Ultrasonic aspiration has become the standard of care for removal of tumors and diseased tissue in neurosurgery and general surgery. Typically, ultrasonic surgical aspirators for fragmenting and aspirating tissue include an ultrasonic transducer supported within a handpiece, an ultrasonically vibrating horn or tip operably connected to the ultrasonic transducer, and a flue positioned about the horn. The horn includes a longitudinally extending central bore having one end located adjacent a distal tip and a second end located adjacent the proximal end of the horn. The proximal end of the horn is adapted to engage a vacuum source to facilitate aspiration of fluid. The flue is positioned about the horn to define an annular passage. Irrigation fluid such as saline is supplied through the annular passage around the horn to the surgical site where it mixes with blood and tissue particles and is aspirated through the bore in the horn. By mixing the irrigation fluid with the blood and tissue particles, coagulation of the blood is slowed down and aspiration thereof is aided. U.S. Pat. Nos. 5,015,227 and 4,988,334 disclose such ultrasonic surgical devices and are incorporated herein by reference. For example, a titanium surgical tip may be powered by a transducer to fragment tissue and suction effluent via a central channel. The transducer vibrates along its length, and ultrasonic horns such as stepped horns and specialty profiles of reduced diameter amplify vibration.
The Ampulla or Gaussian profile was published by Kleesattel as early as 1962, and is employed as a basis for many ultrasonic horns in surgical applications including devices for use in ultrasonic aspiration as described in U.S. Pat. No. 4,063,557 to Wuchinich, et al, 1977, and U.S. Pat. No. 6,214,017 to Stoddard, et al, 2001, which are incorporated herein by reference. The Gaussian profile is used in practice to establish and control the resonance and mechanical gain of horns. A resonator, a connecting body, and the horn act together as a three-body system to provide a mechanical gain, which is defined as the ratio of output stroke amplitude of the distal end of the tip to the input amplitude of the resonator. The mechanical gain is the result of the strain induced in the materials of which the resonator, the connecting body, and the ultrasonic horn are composed.
A magnetostrictive transducer coupled with the connecting body functions as a first stage of the booster horn with a mechanical gain of about 2:1, due to the reduction in area ratio of the wall of the complex geometry. The major diameter of the horn transitions to the large diameter of the Gaussian segment in a stepped-horn geometry with a gain of as large as about 5:1, again due to reduction in area ratio. The uniform strain along the length of the Gaussian provides multiplicative gain of typically less than 2:1. Thus, the application of ultrasonically vibrating surgical devices used to fragment and remove unwanted tissue with significant precision and safety has led to the development of a number of valuable surgical procedures.
Certain devices known in the art characteristically produce continuous vibrations having substantially constant amplitude at a frequency of about twenty to about fifty-five kHz, for example, at a predetermined frequency of 20-36 kHz. Amplitude of vibration of transducer-surgical tip systems decreases with increasing frequency because maximum stress in the material of the horns is proportional to amplitude times frequency, and the material must be maintained to an allowed fraction of its yield strength to support rated life in view of material fatigue limits. For example, U.S. Pat. Nos. 4,063,557, 4,223,676 and 4,425,115, which are incorporated herein by reference, disclose devices suitable for the removal of soft tissue which are particularly adapted for removing highly compliant elastic tissue mixed with blood. Such devices are adapted to be continuously operated when the surgeon wishes to fragment and remove tissue, and generally is operated by a foot switch.
In an apparatus that fragments tissue by the ultrasonic vibration of a surgical tip, efficiency of energy utilization is optimized when the transducer which provides the ultrasonic vibration operates at resonant frequency. The transducer and surgical tip design establishes the resonant frequency of the system, while the generator tracks the resonant frequency and produces the electrical driving signal to vibrate the transducer at the resonant frequency. However, changes in operational parameters, such as changes in temperature, thermal expansion, and load impedance, result in deviations in the resonant frequency. Accordingly, controlled changes in the frequency of the driving signal are required to track the resonant frequency. This is controlled automatically in the generator.
