Laser physics

Lasers are devices that generate light energy. A substance or medium is ‘pumped’ by an external energy source which moves electrons from a stable to an ‘excited’ state. Some of these excited electrons will spontaneously decay to a lower energy state. In the case of lasers, the excited molecule returns to a stable, lower energy state by emitting the extra energy as light in the form of a photon. A photon is a discrete bundle of energy with no mass. Once released, this photon cruises along in a population of like molecules which are also excited. When it strikes another excited molecule, the process of amplification occurs because the excited molecule is stimulated to release its extra energy as another photon of exactly the same wavelength and precisely in phase with the first. When all of this activity occurs in a tube with mirrors at both ends, the energy builds up as the light waves bounce back and forth. If one of the mirrors has a small central aperture, it can be opened in such a way as to release laser light.

Why ‘laser’? It’s an acronym formed from the first letters of the process described above; namely, Light Amplification by Stimulated Emission of Radiation. The concept of radiation in the form of wavelengths is derived from the dual nature of our understanding of light as being characterized by both photons and electromagnetic waves. There are unique features of laser light that make it useful for a variety of applications including medical ones. Laser light is collimated or highly directional which keeps the energy of the beam from dissipating over distance. Laser light is coherent with the peaks and troughs of the sinusoidal wave traveling through space in phase with one another. Coherence allows the energy within the beam to be concentrated with precision.

Finally, the light is monochromatic or emitted at a narrow band of the electromagnetic spectrum. It is this monochromatic property that is used in medicine to achieve specific goals in tissue. Once this laser light has been formed in the generator, it can be manipulated in several useful ways. The release can be controlled by a variety of gating mechanisms ranging from simple shutters to complex pulse generators. For medical uses, the beam is either released as a continuous beam or as a pulsed one. The power output is measured in watts or joules per second. The beam can be focused which is a useful property when cutting or resurfacing by scanning. It can be transmitted through mirrored conduction tubes, wave guides or fiber conductors.

If the beam is released in a pulse with the intent to conduct it to the target without significantly modifying its physical characteristics, the power of the laser must be high enough to produce an appropriate effect when it reaches the target. Once this emitted laser light strikes tissue, it can be reflected, scattered, absorbed or transmitted. While reflection is not a very useful property in medicine, there are some applications where scattering can be employed to achieve therapeutic goals. The most useful current medical applications are absorption and transmission.

Our current concept of what happens when laser light and tissue interact was articulated by Anderson and Parrish¹ in 1984 in their landmark paper describing selective photothermolysis. Specific components in tissue absorb light more intensely at some wavelengths than at others. These components are known as chromophores. For example, absorption by water peaks at 2900 nm and at 10600 nm. The erbium laser emits at 2940 nm and the carbon dioxide laser emits at 10600 nm. The incident energy from both of these lasers is rapidly absorbed by water in the most superficial layers of skin where it is converted to thermal energy. If the laser energy is sufficiently high, it will vaporize the water and in doing so will thereby remove or ablate the tissue. By controlling the incident energy and the duration of application, precise amounts of skin can be removed with limited collateral thermal damage.

Another example of this concept of absorption can be seen in the choice of an argon laser emitting at 514 nm (in the green part of the visible spectrum) where water absorption is very low and melanin absorption is relatively high. Energy emitted from this laser passes through the fluid medium of the eye to be absorbed by the chromophore melanin in the retinal pigment epithelium. The thermal energy generated by this effort can be useful in treating some pathology such as diabetic retinopathy. Figure 22-1 shows the different chromophores and their wavelengths of absorption. The goal of laser skin resurfacing is to vaporize or ablate surface tissue while minimizing thermal damage adjacent to the site of application. The obvious first step is to build the laser with enough power to reach the heat of vaporization, which in the case of water is 100°C. How efficiently this process of vaporizing tissue proceeds will be determined largely by the shape of the laser beam and the time of application. Since most medical lasers emit in either a true Gaussian shape or something similar to it, the most useful bell-shaped configuration will have the energy of the beam highly concentrated in the central portion of the beam. The objective in ablative resurfacing lasers is to exceed the energy of vaporization for the broadest area of the beam possible and to minimize the undesirable formation of thermally devitalized but intact skin on the margins of the beam. In prosaic terms, a beam configuration that looks more like a ‘high top hat’ than a ‘beret’ is desirable.

