Refractive Correction Research Articles

Current concepts in management of amblyopia- A major review

Amblyopia is the leading cause of preventable blindness in children and young. The prevalence of amblyopia in childhood varies between 1-5%, although these values may differ. Most of the visual impairment in amblyopia is reversible if treated early at appropriate time. Various treatment modalities in children include: refractive correction, patching, atropine penalization, binocular therapy, liquid crystal display eyeglasses and transcranial magnetic stimulation. Motive of this article is to give an emphasis on newer management strategy and newer trends in amblyopia therapy. A thorough search of literature was done in PubMed, Scopus, Embase and reference list of articles until December 15, 2020 containing “amblyopia treatment or therapy” was retrieved.

The Basic VRS-Effect Study: Clinical Trial Outcomes and Cost-Effectiveness of Low Vision Rehabilitation in Portugal.

The aim of this study was twofold: (1) to investigate the clinical impact of vision rehabilitation in patients with vision impairment and (2) to investigate the cost-effectiveness of a basic vision rehabilitation service in Portugal. The trial recruited patients diagnosed with age-related macular degeneration or diabetic retinopathy (DR) and visual acuity in the range 0.4-1.0logMAR in the better-seeing eye. Participants were randomised to one of the study arms consisting of immediate intervention and delayed intervention. The intervention included: new refractive correction, optical reading aids, in-office training and advice about modifications at home. Visual ability, health-related quality of life and costs were measured. Economic analysis was performed to evaluate whether the intervention was cost-effective. The trial compared the outcomes 12 weeks after the start in both arms. Of the 46 participants, 34 (74%) were diagnosed with DR, 25 (54%) were female, and mean age was 70.08 years (SD = 8.74). In the immediate intervention arm visual ability increased a mean of 0.523 logits (SE = 0.11) (p < 0.001). Changes in the delayed intervention arm were not statistically significant (p = 0.95). Acuity in the better-seeing eye, near acuity and critical print size also improved during the study. The mean cost of the intervention was €118.79 (SD = 24.37). The incremental cost-effectiveness ratio using the EQ-5D-5L was 30,421€/QALY and 1186€/QALY when using near acuity. The current study gives evidence of positive clinical impact of a basic vision rehabilitation intervention and shows that a basic vision rehabilitation service is cost-effective. These findings are important to clinical and rehabilitation practices and for planning vision rehabilitation services. Retrospectively registered, 21/01/2019. ISRCTN10894889, https://www.isrctn.com/ISRCTN10894889.

Investigation of the relationship between contact lens design parameters and refractive changes in Ortho-K

PurposeTo investigate the relationship between Ortho-K contact lens design parameters and refractive power change of the eye through a parametric mathematical representation. MethodsThe current study utilises fully anonymized records of 249 eyes, 132 right eyes, and 117 left eyes from subjects aged 14.1 ± 4.0 years on average (range 9–38 years) which were selected for secondary analysis processing. The data were split into 3 groups (G1 up to 35 days wear, from 10 to 35 days, G2 up to 99 days wear, more than 35–99 days & G3 more than 100 days wear) according to the length of time, in days, that the lenses were worn. Corneal shape was measured before and after contact lens wear using the Medmont E300 topographer, from which height and distance files were read by a custom-built MATLAB code to construct the corneal anterior surface independently. Changes in refractive power pre and post-Ortho-K wear were determined using constructed tangential refractive power maps from which both centrally flattened and annular steepened zones were automatically bounded, hence used to determine the refractive power change. ResultsOn average, flat Sim-K and steep Sim-K were reduced after Ortho-K lens wear by 1.6 ± 1.3 D and 1.3 ± 1.4 D respectively. The radius of the base curve was correlated with the mean central flattened zone power change strongly in G1 (R = 0.7, p < 0.001) and moderately in G2 (R = 0.4) and G3 (R = 0.4, p < 0.001). Hence, a strong correlation with the base curve was recorded in group G1 and moderate in G2 and G3. The reverse curve was very strongly correlated to the mean central flattened zone power change in G1 (R = 0.8, p < 0.001) and strongly correlated with G2 (R = 0.6, p < 0.001) and G3 (R = 0.7, p < 0.001). The reverse curve was also strongly correlated with the mean annular steepened zone power change among all groups G1, G2, and G3 (R = 0.7, R = 0.6 and R = 0.6) respectively (p < 0.001). ConclusionsAlthough the central corneal refractive power change was strongly correlated to the Ortho-K lens base curve, it characterized only 50% of the target power change. However, the annular steepened zone refractive power change appears to be a clearer predictor of target power change, as there appears to be a one-to-one inverse relationship with the target refractive power correction. Differences between these results and the literature may be a result of the topography software smoothing effect.

