WO1990003150A1

WO1990003150A1 - Phacoemulsification transducer

Info

Publication number: WO1990003150A1
Application number: PCT/US1989/004207
Authority: WO
Inventors: Kevin L. Klug
Original assignee: Storz Instrument Company
Priority date: 1988-09-30
Filing date: 1989-09-28
Publication date: 1990-04-05
Also published as: JPH03502540A; GB2229924B; CH678700A5; EP0389615A4; GB2229924A; SE9001916D0; EP0389615A1; GB9011289D0; NL8921049A; SE9001916L; SE468197B

Abstract

The invention concerns a phacoemulsification probe having an acoustic transducer (3) which vibrates a needle (94), and further comprising a reflector (43) for reflecting energy toward the needle which would otherwise not be transmitted to the needle, and means (46, 66) for maintaining a constant pressure between the transducer (3) and the electrodes (50) which supply power to it.

Description

PHACOE ULSIFICATION TRANSDUCER

The invention concerns a device which vibrates a hollow needle at ultrasonic frequencies. The invention can be used in the medical treatment of cataracts, wherein the vibrating needle shatters the cataract, and the shattered debris is withdrawn through the hollow part of the needle.

Background of the Invention - Prior Art

Figure 1 illustrates a piezoelectric transducer 3 which can be used to set up a standing ultrasonic wave in tube 6. The transducer comprises two piezoelectric crystals 9 and 12 separated by an electrode 15. When the crystals are excited by an alternating current signal applied to surfaces 21A and 21B, the crystals expand and contract at the frequency of the signal. That is, the crystals cycle between the expanded size indicated by phantom lines 18 and the smaller size indicated by surfaces 21A and 21B. This cyclic expansion and contraction applies mechanical pulses to tube 6. If the pulsing frequency equals the resonant frequency of the tube 6, a standing wave is established. The standing wave causes a needle 24 to oscillate between phantom position 24A and solid position 24. The oscillating needle can be used to fracture hard materials, such as cataracts in the human eye.

One type of prior art device uses an electrode 15 manufactured of a beryllium-copper alloy. However, such an electrode suffers from the cyclic compression and relaxation imposed by the vibration of the crystals 9 and 12; over time, the electrode 15 becomes extruded, as illustrated in grossly exaggerated form in Figure 2. This extrusion causes at least three effects. First, it causes minute air gaps to appear, as illustrated by gap 28. These air gaps degrade the acoustic coupling between surfaces 30 and 33, thus reducing the efficiency of transmission of ultrasonic energy into tube 6. Second, the air gaps degrade the electrical contact between electrode 15 and crystals 9 and 12. Good electrical contact is necessary in order to deposit the electrical charge which induces the piezoelectric movement of the crystals 9 and 12. Third, the extrusion unloads the mechanical pressure which was originally applied to the crystals 9 and 12. That is, the crystals are preloaded in compression by mechanical forces illustrated by arrows 36 and 39 in Figure 1. The electrode 15 reacts the forces 36 and 39. The change in thickness of the electrode caused by the extrusion reduces the reaction, decreasing the compression, thus causing the crystals 9 and 12 to become unloaded and to operate under nonoptimal conditions.

A second feature of the probe in Figure 1 is that significant acoustic energy, indicated by waves 40, radiates away from, and not into, tube 6. Waves 40 do not impart energy to the needle 24; their energy is lost.

Objects of the Invention

It is an object of the present invention to provide a new and improved ultrasonic transducer.

It is a further object of the present invention to provide a reflector for an ultrasonic transducer which reduces lost acoustic energy.

It is a further object of the invention to provide an electrode for an ultrasonic transducer which resists extrusion and deformation induced by the cyclic flexing of the transducer. It is a further object of the invention to provide an apparatus which places a substantially constant pressure upon an ultrasonic transducer, irrespective of temperature changes.

It is a further object of the invention to provide a phacoemulsification probe which captures and reflects toward a needle acoustic energy which would otherwise be lost, and which maintains an acoustic transducer in a substantially constant degree of compression, irrespective of temperature changes.