Figure 22-1 Chromophores and absorption wavelengths.

Duration of application of the laser beam can be a confusing parameter to understand. Conduction of any energy away from the site of origin/application requires time. When the water in tissue is the chromophore or target in tissue, the light energy is converted to thermal energy. If the laser energy is applied for a short enough time, thermal energy will be largely dissipated at the site. However if the energy is applied long enough, then the thermal energy will be conducted to the adjacent tissues. If this phenomenon occurs over a long enough time, significant thermal devitalization of the surrounding tissue is possible.² Thermal burns can be produced that can dramatically alter the healing process, to the extent of scarring. This period of time before significant conduction occurs is defined as the thermal relaxation time of tissue. For skin, that time is about 400 microseconds (400 μsec). The challenge is to build a laser with enough power to treat a fairly large area of skin with a ‘short burst.’

An alternative to a single pulse is a scanner that will trace a small focused beam over the target rapidly enough to produce a desirable effect and still not expose the target area for longer than the thermal relaxation time of the tissue. Of course at the sides and depths of the beam, some subablative energy will exist which will produce a residual of thermally devitalized tissue beyond the area of application. Actually, this thermal residual can be helpful because it produces cauterization of small vessels which otherwise might bleed thereby obscuring the field.³ The effect of ablative laser applications can be predicted. The common denominator, at least among lasers of the same wavelength, is fluence which is the amount of energy applied per unit area. Fluence is measured in joules/cm².

For the purpose of comparing lasers, both the carbon dioxide and the erbium laser can be considered to ablate 4 microns (μm) of skin/joule of fluence. Therefore, if the laser beam is applied at 20 J/cm², about 80 μm of skin will be ablated in one pass. 80 μm is about the thickness of eyelid skin. Remembering that the thickness of the papillary dermis is about that of the epidermis, a second pass at 20 J/cm² would place the thermal residual well within the reticular dermis. Thus, two passes over the eyelids at a fluence of 20 J/cm² would be aggressive treatment. For non-ablative devices, the laser or light energy is relatively poorly absorbed by chromophores in the skin. Therefore most of the energy from these devices will pass through the epidermis to be scattered by encounters with molecular structures deeper in the skin before being absorbed by the targeted chromophore. While thermal energy generated by this process is not sufficient to vaporize the tissue, there is enough thermal energy to devitalize the collagen and initiate a reparative process that will ‘renew’ the collagen in the papillary and upper reticular dermis. The entire process can be significantly enhanced by passive and dynamic cooling methods that are mainly designed to protect the epidermis.

The choice of wavelengths, duration of application and modification of effect by superficial cooling can be confusing. There are no simple guides. Furthermore, the results tend to vary considerably among patients, often in unpredictable ways. Perhaps the best approach is skepticism. Use conservative guidelines to gain familiarity. Most importantly, progress must be documented with pre- and postoperative photographs that can be readily accessed. We can all see what is, but our recollection of what was is poor. Remember, the improvement induced by non-ablative devices can take months to reach a maximum effect and that effect can be subtle.

Laser safety

Laser safety is a requirement for all physicians. There are many regulatory levels in laser medicine: federal, state, local, departmental, and institutional. Before offering laser resurfacing or using any laser tool in practice, be sure to check with each level for safety standards. Federal regulations are outlined by the Occupational Safety Health & Administration (OSHA, www.osha.gov) in the form of general industry standards, directives, and compliance letters. Laser safety standards are specifically further delineated by the American National Standards Institute (ANSI). ANSI Z136.1-2000 document covers the general safe use of lasers. ANSI Z136.3-1996 document provides standards for the safe use of lasers in health care facilities. This document is most often used by regulatory groups and in litigation. Most states also maintain regulations as adjuncts to OSHA and ANSI standards, which may be even more rigorous.

While engineering controls are built into laser systems by the manufacturer, each practice must be proactive in pursuing the highest level of safety for both the patients and the staff. ANSI recommends that physicians develop and maintain safety procedures and training policies. Each practice should appoint a Laser Safety Officer, whose duty lies in risk management and all laser safety oversight. Hazards and their control measures must be identified. Protective environmental controls are required including protective eyewear, non-flammable drapes (aluminum foil is common), smoke evacuators, fire extinguishers, and masks. Procedural controls include limited access to the treatment room and regulated laser warning signs at entrances.

