The ablation rate of articular cartilage and fibrocartilage (meniscus), were quantified to examine wavelength and tissue-composition dependence of ablation efficiency for selected mid-infrared wavelengths. The wavelengths tested were 2.9 um (water dominant absorption), 6.1 (protein and water absorption) and 6.45 um (protein dominant absorption) generated by the Free Electron Laser (FEL) at Vanderbilt University. The measurement of tissue mass removal using a microbalance during laser ablation was conducted to determine the ablation rates of cartilage. The technique can be accurate over methods such as profilometer and histology sectioning where tissue surface and the crater morphology may be affected by tissue processing. The ablation efficiency was found to be dependent upon the wavelength. Both articular cartilage and meniscus (fibrocartilage) ablations at 6.1 um were more efficient than those at the other wavelengths evaluated. We observed the lowest ablation efficiency of both types of cartilage with the 6.45 um wavelength, possibly due to the reduction in water absorption at this wavelength in comparison to the other wavelengths that were evaluated.
Osteoarthritis is a heterogeneous disease characterized by progressive loss of cartilage. The earliest biochemical features, which precede gross pathological changes, include non-uniform loss of proteoglycans associated with increase of water content in tissue and finally, fibrillation of the tissue's collagen network. Loss of proteoglycans decreases the ability of cartilage to withstand compressive loading and makes the tissue softer and more susceptible to wear and fibrilation. If the early loss of proteoglycans is detectable by a non-invasive optical technique, progression of the disease may be arrested using, for example, pharmacologic or surgical intervention. When an electric field is applied to cartilage by an electrical stimulator, the current-generated stress gradients are produced and stress deformation occurs. Since differential phase optical coherence tomography is very sensitive to subsurface stress deformation, we propose to stimulate cartilage electrically and detect stress gradients before gross signs of cartilage degeneration appear. Detection of depth-resolved electromechanical stress gradients in cartilage using differential phase optical coherence tomography may be useful to monitor non-invasively cartilage degeneration. Since the streaming potential and other electrokinetic effects in cartilage are directly proportional to proteoglycan density, application of an electric field in cartilage combined with depth-resolved phase sensitive optical measurements may provide a sensitive indicator of cartilage viability on the molecular-level.
During laser irradiation, mechanically deformed cartilage undergoes a temperature dependent phase transformation resulting in accelerated stress relaxation. Clinically, laser-assisted cartilage reshaping may be used to recreate the underlying cartilaginous framework in structures such as ear, larynx, trachea, and nose. Therefore, research and identification of the biophysical transformations in cartilage accompanying laser heating are valuable to develop many potential clinical applications. To observe spectral features in quasi-elastic light scattering from cartilage, light emitted from a Nd:YAG laser (? = 1.32?m) was used for a heating source and a Ti:Al2O3 femtosecond laser (? = 850nm) was used for a light scattering source. A spectrometer and infrared detector were used to monitor the backscattered light spectrum and transient temperature changes from cartilage following laser irradiation. A cartilage sample was irradiated by Nd:YAG laser light over a 6 second time period and the transient temperature was simultaneously measured during laser irradiation. The denaturation of macromolecules within cartilage was observed after 4.5 seconds. The obtained spectral data were analyzed by computing a Fast Fourier transform and converted to the optical path length difference domain. The path length difference of a spectral oscillation within cartilaginous framework was also calculated. Although the measured path length differences were different from the expected values, the results will give rise to distinct patterns of the movement of macromolecules within cartilage following laser irradiation.
Thermodynamic induced changes in birefringence of nasal septal cartilage following Nd:YAG laser irradiation were investigated using a polarization-sensitive optical coherence tomography (PSOCT) system. Birefringence in cartilage is due to the asymmetrical collagen fibril structure and may change if the underlying structure is disrupted due to local heat generation by absorption of laser radiation. A PSOCT instrument and an infrared imaging radiometer were used to record, respectively, depth-resolved images of the Stokes parameters of light backscattered from ex vivo porcine nasal septal cartilage and radiometric temperature following laser irradiation. PSOCT images of cartilage were recorded before (control), during, and after laser irradiation. From the measured Stokes parameters (I,Q,U, and V), an estimate of the relative phase retardation between two orthogonal polarizations was computed to determine birefringence in cartilage. Stokes parameter images of light backscattered from cartilage show significant changes due to laser irradiation. From our experiments we differentiate dehydration and thermal denaturation effects and observe the birefringence changes only in the dehydration effect. Therefore, a dynamic measurement of birefringence changes in cartilage using PSOCT as a feedback control methodology to monitor thermal denaturation is problematic in non-ablative surgical procedures such as laser assisted cartilage reshaping.
We demonstrate application of an IR imaging technique for non-contact determination of thermal diffusivity of biological materials. The IR method utilizes pulsed laser excitation to produce an initial 3D temperature distribution in tissue, and records IR images of subsequent heat diffusion. The theoretical model assumes the time-dependent temperature increase following pulsed laser exposure occurs due to independent heat diffusion in longitudinal and lateral directions. A nonlinear least-squares algorithm is used to compute the lateral point spread function for a pair of recorded IR images and determine thermal diffusivity of a test specimen. Application of the method was demonstrated using tissue phantom s and ex-vivo samples of hydrated cartilage.