refractive keratectomy

Because of the inherent problems of aphakic glasses, contact lens correction, and intraocular lens implantation, ophthalmologists have turned to many refractive problems by altering corneal curvature. Because the corneal surface is the most powerful refractive interface of all refractive media, its mild changes can cause significant refractive changes, which results in keratoplasty. There are two basic approaches to the design of this type of surgery: one is to increase or decrease the curvature of the corneal surface by changing the thickness of the cornea, including corneal augmentation, keratomileusis, and surface keratomileusis; the second is to directly extend or shorten the radius of curvature of the cornea. The purpose of changing the refractive state, including corneal wedge resection, release incision, radial keratotomy. Since these operations are still being explored and developed, they must be carefully selected. Since John Marshall and Stephen Trokel first proposed the reshaping of the anterior surface of the cornea with excimer lasers in 1986, many myopias in the world have been corrected by PRK surgery. The basic method is to remove the epithelial corneal surface and directly cut with excimer laser to achieve the purpose of changing the curvature of the cornea and correcting the refractive error. Treatment of diseases: hyperopic astigmatism myopia Indication Refractive keratectomy is available for: 1. There are requirements for removing glasses, generally 18 years old and under 50 years old. 2. The diopter is stable for 2 years. If a contact lens is worn, the soft lens should be worn for 2 weeks, while the rigid lens should be worn longer. 3. The diopter for correcting myopia, hyperopia and astigmatism should not exceed -6.00D. 4. Both eyes anisometropia are good indications. 5. There are no surgical contraindications for eye examination. 6. Patients who need PRK after penetrating keratoplasty after PRK surgery should be performed at least 1 year later. Radial keratotomy (RK) should be performed 2 years later. Contraindications 1. There is active inflammation of the eye. 2. People with keratoconus, dry eye disease, corneal endothelium, glaucoma, retinal detachment and other eye diseases. 3. The thickness of the cornea is too thin and should be very careful. It is necessary to ensure that the cornea after cutting still maintains normal tensile strength. 4. Correct amblyopia with poor vision. 5. For the myopia with higher correction degree, such as -6.00D or more, although the PRK can be done, the incidence of corneal haze is high and obvious. 6. Surgery can not cooperate or tremors of the eyeball. 7. Scar tissue, diabetes, collagen disease, etc. may affect corneal wound healing. Patients with systemic lupus erythematosus and rheumatoid arthritis are prone to corneal lysis after surgery. 8. Often driving at night, glare discomfort is prone to post-PRK. 9. Diseases such as AIDS. Preoperative preparation 1. Medical history inquiry. 1 The occurrence time of ametropia, the degree of diopter and development, whether it is stable in the past 2 years, etc. 2 The past and present correction methods for refractive errors, such as wearing spectacles or contact lenses, correcting vision. Such as wearing contact lenses, but also to know what kind of contact lens, wearing time, stop time, etc.; 3 eye history including history of ocular trauma, history of eye surgery (including refractive surgery) and various eye history; 4 systemic The history of the disease mainly includes diabetes, collagen diseases such as systemic lupus erythematosus and rheumatoid arthritis and infectious diseases; 5 attention to family history of genetic diseases such as high myopia, glaucoma, keratoconus and corneal dystrophy; No systemic or ocular application history of drugs that may affect corneal wound healing, such as long-term use of immunosuppressants or antimetabolites; 7 history of allergies to drugs and anesthetics. 2. Preoperative examination. If the patient wears a soft contact lens, he should stop wearing it for more than 2 weeks. If you wear a hard contact lens, you need to stop wearing it for a longer period of time. 1 routine examination of the eye and near vision, eye and eye movements, eyelids, conjunctiva, lacrimal membrane, anterior segment of the eye and dilated pupils (especially using indirect ophthalmoscopy to examine the peripheral fundus) and measuring intraocular pressure, except Possible surgical contraindications. If necessary, perform a three-sided mirror to understand the details of the corner or retina. 2 Refractive status examination is one of the most important parts before corneal refractive surgery, and is an important basis for design surgery. Optometry should be performed in non-dug-hole and drug (usually with compound tropicamide or similar other short-acting mydriatic agents), regardless of the method (computer optometry or retinal optometry) The main subject is to test the lens to understand the patient's acceptance of the lens power and astigmatism axis and correct vision. 3 corneal thickness measurement. Ultrasonic thickness gauges are generally used to measure the central cornea multiple times. For some patients, such as patients with corrected presbyopia and suspicious keratoconus, multiple thickness measurements are required for areas outside the center of the cornea. 4 corneal topographic examination. Because it not only can accurately understand the corneal refractive power, but also can reflect and display the visual topographic map and regularity of the cornea surface, and can quantitatively analyze the results, it is very important for subclinical and early keratoconus diagnosis, so it has become corneal refractive surgery. One of the necessary inspection items. 