S. Schutte

A finite-element analysis model of orbital biomechanics

To reach a better understanding of the suspension of the eye in the orbit, an orbital mechanics model based upon finite-element analysis (FEA) has been developed. The FEA model developed contains few prior assumptions or constraints (e.g., the position of the eye in the orbit), allowing modeling of complex three-dimensional tissue interactions; unlike most current models of eye motility. Active eye movements and forced ductions were simulated and showed that the supporting action of the orbital fat plays an important role in the suspension of the eye in the orbit and in stabilization of rectus muscle paths.

Mechanical properties and functional importance of pulley bands or faisseaux tendineux

INTRODUCTION Connective tissue bands connect the horizontal rectus muscles at the level of the posterior pole to the orbital wall. These bands, referred to as pulley bands or faisseaux tendineux, purportedly act like springs to keep the rectus muscle bellies in place during eye movement out of the plane of the muscle. We examined the mechanical properties of these bands in human specimens obtained during surgery. In addition, we examined eye motility and stability of rectus muscles in a patient who had no functional pulley bands. METHODS Exenterations were carried out on two patients with sebaceous gland carcinoma. Pulley bands were identified and force-elongation behavior was examined with a forceps and a force gauge. Stability of rectus muscles was examined in a patient with severe Crouzons syndrome by orbital CT scans and during surgery under local, eye drop, anesthesia. RESULTS The pulley bands showed leash-like mechanical behavior: they were slack over approximately 10mm and became taut when stretched further. In the patient with Crouzons syndrome, both CT and observation of the muscle during surgery showed little sideways displacement of the muscle bellies in eye movement out of the plane of the muscle, despite the lack of functional pulley bands. DISCUSSION The leash-like mechanical behavior of the pulley bands seems unsuited for stabilization of the muscle bellies. The patient with Crouzons syndrome had relatively good eye motility and stable rectus muscle paths despite the lack of functional pulley bands.

An observational study to quantify manual adjustments of the inspired oxygen fraction in extremely low birth weight infants

Aim:  To quantify manual fraction of inspired oxygen (FiO2) adjustments performed by caregivers in extremely low birth weight (ELBW; ≤1000 g) infants, in relation to oxygen saturation (SpO2) and bedside care.

Threshold Amplitude and Frequency for Ocular Tissue Release from a Vibrating Instrument:An Experimental Study

PURPOSE During retinal pigment epithelium (RPE) and choroid graft translocation in the treatment of patients with exudative age-related macular degeneration, the adhesion of the graft to the translocation instrument complicated its submacular release. Vibration of the instrument improved the release of the graft. This study was conducted to validate the effectiveness of the principle of vibration and to determine the threshold amplitude and frequency required for development of an optimized instrument. METHODS An experimental in vitro model with fresh porcine RPE-choroid grafts was used. Release of the graft was studied by a masked observer for amplitudes in the range of 0.05 to 1.2 mm and frequencies in the range of 25 to 200 Hz in the horizontal plane. RESULTS The minimum threshold amplitude required to release the graft was approximately 0.15 mm from a frequency of 100 Hz and higher. CONCLUSIONS This study confirmed the clinical experience that vibration of an instrument induces the release of the RPE-choroid graft. The minimum threshold amplitude and frequency needed for optimum tissue release were estimated.

Accurate Gaze Direction Measurements With Free Head Movement for Strabismus Angle Estimation

We present the Delft Assessment Instrument for Strabismus in Young children (DAISY) a device designed to measure angles of strabismus in young children fast and accurately. DAISY allows for unrestrained head movements by the mean of a triple camera vision system that simultaneously estimates the head rotation and the eye pose. The device combines two different methods to record bilateral eye position: corneal reflections (Purkinje images) and pupillary images. Detailed results are provided on three orthotropic subjects (age 25-27). Three different conditions were tested: 1) gaze ahead, 2) gaze ahead with different head rotations, and 3) fixed head with different eye positions. Systematic errors occurred between subjects that need further study. The system reached sufficient accuracy to be applied for the measurement of angles of strabismus, almost independent from the head pose.

