Crystalline Lens Power Research Articles

In vivo measurement of the human crystalline lens equivalent refractive index using extended-depth OCT

The lens equivalent refractive index (RI) is commonly used in calculations of crystalline lens power. However, accurate determination of the equivalent RI in vivo is challenging due to the need of multiple measurements with different ocular biometry devices. A custom extended-depth Spectral Domain-OCT system was utilized to provide measurements of corneal and lens surface curvatures and all intraocular distances required for determination of the lens equivalent RI. Ocular biometry and refraction were input into a computational model eye from which the equivalent RI was calculated. Results derived from human subjects of a wide age range show a decrease in RI with age and demonstrate the capability of in vivo measurements of the equivalent RI with extended-depth OCT.

Ocular Component Development during Infancy and Early Childhood.

The study fills an important gap by providing a longitudinal description of development of the major structural and optical components of the human eye from 3 months to nearly 7 years of age. Normative development data may provide insights into mechanisms for emmetropization and guidance on intraocular lens power calculation. The purpose of this study was to describe the pattern of development of refractive error and the ocular components from infancy through early childhood. Cycloplegic retinoscopy (cyclopentolate 1%), keratophakometry, and ultrasonography were performed longitudinally on between 162 and 293 normal birth weight infants at 0.25, 0.75, 1.5, 3, 4.5, and 6.5 years of age. Refractive error and most ocular components displayed an early exponential phase of rapid development during the first 1 to 2 years of life followed by a slower quadratic phase. Anterior and vitreous chamber depths, axial length, and crystalline lens radii increased at every visit. The crystalline lens thinned throughout the ages studied. The power of the cornea showed an early decrease, then stabilized, whereas the crystalline lens showed more robust decreases in power. The crystalline lens refractive index followed a polynomial growth and decay model, with an early increase followed by a decrease starting at 1 to 2 years of age. Refractive error became less hyperopic and then was relatively stable after 1 to 2 years of age. Axial lengths increased by 3.35 ± 0.64 mm between ages 0.25 and 6.5 years, showed uniform rates of growth across the range of initial values, and were correlated with initial axial lengths (r = 0.44, P < .001). Early ocular optical and structural development appears to be biphasic, with emmetropization occurring within the first 2 years of infancy during a rapid exponential phase. A more stable refractive error follows during a slower quadratic phase of growth when axial elongation is compensated primarily by changes in crystalline lens power.

Design and recent results of large-scale cohort epidemiology studies on refractive error in children in Shanghai

Between 2011 and 2013, two large-scale cohort epidemiology studies were launched in Shanghai: the SCALE study, which aimed to provide ocular public health services to cover the entire youth population in Shanghai, and the SCES, which was based on sample surveys and aimed to provide information on the prevalence and incidence of visual impairment and different types of refractive errors. A total of 910,245 children and adolescents were finally enrolled in the SCALE study; three possible methods for monitoring refractive error without mydriasis were tested, and the agreement between the refractive outcomes of three commonly used autorefractors were examined to ensure the accuracy of the results of the SCALE study. A total of 8,627 children were enrolled in the SCES, and the baseline prevalence of different refractive errors, different behaviors associated with 1 year myopic shifts, and the different patterns of 2-year myopia progression between internal migrant and local resident school children have been analyzed. In some subset samples of the SCALE study and the SCES, several refraction components such as choroidal thickness (ChT) and crystalline lens power were also measured, to further elucidate the relationships between the refraction components and myopia as well as the mechanism of myopia incidence and development. The three methods used in Shanghai to prevent and intervene with childhood myopia: increasing outdoor time, low concentration atropine, and use of orthokeratology lens are also addressed in this review.

