The neodymium:YAG laser in strabismus and plastic surgery of the face. Wound repair

Abstract

The physical features of a number of laser energy sources presently in use in plastic surgery, such as the Nd:YAG, KTP, Ho:YAG, Er:YAG, and CO2 lasers, are emphasized. Particular attention is paid to the Nd:YAG laser and to an Nd:YAG laser-powered quartz laser scalpel.

Introduction

A large number of laser systems are used in plastic surgery and dermatology. This review is restricted to the Nd:YAG laser delivered in the contact mode, and will be compared to other laser systems.1 For a review, see Table 1.

The neodymium:YAG laser is a solid-state laser containing a crystal of yttrium-aluminum-garnet (YAG) doped in 1-3% neodymium (Nd) ions. Doping is a process by which the crystal is grown in

Table 1. Laser systems used in plastic surgery and dermatology

the presence of an impurity, so that the crystal lattice (YAG) forms with the impurity (Nd) within it. An Nd:YAG rod is placed within the laser cavity where powerful xenon arc lamps or argon lasers excite the Nd:YAG ions that provide emission in the invisible near infrared spectrum at 1064 nm (1.064 µm).

The Nd:YAG is delivered to the tissue either as a direct beam or by fiber optics (Fig. 1).2,3 The distal end of the fiber is typically coupled to a hand piece for focusing. More recently, either a solid sapphire crystal probe or a quartz probe affixed to the end of the fiber optic, or threaded through a lacrimal endoscope, have significantly expanded the versatility of the Nd:YAG laser. These probes attached to the fiber optic are used in direct contact with the tissue, as opposed to the noncontact approach used with the Nd:YAG or other laser systems. These ‘laser scalpels’ combine excellent cutting properties

Fig. 1. a: Conically-shaped clad silica fiber (scalpel). To the right the cladding is intact. To the left, the cladding has been removed. b: Radiation field emitted by a virgin laser scalpel (visualized in a 6G rhodamine solution). c: The beam is somewhat more divergent, but there is no ‘energy leakage’ because the cladding of the scalpel has survived. (Reproduced from Fankhauser et al.5 by courtesy of the publisher.)

with the coagulative hemostatic capability of the Nd:YAG laser. Both the quartz and artificial sapphire probes concentrate the laser energy near the tip of the laser scalpel for cutting and vaporization. A low intensity helium laser, mounted coaxially to the laser beam, serves as the aiming device for most commercial Nd:YAG laser systems.4

The typical cw Nd:YAG laser currently available for medical applications generates up to 80-100 W. However, as has been shown previously,5 Nd:YAG energy sources of 10 W may be sufficient for most tasks involved in plastic surgery of the face. The thermal operation mode (free running and cw) should

not be confused with the photodisruptive operation mode.6

The radiation produced by the Nd:YAG laser is poorly absorbed by water, hemoglobin, and melanin; therefore, its penetration into the tissues is much deeper than either the CO2 (10.6 µm) or argon (488 and 514 nm) lasers. The Nd:YAG laser can produce effects at tissue depths of 4-6 mm into the dermis, resulting in a large volume of coagulated tissue, substantially larger than that with other lasers.7 Hemostatic effects are induced within this irradiated volume. Increased hemostatic effects and tissue vaporization can be achieved when the laser power is increased. At very high power levels, profound thermal damage can be produced unknowingly by an inexperienced physician.

Because of the extensive penetration of the Nd: YAG laser into the skin, its use in cutaneous surgery is somewhat limited. For example, it cannot be used to ablate superficial skin lesions as the CO2 laser can. Furthermore, it cannot be targeted to individual chromophores in the skin, such as melanin or hemoglobin in small blood vessels, as the argon laser can. Most commonly, the Nd:YAG laser is used to treat large hemangiomas,8,9 thick nodular port wine stains, and highly vascular tumors, particularly of the oral mucosal surfaces and tongue, due to its powerful coagulation and hemostatic effects.10-12

Another type of YAG laser is the potassium-ti- tanyl-phosphate (KTP) laser. A number of techniques are available for modifying the wavelength obtained with YAG lasers. The simplest of these is ‘frequency doubling’ or ‘harmonic generation’, where high intensity light propagates through a nonlinear, asymmetric crystal, and generates laser light at twice the input frequency.13 Since the frequency of light is inversely proportional to the wavelength, the result is light emitted from the crystal with twice the frequency, or half the wavelength, of the incident light. In the case of the KTP laser, invisible near infrared 1064 nm wavelength light is passed through a KTP crystal, producing a visible wavelength of 532 nm. These lasers can deliver up to 15-20 W cw, and the light is transmitted through conventional fiber optic delivery systems.

