Patent Application
20070073282
The inventions described below relate to the field of tissue ablation and turbinate reduction.
Chronic nasal obstruction is often the result of enlarged turbinates, which are scroll-like bony projections of the nasal cavity covered with mucus membranes. These mucus membranes are located just inside the nose, and they are subject to chronic swelling and hypertrophy which leads to chronic congestion, sinus infections, sleep disorders and other chronic conditions. Recently, radiofrequency ablation of the turbinates, referred to as somnoplasty, has been adopted as a treatment for enlarged turbinates. In this technique, a slender radiofrequency probe is inserted into the submucosal tissue of the turbinates, and radiofrequency energy is passed through the submucosal tissue to heat and destroy (ablate) a small portion of this tissue. As the injured tissue heals and is resorbed by the body, the submucosal tissue shrinks and the obstruction is alleviated. The healing process takes several weeks.
Similar radiofrequency ablation procedures may also be used to shrink hypertrophied tissue in the palate (to treat snoring and sleep apnea), in vertebral discs (to treat herniated disks), or for various tumor ablations in the brain, liver, prostrate, etc., and various cosmetic surgeries (droopy eyelids).
Because radiofrequency devices pass electrical current through the body, precautions must be taken to avoid excessive current flow and flow of damaging current to areas remote from the devices. Radiofrequency ablation devices depend on thermal feedback or impedance monitoring to control the amount of RF energy applied to achieve the temperature necessary to achieve ablation (60-100° C.). Such feedback systems are intended to ensure that the devices do not deliver excessive amounts of energy into the body and damage nearby anatomy. RF ablation devices can also cause unwanted nerve stimulation, and must be used with caution to avoid interaction with the heart. RF ablation devices may cause unintended tissue damage in nearby anatomical structures and areas remote from the point of application.
The devices and methods described below provide for thermal ablation of hypertrophied tissue, such as turbinates, with a resistive heating element adapted for insertion into the tissue. The device uses DC current to heat the resistive heating element, and is operated at relatively low voltage levels and low current levels. The device is easy to operate, and may be applied for predetermined time periods without feedback control, using a timing circuit or computerized control system. The resistive heating element is covered with a thin, non-stick, coating that is thermally conductive, such as Xylan®, Teflon® or other fluoropolymer or suitable material.
In the embodiment adapted for turbinate ablation, the device components are chosen to provide the desired heating profile and to provide mechanical characteristics which facilitate safe insertion. The tubular resistive heating element (item 21) outer diameter is 0.029 inches (0.74 mm), and the resistive heating elements comprise inconel 625 alloy (a type of stainless steel). The heating segment is coated or covered with a thin (0.001″ (0.025 mm)) layer of non-stick electrically insulative material (ePTFE, Xylan®, etc.) with sufficient thermal conductivity to permit heating through the coating. The resistive heating element extending beyond the mounting tube is about 0.345″ (9 mm) long (the total length of the tube resistor is about 0.46″ (12 mm). The overall resistance of the heating element is 0.1 to 0.25 ohms, preferably about 0.15 ohms. When applying DC current at constant current of about 3 to 3.5 amps, preferably about 3.2 amps, the heating segment will gradually heat turbinate tissue to 80-100° C. over a period of about 60 seconds along the entire length of the heating segment extending beyond the hypotube and mounting tube. Heating occurs at relatively slow rate, starting at a rate of about 20 to 25° C. per five second interval, and slowing to a rate of 1 to 50 per five second interval over the course of a one minute application of current. The control means operates to apply current to the heating segment for a predetermined period. A predetermined period of at least about 30 seconds, and preferably about 60 seconds, is suitable for turbinate ablation. the predetermined period may be set in manufacture, or may be variable by the surgeon just prior to use of the device. The composition of the resistive heating element may also be varied to provide slower or faster heating profiles, to adapt the device to various treatments. The current and/or voltage applied to the heating elements may be varied to obtain slower or faster heating profiles, as indicated by the particular ablation treatment to be performed. Direct current is preferred in this application, in part because it does not interact with nearby nerves, and very little, if any, of the current leaks into the body (the body being much more resistive that the supply wires and the inconel of the resistive heating element). Though direct current is preferred, the resistive heating may also be provided by supplying radiofrequency current or alternating current to the heating segment, as the covering of electrically insulative material will prevent leakage.
The hypotube in this embodiment has an outer diameter of 0.065 inches (1.7 mm) and an inner diameter of 0.057 inches (1.4 mm)(a wall thickness of 0.008″ or 0.2 mm), and is about 4 inches (100 mm) long, with an 180 bend about 2.25 inches (about 60 mm) from the distal tip of the device. The compressive strength of the hypotube (the load at which it buckles), at the bend point, is lower that the compressive strength of the heating segment. The hypotube in this embodiment will kink or collapse at compressive load of about 0.7 to 0.9 lbs, preferable about 0.75 lbs. This feature ensures that, if the surgeon inserts that heating element into the turbinates and encounters excessive resistance and attempts to insert the heating segment with compressive force that might otherwise damage the heating element, the hypotube will buckle instead. In the event the hypotube buckles, the surgeon can withdraw the probe and restart the procedure with a new probe.
While described in the environment of turbinate ablation, the device and method described above may be used in soft palate ablation and somnoplasty generally, in spinal disk reductions, tumor ablation, especially in the brain, and other surgeries currently accomplished with RF ablation. Thus, while the preferred embodiments of the devices and methods have been described in reference to the environment in which they were developed, they are merely illustrative of the principles of the inventions. Other embodiments and configurations may be devised without departing from the spirit of the inventions and the scope of the appended claims.