5 Except for dry eye examination. In addition to paying attention to the patient's main narration such as dry eyes, burning, etc., pay attention to the height of the tear river (normally >0.3mm). It is best to routinely perform corneal fluorescein staining and tear film break time (BUT) examination. On the one hand, understand whether the corneal epithelium is healthy, and understand whether the tear film is stable. A tear secretion test such as the Schirmer test is performed on suspicious patients. Conditional can also be used for fluorescein dilution test. 6 other checks. According to clinical needs, such as axial length measurement, B-mode ultrasound and OCT examination, to understand the changes in eye structure. Through visual field, visual electrophysiology, visual contrast sensitivity, disability glare test, etc., comprehensive evaluation of visual function. 3. Preoperative conversation. 1 For patients who require excimer laser refractive corneal surgery, in addition to propaganda data, imaging materials, etc., they can try to understand the surgical principles, advantages and disadvantages, possible surgical results, and intraoperative results of PRK, LASIK, etc. And knowledge of possible complications after surgery, medical staff should also face-to-face conversations with patients to understand the patient's motivation and expectations. Understand the patient's eye habits, occupations, and the need for near and far vision. Especially for the instability of the refractive state, such as myopia is still progressing requires surgery, it must be said that surgery can only correct the current diopter, can not prevent the development of myopia and the possible progression of the fundus. 2 Make recommendations based on the patient's specific conditions for PRK or LASIK. Due to the improved safety of LASIK, the postoperative visual acuity recovery, pain, small corneal haze, no need to use glucocorticoids for a long time, wider range of corrected diopter, etc., PRK surgery has a significant reduction trend. However, the effectiveness and good predictability of PRK for low and moderate myopia has been widely proven. For ametropia of -8.00D to +4.00D, the long-term effects of the two procedures are comparable, although LASIK recovers faster. In some cases, PRK has its obvious advantages. For example, it does not have serious complications related to the corneal flap. The operation is relatively simple and suitable for patients with too small a cleft palate. In addition, PRK fees are also lower. 3 explain how to cooperate with surgery and preoperative and postoperative medication. 4 explain the possible reactions and symptoms after surgery, the process of vision recovery. 5 other matters after surgery, such as not blinking. 6 to determine the time of postoperative review. 4. Surgical signature. The surgical signature should include all complications that may occur during and after surgery. 1 should emphasize that the goal of surgery is to reduce or eliminate the current refractive error and improve naked eye vision. However, postoperative naked eye vision or even corrected visual acuity may not reach the best corrected visual acuity before surgery. 2 Undercorrection or overcorrection may occur due to differences in individuals and the like. 3 Irregular deviation, astigmatism and poor vision due to irregular cutting and eccentric cutting caused by poor patient cooperation. 4 postoperative corneal haze (haze). 5 postoperative long-term glucocorticoid eye drops, may cause hormonal glaucoma. 6 postoperative infection or even ulcer perforation and even blindness. 7 glare. 8 may experience reading fatigue after surgery, especially those who are older or overcorrected. A very small number of patients may still need to wear lower glasses or even re-operation after surgery. 10 Due to malfunction of the instrument or poor debugging, the operation was rescheduled. In addition to asking the patient to sign it, the doctor concerned should confirm that the patient should sign the above items after they have been clear. It should be pointed out that the doctor introduces and confesses the patient about the operation and possible complications, that is, it is objective and must be measured to enable the patient to make choices and decisions according to his own situation. 5. Eye preparation. Preoperative partial drops of antibiotic eye drops for 3 to 5 days. Train your eyes so that you can work well in your surgery. If necessary, review the eye again on the day of surgery to find out what may happen. For patients with unstable diopter, it is necessary to insert the film again to check the refractive condition. 6. Laser machine debugging. 1 In the laser machine working environment (including temperature, humidity, etc.) and conditions (check various gas pressures, etc.), start preheating. 2 Check the energy state of the laser output, detect the position of the aiming light, and detect the position and uniformity of the laser spot. Adjust the surgical microscope. Surgical procedure 1. The patient is comfortable lying on his back, adjusting the head position, and the order of the eyes is fixed to the first right eye and the left eye. Check the patient and the eyes are correct. 2.0.5% tetracaine or 0.4% benox or other topical anesthetic, eye drops 5 to 10 minutes before surgery, 2 to 3 times. 3. Operate under aseptic conditions, spread the towel, stick the upper and lower eyelid strips, open the sputum, and rinse the conjunctival sac. Let the patient look at the built-in fixation lighting of the machine and be familiar with the sound of the laser machine. 