The Associations of Lens Power With Age and Axial Length in Healthy Chinese Children and Adolescents Aged 6 to 18 Years

To investigate the relationship between lens power and age as well as the relationship between lens power and axial length (AL) in Chinese children and adolescents. The participants underwent a comprehensive ophthalmic examination that included AL, cycloplegic refraction, and Pentacam measurements. The crystalline lens power was calculated using Bennett's formula and then compared among the children of different age groups, refractive statuses, and AL categories. The association of lens power and AL was analyzed using multiple regression. A total of 1992 children and adolescents aged 6- to 18-years old were included. The difference in lens power was greater before 10-years old, followed by a relatively smaller difference in children aged 10 to 14 years and the difference in lens power came to a near plateau in adolescents after 14-years old. The negative association between lens power and AL was found to be more evident in nonmyopes than in myopes irrespective of age (younger than 10 years: nonmyopes: β = -1.499, myopes: β = -0.872; older than 10 years: nonmyopes: β = -1.288, myopes: β = -0.390, all P < 0.001). The lens power in children and adolescents aged 6 to 18 years exhibited three stages. The association between lens power and AL differed between the nonmyopes and myopes. These findings suggested that less reduction in lens power might be associated with both growing age and increasing AL in myopes.

The Relationship between Crystalline Lens Power and Refractive Error in Older Chinese Adults: The Shanghai Eye Study

PurposeTo report calculated crystalline lens power and describe the distribution of ocular biometry and its association with refractive error in older Chinese adults.MethodsRandom clustering sampling was used to identify adults aged 50 years and above in Xuhui and Baoshan districts of Shanghai. Refraction was determined by subjective refraction that achieved the best corrected vision based on monocular measurement. Ocular biometry was measured by IOL Master. The crystalline lens power of right eyes was calculated using modified Bennett-Rabbetts formula.ResultsWe analyzed 6099 normal phakic right eyes. The mean crystalline lens power was 20.34 ± 2.24D (range: 13.40–36.08). Lens power, spherical equivalent, and anterior chamber depth changed linearly with age; however, axial length, corneal power and AL/CR ratio did not vary with age. The overall prevalence of hyperopia, myopia, and high myopia was 48.48% (95% CI: 47.23%–49.74%), 22.82% (95% CI: 21.77%–23.88%), and 4.57% (95% CI: 4.05–5.10), respectively. The prevalence of hyperopia increased linearly with age while lens power decreased with age. In multivariate models, refractive error was strongly correlated with axial length, lens power, corneal power, and anterior chamber depth; refractive error was slightly correlated with best corrected visual acuity, age and sex.ConclusionLens power, hyperopia, and spherical equivalent changed linearly with age; Moreover, the continuous loss of lens power produced hyperopic shifts in refraction in subjects aged more than 50 years.

Lens Power, Axial Length-to-Corneal Radius Ratio, and Association with Diabetic Retinopathy in the Adult Population with Type 2 Diabetes

To calculate crystalline lens power and to determine the relationship between ocular biometry and diabetic retinopathy (DR) in an adult population with type 2 diabetes mellitus (T2DM). Cross-sectional, population-based study. Patients with T2DM from the Beixinjing community, Changning district, Shanghai. Random clustering sampling was used to identify adults with T2DM in the Beixinjing community. Spherical equivalent (SE) was determined by subjective refraction that achieved the best corrected vision. Axial length (AL), corneal power (CP), and anterior chamber depth (ACD) were measured using the IOLMaster. Diabetic retinopathy and diabetic macular edema (DME) were assessed according to the international DR classification. The crystalline lens power was calculated using the Bennett-Rabbetts formula. The AL-to-corneal radius ratio (AL/CR ratio) was defined as the axial length divided by the mean corneal radius of curvature. A total of 4011 eyes of 2057 subjects with T2DM were included in the analysis. In multivariate logistic models adjusting for age, sex, duration of diabetes, glycosylated hemoglobin A1c, serum creatinine, body mass index, systolic blood pressure, and cataract, after categorizing values into quartiles, there were trend associations between lens power and any DR (P= 0.01), between AL/CR ratio and any DR (P= 0.02), and between AL and any DR (P= 0.03), between lens power and moderate DR (P = 0.02), and between AL and moderate DR (P= 0.02); eyes with higher AL/CR ratio were less likely to have any DR (odds ratio [OR], 0.43; 95% confidence interval [CI], 0.24-0.78; P= 0.01 per 1 increase) and moderate DR (OR, 0.44; 95% CI, 0.21-0.93; P= 0.03 per 1 increase), eyes with longer AL were less likely to have any DR (OR, 0.88; 95% CI, 0.81-0.95; P= 0.002 per millimeter increase) or moderate DR (OR, 0.89; 95% CI, 0.80-0.98; P= 0.02 per millimeter increase), and eyes with higher SE were more likely to have any DR (OR, 1.08; 95% CI, 1.03-1.13; P= 0.003 per diopter increase). In persons with T2DM, lens power, AL/CR ratio, and AL were associated with the presenceofany DR and moderate DR. These findings suggested that globe elongation plays a major role inprotectiveeffectsagainstDR, with contributions from lens power and other refractive components.

