眼基线参数对角膜塑形镜设计及其临床效果的影响

角膜塑形镜作为减缓青少年近视快速增长的重要手段之一,被临床广泛认可。角膜塑形镜的参数设计对近视控制效果影响显著。笔者结合角膜塑形镜验配时所需依据的人眼基线参数,对角膜塑形镜镜片关键参数的选择及其相关临床效果进行述评,以期进一步加强验配者对镜片参数设计的认识和提升角膜塑形镜的近视控制效果,为深入开展青少年近视防控工作提供参考和指导。

冷敷疗法缓解经上皮准分子激光角膜切削术后疼痛的效果

目的:观察经上皮准分子激光角膜切削术(TPRK)后眼罩冷敷缓解患者术后不适的临床疗效。方法:前瞻性随机对照研究。选取2018年10月至2019年4月在温州医科大学附属眼视光医院之江院区行TPRK的患者87例(174眼),根据随机数字表随机分成冷敷组和对照组,2组均选取右眼进行观察。冷敷组在术后留观期间给予4℃冷藏硅胶眼罩冰敷袋,冷敷20 min,其余处理同对照组。对照组常规用药不冷敷。分别于术后2 h、1 d、2 d、3 d、5 d每天相对固定时间填写眼部不适感评分量表,包括视物模糊、畏光、异物感、流泪、眼痛、眼痒、眼部分泌物等术后不适情况。并于术后5 d、12 d、1个月、3个月分别评估角膜上皮愈合情况、裸眼视力(UCVA)、等效球镜度(SE)、角膜上皮下雾状混浊(haze)、调制传递函数截止频率(MTF cutoff)、客观散射指数(OSI)等指标。采用重复测量方差分析、独立样本t检验对数据进行分析。结果:术后2 h时,畏光、异物感、眼痛评分等指标在2组间差异均有统计学意义(F=15.93,P<0.001;F=9.52,P=0.003;F=13.57,P<0.001)。2组间视物模糊、流泪、眼痒、眼部分泌物差异均无统计学意义。术后1 d视物模糊指标在2组间差异有统计学意义(F=9.69,P=0.003),畏光、异物感、流泪、眼痛、眼痒、眼部分泌物等指标差异无统计学意义。术后2、3、5 d所有不适症状组间差异均无统计学意义。术后5 d、12 d、1个月、3个月,2组间角膜上皮愈合情况、UCVA、SE、Haze等差异均无统计学意义。术后3个月2组间客观光学质量评估差异均无统计学意义。结论:TPRK术后冷敷疗法能够改善术后早期疼痛、畏光、异物感等刺激症状,但对术后远期视力恢复没有明显作用,且对术后角膜上皮修复、视觉质量恢复等无负面影响。

Consideration of corneal biomechanics in the diagnosis and management of keratoconus: is it important?

Keratoconus is a bilateral,non-inflammatory,degenerative corneal disease.The occurrence and development of keratoconus is associated with corneal thinning and conical protrusion,which causes irregular astigmatism.With the disruption of the collagen organization,the cornea loses its shape and function resulting in progressive visual degradation.Currently,corneal topography is the most important tool for the diagnosis of keratoconus,which may lead to false negatives among the patient population in the subclinical phase.However,it is now hypothesised that biomechanical destabilisation of the cornea may take place ahead of the topographic evidence of keratoconus,hence possibly assisting with disease diagnosis and management.This article provides a review of the definition,diagnosis,and management strategies for keratoconus based on corneal biomechanics.

Ex vivo testing of intact eye globes under inflation conditions to determine regional variation of mechanical stiffness

Background:The eye globe exhibits significant regional variation of mechanical behaviour.The aim of this present study is to develop a new experimental technique for testing intact eye globes in a form that is representative of in vivo conditions,and therefore suitable for determining the material properties of the complete outer ocular tunic.Methods:A test rig has been developed to provide closed-loop control of either applied intra-ocular pressure or resulting apical displacement;measurement of displacements across the external surface of the eye globe using high-resolution digital cameras and digital image correlation software;prevention of rigid-body motion and protection of the ocular surface from environmental drying.The method has been demonstrated on one human and one porcine eye globe,which were cyclically loaded.Finite element models based on specimen specific tomography,free from rotational symmetry,were used along with experimental pressure-displacement data in an inverse analysis process to derive the mechanical properties of tissue in different regions of the eye’s outer tunic.Results:The test method enabled monitoring of mechanical response to intraocular pressure variation across the surface of the eye globe.For the two eyes tested,the method showed a gradual change in the sclera’s stiffness from a maximum at the limbus to a minimum at the posterior pole,while in the cornea the stiffness was highest at the centre and lowest in the peripheral zone.Further,for both the sclera and cornea,the load–displacement behaviour did not vary significantly between loading cycles.Conclusions:The first methodology capable of mechanically testing intact eye globes,with applied loads and boundary conditions that closely represent in vivo conditions is introduced.The method enables determination of the regional variation in mechanical behaviour across the ocular surface.