A paramount concern is always for protection of the eye. The eye is very sensitive to the effects caused by the inadvertent application of laser energy. Ocular tissue damage can occur at different sites depending upon the laser wavelength. The retina and its pigmented cells are potential targets for laser light in the visible and near infrared spectrum (400–1400 nm). The lens and or cornea can be damaged by the ultraviolet (290–400 nm) or the far infrared (1400–10600 nm) spectrum.

Maximum Permissible Exposure (MPE) is defined for specific wavelengths and exposure durations. The Nominal Hazard Zone (NHZ) is the space in which direct, reflected, or scattered laser radiation exceeds the MPE. Appropriate protective eyewear according to wavelength and optical density should be worn by all persons in the treatment room and NHZ. For any laser treatment not involving the eye, the patient must wear protective shields that shield the entire globe. Therefore, corneal shields alone are inadequate.

Much has been written about plumes from lasers and cautery devices that contain viral and potentially mutagenic materials, hazardous gases, and bloodborne pathogens.⁴ Avoidance of such contaminants is obviously important. These plume hazards can be mitigated by laser specific surgical masks and smoke evacuators. The evacuator should be positioned within 2 cm of the smoke production for its highest efficiency. Specific laser masks should also be employed. The standard surgical mask has a 5 micron filter. Laser surgical masks have filtering capacity of 0.1 microns or less.

Table 22-1 Clinical laser safety checklist for intraoperative ablative facial laser resurfacing

Preoperative evaluation and patient selection

Facial resurfacing is an elective cosmetic procedure. Counseling patients about realistic expectations and postoperative care is critical before undertaking any such procedure. Special care should be taken to understand which areas of the face are of concern to the patient. Indications for facial laser resurfacing include facial rhytids, skin laxity (Fig. 22-2), dyschromia, atrophic acne scarring, rhinophyma (Fig. 22-3) and scar revision (Fig. 22-4). Benign skin growths may also be ablated; however, all suspicious skin lesions must be biopsied first. Education of the patient before the procedure is crucial. Show the prospective patient a photo album of pictures taken daily of actual patients who have undergone laser skin resurfacing during their first two postoperative weeks. This review can be most helpful in preparing the patient for what can be a challenging postoperative recuperation.

Figure 22-2 59-year-old man who underwent full face resurfacing along with adjunctive endoscopic brow lift and upper eyelid blepharoplasty. A, Preoperative; B, 6 weeks postoperative.

Figure 22-3 Rhinophyma significantly improved after resurfacing. A, Preoperative; B, postoperative.

Figure 22-4 Scar modification, another application of resurfacing. A, Preoperative; B, postoperative.

Complications and how they will be treated should also be discussed. Photographs must be taken to document preoperative anatomy. Both the patient and practitioner must have appropriate expectations of results. Emphasize that the goal is to modify and not erase. Laser facial resurfacing can usually improve severely photoaged skin to a moderate level, and a moderately photoaged skin to a minimal level and so on.⁵ Laser resurfacing is also a fine complementary procedure to other surgical modalities. Figures 22-5 and 22-6 show a patient who underwent full face resurfacing combined with a brow lift, midface lift, and four-lid blepharoplasty.

Figures 22-5 and 22-6 78-year-old woman with severely photoaged skin. She underwent full face resurfacing along with endoscopic brow and midface lift, and CO₂ laser assisted 4-lid blepharoplasty. Also note some expected mild erythema of skin at postop week 6. A, Preoperative; B, postoperative.

A routine medical and dermatologic history should be obtained. The skin type must be noted as part of the normal physical exam. Fitzpatrick Class IV or darker has an increased risk of pigmentation change and scarring. Oral retinoids within the previous 1 to 2 years may have depleted the density of pilosebaceous appendages. These structures provide the reservoir for reepithelialization. Active acne should be treated with oral antibiotics before resurfacing. Tobacco abuse may cause delayed healing which could lead to hypertrophic scarring. Table 22-2 shows contraindications to resurfacing.

Table 22-2 Contraindications to resurfacing

A pretreatment consultation should include pro-viding prescriptions for complication prophylaxis. A common protocol includes an anti-herpetic, beginning 1 day preoperatively and continued until fully epithelialized (7–10 days).⁶ Most experienced surgeons also prescribe an oral antibiotic. More controversial are topical retinols or alphahydroxy acids in the immediate preoperative period which may prime the skin for healing, and the routine use of hydroquinones postoperatively to decrease possibility of hyperpigmentation.

