A corneal model for slit-scanning elevation topography

Computerised videokeratography is increasing in sophistication and accuracy. The Orbscan II Videokeratographer combines the advantages of Placido derived and slit-scanning derived topography, to acquire shape information on the posterior surface of the cornea. The purpose of this paper is to construct a model of slit-scanning elevation topography and highlight potential sources of variation in this methodology. Ray tracing calculations were performed on a defined, theoretical, spherical model to obtain a pachymetric measurement. These calculations were tested by comparing the pachymetry measurement derived from the model, over a refractive index shift from 1.376 to 1.400 with the pachymetry measurement obtained by Orbscan II, of a human eye, with an identical alteration in refractive index. The two methods of measurement differed by only 0.157 microm, suggesting that the model is a good representation of slit-scanning elevation topography, and that refractive index variations have only a minimal effect on Orbscan II pachymetry measurement.     

The effect of central corneal thickness on estimates of the anterior chamber depth

Background: The slitlamp can be used to estimate the anterior chamber depth (ACD). The length of a slit object is increased until the corneal and iris/lens images appear to just touch. Multiplying the just‐touching‐slit‐length (JTSL) by a conversion factor gives an estimate of the ACD as measured by ultrasonography. The purpose of this study was to determine if central corneal thickness (CCT) affects the accuracy of this technique. Methods: The ACD of 50 subjects was measured by A‐scan ultrasonography and estimated by the slitlamp technique. CCT was measured by ultrasonic pachometry. The refractive error was determined subjectively. Results: The average ultrasonographic ACD for all subjects was 3.32 ± 0.65 mm. The average JTSL was 2.46 ± 0.38 mm. The conversion ratio between the ultrasonographic ACD and the average JTSL was 1.35. The predicted ACD using the regression equation of JTSL on the ultrasound anterior chamber depth (USACD) was 3.32 ± 0.54 mm. The corresponding value using the regression equation of JTSL and CCT on USACD was exactly the same, that is, 3.32 ± 0.54 mm. Conclusion: Incorporation of CCT into a regression equation does not improve the accuracy of the Smith technique.     

Contact lens fitting following corneal graft surgery

Contact lens fitting may be required following keratoplasty for either optical or thera‐peutic reasons. Optical indications for contact lens fitting include the correction of irregular astigmatism, high regular astigmatism, anisometropia and secondary aniseikonia, as well as simple ametropia, where the patient desires to wear contact lenses in preference to spectacles. Therapeutic lenses are not routinely fitted following kerate plasty, although this management is advised in certain cases, such as when there are protruding sutures or epithelial healing is impaired. Designing a contact lens for a patient who has undergone keratoplasty will require the practitioner to carefully assess all the relevant features of the corneal graft. In this regard, there are many factors that need to be considered including the diameter of the graft zone, the topographical relationship between the host cornea and donor cornea, the corneal (graft) toricity and the location of the graft. Special designs, such as reverse geometry lenses, or more complex contact lens modalities, such as piggyback contact lens systems, may be required to achieve success in fitting.     

高度近视LASIK治疗中角膜瓣厚度的探讨

目的:探讨高度近视LASIK治疗中角膜瓣厚度问题。方法:142例(283眼)屈光度-9.00～-12.00 D的近视患者用日本NIDEK MK-2000全自动微型板层角膜切割刀切开角膜瓣,厚度71～184μm,激光切削方式一致。将角膜瓣厚度≤110μm分为Ⅰ组,72例(144眼);角膜瓣厚度>110μm分为Ⅱ组,70例(139眼)。对术前、术后屈光度、视力以 及剩余角膜基质床厚度进行比较。结果:术前平均角膜厚度:Ⅰ组为(531.76±30.28)μm(469～575μm),Ⅱ组为(550.66±24.45)μm(506～584μm),两者有明显差异(P<0.05)。术后剩余角膜基质床厚度:Ⅰ组为(333.01±3.30)μm(273～452μm),Ⅱ组为(309.71±31.41)μm(254～368μm),两者有明显差异(P<0.05)。术前屈光度两组分别为(-10.61±1.01)D及(-10.13±0.73)D,两组间无明显差异(P>0.05),术后7 d时分别为(+1.05±1.38)D及(+1.32±1.70)D,两组间也无明显差异(P>0.05),而术后3m Ⅰ组为(-0.13±0.94)D,Ⅱ组为(-1.06±0.96)D,两组有明显的差异(P<0.01)。术前、术后不论UCVA还是BCVA,两组无明显差异(P>0.05)。结论:治疗高度近视时70～110μm的角膜瓣的制作是合理的,并不影响术后视力,对防止术后屈光回退是有效的,对一个有经验的医生来说是可以很好完成的。     

Dynamics of ablation plume particles generated during excimer laser corneal ablation

