Biomedical subjects

Mark C M Dunne

Publications and source records attributed to Mark C M Dunne.

5 recordsLinked to original sources

PubMed2007

Operator-induced errors in Hartmann-Shack wavefront sensing: model eye study.

PURPOSE: To evaluate the effects of instrument realignment and angular misalignment during the clinical determination of wavefront aberrations by simulation in model eyes. SETTING: Aston Academy of Life Sciences, Aston University, Birmingham, United Kingdom. METHODS: Six model eyes were examined with wavefront-aberration-supported cornea ablation (WASCA) (Carl Zeiss Meditec) in 4 sessions of 10 measurements each: sessions 1 and 2, consecutive repeated measures without realignment; session 3, realignment of the instrument between readings; session 4, measurements without realignment but with the model eye shifted 6 degrees angularly. Intersession repeatability and the effects of realignment and misalignment were obtained by comparing the measurements in the various sessions for coma, spherical aberration, and higher-order aberrations (HOAs). RESULTS: The mean differences between the 2 sessions without realignment of the instrument were 0.020 microm +/- 0.076 (SD) for Z(3)(-1)(P = .551), 0.009 +/- 0.139 microm for Z(3)(1)(P = .877), 0.004 +/- 0.037 microm for Z(4)(0) (P = .820), and 0.005 +/- 0.01 microm for HO root mean square (RMS) (P = .301). Differences between the nonrealigned and realigned instruments were -0.017 +/- 0.026 microm for Z(3)(-1)(P = .159), 0.009 +/- 0.028 microm for Z(3)(1) (P = .475), 0.007 +/- 0.014 microm for Z(4)(0)(P = .296), and 0.002 +/- 0.007 microm for HO RMS (P = 0.529; differences between centered and misaligned instruments were -0.355 +/- 0.149 microm for Z(3)(-1) (P = .002), 0.007 +/- 0.034 microm for Z(3)(1)(P = .620), -0.005 +/- 0.081 microm for Z(4)(0)(P = .885), and 0.012 +/- 0.020 microm for HO RMS (P = .195). Realignment increased the standard deviation by a factor of 3 compared with the first session without realignment. CONCLUSIONS: Repeatability of the WASCA was excellent in all situations tested. Realignment substantially increased the variance of the measurements. Angular misalignment can result in significant errors, particularly in the determination of coma. These findings are important when assessing highly aberrated eyes during follow-up or before surgery.

Calibration↗

PubMed2005

Non-invasive phakometric measurement of corneal and crystalline lens alignment in human eyes.

We describe a non-invasive phakometric method for determining corneal axis rotation relative to the visual axis (beta) together with crystalline lens axis tilt (alpha) and decentration (d) relative to the corneal axis. This does not require corneal contact A-scan ultrasonography for the measurement of intraocular surface separations. Theoretical inherent errors of the method, evaluated by ray tracing through schematic eyes incorporating the full range of human ocular component variations, were found to be larger than the measurement errors (beta < 0.67 degrees , alpha < 0.72 degrees and d < 0.08 mm) observed in nine human eyes with known ocular component dimensions. Intersubject variations (mean +/- S.D.: beta = 6.2 +/- 3.4 degrees temporal, alpha = 0.2 +/- 1.8 degrees temporal and d = 0.1 +/- 0.1 mm temporal) and repeatability (1.96 x S.D. of difference between repeat readings: beta +/- 2.0 degrees , alpha +/- 1.8 degrees and d +/- 0.2 mm) were studied by measuring the left eyes of 45 subjects (aged 18-42 years, 29 females and 16 males, 15 Caucasians, 29 Indian Asians, one African, refractive error range -7.25 to +1.25 D mean spherical equivalent) on two occasions.

Adolescent↗

PubMed2004

Cling film as a barrier against CJD in Goldmann-type applanation tonometry.

PURPOSE: To determine the feasibility of taking intra-ocular pressure (IOP) readings with Goldmann-type applanation tonometer probe covered with cling film to avoid transmission of Creutzfeldt-Jakob disease (CJD). METHODS: Data were collected on two occasions from the right eyes of 30 healthy subjects using a Perkins applanation tonometer. On each occasion, a single tonometry measurement was made with and without cling film covering. RESULTS: Cling film covering brought about a statistically significant IOP bias (-0.8 mmHg on first occasion and -1.17 mmHg on second occasion). The 95% limits of agreement, between readings made with and without cling film covering, were +/-3.4 mmHg on first occasion; +/-2.5 mmHg on second occasion. A statistically significant reduction of inter-sessional repeatability was observed for readings taken with cling film (+/-5.2 mmHg) compared to those made without (+/-3.9 mmHg). CONCLUSIONS: Even if the above findings are found to be acceptable, given the known intra-subject variations of this type of tonometry (+/-4 mmHg), ethical and legal considerations relating to the barrier properties of cling film are likely to prevent its routine use by optometrists.

Adult↗

PubMed2004

Posterior retinal contour in adult human anisomyopia.

PURPOSE: It is well documented that myopia is associated with an increase in axial length or, more specifically, in vitreous chamber depth. Whether the transverse dimensions of the eye also increase in myopia is relevant to further understanding of its development. METHODS: The posterior retinal surface was localized in two-dimensional space in both eyes of young adult white and Taiwanese-Chinese iso- and anisomyopes (N = 56), from measured keratometry, A-scan ultrasonography, and central and peripheral refraction (+/-35 degrees) data, with the aid of a computer modeling program designed for this purpose. Anisomyopes had 2 D or more interocular difference in their refractive errors, with mean values in their more myopic eyes of -5.57 D and in their less myopic eyes of -3.25 D, similar to the means of the two isomyopic groups. The derived retinal contours for the more and less myopic eyes were compared by way of investigating ocular shape changes that accompany myopia, in the posterior region of the vitreous chamber. The presence and size of optic disc crescents were also investigated as an index of retinal stretching in myopia. RESULTS: Relative to the less myopic eyes of anisometropic subjects, the more myopic eyes were more elongated and also distorted into a more prolate shape in both the white and Chinese groups. However, the Chinese eyes showed a greater and more uniform relative expansion of the posterior retinal surface in their more myopic eyes, and this was associated with larger optic disc crescents. The changes in the eyes of whites displayed a nasal-temporal axial asymmetry, reflecting greater enlargement of the nasal retinal sector. CONCLUSIONS: Myopia is associated with increased axial length and a prolate shape. This prolate shape is consistent with the proposed idea that axial and transverse dimensions of the eye are regulated differently. The observations that ocular shape changes are larger but more symmetrical in Chinese eyes than in eyes of whites warrant further investigation.

Adolescent↗

PubMed2003

Cling film as a barrier against CJD in corneal contact A-scan ultrasonography.

PURPOSE: To determine the validity of covering a corneal contact transducer probe with cling film as protection against the transmission of Creutzfeldt-Jakob disease (CJD). METHODS: The anterior chamber depth, lens thickness and vitreous chamber depth of the right eyes of 10 subjects was recorded, under cycloplegia, with and without cling film covering over the transducer probe of a Storz Omega Compu-scan Biometric Ruler. Measurements were repeated on two occasions. RESULTS: Cling film covering did not influence bias or repeatability. Although the 95% limits of agreement between measurements made with and without cling film covering tended to exceed the intrasessional repeatability, they did not exceed the intersessional repeatability of measurements taken without cling film. CONCLUSIONS: The results support the use of cling film as a disposable covering for corneal contact A-scan ultrasonography to avoid the risk of spreading CJD from one subject to another.

Adult↗