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.

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 (FiO(2)) adjustments performed by caregivers in extremely low birth weight (ELBW; ≤1000 g) infants, in relation to oxygen saturation (SpO(2)) and bedside care. METHODS: In a single-centre study, FiO(2) , SpO(2) and alarm limits of ELBW infants were collected for 3 days continuously, while caregivers were filmed. A descriptive analysis, focused on manual FiO(2) adjustments, was performed. RESULTS: Twelve ELWB infants were included. Total recording time was 726 h. FiO(2) was increased 851 times and decreased 1309 times; median (range) step size was 5% (1% to 65%) and -3% (-1% to -65%), respectively. Wide variation of FiO(2) adjustments for equal levels of SpO(2) was observed in all included infants. One hundred and twenty-six of 136 FiO(2) adjustments with a step size ≥15% and 111 of 171 desaturations <70% were associated with medical or nursing procedures. When FiO(2) was >21%, alarm limits for SpO(2) were set according to protocol (88-94%) in 64% of the time. Within these periods, SpO(2) was >94% for 30% and <88% for 16% of the time. CONCLUSIONS: Manual FiO(2) adjustments varied widely in frequency and step size. Deep desaturations and large FiO(2) adjustments were associated with medical or nursing procedures. When large adjustments are really necessary, it will be challenging to implement them in an automatic adjustment device.

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

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 F(a) depended strongly on the amount of generated energy (F(a) = 0.03-2.5 mN) and contact force (F(a) = 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.

Human error in strabismus surgery: quantification with a sensitivity analysis.

BACKGROUND: Reoperations are frequently necessary in strabismus surgery. The goal of this study was to analyze human-error related factors that introduce variability in the results of strabismus surgery in a systematic fashion. METHODS: We identified the primary factors that influence the outcome of strabismus surgery. For each of the human-error related factors we quantified variation with clinical assessments: measurement of the angle of strabismus, surgical strategy and surgical accuracy. Firstly, six patients were examined by six orthoptists, and accuracy of prism cover tests was assessed. Secondly, a questionnaire with sample cases (10 degrees, 15 degrees and 20 degrees of infantile esotropia) was put to orthoptists, to determine variation in current surgical strategy. Finally, photographs made during surgery were analyzed to assess surgical accuracy. The influence of human-error related factors was related to the influence of inter-patient differences with a mechanical model. The relative contribution of all factors was assessed with a sensitivity analysis, and results were compared to clinical studies. RESULTS: The surgical trajectory of strabismus surgery could be modeled mathematically. Measurement of angle of strabismus, surgical technique, anatomy and physiology were considered. Variations in the human-error related factors were: (1) the latent angle at distant fixation was measured with a 90% confidence interval of 5 degrees , and comprised 20% of the total variance of the postoperative angle, (2) orthoptists decided for bilateral recessions of, respectively, 7.3 +/- 1.7 mm (total amount of two recessions), 9.1 +/- 1.2 mm and 10.3 +/- 1.3 mm, which comprised 15% of the total variance, and (3) surgical accuracy was estimated at +/-0.5 mm, which comprised 20% of the total variance. CONCLUSION: The human error in strabismus surgery could be quantified with a sensitivity analysis. Approximately half of the reoperations in strabismus surgery are caused by inaccuracy in the measurement of the angle of strabismus, variability in surgical strategy and imprecise surgery.

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.

Elasticity, viscosity, and deformation of orbital fat.

PURPOSE: For development of a finite element analysis model of orbital mechanics, it was necessary to determine the material properties of orbital fat and its degree of deformation in eye rotation. METHODS: Elasticity and viscosity of orbital fat of eight orbits of four calves and two orbits of one rhesus monkey were measured with a parallel-plate rheometer. The degree of deformation of orbital fat was studied in two human subjects by magnetic resonance imaging (MRI) through the optic nerve in seven (first subject) or fourteen positions of gaze from left to right. Bifurcations of veins in the fat were used as markers for displacement of the fat. RESULTS: The elastic shear modulus (G') of calf orbital fat was between 250 Pa and 500 Pa, and of monkey orbital fat it was between 500 Pa and 900 Pa. The viscous shear modulus (G'') of calf orbital fat was between 80 Pa and 150 Pa, and for monkey orbital fat it was between 300 Pa and 500 Pa. In the MRI scans, it was found that markers in the fat, 1 to 5 mm posterior to the sclera, rotated with the eye for 36% to 53% of eye rotation; the remainder was accounted for by sliding of the eye within the Tenon capsule and within the orbital fat. CONCLUSIONS: Elastic and viscous shear moduli of orbital fat are low. Little energy is dissipated in the fat. The required deformation of the fat during eye rotation is limited because the eye slides, to some extent, within the Tenon capsule.

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.