Dynamic contour tonometry versus Goldmann applanation tonometry: challenging the gold standard

The accurate measurement of intraocular pressure (IOP) is fundamental to almost any ophthalmic examination. Dynamic contour tonometry (DCT) was introduced 5 years ago as an entirely novel contact tonometry principle designed to measure IOP largely independently of corneal properties. Since then, many studies have compared the performance of this tonometer to the Goldmann applanation tonometer (GAT) and other tonometers in healthy eyes, as well as eyes with glaucoma or corneal diseases, and after corneal surgery. There is now strong evidence that DCT measures IOP very accurately, with very low inter- and intraobserver variability. This article summarizes the findings of these studies and analyzes the role of DCT in challenging GAT as the gold-standard tonometer for IOP measurements

A national survey of diagnostic tests reported by UK community optometrists for the detection of chronic open angle glaucoma

Purpose:  In the UK, the majority of cases of chronic open angle glaucoma are detected by community optometrists following a routine sight test. However, there is potential for variability in case finding strategies used. The aim of this study was to carry out a national web-based survey to determine current diagnostic tests used by optometrists in glaucoma case finding. / Methods:  Optometrists on the Association of Optometrists (AOP) electronic database were invited to participate. The survey was open for 16 weeks between April and July 2008. / Results:  A total of 1875 optometrists were eligible to enter the survey, of which 1264 answered the questions relating to diagnostic equipment. Respondents were asked to indicate their usual method of examining the optic nerve head. Direct ophthalmoscopy only was used by 25% with the majority (62%) using a combination of direct and slit-lamp binocular indirect methods. The vast majority of optometrists (78%) used non-contact tonometry to measure intraocular pressure, with only 16% routinely using a Goldmann or Perkins applanation tonometer. The perimeter most frequently used was either one of the Henson range of instruments (39%) or the Humphrey Field Analyser (22%). A smaller number of optometrists (<5%) had access to more specialised imaging equipment, such as HRT, GDx or OCT. / Conclusions:  The results of the survey demonstrate that UK optometrists are well equipped to carry out case finding for chronic open angle glaucoma, although there is a lack of standardisation with respect to equipment used

Tonometry:a study in biomechanical modelling. Appraisal and utility of measurable biomechanical markers.

Goldmann Applanation Tonometry (GAT) is the recognised ‘Gold Standard’ tonometer.However this status is refuted by eminent authors. These contradictory views have driventhe initial goal to assess, from first principles, the evolution of GAT and to experimentallyevaluate its utility and corrections. Subsequently, an important caveat became theevaluation of Corneal Hysteresis and Corneal Resistance Factor.Chapter 1. Biomechanical building blocks are defined and constitutive principlesincorporated into continuum modelling. The Imbert-Fick construct is re-interpreted asimple biomechanical model. GAT corrections are also appraised within a continuumframework; CCT, geometry and stiffness. These principles enable evaluation ofalternative tonometer theory and the evolving biomechanical markers, CornealHysteresis (ORA-CH) and Corneal Resistance Factor (ORA-CRF).Chapter 2 appraises corneal biomechanical markers, CCT, curvature, ORA-CH andORA-CRF in 91 normal eyes and the impact these have on three tonometers: GAT,Tonopen and Ocular Response Analyser (ORA). Tonopen was the sole tonometer notaffected by biomechanics. CCT was confirmed the sole measurable parameter affectingGAT. ORA did not demonstrate improved utility. ORA-CH and ORA-CRF do not appearrobust biomechanical measures.Chapter 3 assessed agreement between GAT, the ORA measures and Tonopen.Tonopen is found to measure highest and raises the question should a development goalemphasise GAT agreement or improvement?Chapter 4 assessed repeatability of the three tonometers and biomechanical measureskeratometry, pachymetry, ORA-CH and ORA-CRF on 35 eyes. Coefficients ofRepeatability (CoR) of all tonometers are wide. Effects assessed in Chapter 5 may bemasked by general noise. ORA does not appear to enhance utility over GAT.Isolation of corneal shape change via Orthokeratology (Chapter 5) demonstrate ORACHand ORA-CRF reflect, predominantly, a response to corneal flattening. It is proposedthey do not significantly reflect corneal biomechanics.After reviewing models for tear forces (Chapter 6), a refined mathematical model ispresented. Tear bridge attraction is minimal and cannot explain under-estimation of IOPby GAT in thin corneas. CCT corrections and the Imbert-Fick rules are incompatible.Chapter 7 summarises findings. The supremacy of GAT is likely to remain for some time,reflecting the sheer magnitude of overturning 60 years of convention, historicalprecedent, expert opinion as well as the logistical and educational difficulties ofredefining standards and statistical norms