Both the argon and KPT lasers produce green light at somewhat different wavelengths (514 and 532 nm, respectively), although the 532-nm KTP radiation is more highly absorbed by the RPE and hemoglobin than argon green light (514 nm). In fact, the KTP laser is used for many of the same procedures as the argon laser.

Finally, there are a number of YAG lasers doped with other elements, such as holmium (Ho:YAG) or erbium (Er:YAG), that produce emissions in the near infrared portion of the spectrum at wavelengths of 2.15 and 2.94 µm, respectively. Because these emission wavelengths correspond to the absorption maxima of water,14 they may, in fact, replace the CO2 laser.15 Since this radiation is absorbed so intensely by water, the penetration depth is only a few

µm. Many photons being carried in such a small volume produce a rapid rise in the temperature of the substrate. As energy is added, the water in the substrate is raised to its boiling point. Internal vapor pressure builds up until a microexplosion occurs. The rapid expansion created by this excitation gives rise to the actual ejection of microscopic tissue fragments at high velocities. Since most of the energy of the laser pulse goes into the thermal phase change associated with tissue vaporization, ablated fragments ejected from a tissue surface carry most of the energy with them, leaving little energy in the form of heat to damage the surrounding material.15 Therefore, ablation of the exposed material can, in principle, be performed more easily. This phenomenon prevents undesirable melting or carbonization of organic molecules, a problem that exists when CO2 laser irradiation is used.

It only remains to be said that, although the laser appears to offer distinct advantages, many oculoplastic procedures may be performed by non-laser methods just as well.

A hemostatic laser scalpel

General aspects

The main advantage of a laser scalpel (Fig. 1) working in the contact mode is its hemostatic action, which allows it to be operated without inducing serious hemorrhage. There are limitations in as far as the diameters of arteries greater than 2 mm, as well as the diameters of veins greater than about 5 mm, cannot be closed by a contact laser scalpel.16 The physical parameters of various laser scalpels vary within wide limits (Table 1). Laser energy must be fed into the scalpel (probe) by optical fibers. Because there is a more or less distinct selectivity of the fiber material, the choice of the physical properties of the fibers is important.2,3,17 Quartz (SiO2) transmits wavelengths within a range of about 0.3- 2.0 µm with little loss, although, in the longand shortwave ranges, specially-manufactured quartz fibers are required.17,18

The transfer of radiated energy from fiber to tissue is either directly from the fiber end,19 or optical probes are used. These are made of either sapphire or quartz. The radiation profile of the emitted rays from the end of a probe can be adapted to a specific surgical task.18,20-23 When laser energy enters the tissue, it is redistributed.24-28 With increasing power being radiated by the probe, the temperature increases, and this first leads to coagulation and then to evaporation.27,28

Not all the laser energy transmitted to the probe is radiated to the tissues; part of it is transformed into heat and is transmitted as heat to the tissues. As a consequence, a higher temperature level is generated in the immediate vicinity of the probe, resulting in tissue dissection and, even more so, in hemostasis, while tissue temperatures may rise con-

siderably.29-31 Further away from the probe, temperature is generated by absorbed radiated laser energy, while heat conduction is negligible. The width of the evaporation and coagulation effect may be influenced by cooling the probe by various means.32,33 It remains to be determined whether the geometry of the probe is instrumental in determining the distribution of its cutting properties, and it has been noted that, as was previously believed, a strict relationship is only partially produced. This insight has allowed cheap probes with plain endfaces to be adopted.34-39

Hemostasis

In all applications of the laser scalpel in surgery of the face and its appendages, hemostasis is of great importance. Because CO2 radiation is very highly absorbed by water, and that of argon and KTP lasers by sanguineous structures, their penetration is poor (Fig. 2).40-42 This means that the depth of an

