Types Of Refractive Errors:
Low Order Aberrations :
An eye that has refractive error when viewing distant objects is said to have ametropia or be ametropic. This eye, when not using accommodation, cannot focus parallel rays of light (light from distant objects) on the retina.
The word “ametropia” can be used interchangeably with “refractive error” as they refer to the same thing. Types of ametropia include myopia, hyperopia and astigmatism. They are frequently categorized as spherical errors and cylindrical errors which are generally considered as Lower order aberrations . They make up about 85 percent of all aberrations in an eye.
Spherical errors occur when the optical power of the eye is either too large or too small to focus light on the retina. People with refraction error frequently have blurry vision.
- Myopia: When the optics are too powerful for the length of the eyeball one has myopia or nearsightedness.
- Hyperopia or Farsightedness: This can arise from a cornea with not enough curvature or an eyeball that is too short.
- Astigmatism: People with a simple astigmatic refractive error see contours of a particular orientation as blurred, but see contours with orientations at right angles as clear. When one has a cylindrical error, one has astigmatism.
High Order Aberrations :
No eye is perfect, which means that all eyes have at least some degree of higher-order aberrations. HOA are more complex vision errors than lower-order aberrations. These types of aberrations can produce vision errors such as difficulty seeing at night, loss of contrast, glare, halos, blurring, starburst patterns or double vision Higher-order aberrations comprise many varieties of aberrations. Some of them have names such as coma, trefoil and spherical aberration, but many more of them are identified only by mathematical expressions (Zernike polynomials). An eye usually has several different higher-order aberrations interacting together. They make up about 15 percent of the total number of aberrations in an eye. People with larger pupil sizes generally may have more problems with vision symptoms caused by higher-order aberrations, particularly in low lighting conditions when the pupil opens even wider.
Treatment Options :
The treatment options will vary from person to person depending upon the uniqueness of the eye in terms of amount of refractive power, the central corneal thickness (pachymetry) and the corneal curvature (Topography). The recommended treatment type will be personalized according to your corneal thickness, refractive errors, pupil size and other factors and. The best treatment option will be advised after detailed evaluation keeping in mind the patient’s preoperative power, patient’s expectations and requirements and counseled accordingly.
By way of Glasses or Contact lenses: Usually the preferred method for children till the refractive stability is achieved, which usually takes 18 to 19 years.
By way of Excimer laser(LASIK/PRK) or Implantable Collamer Lenses (ICL) depending on the Patients Native Corneal thickness, Amount of refractive errors and other eye conditions, which will be dealt with in detail in respective topics.
Laser Vision Correction
Laser Vision Correction is a permanent treatment. However, patients who are 40 years and above, may require reading glasses. During your consultation, we will give you can an idea of the procedure and the level of vision you can expect. We need to ensure the patient is left with a definitive safety margin of Residual Corneal thickness after the end of the refractive surgery. More the refractive error, the more the amount of issue that is to be ablated to reshape the cornea, hence lesser the Residual corneal thickness after the surgery
- If the corneal thickness is quite adequate for the power to be treated the patient is advised to undergo LASIK.
- If the Corneal thickness is just enough or borderline for the amount of power to be treated, then Photorefractive Keratectomy is advocated.
- If the Corneal thickness is too less even for PRK, then we defer Laser vision correction. The Patients are counseled then for Implantable Collamer Lens(ICL) Surgery.
Lasik stands for laser assisted in-situ keratomileusis. It is widely considered as the procedure of choice for correction of most cases of myopia. It combines using microkeratome for making a circular superficial cornea flap followed by application of extremely precise excimer laser to reshape the deeper layers of the cornea according to the patient’s spectacles prescription. The flap is then folded back and will adhere itself naturally without the need for stitches. This Procedure is described in detail in the relevant sections.
Advantages of LASIK
- It is the preferred choice for refractive surgery by surgeons worldwide
- Relatively quick procedure -Procedure is done on an out-patient basis
- Quick visual recovery-Clarity of vision within hours of surgery
- Minimal discomfort
- Both eyes treated on the same day
- Extremely predictable
- Low enhancement rate
- Broad range of treatable prescriptions
- Very low infection rate
- Very low risk of scarring
- Preservation of all corneal layers
Laser Epithelial Keratomileusis or LASEK is a modified form of Photo Refractive Keratectomy (PRK).This procedure involves loosening the outer layer of the cornea, called the epithelium with an alcohol solution for around 30 seconds. The loosened epithelium is folded back so that the laser can reshape the exposed cornea. After laser application, the “flap” of epithelium is replaced back over the corneal bed and placed a bandage soft contact lens on top.
LASEK combines the benefits of the two most commonly performed procedures Lasik and PRK. Visual recovery after LASEK is generally faster than in PRK but slower than in LASIK.
However, LASIK is not an option if a persons cornea is too thin. For such persons, LASEK is the safer alternative.
Although all the epithelium is present immediately following LASEK, it needs time to become adherent. The contact lens keeps the eyelid from disturbing this adhering process. A smooth, central epithelium is crucial for clear vision. Following LASEK, although the epithelium is present, it may be slightly irregular and needs time to become perfectly smooth again Typically; it takes a day or two to obtain functional vision following LASEK. While some patients can go back to work the next day, most feel more comfortable waiting a few days.