High Sensitive MEMS Intraocular Capacitive Pressure Sensor (Glaucoma)

الأنظمة الميكانيكية الكهرو ميكانيكية الدقيقة(MEMS) هي تقنية صغيرة الحجم تم تبنيها بشكل كبير من قبل صناعة الدوائر المتكاملة (IC) وتطبيقها على تصغير جميع الأنظمة (الأنظمة الكهربائية والميكانيكية والضوئية والموائع والمغناطيسية وغيرها). تم تحقيق الحد الأدنى من خلال عمليات التصنيع الصغيرة. مستشعر الضغط السعوي هو ببساطة جهاز من نوع الحجاب الحاجز يتم فيه تحديد إزاحة الحجاب الحاجز عن طريق قياس تغير السعة بين الحجاب الحاجز ولوحة معدنية قريبة منه. لهذا الغرض، وأجهزة استشعار الضغط داخل العين مهمة في الكشف عن وعلاج مرض عضال يسمى الجلوكوما. لتحسين حساسية مستشعر الضغط بالسعة، يتم استخدام مادة البولي سيلكون منخفضة التوتر المخدر كمادة متوافقة حيويا. الجلوكوما هو مجموعة من أمراض العيون التي تحدث بسبب ارتفاع ضغط العين (IOP). IOP هو الضغط الذي يمارسه سائل العين يسمى الفكاهة المائية (السائل الواضح داخل العين) الذي يملأ الغرفة الأمامية للعين تظهر النتائج العلاقة المحاكاة بين السعة والضغط لـ ++ clamped silicon وpolysilicon. يمكن أن نرى من الشكل أن السعة الأولية لسيليكون p ++ المشكل هي حوالي 1.81 pF تتراوح السعة من 1.81 إلى 2.162 pF للسليكون p ++ المشدد والحجاب الحاجز polysilicon، على التوالي وبالتالي فإن التغير الكلي للسعة. هذه النتيجة تبين استخدام مادة البولي سيليكون في الحجاب الحاجز حساسية عالية من السيليكون p + +.Micro Electro Mechanical Systems (MEMS) are a small-scale technology that was largely adopted by the IC industry and applied to miniaturize of all systems (electrical systems, mechanical, optical, fluidic, magnetic, etc.). Minimization has been accomplished with small manufacturing processes. A Capacitive pressure sensor is simply a diaphragm-type device in which the diaphragm displacement is determined by measuring the capacitance change between the diaphragm and a metal plate that is close to it. For this purpose, intraocular pressure sensors are important in detection and treatment of an incurable disease called glaucoma. To improve the sensitivity of the capacitive pressure sensor, low stress doped polysilicon material is used as a biocompatible material. Glaucoma is a group of eye diseases that occurs by high intraocular pressure (IOP). IOP is the pressure exerted by the ocular fluid called aqueous humor (the clear fluid inside the eye) that fills the anterior chamber of the eye The results Shows the simulated relation between capacitance and pressure for clamped ++silicon and polysilicon clamped. It can be seen from figure that the initial capacitance for clamped p++ silicon is about 1.81 pF the capacitance varies from 1.81 to 2.162 pF for clamped p++silicon and clamped polysilicon diaphragm, respectively, so the total variation of the capacitance. This result shows the use of poly silicon material in diaphragm is high sensitivity than the p++ silicon

Corneal Biomechanics in Ectatic Diseases: Refractive Surgery Implications.

