Accuracy of Surgeon-Selected Ablation Center in Active Eye-Tracker-Assisted Advanced Surface Ablation-Photorefractive Keratectomy (ASA-PRK)

Sang Bumm Lee; Myung Jin Cho

doi:10.3341/jkos.2007.48.9.1177

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Lee and Cho: Accuracy of Surgeon-Selected Ablation Center in Active Eye-Tracker-Assisted Advanced Surface Ablation-Photorefractive Keratectomy (ASA-PRK)

Original Article

J Korean Ophthalmol Soc 2007;48(9):1177-1188.

Published online: 30 August 2007

DOI: https://doi.org/10.3341/jkos.2007.48.9.1177

Accuracy of Surgeon-Selected Ablation Center in Active Eye-Tracker-Assisted Advanced Surface Ablation-Photorefractive Keratectomy (ASA-PRK)

Sang Bumm Lee, M.D., Ph.D., Myung Jin Cho, M.D.

Department of Ophthalmology, Yeungnam University College of Medicine, Daegu, Korea

Address reprint requests to Sang Bumm Lee, M.D., Ph.D., Department of Ophthalmology, Yeungnam University College of Medicine, #317-1 Daemyung-dong, Nam-gu, Daegu 705-717, Korea, Tel: 82-53-620-3445, Fax: 82-53-626-5936, E-mail: sbummlee@med.yu.ac.kr

Received 27 February 2007 Accepted 20 June 2007

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Purpose

To evaluate the accuracy of the surgeon-selected ablation center in active eye-tracker-assisted ASA-PRK and to identify factors influencing the ablation center.

Methods

This retrospective study included 109 eyes of 62 patients who underwent active eye-tracker-assisted ASA-PRK (VISX STAR™ S4 with ActiveTrak™ System). The location the surgeon-selected ablation center and its distance from the center of the entrance pupil were analyzed by corneal topography (EyeSys Corneal Analysis System^TM with pupil finding software). The factors influencing centration were investigated.

Results

The mean decentration was 0.24±0.13 mm (range 0.04 to 0.83 mm). One-hundred and five eyes (96%) were within 0.5 mm of the pupillary center. Supero-nasal displacement of the ablation center occurred most frequently in 44 eyes (40%) after ASA-PRK. The decentration amount was not dependant on factors related to the patient, the surgeon, or the surgery. There was no significant correlation between the amount of decentration and the vertical scale bar of the ActiveTrak^TM icon on the screen representing the distance from the pupil center determined by the tracking system, to the surgeon-selected ablation center

Conclusions

This method of tracking the ablation center, which was selected by the surgeon according to each patient's specific pupil decentration with the active eye tracking system, was highly accurate and effective in avoiding severe decentration in ASA-PRK.

Keywords: Ablation center, Advanced surface ablation, Decentration, Eye-tracker, Photorefractive keratectomy

References

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Figure 1.

Vertical scale bar of ActiveTrak™ system. The vertical scale bar to the right of the ActiveTrak™ icon represents the distance between tracking system determined pupil center and surgeon-selected ablation center. For example, this vertical scale bar shows pupil offset by 0.5 mm.

Figure 2.

Cumulative percentage graph according to decentration groups in 0.1 mm steps.

Figure 3.

Scatter plot of the amount of ablation decentration over time.

Figure 4.

Decentration amount of the center of ablation zone relative to the center of entrance pupil according to gender, age, laterality, order of procedure, attempted correction, type of procedure, learning effect and vertical scale bar. In each matched group, there were no statistically significant factors which influence the amount of decentration.

Figure 5.

Scatter plot of the amount of ablation decentration versus the amount of myopic correction. No significant correlation was observed between the amount of myopic correction and the measured amount of decentration (Pearson correlation coefficient, r=-0.019, p=0.85).

Figure 6.

Scatter plot of the amount of ablation decentration versus vertical scale bar percentage. There was no significant correlation between the measured amount of decentration and the vertical scale bar of ActiveTrak™ icon on the computer screen representing the distance from tracking system determined pupil center to surgeon-selected ablation center (Pearson correlation coefficient, r=0.033, p=0.73).

Table 1.