Fig. 2. A: Absorption of deoxygenated blood with a hemoglobin concentration of 150 g/l as a function of wavelength for various thickness blood layers. The number of erythrocytes is 4.9 x 106 mm-3. Absorption at wavelengths beyond 1000 nm are extrapolated. B: Absorption of oxygenated blood with a hemoglobin concentration of 150 g/l as a function of wavelengths for various thicknesses of blood layer. The number of erythrocytes is 5.2 x 106 mm-3. Values for absorption at wavelength for various thickness blood layers. (Adapted from Welsch et al.122 by courtesy of the publisher.)

incision with these lasers is not great, and that the hemostatic effects are limited to the edges of the wound, despite the high laser powers that may be applied. In contrast, both the diode and even more so, by virtue of its great penetration, the Nd:YAG laser, have excellent hemostatic and cutting prop- erties.40-42

A first approximation for the elucidation of hemostatic mechanisms is given in the Appendix.

Laser-tissue interaction

The mechanism of laser-tissue interaction has been described in a number of publications.43-50 Cutting with laser energy in the contact mode requires highly localized heat to vaporize small tissue volumes, rapidly creating a carbonized (charred) incision with some damage to adjacent areas, which decreases as a function of the distance from the charred border (Fig. 3).

The hemostatic effect of Nd:YAG laser light is illustrated in Figures 4, 11, 12, and 13.

Fig. 3. Site of excision of the upper lid (resection of skin and orbicularis oculi in case of blepharochalasis). Three zones are shown: S: A zone of charred tissue of unequal width forms the wound edge. This layer is shed off in the postoperative phase. Below F, collagenous tissue, displaying a variable amount of heat damage. Further below M, a wedge-shaped zone of muscle fibers which show only a modest degree of heat damage. Within both the zone of muscle fibers and, even more so, in the zone of connective tissue damage, F, both heat and hemostatic effects upon vessels can be seen. Semithin section. Negative magnification x 100. (By courtesy of E. Van der Zypen.)

Fig. 4. An intense, vaso-occlusive heat effect upon a vessel (artery) of the lid is shown. The vessel is constricted and the lumen, C, is obstructed by a conglomerate of coagulated protein and lysed erythrocytes and other blood cells. The wall of the vessel, M, has lost its typical structure and is partly homogenized. The surrounding collagenous tissue, F, shows intense heat effects. Semithin section. Negative magnification x 160. (By courtesy of E. Van der Zypen.)

The wound

One of the attractive features of the surgical CO2 laser beam is that, beneath the excised surface of soft tissue, there is only a thin layer of coagulation damage, typically 200-300 µm thick.46 This allows good postoperative healing with minimal edema formation and risk of infection. On the other hand, such shallow depths of coagulation have implications for the effectiveness of the hemostatic quality of the beam, by limiting the size of vessels which may be sealed adjacent to the laser excision. In contrast, the penetration of Nd:YAG laser light is greater and hence the hemostatic efficiency much greater, due to superior tissue heating: this is the price that must be paid for increased hemostatic efficiency.

Wound healing

Wound healing mechanisms (Fig. 5) take place in the thermally damaged regions. Wound healing mechanisms may be related to the primary insult. Here, the lack of tissue edema, typical of some la- ser-induced wounds, may be an important factor. Apart from the type of the primary insult,51 the final architecture of the wound depends on a number of factors, which are partly determined genetically. Basically, it should not be overlooked that there does not seem to be any escape from thermal damage, whatever surgical approach is chosen: the necessity of hemostasis forces the surgeon, working with the cold scalpel, into hemostatic action, e.g., clamping, and/or a high frequency current. A number of competent papers may provide insight into the staggering complexity of redundant, mutually exclusive, and antagonistic mechanisms of wound healing.

Coagulation begins immediately after injury, and involves both humoral elements and platelets. Within a few hours, neutrophils appear within the wound;

Fig. 5. Flow diagram of wound healing. (Adapted from Ross51 by courtesy of the publisher.)

in the absence of infection, they rapidly diminish in number. Monocytes, which transform into tissue macrophages, attain their numerical peak somewhat later than neutrophils, but persist for a longer period of time. Fibroblasts (fibroplasia and extracellular matrix deposition) and capillaries (angiogenesis) generally make their appearance during the first day, and achieve maximum numbers after between seven and ten days. Epithelialization also starts around that time. With the onset of remodelling, fibroblasts and capillaries diminish in number, leaving a relatively acellular mass of collagen and glycosaminoglycans, which form the scar tissue.

