The application of laser beams in the treatments of biomedical media has always been investigated by both industry and clinical research laboratories since more than 35 years.
Two major families of commercially available medical lasers differ themselves from the operative wavelength of the electromagnetic energy emitted by the cavity of the device. These are the following (Figure 1): the beams showing “absorption” modalities by the tissue (CO2 lasers-like) in the region of 10 m wavelength and the other ones showing “scattering” characteristics into the tissue (YAG lasers-like) in the region of 1m. The first one shows strong cutting and haemostatic characteristic especially in low-water content conditions, while the second one shows strong concentration (“pile-up”) of energy below the surface with consequent heating (for pure thermodynamic applications), cutting and haemostasis of the irradiated volume in the depth of the same. One key aspect always to be considered is the combination of: the water/liquid content of the irradiated tissue, the biochemical/anatomical structure of the tissue, the beam wavelength and the emitted energy on the spot (power density - Joules/sec.cm2 ) located on the surface to be treated. Therefore, one obvious goal is to effectively control this mix in order to reduce lateral thermal damages and avoid uncontrolled large cuttings or carbonization effects.
Figure 1. CO2 versus YAG Laser Beam-Tissue Interactions
The evolution of the clinical usage of the laser is here pressented, providing also an overview of the main technological milestones and an outlook to future challenges for MIS / UCMIS.
The first ruby laser device, following the theoretical investigation by Albert Einstein.
In 1917 , has been constructed by Maiman several years later  and later in the 60’es used to treat dermatologic pathologies [3,4]. The world of Medicine, however, had to wait another 2 decades to see the laser beam successfully implemented in the treatment of the musculoskeletal system : in 1981 the CO2 laser has been tested in arthroscopic surgery  and in 1987  to remove herniated spinal disc tissue.
Some disappointing results were also reported during the early utilization of the neodymium: YAG and argon lasers lasers in meniscectomy causing cartilage degeneration [7,8], leading to the first widely recognized successes of the CO2 laser in arthroscopic surgery  and meniscectomy . This type of laser got the FDA approval for these surgical applications in the mid 1980s. In parallel, several YAG laser types were introduced: erbium: YAG, holmium: YAG, thulium:YAG and others [11,12]. Although their wavelengths are similar within +/-0.5m, the effects on tissue compared to the original neodymium:YAG are different : ultimately, from the late 80’es on, the 2.1 m wavelength of the holmium: YAG laser and the CO2 laser at 10.6 m are still the two most accepted devices for arthroscopic surgery [14,15]. Other wavelengths tested in the treatments of the knee, the capsule of the shoulder and other smaller joints, such as the KTP laser and the excimer laser, reported also positive results in clinical laser-assisted arthroscopy [16,17]. Spinal disorders, such as spinal disc hernia and relative complications including postdiscectomy syndrome, suggested the implementation of less invasive techniques [18,19] via non-endoscopic and endoscopic laser-assisted procedures using CO2 , neodymium: YAG, holmium: YAG and KTP lasers . These techniques are now routinely used for intra-discal and blindintra-discal methods via endoscopic approaches . Additionally, the holmium laser at 2.1 microns wavelength is gaining wide acceptance among orthopedic surgeons as a useful arthroscopic surgical tool. This laser type provides a minimal amount of thermal necrosis and is able to cut and ablate tissues with great ease.
Based on the considerations above, the combined hygienic, ablative and haemostatic effects of the laser beam have encouraged physicians to move forward, especially in orthopedic surgery (low -water / liquid content – Figure. 2), keeping an eye on the conservative aspects of the procedures via low-energy delivery on the laser focal spotfocused on the bone tissue. Depending on the type of bone structure, the resulting therapeutic effects are different . Cancellous and compact bone types can be defined as follows: compact bone, or cortical bone, is very dense, with almost no holes in its structure, while spongy bone, or cancellous bone, is much less dense, and it is typically very porous, with several large cavities. Both types contribute to the stability and robustness of the skeletal system while providing a good mix of flexibility and lightness. The thermodynamic respond to the irradiation of laser beam is different due to the separate modalities of heath spread inside different bone structures.
Current investigation with laser tissue agglutination and tissue welding may make it possible to repair torn tissues with laser energy . In the past, Canestri has published several studies [23,24 and Cross References] on both mechanical and thermodynamic measurements of models for laser beam spreading in biological tissues. Also media which simulate the characteristics of a given biological substance exposed to laser radiations can be used for clinical investigations in-vitro, such as the poly methyl methacrylate (PMMA). The PMMA is a polymer that has several applications in medicine and biology: its thermal properties and characteristics are very similar to the ones of human and animal compact tissues (examples: compact bones, dentins ). This makes it as ideal tool for both direct clinical utilization (examples: surgery in orthopedic applications, dentistry ) and research activities as simulation medium for human tissue, in particular for the low-water content ones. It also allows capturing craters and cuts produced by a laser-beam for further geometrical investigations. One of these research applications relates to the study of thermodynamic models which describe the heat transfer patterns across biological media. Here the ‘source of heat’ can be any device which provides heat generation, including also medical lasers .