Conventional ultrasonic surgical aspirating tips employed in surgery for many years typically present a longitudinally vibrating annular surface with a central channel providing suction or aspiration, which contacts tissue and enables fragmentation via described mechanisms of mechanical impact (momentum), cavitation, and ultrasound propagation. Mechanical impact may be most useful in soft tissue and cavitation clearly contributes to the fragmentation of tenacious and hard tissue in situations where liquids are present and high intensity ultrasound exceeds the cavitation threshold.
Ultrasound propagation is concerned with the transmission of pressure across the boundary of a surgical tip and tissue, which leads to the propagation of pressure and, perhaps more importantly, particle displacement. Acoustic impedance is the total reaction of a medium to acoustic transmission through it, represented by the complex ratio of the pressure to the effective flux, that is, particle velocity times surface area through the medium. As discussed in the classic text of Krautkramer J. and Krautkramer H, Ultrasonic Testing of Materials, Berlin, Heidelberg, N.Y., 1983, for the case of a low to high acoustic impedance boundary, it may seem paradoxical that pressure transmitted can exceed 100%, but that is what results from the build-up of pressure from a low to high acoustic impedance boundary. In the case of a high to low acoustic impedance mismatch, such as with a high impedance titanium ultrasonic horn to low impedance fibrous muscle, soft tissue, or water, the pressure transmitted decreases (e.g., less than 15% for titanium to fibrous muscle) and particle displacement increases (e.g., as great as 186% for titanium to muscle).
U.S. Pat. No. 4,827,911 to Broadwin et al., 1989, described the use of a periodically interrupted power supply to limit thermal rise of tissue at the surgical site. This is referred to as pulse width modulation hereinafter. The patent described the use of periodic interruption of power, or pulse width modulation, to control reserve power independently of stroke, such that fragmentation power, now known to be associated with velocity squared or stroke times frequency all squared, could be maintained for efficacy in tissue removal. A system was implemented to modulate between only a high amplitude and low amplitude. It improved potential for safe use of the ultrasonic aspirators with implementation of periodic interruption of power supplied with an electronically derived analog duty cycle. The duty cycle could simply be on and off, but was implemented as a duty cycle from a high amplitude to lower amplitude versus an on and off state. It is noted that the steady state stroke is also shown as decreasing with increased tissue selectivity settings. It appeared that the primary interest was to limit reserve power or transducer power to the surgical site, which could cause heating. Temperature feedback was discussed in the patent.
U.S. Pat. No. 6,083,191 to Rose et al., 2000, described the introduction of a sense ceramic in the piezoelectric stack, and described its use as a means of controlling stroke. In the later implementation, a limiter was used to control power electronically, and it was said this provided tissue selectivity enhancement.
A known instrument on the market for the ultrasonic fragmentation of tissue at an operation site and aspiration of the tissue particles and fluid away from the site is the CUSA® Excel Ultrasonic Surgical Aspirator (Integra LifeSciences Corporation, Plainsboro, N.J.). When the longitudinally vibrating tip in such an aspirator is brought into contact with tissue, it fragments and removes the tissue. In simple harmonic motion devices, the frequency is independent of amplitude. CUSA Excel magnetostrictive transducers utilized a magnetic feedback coil and circuit to monitor stroke.
The devices described above suffered from one or more drawbacks. For example, such devices did not provide proper tissue selectivity over the full amplitude range of operation. For instance, they did not necessarily enable precise tissue selectivity at low amplitudes (e.g., below 50% amplitude settings). Also, for instance, control of reserve power in such devices does not function correctly at low amplitudes, and these low amplitudes are often employed in neurosurgery. In neurosurgery practice, such as removal of glioma tumors, it is found 30% or lower amplitude is often employed and the control of the reserve power with analog control of such devices is not functional. In fact, higher reserve power could occur with some settings of higher selectivity.
Hence, for these and other reasons, there is a need for a surgical handpiece with improved tissue selection function. The present disclosure fulfills this need and others.