Abstract
Cartilage undergoes characteristic mechanical stress relaxation following laser irradiation below the ablation threshold. Porcine auricular cartilage (1–2 mm thickness) was irradiated with a Nd:YAG laser (λ=1.32 μm) at two power levels (W/cm2). Surface temperature (S c (t) (°C)) (monitored using a single element HgCdTe infrared detector, 10-14 μm spectral range), and integrated back scattered light intensityI(t) were measured during laser irradiation. A HeNe laser beam (λ=632.8 nm) was incident on the back surface of the cartilage specimen and fractional integrated backscattered light intensity was measured using an integrating sphere anda silicon photodiode. Laser irradiation (5.83 W/cm2, 50 Hz pulse repetition rate (PRR)) continued until surface temperature reached approximately 70°C, during which cartilage mechanical stress relaxation was observed. Integrated back scattered light intensity reached a plateau at about 70°C). At higher laser power (39.45 W/cm2, 50 Hz PRR), a feedback-controlled cryogen spray was used to maintain surface temperature below 50°C. A similar plateau response was noted in integrated backscattered light intensity. This signal may be used to optimise the process of stress relaxation in laser cartilage reshaping. Several clinical applications involving reconstructive surgery are proposed.
Highlights
Cartilage is a complex macromolecular tissue composed of 80% water, 13% collagen (Type II), and 7% protein-polysaccharide
These changes were produced by irradiating the cartilage specimens while they were wrapped around the wooden dowel
The resultant change in slope of AI(t)/I o needs to be identified during sustained laser irradiation with cryogen cooling and such studies are presently underway in our laboratories. Results of these studies demonstrate that the process of laser-induced stressrelaxation in cartilage is accompanied, and may be monitored, by changes in the fractional change of integrated backscattered light intensity AI(t)/Io
Summary
Cartilage is a complex macromolecular tissue composed of 80% water, 13% collagen (Type II), and 7% protein-polysaccharide (proteoglycans). Collagen Type II forms a rigid framework that encases large meshes of proteoglycan macromolecules (100-200 MDa) containing copious numbers of charged species, COO- and SO 3- moieties at physiologic pH. As a Correspondence to: B.J.F. Wong, Beckman Laser Institute and Medical Clinic, 1002 Health Sciences Road East, University of California, Irvine, CA 92697, USA, consequence, electrostatic repulsion exists between negatively charged ion species that is only partially balanced by free counter ions in solution. Beckman Laser Institute and Medical Clinic, 1002 Health Sciences Road East, University of California, Irvine, CA 92697, USA, consequence, electrostatic repulsion exists between negatively charged ion species that is only partially balanced by free counter ions in solution This electrical imbalance results in an intrinsic tissue turgor termed the Donnan osmotic pressure [I-3]. The extrinsic morphology of cartilage is determined by the interplay of these ionic forces, ion and fluid flow in the matrix, and the tensile properties of the collagen mesh [2,3]
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