It is a further object of the invention to provide a unique acoustic power delivery system which incorporates both an automatic frequency tracking capability that maintains stable oscillation signals over a specified frequency range and a load compensation mechanism that follows the changing power demand of the acoustic load and ensures maximum power transfer to the load over a specified range of load condition variance.

Summary of the Invention

In one form of the invention, a piezoelectric crystal transducer sandwich is located between a high acoustic impedance medium, called a reflector, and a low acoustic impedance medium, called a resonator. The resonator impedance is matched to the acoustic load thereby transferring maximum power from the transducer to the load. The high impedance reflector recovers acoustic energy which would be otherwise lost and redirects it through the resonator acoustic path towards the load. The result is a higher efficiency energy transfer compared to prior art mechanisms.

Automatic frequency and load tracking capability is provided electronically by a phacoemulsification control circuit. An optimum, nearly constant mechanical stress environment is maintained for the piezoelectric transducers over a specified operational temperature range by a unique flexible clamping mechanism. An optimum mechanical stress environment is maintained for the needle support by the resonator which is an acoustic horn and tube combination having a shape that closely approximates the ideal catenoidal horn assembly. Brief Description of the Drawing

Figure 1 illustrates a phacoemulsification probe as used in the prior art.

Figure 2 illustrates the extrusion which can occur in electrode 15 in Figure 1.

Figures 3 and 4 illustrate one form of the invention.

Figure 5 illustrates reflection of acoustic waves by reflector 43 in Figure 4. Figure 6 illustrates schematically the compression of transducer 3 of Figure 3.

Figure 7 illustrates schematically the expansion of rod 66B, which represents rod 66 in Figure 3, which occurs in order to maintain constant pressure upon transducer 3B in Figure 7.

Figure 8 illustrates a circuit which provides a signal to a transducer which is of the same frequency as the resonant frequency of a load on the transducer.

Detailed Description of the Invention Figure 3 illustrates one form of the invention, while Figure 4 illustrates the invention of Figure 3, but in exploded, simplified, schematic form. In these figures, an ultrasonic transducer 3 is located between a reflector 43 and a resonator 46. The transducer comprises an electrode 50, constructed of unhardened #01 carbon steel, and two piezoelectric crystals 53 and 56, constructed of a modified lead zirconate titanate ceramic material, formed into rings, silver coated for electrical conductivity, and marketed under the trade name PXE by the Electronic Components and Materials Division of North American Phillips Corporation. A lug 59, fastened to the electrode, allows connection to a power supply. An insulating tube 61 fits within a bore 63 within transducer 3. The reflector 43 is fastened to the resonator 46 by a hollow threaded tube 66, which mates with threaded regions 68 and 70 in the reflector and resonator. Both the hollow tube 66 and the resonator 46 are constructed of 6AL-4V titanium. Reflector 43 is constructed of #17 tungsten. Insulating sleeve 61 is constructed of Teflon, Teflon being a trademark of the DuPont Chemical Corporation.

In assembling the components of Figure 4 into the completed assembly of Figure 3, threaded tube 66 is first threaded into resonator 46 until an end 72 in Figure 3 becomes seated against shoulder 75. Then, reflector 43 is threaded onto threaded tube 66, in order to compress the transducer 3. The amount of compression is determined by the following method.

A two microfarad capacitor 77 is connected across piezoelectric crystal 56, as indicated in Figure 3. This connection places capacitor 77 in parallel with crystals 53 and 56. This parallel arrangement exists because the threaded tube 66 electrically connects reflector 43 with resonator 46, thus placing resonator 43 and reflector 46 at the same electrical potential. (That is, surfaces 79 and 80 of the crystals are both electrically connected to lead 83 of the capacitor 77, while surfaces 85 and 86 are connected with lead 89.)

After placement of capacitor 77 in parallel with crystals 53 and 56, reflector 43 is advanced toward resonator 46 by rotation upon threaded tube 66 until the piezoelectric crystals are compressed to the extent that the voltage across capacitor 77 reaches 0.75 volts. At this time, advancement of reflector 43 is stopped, and the piezoelectric crystals 53 and 56 are now properly compressed.

