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Title
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Factors which affect refractive outcome following LASIK for myopia.
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| Author(s) |
Feltham, Mark Hayes, Optometry & Vision Science, Faculty of Science, UNSW
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| Resource Type |
Thesis
PhD Doctorate
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| Keyword(s) |
LASIK (Eye surgery)
Myopia
Eye
Refractive errors
laser in-situ keratomileusis
Eye surgery
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| Date |
2004 |
| School/Centre |
University of New South Wales. School of Optometry and Vision Science |
| Description/Abstract |
Purpose: To improve the predictability, accuracy and stability of laser in-situ keratomileusis
(LASIK), by evaluating the clinical, procedural and tissue response factors that affect refractive
outcomes.
Methods: Myopic LASIK surgeries (n=5,978) were carried out using the Technolas planoscan
and Nidek EC-5000 excimer lasers. Clinical variables associated with a refractive outcome of
within ±0.50 D of the target were identified using regression analysis. Possible procedural
variations such as the timing of the procedure and accuracy of both the chosen keratome and
excimer laser were evaluated. The predictability and accuracy of the ablation was assessed by
measuring changes in corneal thickness during and after the procedure. Factors influencing the
stability of refractive outcome were assessed.
Results: Clinical factors associated with a refractive outcome of within ±0.50 D of the target
included; corrections less than 5.00 DS (OR 0.21x, 95% CI 0.11-0.40x compared with
corrections over -5.00 DS[referent]), patients younger than 40 years (patients over 50 OR 8.27x,
95% CI 3.41-20.03x, patients 40 to 50 years OR 1.93x, 95% CI 0.96-3.90x, compared with
patients under 40[referent]) and average pre-operative curvatures between 43.50 and 45.50D
(OR 0.39x, 95% CI 0.18-0.83 compared with curvatures of less than 43.50D [referent]).
Refractive stability was improved using optic zone sizes between 5.5-6.0 mm, reduced myopic
corrections, flatter pre-operative corneal curvatures and thicker corneal flaps (R²=25%,
p<0.001).
Procedural factors associated with poorer outcomes included: thinner measured flap thickness,
deeper ablations and the use of the automated corneal shaper (ACS) microkeratome with a
novice surgical team (R²=34%, p < 0.001). Delaying the ablation from 20 to 90 seconds (s) after
flap lift was associated with a more stable refractive outcome at three months (p=0.017).
In the 90 s following flap lift, the cornea thinned by 5±3%. The ablation rate per scan varied
between procedures, however, the effect on refractive outcome was small (r=0.15, p=0.267).
Changes in central corneal thickness indicated refractive stability (p=0.039).
Conclusions: Applying the optimal clinical and procedural factors as described afforded a
refractive outcome in a further 8% of cases, resulting in 94% to 96% of cases within ±0.50 DS
of target. Refractive predictability was limited due to the inability of the keratome to produce a
consistent corneal flap thickness and unexpected changes in corneal thickness. The accuracy of
refractive outcome will decrease with larger ablations. The degree of refractive inaccuracy with high refractive corrections (> -10.00 D) can be over 1.00 D. |
| Language |
EN |
| Rights |
Please click here to view the rights |
| Citation Link |
Please use this identifier to cite or link to this item: http://handle.unsw.edu.au/1959.4/20800 |
| Full Text | 
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| Total Attachment(s) | 2 |
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