A documented, preoperative consultation with informed consent including potential complications is paramount. Risks of the procedure as well as anesthesia complications must be documented. Photographs must also be performed preoperatively for the medical record.

Operative techniques for ablative facial resurfacing

Postoperative care for ablative laser resurfacing

Immediately after surgery, an occlusive or semiocclusive wound environment must be initiated. One possibility is a biosynthetic dressing¹⁰ (e.g. Flexzan) (Fig. 22-7). As an alternative, petrolatum ointment may be applied. Topical antibiotics should be avoided, especially during the early phases of wound healing, because the epithelium has been removed by the surgical procedure thereby reducing the barrier effect of the epidermis. If a biosynthetic dressing is used, it is usually removed on postoperative day 1–3 by ‘soaking it off’ in a long shower. Once the dressing is removed, vigorous open wound care is begun using soaks and ointments. Aquaphor is a common choice although many other healing balms have been advocated including Crisco and Vaseline. The most important objective is keeping the wound moist which promotes rapid re-epithelialization. Soaks containing a weak acid (1 teaspoon of white vinegar/cup of tap water produces a concentration of about 0.25% acetic acid) will ‘cut’ the residual ointment and be a weak antibacterial. The soaks should be applied as ‘sopping wet’ 6–8 times/day. Using clean paper towels provides an alternative to the accumulation of soiled hand towels. After soaking for 15–30 minutes, reapply the barrier ointment. The treated skin should not be allowed to ‘dry out.’ A yellowish serous transudate will be prominent on the treated dermal surface for 3–5 days. Keeping the wounds clean will help to prevent accumulation of a nutrient bed for fungal or bacterial infection. Expect moderate erythema and some edema, especially in periorbital and perioral zones.

Figure 22-7 Postoperative patient with bio-occlusive dressings.

Pain is usually minimal. Many patients need only over-the-counter medicines. Most physicians prescribe an anti-herpetic for prophylaxis because the trauma of the surgery may reactivate dormant herpes simplex virus which could lead to a herpetic infection. Oral antibiotics may also be prescribed. The patient should be seen frequently in the first 10 days to monitor for infection or other complications. It is important to assess how meticulous the patient is with their post-operative skin care. Since this early recovery period is a vulnerable one for the patient, emotional support and encouragement are important during this time.

Complete re-epithelialization should occur around day 7 to 10 and will be manifested by smooth pink skin. It will take a month for the epithelium to regain its full thickness. Make-up and other cosmetics can be slowly introduced, usually one at a time to facilitate identification of potential irritants. Inform patients that contact dermatitis (Fig. 22-8) can occur even to topicals that they have successfully used in the past. Sun block of at least SPF 30 should be used. Acne and milia can also occur after the epidermis has been re-established. The new epidermis is thin, so avoid unnecessary chemicals, perfumes, or abrasive scrubs for the first few weeks. The skin is usually pink for 8–12 weeks (Figs 22-5, 22-6). New collagen and elastin fiber production may continue for up to 12 months after resurfacing. Figure 22-9 represents one year post-op follow-up results. As with other anti-aging procedures, most results should be longstanding.

Figure 22-8 Contact dermatitis after laser resurfacing.

Figure 22-9 Lasting results, one year postoperative A, Preoperative; B, 1 year postoperative.

Complications of ablative laser resurfacing

Postoperative care and complication recognition are perhaps the most difficult part of laser facial resurfacing. A preceptorship or formally arranged partnership with an experienced surgeon will be invaluable to the beginning laser surgeon. Complication avoidance is the goal of appropriate postoperative care as outlined above. However, complications do occur and most can be overcome with patience and treatment.

Bacterial cellulitis can occur in the first 1–2 weeks following resurfacing.¹¹ Typical signs are redness, pain, infectious exudates, and even foul odor. Gram stain, cultures, and sensitivities are necessary in all cases. IV antibiotics may be needed depending on severity. If bacterial infection does occur, polymicrobial infections are not uncommon. Staph, strep, and pseudomonas are the typical pathogens. Some surgeons advocate pretreatment prophylaxis with antibiotics such as cephalosporin.