One reason for making this particular type of voltage measurement, using capacitor 77, is that the total capacitance of crystals 53 and 56 is approximately 600 to 700 picofarads. The electric charge separation, which is induced by compression between reflector 46 and resonator 43 of the crystals, with such a small capacitance, would produce a large voltage, of the order of hundreds of volts. Measurement of such a voltage under these conditions is difficult, at least for the reasons that a very small RC time constant results from the combination of -inherent crystal capacitance and the input resistance of the voltmeter.

The assembly of Figure 3 can be used as follows. A phacoemulsification needle 94, known in the art, such as Model Number IA-145, available from Storz Instrument Company, located in St. Louis, Missouri, is screwed into threaded end 96 of resonator 46. In use, the needle vibrates in a longitudinal mode by alternately compressing to solid position 94 and expanding to phantom position 98. The vibrational displacement, indicated by dimension 101, is about 5/1000ths of an inch. The vibration of the needle occurs at the oscillation frequency of the piezoelectric crystals 53 and 56, which are coupled to the needle 94 through resonator 46. Curved region 104 of the resonator 46 acts as a horn in order to impedance-match crystal 56 with needle 94, in order to maximize energy flow toward the needle 94. Resonator 46, as a whole, functions as a 1/4 wavelength transmission line (at the crystal frequency) on which needle 94 acts as a load.

Crystals 53 and 56 in Figure 3 are driven by a signal applied to electrode 50 and reflector 43. The application of an alternating current signal to the crystals 53 and 56 causes them to cyclically expand to the phantom position 107, shown in exaggerated form in Figure 4, and then contract to the solid position shown. This cyclic expansion and contraction applies mechanical pulses to the resonator 46, at the signal frequency.

The signal frequency which drives electrode 50 and reflector 43 is preferably 28.0 to 29.0 kilohertz. One system for applying such a driving signal to crystals 53 and 56 is described in U.S. patent application entitled "Control System For Ophthalmic Surgical Instruments," Serial No. 928,170, filed November 6, 1986, in which the inventors are Gregg Scheller, et al., and which is assigned to the assignee of the present invention. This application is hereby incorporated by reference. One embodiment of such a system is available from Storz Instrument Company, St. Louis, Missouri, under the product name of "DAISY." One type of circuit that is utilized in the DAISY system to apply an electrical signal to drive the transducer at its resonant frequency is shown in the block diagram of Figure 8. In the present explanation, the transducer 3 is modeled as an RLC series resonant network in parallel with a capacitance when operating under load and near the transducer's resonant frequency. This model of the transducer is not shown in Figure 8.

Being a closed loop system, the driving circuit is essentially an oscillator which fulfills the Barkhausen criteria for oscillation: zero phase shift and unity loop gain. The design frequency of the oscillator is 28,500 ⁺- 500 hertz.

The feedback portion of the loop consists of an injection oscillator 203, a band pass active filter 205, a low pass active filter 207, and a variable gain amplifier 209. The injection oscillator 203 provides an initial voltage signal at a frequency near the transducer resonant frequency. That signal will be disengaged from the feedback loop path once the driving circuit provides a signal strong enough to maintain the transducer oscillations. The band pass and low pass filters provide the appropriate frequency selectivity and phase shift characteristics to maintain the strength of the transducer feedback signal while the transducer phase characteristics vary over a normal operating range. The signal fed back from the transducer is derived over a compensation network 213 which provides additional frequency selectivity and phase shift stability. The variable gain amplifier 209 establishes the loop gain during initial calibration of the filter circuits, and remains essentially fixed after the filter circuit calibration is complete. The power amplifier and transformer 215 provide a maximum driving voltage of about 380 volts rms with a maximum current of about 10 milliamps rms. A gain control network 218 provides a stable voltage signal output by comparing the driving voltage, on line 221, with a voltage command reference level, provided by a user on line 223,^' and then compensating for any differences by adjusting the gain of the power amplifier 215.

Vibration of the needle 94 in Figure 3 can be used in the medical treatment of hardened objects, such as cataracts in the human eye. The vibrating needle 94, when brought near a cataract, causes the cataract to shatter, and the shattered debris is withdrawn through channel 110, under the influence of a vacuum source 115 attached to nipple 117.

