• General Information

Effects of laser on subdermal fat tissue

Dan Mon O´Dey, A. Prescher, R. Poprawe, S.Gaus, S.Stanzel, N. Pallua. Ablative Targeting of Fatty-Tissue Using a High-Powered Diode Laser. Lasers in Surgery and Medicine 40:100-105 (2008)

• He aim of this study is to evaluate the thermal effects of the interaction of fat tissue of a laser delivery within the near-infrared and in reproducible situations.

• To this end, a study was performed based on the optical spectrograph of the fat tissue before high power diode laser delivery at 940 nm and the effect of its delivery is studied on 59 fresh corpses in non-contact mode. The histological effects are assessed through a computer system based on the following measurements and parameters:

A- Ablation Rate - AR

B- Cavity diameter ratio / Collateral damage ratio (CCDratio)

• Results show that from 250 to 400 w/cm2, a greater ablative effect and less collateral damage is produced in the tissue. So that an irradiation of 270 w/cm2 displays a CCD ratio of 2:1 and an AR of 9.98 ± 7.65 mm3/sec.

• The authors conclude that the laser used is susceptible to ablation in human fat tissue and the efficiency and safety should be improved for this procedure.

• Soft tissue repair often requires the use of flaps that on occasion need a major slimming procedure, the handling of a laser tool is not indispensable but it could be of interest to improve knowledge of the effect of diode lasers on fat tissue. Diode laser could be of interest for several reasons, the fibre could be guided to work through endoscopy, that could work with power intensities of 100 to 10,000 w/cm2 to produce a heat effect, preventing photochemical interaction, ablation induced by plasma and the photo ablative interactions at higher densities as well as the photo disruption effects. As well as tissue penetration of several millimetres, low production cost and high efficiency. Based on the study of the optical properties of fat, this study is performed with a 940 nm diode laser output.

• Emissions on 59 female corpses are performed before 48 hours after death, and the irradiation employed is from 93 to 1579 w/cm2, based on 100-500 W and a spot size of 0.17 to 1.08 cm2 and total delivery times of 30 to 90 seconds.

• Then, over 1,000 samples are performed and stained with Masson-Goldner trichrome staining, highly interesting for connective fat tissue staining. The measurements taken to quantify the heat effect on the fat tissue are performed at different depths for each cavity generated and include measurements of the:

1.- Cavity width
2.- Cavity depth
3.- Carbonisation fringe
4.- Damage fringe in moderate dermal collagen
5.- Collateral damage
6.- Cavity volume
7.- Ablation degree (mm3/sec), vaporised fat volume per second
8.- Ratio between the cavity/diameter of collateral damage

Evaluation of the optical properties of fat tissue

• Performed on lipoaspirated specimens after elimination of the haematic stage and free fat, pure fat samples are analysed (BFT) as well as non-blood tinged fat tissue (NBFT) and absorption property of water.

• The highest absorption peak was found at a visible light wavelength of lambda 460 nm, the other two absorption peaks were lower, seen at wavelengths within the near-infrared range of 930 and 1,030 nm. Water revealed an absorption peak at a wavelength of 975 nm. The operating wavelength of the high-powered diode laser used, is between the secondary peaks of fat and water.

Evaluation of laser ablation of fat tissue

The specimens presented characteristics of collateral damage, vacuoles and condensation areas that surround the cavity caused by the tissue ablation. For emissions of 1,300 W/cm2, a major ablation area is produced and a fairly larger index of collateral damage, it therefore seems that very high irradiations cause greater heat conduction and collateral damage in proportion with the total ablated fat. This effect could be due to that an increase in power absorption occurs in the fat tissue and a decrease in heat transmission. It can then be deduced that in the fat tissue, the changes in its optical properties can reduce the ablative capacity of the diode laser.

• For emissions of 360 W/cm2, the ratio between the degree of ablation and the collateral damage is 3:1, while for emissions at 1,300 W/cm2 the ratio is 1:2.

• For emissions of 360 W/cm2, the AR ratio (elimination of fat volume) by time is 9.98 ± 7.65 mm3/second.

• According to these parameters, the following is deduced:

A- With 360 J, 10 mm3 are eliminated and with 1 kJ, 27.7 mm3 of fat is eliminated. 36 kJ are needed to eliminate 1 cm3 of fat.

B- Delivery at 370 W/cm2 corresponds with a total power of 58.96 ± 41.38 joules/mm3.

The authors confirm that irradiances from 250 to 400 W/cm2 may reflect the ideal power setting for fat tissue removal according to the conditions examined.

Concerning the statistical results, there is a poor correlation between the irradiance and power density with the dependent variables. However, a statistical relation between these parameters and severe collateral damage and the stained area with total collateral damage has been found.

Exposure time displays a significant relation between AR (ablation ratio per second), the width of the generated cavity and the total volume of the generated cavity.

The irradiance and density of power show significant statistical relationship on the CCD ratio, ratio between the cavity diameter and the diameter of collateral tissue damage.

No statistical relation has been found between the irradiance, power density or the delivery time with collateral tissue damage.

Discussion

• The authors state that fat does not respond as homogenously as other tissue such as muscle, to laser energy. The heterogeneous response of fat to laser energy depends on energy requirements of phase changes of water from liquid to steam, tissue desiccation formed by rapid expansion of steam vacuoles, fat liquefaction and vaporisation, and changes in optical properties of fat tissue.

• Concerning the study at 940 nm, this wavelength achieves and increased absorption of both water and fat tissue while maintaining a penetration depth of several millimetres into the tissue. To obtain constant ablation, an indispensable requirement is to maintain unvarying optical properties of the tissue. Transmission is complicated in the fat tissue by carbonisation decreasing penetration depth and increasing surface temperature. Carbonisation promotes a mismatch between ablation and the effect of collateral damage in favour of larger area and intensity of the collateral tissue damage.

• The authors confirm that the low water content of fat, measured at 35 ± 6 ml/mol triglyceride and its very high flash point, at some 189 ºC, makes high energy necessary for vaporisation of fat. However, vaporisation of water begins at temperatures above 100 ºC and therefore reflects the most important part of the ablation process. The proportion of localised water deposits within connective tissue septa, containing approximately 70% of water, may be responsible for both the side effects observed on the septalised zones and that of stained collateral tissue damage. Such deposits show an increased affinity to conducted heat than surrounding fat cells which contain less water.

• They also confirm that pulsed lasers are proposed to be beneficial by reducing total energy spread. Collateral tissue damage cannot be avoided. However, improved efficiency of laser ablation should result from a wavelength matching the absorption properties of the tissue. Finally, it seems possible to achieve ablation of the fat tissue through diode laser with higher effectiveness in the ablation volume/second and less collateral damage.

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