinite variable transmission proposed in this article is based on the actuation of one half-sided pulley, which has a translational movement in a direction, and the same amount of movement is reproduced by the other half-sided orbital pulley in reverse to secure that the length of the belt remains constant. The fixed constant horizon enables the infinite variable transmission to change the transmission ratio from negative values to positive passing through zero in a continuous manner without using a clutch or interrupting the system. The dynamic model and prototype of the infinite variable transmission with orbital pulleys are developed for designing, controlling, and validating purposes. The model is obtained using the Euler–Lagrange methodology, and it is experimentally validated by comparing the proposed model with the experimental measures. The infinite variable transmission with orbital pulleys is controlled under different conditions; the experimental results show that the proposed design of infinite variable transmission provides robustness to maintain constant speeds at the output to changes at the input velocity.

Causes, background, and characteristics of binocular diplopia in the elderly.

We evaluated the background and characteristics of elderly patients with binocular diplopia including diseases caused by abnormalities in orbital pulleys. Retrospective. The participants were 236 patients aged 60 years or older who visited Kitasato University Hospital complaining of binocular diplopia. We classified strabismus by types and investigated the causes for each group. We diagnosed orbital pulley disorders using magnetic resonance imaging (MRI). Patients with orbital pulley disorders exhibited esotropia and/or vertical strabismus and did not present with cranial nerve palsy, systemic illness, or ocular injury. Classification of strabismus types was: exotropia (24.2%); esotropia (25.0%); vertical strabismus (30.1%), combined strabismus (20.8%). There were 50.9% cases of strabismus associated with vertical deviation. The causes of disease in each group were as follows: in the exotropia group, 50.9% convergence insufficiency exotropia, 21.1% basic exotropia; in the esotropia group: 35.6% orbital pulley disorder 33.9% sixth cranial nerve palsy; in the vertical strabismus group: 32.4% forth cranial nerve palsy, 31.0% orbital pulley disorder; in the combined strabismus group: 28.6% orbital pulley disorder, 28.6% forth cranial nerve palsy. About half of the elderly patients with binocular diplopia exhibited vertical deviation. In addition, binocular diplopia was often caused by orbital pulley disorders. It is the first epidemiological study focusing on orbital pulley disorders diagnosed on the basis of MRI.

Degeneration of orbital pulleys in elderly patients with diplopia

In recent years, with the development of diagnostic imaging, diagnosis of diplopia induced by the degeneration of orbital pulleys has increased. We studied elderly patients with diplopia who could not be diagnosed by conventional testing and compared their MRI to those of controls.

Cross-coupled eye movement supports neural origin of pattern strabismus.

Pattern strabismus describes vertically incomitant horizontal strabismus. Conventional theories emphasized the role of orbital etiologies, such as abnormal fundus torsion and misaligned orbital pulleys as a cause of the pattern strabismus. Experiments in animal models, however, suggested the role of abnormal cross-connections between the neural circuits. We quantitatively assessed eye movements in patients with pattern strabismus with a goal to delineate the role of neural circuits versus orbital etiologies. We measured saccadic eye movements with high-precision video-oculography in 14 subjects with pattern strabismus, 5 with comitant strabismus, and 15 healthy controls. We assessed change in eye position in the direction orthogonal to that of the desired eye movement (cross-coupled responses). We used fundus photography to quantify the fundus torsion. We found cross-coupling of saccades in all patients with pattern strabismus. The cross-coupled responses were in the same direction in both eyes, but larger in the nonviewing eye. All patients had clinically apparent inferior oblique overaction with abnormal excylotorsion. There was no correlation between the amount of the fundus torsion or the grade of oblique overaction and the severity of cross-coupling. The disconjugacy in the saccade direction and amplitude in pattern strabismics did not have characteristics predicted by clinically apparent inferior oblique overaction. Our results validated primate models of pattern strabismus in human patients. We found no correlation between ocular torsion or oblique overaction and cross-coupling. Therefore, we could not ascribe cross-coupling exclusively to the orbital etiology. Patients with pattern strabismus could have abnormalities in the saccade generators.

Author Response: Motion-Encoded MRIs Provide Evidence against Orbital Pulleys

Author Response: Motion-Encoded MRIs Provide Evidence against Orbital Pulleys

Motion-Encoded MRIs Provide Evidence against Orbital Pulleys

Motion-Encoded MRIs Provide Evidence against Orbital Pulleys

Evidence supporting extraocular muscle pulleys: refuting the platygean view of extraocular muscle mechanics.