Herpetic and fungal infections can be seen in the same time frame. It can be difficult to properly identify herpetic infection in the early postoperative stage because herpetic dermatitis usually presents with epithelial signs. The typical vesicular stage is not seen when the epidermis is gone. A row or crop of raw, red lesions may be observed instead. Significant scarring can occur if untreated. Figure 22-10 represents a severe herpetic infection following perioral resurfacing. Fungal infections, usually candida, appear as soft white plaques with erythema and satellite lesions. Diagnostic evidence with KOH prep can be vital to identification and differentiation. Prophylaxis with a single dose of fluconazole may be prudent.

Figure 22-10 Severe herpetic infection after perioral resurfacing.

Pruritis is a common side effect of the healing response. Symptoms can be strong, but thankfully are transient. The mandibular border and the forehead are the most affected areas. An oral antihistamine, topical mild steroid, ice pack or cool pack should give relief.

Acne may occur after resurfacing. An oral tetracycline, such as Minocin, may be prescribed. Milia are often present in the first 3–4 weeks postoperative (Fig. 22-11). Treatment with low concentrations of retinoic acid, alphahydroxy acids, or glycolic peels is usually beneficial.

Figure 22-11 Milia are possible sequelae of resurfacing.

Post-inflammatory hyperpigmentation occurs transiently in about 30 percent of resurfacing patients (Fig. 22-12). It typically presents in the first 3–4 weeks. It is more common in Fitzpatrick Type III to VI skin types.¹² Melanin builds up in macrophages and production is increased during the postoperative period. There are many treatment options, including observation (resolves usually within 6 months), topical bleaching agents (i.e. hydroquinones), topical steroids, sun avoidance and sun block (SPF 30 and higher). Some surgeons pretreat those at risk for hyperpigmentation with hydroquinones and possibly topical steroids.

Figure 22-12 Post-inflammatory hyperpigmentation may present in patients with Fitzpatrick skin types III to VI.

Hypopigmentation occurs in less than 1 percent of treated patients. It is thought to be caused by an overall reduction in melanocytes following resurfacing. Patients who have undergone previous deep chemical peels or dermabrasion are most at risk. Unfortunately, there is no simple or reliable treatment. Cosmetic camouflage may be the most practical answer.

Table 22-3 Advantages and disadvantages of ablative laser resurfacing

Erythema is normal during the first 12 weeks. It should gradually lessen. After 3 or more months of persistent erythema, mild topical steroids may be necessary to hasten improvement. Most of the redness can be neutralized after re-epithelialization by using make-up with a yellow or green color because these colors are complimentary to red.

Scarring may result as a rare complication in the resurfaced face.¹³ It is probably the most significant iatrogenic complication, but is preventable with proper technique. The resurfacing must not be performed too vigorously. Stay within the superficial reticular dermis. In addition, careful cleaning of the residual desiccated tissue during the resurfacing will avoid char.¹⁴ Char is formed by the carbonization of residual treated tissues. Char then becomes the chromophore for additional passes of the ablative resurfacing laser. When char becomes the target, there is a change in thermal dynamics from the temperature needed to vaporize water (100°C) to a much higher temperature. This additional thermal energy has the potential to produce side effects that may result in scars. Scarring can present unpredictably in certain skin types. The surgeon must be aware of previous unfavorable healing and or keloids. Treatment of scarring may consist of topical steroids, intralesional steroids, 585 nm pulse dye laser, or surgical excision. The risk of scarring can be best reduced by appropriate preoperative evaluation, safe surgical technique, close follow up of the patient after surgery, and good compliance with aftercare instructions.

Cicatricial ectropion secondary to anterior lamellae scarring or shrinkage with or without punctal ectropion can also occur. This is not to be confused with the early, likely lower lid eyelid edema and skin tightening, which will spontaneously resolve. True cicatricial ectropion may be seen in the later postoperative period. Again, carefully examine the patient in the preoperative period looking for other potential factors in ectropion: gravitational cheek descent, excessive lid laxity, and canthal tendon dehiscence. Some surgeons incorporate a lateral tarsal strip procedure into the operation in those at risk.

Facial laser resurfacing is an effective tool in skin rejuvenation. It may seem daunting at first to the laser inexperienced. However, once implemented, it can be a very satisfying and valuable procedure for the oculoplastic surgery practice.