Several important aspects of the invention are the following:

(1) As stated above, reflector 43 is constructed of tungsten. Tungsten has a very high acoustic impedance, of the order of 90 x 10⁶ kg/(m²-sec) to 105 x 10⁶ kg/(m²- sec) . Consequently, the acoustic energy reflected at the interface 79 in Figure 3 is reflected (1) in phase, with

(2) a reflection coefficient of almost unity, meaning that almost 100% of the energy is reflected, with minimal transmission into reflector 43. This high reflection recovers and redirects toward resonator 46 energy which would otherwise have been lost into reflector 43. Such lost energy is indicated as waves 40 in Figure 1. The high reflection attained by the invention can be explained as follows.

As the traveling acoustic waves move through the transducer 3, they encounter different levels of acoustic impedance, depending upon the density and moduli of elasticity of the different materials composing the transducer assembly. As an acoustic wave crosses a boundary between two such different materials, it is probable that the wave will experience a reflection phenomenon. The theory of energy transmission quantifies this reflection phenomenon, as the following discussion will explain.

The reflection coefficient, which is a complex number having both real and imaginary parts (both being possibly nonzero) , describes the amount of the incident wave energy which is reflected at the boundary between the different materials. It also describes the phase relationship between the incident and reflected waves, that relationship being either in phase (zero degrees phase shift) or out of phase (by up to 180 degrees) .

The principal design methods employed for the transducer assembly used an initial assumption that the transmission media for the acoustic waves are lossless. This assumption provides the benefit that the mathematical manipulations required to implement an acoustic transmission design are far more manageable, and incur little cost in terms of accuracy of the final result.

Consistent with the lossless transmission assumption is the method by which the reflection coefficient is calculated. In its most general form, that calculation is simply a ratio of (1) the differences between two acoustic impedances and (2) the sum of those same two impedances. A potentially confusing situation arises when trying to assign numerical values to each impedance before the ratio is computed. The calculation for the reflection coefficient, R, is the following:

R = (Z_L - Z₀)/(Z_L + Z₀)

In this form, Z_L represents the specific acoustic impedance presented to the acoustic wave as it travels from a medium having a characteristic acoustic impedance Z₀ into a medium having a characteristic acoustic impedance Z_x. The numerical value of the specific acoustic impedance is a function of the characteristic acoustic impedance, the length of the material from the incident wave interface to the acoustic termination of that material section, and the numerical value (possibly complex) of the specific acoustic impedance presented to the transmitted wave when it reaches the termination. Also important, in the most general sense, are the attenuation properties of the material. However, as stated earlier, those properties are ignored for the purposes of this design because it is felt that sufficient dimensional constraints have been placed on the component parts so that the no loss assumption remains valid.

One important dimension on which this design is based is the wavelength of the acoustic wave as it passes through the ceramic crystal material. In order for the tungsten reflector to have the desired in phase reflection properties, its length must be close to 1/4 wavelength with a low acoustic impedance backing, that is, with a backing which is nearly an acoustic short circuit over the operating frequency range of the transducer assembly. The acoustic impedance of air is usually considered to be an acoustic short circuit.

Under these conditions, the specific acoustic impedance presented to the transmitted acoustic wave at the tungsten reflector termination is nearly zero. Therefore, the specific acoustic impedance presented to the incident acoustic wave at the interface between the ceramic crystal and the tungsten reflector is nearly infinite. As a consequence, the numerical value of Z_L is very large compared with Z₀, and the reflection coefficient, defining the pressure amplitude and phase shift of the incident acoustic wave occurring upon reflection, will nearly equal unity. This is, nearly 100% of the incident acoustic wave will be reflected in phase, thereby increasing the net pressure amplitude of the acoustic wave in the primary direction of acoustic power delivery, that is, towards the resonator and ultimately to the needle tip.

