Skin contains a very high percentage of water, the fractional delivery of CO2 laser to the skins surface is attracted to the water molecules in the skin. This absorbs the laser energy creating heat - Selective Photothermolysis. The laser energy is applied to the skin in a fractional way, by creating longitudinal microthermal zones in the skin penetrating to approximately 2mm in depth through to the dermis. This thermal damage creates trauma to the existing cells in the skin thus promoting collagen formation. The space between the columns of intact epidermis allows the skin to trigger a faster healing response of the damage tissue achieving better results with less down time. As a result, production of new skin is stimulated within the dermis and epidermis.
The delivery of CO2 produces 2 effects on the skin:
1. Ablation Effect: The high power of the laser emission releases over a short time for rapid ablation of the epidermis and first layers of the dermis. This is beneficial for layers of the skin with low water content.
2. Thermal effect: After the epidermal ablation the energy pulse spreads into the dermal tissue causing contraction of collagen in turn initiating collagen repair.
The energy of the laser will be absorbed by the water (chromophore) in the skin to achieve the clinical end point of white dots where the laser has delivered. This shows the water has been absorbed and heat trauma has been achieved.
This clinical end point shows the heat caused trauma to the tissue, increasing the temperature to approximately 70- 80°. This damages the collagen beyond repair. The body's immune system then picks up the damaged cells encouraging a fibroblast response to the injury in both the initiation and the resolution phases. Due to the spacing between each laser emission, the tissue in-between is spared from the trauma, resulting in a faster healing process.
The objective in CO2 laser resurfacing, as opposed to pure erbium laser resurfacing, is to accurately ablate the epidermis while applying a controlled amount of heat to the dermis to induce collagen remodelling. The energy that is applied to a surface area of tissue is represented by fluence (J/cm2). For resurfacing, a high-energy beam with a very small spot size is applied to the skin with an ultrashort pulse duration (1000 ms). After each pulse, the tissue is allowed to cool for a time equal to or greater than the tissue thermal relaxation time before the next pulse is delivered. This prevents unwanted conduction of heat to adjacent areas.
Once the epidermis has been ablated in this first pass, one encounters the dermis. The dermis has a lower water content than the epidermis, so at the same fluence there is less absorption of the laser energy. Therefore, multiple passes over the same area result in more thermal injury with less tissue vaporization. It is suggested that heat-related collagen denaturization leaves collagen fragments that stimulate new collagen deposition and directed migration of fibroblasts. In addition, the initial contraction from the heat, forces created partially by dehydration of the skin on first pass of the CO2 laser can assist in the alignment of the newly synthesized collagen.