Distribution of the Crystalline Lens Power In Vivo as a Function of Age

To observe the age-related changes in crystalline lens power in vivo in a noncataractous European population. Data were obtained though Project Gullstrand, a multicenter population study with data from healthy phakic subjects between 20 and 85 years old. One randomly selected eye per subject was used. Lens power was calculated using the modified Bennett-Rabbetts method, using biometry data from an autorefractometer, Oculus Pentacam, and Haag-Streit Lenstar. The study included 1069 Caucasian subjects (490 men, 579 women) with a mean age of 44.2 ± 14.2 years and mean lens power of 24.96 ± 2.18 diopters (D). The average lens power showed a statistically significant decrease as a function of age, with a steeper rate of decrease after the age of 55. The highest crystalline lens power was found in emmetropic eyes and eyes with a short axial length. The correlation of lens power with different refractive components was statistically significant for axial length (r = -0.523, P < 0.01) and anterior chamber depth (r = -0.161, P < 0.01), but not for spherical equivalent and corneal power (P > 0.05). This in vivo study showed a monotonous decrease in crystalline lens power with age, with a steeper decline after 55 years. While this finding fundamentally concurs with previous in vivo studies, it is at odds with studies performed on donor eyes that reported lens power increases after the age of 55.

Calculation of crystalline lens power using a modification of the Bennett method.

We present a method for measuring lens power from extended depth OCT biometry, corneal topography, and refraction using an improvement on the Bennett method. A reduced eye model was used to derive a formula for lens power in terms of ocular distances, corneal power, and objective spherical equivalent refraction. An error analysis shows that the formula predicts relaxed lens power with a theoretical accuracy of ± 0.5 D for refractive error ranging from -10 D to + 10 D. The formula was used to calculate lens power in 16 eyes of 8 human subjects. Mean lens power was 24.3 D ± 1.7 D.

Prediction of Juvenile-Onset Myopia.

