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Biomedical subjects

M C Dunne

Publications and source records attributed to M C Dunne.

11 recordsLinked to original sources
PubMed1992

Normal variations of the posterior corneal surface.

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 implant calculations was also considered.

Adult↗
PubMed1992

Scheme for the calculation of ocular components in a four-surfaced eye without need for measurement of the anterior crystalline lens surface Purkinje images.

The computing scheme described in this paper allows calculation of the internal radii of the human eye on the assumption that it possesses four (two corneal and two lenticular) axially aligned spherical surfaces separated by homogeneous optical media. It involves measurement of Purkinje images I, II and IV. This approach is novel in that it accounts for the contribution of the posterior corneal surface and does not require measurement of the Purkinje images arising from the anterior crystalline lens surface which are notoriously poor in quality.

Anthropometry↗
PubMed1991

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%.

Adult↗
PubMed1991

Effect of iris displacement on oblique astigmatism in aphakic eyes.

Aphakic eyes possess considerably less oblique astigmatism than phakic eyes. The possibility that posterior iris displacement after lens extraction accounts for the observed reduction in oblique astigmatism is tested in this study. Oblique astigmatism was calculated for a phakic and an aphakic schematic eye. With no iris displacement little reduction of oblique astigmatism occurred contrary to experimental findings. However, introduction of varying levels of iris displacement reduced this aberration to values found experimentally. Posterior iris displacement (mean = 1.19 mm) was confirmed in vivo by pre- and postoperative anterior chamber pachometry in eight patients.

Adult↗
PubMed1990

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.

Cornea↗
PubMed1990

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.

Anthropometry↗
PubMed1989

An evaluation of Bennett's method for determining the equivalent powers of the eye and its crystalline lens without resort to phakometry.

Refraction and optical components have been measured in 36 eyes using objective optometry, keratometry, ophthalmophakometry and A-scan ultrasonography. A computing scheme was used to confirm that the accumulated experimental errors for a phakometric technique tend to exceed the inherent errors of a new method which calculates the equivalent powers of the eye and its crystalline lens without resort to phakometry. Bennett's method was shown to be a valuable technique in biometric studies which obviates the need for lengthy experimental investigations.

Adult↗
PubMed1989

An analysis of three meridional keratometric measurement of the anterior corneal surface.

Analysis of measurements taken in three meridians of a toroidal optical surface can produce a spherocylindrical result without having to locate the principal meridians. This technique is assessed for the anterior corneal surface using keratometry. Good agreement is found with measurements taken in the principal meridians making this a suitable method for the assessment of the eye's optical surfaces by Purkinje image measurement.

Adult↗
PubMed1989

Calculation of crystalline lens radii without resort to phakometry.

A computing scheme was developed in 1988 to determine the equivalent powers of the eye and its crystalline lens without resort to phakometry. An adaptation to this scheme is described which allows calculation of the crystalline lens radii using an assumed average value for Q. The values obtained for lens radii using this method compare well with phakometric measurements made on 36 eyes (ocular refraction range -9.37 D to + 3.14 D) which suggests that the variations in Q for a large proportion of the population are small.

Accommodation, Ocular↗