Mark Dunne

Phakometric measurement of ocular surface radii of curvature, axial separations and alignment in relaxed and accommodated human eyes.

Measurements (autokeratometry, A‐scan ultrasonography and video ophthalmophakometry) of ocular surface radii, axial separations and alignment were made in the horizontal meridian of nine emmetropes (aged 20–38 years) with relaxed (cycloplegia) and active accommodation (mean ± 95% confidence interval: 3.7 ± 1.1 D). The anterior chamber depth (−1.5 ± 0.3 D) and both crystalline lens surfaces (front 3.1 ± 0.8 D; rear 2.1 ± 0.6 D) contributed to dioptric vergence changes that accompany accommodation. Accommodation did not alter ocular surface alignment. Ocular misalignment in relaxed eyes is mainly because of eye rotation (5.7 ± 1.6° temporally) with small amounts of lens tilt (0.2 ± 0.8° temporally) and decentration (0.1 ± 0.1 mm nasally) but these results must be viewed with caution as we did not account for corneal asymmetry. Comparison of calculated and empirically derived coefficients (upon which ocular surface alignment calculations depend) revealed that negligible inherent errors arose from neglect of ocular surface asphericity, lens gradient refractive index properties, surface astigmatism, effects of pupil size and centration, assumed eye rotation axis position and use of linear equations for analysing Purkinje image shifts.

Posterior corneal surface toricity and total corneal astigmatism

The influence of posterior corneal surface toricity upon total corneal astigmatism was investigated in 60 young subjects (mean age 22.04 ± 3.24 years). Both surfaces were found to be flatter horizontally than vertically. Astigmatism arising from the anterior corneal surface was therefore reduced by astigmatism of the opposite sign arising from the posterior surface. Had the toricity of the posterior corneal surface been purely governed by that of the anterior surface this reduction would have amounted to about 5%. However, the posterior surface was found to exhibit additional toricity bringing about a greater reduction of total corneal astigmatism amounting to approximately 14%.

Normal variations of the posterior corneal surface.

Abstract. Measurements of the sphero‐cylindrical components of the rear corneal surface were taken from 80 healthy right eyes in order to determine normal variations. Dimensions of this surface were strongly influenced by the front corneal surface with the exception that the rear surface exhibited more toricity. Both surfaces tended to be flatter in males compared to females and in myopes compared to hyperopes. The corneal surfaces were also found to be flatter in younger eyes compared to older eyes, but this finding was most likely due to the preponderance of myopes in the young and hyperopes in the older group. The influence of the ratio of anterior: posterior corneal surface radius upon the estimation of total corneal power as required for intraocular lens inplant calculations was also considered.

Peripheral astigmatic asymmetry and angle alpha

The association between peripheral astigmatic asymmetry and angle alpha was tested in the present study. Measurements were made in 34 eyes. Peripheral astigmatism was measured over the horizontal meridian using a Zeiss (Jena) Hartinger coincidence optometer and a Canon R‐1 autorefractometer. Curves were fitted to the measured data of each eye and the minima determined by differentiation. Angle alpha was estimated by alignment of Purkinje images 1 (anterior cornea) and IV (posterior crystalline lens). Peripheral astigmatism was found to be symmetrical about a point on the nasal retina. This point departed from the visual axis by 8.8 ± 7.0° (Hartinger) and 9.4 ± 9.8° (Canon). Both values were found to he significantly higher than angle alpha 5.0 ± 1.2°. The results indicate that either peripheral astigmatic asymmetry is due to additional factors such as lack of symmetry in the peripheral curvature of individual optical surfaces, or that there is further misalignment of optical surfaces away from an optical axis.

Three-Dimensional Magnetic Resonance Imaging of the Phakic Crystalline Lens during Accommodation