The first important phase of wound healing starts with local bleeding and tissue trauma, which, in turn, precipitate the clotting cascade. The final phase of wound healing is related to wound remodelling. This involves (a) increasing the cross-linkage of collagen, which enhances its strength; (b) the breakdown of collagen (collagenase activity) to meet excess accumulation; (c) regression of the lush network of the surface capillaries as the metabolic demand diminishes; (d) decrease in the wound level of proteoglycans and, hence, of water content.

Other important steps in wound modelling are fibroplasia, collagen synthesis, and collagenolysis. Increased collagenase activity is encountered by increased collagen production, and it is the balance between these two processes that is critical in determining the final quality of the scar. Most importantly, apart from growth factors, macrophages are known to play a key role in wound healing, and a thorough understanding of the numerous mechanisms involved could offer a means for the effective regulation of these processes by pharmacological intervention.

Important factors in wound healing that should not be ignored are differences in species, and age, 112 and health state and nutrition.113,114 The effects of the physiology of blood have been recognized. 117 Another important insight into wound healing mechanisms stems from models of wound healing, and it should be recognized that the investigation of any physiological process depends upon the use of models that attempt to simulate complex phenomena. A variety of animals has been used as models

for human wound healing. Although most mammalian species simulate the stages of wound healing, with collagen deposition being a prominent feature, the healing processes are certainly not identical. Nevertheless, such attempts have revealed the important role of apoptosis in the developing wound.118,119

An Nd:YAG laser scalpel

We have described a laser scalpel consisting of a quartz (silica, SiO2) probe with a diameter of 0.6 mm and a length of 40 mm, and the diameter of the cutting edge being 0.15 mm. The probe is screwed into a universal hand piece,38,39 and is connected by a light cable to a cw laser module of 10 W. The dynamic range, i.e., the maximum irradiance at the cutting edge, is 57 kW/cm2 (Figs. 1a and b).

This probe has been used in a number of pathological features in the facial region: benign tumor of the face (Case 1); strabismus (Case 2); dermachalasis (Case 3); massive angioma of the face (Case 4); capillary vascular malformation (Case 5).

Case reports

Case 1 (Fig. 6)

A patient presented with a coarse, vascularized skin tumor of the left upper cheek, which was considered to be benign (Fig. 6a). The tumor was excised under local anesthesia. Healing was rapid and the cosmetic result satisfactory (Fig. 6b). The various phases of tumor resection are shown in Figures 6c-f. Dissection was only disturbed by a single large vessel, which was stopped by Joffe’s technique,37 and the precision of the dissection procedure was high. Figure 6g,h shows thermal damage to the connective tissue. Total energy about 60 J.

Case 2 (Fig. 7)

A case of a convergent squint was operated on using the laser scalpel (Fig. 7a). The conjunctiva was cut by driving the laser scalpel at a power of 3 W. The lateral rectus was dissected using power in the range of 3-10 W, at pulse durations of about six seconds. (Fig. 7a,b,c) and reinserted (Fig. 7d). The energy required for one complete cut through one muscle amounted to about 120 J. The total energy dose for the entire operation amounted to about 300 J. During the intervention, bleeding was minimal, although not completely absent.

Case 3 (Fig. 8)

This patient presented with blepharochalasis of the left upper eyelid (Fig. 8a). Figure 8b displays the resection of a skin flap without bleeding. In Figure 8c, the cosmetic result can be seen to be good. Power levels used varied from 3-10 W and pulse durations of about six seconds were used. Bleeding was minimal, although not completely absent. Scarring of the skin was also minimal. Total energy about 200 J.

The hemostatic effect of Nd:YAG laser light upon an artery is shown in Figure 4.