Background and Objective: Although the empirical characteristics of ArF excimer laser corneal ablation have been well documented, the exact ablation mechanisms are not well understood. The present paper reports a quantitative analysis of corneal ablation plumes using in situ time resolved laser light scattering and Raman spectroscopy. Study Design/Materials and Methods: Bovine corneas were used as the ArF excimer laser ablation targets. Light scattering data were recorded from the ablation plume as a function of height above the tissue surface and as function of delay time with respect to the ablative ArF laser pulse. Results: Raman spectra of the ablation plume allow identification of the particles as water. Mean plume particle diameters are found to decrease with height, while the particle volume fractions are relatively constant. The total volume of plume particles correlates well with the total volume of water in the ablated corneal tissue. Conclusion: The finding of a non-evolving plume composed of water spherules, combined with the excellent agreement between total volume of water in the plume and the content of water in the ablated corneal tissue, support the concept of photodecomposition or “cold ablation” for corneal tissue during ArF excimer laser ablation. © 1995 Wiley-Liss, Inc.     

Measurement of axial edge lift of rigid corneal lenses*

A simple but reliable method was used to measure the axial edge lift at the total diameter over a range of back optic zone radii of several proprietary designs of rigid gas-permeable corneal lens. One of these was found to have a constant axial edge lift construction.     

Some Guidelines for the Use of Stimulus Modifications in Teaching Response Topography to People with Developmental Disabilities

The use of stimulus modifications in teaching involves altering the materials of a task in order to simplify its execution. Though their use in teaching stimulus discriminations to people with developmental disabilities has been extensively investigated, less attention has been given to their utilization in teaching response topography. This paper offers some general guidelines about their use for this purpose. In relation to their design, it is suggested that the aim should be to reduce the demands made by task components, to eliminate components, and/or to increase the attentional value of the discriminating features of the response topography. In relation to their application, guidelines are suggested about when they should be employed in conjunction with response prompts; when they should be used, either alone or in conjunction with response prompts, in preference to response prompts alone; whether one or a series of modifications is required; and at what point during teaching should probe trials on the unmodified task be introduced.     

Corneal conjunctivalisation in long‐standing contact lens wearers

Background: There are many contact lens‐related ocular surface disorders. Some can damage the limbal region where stem cells are thought to be located in its basal cell layer. This damage can result in destruction and a deficiency of corneal stem cells. One important sign of this complication is corneal conjunctivalisation. The purpose of this study is to describe clinical characteristics of a series of long‐standing contact lens (CL) wearers with corneal conjunctivalisation (CC). Methods: In a one‐year (March 2004 to March 2005) retrospective unmasked study, 591 CL clinical histories (195 new patients and 396 review patients) were analysed. Results: There were 24 eyes of 14 myopic patients (93 per cent women) with CC without a specific disease entity known to cause limbal stem cell deficiency (LSCD). Conjunctivalisation occurred in the inferior limbus of three eyes (12 per cent). Only four patients (28.6 per cent) reported previous ocular symptoms. All were myopic with a mean spherical equivalent of ‐8.80 ± 5.00 (SD) dioptres (range from ‐1.75 to ‐21.50 D) and mean visual acuity 0.9 ± 0.2 (range from 0.4 to 1.2). The mean years of CL wear was 17.6 ± 8.5 (CI 95% 13.2 to 22; range six to 30). All were daily‐wear patients with a mean daily‐wear time of 12.5 ± 1.8 hours per day (CI 95% 11.6 to 13.4). Conclusion: Corneal conjunctivalisation is a contact lens‐related complication in asymptomatic patients. Optometrists can play an important role in early diagnosis, education and management of these patients.     

纤维蛋白胶体外重建兔角膜上皮的实验研究

目的利用组织工程技术体外重建角膜上皮组织,为重建角膜表面提供良好的移植材料和方法。方法以等量凝血酶和纤维蛋白原作用生成的纤维蛋白胶为载体(实验组),种植兔角膜缘组织块,体外培养重建角膜上皮层,每天记录细胞生长覆盖面积,从生长特性、光镜特征、超微结构特点以及免疫细胞化学方面进行观察检测。结果角膜缘上皮细胞在纤维蛋白胶上黏附生长增殖,体外培养7d左右即形成单层上皮细胞,2周左右形成的复层上皮细胞与纤维蛋白胶构成角膜上皮组织经检测与生理状态下的角膜上皮组织相近。实验组与对照组(置盖玻片为载体)比较细胞生长覆盖面积第4天至第7天差异有显著性(P<0.05)。结论以纤维蛋白胶为载体,体外培养的角膜上皮组织片可作为眼表重建角膜上皮移植良好的来源。     

对LASIK手术中游离瓣的处理

目的 探讨LASIK手术中出现游离瓣的原因及处理方法。方法 对1500例（2900眼）LASIK手术中出现的9例（9眼）游离瓣进行随访观察。结果 术后1天，9例（9眼）角膜瓣复位良好，边缘整齐，瓣下干净。术后3天取出角膜接触镜，其中7眼视力≥1．0，2眼视力≥0．6。1月视力均达到最佳矫正视力，6月～1年视力稳定。结论LASIK手术中出现游离瓣只要及时正确处理，仍能获得最佳矫正视力。