Fast segmentation of anterior segment optical coherence tomography images using graph cut

Background:Optical coherence tomography(OCT)is a non-invasive imaging system that can be used to obtain images of the anterior segment.Automatic segmentation of these images will enable them to be used to construct patient specific biomechanical models of the human eye.These models could be used to help with treatment planning and diagnosis of patients.Methods:A novel graph cut technique using regional and shape terms was developed.It was evaluated by segmenting 39 OCT images of the anterior segment.The results of this were compared with manual segmentation and a previously reported level set segmentation technique.Three different comparison techniques were used:Dice’s similarity coefficient(DSC),mean unsigned surface positioning error(MSPE),and 95%Hausdorff distance(HD).A paired t-test was used to compare the results of different segmentation techniques.Results:When comparison with manual segmentation was performed,a mean DSC value of 0.943±0.020 was achieved,outperforming other previously published techniques.A substantial reduction in processing time was also achieved using this method.Conclusions:We have developed a new segmentation technique that is both fast and accurate.This has the potential to be used to aid diagnostics and treatment planning.

Unintended changes in ocular biometric parameters during a 6-month follow-up period after FS-LASIK and SMILE

Background:Corneal refractive surgery has become reliable for correcting refractive errors,but it can induce unintended ocular changes that alter refractive outcomes.This study is to evaluate the unintended changes in ocular biometric parameters over a 6-month follow-up period after femtosecond laser-assisted laser in situ keratomileusis(FS-LASIK)and small incision lenticule extraction(SMILE).Methods:156 consecutive myopic patients scheduled for FS-LASIK and SMILE were included in this study.Central corneal thickness(CCT),mean curvature of the corneal posterior surface(K_(pm)),internal anterior chamber depth(IACD)and the length from corneal endothelium to retina(ER)were evaluated before and after surgery over a 6-month period.Results:Both the FS-LASIK and SMILE groups(closely matched at the pre-surgery stage)experienced flatter Kpm,shallower IACD and decreased ER 1 week post-surgery(P<0.01),and these changes were larger in FS-LASIK than in SMILE group.During the 1 week to 6 months follow up period,K_(pm),IACD and ER remained stable unlike CCT which increased significantly(P<0.05),more in the FS-LASIK group.Conclusions:During the follow up,the posterior corneal surface became flatter and shifted posteriorly,the anterior chamber depth and the length from the corneal endothelium to retina decreased significantly compared with the pre-surgery stage.These unintended changes in ocular biometric parameters were greater in patients undergoing FS-LASIK than SMILE.The changes present clear challenges for IOL power calculations and should be considered to avoid affecting the outcome of cataract surgery.

Effect of orthokeratology on precision and agreement assessment of a new sweptsource optical coherence tomography biometer

Background:To evaluate the effect of orthokeratology on precision of measurements in children using a new swept-source optical coherence tomography(SS-OCT)optical biometer(OA-2000),and agreement between its measurements and those provided by the commonly used IOLMaster based on partial coherence interferometry(PCI).Methods:This study recruited fifty-one eyes of 51 normal children(8–16 years).An operator took measurements with the two biometers.Then,a second operator took measurements with the SS-OCT biometer.After orthokeratology was performed for one month,the same operators repeated the same procedures.Axial length(AL),mean keratometry(Km)at 2.5 mm and 3.0 mm diameters(Km2.5 and Km3.0),central corneal thickness(CCT),anterior chamber depth(ACD),lens thickness(LT)and corneal diameter(CD)were analyzed.Results:With the SS-OCT optical biometer,the test-retest repeatability of AL measurements was<0.06 mm.For all parameters,the coefficients of variation were<1.23%and the intraclass correlation coefficients were>0.95.The 95%limits of agreement of difference between the two devices for CD parameter were up to 1.53 mm.After orthokeratology,the fluctuation ranges of difference for Km3.0 measurement was 1.11 times higher than before orthokeratology,while the absolute values of difference for AL,Km2.5,ACD and CD measurements were comparable.Conclusions:Before and after orthokeratology,the SS-OCT biometer showed high repeatability and reproducibility for all measurements.Wearing orthokeratology contact lenses affected the agreement between SS-OCT and PCI biometers for Km3.0 measurements.The CD measurement showed poor agreement between the two devices.

