Full femtosecond and excimer comparison 17: transition zone

What is the transition zone in refractive surgery? The transition zone is actually an important concept and indicator of surgical design in refractive surgery. This is a schematic diagram of excimer laser cutting. The light zone is 6.0 mm, the transition zone is 2 mm, and the entire cutting zone is 8 mm. Simply speaking, it is like a tropical zone and a cold zone on the earth. There is a temperate zone between them, neither hot nor cold. This area is called the transition zone, and its role is an area where the temperature gradually decreases. The tropical zone is like the optical zone in our refractive surgery. The temperate zone is similar to the transition zone. In myopia surgery, there is a corresponding transition zone. This transition zone is separated from the optical zone and the non-optical zone, or the refractive power zone and the The transition zone without changing the refractive power area has the effect of slowly transitioning the changed refractive power to none. What if there is no transition zone? It may cause the patient’s positive spherical aberration to increase and the visual quality to deteriorate. Therefore, every excimer laser device has a transition zone design, such as a light zone of 6.0 mm and a transition zone of 2.5 mm, then the treatment zone is 8.5 mm. Here, 6.0 is like a convex lens, and 2.5 is like the edge of the convex lens extending to the peripheral flat area. So, the question is coming, is there a transition zone for all femtoseconds? The answer is, almost nothing. It is hardly said because there is one more thing. For myopia patients with astigmatism, the full femtosecond has a 0.1 mm transition zone. This 0.1 mm is just like soy sauce, better than nothing. SMILE has a light zone of 6.5 mm, a transition zone of 0.1 mm, and an entire treatment zone of 6.6 mm. SMILE has a light zone of 6.5 mm and a transition zone of 0 mm. Why is the entire treatment zone still 6.5 mm? This is determined by the characteristics of full femtosecond surgery. Because full femtosecond surgery is to scan the two layers between the corneal layers by femtosecond, and then use tweezers to remove a lens to complete the operation. Lenses that are too thin must be added with a base to increase the difficulty of taking out the lens. If you add the thin edge of the transition lens around the lens, complete removal becomes almost an impossible task. In contrast, excimer lasers ablate corneal tissue with laser pulses, so no matter how fine the treatment target is, there is nothing more than a laser gun to adjust the direction and aim to launch. The light area of ​​the Armstrong excimer laser machine is 6.3 mm, the transition area is 1.63 mm, and the entire treatment area is 7.93 mm. Therefore, the existence of the transition area makes the excimer treatment area larger, and some laser machines can even achieve 9 mm. , And the largest treatment area of ​​all femtoseconds is only 6.6 mm. It means that there is no treatment area except 6.6mm in diameter. Isn’t light outside 6.6mm participating in our visual experience? Or do some people’s pupils not exceed 6.6 mm at night? In summary, the design of the transition zone is very important for refractive surgery. Even in the earliest RK surgery, the refractive power around the cornea also has a similar transition zone as the cutting depth of the blade becomes smaller. Surgery here, due to the radical transformation of the surgical method, the design and function of the transition zone cannot be achieved. Therefore, it is theoretically determined that the postoperative effect will be affected, especially for patients with excessive dark pupils and degrees Patients who are large and have to shrink the light area. The world of all femtoseconds has no temperate zone.