4. Remove the central corneal epithelium 1 mm larger than the laser cutting range. There are three methods commonly used. 1 mechanical method: the corneal epithelium was scraped from the outside to the inside with a corneal epithelium scraper. Note that the scraper should be perpendicular to the corneal surface, uniform in strength, suitable in depth, and completed in a short period of time to avoid drying of the corneal surface (required to be completed within 1 min). Gently wipe the epithelial debris remaining in the epithelial scraping area; 2 chemical method: apply 4% cocaine or other liquid tissue that can release the epithelium to the cornea. After the corneal epithelial edema, remove the epithelium with the instrument. . This method is currently rarely used; 3 brush method: 7mm diameter special electric brush, can quickly remove the epithelium without damaging the corneal stroma. 4 laser cutting method: generally using the therapeutic laser keratectomy (PTK) program, set the diameter of 6 ~ 6.5mm, depth of 40 ~ 45m, if there are residual epithelial basal cells, manually removed with a blunt instrument. It is also possible to set the procedure directly with the amount of PRK surgical cut, and the corneal epithelium and the matrix are simultaneously cut. The average amount of epithelial cutting is 50 m. The laser cutting method is lighter than the haze produced by mechanical methods because of its low interference to the matrix, low levels of stromal cell apoptosis, cell proliferation, and irregular collagen production. Therefore, it is a method that is strongly advocated at present. 5. Laser cutting. 1 Determine the corneal optical center. Let the patient look at the fixation light and adjust the aiming laser so that the midpoint of the focal plane is just at the front surface of the cornea where the center of the pupil is located. This is the corneal optical heart. 2 laser cutting. The eye position, aiming light position, and focus state should be closely monitored. Once the eyeball has rotated a lot, stop cutting immediately to avoid off-center cutting. At the same time, attention should be paid to the distribution and tissue reaction of laser cutting. When the surface of the cornea is hydrated unevenly or the liquid is too much, it should also be wiped dry with a sponge before light cutting. When the cutting amount is large and the continuous cutting gas mist is large, it may be paused in the middle of the cutting as the case may be. 6. Surgical eye drops of antibiotic eye drops, eye pads or disposable contact lenses. The paralyzed patient should not blink and review it the next day. The corneal epithelium is mechanically scraped off with a corneal epithelium scraper or epithelial brush. Figure 4 The corneal anterior surface is cut. The focus of the laser treatment is concentrated on the corneal surface. Aligning with the center of the pupil, the computer treatment program is started. At this time, the computer will control the cutting of the cornea by the excimer laser according to the data input in advance. complication 1. Best corrected vision loss. It is one of the most significant complications of refractive surgery. It can be a complication of the surgery itself or it can be caused by other problems. Possible causes include: irregular astigmatism (irregular corneal epithelium, irregular light cutting or off-center cutting), corneal matrix haze, retinal abnormalities, etc. 2. Undercorrection. More nearsightedness (-0.50D) is called undercorrection. The most common cause is preoperative diopter measurement inaccurate, and a few are caused by steeper corneal center. Sometimes it may be caused by too short a contact lens. Glucocorticoid eye drops can usually be continued locally, and if the drug has been discontinued, the drug can be restarted in an attempt to reduce undercorrection. The part that is still under-corrected can be corrected by wearing a mirror or re-surgery. 3. Overcorrection. Surgery for 1 month or longer is more distant than correction. It has been found that at 6 weeks postoperatively, if overcorrection exceeds 1.00D and continues, corneal haze often does not occur. It shows that the corneal healing process of these patients with laser damage is different from that of the average person, and the refractive regress is small. Therefore, glucocorticoid eye drops can be stopped in advance for such patients (stop immediately or quickly stop according to the situation). For patients who are still overcorrected, they can be corrected by wearing glasses, hyperopic refractive laser ablation or holmium laser corneal thermoplasty. 4. Retreat. Within 6 weeks after surgery, it is normal for the diopter to retreat to 1.00D in the direction of myopia. However, if it occurs after 6 weeks, the fallback is not normal. This abnormal regression requires treatment with a larger dose of glucocorticoid eye drops, such as 0.1% dexamethasone once every 2 to 3 hours, for 2 weeks. If the cornea is significantly flattened, the myopic diopter is reduced by 1 to 3.00D, and the hormone can be gradually reduced, usually up to 4 months. Attention must be paid to changes in intraocular pressure during the period. If the hormone is not effective for 2 weeks, it should be quickly reduced and stopped immediately. The reoperation must be performed after the refractive state is completely stabilized. 