In practice, the actual length of the reflector section is less than 1/4 wavelength. However, the no loss assumption results in a pure imaginary number representing the specific acoustic impedance presented by the reflector. The net result is that the magnitude of the reflection coefficient will always be 1, even for reflector lengths other than 1/4 wavelength. The noticeable difference in the reflected wave will be the phase relation between it and the incident wave. Varying the length of the reflector will change that phase relationship. For the present configuration, that phase shift should be less than 30 degrees over the normal operating frequency range of the transducer.

In the preferred embodiment, Z_x is approximately 100 x 10⁶ kg/(m²-sec) and Z₀ is approximately 30 x 10⁶ kg/(m²-sec). Both of these numbers are real, that is, complex numbers with a zero imaginary part. Z_L is approximately 130 x 10⁶ kg/(m²-sec) . This number is imaginary, that is, a complex number with a zero real part. Z_L is derived from standard distributed transmission line methods which incorporate the length, acoustic velocity and attenuation characteristics of the material, as well as the characteristics of the acoustic load making contact with the material.

Z_L = j tan (βl) Z_x

where β is the phase shift constant, 1 is the material length and j is the square root of -1.

(2) As stated above, threaded tube 66 in Figures 3 and 4 is constructed of 6AL-4V titanium. This alloy of titanium has a small modulus of elasticity. Modulus of elasticity is commonly defined as the ratio of unit stress to unit strain, or tensile force per square inch divided by elongation per unit length. Stated another way, with a low modulus, a small tensile force causes a large elongation of the threaded tube 66. In oversimplified terms, the titanium threaded tube 66 stretches easily. The small modulus of elasticity is important because thermal expansion and contraction of the threaded tube 66, which hold together reflector 43 and resonator 46

(thereby applying pressure to the piezoelectric crystals 53 and 56) , could cause the tube 66 to change in length, thus changing the pressure applied to piezoelectric crystals 53 and 56, which is undesirable. The small modulus of elasticity accommodates thermal dimension changes. An example will illustrate this. Thermal growth only affects components lying to the left, in the direction of arrow 130, of surface 80 in Figure 3 because resonator 46 is constructed of the same material as threaded tube 66, and thus the coefficients of thermal expansion of tube 66 and resonator 46 are the same. As to components on the left of surface 80, if a cooling of transducer 3 occurs, and if threaded tube 66 tends to contract more than does transducer 3, then reflector 43 and resonator 46 tend to compress the crystals 53 and 56 by applying more pressure. However, a low modulus of elasticity allows the threaded tube 66 to stretch, maintaining the pressure substantially constant. This is further explained with reference to Figure 6. Walls 132 and 134 represent the ends in Figure 3 of reflector 43 and resonator 46 respectively which compress transducer 3. Springs 137, which tend to pull the walls 132 and 134 together, represent the threaded tube 66 which holds together the reflector and the resonator.

In general, the force applied by a spring 137 is proportional to its percentage change in elongation, but, however, it may be assumed that, for small elongations (of the size involved in thermal expansions) the force is relatively constant. Therefore, if thermal expansion of the transducer 3 tends to drive wall 134 into phantom position 134A with respect to wall 132, springs 137 stretch, maintaining a relatively constant opposing force, which compresses transducer 3. Threaded rod 66 in Figure 3, in acting like spring 137, maintains the pressure upon crystals 53 and 56 at a relatively constant value.

Rod 66 in Figure 3 has an outer diameter of 0.164 inches, an inner diameter of 0.0625 inches, and has a length between threaded junctions (i.e., dimension 130, representing the distance between junctions 68 and 70) of 0.580 inches. These dimensions of rod 66 give it an approximate modulus of elasticity of 16.5 x 10⁶ psi, which is considered appropriate for the diameter of transducer 3, which is 0.394 inches, and for temperature excursions from 60 degrees Fahrenheit to 270 degrees Fahrenheit.

Rod 66 has been described as a spring which experiences a small extension, in response to thermal expansion of transducer 3, thus applying only a small change in pressure to crystals 53 and 56. It will now be shown that the particular configuration of the invention in Figure 3 causes an even smaller change in pressure, as compared with the schematic configuration of Figure 6.