Changes in Light Disturbance Analyzer Evaluation in SMILE for High Myopia and Astigmatism.

To study the objective change in light disturbance around a glare source using the Light Disturbance Analyzer (LDA) (Binarytarget Lda) and correlate this with the validated subjective quality of vision (QoV) questionnaire in small incision lenticule extraction (SMILE) for high compound myopic astigmatism. This was a prospective study recruiting 114 patients undergoing SMILE with attempted spherical equivalent refraction (SEQ) correction from -9.00 to -13.00 diopters (D), and cylinder up to 5.00 D. The LDA was used before and 3 and 12 months after surgery to evaluate the Light Disturbance Area, Light Disturbance Index, Best Fit Circle Radius, and Best Fit Circle Irregularity. Patients completed the Rasch-validated QoV questionnaire before and 12 months after surgery. Correlation analysis was performed between the LDA parameters and the QoV frequency, severity, and bothersomeness for halos and starbursts. At 12 months, there was a non-statistically signifcant change of 11.9% for Light Disturbance Area and Light Disturbance Index and 6.5% for Best Fit Circle Radius. There was an increase of 81.9% for Best Fit Circle Irregularity (P = .017). Light Disturbance Area, Light Disturbance Index, and Best Fit Circle Radius correlated with bothersomeness for QoV halos, and with frequency, severity and bothersomeness for QoV star-bursts (P < .05), albeit with a low correlation coefficient (R2 < 0.13). There was no correlation between increase in Best Fit Circle Irregularity and the halo and starburst scores from the QoV questionnaire. LDA Best Fit Circle Irregularity was found to increase following high myopic SMILE. Other LDA parameters appeared to increase, but a much larger sample would be required to find statistical significance. The change in LDA parameters was only weakly correlated with QoV halo and starburst metrics, representing the wide variation in inter-subject perception. [J Refract Surg. 2022;38(11):725-732.].

Regional Changes in Posterior Corneal Surface During a 6-Month Follow-up Period After tPRK, FS-LASIK, and SMILE.

To investigate changes in corneal curvature in different zones of the posterior corneal surface during a 6-month follow-up period after transepithelial photorefractive keratectomy (tPRK), femtosecond laser-assisted excimer laser in situ keratomileusis (FS-LASIK), and small incision lenticule extraction (SMILE). The study included a total of 202 eyes, including 65, 77, and 60 that underwent tPRK, FS-LASIK, and SMILE, respectively. Elevation data for the posterior surface were obtained preoperatively (pre), as well as 1 week (pos1w), 1 month (pos1m), 3 months (pos3m), and 6 months (pos6m) postoperatively. Changes in posterior corneal curvature (M) were analyzed in the central (diameter: 0 to 3 mm), paracentral (diameter: 3 to 6 mm), and peripheral (diameter: 6 to 9 mm) regions. Over all follow-up periods, the central region of the posterior surface in all patients became flatter (P < .05), with FS-LASIK showing the largest change, whereas the paracentral and peripheral regions became steeper. The posterior curvature changes between pre and pos6m, determined before and after correction for ablated stromal depth, tended to follow similar trends in the three regions and after the three surgeries. There was also no significant correlation (P > .05) between the changes in the mean curvature (M, recorded between pre and pos6m) and each of the refractive error corrections, the changes in spherical aberration postoperatively, the optical zone diameter, ablated stromal depth, and residual stromal bed thickness in the central and peripheral regions, but the correlation was significant in the paracentral region. The postoperative changes in posterior corneal shape followed different trends in the central, paracentral, and peripheral regions. The FS-LASIK group exhibited the most notable changes in posterior corneal curvature, especially in the central region. These changes were statistically correlated with variations in spherical aberration, and ablated and residential stromal thickness in the paracentral region. [J Refract Surg. 2022;38(11):708-715.].