Characteristics of 109 eyes before ASA-PRK

Parameters	Mean±SD (range)
Age (years)	25.5±5.1 (19 to 43)
Gender Male	28 patients 47 eyes
Female	34 patients 62 eyes
Sphere (D)	−4.42±1.45 (−1.60 to −9.00)
Cylinder (D)	−0.57±0.60 (0 to −2.50)
Spherical equivalent (D)	−4.71±1.52 (−1.75 to −9.25)

Table 2.

Distribution of center of the ablation zone relative to the center of entrance pupil

Laterality	Location of ablation zone center, eyes (%)				Total
Laterality	SN^∗	ST^†	IN^‡	IT^§	Total
Right eye	15 (13.8)	24 (22.0)	7 (6.4)	7 (6.4)	53 (48.6)
Left eye	29 (26.6)	11 (10.1)	10 (9.2)	6 (5.5)	56 (51.4)
Total	44 (40.4)	35 (32.1)	17 (15.6)	13 (11.9)	109 (100)

^∗ SN: Superior-Nasal,

^† ST: Superior-Temporal,

^‡ IN: Inferior-Nasal,

^§ IT: Inferior-Temporal.

Table 3.

Decentration amount of the center of ablation zone relative to the center of entrance pupil according to the patient-related factors, the surgery-related factors and the surgern-related factors

Characteristics		N	Mean±SD (mm)	P-value
Patient-related factors
Gender	Male	47	0.26±0.15	0.22
	Female	62	0.23±0.11
Age (years)	< 25	60	0.24±0.12	0.66
	≥ 25	49	0.25±0.13
Laterality	Right eye	53	0.23±0.11	0.21
	Left eye	56	0.26±0.15
Order of procedure	1st	47	0.24±0.12	0.71
(in cases of bilateral surgery)	2nd	47	0.23±0.10
Surgery-related factors
Attempted correction (SE)	≤ −4.71D	58	0.25±0.14	0.69
	>−4.71D	51	0.24±0.12
Type of procedure	PARK^∗	73	0.26±0.14	0.37
	PRK^†	36	0.23±0.10
Surgeon-related factors
Learning effect	First 59 procedures	59	0.25±0.15	0.36
	Recent 50 procedures	50	0.23±0.11
Vertical scale bar	< 50%	58	0.23±0.12	0.50
	≥ 50%	51	0.25±0.13

^∗ PARK: photoastigmatic refractive keratectomy,

^† PRK: photorefractive keratectomy.

The decentration amount was not depending on the patient-related factors (gender, age, laterality, and order of procedure), the surgery-related factors (amount of attempted correction, type of procedure), and the surgeon-related factors (learning effect, vertical scale bar) in this study.

Table 4.

Comparison of decentration of the ablation zone with the result of other studies

					Decentration
Study	No of case	Type of procedure	Type of Excimer laser	Eye-tracker	Mean±SD (mm)	Cumulativem percentage (%)		Shift of Decentration
Study	No of case	Type of procedure	Type of Excimer laser	Eye-tracker	Mean±SD (mm)	0.5 mm ↓	1 mm ↓	Shift of Decentration
Cavanaugh^10,11	110	PRK	Summit UV200	No	0.52	57	93	down, right
Lin³⁰	97	PRK	Visx 20/20 B	No	0.36±0.25	85	98
Amano¹²	60	PRK	Summit UV200	No	0.51±0.31	60	93	down
Lee²⁹	190	PRK	Visx 20/20 B	No	0.33±0.21	83	99	sup-nasal
Tsai²¹	16	PRK	Schwind Keratom	Yes	0.33±0.32	69	100	no
	161	LASIK		Yes	0.35±0.26	70	100	no
Pineros²⁸	18	LASIK	Chron Keracor 117-C	No	0.55±0.30
	20	LASIK		Yes	0.43±0.23
Choi²²	41	LASIK	LaserScan LSX	No	0.49±0.24	51	97	inferior
	41	LASIK		Yes	0.38±0.23	66	100	superior
Giaconi²⁷	25	LASIK	Visx Star S2/S3	No	0.21±0.26	96	100	no
	25	LASIK		Yes	0.22±0.20	88	100	left
This study	109	ASA-PRK	Visx Star S4	Yes	0.24±0.13	96	100	sup-nasal

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