Case 4 (Fig. 9)

An eleven-and-a-half-month-old child presented with a hemanioma of the face occupying her left cheek, left lower eyelid, left side of the nose, and left upper eyelid (Fig. 9A). MRI revealed diffuse involvement of all the structures in the left cheek down to and including the maxillary bone (Figs. 9B and C). The patient underwent two sessions of YAG laser photocoagulation, plus direct steroid injection (5 ml

celestone, 5 ml triamcinolone). These procedures produced partial blanching and 25% shrinkage of the hemangioma (Fig. 9D). The patient underwent an arteriogram with superselective embolization under general anesthesia four months before resection. Successful embolization of 95% of the hemangioma was achieved. The predominant vessels to be identified and injected were the left maxillary artery and the right and left facial/lingual arteries. This embolization produced marked shrinkage of the hemangioma. Excision of the hemangioma was accomplished using a YAG laser with a 0.8-mm sapphire

scalpel. Blood loss was 300 ml, and the postoperative healing and subsequent course have revealed total hemangioma removal and a satisfactory cosmetic result (Fig. 9E). The initial hemangioma resection was followed one year later by scar revision with a Mustarde cheek flap being used to resurface the cheek area (Fig. 9F).

Case 5 (Fig. 10)

A capillary vascular malformation was irradiated with a frequency-doubled cw (KTP) laser in the contact mode (Figs. 10a and b).

Ultrastrutural results

Immediate thermal occlusion with the Nd:YAG laser working in the thermal mode is achieved in both venous and arterial vessels and capillaries. This is shown in Figures 11, 12, and 13. Immediate thermal effects are characterized by occlusive obstruction of the lumen of the vessels, immobilization, and, most of the time, disintegration of the blood cells.

Fig. 10. (Case 5). Capillary vascular malformation of the cheek. A: Before; and B: following irradiation with KTP laser. (By courtesy of M.W. Berns.136)

Rationale

Cases 1 to 5 emphasize a wide spectrum of the Nd: YAG laser working in the contact and non-contact modes for the treatment of various applications. Cases 1, 2, 3 and 4 show the excellent cutting and hemo-

Fig. 11. Electron micrograph of a venule of the mesenterium of a rabbit. An immobilized granulocyte, G, and coagulated plasma, P, can be seen in the lumen of the vessels. The plasma of the granulocyte is vacuolated and there is heavy damage to the endothelium, E. The muscularis and the perivascular collagenous tissues, F, show serious heat damage. TEM. Negative magnification x 3400. (By courtesy of E. Van der Zypen.)

F M

Fig. 12. Electron micrograph of the mesenteric artery of a rabbit. The lumen of the vessel is obstructed by coagulated plasma proteins, P. Shadows of the shattered erythrocytes can be seen, Er. The endothelial cells, E, show serious thermal damage and are shattered, and there is moderate damage to the perivascular collagenous fibrils, F, while the muscle cells, M, appear to be ultrastructurally intact. TEM. Negative magnification x 7000. (By courtesy of E. Van der Zypen.)

Fig. 13. Rabbit mesenterium capillary (diameter: 8 µm) irradiated with a free-running Nd:YAG laser (focal spot size: 80 µm; pulse energy: 1 J; pulse duration: 20 msec). Erythrocytes, Er, can be seen among plasma coagulates, P, and have dissolved. The endothelial cells, E, appear to be morphologically intact. TEM x 7000. (Reproduced from Fankhauser et al.123 by courtesy of the publisher.)

static properties of an Nd:YAG laser knife for the excision of a tumor of the face and in cosmetic surgery (Case 4). In Case 2, these properties were utilized in squint surgery. Case 4 shows how the laser scalpel can be used in the non-contact and contact modes to eliminate a large hemangioma of the face. Case 5 shows the Nd:YAG laser working in the frequencydoubled mode (KTP laser) and non-contact configurations to eliminate a small-volume hemangioma.

Conclusions

The Nd:YAG laser operating in the cw mode is well suited for cosmetic operations of the face and its adnexa. Low power levels such as 7-10 W are usually sufficient, except for structures where mass volume irradiation is essential (Case 4, Fig. 9). Apart from such extreme situations, the hemostatic efficiency is limited to vessel diameters equal to or smaller than about 2-3 mm, and larger vessels must be prevented from bleeding either by special laser manipulation37 or by ligation, electrothrombosis, etc.

