Astigmatism
Conditions
Keywords
Excimer Laser, photorefractive keratectomy, astigmatism
Brief summary
The purpose of this study is to determine whether cross-cylinder approach is more effective than routine method in laser correction of astigmatism.
Detailed description
Both eyes of 50 patients(100 eyes) with medium-high compound myopic astigmatism and inclusion criteria,will be treated by photorefractive keratectomy (PRK) using excimer laser.It breaks chemical bonds within corneal stroma.It's wavelength is 193 nm that is in the ultraviolets range with high energy and very low tissue penetrance. The right or left eye of every patient would be allocated randomly to one of the study groups using RandList 1.1. In spite of recent advances in excimer laser machines and surgical techniques,correction of astigmatism is not more successful compared to myopia. In this study we will compare two methods of laser correction of astigmatism regarding to visual acuity,residual error and aberrations.
Interventions
PROCEDUREphotorefractive keratectomy (PRK)
comparison of PRK by cross-cylinder approach in one eye and conventional (single) method in the other eye for laser correction of astigmatism
Sponsors
Tabriz University
Study design
Allocation
RANDOMIZED
Intervention model
PARALLEL
Primary purpose
TREATMENT
Masking
SINGLE (Subject)
Eligibility
Sex/Gender
ALL
Age
20 Years to No maximum
Healthy volunteers
Yes
Inclusion criteria
* \>20 years of age * refractive stability for at least one year * regular and symmetric astigmatism of \>1.5 diopters
Exclusion criteria
* corneal thickness\<470 microns * keratoconus or suspected keratoconus * active ocular disease and systemic collagen disease
Design outcomes
Primary
| Measure | Time frame | Description |
|---|---|---|
| Correction of Astigmatism | six months after surgery | It shows the result of correction of astigmatism .In compound myopic astigmatism the meridians of maximum and minimum power are both too strong.So both line images fall short of the retina.It is measured by refraction(using autorefractor).Diopter is a unit of measurement of the optical power of any refracting surface ,which is reciprocal of the focal length measured in meters. |
Secondary
| Measure | Time frame | Description |
|---|---|---|
| Higher Order Aberrations | six months postoperative | Although lower- order aberrations decrease after laser vision correction,higher -order aberrations may increase after conventional PRK or LASIK(Laser in situ keratomileusis).Higher-order aberration is Composed of delicate irregularities within the cornea not correctable by spectacles.It is measured using aberrometer.In this study OPD scan was used that is one of the methods of wavefront analysis . |
| Corrected Distance Visual Acuity(CDVA) | six months | CDVA is attained after correction of refractive errors using lenses of varying powers .the data are the LogMAR values calculated from the snellen chart using the visual acuity conversion chart(the units would be LogMAR).Any patient with better vision can see the smallest letters at 20 feet(20/20=0.00 LogMAR).Higher LogMAR values represent worse outcome.Log MAR(logarithmic minimum angle of resolution)notation ,has gained widespread use in statistical calculations. |
| Uncorrected Distance Visual Acuity (UCDVA) | six months | UCDVA is attained without correction of refractive errors .the data are the LogMAR values calculated from the snellen chart using the visual acuity conversion chart( the units would be LogMAR).Any patient with better vision can see the smallest letters at 20 feet(20/20=0.00 LogMAR).Higher LogMAR values represent worse outcome.Log MAR(logarithmic minimum angle of resolution)notation ,has gained widespread use in statistical calculations. |
| Surgically Induced Astigmatism | six months after surgery | It is the vector of the astigmatic change actually induced by the surgery.Participant population was the same as baseline participants. It was measured by refraction(using autorefractor) and vector analysis(using special software).Diopter is a unit of measurement of the optical power of any refracting surface ,which is reciprocal of the focal length measured in meters |
Countries
Iran
Participant flow
Recruitment details
Both eyes of Fifty patients with medium-high compound myopic astigmatism(1.5- 5 D) were enrolled in a prospective randomized study between sep 2007 and sep 2008 at Nikookari eye hospital and Mehr eye center
Pre-assignment details
All enrolled patients having inclusion criteria accepted both available techniques for correction of astigmatism.