A review of active and passive optical methods in hydrography

The history of using optical methods for hydrographic applications dates back to the mid of the last century. Since these early days, both passive and active techniques have evolved considerably and with ever new sensors and platforms available, there is unbroken scientific progress in this field capable of capturing the submerged topography of shallow water bodies. This review article gives an overview of the employed optical methods, which can be categorized into passive and active techniques. Passive methods comprise spectrally derived bathymetry as well as multimedia stereo photogrammetry, also referred to as photo bathymetry, and the active method is laser bathymetry. Another approach for categorizing optical methods is based on the used carrier platform, where satellites, crewed aircraft, uncrewed aerial systems and lately also remotely piloted underwater vehicles are used for mapping water bodies at different scales, resolution and accuracy. Another attempt to classify the approaches is whether the sensor is above or below the water table, referred to as through-water or underwater, respectively. Another focus of the paper is on data processing techniques including refraction correction, which is crucial for most optical hydrography approaches, signal processing of the radiometric content in spectrally derived bathymetry, and analysis of the backscattered waveform signal in laser bathymetry. In the recent past, machine learning techniques play an increasingly important role not only for object classification but also for regression-based depth estimation. And finally, the paper discusses the huge variety of applications ranging from mapping navigable channels for save ship navigation, disaster documentation, flood simulation, coastal protection, benthic habitat modeling, river restoration, monitoring of hydromorphological changes, maintenance of hydroelectric power plants, mapping of off-shore infrastructure, coral reef mapping to underwater archaeology. The paper presents seminal work in the field but also focuses on the multitude of recent scientific work demonstrating the unbroken relevance of optical hydrography.

Efficacy of a One-Piece Aberration Neutral Hydrophobic Acrylic Toric Intraocular Lens.

To assess the refractive outcomes, intraocular centration and rotational stability of the enVista toric intraocular lens (IOL). This study was a prospective, multi-centre, double-masked, partially randomized and partially controlled clinical trial. A total of 191 participants were implanted with toric IOL (1.25, 2.00, or 2.75D) or non-toric IOL (control). The lowest range of corneal astigmatic eyes were randomized to 1.25D toric or control. Higher astigmatic powers were allocated to the treatment arm. Subjects were assessed immediately postoperatively, 1-2 (V1), 7-14 (V2), 30-60 (V3) and 120-180 (V4) days postoperatively. Unaided (UDVA) and distance corrected visual acuity (CDVA), manifest refraction and corneal curvature were assessed. Vector analysis was used to calculate surgically induced refractive correction (SIRC), correction ratio (CR), error magnitude (EM) and error vector (EV). Slit-lamp photography was used to measure centration and rotational stability. UDVA was better in the low toric IOL group in comparison with the control group at V4 (p<0.001). There was an undercorrection in the control group, whereas the average CR for all toric subjects was 1.00 ± 0.32: V2, 0.98 ± 0.34: V3 and 0.98 ± 0.35: V4. The absolute IOL rotational stability in comparison to the position of the IOL at V1 was 1.35° ± 0.97°: V2, 1.35° ± 1.07°: V3 and 1.38° ± 1.25°: V4. Decentration was generally inferior (V1: 0.04 ± 0.22mm, V2: 0.05± 0.20mm, V3: 0.08 ± 0.22mm, V4: 0.04 ± 0.21mm) and nasal (V1: 0.19 ± 0.23mm, V2: 0.20 ± 0.20mm, V3: 0.20 ± 0.21mm, V4: 0.17 ± 0.22mm). Participants with low levels of corneal astigmatism achieved superior vision and refractive outcomes in the low toric group over the control. Moderate and high levels of astigmatism achieved excellent refractive outcomes. The toric IOL demonstrated high levels of both rotational and centrational stability.

