A Comparative Study On The Effect Of Femtosecond Laser Versus Mechanical Microkeratome Flap Creation On Corneal Biomechanics
Moataz Mohammad Wessam Taha;
Abstract
The biomechanical properties of the cornea affect the
predictability and stability of refractive surgery outcomes. Abnormal corneal biomechanics plays a critical role in the development of the post refractive surgery complications. Quantifying the biomechanical state of a cornea before refractive surgery would be a welcome addition to the diagnostic tools currently available and may help reduce the incidence of keratectasia by improving refractive surgery screening.
Femtosecond laser and mechanical microkeratome systems have different mechanisms of action to create corneal flaps for LASIK surgery. The mechanical microkeratome uses an oscillating blade for sharp dissection as the blade passes across the cornea in a rotational or translational approach. The femtosecond laser creates a corneal resection plane by delivering laser pulses at a predetermined depth in the cornea. These pulses physically define the cleavage plane through photodisruption, producing thousands of microscopic expanding bubbles of carbon dioxide and water, which separate the corneal lamellae.
The Ocular response analyzer quantifies 2 biomechanical parameters; CH and CRF. During the measurement procedure, a rapid air impulse is used to applanate the cornea. Using an electrooptical system, applanation pressure measurements are recorded. The first measurement occurs when the cornea is flattened and moving inward and the other as the cornea flattens and is moving outward after moving through a concavity at maximum applanation. This process takes approximately 20 milliseconds. Due to its viscoelastic properties, the cornea resists the dynamic air puff differentially on the inward and outward applanation events, resulting in 2 different pressure values. Corneal hysteresis is defined as the difference between these 2 pressure values, corneal resistance factor is derived from corneal hysteresis.
Our prospective study included 50 patients with mild to moderate myopia and myopic astigmatism; range of preoperative manifest refractive spherical equivalent (MRSE) is from -1.25 to -8.00 diopters. They are randomly distributed into two groups; microkeratome and femtosecond. All patients are between 20 – 40 years and had at least one year stable refraction. All baseline evaluations including medical history taking, visual acuity both unaided and best corrected, manifest and cycloplegic refraction, slit lamp examination of anterior segment, intraocular pressure check using Goldmann applanation tonometer, fundus examination by indirect ophthalmoscopy and slit lamp biomicroscopy, preoperative Pentacam and preoperative Ocular Response Analyzer® for corneal hysteresis and corneal resistance factor.
predictability and stability of refractive surgery outcomes. Abnormal corneal biomechanics plays a critical role in the development of the post refractive surgery complications. Quantifying the biomechanical state of a cornea before refractive surgery would be a welcome addition to the diagnostic tools currently available and may help reduce the incidence of keratectasia by improving refractive surgery screening.
Femtosecond laser and mechanical microkeratome systems have different mechanisms of action to create corneal flaps for LASIK surgery. The mechanical microkeratome uses an oscillating blade for sharp dissection as the blade passes across the cornea in a rotational or translational approach. The femtosecond laser creates a corneal resection plane by delivering laser pulses at a predetermined depth in the cornea. These pulses physically define the cleavage plane through photodisruption, producing thousands of microscopic expanding bubbles of carbon dioxide and water, which separate the corneal lamellae.
The Ocular response analyzer quantifies 2 biomechanical parameters; CH and CRF. During the measurement procedure, a rapid air impulse is used to applanate the cornea. Using an electrooptical system, applanation pressure measurements are recorded. The first measurement occurs when the cornea is flattened and moving inward and the other as the cornea flattens and is moving outward after moving through a concavity at maximum applanation. This process takes approximately 20 milliseconds. Due to its viscoelastic properties, the cornea resists the dynamic air puff differentially on the inward and outward applanation events, resulting in 2 different pressure values. Corneal hysteresis is defined as the difference between these 2 pressure values, corneal resistance factor is derived from corneal hysteresis.
Our prospective study included 50 patients with mild to moderate myopia and myopic astigmatism; range of preoperative manifest refractive spherical equivalent (MRSE) is from -1.25 to -8.00 diopters. They are randomly distributed into two groups; microkeratome and femtosecond. All patients are between 20 – 40 years and had at least one year stable refraction. All baseline evaluations including medical history taking, visual acuity both unaided and best corrected, manifest and cycloplegic refraction, slit lamp examination of anterior segment, intraocular pressure check using Goldmann applanation tonometer, fundus examination by indirect ophthalmoscopy and slit lamp biomicroscopy, preoperative Pentacam and preoperative Ocular Response Analyzer® for corneal hysteresis and corneal resistance factor.
Other data
| Title | A Comparative Study On The Effect Of Femtosecond Laser Versus Mechanical Microkeratome Flap Creation On Corneal Biomechanics | Other Titles | دراسة مقارنة بين تأثير ليزر الفمتوثانية و قاطع القرنيه الآلى فى عمل سديلة القرنيه على النشاط الحيوى الحركى للقرنيه | Authors | Moataz Mohammad Wessam Taha | Issue Date | 2016 |
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