Sudden and Unpredictable Below-Surface Ablation Pattern Changes by CO 2 Laser Beams: A Qualitative Description of Five Macroscopic Cases Observed in PMMA with High Probability to Occur during Surgery in Low-Water-Content Tissues

Abstract

Objective: This paper describes five cases of macroscopic irregular CO₂ laser-beam ablation patterns that can generate below-surface complications during surgery. These five cases are related to curved reflected beams, curved craters generation with abnormal superficial thermal damage, and craters that show irregular wall contours. Although these alterations have been observed during irradiation in PMMA samples (polymethilmethacrylate), it is possible that similar unpredictable changes also happen in low-water-content, hard and uniform biological tissues such as compact bone, enamel, and dentin. This fact can predict severe impacts on the quality of the final surgical outcome, especially there where precision surgery techniques are required. A qualitative description about the possible causes of these effects and how to avoid them during surgery have been suggested too. Background Data: In the past decades, daily surgery and research studies have provided useful information about the interaction between medical CO₂ laser beams and animal, human, and other biological tissues. Several mathematical models describe with acceptable accuracy all the ablative properties of the 10.6 µm laser beam. Very few studies describe the presence and address the consequences of the ablative aberrations, which can frequently and randomly happen during laser surgery. The probability that these changes happen in below-surface, therefore invisible, parts of the biologic media under treatment makes the whole matter crucial, even in cases of traditional surgery. Where gross mass removals are considered, the presence of unpredictable and sudden deviations from the expected traditional cone-shaped patterns raise several questions about safety. The continuous need for properly engineered medical laser-beam devices, online laser-beam monitoring, and real-time control becomes mandatory in modern surgery. Materials and Methods: The equipment used in this study was provided by the National Cancer Institute of Milan, Milan, Italy, and by the Sackler School of Medicine, Tel Aviv University, Tel Aviv, Israel. A large TEM-mode laser-beam device (Valfivre, Italy) and TEM00 laser-beam device (Synrad, USA), both coupled to 2.5- and 5-inch focusing lenses, have been used to irradiate, at 10 Watts nominal output, on the focal spot, several PMMA blocks (3 × 2 × 4 × 2 × 2 cm) up to 10 sec CW. For one set of experiments, a metallic, well-polished mirror was placed against one surface of each sample to simulate possible internal beam reflections caused by generic metallic surgical instruments, such as conventional scalpels or clamps. Results: The experimental evidence of five major and unpredicted changes in the shape of craters produced by CO₂ laser beams in PMMA are shown in photos and discussed in a qualitative way. Several physical and thermodynamic phenomena are proposed to identify and therefore minimize or avoid the causes of these events. Conclusion: The results discussed in this paper show how important it is to constantly and carefully observe both the irradiated tissue's or media's structure and the beam's broadening under surface during the ablation process. Fume attenuation of the incoming laser beam, fume escape paths, internal beam interference phenomena, media's microstructural changes and, perhaps more importantly, their combination can offer good explanations of all the described phenomena. In summary, the presence of fumes in at least two out of the five reported cases plays a key role in aberrant crater generation processes. Therefore, the author recommends paying extra attention to them in order to provide a higher quality outcome in Surgery where CO₂ laser beams are deployed.