PhotoDynamic Diagnostic (PDD) and PhotoDynamic Therapy (PDT) are modern and very promising methods of diagnosing and treating cancer. They were used in clinical examination for the first time by T. J. Dougherty. He used them mainly to treat squamous epithelium carcinoma of skin, melanotic cancer, mycosis, and Kaposy's tumors.
PDT treatment allows for selective elimination of cancer tissue without any harm to the healthy cells and tissue. The PDD method, which utilizes properties of stains that glow in laser light, can be used for detecting and pinpointing cancer and pre-cancer lesions in very early stages when they are still small and therefore impossible to detect by traditional methods. PDD also makes it possible to precisely locate places where it is suspected cancer processes take place, which in turn allows for specimens to be taken for histopathological examination and the range of the lesions to be precisely determined, which is useful for operation planning. The process of conducting the diagnosing method and the photodynamic therapy is presented in the figure 14.
The first step in PDD method is injection of sensitizer (a chemical substance, e.g. derivative of HpD). It causes biological tissue to be sensitive to light of particular wavelength. After injection it
is distributed throughout the body and after 12 to 24 hours (depending on the sensitizer used) it is evacuated from the healthy cells and accumulated exclusively in the pathologically changed ones. Next, the appropriate wavelength of the radiation is chosen taking into consideration the absorption properties of the sensitizing substance. What should be also considered is the depth to which tissue can be penetrated by the radiation of particular wavelength.
The cancer lesions are diagnosed through the activation of the sensitizing substance by laser light the sensitizer glows fluorescently in that light. The fluorescent light is usually red. For example, irradiation of cancer tissue saturated with HpD sensitizer causes red fluorescence of 670nm, easily visible to human eye.
In cancer therapy the lesions, in which sensitizer is accumulated, are irradiated with laser light. In the case when the solution of HpD is used the wavelength of light used to irradiate the diseased place should be 630nm. This particular wavelength can penetrate the soft tissue to the greatest depth and it corresponds to the solution's absorption. This relation is displayed in the diagram shown in the figure 14. On the other hand, the wavelength adequate for another solution, for example PHOTOFRIN II, will be ë = 650nm, which is also characterized by red light.
The following figure illustrates the conduction of the PDD diagnosing procedure and the PDT therapy:
The scientists are acquainted with many substances used as photosensitizers. Unfortunately, some of them may cause damage in healthy cells. For this reason for PDD and PDT goals only those photosensitizers can be used which are evacuated from the healthy cells very fast, but at the same time can be accumulated in cancer tissue. It is also necessary that the photosensitizer decomposes when acted upon by the laser light photons of particular energy. In the photochemical reactions the singleton oxygen or other free radicals, which can eliminate the saturated with sensitizer cancer cells, must be released.
Recapitulating, it can be said that the spectral characteristic of sensitizer absorption and the ability of radiation permeability through the main components of tissue define the requirements for the source of laser light in the PDT method. In the case when hematoporphyrin is used as the sensitizer the optimal wavelength of the laser light used for pinpointing the pathologically changed tissue should be 405nm, and for eliminating the cancer cells on the deepest possible depth - 630nm (red radiation). The additional condition that has to be met by the laser light is radiation power density suitable to the above mentioned wavelength.
 |
| Fig. 1. The PDT method. |
Threshold power density required for starting the photochemical reaction with photosensitizer is 0.025W/cm2. For this reason, depending on the clinic form of cancer, the density of power used ranges between 0.05 1.0W/cm2. The time of irradiation needed for the method to be effective ranges between 3 and 60 minutes. Consequently, the level of laser light dose of energy that can be employed varies between 50 and 500J/cm2. For small cancers and for their massive changes reduced levels of energy dose are used. Nevertheless, the doses of energy should be individually matched for each case.
The power density of 100 200J/cm2 is used in early stages of malignant cancer. When dealing with small (up to 1cm) flat cancers without malignant infiltration the power of 100J/cm2 is sufficient. The basiliar cell cancers are treated using the density of 200J/cm2; the squamous cell, metastatic ones: 300J/cm2. When dealing with recurrent, vast cancers the power density can be raised to 350 400J/cm2.