PURPOSE To quantify changes in crystalline lens curvature, thickness, equatorial diameter, surface area, and volume during accommodation using a novel two-dimensional magnetic resonance imaging (MRI) paradigm to generate a complete three-dimensional crystalline lens surface model. METHODS Nineteen volunteers, aged 19 to 30 years, were recruited. T(2)-weighted MRIs, optimized to show fluid-filled chambers of the eye, were acquired using an eight-channel radio frequency head coil. Twenty-four oblique-axial slices of 0.8 mm thickness, with no interslice gaps, were acquired to visualize the crystalline lens. Three Maltese cross-type accommodative stimuli (at 0.17, 4.0, and 8.0 D) were presented randomly to the subjects in the MRI to examine lenticular changes with accommodation. MRIs were analyzed to generate a three-dimensional surface model. RESULTS During accommodation, mean crystalline lens thickness increased (F = 33.39, P < 0.001), whereas lens equatorial diameter (F = 24.00, P < 0.001) and surface radii both decreased (anterior surface, F = 21.78, P < 0.001; posterior surface, F = 13.81, P < 0.001). Over the same stimulus range, mean crystalline lens surface area decreased (F = 7.04, P < 0.005) with a corresponding increase in lens volume (F = 6.06, P = 0.005). These biometric changes represent a 1.82% decrease and 2.30% increase in crystalline lens surface area and volume, respectively. CONCLUSIONS; The results indicate that the capsular bag undergoes elastic deformation during accommodation, causing reduced surface area, and the observed volumetric changes oppose the theory that the lens is incompressible.

Measurement of the posterior corneal radius using slit lamp and Purkinje image techniques

A method for measuring the radius of the posterior corneal surface using the first and second Purkinje images has recently been developed. Values for the posterior corneal radius in the vertical meridian obtained using this method are compared with those obtained using a more conventional slit lamp method. A good correlation was found between values obtained using the two methods.

Statistical guidelines for clinical studies of human vision.

Citation information: Armstrong RA, Davies LN, Dunne MCM & Gilmartin B. Statistical guidelines for clinical studies of human vision. Ophthalmic Physiol Opt 2011, 31, 123–136. doi: 10.1111/j.1475‐1313.2010.00815.x

Measurement of posterior corneal surface toricity.

We describe a method in which posterior corneal surface toricity can be determined by photographing catoptric (Purkinje) images in three fixed meridians simultaneously using infrared light sources. Keratometry, the comparison of anterior and posterior corneal catoptric (Purkinje) images and pachometric results, are applied to three meridional analysis equations, allowing the posterior corneal surface to be described in spherocylindrical form. The technique is evaluated on five subjects with anterior corneal radii varying from 7.11 to 8.52 mm.

A study of the axis of orientation of residual astigmatism

Abstract. The aim of this study was to determine the axis of orientation of residual astigmatism in a sample of human eyes applying the principle of astigmatic decomposition. Calculations were carried out on keratoscopic and refractive data collected from the right and left eyes of 70 subjects (37 male and 33 female students) of mixed race (including 25 Asians and 43 Caucasians). No statistically significant difference was found for mean levels of residual astigmatism measured in the right (0.46 DC × 98.2°) and left (0.50 DC × 99.4°) eyes. Residual astigmatism was predominantly against‐the‐rule (83% of right eyes and 66% of left eyes) and was within ± 20° of being perpendicularly disposed relative to the corneal astigmatic power axis in two thirds of the eyes measured. No statistically significant differences were found for either gender or race.

Accuracy of cornea and lens biometry using anterior segment optical coherence tomography

We assess the accuracy of the Visante anterior segment optical coherence tomographer (AS-OCT) and present improved formulas for measurement of surface curvature and axial separation. Measurements are made in physical model eyes. Accuracy is compared for measurements of corneal thickness (d(1)) and anterior chamber depth (d(2)) using-built-in AS-OCT software versus the improved scheme. The improved scheme enables measurements of lens thickness (d(3)) and surface curvature, in the form of conic sections specified by vertex radii and conic constants. These parameters are converted to surface coordinates for error analysis. The built-in AS-OCT software typically overestimates [mean+/-standard deviation(SD)]d(1) by +62+/-4 mum and d(2) by + 4 +/- 88 microm. The improved scheme reduces d(1) (-0.4 +/- 4 microm) and d(2) (0 +/- 49 microm) errors while also reducing d(3) errors from +218 +/- 90 (uncorrected) to +14 +/- 123 microm (corrected). Surface x coordinate errors gradually increase toward the periphery. Considering the central 6-mm zone of each surface, the x coordinate errors for anterior and posterior corneal surfaces reached +3 +/- 10 and 0 +/- 23 microm, respectively, with the improved scheme. Those of the anterior and posterior lens surfaces reached +2 +/- 22 and +11 +/- 71 microm, respectively. Our improved scheme reduced AS-OCT errors and could, therefore, enhance pre- and postoperative assessments of keratorefractive or cataract surgery, including measurement of accommodating intraocular lenses.