Late in the 20th century, it was recognized that connective tissue structures in the orbit influence the paths of the extraocular muscles and constitute their functional origins. Targeted investigations of these connective tissue "pulleys" led to the formulation of the active pulley hypothesis, which proposes that pulling directions of the rectus extraocular muscles are actively controlled via connective tissues. This review rebuts a series of criticisms of the active pulley hypothesis published by Jampel, and Jampel and Shi, in which these authors have disputed the existence and function of the pulleys. This article reviews published evidence for the existence of orbital pulleys, the active pulley hypothesis, and physiological tests of the active pulley hypothesis. Magnetic resonance imaging in a living subject and histological examination of a human cadaver directly illustrate the relationship of pulleys to extraocular muscles. Strong scientific evidence is cited that supports the existence of orbital pulleys and their role in ocular motility. The criticisms of the hypothesis have ignored mathematical truisms and strong scientific evidence. Actively control led orbital pulleys play a fundamental role in ocular motility. Pulleys profoundly influence the neural commands required to control eye movements and binocular alignment. Familiarity with the anatomy and physiology of the pulleys is requisite for a rational approach to diagnosing and treating strabismus using emerging methods. Conversely, approaches that deny or ignore the pulleys risk the sorts of errors that arise in geography and navigation from incorrect assumptions such as those of a flat ("platygean") earth.

Evidence Against Mobile Pulleys on the Rectus Muscles and Inferior Oblique Muscle: Central Nervous System Controls Ocular Kinematics

To provide evidence against the existence of orbital pulleys. Interpretation of magnetic resonance imaging (MRI) scans; video eye tracking; ocular motor nerve stimulations; and clinical observations. No pulleys or planes splitting the extraocular muscles into layers were noted on MRI scans. Smooth muscle does not antagonize striate muscle. There is no physiological evidence that human rectus pulleys shift the ocular rotational axes to attain commutative behavior. In the monkey and humans, the axes of rotation are not determined by eye position. Operations on the extraocular muscles reveal no pulleys. The somatosensory system of the central nervous system controls the extraocular muscles. The autonomic nervous and the hormonal systems control the infrastructure of the orbit vital for the function of the extraocular muscles. The three systems are integrated and controlled by the central nervous system. Neural circuits are necessary to compensate for extraocular muscle abnormalities. There are no pulleys.

Commutative saccadic generator is sufficient to control a 3-D ocular plant with pulleys.

One-dimensional models of oculomotor control rely on the fact that, when rotations around only one axis are considered, angular velocity is the derivative of orientation. However, when rotations around arbitrary axes [3-dimensional (3-D) rotations] are considered, this property does not hold, because 3-D rotations are noncommutative. The noncommutativity of rotations has prompted a long debate over whether or not the oculomotor system has to account for this property of rotations by employing noncommutative operators. Recently, Raphan presented a model of the ocular plant that incorporates the orbital pulleys discovered, and qualitatively modeled, by Miller and colleagues. Using one simulation, Raphan showed that the pulley model could produce realistic saccades even when the neural controller is commutative. However, no proof was offered that the good behavior of the Raphan-Miller pulley model holds for saccades different from those simulated. We demonstrate mathematically that the Raphan-Miller pulley model always produces movements that have an accurate dynamic behavior. This is possible because, if the pulleys are properly placed, the oculomotor plant (extraocular muscles, orbital pulleys, and eyeball) in a sense appears commutative to the neural controller. We demonstrate this finding by studying the effect that the pulleys have on the different components of the innervation signal provided by the brain to the extraocular muscles. Because the pulleys make the axes of action of the extraocular muscles dependent on eye orientation, the effect of the innervation signals varies correspondingly as a function of eye orientation. In particular, the Pulse of innervation, which in classical models of the saccadic system encoded eye velocity, here encodes a different signal, which is very close to the derivative of eye orientation. In contrast, the Step of innervation always encodes orientation, whether or not the plant contains pulleys. Thus the Step can be produced by simply integrating the Pulse. Particular care will be given to describing how the pulleys can have this differential effect on the Pulse and the Step. We will show that, if orbital pulleys are properly located, the neural control of saccades can be greatly simplified. Furthermore, the neural implementation of Listing's Law is simplified: eye orientation will lie in Listing's Plane as long as the Pulse is generated in that plane. These results also have implications for the surgical treatment of strabismus.