It has been proved that photo biological changes in cancer tissue when photodynamic therapy is used depend on histopathological type of the cancer considered. The best effects have been achieved with basiliar cell skin cancer for total elimination the sufficient power density was 100 - 150J/cm2. Typically, the eradication is completed right after the therapy is concluded or in 6 hours thereafter. For full elimination of adenoma and squamous epithelium cancer the power density required is 100 200J/cm2. In this case full elimination is accomplished in 3 4 days after irradiation of the pathologically changed tissue. When treating melanotic cancer only modified photodynamic method seems to be effective: during the first 24 hours after the injection of sensitizer the diseased place is irradiated with light of 20 50W/cm2 power density up to the point when 1/4 of the energy required for the total elimination of the cancer (400J/cm2) is achieved; during the following 24 hours melanotic cancer is irradiated with power density of 150 300W/cm2 (up to the point when 1/2 of the total energy needed is reached); in the third consecutive 24 hours the cancer is irradiated with light of power density of 150 300W/cm2, raised till the quantity of 400J/cm2 is attained. The total elimination of melanotic cancer is accomplished after 7 10 days.
 |
| Fig. 2. The conduction of the PDD diagnosing procedure and the PDT therapy. |
PDT can be combined with traditional malignant cancer treatments: operation, radiotherapy, or chemotherapy. The phototherapy gives best effects when applied in early stages of the disease, when operation does not come into play yet as well as in the cases of radio- and chemo resistant forms of cancer. Among the advantages of the photodynamic method are its efficiency, non-toxicity and non-invasiveness. It can be used for treatment of extensive cancers of diverse configurations as well as
of cancers situated in inaccessible locations. The therapy can be used in cases of cancers resistant to traditional treatment. It can be repeated many times. It is especially suited for elderly patients. The laser beam here can be guided by a flexible optical fiber inserted into an organism through an endo-scope.
As it was mentioned before, the greatest advantage of PDT is its selectivity. On the other hand, when the case of cancer cells scattered throughout the organism is taken into consideration then this method is of not much use. In a few days time after the sensitizer is injected undesirable effects may appear, e. g. blanching of skin, and later on irritation and oedema. Oversensitiveness to sunlight has also been recorded. For this reason patient should be protected against direct light.
Lasers applied in prosthetics.
This chapter deals with processes and technologies of applying lasers in welding dentures and orthodontic devices. At present the use of soldering and gluing techniques of joining dental elements is widely popular. The imperfections of these methods are strongly connected with the negative influence of the solder and the glue components on the oral cavity and the digestive tract. Also the following flaws are of great importance:
- insufficient joint resistance,
- possibility for the joined elements to be deformed by heat,
- diminishing of the elements' mechanical properties when vast areas are heated,
- low corrosion-resistance.
The laser technique of welding dentures and orthodontic materials is to the great extent free from the above listed imperfections.
A) Laser welding of ceramic materials
A CO2 molecular laser system is used for joining ceramics. It is connected with the fact that the radiation beam of 10600nm wavelength generated by the laser is almost entirely absorbed by the layers of the ceramic components.
In addition, it is possible to obtain a smooth (polished) surface of the dental ceramics in the area affected by the CO2 laser defocused beam. The surface quality is better than in the case when traditional methods are used.
In practice a modified laser welding must be applied because strong local heating of ceramics
with the laser beam may result in local cracs and stresses. This method deploys preceding heating
of the targeted area. The scheme of the process is presented in Fig.17.
 |
| Fig. 3. Scheme of laser welding of dental ceramics with prior laser beam (W2) heating up of a surface. |
The welding laser beam (W1) covers the area approximately ten times smaller than the laser beam (W2) that heats the ceramics before the welding action takes place. The laser beam (W1) operating in the continuous or pulse mode then welds together two M1 and M2 materials previously heated by the defocused pulsed laser beam W2. Thanks to this process the structural thermal press-es of the material are noticeably reduced. The risk of the material cracking is also diminished.
Ceramic welds formed in such a manner have homogeneous structure without any visible mat-erial inclusions. The scientific research has proven their higher density in comparison to the joined materials. Thanks to that fact ceramic welds formed in this method have higher bending strength than the preform (original) materials.
The examples taken from the research results examining dental and organic materials joined with the laser welding are shown in Fig. 18a) and 18b).
 |
| Fig. 4. The comparison of bending strength between: a) homogeneous dental ceramics and laser welded ceramics. b) homogeneous organic ceramics and laser welded ceramics. |
In the case of organic ceramics welds have been reported to have only slightly higher bending strength than the preform material. The dental ceramics strength was raised by about 20% as compared to the preform material. Laser welding of ceramics gives excellent results. Apart from that the laser system made it possible to remove local surface defects of the material, making it more homogeneous and smooth. This fact is extremely important for the dental and orthodontic adaptations.
B) Welding of dental metal alloys
The efficacy of laser welding depends on the material's ability to absorb laser radiation of a spe-cific wavelength. Metals and their alloys alike usually do not absorb well the far infrared light radiation generated, for example, by the CO2 lasers. For that reason the Nd:YAG lasers that generate radiation generated, for example, by the CO2 lasers. For that reason the Nd:YAG lasers that generate radiation of 1064nm wavelength are excellent for durable welding of metals and their alloys.