Assessing Through-Water Structure-from-Motion Photogrammetry in Gravel-Bed Rivers under Controlled Conditions

Structure-from-Motion (SfM) photogrammetry has become a popular solution for three-dimensional topographic data collection in geosciences and can be used for measuring submerged bed surfaces in shallow and clear water systems. However, the performance of through-water SfM photogrammetry has not been fully evaluated for gravel-bed surfaces, which limits its application to the morphodynamics of gravel-bed rivers in both field investigations and flume experiments. In order to evaluate the influence of bed texture, flow rate, ground control point (GCP) layout, and refraction correction (RC) on the measurement quality of through-water SfM photogrammetry, we conducted a series of experiments in a 70 m-long and 7 m-wide flume with a straight artificial channel. Bed surfaces with strongly contrasting textures in two 4 m-long reaches were measured under five constant flow regimes with three GCP layouts, including both dry and underwater GCPs. All the submerged surface models with/without RC were compared with the corresponding dry bed surfaces to quantify their elevation errors. The results illustrated that the poorly sorted gravel-bed led to the better performance of through-water SfM photogrammetry than the bed covered by fine sand. Fine sediment transport caused significant elevation errors, while the static sand dunes and grain clusters did not lead to noticeable errors in the corrected models with dry GCPs. The elevation errors of the submerged models linearly increased with water depth for all the tested conditions of bed textures, GCP layouts, and discharges in the uncorrected models, but the slopes of the increasing relations varied with texture. The use of underwater GCPs made significant improvements to the performance of direct through-water SfM photogrammetry, but counteracted with RC. The corrected models with dry GCPs outperformed the uncorrected ones with underwater GCPs, which could still be used to correct the underestimation in surface elevation caused by RC. Based on the new findings, recommendations for through-water SfM photogrammetry in measuring submerged gravel-bed surfaces were provided.

Establishment of Personalized Finite Element Model of Crystalline Lens Based on Sweep-Source Optical Coherence Tomography

The virtual lens model has important value in ophthalmic research, clinical diagnosis, and treatment. However, the establishment of personalized lens models and the verification of accommodation accuracy have not been paid much attention. We proposed a personalized lens model establishment and the accommodation accuracy evaluation method based on sweep-source optical coherence tomography (SS-OCT). Firstly, SS-OCT is used to obtain a single lens image in the maximum accommodation state. After refraction correction, boundary detection, and curve fitting, the central curvature radius, thickness, and lens nucleus contour of the anterior and posterior surfaces of the lens were obtained. Secondly, a personalized finite element model improved from Burd’s model was established using these individual parameters, and the adaptation process of the lens model was simulated by pulling the suspensory ligament. Finally, the shape and refractive power changes of the real human lens under different accommodation stimuli were collected and compared with the accommodation process of the finite element model. The results show that the accommodation process of the finite element model is highly consistent with that of the real lens. From the un-accommodation state to the maximum-accommodation state, the difference rate of all geometric and refractive parameters between the two is less than 5%. Thus, the personalized lens finite element model obtained by the calibration and correction of the existing model can accurately simulate the regulation process of a specific human lens. This work helps to provide a valuable theoretical basis and research ideas for the study of clinical diagnosis and treatment of related diseases.

A Spatiotemporal Atmospheric Refraction Correction Method for Improving the Geolocation Accuracy of High-Resolution Remote Sensing Images

Atmospheric refraction is one of the most significant factors that affect the geolocation accuracy of high-resolution remote sensing images. However, most of the current atmospheric refraction correction methods based on empirical data neglect the spatiotemporal variation of pressure, temperature, and humidity of the atmosphere, inevitably resulting in poor geometric positioning accuracy. Therefore, in terms of the problems mentioned above, this study proposed a spatiotemporal atmospheric refraction correction method (SARCM) based on global measured data to avoid the uncertainty of traditional empirical models. Initially, the atmosphere was stratified into 42 layers according to their pressure property, and each layer was divided into 1,042,560 grid cells with intervals of 0.25 longitude and 0.25 latitude. Then, the atmospheric refractive index of each grid in the imaging region was accurately calculated using the high-precision Ciddor formula, and the result was interpolated using three splines. Subsequently, according to the rigorous geometric positioning model, the line-of-sight of each pixel and the viewing zenith angle outside the atmosphere in WGS84 were derived to provide input for atmospheric refraction correction. Finally, the coordinates of the ground control points were corrected with the calculated atmospheric refractive index and Snell’s law. The experimental results showed that the proposed SARCM could effectively improve the positioning accuracy of the image with a large viewing zenith angle, and especially, the improvement percentage for a viewing zenith angle of 34.2426° in the x-direction was 99.5%. Moreover, the atmospheric refraction correction result of the SARCM was better than that of the primary state-of-the-art methods.