BACKGROUND: Ectasia development occurs due to a chronic corneal biomechanical decompensation or weakness, resulting in stromal thinning and corneal protrusion. This leads to corneal steepening, increase in astigmatism, and irregularity. In corneal refractive surgery, the detection of mild forms of ectasia pre-operatively is essential to avoid post-operative progressive ectasia, which also depends on the impact of the procedure on the cornea. METHOD: The advent of 3D tomography is proven as a significant advancement to further characterize corneal shape beyond front surface topography, which is still relevant. While screening tests for ectasia had been limited to corneal shape (geometry) assessment, clinical biomechanical assessment has been possible since the introduction of the Ocular Response Analyzer (Reichert Ophthalmic Instruments, Buffalo, USA) in 2005 and the Corvis ST (Oculus Optikgerate GmbH, Wetzlar, Germany) in 2010. Direct clinical biomechanical evaluation is recognized as paramount, especially in detection of mild ectatic cases and characterization of the susceptibility for ectasia progression for any cornea. CONCLUSIONS: The purpose of this review is to describe the current state of clinical evaluation of corneal biomechanics, focusing on the most recent advances of commercially available instruments and also on future developments, such as Brillouin microscopy.(undefined)info:eu-repo/semantics/publishedVersio

A theoretical and experimental study of the mechanical behavior of the cornea with application to the measurement of intraocular pressure

Biomechanics of cornea - application to intraocular pressure measuremen

Biomechanical aspects of the anterior segment in human myopia

The thesis investigates the relationship between the biomechanical properties of the anterior human sclera and cornea in vivo using Schiotz tonometry (ST), rebound tonometry (RBT, iCare) and the Ocular Response Analyser (ORA, Reichert). Significant differences in properties were found to occur between scleral quadrants. Structural correlates for the differences were examined using Partial Coherent Interferometry (IOLMaster, Zeiss), Optical Coherent tomography (Visante OCT), rotating Scheimpflug photography (Pentacam, Oculus) and 3-D Magnetic Resonance Imaging (MRI). Subject groups were employed that allowed investigation of variation pertaining to ethnicity and refractive error. One hundred thirty-five young adult subjects were drawn from three ethnic groups: British-White (BW), British-South-Asian (BSA) and Hong-Kong-Chinese (HKC) comprising non-myopes and myopes. Principal observations: ST demonstrated significant regional variation in scleral resistance a) with lowest levels at quadrant superior-temporal and highest at inferior-nasal; b) with distance from the limbus, anterior locations showing greater resistance. Variations in resistance using RBT were similar to those found with ST; however the predominantly myopic HKC group had a greater overall mean resistance when compared to the BW-BSA group. OCT-derived scleral thickness measurements indicated the sclera to be thinner superiorly than inferiorly. Thickness varied with distance from the corneolimbal junction, with a decline from 1 to 2 mm followed by a successive increase from 3 to 7 mm. ORA data varied with ethnicity and refractive status; whilst axial length (AL) was associated with corneal biometrics for BW-BSA individuals it was associated with IOP in the HKC individuals. Complex interrelationships were found between ORA Additional-Waveform-Parameters and biometric data provided by the Pentacam. OCT indicated ciliary muscle thickness to be greater in myopia and more directly linked to posterior ocular volume (from MRI) than AL. Temporal surface areas (SAs, from MRI) were significantly smaller than nasal SAs in myopic eyes; globe bulbosity (from MRI) was constant across quadrants

Tonometry and biomechanics of the cornea in contact lens wear

Aims: Research on use of contact lenses as drug delivery systems continues. Disposable lenses are often used to treat corneal injuries. Accurate intraocular pressure (IOP) measurements with lenses in situ will enhance patient care and save valuable chair time.Inter- and intraobserver reliability of rebound tonometer (RBT) and intraobserver reliability of ocular response analyser (ORA) with and without contact lenses of 50 (15 male, 35 female) healthy well adapted contact lens wearers between 18 – 55 years (M = 38.90, SD = 9.23) were examined. Clinical comparisons of IOP measurements with ORA and RBT were done. Accuracy of IOP measurements with four commonly prescribed disposable contact lenses (Acuvue Oasys, Frequency XC, Acuvue 1-Day Moist, and Pure Vision with powers -6.00 − +6.00 D) in situ was evaluated. Physiological and physical factors influencing IOP measurements with both instruments were determined. Findings: Intraobserver reliability of RBT without and with lenses was excellent (ICC > 0.88; > 0.92 respectively). Interobserver reliability of RBT was excellent without or with lenses (ICC 0.81; 0.88 respectively). Intraobserver reliability of ORA was good for all metrics measured except for corneal hysteresis (CH) (ICC: CH 0.63; corneal resistance factor (CRF) 0.79; corneal compensated IOP (IOPcc) 0.77; IOPg 0.87).RBT and ORA IOPg (Goldmann equivalent IOP) measurements were clinically and statistically comparable without or with lenses (differences < 0.6 mmHg). ORA IOPcc and RBT were less comparable (differences < 1.45 mmHg).Accurate RBT and ORA tonometry (within 2 mmHg) was possible with low minus power (range -0.50 to -6.00 D); moderate modulus of elasticity (< 0.75 MPa); thin silicone hydrogel (Acuvue Oasys) and hydrogel (Frequency XC; Acuvue 1-Day Moist) contact lenses in situ.Multiple regression analyses showed biomechanical metrics CRF and CH affected RBT and ORA (IOPcc and IOPg) measurements strongly (p < 0.0001). Therefore, cornea’s biomechanical properties had greater influence on accuracy of IOP measurements with these two instruments than other variables examined