For the research from a group of precious metals silver and palladium (Sipal) alloys have been chosen. And from the group of common metals the following alloys were chosen: cobalt-chrome-molybdenum (CCM), nickel-chrome-aluminium (NCA) and pure titanium.
The Nd:YAG laser system used in the research was equipped with regulation of: pulse width ranging between 2 to 5msec, pulse radiation energy ranging between 1-8J and the pulse repetition rate ranging between 0.1Hz -10Hz. The final results of diameter and joint depth dependence on pulse energy are shown in Fig. 19a) and 19b).
 |
 |
| Fig. 5. Nd:YAG laser pulse energy influence on the results of joining four typical dental materials: a) on weld diameter b) on weld depth. |
The analysis of the research results revealed that the maximum difference of the joint diameter did not exceed 0.4mm (Sipal and NCA). The joint depth, on the other hand, depended more on the absorption coefficient and was the biggest in the case of titanium. The depth reached 1.2mm when the 8J pulse energy was used. The typical laser welding parameters for dental metal alloys are pre-sented below in the Table 3.
 |
| Table 1. Laser radiation parameters for efficient typical dental metal alloys welding. |
The quality of laser joints and soldered joints was compared. In order to make a comparative evaluation of various material joining methods the following criteria were chosen: the homogeneity of material structure after joining, weld homogeneity, weld depth and width, weld's bending and tensile strength.
The comparative evaluation research of laser joints and joints made with soldering proved the former to be superior. For example joining CCM and CCM with laser proved that such a joint has more than three times higher tensile strength, and more than two times grater bending strength than in the same joint made with the use of soldering.
The resistance to corrosion of the welded joints is the essential criterion of laser welding use-fulness of laser welding for dental materials. It is commonly known that in the oral cavity the innate materials and the artificial ones comprising various dentures are exposed to different destructive factors. Apart from that the prosthetic materials formed with the use of any of the joining method (welding, soldering, gluing, etc) can cause allergic reactions and introduce toxins into the organism.
In contrast to traditional soldering methods laser welding of prosthetic materials provide better "biocompatibility" of the joints. This fact is connected with higher resistance to corrosion of the jointed areas.
In order to estimate their level of resistance to corrosion the dental materials joined with laser technique and the comparative material joined with solder were placed in a 0.1mol solution of lac-tic acid and sodium chloride with pH=2.4 and the temperature of 37°C for two days period. The microscopic research confirmed that corroding of laser made welds was postponed. In addition an analysis of oxygen content accelerating the process of corrosion for Sipal, CCM and NCA alloys was conducted. Even though the oxygen level for the initial materials was similar, the content of oxygen in materials in their transient phase was 2 times lower (Sipal), and in laser weld it was 3-4 times lower than in the weld made by soldering.
When compared with soldering laser welding caused also negligibly small changes, in the content of every alloy that was examined.
Therefore laser welding guarantees higher mechanical resistance of joints than it can be expected with traditional methods. And also thanks to its high corrosion resistance it guarantees better coexistence of the joints with the environment of the oral cavity.
C) Titanium welding with the Nd:YAG laser
Titanium is a metal characterised by minimal tendency for corroding. For this reason it is used in vivo (within the living organism), especially for implants. It also becomes more popular in pros-thetics too (bridges and crowns). Because of its high melting temperature (ca. 1.700ºC) the tradi-tional method of joining titanium elements are strongly limited. This is the reason the effective joining of titanium elements can be achieved by means of high power density technologies: for example electron beam, plasma arc or laser welding. The greatest practical meaning has the laser welding technology. It ensures among others:
. high durability of joint and high corrosion resistance because of not changing the material's
structure;
. reducing the material's oxydation when argon shield is used during the welding procedures;
. minimalizing the area on which thermal effect is exerted by melting material with short laser
pulses;
. gaining grater precision of joining than with the traditional methods.
Recapitulating, it could be said that laser welding of the materials used in prosthetics and orthodontics is free from above mentioned imperfections of "classic" soldering and gluing. Laser welding of dental materials has the following advantages:
. the possibility of joining different dental materials, for example high-melting titanium of different thickness;
. the homogeneity of joined materials within the weld area, without changes in structure of
alloys,
. high resistance to corrosion of laser welds and thus better homeostasis in the oral cavity,
. very high welds endurance, even 4 times higher, than in the case of soldering welds,
. non-contact extreme precision in joining procedure, with only minimal overheated area and
minimal material stresses,
. very high speed of joining dental materials, time saving and economic as far as expensive
joining materials e.g. solder are concerned.
Teeth whitening has become one of the faster developing fields of aesthetic dentistry. Traditionally, the teeth whitening substances available in drug stores often prove ineffective, they have to be applied for a very long time, often even for several weeks, and the outcomes frequently are unsatisfactory.