MRI-based Visualisation of Orbital Fat Deformation During Eye Motion

Orbital fat, or the fat behind the eye, plays an important role in eye movements. In order to gain a better understanding of orbital fat mobility during eye motion, MRI datasets of the eyes of two healthy subjects were acquired respectively in seven and fourteen different directions of gaze. After semi-automatic rigid registration, the Demons deformable registration algorithm was used to derive time-dependent three-dimensional deformation vector fields from these datasets. Visualisation techniques were applied to these datasets in order to investigate fat mobility in specific regions of interest in the first subject. A qualitative analysis of the first subject showed that in two of the three regions of interest, fat moved half as much as the embedded structures. In other words, when the muscles and the optic nerve that are embedded in the fat move, the fat partly moves along with these structures and partly flows around them. In the second subject, a quantitative analysis was performed which showed a relation between the distance behind the sciera and the extent to which fat moves along with the optic nerve.

Micro-scale thermal tissue gripper.

During eye surgery translocating a graft of retinal tissue without damaging the vulnerable top layer is highly problematic using the currently available instruments. This study presents indirect Heat Induced Attachment and Detachment (HIAD) as a new concept for single side attachment of tissue to, and detachment from a heated metal wire. A small-scale prototype was built, having a 50 μm molybdenum wire that could be heated via an electric current. Tests (n = 60) were performed on submerged chicken meat to study the attachment and detachment properties of the prototype at different electric signal lengths. A 9V signal was applied to the prototype, with amplifier input signal lengths varying from 0.6–1.5 ms. Voltages and currents were sampled at 250 kHz to obtain energies. Both attachment and detachment occurred with 98% and 90% certainty, respectively, supplying 41 and 110 mJ of energy to the prototype in 0.7 and 1.5 ms. The attachment strength was estimated at 0.2 mN. Visible damage appeared to be approximately 0.005 mm2. The concept of indirect heating of the instrument-tissue interface proved to be effective as the prototype could induce attachment and detachment of tissue. Indirect HIAD may be applicable in many different surgical applications.

Dynamic visualisation of orbital fat deformation using anatomy-guided interaction

The human eye is a biomechanical system. Orbital fat plays an important role in the working of this system, but its behaviour during eye movement is not well understood. To give insight into this behaviour, visualisation is a useful tool. This paper presents a complete pipeline for interactive particle-based visualisation and exploration of orbital fat deformation from MRI data. Sensible 3D particle seeding is important in this type of visualisation. We address that problem with a two-step process: Interactive, anatomy-guided slice positioning, and contour-based region of interest specification. Since the deformation calculation is unlikely to be correct everywhere, we derive and visualise an uncertainty measure based on deformed and original MRI data. We also performed a case study evaluation to investigate the benefits of our approach towards orbital fat deformation visualisation.

Single-sided and small-scaled grasping of delicate tissues: effectiveness of indirect heat-induced attachment and detachment.

Abstract Introduction: Indirect heat-induced attachment and detachment (iHIAD) is a promising concept for gripping delicate tissues in microsurgery. However, the optimal settings of iHIAD are unknown. This study evaluates the effects of the instrument heating properties and initial contact force on the adhesion force, detachment success and thermal damage. Material and methods: An instrument prototype was developed to test attachment and detachment for different combinations of generated energy (3.5–20.0 mJ) and pulse length (0.25–2.50 ms). The tissues tested on were kidney and eye from the pig. Thermal tissue damage was estimated with a histological analysis. Results: The adhesion force Fa depended strongly on the amount of generated energy (Fa = 0.03–2.5 mN) and contact force (Fa = 0.25–1.0 mN). Pulse length played a minor role. Detachment success (0–100%) was determined by generated energy (3–16 mJ). Histological analysis showed minimal damage. Conclusion: Adhesion forces increased with increasing contact forces. iHIAD proved sensitive to heating characteristics. Detachment success increased with generated energy. Thermal damage was minimal. iHIAD creates a potential to build better performing tissue manipulators.

Orbital Soft Tissue Biomechanics

The human eye is suspended in the orbit. A miraculous interaction between pressure and tensile forces in the soft tissues keeps the eye in place, while a large range of rotational motion is enabled. This article discusses the factors (the geometrical shapes of the tissues, their material properties, their mechanical interactions, and the mechanical loads) that play a role in understanding soft tissue biomechanics and the suspension of the eye in the orbit.