ABSTRACT
Strong evidence that short-pulse laser ablation of biological tissues is a photomechanical process is presented. A full three-dimensional, time-dependent solution to the thermoelastic wave equation is compared to the results of experiments using an interferometric surface monitor to measure thermoelastic expansion. Agreement is excellent for calibrations performed on glass and on acrylic at low laser fluences. For cortical bone, the measurements agree well with the theoretical predictions once optical scattering is included. The theory predicts the presence of the tensile stresses necessary to rupture the tissue during photomechanical ablation. The technique is also used to monitor the ablation event both before and after material is ejected.
Subject(s)
Laser Therapy , Lasers , Models, Theoretical , Animals , Elasticity , Hot Temperature , HumansABSTRACT
A new experimental technique was developed to study short-pulsed laser ablation of biologic tissues (human meniscus and bovine tibial bone), water, and acrylic. The experimental technique was based on interferometric monitoring of the motion of the tissue surface to measure its laser-induced expansion after irradiation. The thermoelastic expansion of these materials after laser irradiation under subablation threshold was examined to determine its role in the initiation of ablation. The experimentally observed surface expansion of cortical bone and acrylic was in agreement with theoretical predictions. The movement of meniscal tissue was similar to that shown by water. The latter 2 materials showed additional features consistent with the growth and collapse of cavitation bubbles. The exact role of cavitation in the irradiation of meniscal tissue by laser light remains unknown, but may represent a clinically important mode of tissue ablation and postirradiation trauma.
Subject(s)
Laser Therapy/methods , Menisci, Tibial/surgery , Animals , Cattle , Elasticity , Evaluation Studies as Topic , Humans , Interferometry , Laser Therapy/adverse effects , Light , Menisci, Tibial/physiology , ThermodynamicsABSTRACT
Under certain conditions, laser light incident on a target material can induce an explosive removal of some material, a process called laser ablation. The photomechanical model of laser ablation asserts that this process is initiated when the laser-induced stresses exceed the strength of the material in question. Although one-dimensional calculations have shown that short pulsed lasers can create significant transient tensile stresses in target materials, the stresses last for only a few nanoseconds and the spatial location of the peak stresses is not consistent with experimental observations of material failure in biological tissues. Using the theory of elasticity, analytical expressions have been derived for the thermoelastic stresses and deformations in an axially symmetric three-dimensional solid body caused by the absorption of laser light. The full three-dimensional solution includes three stresses, radial, circumferential and shear, which are necessarily absent in the simple one-dimensional solution. These stresses have long-lived components that exist for eight orders of magnitude longer in time than the acoustic transients, an important point when the details of dynamic fracture are considered. Many important qualitative features are revealed including the spatial location of the peak stresses, which is more consistent with experimental observations of failure.
Subject(s)
Laser Therapy , Lasers , Mathematics , Models, Biological , Animals , Elasticity , Radiation Effects , Solutions , Thermodynamics , Time FactorsABSTRACT
We present results from the application of laser interferometry to the study of short-pulsed laser ablation of biological tissue. The mechanical response of tissue to laser-induced stress is examined under subthreshold conditions to determine its role in initiating the ablation process. A theoretical model is developed to relate this surface displacement to the pressure within the tissue and the mechanical properties of the tissue. In the experiment, a 7.5 ns pulse of 355 nm light was used to irradiate bovine shank bone, human meniscus, and an aqueous dye solution. Interferometric monitoring of the tissue surface was used to determine its motion after laser irradiation. The surface movement of bone was qualitatively consistent with the theoretical predictions of the model. The movement of meniscus and an aqueous dye solution showed additional features that are consistent with the growth and collapse of cavitation bubbles.
Subject(s)
Interferometry , Lasers , Animals , Bone and Bones/radiation effects , Cartilage/radiation effects , Cattle , Coloring Agents , Humans , In Vitro Techniques , Laser Therapy , Models, TheoreticalABSTRACT
The photomechanical model of laser ablation of biological tissue asserts that ablation is initiated when the laser-induced tensile stress exceeds the ultimate tensile strength of the target. We show that, unlike the one-dimensional thermoelastic model of laser-induced stress generation that has appeared in the literature, the full three-dimensional solution predicts the development of significant tensile stresses on the surface of the target, precisely where ablation is observed to occur. An interferometric technique has been developed to measure the time-dependent thermoelastic expansion, and the results for subthreshold laser fluences are in precise agreement with the predictions of the three-dimensional model.