Because of the ever present aim of arriving at the fastest, the longest lasting, and the most effec-tive methods the scientists also here finally turned their attention to laser.
The colour of teeth is the resultant of the combination of natural teeth pigmentation and of forming discolourments thereon throughout one's life. The discolourments are connected with the presence of chromophores in human mouth. They can be of organic or inorganic nature, but they can also be caused by other factors. The pigment substances (the chromophores) constitute of non-saturated double bonds. When the chromophores are oxidized the non-saturated bonds become saturated. At the same time the chromophores turn lighter in colour or they even become transparent.
The active bond responsible for oxidization of the chromophores and the whitening effect on teeth is hydrogen peroxide (H2O2). It can be found in different preparations by itself or in combina-tion with urea in the form of carbamide peroxide. The traditional whitening solutions (usually in the form of gel) contain about 10% of hydrogen peroxide.
In laser teeth whitening method the whitening gels used contain greater concentration of the H2O2, namely 30% or even 35%. The laser light's role in this method is to accelerate the activation
of the hydrogen peroxide which, in reaction to the absorption of photons, breaks down into a particle of water and radical of oxygen. The free oxygen radicals cause the oxidization of the chro-mophores particles, which are part of the teeth enamel. The utilization of appropriate laser light (wavelength, power, power density and energy density) in this procedure is responsible for achieving an outstanding outcome after only 20 minutes. What is interesting and important about the laser teeth whitening method is the fact that it can be accomplished during just one dental appointment.
The laser teeth whitening method is absolutely save. It is not thermic - it uses special gel, which is activated by laser light of minor power density. Moreover, this procedure is painless, and the only undesirable effect experienced during or after the surgery is the over sensitivity of gums, which can be caused by some of the chemical components of gel. Nevertheless, the gums can be protected by special liquid preparation. The typical full procedure of laser teeth whitening is demonstrated on the following photographs (Ill. No 16).
 |
| Fig 6. The full sequence of laser teeth whitening showing the order in which this process is carried out. |
References
1. Bringmann W. Lasertherapie-Licht kann heilen. Berlin 2002.
2. Bürger F. Lasereinsatz in der ästhetischen Zahnheilkunde. Laserzahnheilkunde Einsteiger-Handbuch 2003.
3. Catone G.A., Alling Ch.C. Laser Applications in Oral and Maxillofacial Surgery. W.B. Saunders Company, 1997.
4. Frentzen M., Koort H.J. Photoablation of dental tissues: a new method for caries removal?, J. Dent. Res., 1989; 68: 1982.
5. Grevers G ai all. Laser im Kopf-Hals-Bereich. Springer-Verlag Berlin 2001.
6. Gutknecht N. Lasertherapie in der zahnärztlichen Praxis. Auitessenz Bibliothek, 1999.
7. Henderson B.W., Dougherty T.J. Photodynamic Therapy-Basic Principles and Clinical Applications. Roswell Park Center Institute
Buffalo, New York, 1992.
8. Hopp M. Laserassistiertes Bleaching. Zahnarzt Praxis International, No 6, November 2001.
9. Kert J., Rose L. Clinical laser therapy. Low level laser therapy. Scandinavian Medical Laser Technology, Copenhagen 1989.
10. L. Clayman at all. Oral and Maxillofacial Surgery Clinics of North America. W.B. Saunders Company, Volume 9, Number 1,
February 1997.
11. Miserendino L.J., Pick R.M. Lasers in Dentistry. Quintessence Publishing Co, 1995.
12. Pokora L., Puzewicz Z. Excimer laser system for diagnosis and therapy of cancer. The International Congress on Optical Science
and Engineering, The Hague, The Netherlands. Proc. SPIE 1503,53 1991.
13. Proc. of the 2nd International Congress on Lasers in Dentistry presented by the International Society for Lasers in Dentistry, Paris,
France, May 28-31, 1990.
14. Proc. of the 3rd International Congress on Lasers in Dentistry presented by the International Society for Lasers in Dentistry, Salt
Lake City, USA, August 6-8, 1992.
15. Romanos G. Atlas der chirurgischen Laser-zahnheilkunde. Urban & Fischer, München 1999.
16. Spinelli P., Dal Fante M., Marchesini R. Photodynamic Therapy and Biomedical Lasers. Excerpta Medica. International Congress
Series 1011, 1992.
17. Vahl J. at all. Laser in der Zahnmedizin. Quintessenz Verlags-GmbH. Berlin 1992
18. Willenborg G.C. The future of lasers in clinical dentistry. Laser Institute of America International Conference on Applications of
Lasers and Electro-Optics, 1989; 64 :170-183.