We did not observe any excessive scarring in a large series of patients. The diode laser is known to be equally as effective as the Nd:YAG laser. The diode laser presently being used for plastic surgery has a dynamic range of 25 W.120 There are differences between Nd:YAG and diode lasers in as far as the absorption coefficient of the diode laser by water is 3.5 greater, leading to a more superficial effect and less hemostatic efficiency in depth.121

It only remains to be emphasized that, although many of these laser systems are used with completely different parameters (i.e., pulse duration, spot size, and power), they can give comparably good-to-ex- cellent cosmetic results, with few complications. Some lasers have more specific vascular effects and some techniques are slower, requiring more skill on the part of the operator. Therefore, there is no way of deciding that one system is unequivocally superior to another. Nevertheless, clear progress would appear to have been achieved with the hemostatic scalpels.

Appendix

Light conduction and diffusion in blood, effect of heat: Improvements in the phototherapy of vascular tumors and neovascular formations require precise consideration of the optical properties of blood. These are necessary for understanding the course in time of thermal changes resulting from irradiation. In the discussion on the propagation of photons, there are two analytical models: a model of multiple scattering and a model of diffusion.125 In this study, we emphasize the diffusion theory. This considers photons as particles that obey the laws of diffusion of the medium.126

Changes in the state of oxygenation, as well as

hemolysis of hemoglobin and coagulation of blood plasma, are the first effects to be observed after irradiation of native blood. Other thermal effects running simultaneously or successively are: the increase of hemoglobin concentration or hematocrit followed by disintegration of the hemoglobin molecule into hem and globin. A simplified approach for describing these processes has been mentioned earlier.40

It is known that the absorption conditions of native blood differ markedly from those of hemolysed blood. Multiple scattering effects complicate the situation of native blood.124 The experiments of Kramer et al.125 with regard to transmission at various wavelengths and layer thicknesses show that transmission through blood does not obey Beer-Lambert’s simple law, i.e.,

I = I0 exp (-αd)

where I denotes the transmitted intensity, I0 the initial intensity, α a loss factor, and d the layer thickness of blood. It has been shown that the losses of power, in contrast to the prediction of the Beer-Lambert relationship, are much greater and do not change as a function of d alone. For example, Anderson and Sekelj127,128 measured the transmission and reflection of fully oxygenized erythrocytes. The hematocrit, H, which is defined as the volume of the blood cells in relation to whole blood, has thereby been varied over a large range. These results confirm the unreliability of Beer-Lambert’s law, in that Anderson and Sekelj’s measurements correspond better with the results of Twersky’s multiple scatter model129 for blood layers thicker than 0.0025 cm.

Loewinger et al.130 measured the transmission of rabbit blood as a function of layer thickness and hematocrit. Their results were in accordance with the measurements of Kramer et al.125 and Anderson and Sekelj.127 Yet, in contrast to these authors, Loewinger et al.130 found that the optical density (OD), defined as

OD = log10 (I0/I)

behaves approximately linearly, as a function of blood layer thickness at high hematocrits.

At very small layer thicknesses, i.e., << 1 mm, the multiple scatter model has proved to be valid,131,132 whereas at layer sizes ≈ 1 mm, diffusion mechanisms must be assumed. Furthermore, Janssen131 has investigated the scattering behavior of very diluted solutions of erythrocytes. With increasing hematrocrit, intense backscattering has been detected. These experiments lead to the conclusion that diffusion processes prevail at a hematocrit greater than 0.5. Finally, the transmittance, T, of blood layers can be measured as a function of the layer thickness, with the aid of light emitted by a fiber that is dipped into blood and detected by a second fiber located nearby. When this experiment is performed

Fig. 14. Relative transmittance, A, versus d for H = 0.08; b = 1 mm; d = 2.25 mm. Relative transmittance, B, versus d for H = 0.40; b = 1 mm; d = 0.66 mm.

at a hematocrit, H, of 0.40, an optical penetration depth of the scattered photon of d = 0.66 mm is measured, while for H = 0.08, d = 2.25 mm is obtained (Fig. 14).

In summary, the diffusion of light rays in native blood depends on a variety of factors, including the photon density of the laser beam, as well as diffusion, scattering and absorption of photons, optical penetration depth of the scattered photons, hematocrit, H, and transmittance, T. For further information, the reader is referred to Kramer et al.,125 Anderson and Sekelj,128 Twersky,129 Welsch et al.,134 and Zdrojkowski and Pisharoty.135

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