Late surgery for unilateral persistent fetal vasculature: long-term visual and anatomic outcomes

BackgroundPersistent fetal vasculature (PFV) is a congenital anomaly caused by failure of the ocular vasculature to regress. We report the visual and anatomic outcomes in patients who underwent surgery for PFV between 7-48 months of age. MethodsThe medical records of patients who underwent lensectomy and anterior or core vitrectomy for unilateral PFV, with or without intraocular lens implantation, were reviewed retrospectively. Inclusion criteria were surgery between 7-48 months of age and at least 12 months of follow-up. Patients with severe posterior segment involvement were excluded. The primary outcome was the final visual acuity using age-appropriate tests converted to logMAR. Secondary outcomes included the rate of adverse events and number of subsequent intraocular procedures. ResultsA total of 20 patients with a mean age at surgery of 19.3 ± 10.5 months and mean follow-up of 73.7 ± 46.7 months were included. Sixteen patients had delayed surgery due to late presentation; the remaining 4 were managed initially with refractive correction and occlusion. Eight patients (40%) achieved a final visual acuity better than 1.0 logMAR. Four eyes had adverse events, including one retinal detachment. No patient developed glaucoma. Four eyes underwent subsequent procedures. ConclusionsIn our study cohort, surgery for unilateral PFV between 7-48 months of age achieved functional visual acuity in over one-third of patients. This is comparable to results achieved with surgery before 7 months of age, but with fewer adverse events. Differences in disease severity could account for later PFV presentation and surgery as well as outcomes.▪

Numerical refractive index correction for the stitching procedure in tomographic quantitative phase imaging.

Tomographic quantitative phase imaging (QPI) lacks an absolute refractive index value baseline, which poses a problem when large dense objects extending over multiple fields of view are measured volume by volume and stitched together. Some of the measurements lack the natural baseline value that is provided by the mounting medium with a known refractive index. In this work, we discuss the problem of the refractive index (RI) baseline of individual reconstructed volumes that are deprived of access to mounting medium due to the extent of the object. The solution of this problem is provided by establishing the RI offsets based on the overlapping regions. We have proven that the process of finding the offset RI values may be justifiably reduced to the analogous procedure in the 2D baseline correction (2D-BC). Finally, we proposed the enhancement of the state-of-the-art 2D-BC procedure previously introduced in the context of 2D QPI. The processing is validated at the examples of a synthetic dataset and a liver organoid.

Prevalence of myopia and uncorrected myopia among 721 032 schoolchildren in a city-wide vision screening in southern China: the Shantou Myopia Study

AimsTo explore the prevalence and risk factors for myopia and uncorrected myopia in schoolchildren in southern China.MethodsThe government-led Shantou Myopia Study was conducted from September 2020 to June 2021. Non-cycloplegic...

A tale of two bridges and a round Earth

Columbus concluded that Earth is round while watching a distant ship sink beneath the horizon. You too can observe the curvature of Earth on the San Francisco Bay (and other places); not only that: you can determine Earth’s radius from such observations. I present here a method to use the vertical shift between two bridges over the Bay due to Earth’s curvature as seen in a photograph, where the more distant bridge is apparently lower than the proximate one (as shown in Fig. 1). Starting with each bridge’s known dimensions and location and applying atmospheric refraction corrections and then measuring the vertical shift between the two bridges on the image allows a calculation of Earth’s radius within a surprising 1% of the accepted value.