Myopia (nearsightedness) has its onset in childhood and affects about one-third of adults in the United States. Along with its high prevalence, myopia is expensive to correct and is associated with ocular diseases that include glaucoma and retinal detachment. To determine the best set of predictors for myopia onset in school-aged children. The Collaborative Longitudinal Evaluation of Ethnicity and Refractive Error (CLEERE) Study was an observational cohort study of ocular development and myopia onset conducted at 5 clinical sites from September 1, 1989, through May 22, 2010. Data were collected from 4512 ethnically diverse, nonmyopic school-aged children from grades 1 through 8 (baseline grades 1 through 6) (ages 6 through 13 years [baseline, 6 through 11 years]). We evaluated 13 candidate risk factors for their ability to predict the onset of myopia. Myopia onset was defined as -0.75 diopters or more of myopia in each principal meridian in the right eye as measured by cycloplegic autorefraction at any visit after baseline until grade 8 (age 13 years). We evaluated risk factors using odds ratios from discrete time survival analysis, the area under the curve, and cross validation. A total of 414 children became myopic from grades 2 through 8 (ages 7 through 13 years). Of the 13 factors evaluated, 10 were associated with the risk for myopia onset (P < .05). Of these 10 factors, 8 retained their association in multivariate models: spherical equivalent refractive error at baseline, parental myopia, axial length, corneal power, crystalline lens power, ratio of accommodative convergence to accommodation (AC/A ratio), horizontal/vertical astigmatism magnitude, and visual activity. A less hyperopic/more myopic baseline refractive error was consistently associated with risk of myopia onset in multivariate models (odds ratios from 0.02 to 0.13, P < .001), while near work, time outdoors, and having myopic parents were not. Spherical equivalent refractive error was the single best predictive factor that performed as well as all 8 factors together, with an area under the curve (C statistic) ranging from 0.87 to 0.93 (95% CI, 0.79-0.99). Future myopia can be predicted in a nonmyopic child using a simple, single measure of refractive error. Future trials for prevention of myopia should target the child with low hyperopia as the child at risk.

Lens Power in a Population-Based Cross-Sectional Sample of Adults Aged 40 to 64 Years in the Shahroud Eye Study

To report on calculated lens power in adults in relation to other ocular components of refraction, analyzed in relation to secular change in height and education. The first phase of the Shahroud Eye Cohort Study is a cross-sectional population-based study in Iranian subjects 40 to 64 years old. Data on cycloplegic spherical equivalent refraction and the ocular components of the right eyes were used for the calculation of crystalline lens power with Bennett's formula. Interactions between sex and age as independent variables were analyzed by two-way analysis of variances. Cycloplegic refraction data and biometry were obtained from 4592 subjects, of whom 2666 (58%) were women. The mean lens power showed a biphasic trend. Up to the age of 50, younger subjects had lower lens power than older subjects, but after the age of 50, older subjects had lower lens power. A secular trend in height was found, with younger subjects significantly taller than older ones. Taller men or women had longer eyes, with flatter corneas and less powerful lenses, independent of refractive error. In multiple regression models, corneal power (P < 0.001), axial length (P < 0.001), and lens power (P < 0.001) were all associated with height, independent of age and sex. There was an unexpected biphasic pattern of the distribution of lens power with age in this cross-sectional study. Younger subjects were taller, and despite having longer axial lengths, their bigger eyes were still predominantly emmetropic. The greater axial lengths were counterbalanced by both lower corneal and lower lens powers.

Calculation of crystalline lens power in chickens with a customized version of Bennett’s equation

This paper customizes Bennett's equation for calculating lens power in chicken eyes from refraction, keratometry and biometry. Previously published data on refraction, corneal power, anterior chamber depth, lens thickness, lens radii of curvature, axial length and eye power in chickens aged 10-90 days were used to estimate Gullstrand's lens power and Bennett's lens power for chicken eyes, and to calculate the lens equivalent refractive index. Bennett's A and B constants for the front and back surface powers of the lens were calculated for data measured from day 10 to 90 at 10 day intervals, and mean customized constants were calculated. The mean customized constants for Bennett's equation for chicks were A=0.574±0.023 and B=0.379±0.021. As found previously, lens power decreases with age in chicks, while corneal power decreases and axial length increases. The lens equivalent refractive index decreases with age from 10 to 90 days after hatching. Bennett's equation can be used to calculate lens power in chicken eyes for studies on animal myopia, using standard biometry.