High intercorneal symmetry in corneal biomechanical metrics

Backgroud:To evaluate the symmetry of corneal biomechanical metrics,measured using an ocular response analyzer(ORA)and self-built corneal inflation test platform,in bilateral rabbit corneas and to investigate their relationship with physical intraocular pressure(IOPp).Methods:Twenty fresh enucleated eyes from ten rabbits were used for ex vivo whole ocular globe inflation.IOP was increased from 7.5 to 37.5 mmHg with 7.5 mmHg steps and biomechanical metrics were acquired using the ORA.At least 3 examinations were performed at each pressure stage.Two biomechanical metrics,corneal hysteresis(CH)and corneal resistance factor(CRF)were recorded and analyzed as a function of IOPp.Corneal specimens were then excised from the intact ocular globe and tested under inflation conditions up to 45.7 mmHg posterior pressure.The experimental pressure-deformation data was analyzed using an inverse modeling procedure to derive the stress-strain behavior of the cornea.Results:A comparison of corneal shape parameters showed no statistically significant difference(P>0.05)between bilateral eyes.Similarly,there were no statistically significant differences in values of CH,CRF and corneal stiffness(as measured by the tangent modulus,Et)between bilateral eyes(CH:F=0.94,P=0.54;CRF:F=4.42,P=0.35;Et:F=3.15,P=0.12)at different pressure levels.IOPp was highly correlated with CRF while the relationship with CH was less pronounced.Conclusions:An obvious interocular symmetry in biomechanical metrics is found in this research.IOP has been shown to have important influences on the value of CRF provided by ORA.

Review of in-vivo characterisation of corneal biomechanics

The study of corneal biomechanics in vivo has been evolving fast in recent years.While an organised corneal structure is necessary for its transparency,resistance to occasional external insults and bearing the intraocular pressure(IOP),which several clinically relevant events can disturb.This review focuses on three techniques that are available for clinical use,namely the Ocular Response Analyzer(Reichert Ophthalmic Instruments,Buffalo,NY,USA),the Corvis ST(Oculus Optikgerate GmbH,Wetzlar,Germany)and the Brillouin Optical Scattering System(Intelon Optics Inc.,Lexington,MA,USA).The principles and the main parameters of each device are discussed along with their strategies to improve accuracy in the IOP measurement,corneal ectasia diagnosis,evaluation of corneal cross-linking procedures,and planning of corneal refractive surgeries.

Accuracy and reliability of orthogonal polynomials in representing corneal topography

Fitting of corneal topography data to analytical surfaces has been necessary in many clinical and experimental applications,yet absolute superiority of fitting methods was still unclear,and their overfitting risks were not well studied.This study aimed to evaluate the accuracy and reliability of orthogonal polynomials as fitting routines to represent corneal topography.Four orthogonal polynomials,namely,Zernike polynomials(ZPs),pseudo-Zernike polynomials(PZPs),Gaussian-Hermite polynomials(GHPs)and Orthogonal Fourier-Mellin polynomials(OFMPs),were employed to fit anterior and posterior corneal topographies collected from 200 healthy and 174 keratoconic eyes using Pentacam topographer.The fitting performance of these polynomials were compared,and the potential overfitting risks were assessed through a prediction exercise.The results showed that,except for low orders,the fitting performance differed little among polynomials with orders
10 regarding surface reconstruction(RMSEs~0.3μm).Anterior surfaces of normal corneas were fitted more efficiently,followed by those of keratoconic corneas,then posterior corneal surfaces.The results,however,revealed an alarming fact that all polynomials tended to overfit the data beyond certain orders.GHPs,closely followed by ZPs,were the most robust in predicting unmeasured surface locations;while PZPs and especially OFMPs overfitted the surfaces drastically.Order 10 appeared to be optimum for corneal surfaces with 10-mm diameter,ensuring accurate reconstruction and avoiding overfitting.The optimum order however varied with topography diameters and data resolutions.The study concluded that continuing to use ZPs as fitting routine for most topography maps,or using GHPs instead,remains a good choice.Choosing polynomial orders close to the topography diameters(millimeters)is generally suggested to ensure both reconstruction accuracy and prediction reliability and avoid overfitting for both normal and complex(e.g.,keratoconic)corneal surfaces.

Review of ex-vivo characterisation of corneal biomechanics

The evaluation of the corneal biomechanical behaviour has important clinical applications.To name a few,the accuracy of the intraocular pressure measurement,the study of corneal ectatic diseases and the assessment and optimisation of corneal surgical procedures are all highly influenced by corneal biomechanics.Over the last 45 years different ex-vivo methods were developed to study corneal biomechanical behaviour.Different tissue maintenance,support,loading systems,as well as different monitoring strategies of corneal deformations were employed.In this review,the most important and commonly used methods are outlined,including strip extensiometry,inflation,compression,indentation and tissue separation testing.Their particularities,applications,pros and cons and main applications are discussed.