Development of myopia in laser-treated ROP infants: prematurity or laser photocoagulation?

To investigate the relationship between the development of corrected 1-year-old refraction values and risk factors in preterm infants who underwent laser photocoagulation (LPC) therapy for retinopathy of prematurity (ROP). The ophthalmic examination findings of preterm neonates who had developed Type I ROP and received LPC therapy were evaluated retrospectively. The association between spherical equivalent (SE) values and clinical findings were analyzed by using multivariable linear regression analysis. The study included 157 eyes of 80 neonates with a mean birth week of 27.1 ± 2.2weeks (23 to 32weeks) and a mean birth weight of 995 ± 273g (565 to 1760g). The treatments were administered on an average of 36.8 ± 2.7 (32 to 45weeks) postmenstrual age. LPC treatment was applied bilaterally to 77 of the 80 neonates included in the study, and unilaterally to 3 of them. The mean ± standard deviation of the SE value was 0.31 ± 1.89 diopters (D) (-8.00 to 4.63 D) according to the results of the 1-year corrected age refraction examination. In univariate analysis, no significant association between GA, BW, and ROP zone and SE value, while the number of laser spots (ß = - 0.27 ± 0.00 D, p = 0.00) and stage 3 ROP (ß = - 0.29 ± 0.37 D, p = 0.00) were significantly associated with the SE value. In multivariable linear regression analysis, a significant association between number of laser spot, stage 3 ROP and SE value (ß = - 0.25 ± 0.00 D, p = 0.01 for number of laser spot, ß = - 0.28 ± 0.36 D, p = 0.00 for stage 3 ROP). In conclusion, this study supports that stage of ROP and the number of laser spots count applied in photocoagulation treatment for ROP is significantly correlated with degree of myopia (p < 0.05).

Assessment of the Refractive Error and Stabilisation of Refraction after Cataract Surgery in Relation to the Length of the Eyeball.

The aim of this study was to analyse the changes in refraction, depending on the length of the eyeball, in patients who had undergone cataract surgery using the phacoemulsification method and to assess the stability of refraction. A total of 90 patients (46 to 85 years of age) took part in the study and were divided into three groups: emmetropic, hypermetropic, and myopic. Two types of intraocular lenses were used: Bausch (Akreos AO) and Rayner (C-flex). In conclusion, stabilization of refraction was achieved in the third week in 91% of the emmetropic, 77% of the myopic, and 46% of the hypermetropic patients, respectively. The correct postoperative refraction was achieved using optical biometry and the Barrett Universal II formula to calculate the power of the lens implant.

Prevalence of Reduced Vision among UK Elderly Drivers: The Bridlington Eye Assessment Project (BEAP)-A Cross-Sectional Study.

Self-assessment of driving fitness is mandatory in the United Kingdom. A paucity of data on visual function among drivers exists. We report prevalence of elderly drivers below legal visual acuity (VA) standard from a population study (The Bridlington Eye Assessment Project (BEAP)) conducted from 2002 to 2006. All residents aged ≥65 years were invited, 3459 undergoing structured interviews/ophthalmic examinations. Driving status was recorded, VA measured, and visual field (VF) testing performed. Outcomes were prevalence and characteristics of drivers below VA legal standard and prevalence of bilateral VF defects. Conditions causing reduced VA were explored and those with treatable conditions allowing visual improvement identified. Duration since last optometry review was recorded. Associations were explored using unpaired t-tests for continuous and chi-squared for discrete variables. Logistic regression was used for multivariate analysis and to determine odd ratios in the final adjusted model. Statistical analysis was performed using Stata 14.0 (Stata Corp, Tx). Within this sample, 7.1% (95% CI 6.0–8.3) of drivers fell below the VA legal driving standard (6/12) in their better eye, with 20% not having seen an optometrist for 2 years, including 8.2% who had not attended for over 5 years. The percentage of drivers falling below the VA minimum increases with age reaching 22.8% (95% CI 13.7–35.3) among those aged 85–89 years. 7.2% (95% CI 6.2–8.6) of drivers had bilateral visual field defects. 93% of drivers with reduced VA below legal standard had a cataract, refractive error or both in at least one eye. Significant numbers of elderly drive with VA below legal standard, most having easily correctable causes. Poor attendance with optometrists appears commonplace. Public education raised awareness of legal driving standards and encouraged compliance are required. Regular eye tests, appropriate refractive correction, and cataract surgery when needed should be encouraged.

