Basic Properties of Collagen Shrinkage and Laser-Collagen Interactions
The thermal properties of collagen have been extensively studied in a variety of experimental models since the mechanism of collagen shrinkage was first proposed by Flory et al. [7–9] in the 1950s. Flory et al. [7–9] described how thermal contraction of c
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Introduction The thermal properties of collagen have been extensively studied in a variety of experimental models since the mechanism of collagen shrinkage was first proposed by Flory et al. [7-9] in the 1950s. Flory et al. [7-9] described how thermal contraction of collagen is brought about by a transition between the crystalline and amorphous phases of collagen. The investigators applied the thermodynamic theory of high-polymeric materials described by Gee [11] to collagenous shrinkage and demonstrated that the transition from a crystalline to an amorphous phase is fully analogous to that involved in the melting of other crystalline polymers. The transition to an amorphous state where polypeptide chains are disordered and randomly coiled was deduced from the disappearance of X-ray diffraction and the loss of optical birefringence of native collagen [9]. The process was described as "degradation to gelatin" or as "denaturation." It has been shown that the thermal properties of collagen vary with the age of the animal and the
environmental condition [1, 19,30, 33]. Rosenbloom et al. [26[ showed that hydroxyproline content determines the denaturation temperature of collagen using a chick tendon model. More recently, Allain et al. [1] described collagen network behavior under the influence of heat during hydrothermal shrinkage and swelling in rat skin. These investigators proposed that the onset of denaturation of collagen starts with the unwinding of the triple helix. During the course of the temperature rise, hydrolysis of the heat-labile crosslinks occurs, while the maintenance of heat-stable crosslinks is responsible for the residual tension within collagen fibrils (Fig. 1). The investigators concluded that swelling and shrinkage of collagen fibrils are secondary to unwinding of the triple helix with maintenance of heat-stable intermolecular crosslinks. Hogan et al. [19] reported a strong correlation between thermal properties of tendon and the concentration of nonreducible crosslinks. To date, the thermal properties of collagen have been explained mainly in terms of the crosslinks. A number of different methods have been used to study this parameter, including differential
A
Fig. lA, B. Effect of heat on the strcutural properties of type I collagen. A shows the native structure of collagen. In B, the triple helix of collagen has undergone thermal unwinding due to disruption of the hydrogen bonds between the a-chains of the triple helix. Shrinkage occurs because of the combination of thermal unwinding and the maintenance of heat-stable crosslinks during this process
B. E. Gerber et al. (eds.), Lasers in the Musculoskeletal System © Springer-Verlag Berlin Heidelberg 2001
Basic Properties of Collagen Shrinkage and Laser-Collagen Interactions
scanning calorimetry, ultraviolet difference spectroscopy, isometric tension measurement, and isotonic contraction measurement [1, 5, 19,30, 33]. Laser Tissue Welding
The joining of tissues together requires uniform heating of
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