Le pseudoxanthome élastique et la myopie

Refraction in patients with pseudoxanthoma elasticum (PXE) has not yet been studied in the literature. The purpose of our study was to determine if PXE may be associated with myopia, similarly to Marfan syndrome, another collagen-vascular disease. This cross-sectional study evaluated 53 consecutive patients (32 women and 21 men, mean age 50 ± 16) from September 2009 to June 2011, with 53 age- and sex-matched controls. For each patient, we measured spherical equivalent, visual acuity, keratometry, axial length and crystalline lens power of each eye. Results were compared with Student's t-test. There was a significant difference between the mean values of myopia in the PXE population (-0.98) compared to the group of healthy controls (-0.33, P<0.0093). There was no significant difference between the two groups for the axial length (23.9 mm versus 23.69 mm; P=0.236), keratometry (43.53 versus 43.44; P=0.68), lens power (22.03 versus 21.71; P=0.225). PXE induces calcification of elastic fibers in various human tissues (skin, retina, vascular walls). Since there are no elastic fibers in the cornea or lens, we assume that the elastic zonular fibers may be pathologic in PXE patients. This could result in an anterior displacement of the lens in PXE, which could explain the moderate myopia. PXE may be associated with mild myopia compared to healthy controls. Additional studies, including more patients and evaluating anterior chamber depth and zonular histology are necessary to confirm these preliminary findings.

Changes in Lens Power in Singapore Chinese Children during Refractive Development

To examine changes in lens power during refractive development in Singapore Chinese children. Children aged six to nine years from three Singapore schools were invited to participate in the Singapore Cohort study Of the Risk factors for Myopia (SCORM) study. Cycloplegic refractions and biometry were measured annually in the schools over a five year period from 1999. Children were classified into five refractive error groups: persistent hyperopia, emmetropizing hyperopia, persistent emmetropia, newly developed myopia, or persistent myopia. Crystalline lens power was calculated using Bennett's formula. The rate of change per year across the refractive groups was adjusted for age and sex using General Linear Models. There were 1747 children with at least three sets of measurements for lens power calculations. The mean age at baseline was 7.94 ± 0.84 years and the mean spherical equivalent refraction was -0.41 ± 1.71 diopters (D). Lower lens power and lower lens thickness were associated with persistent myopia. As expected, the newly developed myopes and the persistent myopes showed the largest changes in axial length (AL). Changes in lens power and thickness at follow-up were similar in all refractive groups, except for the newly developed myopes, who showed significantly greater decreases in lens power (0.36 vs. 0.29 D/year; P < 0.001) and lens thickness (0.015 vs. 0.0003 mm/year; P < 0.001) than the persistently emmetropic group. Newly developed myopes showed a significantly greater decrease in lens power than other refractive groups, which may be linked to rapid changes in AL and refraction that occur around the onset of myopia.

Crystalline Lens Power and Refractive Error

To study the relationships between the refractive power of the crystalline lens, overall refractive error of the eye, and degree of nuclear cataract. All phakic participants of the population-based Central India Eye and Medical Study with an age of 50+ years were included. Calculation of the refractive lens power was based on distance noncycloplegic refractive error, corneal refractive power, anterior chamber depth, lens thickness, and axial length according to Bennett's formula. The study included 1885 subjects. Mean refractive lens power was 25.5 ± 3.0 D (range, 13.9-36.6). After adjustment for age and sex, the standardized correlation coefficients (β) of the association with the ocular refractive error were highest for crystalline lens power (β = -0.41; P < 0.001) and nuclear lens opacity grade (β = -0.42; P < 0.001), followed by axial length (β = -0.35; P < 0.001). They were lowest for corneal refractive power (β = -0.08; P = 0.001) and anterior chamber depth (β = -0.05; P = 0.04). In multivariate analysis, refractive error was significantly (P < 0.001) associated with shorter axial length (β = -1.26), lower refractive lens power (β = -0.95), lower corneal refractive power (β = -0.76), higher lens thickness (β = 0.30), deeper anterior chamber (β = 0.28), and less marked nuclear lens opacity (β = -0.05). Lens thickness was significantly lower in eyes with greater nuclear opacity. Variations in refractive error in adults aged 50+ years were mostly influenced by variations in axial length and in crystalline lens refractive power, followed by variations in corneal refractive power, and, to a minor degree, by variations in lens thickness and anterior chamber depth.