5. Off-center cutting. A common cause is that the patient does not have a good fixation or the doctor does not find the patient's fixation in time. Generally, the eccentricity within 1mm has little effect on vision, and vice versa can increase regular or irregular astigmatism. Patients often have unclear vision, glare, halo, phantom or monocular double vision. A detailed examination of this patient is required. The corneal topography shows a good eccentricity of the cornea, but the eccentric results shown are not all caused by eccentric cutting. Sometimes the corneal topography does not align with the apex of the cornea, or the asymmetry of the cornea also shows eccentric results, which should be carefully distinguished. If it is indeed caused by partial center cutting, the clinical symptoms are obvious, and it may be considered to perform surgery again under the guidance of corneal topography. The distance from the center of the pupil should be adjusted in the opposite direction as the cutting diameter selected for the first surgery. The method of cutting the epithelium together with the matrix is generally employed. After re-treatment, although regular astigmatism may occur, it can effectively eliminate the symptoms. At present, the advent of the eye tracking system and the application of intraoperative cutting have greatly reduced the eccentricity. 6. The center island is formed. The relatively steep central island shown on the corneal topography is generally considered to have a diameter of 2 to 4 mm and a diopter of 1.50 to 3.50 D. Most central islands have an under-corrected regular astigmatism of 1.00 to 3.00D. The central island can cause optimal correction of vision abnormalities such as decreased vision, glare, and halos. Over time, most central islands disappear on their own, and only a few patients still exist after half a year. A persistent symptomatic central island that can be treated with topical PRK. The diameter of the light cutting is slightly larger than the diameter of the central island, such as 2.8 mm for the central island and 3 to 3.2 mm for the cutting diameter. At present, many new laser machine softwares have taken into account the central island problem, pre-treatment of about 2.5 mm in the center during light cutting, which basically eliminates the formation of the central island. 7. Corneal haze. It refers to the turbidity of the corneal epithelial and matrix interface in the cutting area after excimer laser refractive corneal surgery. Haze is a common complication within 4 months after PRK, which can occur as early as 10 days after surgery and peaks in 3 to 6 months. Large cutting volume, scar constitution, insufficient glucocorticoid medication, etc. are prone to occur or are heavier. The difference in individual and gender (male is more important than female), corneal injury and uneven epithelial removal during mechanical scraping of the corneal epithelium are all related to its occurrence. Clinically, the corneal haze is reticular or marble. According to the degree is divided into 5 levels: 0 level - corneal transparent without turbidity; 1 level - mild haze, with slit lamp oblique method to find spot turbidity, does not affect refraction; 2 - moderate turbidity, under the slit lamp Easy to see turbidity, does not affect iris texture observation; grade 3 - corneal opacity affects iris texture observation; grade 4 - obvious corneal opacity makes it impossible to see iris texture. Level 2 or above affects refraction and affects vision. The material basis of corneal haze is type III collagen and keratan sulfate produced by activated corneal cells. Therefore, it is different from corneal scars (such as phlegm and cantharidin) and may be alleviated and disappeared after treatment. In addition to preoperative screening of patients prone to corneal haze (such as scar physique, high myopia with large amount of cut), surgery using laser cutting (PTK) to remove the epithelium, the application of hormones after surgery, can effectively prevent corneal turbidity . Once the corneal haze eventually turns into a scar and cannot resolve itself, the turbid part can be cut again with a laser. 8. Corneal infection. The stromal infiltration of the cornea after surgery can be infective or sterile. However, as long as inflammation occurs in the clinic, it should be treated as infection first. Remove contact lenses, corneal scrapers and swabs to find pathogenic microorganisms, suspend glucocorticoids and immediately apply broad-spectrum or sensitive antibiotics. Glucocorticoids can be reused after a few days of administration and exclusion of infection. 9. Hormone high intraocular pressure and glaucoma. Long-term application of glucocorticoids in the eye, elevated intraocular pressure 22mmHg, no visual field and optic nerve pathological changes are called hormonal high intraocular pressure. Timely withdrawal of hormones, and effective intraocular pressure reduction measures, eye pressure can return to normal. Some patients may have visual field and optic nerve damage, called hormonal glaucoma. If the eye pressure is not effectively controlled, it may seriously impair visual function or even blindness. 4 to 6 weeks after surgery, the intraocular pressure is increased. Therefore, the intraocular pressure should be measured regularly after 3 to 4 weeks of treatment, and the fundus should be observed. If necessary, the visual field should be examined. According to reports, the incidence of increased intraocular pressure is related to the type, concentration, frequency of glucocorticoids used, sensitivity of patients to hormones, and high-risk groups of glaucoma. Dexamethasone and prednisone are likely to cause an increase in intraocular pressure, while effluent causes less increase in intraocular pressure.