Biology and the Design of Surgical Laser Systems


Doug Stinson (was Director of R&D and Director of New Product Development for the laser therapy unit of American Medical Systems, now a division of ENDO Health Solutions) spoke about Biology and Design of Surgical Laser Systems at http://www.bio2devicegroup.org .

Surgical lasers have been in use for over 50 years and have been used in almost every part of the body. The results however, have been mixed and the medical profession has alternated between enthusiastically embracing laser technology and shuning it. However, good results can be obtained if the use of the laser systems is matched with clinical objectives and better understanding of the needs of the patients, said Stinson. In addition, the characteristics of the lasers must be tailored to achieve the specific tissue response desired. Using the example of laser treatment for Benign Prostate Hyperplasia (BPH), Stinson further elaborated these points. BPH refers to the excessive growth or enlargement of the prostate gland, which is a common occurrence as men get older. BPH can cause bothersome urinary symptoms. Untreated prostate gland enlargement can block the flow of urine out of the bladder and can cause bladder, urinary tract or kidney problems. Current surgical treatment options include open prostatectomy and TURP or trans-urethral resection of the prostate. In TURP procedure, a wire with a loop at the end is inserted and electric current is passed to cut off the tissue. For obvious reasons and risks associated with open prostatectomy, it is much more risky and costlier procedure. TURP is also associated with significant complications that include impotence, incontinence, clot retention etc. and requires overnight hospitalization.

Laser surgery for BPH is becoming more common. Laser procedures are done on outpatient basis and overall result in fewer complications, compared to TURP. If an appropriate laser wavelength is used, when tissue is burned off with a laser, it heals well and looks almost as good as new. In surgery, three laser parameters are critical and include, power density, exposure time, and penetration depth. Power density relates to the brightness, is very controllable, and can be targeted to a very small area. Exposure time simply refers to the length of time the tissue is exposed to the laser. Penetration depth refers to how deep below the skin’s surface the tissue is exposed to the laser and here is where particularly, the biology comes into play. How the tissue is impacted depends on laser power density and pulse duration and there is a tradeoff. This trade off between power density and exposure time works well over a limited range. Over larger ranges, other factors come in to play and different chemical reactions occur. How the tissue reacts depends on temperature. Around 60 degrees, one can cause coagulation of the tissue. Below 60 degrees, the changes are reversible, but above 60 degrees the changes are not reversible. When lasers are used on the live tissue, all of this needs to be taken into account. Additionally, the optical penetration depth characterizes how deep into the tissue the light penetrates. As light penetrates tissue is is absorbed, Optical Penetration Depth is defined as the distance into tissue at which all but 37% of the light is absorbed. Water and oxyhemoglobin in the tissue absorbs light, but the do so at different wavelengths. Therefore, penetration depth depends on the absorption coefficient which is wavelength dependent. Different wavelengths have different penetration depths, depending on absorption and scattering of the light. A biological fact is that the depth of penetration determines the thickness of the coagulated zone.

Stinson demonstrated an example of the in-vivo vaporization of the canine prostate with a 180W, 530nm laser that resulted in perfect healing at 8 weeks. It is clear that laser systems have their usefulness. However, the usefulness of the lasers and the development of the laser systems depend upon the specific clinical objectives. The characteristics of the lasers must be tailored to achieve the specific tissue response desired. The talk was followed by Q&A. For more information, please contact Doug Stinson at douglas-stinson@comcast.net.

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