Influence of Biometric Data on Planning Strabismus Surgery. / Einfluss von biometrischen Daten auf die Planung der Augenmuskeloperation.

PURPOSE: To evaluate the influence of optical biometry data (axial eye length, anterior chamber depth) on planning strabismus surgery using a simulation software and partial least squares regression. METHODS: This retrospective study included patients who had undergone strabismus surgery in one eye involving only the horizontal eye muscles. Furthermore, optical biometry had been performed and the extent of strabismus had been measured pre- and postoperatively. In the next step the strabismus surgery was simulated (See++, RISC, Austria) with and without axial eye length data. In the last step, anatomical data of the eye were used and their influence on the postoperative extent of strabismus was evaluated using partial least squares regression and boot strapping. RESULTS: Of 97 patients, 92 were included in the analysis. In all cases the extent of strabismus was reduced by at least 25% and in 60% of the cases the reduction was at least 75%. Taking the axial eye length into account improved the simulation slightly (change of surgical planning: 0.30 mm, standard deviation 1.65 mm). DISCUSSION: The simulation model used showed that including the axial eye length is useful for strabismus surgery planning. However, the anterior chamber depth/axial eye length was found to have a significantly greater impact.

Development of a Generic Tubular Tree Structure for the Modeling of Orbital Cranial Nerves.

We developed a generic approach for modeling tubular tree structures as triangle meshes for the extension of our biomechanical eye model SEE-KID with a visualization of the orbital cranial nerves. Since three of the orbital nerves innervate extraocular eye muscles and move together with them, the structure must also support the partial translation and rotation of the nerves. For the SEE-KID model, this extension allows a better parameterization as well as an easier simulation of innervational disorders. Moreover, it makes the model even more useful for education and training purposes in contrast to other anatomical models. Due to its generic nature, the developed data structure and the associated algorithms can be used for any tubular tree structures, even in non-medical application areas.

Computer-assisted diagnosis of monocular elevation deficiency.

Our aim is to demonstrate the benefits of using a computer model to support the clinical diagnosis of complex eye motility disorders. For diagnosis and differential diagnosis we compared the clinical data of a patient with suspected monocular elevation deficiency (MED) and the corresponding computer simulation with the simulations of rectus superior palsy, vertical Duane miswiring syndrome and two simulations of asymmetric gaze palsy. We used our biomechanical eye model SEE-KID for the computer simulations, which is partly based on ideas and concepts of the software system Orbit™ by Joel Miller. A young patient with the clinical characteristics of congenital MED, unilateral limitation of up-gaze above midline, with accompanying ptosis on the affected right side, mild head posture, chin-up position, partial binocular functions and Bell's phenomenon was examined. Pupillary situation, cover test, version and duction movements, saccadic test, Parks-Bielschowsky phenomenon and head tilt test, stereopsis test, Bagolini striated lens test and forced duction test were assessed. Up to the age of 5 years we used the prism cover test in the nine main gaze positions; later we switched to the Hess-Lancaster test for analyzing deviations. We also used our computer model for evaluating the diagnosis and for differential diagnosis of our patient. The simulation results from the SEE-KID model support the diagnosis of supranuclear MED, which can be achieved in the model by varying central innervations, contrary to the modification of muscle forces in the simulation of a rectus superior palsy. It is necessary to distinguish between supranuclear, nuclear, interstitial or peripheral lesions with regard to monocular elevation deficit. Simulations of patients with similar pathologies in a way that the simulations correspond to the patient-measured values support (beside the clinical signs) the diagnosis of supranuclear or infranuclear lesions.

Biomechanical analysis of x-pattern exotropia.

PURPOSE: To simulate and check the plausibility of the proposed mechanisms of X-pattern exotropia and to determine the least invasive surgical method that can be used to treat the disorder. DESIGN: Computational supported analysis and retrospective study. METHODS: The oculomotor model SEE++ was used to simulate the effects of the different causes that have been proposed for the X-phenomenon. In addition, a retrospective study was conducted using preoperative and postoperative measurements of 10 patients with X-pattern exotropia. Eye movements and surgery of these patients were simulated and analyzed statistically. RESULTS: Our computer-based simulations showed that only 1 of the 4 proposed theories can account successfully for the observed X-patterns: an overaction of all 4 oblique muscles can induce divergent exotropia in upgaze and downgaze, and an alteration of horizontal muscles can cause the additional divergence in all gaze positions. The simulation of eye muscle surgery confirmed that a sufficient correction of the divergent deviation in all gazes already can be achieved by a recession and resection of 2 horizontal eye muscles. CONCLUSIONS: In case of X-pattern exotropia, recession and resection of 2 horizontal muscles can be used as a first-line therapy, leading to a simplification of the therapy.