Assume that the transducer 3 in Figure 6 expands thermally by 0.001 inches (i.e., dimension 132 is 0.001 inches). In order to maintain constant pressure on transducer 3, rod 66 is Figure 3 must both (1) expand 0.001 inches and (2) maintain the same spring force on transducer 3, as explained above. According to Hooke's law, it is the percentage change (not absolute change) in length of a spring that determines the absolute change in spring force. In this example, if the stretching region of rod 66 were coextensive with transducer 3 (i.e., threaded junction 68 ended at point 135, so that the stretching region of rod 66 is as long as transducer 3) , and if the stretching region of rod 66 were 1 inch long, then the percentage change of rod 66 is 0.001/1.0 or 0.1 percent.

On the other hand, when the stretching region is as shown in Figure 3 (extending from threaded junction 68 to threaded junction 70) , the percentage change is reduced. If the stretching region, distance 130, is 3 inches long, then the percent change is 0.001/3.0 or 0.033 percent.

Therefore, the configuration of Figure 3 provides a change in spring force which is three times smaller than when the stretching region of rod 66 is coextensive with transducer 3 (i.e., 0.033 v. 0.1). One reason for this^* reduction in change is that the length of spring involved (length 130) is longer than transducer 3, whose thermal expansion, if unaccommodated, tends to increase pressure on crystals 53 and 56.

Viewed another way, the thermal expansion of one element (i.e., the transducer) which tends to increase pressure on itself (because of being located in a vise having jaws in the form of reflector 43 and resonator 46) is accommodated by stretching of the rod 66 which holds the jaws together. Further, the stretching rod 66 is longer than the expanding transducer 3. Thus, the percentage elongation of the rod is less than the percentage elongation of the transducer. The difference in elongation is further illustrated in Figure 7 wherein reflector-jaw 43B and resonator-jaw 46B squeeze transducer 3B between them. Rod 66B holds the jaws together. If the transducer 3B expands from dimension 140 to dimension 144, rod 66B expands from dimension 146 to dimension 148. The absolute expansion of transducer (dimension 150) equals the absolute expansion of rod 66 (dimension 152) , yet the percentage expansion of rod 66B (dimension 152/dimension 146) is less than the percentage expansion of transducer 3 (dimension 150/dimension 140) . Consequently, the change in spring force applied by rod 66 is less than if the percentage change in length of rod 66 were equal to that of transducer 3.

This small change in spring force provides a more constant compression applied to transducer 3 in Figure 3: the thermal expansion of the transducer 3 is distributed over a longer spring, namely, over a spring of length 130, which is 0.580 inches in the preferred embodiment, as compared with the length of transducer 3, which is 0.222 inches, dimension 140 in Figure 4.

It is to be noted that both the forces of thermal expansion and the spring force of threaded rod 66 are significantly greater than the pressure forces applied by the acoustic pulses. That is, threaded rod 66 in region 130 in Figure 3 is not significantly elongated at the acoustic frequency of about 29 kilohertz by the acoustic pulses.

(3) The surfaces of the elements involved in meeting at interfaces 79 and 80 in Figure 3 are lapped and polished to within 1/10,000 inch flatness, or stated another way, to a number 2 microfinish. (4) The effective impedance of the resonator 46, as seen by crystal 56, is affected by the loading upon needle 94. (When the needle delivers energy to a cataract, the needle becomes "loaded.") From one point of view, the impedance of resonator 46 changes upon loading. Similarly, the presence of debris in tube 110 within resonator 46 affects resonator impedance. Given that the Q of the resonator 46 is very sharp, of the order of 1,000 to 2,000, as a result, the band width is very narrow, of the order of 15 to 30 hertz. Therefore, the frequency of the input signal applied to transducer 3 must be continually matched to the changing resonant frequency of the resonator 46. The apparatus described in the patent application identified above accomplishes such matching.

Numerous modifications and substitutions can be undertaken without departing from the true spirit and scope of the invention as defined in the following claims.

Claims

Clai s

1. In a phacoemulsification probe having a waveguide which couples a mechanical oscillator with a needle which is vibrated by the oscillator, the improvement comprising: a) a reflector coupled to the oscillator for increasing the amount of acoustic energy delivered to the needle.

