I. Introduction of femtosecond laser
Femtosecond, a unit of measurement for the length of time, one femtosecond is 10 negative 15 times the second, that is, 1/1000 trillion seconds. Femtosecond laser is a kind of laser in the form of pulse operation, its duration is very short, only a few femtoseconds, it is thousands of times shorter than the shortest pulse obtained by using electronic methods, is the shortest pulse of human technology in laboratory conditions, this is a significant feature of the femtosecond laser. The second distinctive feature of femtosecond laser is the instantaneous high power, which can reach one million billion watts, a hundred times more than the total power generated worldwide.
Femtosecond lasers are widely used in physics, biology, chemical control reactions, optical communications, and other fields. It is particularly worth to propose that because of its fast and high-resolution characteristics, the femtosecond laser has its unique advantages and irreplaceable role in the early diagnosis of lesions, medical imaging and biological in vivo detection, surgical medicine and the manufacture of ultra-small satellites. A very peculiar phenomenon occurs when matter is subjected to a high-intensity femtosecond laser: gaseous, liquid, or solid matter instantaneously becomes a plasma. This plasma can radiate laser light of various wavelengths of rays. High power femtosecond lasers colliding with electron beams are capable of producing hard X-ray femtosecond lasers, producing beta-ray lasers, and producing positive and negative electron pairs. High-power femtosecond lasers have promising applications in medicine, ultra-fine micromachining, and high-density information storage and recording. High-power femtosecond lasers can also pierce the atmosphere, thus creating discharge channels to artificially induce lightning and avoid catastrophic damage caused by natural lightning strikes on aircraft, rockets, and power plants. The use of femtosecond lasers can accelerate electrons very effectively, allowing thousands of times compression in the size of the gas pedal. High-power femtosecond lasers interact with matter and are able to produce sufficient numbers of neutrons to achieve rapid ignition of laser-controlled nuclear fusion. Thus opening up a brand new way for mankind to achieve a new generation of energy.
Second, femtosecond laser in medical applications
1, femtosecond laser has a considerable precise directionality and positioning
”Femtosecond laser” surgery has two principles, one is the principle of light transmission, one is the principle of light blasting. First, let’s look at its light transmission principle: before the surgery, the doctor will enter the patient’s basic information and surgical data into the computer. During surgery, the surgeon operates the “femtosecond laser” machine, which uses a flattening cone lens to flatten the cornea and maintain the precise distance between the laser head and the laser focus in the corneal tissue. The femtosecond laser transmits laser pulses in a pattern set by the surgeon to perform various targeted cuts in the cornea.
2. Femtosecond laser blade makes a flatter and more precise corneal flap
Femtosecond laser light blasting principle: laser pulses are focused into the corneal tissue, producing light blasting; each pulse of light blasting, producing a micro-ion; each micro-ion, evaporating about 1 micron of corneal tissue; evaporating corneal tissue to produce extended blisters and CO2 bubbles, the blisters and bubbles are absorbed by the corneal tissue, the corneal tissue is thus separated. The computer-controlled optical delivery system generates thousands of femtosecond laser pulses, which are focused in a dense, equal-width, equal-spacing fence-wall grating pattern to produce a light burst at the same depth, creating a layer of tiny diameter bubbles in the corneal tissue, which separates the corneal tissue and forms the cutting surface of the femtosecond laser. The cutting pattern creates a horizontal separation surface and a vertical surface.
Femtosecond laser pulses can also be focused in the corneal tissue at any angle and in any range of stacking, forming different angles and ranges of tissue separation, so the femtosecond laser can be used in corneal transplantation and lasik surgery to perform lamellar cutting of the cornea, making fine implants and corneal flaps.
Third, the advantages of femtosecond laser for myopia treatment
1.Significantly reduce the risk of surgery
In the traditional laminate knife surgery, if the blade rotation process occurs during the negative pressure ring loosening and other accidents, the production of corneal flap will not be completed, the surgery can only wait for three months to do again, bringing great psychological pressure to patients and doctors. In the case of a similar accident with the femtosecond laser, the surgeon can simply put the negative pressure ring back on and immediately replenish the laser without interrupting the surgery. Because the femtosecond laser only produces some water and bubbles to push away the corneal tissue when creating the flap, there is no damage to the tissue, and multiple surgeries can be performed on the same affected area, greatly improving safety. It is also worth noting that the transient high intraocular pressure may cause some damage to the fundus. Compared to the transient high intraocular pressure of the laminar knife, the negative attraction pressure of the femtosecond laser is much lower, thus reducing the possible damage to the fundus.
2. Embedded corneal flap, more accurate reset
The lamellar knife is horizontal cutting, and the surface of the eye is a sphere, so the corneal flap profile is “cup and lid” type, and the adhesion with the eye matrix is not good enough; while the corneal flap made by femtosecond laser and the eye matrix is “underground well lid” type embedded occlusion. The flap is easily repositioned and tightly occluded. At the same time, its laser light source also avoids the residual metal debris that may be caused by the use of lamellar knife.
3. Perfect post-operative visual quality
”Femtosecond laser can precisely open the molecular chain of the eye tissue to produce a more uniform and perfect corneal flap, effectively avoiding the medical aberration that may occur with laminate flap making, and avoiding glare and blurring in foggy days, rainy days and night driving conditions, allowing myopic patients to obtain a perfect visual quality.
4.Wider correction range
In the past, the thickness of the cornea affected nearly 10% of patients who were too deep and too thin to undergo traditional laser surgery such as LASIK. Femtosecond laser surgery is not affected by corneal curvature, which is a great blessing for myopic patients with thin corneas and large corneal curvature variation.
Types of femtosecond laser surgery for clinical use in ophthalmology
There are two types of femtosecond laser surgery for refractive errors: partial femtosecond laser surgery and full femtosecond laser surgery. As we know, the traditional excimer laser procedure for refractive errors is composed of two parts: the laminar flap creation process and the excimer laser treatment process. The femtosecond laser used for the first type of femtosecond laser surgery is the flap-making instrument, which only replaces the flap-making part of the laminar knife, while the latter part of the laser treatment is still done by the traditional excimer laser. Therefore, this type of surgery can only be called all-laser surgery, but not all-femtosecond surgery. So it is not strictly speaking a revolutionary surgical procedure. The latter uses the femtosecond laser to replace the entire process of traditional surgery, i.e., laminar flap creation and excimer laser treatment, so that it can be called all-femtosecond laser surgery.
At present, the only femtosecond laser that can perform all-femtosecond surgery is the VisuMax femtosecond laser from Zeiss, Germany. the VisuMax femtosecond laser’s all-femtosecond surgery system is called RELEX (including Flex and Smile surgery), and its features and advantages are obvious: 1.
1. The unique 3D stereoscopic cutting can change various cutting parameters for femtosecond laser flap making, as well as femtosecond corneal transplantation and femtosecond refractive surgery. Clearer vision, better treatment effect, especially for medium to high myopia without regression, better night vision, less postoperative complications.
2. The unique curved contact lens design only contacts the cornea, without flattening the cornea so that the corneas that are too steep or too flat can also be operated for more indications and more in line with the biomechanical flap making. Minimal intraocular pressure elevation, less potential risk to the retina and optic nerve during surgery, and avoid inflammatory reactions.
3. The patient has no loss of fixation during the whole surgery, can see the fixation light, and use the patient’s fixation point as the best point for flap positioning to make the flap center more accurate. The patient is more comfortable intraoperatively.