Surgical treatment for myopia

This article briefly explains surgical treatments of myopia. Laser refractive surgery using the excimer laser and/or femtosecond is the most commonly operated one. As the cornea has the greatest refractive power out of the ocular structures, the laser refractive surgery ablates the corneal stroma. Laser refractive surgeries are categorized by the surgical procedures basics: photorefractive keratectomy, laser in situ keratomilieusis (LASIK), laser epithelial keratomileusis, and Epi-LASIK. There are also some surgical treatment of myopia that does not use the laser ablation of the stroma. Intrastromal corneal ring segments implantation changes corneal shape which leads to the change of refractive power. Clear lens extraction reduces refractive power of crystalline lens by removal, and phakic intraocular lens implantation adds negative refractive power by implantation of intraocular lens. Patients are recommended to have a detailed consultation before choosing the surgical treatment based on their ocular status. There are several methods to treat myopia surgically, Laser reftractive surgery on cornea is most common method currently.

Model of accommodation: Contributions of lens geometry and mechanical properties to the development of presbyopia

To determine the relative importance of lens geometry and mechanical properties for the mechanics of accommodation and the role of these elements in the causes and potential correction of presbyopia. Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey, USA. Experimental study. Finite element methods and ray-tracing algorithms were used to model the deformation and optical power of the human crystalline lens during accommodation. The mechanical model treats the lens as an axisymmetric object, and the optical model incorporates a gradient refractive index. Using these models, the accommodation of a broad range of lenses with different geometries and mechanical properties were investigated. The most significant result was that reshaping the 45-year-old lens to the geometry of the 29-year-old lens, while retaining the mechanical properties, restored the former's accommodation amplitude to 72% to 94% of that of the 29-year-old lens, depending on ciliary body displacement. That is, reshaping can add 1.8 to 3.7 diopters of accommodation. A sensitivity analysis showed that this result was robust over a wide range of mechanical and geometrical properties. The study results suggest that a significant amount of the loss of accommodation is due to changes in lens geometry.

Refração e seus componentes em anisometropia

To compare the individual means of ocular components of both eyes in patients with anisometropia; to correlate the differences of the components with refractive differences; and to identify the smallest number of factors that contain the same level of information expressed in the set of variables that influence refractive difference. An analytical transversal study was carried out in 77 patients with anisometropia of two or more dioptres seen at the Ophthalmologic Clinic, University Hospital, Nilton Lins Medical School, Manaus. All participants were submitted to ophthalmologic examination which included objective and subjective cycloplegic refractometry, keratometry and ultrasound biometry. Data analysis comprised the following statistical models: univariate, multivariate, multiple and factorial regression analyses. There were no significant differences in the comparison between the individual means of the ocular components. There was negative correlation between refractive difference and difference of axial length (r= -0.64; p<0.01) and weak negative correlation between refractive difference and crystalline lens power difference (r= -0.34; p<0.01). The analyzed variables reached 78% of the total variation of refractive difference. Three factors were identified for refractive differences: a) factor 1 (refraction, axial length); b) factor 2 (anterior chamber depth, cornea power), and c) factor 3 (crystalline lens power).