Refractive prescribing for preschool children by optometrists in England.

Correction of refractive error in children is important for visual and educational development. The aim of this questionnaire-based study was to explore paediatric refractive correction by optometrists in England. An online questionnaire was piloted and distributed to optometrists in England. The questionnaire asked about respondents' characteristics (such as type of practice), management of refractive error in 1- and 3-year-old children and sources of information used as a basis for decisions on prescribing refractive error in children. Two hundred and ninety-three questionnaires were returned, although only 139 (47%) were fully completed. In an average month, about half of respondents examined no children between 0 and 2 years of age, and about half examined no more than five children aged 3-4 years. A significant proportion indicated they would refer children aged 1 or 3 years with refractive error and no other signs or symptoms into the hospital eye service. Almost a quarter would prescribe in full or in part an isometropic refractive correction of +2.00 D for a 3-year-old (within the normal range) with no other signs or symptoms, suggesting a degree of unnecessary prescribing. Almost all would act in cases of clinically significant refractive error. Respondents made similar use of their colleagues, optometric or postgraduate/continuing education, professional guidance and peer-reviewed research as sources of evidence on which to base decisions about prescribing for paediatric refractive errors. Most reported 'never' or 'rarely' using Cochrane reviews. These results suggest optometrists often defer management of paediatric refractive error to the hospital eye service, with implications in terms of underutilisation of community optometric expertise and burden on the National Health Service. In some cases, the results indicate a mismatch between respondents' reported management and existing guidance/guidelines on paediatric prescribing.

Biomechanical Characterization of Human Corneal Lenticules

Abstract In surgical refractive interventions over-correction or under-correction still occur in 10-15 % of cases. Improved outcomes could be achieved by incorporating patient-specific corneal biomechanics into surgical planning. As a first step, this study mechanically characterizes and numerically simulates corneal lenticules harvested during a refractive intervention called Corneal Lenticule Extraction for Advanced Refractive Correction (CLEAR). These human corneal lenticules were mechanically tested in uniaxial extension in nasal-temporal and 45 deg orientations. An HGO material model that accounts for the unique collagen arrangement of the human cornea was developed and used to identify the material parameters using a finite element model of the experimental setup. The material parameters that best represent the experimental force/displacement were obtained by parameter estimation using Bayesian optimization. The uniaxial experiments performed on 3 patients did not show distinctive differences in orientation. The estimated material parameters fit well with the experimental data, representing a promising material model. In the future, Brillouin measurements will be performed on the CLEAR patients before lenticule extraction so that material parameters can be estimated in vivo, providing the surgeon with a patient specific-surgical planning tool.

Optical coherence tomography refraction and optical path length correction for image-guided corneal surgery.

Optical coherence tomography (OCT) may be useful for guidance of ocular microsurgeries such as deep anterior lamellar keratoplasty (DALK), a form of corneal transplantation that requires delicate insertion of a needle into the stroma to approximately 90% of the corneal thickness. However, visualization of the true shape of the cornea and the surgical tool during surgery is impaired in raw OCT volumes due to both light refraction at the corneal boundaries, as well as geometrical optical path length distortion due to the group velocity of broadband OCT light in tissue. Therefore, uncorrected B-scans or volumes may not provide an accurate visualization suitable for reliable surgical guidance. In this article, we introduce a method to correct for both refraction and optical path length distortion in 3D in order to reconstruct corrected OCT B-scans in both natural corneas and corneas deformed by needle insertion. We delineate the separate roles of phase and group index in OCT image distortion correction, and introduce a method to estimate the phase index from the group index which is readily measured in samples. Using the measured group index and estimated phase index of human corneas at 1060 nm, we demonstrate quantitatively accurate geometric reconstructions of the true cornea and inserted needle shape during simulated DALK surgeries.