Patients have different risk in experiencing post-operative dry eye. Typically, most patients experience mild dryness of the eyes (usually upon awakening or after extended reading) for a month or two. Artificial tear drops are usually all that is needed to relieve these symptoms.
Disadvantages of LASEK
- Longer visual recovery time compared to LASIK. Many LASEK patients will not fully recover functional vision for 1 to 2 weeks while their eye heals, which is similar to the healing time experienced in PRK .
- LASIK patients often have good vision by the day after surgery.
- LASEK may cause more pain and discomfort than LASIK, but less pain than PRK.
- Most LASEK patients say the discomfort lasts about 2 days or less.
- Patients need to wear a “bandage contact lens” for about 3 or 4 days after LASEK to serve as a protective layer between your blinking eyelids and the treated eye surface, which is not necessary after LASIK.
- Patients must use topical steroid drops for several weeks longer than that used after LASIK laser eye surgery.
Side Effects of LASEK
LASEK shows side effects less frequently than is seen with PRK, however side effects may occur. These may include:
- Sensation of having a foreign object in your eye (can last anywhere from 1 to 4 days)
- Temporary reduced vision under poorly lit conditions (up to 12 months)
- Dry eyes, requiring the use of moisturizing drops (up to 6 months)
- Hazy or cloudy vision (should disappear within 6 to 9 months)
Epi-LASIK Laser-Assisted Keratomileusis is a new laser vision correction technique that combines the advantages of PRK and LASIK and eliminates their disadvantages.
Similar to PRK and LASEK, Epi-Lasik creates a flap of the epithelium The Epi-LASIK procedure employs a unique microkeratome, called the “Epikeratome”, to mechanically “separate” the epithelium from the stroma, creating a flap of epithelial cells only (Similar in appearance to a LASIK flap). Epi-Lasik uses a blunt epikeratome blade to lift the epithelium in one sheet of cells that is moved aside and replaced over the area treated with the excimerlaser to reshape the corneal surface and the epithelial flap is repositioned without any stitches required. Unlike LASIK, no sharp blades or knives are required and unlike LASEK, no alcohol is required.
In this procedure, the first step of creating flap is using a highly precise laser, the Femtosecond Laser. The rest of the surgical steps are routine as with LASIK. The main advantages of the Femtosecond laser are Safety, Precision and Predictability – Flap measurements are highly accurate and uniform resulting in decreasing the risk of Ectasia and No induced changes in refraction which can occur with irregularly thick flaps.
Benefits Of Femtosecond Laser
Of course the biggest benefit of Femtosecond laser is the increased SAFETY and PRECISION.
- Enhanced patient comfort and safety during the procedure.
- Smoother corneal surface for excellent visual outcome.
- Improved flap quality.
- Thinner flaps for tissue saving.
- Rapid healing & visual recovery.
In short there is no longer any worry about irregular or non-perfect flaps for Lasik.
Types Of Laser Refractive Surgeries
Conventional LASIK corrects only prescription (sphere and cylinder) and a single central reading of corneal curvature map. For those people who have lower prescriptions, typically those up to −3.0 to +2.0 dioptres without astigmatism or irregular corneal curvature, conventional LASIK may suffice and achieves very good results in most patients. For higher prescriptions, Standard LASIK has the potential for inducing new higher-order abrerrations.
Unpredictable results may occur, more so in treatments with lasers that are not technologically advanced. Hence, it is very important to screen the right candidate for Standard LASIK.
The Excimer laser delivery is programmed in such a manner that after the completion of the procedure the optical system of the eye is maintained in the same manner without inducing any higher order aberrations. The Prolate vs. Oblate Cornea configuration is unaltered and hence its the Effect on quality of vision is unaffected.
Every patient’s eye differs in corneal shape, corneal thickness and pupil size. Wavefront technology analyses the entire visual system from the corneal surface through the crystalline lens of the eye, all the way back to the retina . Customised LASIK uses equipment called the wavefront analyzer (aberrometer) to accurately measures the way light travels through the eye. The resulting map of the eye is then programmed into the laser, and the laser treats the eye based on the personalized 3D map enables your surgeon to customize the Conventional LASIK procedure to your individual eye Customised LASIK helps to treat “higher order” aberrations, which are tiny imperfections in the optical system of the eye. They have a significant impact on quality of vision. In fact, higher order aberrations have been linked to visual glare & haloes. The customized procedure can result in patients seeing clearer and sharper than even before.
- No accommodation influence.
- No limitation due to pupil size (large treatment-zones possible).
- Experience of surgeon with topographic systems can be used.
- 80% of Aberrations / Errors are on the cornea – laser treats on cornea.
- No dilation of the pupil necessary.
- Ability to measure very irregular corneas, high astigmatism and decentrations.
- Selective simulation of corneal aberrations (PSF, acuity chart).
- Comparison of total and corneal aberrations (if Ocular Wave front Analyzer is used).
- High resolution: 230µm Shack-Hartmann wave front sensor.
- Widest wave front measuring range available: Sphere: −15 to +20D, astigmatism up to 10D.
- Capturing larger pupils.
- Expert analysis functions: Zernike, MTF, PSF, etc….
- Accommodative response simulation of each single coefficient for optimizing correction taking accommodation into account.
- Analyzing accommodative refractive components in vivo (considering new generation of IOL’s, multifocal contact lenses,..).