A biometric investigation of ocular components in amblyopia

The aim of this study was to investigate the ocular parameters and their contribution to total axial length, and any inter-ocular differences when compared to the fellow non-amblyopic eye, in a population of paediatric amblyopes. Inter-ocular differences in visual acuity, refractive error, corneal curvature, anterior chamber depth and their contribution to total axial length were analysed in four populations: adult controls (n=26), paediatric controls (n=24), paediatric strabismic amblyopes (n=18) and paediatric anisometropic amblyopes (n=27). In the two control groups, there were small inter-ocular differences between the components, none of which were statistically or clinically significant. Anisometropic amblyopic eyes were found to have statistically and clinically significant differences in refractive error, crystalline lens power, vitreous chamber depth and total axial length when compared to the fellow eye. Anterior and vitreous chamber depths, when expressed as a percentage of the anterior and vitreous chambers in the non-amblyopic fellow eye, were approximately 95% of the size. When parameters were normalised by expressing as a percentage of total axial length, the contribution of the anterior chamber, crystalline lens and vitreous chamber to total axial length in both eyes were comparable, suggesting that all components of the anisometropic amblyopic eye are proportionately reduced in size. The strabismic amblyopic eyes were in the main isometropic. There were however statistically significant differences in anterior chamber depth, crystalline lens power, vitreous chamber depth and total axial length, but not in crystalline lens thickness. When the components were expressed as a percentage of the component in the non-amblyopic fellow eye, the anterior and vitreous chambers were approximately 90% of the size of the fellow non-amblyopic eye and made a significantly smaller contribution to total axial length when normalised. Crystalline lens thickness contributed significantly more to the total axial length in a strabismic eye. This suggests that, unlike control eyes or anisometropic amblyopic eyes, the strabismic eye is physically, though not proportionately, reduced in size.

Optical Power of the Isolated Human Crystalline Lens

To characterize the age dependence of isolated human crystalline lens power and quantify the contributions of the lens surfaces and refractive index gradient. Experiments were performed on 100 eyes of 73 donors (average 2.8 +/- 1.6 days postmortem) with an age range of 6 to 94 years. Lens power was measured with a modified commercial lensmeter or with an optical system based on the Scheiner principle. The radius of curvature and asphericity of the isolated lens surfaces were measured by shadow photography. For each lens, the contributions of the surfaces and the refractive index gradient to the measured lens power were calculated by using optical ray-tracing software. The age dependency of these refractive powers was assessed. The total refractive power and surface refractive power both showed a biphasic age dependency. The total power decreased at a rate of -0.41 D/y between ages 6 and 58.1, and increased at a rate of 0.33D/y between ages 58.1 and 82. The surface contribution decreased at a rate of -0.13 D/y between ages 6 and 55.2 and increased at a rate of 0.04 D/y between ages 55.2 and 94. The relative contribution of the surfaces increased by 0.17% per year. The equivalent refractive index also showed a biphasic age dependency with a decrease at a rate of -3.9 x 10(-4) per year from ages 6 to 60.4 followed by a plateau. The lens power decreases with age, due mainly to a decrease in the contribution of the gradient. The use of a constant equivalent refractive index value to calculate lens power with the lens maker formula will underestimate the power of young lenses and overestimate the power of older lenses.

On the ocular refractive components: the Reykjavik Eye Study

To study the correlation between ocular refraction and the refractive components (corneal power, lens power and axial length) in a population-based sample of normal subjects. We analysed the refractive and biometric findings for 723 right eyes (325 males and 398 females) comprising a population-based random sample of citizens 55 years and older participating in the Reykjavik Eye Study. Measurements of refraction, corneal curvature (by keratometry), anterior chamber depth, lens thickness and axial length (all by ultrasound biometry) were used to calculate crystalline lens power. The correlation and regression between refraction and ocular refractive components (corneal power, anterior chamber depth, lens power and axial length) were studied by distributional statistical methods. Refraction (spherical equivalent) showed a significant negative correlation with axial length (r = -0.59, P < 0.0001), lens power (r = -0.26, P < 0.0001) and corneal power (r = -0.16, P < 0.0001). There were significant negative correlations between axial length and corneal power (r = -0.44, P < 0.0001) and between axial length and lens power (r = -0.44, P < 0.0001). Based on multiple linear regression analysis, refraction could be correlated with corneal power, lens power and axial length in combination with a correlation coefficient of 0.98 (P < 0.0001). This study confirms that ocular refraction is statistically significantly correlated with not only axial length but also lens power and (to a lesser extent) corneal power. The variation and correlations of crystalline lens power were considerable -- possibly indicating this component's modulatory effect on ocular refraction during growth. We conclude the refractive error of the eye to be a multifactorial condition involving a complex interplay between the cornea, the lens and the length of the eye.