Pressure cell for investigations of solid-liquid interfaces by neutron reflectivity.

We describe an apparatus for measuring scattering length density and structure of molecular layers at planar solid-liquid interfaces under high hydrostatic pressure conditions. The device is designed for in situ characterizations utilizing neutron reflectometry in the pressure range 0.1-100 MPa at temperatures between 5 and 60 °C. The pressure cell is constructed such that stratified molecular layers on crystalline substrates of silicon, quartz, or sapphire with a surface area of 28 cm(2) can be investigated against noncorrosive liquid phases. The large substrate surface area enables reflectivity to be measured down to 10(-5) (without background correction) and thus facilitates determination of the scattering length density profile across the interface as a function of applied load. Our current interest is on the stability of oligolamellar lipid coatings on silicon surfaces against aqueous phases as a function of applied hydrostatic pressure and temperature but the device can also be employed to probe the structure of any other solid-liquid interface.

Collision and containment detection between biomechanically based eye muscle volumes.

Collision and containment detection between three-dimensional objects is a common requirement in simulation systems. However, few solutions exist when exclusively working with deformable bodies. In our ophthalmologic diagnostic software system, the extraocular eye muscles are represented by surface models, which have been reconstructed from magnetic resonance images. Those models are projected onto the muscle paths calculated by the system's biomechanical model. Due to this projection collisions occur. For their detection, three approaches have been implemented, which we present in this paper: one based on image-space techniques using OpenGL, one based on the Bullet physics library and one using an optimized space-array data structure together with software rendering. Finally, an outlook on a possible response to the detected collisions is given.

Computer-based simulation of the Bielschowsky head-tilt test using the SEE++ software system.

INTRODUCTION: Latest measurements of the vestibulo-ocular reflex (VOR) allowed the integration of the simulation of the Bielschowsky head-tilt test (BHTT) into the SEE++ software system. SEE++ realizes a biomechanical model of the human eye in order to simulate eye motility disorders and strabismus surgeries. With the addition of the BHTT it can now also be used for differential-diagnostic simulations of complex disorders (e.g., superior oblique palsies). METHODS: In order to simulate the BHTT in SEE++, the user can freely choose the desired head-tilt angle from -45 degrees to +45 degrees. The chosen angle is shown in the 3D view with a human body model and is also used in the calculation of the Hess-Lancaster test. RESULTS: The integration of the BHTT offers an additional improvement of the possibilities for simulating eye motility disorders. Moreover, SEE++ allows the creation of a video of the "virtual patient" while tilting the head from one side to the other, which shows dynamic changes in the simulated Hess-diagrams. DISCUSSION: Comparisons of simulation results with patient-measured data showed a good correlation between the simulated and the measured data. Further comparisons with patient data are planned.

Model-based improvements in the treatment of patients with strabismus and axial high myopia.

PURPOSE: Eye motility disorders with axial high myopia and an enlarged globe are often characterized by a hypotropia of the affected eye, usually referred to as heavy-eye syndrome. Based on an intuitive interpretation of magnetic resonance (MR) images, the cause of the hypotropia has typically been assigned to the rectus muscles. In this study, the hypothesis that the oblique muscles play an important role in the underlying biomechanical disorder of this type of strabismus was investigated. METHODS: The hypothesis was tested by (1) a retrospective analysis of surgical results in one patient with unilateral axial high myopia; and (2) MR images of orbital tissues in two further patients with unilateral axial high myopia. RESULTS: MR images demonstrated a pattern of extraocular muscle path displacements similar to those described previously, but also a uniform decrease in the cross-sectional area of the inferior oblique muscles. Computer modeling required decreased inferior oblique contractility in addition to displaced extraocular muscle paths to recreate the observed motility pattern accurately. CONCLUSIONS: Patients with axial high myopia regularly show a reduction in the diameter of the inferior oblique. The resultant reduction in muscle-strength is important for the correct explanation of this complex eye movement disorder.