| There are numerous applications of thermal energy in medicine such as in tissue welding, thermokeratoplasty, skin resurfacing, elimination of discogenic pain in the spine and treatment of joint instability. We are interested in developing computer models to simulate the thermomechanical responses of collagenous tissues to help the physician with pre-operational planning.  Soft tissue thermotherapy based on sub-ablative
(Ttissue<100oC) heating of collagenous tissues finds widespread application in medicine such as tissue welding, thermokeratoplasty, skin resurfacing, elimination of discogenic pain in the spine and treatment of joint instability. One such sub-ablative therapy of interest is thermal capsulorrhaphy. In this procedure, mechanically deformed, lax collagenous tissues surrounding the shoulder joint (the shoulder capsule and the underlying ligaments) are shrunk by means of laser or radiofrequency heating (Figure 9).
Thermal capsulorrhaphy utilizes heat for denaturing the underlying collagenous network of the soft tissues. When subjected to hyper-physiologic temperatures, a collagen molecule transforms by unwinding from its ordered, triple-helical structure to an amorphous state. This transformation is caused by the destruction of the heat-labile intramolecular hydrogen bonds. The rate of the transformation is shown to be dependent on temperature and the hydroxyproline content of the collagen molecule (which is tissue and species specific). The heat-labile hydrogen bonds are responsible for the structural and mechanical stability of the collagen molecule and thus of the tissue proper. In the macro-scale, the outcome of collagen denaturation is tissue shrinkage (given that the traction boundary conditions are zero) in the direction of primary collagen fiber orientation accompanied by changes in the mechanical properties of the tissue. It has been shown that the thermal history as well as the mechanical stress state of the tissue during heating is the major factor in determining the heat-induced tissue response.
We have developed a numerical model to quantify the thermal damage fields created in collagenous tissues by three different clinical heating modalities (Ho:YAG laser, Monopolar and Bipolar Radiofrequency) using finite element analysis (Figure 10). Heating rate dependent denaturation of the collagenous tissue was determined experimentally and incorporated into the model. Differences among heating modalities were established. We are currently focused on expanding the capabilities of the numerical algorithm to predict the heat-induced mechanical deformation of the collagenous tissue with special focus on laser eye surgery.
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