Figure 2. Structure of compact and spongy bone.
The following 4 parameters play key roles in the quality of the outcome of the laser therapy :
1) the lateral “size” of the beam (“Transverse Electromagnetic Mode” or TEM–Figure 3) regulating ultimately the size of the focal spot where the laser beam energy is focused on the surface to be treated; 2) the total time duration of the exposure of the tissue to the beam itself; 3) the modality of the beam delivery, which can be either configured in continuous- or pulsed-mode and 4) the amount of beam energy in Joules or Watt (Joules / sec.) delivered to tissue.
Figure 3. TEM-dependence of the resulting crater shapes on PMMA at the on-set evaporation. After longer exposure time, all these three profile will evolve into a conic shape.
Figure 3. Another technical challenge is the quantification of the crater and cut generation processes which bring a lot of new insights into both thermodynamic and mechanical phenomena of laser irradiation. All this guarantees good quality outcomes to ensure good safety margins for the patient. A unified approach which correlates and takes into account all the complex thermodynamic interrelated phenomena taking place during the production of laser beam craters in low-water-content tissues must be defined. The optical absorption coefficient of the PMMA material is a key element for this approach. Currently, the data available on recent literature  allow to identify the numerical value of the optical absorption coefficient (cm-1 ) of biological media with enough, and therefore satisfactory, accuracy. Additionally, the correct identification of the optical absorption of PMMA allows isolating with better accuracy other key time-dependent coefficients, such as the relaxation time, the volumetric threshold time and the heat incubation time . Thanks to these elements, several clinical applications can be better accessed and analyzed with more precision. For example, Scholz etal.  claim the fact that colorant dopants of the bone cement strongly bound to the PMMA, and that, in case the absorption coefficient of the PMMA would be larger than 500 , the dopants themselves could be reduced or abolished completely in the preparation of the mixture of the bone cement itself. This would then allow reduce the trauma to the bone caused by high percentage of colorants which occur during the removal procedure of the cement using laser techniques. Based on these and other considerations, several mathematical aspects of these analyses have been already published in the past by Canestri by both isolating the LCA(“Laser Characterization Algorithm”) model and numerically solving the Canestri-Langerholc equation , both describing the geometric evolution of the early phase of the crater evolution which develop itself from a cylindrical geometry onto a conic one (Figure 6).
Figure 6. The Relation Between real and calculated (geometrical) generatedb volume versus time.
The LCA Algorithm by Canestri  predicts the existence of a ‘volumetric threshold time’ t1b which is linked to the production of a the first measurable volumetric crater V1b (Fig.6). As shown in Figure 4, 5 and 6, several important parameters have been identified as part of the thermodynamic process regulating all the phases from the on-set to the development of laser craters or cuts into biological or quasi-biological in-vitro media [23, 24, 30]. This is one contribution to Minimally Invasive Surgery (MIS), which is a type of surgery aiming to minimize the size of surgical incisions. This type of surgery is performed using thin-needles combined to endoscopes to visually control the surgical operation via several smaller incisions rather than more radical and larger ones. The goal of MIS is to reduce postoperative pain, speed recovery, minimize blood loss and reduce tissue scaring : the LCA Algorithm is an ideal approach to all this.Additionally, the most important principle of Ultra Conservative Minimally Invasive Surgery applications (UCMIS) is the quantification of the smallest possible crater and cut geometries to reduce as much as possible the risk of potential unwanted damage during operation and to maximize the patient’s comfort after surgery . Examples are the treatments in orthopedic applications of fine human bone structures, in neurology the micro dissection of nerves and in some conservative procedures also required in general surgery. In order to define a unified theory which addresses all the complex correlated thermodynamic phenomena taking place during the production of laser beam craters in low-water-content tissues during MIS, more investigations about minimal delivery of laser energy into tissue are needed.
Currently, the data published by Canestri  on Literature report numerical values about the smallest focal length and the relative CO2 laser beam spot size with still surgical with reduced side thermodynamic damage at 10.6m. The procedure to obtain both parameters has helped to further improve the overall quality of UCMIS protocols via endoscopic scalpels. These use both mechanical focusing heads and fiber optic instrumentation to deliver ablative energy on tissue (Figure 7).
Fig. 4 : The laser parametersset-up for the absolute UCMIS limiting crater profile for 0.013“ focal at Wmin . Due to the limiting conditions, tR = 0 for the second pulse on.