Participants by arm
| Arm | Count |
|---|---|
| Photorefractive Keratectomy( PRK) Right or left eye of 50 patients was randomly enrolled(data provided in this module refer to100 eyes of 50 participants)to undergo PRK by cross-cylinder or conventional(single) approach using Excimer laser.In cross-cylinder method , half of the cylinder is treated along the steepest meridian and the other half is ablated along the flattest meridian .In single method the steepest meridian is treated by elliptical ablation to match the flattest meridian. | 50 |
| Total | 50 |
Withdrawals & dropouts
| Period | Reason | FG000 |
|---|---|---|
| Overall Study | Lost to Follow-up | 2 |
Baseline characteristics
| Characteristic | Photorefractive Keratectomy( PRK) |
|---|---|
| Age, Categorical <=18 years | 0 Participants |
| Age, Categorical >=65 years | 0 Participants |
| Age, Categorical Between 18 and 65 years | 50 Participants |
| Age Continuous | 28.46 years STANDARD_DEVIATION 7.05 |
| astigmatism | -2.49 diopter STANDARD_DEVIATION 0.83 |
| corrected distance visual acuity | 0.06 logMAR STANDARD_DEVIATION 0.11 |
| high order aberration | 0.36 micron STANDARD_DEVIATION 0.1 |
| Region of Enrollment Iran, Islamic Republic of | 50 participants |
| Sex: Female, Male Female | 39 Participants |
| Sex: Female, Male Male | 11 Participants |
| Spherical equivalent | -5.19 Diopter STANDARD_DEVIATION 2.47 |
| Target induced astigmatism(TIA) | 1.86 Diopter STANDARD_DEVIATION 0.6 |
| uncorrected distance visual acuity | 1.11 log MAR STANDARD_DEVIATION 0.36 |
Adverse events
| Event type | EG000 affected / at risk | EG001 affected / at risk |
|---|---|---|
| deaths Total, all-cause mortality | — / — | — / — |
| other Total, other adverse events | 0 / 48 | 0 / 48 |
| serious Total, serious adverse events | 0 / 48 | 0 / 48 |
Outcome results
Primary
Correction of Astigmatism
It shows the result of correction of astigmatism .In compound myopic astigmatism the meridians of maximum and minimum power are both too strong.So both line images fall short of the retina.It is measured by refraction(using autorefractor).Diopter is a unit of measurement of the optical power of any refracting surface ,which is reciprocal of the focal length measured in meters.
Time frame: six months after surgery
Population: The sample size was determined by the one proportion formula where power = 0.8, d=0.02 , p= 0.5(primary outcome) and type I error probability associated with this test of null hypothesis is 0.05.~The analysis was per protocol
| Arm | Measure | Value (MEAN) | Dispersion |
|---|---|---|---|
| Cross-cylinder PRK | Correction of Astigmatism | -0.54 diopter | Standard Deviation 0.43 |
| Single PRK | Correction of Astigmatism | -0.59 diopter | Standard Deviation 0.33 |
p-value: <0.0595% CI: [-0.62, -0.001]t-test, 2 sided
Secondary
Corrected Distance Visual Acuity(CDVA)
CDVA is attained after correction of refractive errors using lenses of varying powers .the data are the LogMAR values calculated from the snellen chart using the visual acuity conversion chart(the units would be LogMAR).Any patient with better vision can see the smallest letters at 20 feet(20/20=0.00 LogMAR).Higher LogMAR values represent worse outcome.Log MAR(logarithmic minimum angle of resolution)notation ,has gained widespread use in statistical calculations.
Time frame: six months
Population: the analysis was per protocol
| Arm | Measure | Value (MEAN) | Dispersion |
|---|---|---|---|
| Cross-cylinder PRK | Corrected Distance Visual Acuity(CDVA) | 0.02 LogMAR | Standard Deviation 0.02 |
| Single PRK | Corrected Distance Visual Acuity(CDVA) | 0.03 LogMAR | Standard Deviation 0.03 |
Secondary
Higher Order Aberrations
Although lower- order aberrations decrease after laser vision correction,higher -order aberrations may increase after conventional PRK or LASIK(Laser in situ keratomileusis).Higher-order aberration is Composed of delicate irregularities within the cornea not correctable by spectacles.It is measured using aberrometer.In this study OPD scan was used that is one of the methods of wavefront analysis .
Time frame: six months postoperative
Population: the analysis was per protocol.
| Arm | Measure | Value (MEAN) | Dispersion |
|---|---|---|---|
| Cross-cylinder PRK | Higher Order Aberrations | 0.51 micron | Standard Deviation 0.21 |
| Single PRK | Higher Order Aberrations | 0.51 micron | Standard Deviation 0.2 |
Secondary
Surgically Induced Astigmatism
It is the vector of the astigmatic change actually induced by the surgery.Participant population was the same as baseline participants. It was measured by refraction(using autorefractor) and vector analysis(using special software).Diopter is a unit of measurement of the optical power of any refracting surface ,which is reciprocal of the focal length measured in meters
Time frame: six months after surgery
| Arm | Measure | Value (MEAN) | Dispersion |
|---|---|---|---|
| Cross-cylinder PRK | Surgically Induced Astigmatism | 1.98 Diopter | Standard Deviation 0.57 |
| Single PRK | Surgically Induced Astigmatism | 1.51 Diopter | Standard Deviation 0.43 |
Secondary
Uncorrected Distance Visual Acuity (UCDVA)
UCDVA is attained without correction of refractive errors .the data are the LogMAR values calculated from the snellen chart using the visual acuity conversion chart( the units would be LogMAR).Any patient with better vision can see the smallest letters at 20 feet(20/20=0.00 LogMAR).Higher LogMAR values represent worse outcome.Log MAR(logarithmic minimum angle of resolution)notation ,has gained widespread use in statistical calculations.
Time frame: six months
Population: the analysis was per protocol
| Arm | Measure | Value (MEAN) | Dispersion |
|---|---|---|---|
| Cross-cylinder PRK | Uncorrected Distance Visual Acuity (UCDVA) | 0.08 LogMAR | Standard Deviation 0.15 |
| Single PRK | Uncorrected Distance Visual Acuity (UCDVA) | 0.10 LogMAR | Standard Deviation 0.16 |