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A metallic probe making contact with the tissue is cooled to a very low temperature (e.g. -196[d]C for liquid nitrogen). The effect is to freeze the surrounding tissue so that it dies. In the tissue immediately beyond the killed zone a degree of coagulation occurs thus limiting the resulting bleeding. The technique is used for freezing and breaking off small polyps, and also for resecting the prostate gland. For larger lesions liquid nitrogen cooling is required since a large ice ball is necessary. Other cryogenic surgery devices utilize the Joule-Thomson effect, usually by the expansion of nitrous oxide from a cylinder through an orifice in the tip of the probe, although low power types for ophthalmology use carbon dioxide.
In devices using liquid nitrogen a dewar or thermos bottle is required as a reservoir from which the liquid is piped to the probe and the resulting boiled-off gas drawn off through a tube. In order to localize the cooling at the tip, the shank of the probe may contain a heater to prevent freezing of tissue distal to the treatment site. Gas expansion devices are relatively easy to control by interrupting the gas flow but they cannot remove heat at the rate achieved with liquid nitrogen.
They are used in operating theatres, special clinics and chiropody clinics.
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