Clinical Trial: Comparison of Cross-cylinder and Conventional Photorefractive Keratectomy(PRK) in Correcting Medium-high Astigmatism

Study Status: Completed
Recruit Status: Completed
Study Type: Interventional

Official Title: Comparison of Cross-cylinder and Conventional PRK for Laser Correction of Astigmatism: A Randomized Clinical Trial.

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 Summary: 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.
Sponsor: Tabriz University

Current Primary Outcome: Correction of Astigmatism [ Time Frame: 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.


Original Primary Outcome: Improvement of visual acuity (snelen chart), refraction, aberrometry [ Time Frame: six months after surgery ]

Current Secondary Outcome:

  • Higher Order Aberrations [ Time Frame: 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) [ Time Frame: 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) [ Time Frame: 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 [ Time Frame: 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



Original Secondary Outcome: aberrometry for higher order aberrations, refraction for residual and regressed astigmatism, snelen chart for visual acuity test [ Time Frame: six months postop ]

Information By: Tabriz University

Dates:
Date Received: April 18, 2008
Date Started: November 2007
Date Completion:
Last Updated: March 10, 2012
Last Verified: March 2012