Surgery – Instruments – Cyrogenic application
Patent
1997-07-07
1999-01-19
Cohen, Lee
Surgery
Instruments
Cyrogenic application
A61B 1736
Patent
active
058609709
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to cryosurgical instruments.
BACKGROUND
It is known to use cryosurgical instruments in surgical operations in which living tissue is destroyed (or ablated) by cooling the tissue to well below the freezing point of water (e.g. -20.degree. to -30.degree. Centigrade).
WO 93/04647 discloses a cryosurgical instrument in which a metal tip of a cryosurgical probe is cooled by supplying liquid nitrogen to the interior of the tip.
In such an instrument, when the liquid nitrogen reaches the interior of the metal tip, it starts to boil. There are two possible modes of boiling which can take place. If so-called `film` boiling occurs, a film of nitrogen gas forms between the inner surface of the metal tip and the boiling liquid nitrogen. This film of gas reduces the heat conduction between the boiling liquid and the body of the tip and thus reduces the cooling efficiency of the instrument. The other possible mode of boiling is referred to as `nucleate` boiling. In this mode the film of gas does not form, so the heat conduction between the boiling liquid and the body of the tip is much better, and the cooling efficiency of the tip is thus improved.
Although nucleate boiling provides a more efficient cooling operation, it is often difficult to control the mode of boiling reliably so that nucleate boiling takes place. In particular, film boiling often occurs for some time at the start of a period of operation of a cryosurgical instrument. Because the cooling efficiency is lower when film boiling takes place, surgical procedures in which tissue has to be cooled can take longer. This can be both hazardous (by increasing the time during which a patient must be anaesthetized), expensive and inconvenient.
SUMMARY OF THE INVENTION
This invention provides a cryosurgical instrument comprising a metal cooling tip operable to be cooled by the boiling of a cryogenic fluid, predominantly within a boiling region of the tip, in which the exterior of at least the boiling region is coated with a heat-insulating coating.
The invention addresses the problem of achieving more reliable nucleate boiling in a cryosurgical instrument by the counter-intuitive step of adding a heat-insulating coating to the exterior of the cooling tip. It has been found that such a coating can actually promote nucleate boiling (rather than film boiling) within the cooling tip.
The invention involves coating substantially the entire boiling region of the tip. In fact, for ease of application of the coating, in preferred embodiments the entire exterior surface of the tip can be coated. This contrasts with precious tips (e.g. those disclosed in U.S. Pat. No. 3,827,436) in which cooling regions of the tip are masked to prevent unwanted freezing of tissue by those particular regions. In the invention, however, the coating extends over active (cooling) regions of the tip which are still used for tissue ablation.
Preferably the instrument comprises means for supplying the cryogenic fluid to the interior of the tip.
Although the use of other cryogenic fluids could be envisaged (such as freon 14, freon 22, freon 13, liquefied air and normally gaseous hydrocarbons), it is preferred that the cryogenic fluid is liquid nitrogen. Liquid nitrogen is reasonably cheap, readily available, inert and relatively innocuous when vented to the atmosphere.
Various heat insulating coatings could be used, such as thin plastics coatings. However, it is preferred that the coating is a polytetrafluoroethane coating.
Preferably the coating is less than about 30.times.10.sup.-6 meters thick.
Preferably the tip is made of silver. Silver is a good heat conductor and can also tend to discourage the growth of potentially harmful bacteria on the tip.
It is preferred that the instrument comprises a probe handle and a silver probe shaft linking the probe handle and the tip. The silver shaft has the advantages described above and can also act to conduct body heat to the tip when it is necessary to thaw the tip.
To assist in thawing the tip when requi
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Goddard Robert William
Penny Leslie John
Cohen Lee
Spembly Medical Limited
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