2. Apparatus for vibrating a needle, comprising: a) a mechanical oscillator; b) a coupler in acoustic contact with the oscillator and which i) is resonant at a frequency attainable by the oscillator, and ii) supports the needle; and c) a reflector in acoustic contact with the oscillator for increasing the efficiency of delivery of acoustic energy to the needle.

3. Apparatus for vibrating a needle, comprising: a) transducer means for radiating ultrasonic energy in first and second directions; b) means for coupling ultrasonic energy radiating in the first direction with the needle; and c) reflector means for reflecting ultrasonic energy radiating in the second direction toward the needle.

4. In a phacoemulsification probe having a piezoelectric oscillator compressed between two elements, the improvement comprising: a) a member in tension extending between the two elements and which has a modulus of elasticity such that the tension remains substantially constant as temperature changes.

5. A phacoemulsification probe, comprising: a) a piezoelectric element having first and second faces; b) an impedance matching horn for transmitting acoustic energy from the first face to a needle; and c) a reflector contacting the second face and having an acoustic impedance such that the reflection coefficient at the region of contact has a real component exceeding about 0.9.

6. A phacoemulsification probe, comprising: a) a piezoelectric crystal pair, separated by an electrode, and having first and second faces; b) an impedance matching horn for transmitting acoustic energy from the first face to a needle; and c) a tungsten reflector in contact with the second face for reflecting acoustic energy toward the impedance matching horn.

7. Apparatus according to Claim 6 and further comprising: d) compression means for compressing the piezoelectric crystal pair between the impedance matching horn and the tungsten reflector, the compression means having a modulus of elasticity such that the compression of the piezoelectric crystal pair remains substantially constant as temperature changes.

8. In a phacoemulsification probe having an oscillator compressed between two elements, the improvement comprising: a) spring means connecting the two elements for maintaining pressure on the oscillator substantially constant as temperature changes.

9. Apparatus according to Claim 8 in which the spring means has a spring constant which is substantially unchanging over the extension of the spring means caused by temperature changes.

10. In a phacoemulsification probe, the improvement comprising: a) a piezoelectric oscillator; b) an acoustic resonator, abutting a first face of the piezoelectric oscillator, for transmitting acoustic energy out of the oscillator; c) an acoustic reflector, abutting a second face, opposite the first face, of the piezoelectric oscillator, for reflecting acoustic energy into the oscillator; d) a member connecting the resonator and the reflector such that tension in the member causes compression of the oscillator, the member having a modulus of elasticity such that temperature changes cause no substantial change in the compression of the oscillator.

11. A phacoemulsification probe comprising: a) the following elements in the following spatial sequence on an axis: i) a mount for a needle; ii) an acoustic resonator having a Q in excess of 1,000 and supporting the mount; iii) a first piezoelectric crystal abutting the acoustic resonator; iv) an electrode abutting the first piezoelectric crystal; v) a second piezoelectric crystal abutting the electrode and having an acoustic impedance Z₀; vi) a reflector abutting the second piezoelectric crystal and having an acoustic impedance such that the real part of the reflection coefficient between the reflector and the second piezoelectric crystal is positive and greater than about 0.9.

12. Apparatus according to Claim 11 and further comprising: b) a channel contained within the elements of paragraph (a) ; and c) a tube surrounding the channel and connecting the resonator to the reflector, and which is under tension, causing the resonator and the reflector to compress the piezoelectric crystals, and which has a modulus of elasticity such that the compression of the piezoelectric crystals remains substantially constant as temperature changes.

13. Apparatus according to Claim 11 in which the electrode is nondeformable by vibration of the piezoelectric crystals.

14. In the operation of a phacoemulsification probe, which includes a mechanical oscillator coupled to a needle, the improvement comprising the following step: a) reflecting, toward the needle, oscillator energy which is traveling away from the needle.

15. The method of Claim 14 in which the reflected energy is placed into substantially the same phase relationship with other energy traveling toward the needle.

16. The method of Claim 14 in which the reflection occurs at an interface having a positive acoustic reflection coefficient at the frequency of the mechanical oscillator.