Fig. 7 : Computer system components for laparoscopic remote control
The quantification of the overall conditions to avoid small injuries is mathematically determined by using several laws of thermodynamic in conjunction to the mechanical interaction of the laser beam with the biological media being irradiated. Additionally, these minimal conditions are dependent on the type of surgical operation required case by case and on the type of exposed tissue. These concepts are important for both the industry and the surgeons communities: the first one can better design future medical equipment based on the calculated physical limits intrinsically present in each operative procedure, while the second one can better foreseen the boundaries of a given surgical operation during the planning phase of the same and consequently better estimate all the associated risks.
UCMIS brings lots of new insights into both early thermodynamic and mechanical ablation phenomena associated to the smallest possible thermal injury and avoidable collateral complications. Examples are the treatments, in orthopedic applications, of fine human bone structures, in neurology the micro dissection of nerves and in general surgery the generic treatment of other small anatomical structures as required case by case.
The details of clinical identification tools designed to : a) precisely quantify and forecast the ablation capabilities of the CO2 laser beam, b) to optimize its usage in Operating Room and c) to particularly address all the safety issues related to surgical interventions on human tissue exposed to = 10.6 μm radiation of CO2 laser-beams has been reported. As seen in other Studies by the same Author, the correct identification of the optical absorption of PMMA allows investigating with better accuracy other key timedependent coefficients, such as the relaxation time, the surface threshold time and the heat incubation time. These ones are all described on Literature in rather qualitative than quantitative fashion.
Several tests on other biological media have been performed and suggested as potential contributions for Minimal Invasive Surgery (MIS) procedures. Other tissues and power densities can be carefully but meaningfully interpolated in order to achieve a wider understanding of the interaction processes, such as with TMLR protocols (Trans-Myocardial Laser Revascularization) using MIS  : the liquid content of the myocardial muscle reduces absorption, resulting in effective treatment with quite limited collateral thermal damages. Unwanted side effects, such as tissue necrosis, have given further incentives to develop even less invasive laser procedures, such as the excimer cold laser  : however, one has to point out the fact that the low ablation rate of this and other similar lasers in the near-infrared region of the electromagnetic spectrum reduces their effectiveness in orthopedic surgery. More effective lasers using low power energies (heliumneon, gallium-arsenic and argon lasers) are used to treat rheumatoid arthritis and tissue welding via photodynamic therapy [33,34]. Very optimistic outlook can be found in several laser minimally invasive surgical procedures of the vertebrae and spinal cord. It is important to mention here the laminotopy and foraminotopy procedures. The lamina is the part of a vertebra that helps protect the spinal cord : several pathological conditions can create impinging material intruding the spinal canal, causing pressure on the spine itself, resulting into pain and discomfort. The foramina are a channel through the walls of the vertebrae of the spine that allows nerve bundles to connect the body to the spinal cord: these inter-vertebral passages can also suffer from pathologies which increase the pressure on the nerve bundles themselves. An arthroscopic procedure allows the laser tip to be placed in situ and remove bone or discs material from the lamina and from the foramina to relieve the symptoms of nerve root compression or pinched nerve. In general terms, endoscopic thermal ablation and minimal invasive stabilization will play more and more a significant role in the usage of the laser beam in surgery.
There is still room for improvement in the surgical methods and in the design of the instrumentation of the laser systems: many questions on the biophysics, biomechanical and biochemical interaction between the laser beam and the biological tissue remain unsolved. These open questions represent a potential source of unwanted side effects and complications in the routine utilization of the medical lasers for both conservative and current surgical therapy of the musculoskeletal system: do lasers in MIS always represent an alternative to traditional methods? Can thermodynamic side effects always be controlled? Can further technical evolution of medical lasers ensure the growth of their utilization in surgery and related medical therapies?
An obvious goal is therefore to develop more focused research programs focused in all these directions in order to increase the overall effectiveness and safety of modern medical lasers during routine clinical treatments.
The Author would like to thank the University of Tel Aviv and the Sackler Faculty of Medicine for the support given to him during the years 1993 and 1994 (measurements and data acquisition with Gaussian and non-Gaussian laser beam profiles on in-vivo / in-vitro samples) and for the continuous contact and exchange of information since then.
5) Whipple T. Application of the CO2 laser to arthroscopic meniscectomy in a gas medium. Meeting of the International Arthroscopy Association, American Arthroscopy Association of North America – Rio de Janeiro, Brazil – 1981.
6) Choy D, Case R, Ascher P. Percutaneous laser ablation oflumbar discs. A preliminary report of in-vitro and in-vivo experience in animal and four human patients”, 33rd Annual Meeting of the Orthopaedic Research Society. 1987, 1: 19.
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