Medical lasers are advanced surgical instruments that use highly focused light to precisely cut, vaporize, or remove tissue in a non-contact manner. Unlike a traditional scalpel, which physically separates tissue layers, the laser acts by delivering concentrated energy to the target cells. This focused energy allows surgeons to make incisions with high accuracy while minimizing bleeding and reducing post-operative swelling.
Prerequisites for Tissue Interaction
The ability of a laser to cut tissue begins with the principle of selective absorption, which governs how light energy is taken up by biological material. Tissues contain specific molecules known as chromophores that are preferential targets for certain light wavelengths. Common chromophores include water, hemoglobin, and melanin, each absorbing light most efficiently at different points along the electromagnetic spectrum. For a surgical laser to be effective, its specific wavelength must match the absorption profile of the targeted chromophore. For example, lasers used to cut soft tissue, which is primarily water, often utilize wavelengths that water strongly absorbs, such as those produced by CO2 lasers. If the laser’s wavelength is not readily absorbed, the light will simply scatter or pass through the tissue without causing the desired effect.
The Three Ways Lasers Remove Tissue
Once the laser energy is absorbed by the target chromophores, the light is converted into other forms of energy that physically remove the tissue through one of three distinct mechanisms.
Photothermal Vaporization
The most common method for bulk tissue cutting is photothermal vaporization, where the absorbed light energy is quickly converted into intense heat. This rapid temperature increase causes the intracellular water within the targeted cells to boil, leading to an explosive expansion and vaporization of the tissue. The resulting steam and smoke plume carries the cellular material away, creating a clean incision line, such as with a CO2 laser.
Photoablation
A second, more precise method is photoablation, sometimes called photodisruption, which relies on extremely short, high-energy pulses to break molecular bonds directly. This process occurs so rapidly, typically in nanoseconds or less, that there is no time for significant heat to transfer to the surrounding tissue. The intense energy creates a localized plasma state, where tissue molecules are instantly decomposed and ejected with minimal thermal damage. This non-thermal mechanism is highly valued for delicate procedures, such as corneal reshaping in eye surgery.
Photochemical Effects
The third mechanism involves photochemical effects, which utilize lower energy light to initiate a chemical reaction within the tissue without causing immediate heat damage or mechanical disruption. In this process, the absorbed photons trigger a specific cascade of events that can lead to the breaking of molecular bonds over time. This technique is often used in therapies where the goal is a highly specific molecular change, rather than bulk removal. The specific effect is entirely dependent on the chemical composition of the tissue and the exact wavelength of light used.
Factors Determining Cutting Depth and Precision
Surgeons control the physical effects of the laser primarily by modulating three interrelated operational parameters to achieve the desired cutting depth and precision.
Pulse Duration
Pulse duration is the length of time the laser is on for a single burst of energy. Delivering energy in very short pulses, often shorter than the tissue’s thermal relaxation time, ensures the energy remains localized and minimizes the spread of damaging heat to nearby cells. Conversely, using longer or continuous pulses allows heat to diffuse outward, resulting in a broader zone of coagulation, which is useful for sealing blood vessels.
Energy Density (Fluence)
Energy density, also referred to as fluence, represents the total amount of energy delivered to a specific area of the tissue surface. Measured in joules per square centimeter, a higher energy density increases the rate of tissue removal and the depth of the cut. The surgeon must carefully balance this parameter to ensure the cutting threshold is met without causing excessive collateral damage.
Spot Size
The spot size is the diameter of the focused laser beam on the tissue surface. By adjusting the focusing optics, the surgeon can concentrate the total energy into a smaller spot, dramatically increasing the power density at the incision site. A smaller spot size allows for a finer, more delicate incision, while a larger, less-focused spot distributes the energy over a wider area, leading to a broader, shallower effect often used for surface treatment or coagulation.