Development and in-vitro evaluation of a potentially implantable fibre-optic glucose sensor probe

Type I diabetics need regular injections of insulin to survive. Insulin allows the cells of the body to extract glucose from the blood supply to use as fuel. Without insulin the cells turn to other backup fuel sources,this can cause side effects that are quickly fatal or gradual wasting of the bodies tissues. The use of insulin, however, is not danger free, as an incorrect dosage can quickly lead to the reduction of glucose circulating in the blood to drop to a dangerously low level. Without glucose circulating in the blood supply the brain quickly runs out of fuel causing coma and death. Because of this, a means to constantly monitor blood glucose levels has been sought for the last two decades. With such a device, diabetics could judge the correct amount of insulin to inject and be warned of low blood glucose levels. However, to date no reliable portable system has been produced. Recent developments in fibre optic biosensor technology, suggested a possible route to achieves this goal. The work in this thesis presents the development and testing of such a sensor. The sensor presented in this thesis is based around a commercial fibre optic blood gas sensor, the Paratrend 7. The oxygen-sensing element of this device was modified into a glucose sensor using polymer membranes incorporating the enzymes glucose oxidase and catalase. The research was aimed at building a glucose sensor that could be developed into a working blood glucose sensor in the minimum amount of time if the research proved successful. For this reason the Paratrend 7 sensor system was chosen to provide a clinically tested sensor core around which the glucose sensor could be built. The initial experiment, which used a Paratrend7 sensor coated in polyHEMA and glucose oxidase, produced a sensor of diameter of 700µm with a range of 0 to 4mM/1 of glucose and a 90% response time of <100 seconds in a solution with a 15% oxygen tension. The sensor design was then developed to incorporate the enzyme catalase to protect the glucose oxidase and an outer diffusion limiting polyHEMA membrane. This produced a sensor with a range of 0 to 6 mM/l and a response time of <100 seconds. The method of coating the sensors was'then improved, through a series of stages, until an optomised dip coating technique was developed. This technique produced sensors with ranges (in 7.5KPa oxygen tension solutions) between 0 to 3mM/l and 0 to lOmM/1, responsetimes of <100 seconds in some cases and with diameters of 300µm. By using a partial polyurethane outer coat the range of the sensors was increased form 0 to 4mM/l up to 0 to 24mM/1, in one case, with 90% response times in the 100to 500 second range. The sensors were then sterilised using gamma radiation and their performance before and after sterilisation examined. The gamma sterilisation was found to cause a reduction in the range of the sensors,for example 0 to 24 m /I down to 0 to 14mM/l in one case. The affect of 24 hour operation in a 5mM/1 solution of glucose and storage, for up to three months, was then investigated. Both processes were found to reduce the operational range of the sensors,0 to 20 reduced to 0 to 15 mM/i, in one case,for 24 hour operation and form 0 to 15mM/1 reduced to 0 to 11mM/1in one case for a storage time of three months. The use of the enzymes glucose oxidase and catalase together in a fibre optic as can sensor has not been previously reported in the literature as far be ascertained. The comparison of sensor performance before and after gamma sterilisation also appears to be unique as does the gamma sterilisation of a fibre optic glucose sensor