SYSTEMS AND METHODS FOR THE MEASUREMENT OF THE STATE OF CURE OF PMMA IN A SURGICAL DISPENSING APPLICATOR

Abstract

The invention comprises a system for aiding in the in-situ determination of the state of cure (or progress of cure) in bone cement which is a composition that cures in an exothermic reaction and having a device which comprises a frequency sensor joined to a circuit and to an indicator that emits a signal in response to a current emitted to the circuit by the sensor to monitor a state of cure. The invention also relates to surgical methods using the system.

Claims

1. A system for sensing the state of cure progress of PMMA cement, comprising: a applicator for use during a surgery having a reservoir that contains a quantity of a cement during cure, a sensor assembly having a sensor probe which is in contact with the quantity of cement and which monitors one or more of the mechanical resonance spectra and frequency peaks of the sensor probe in mechanical contact with the quantity of cement and a signal processing system which is in communication with the sensor probe to determine and communicate a state of cure of the cement using one or more of either the mechanical resonance spectra and the frequency peaks of the sensor probe.

2. A system for sensing the cure progress of PMMA cement as set forth in claim 1, wherein the sensor assembly includes a first electromechanical transducer in mechanical series contact with a second electromechanical transducer in mechanical series contact with the sensor probe which has a distal tip in contact with the quantity of cement.

3. A system for sensing the cure progress of PMMA cement as set forth in claim 2, wherein the first electromechanical transducer acts as a mechanical excitation or transmitter of mechanical vibrations.

4. A system for sensing the cure progress of PMMA cement as set forth in claim 3, wherein the second electromechanical transducer acts as a mechanical excitation receiver of vibrations.

5. A system for sensing the cure progress of PMMA cement as set forth in claim 4, wherein the first and second electromechanical transducers in mechanical contact and in series with the distal tip of the sensor probe form a sensor probe assembly with a self-resonant frequency spectrum and natural frequency peaks.

6. A system for sensing the cure progress of PMMA cement as set forth in claim 1, wherein the hardness of the quantity of cement represents its state of cure and this hardness in mechanical contact with the distal tip of the sensor probe, modifies the natural resonance of spectra of the sensor probe assembly.

7. A system for sensing the cure progress of PMMA cement as set forth in claim 6, wherein the hardness modifies the resonance frequency of the sensor probe assembly by shifting the resonance frequency.

8. A system for sensing the cure progress of PMMA cement as set forth in claim 7, wherein the sensor probe is in contact with the quantity of cement during cure.

9. A system for sensing the cure progress of PMMA cement as set forth in claim 1, wherein the sensor assembly comprises electromechanical transducers which are electromagnetically actuated voicecoil motors.

10. A system for sensing the cure progress of PMMA cement as set forth in claim 1, wherein the sensor assembly comprises electromechanical transducers which are electrically actuated piezoelectric crystals.

11. A system for sensing the cure progress of PMMA cement as set forth in claim 1, wherein the sensor assembly comprises a plurality of electromechanical transducers in series which include an electromagnetically actuated voicecoil motor and an electrically actuated piezoelectric crystal transducer.

12. A system for sensing the cure progress of PMMA cement as set forth in claim 11, wherein the sensor assembly includes a dual voicecoil transducer or a dual PZT transducer.

13. A system for sensing the cure progress of PMMA cement as set forth in claim 1, wherein the sensor assembly uses a sweep signal can be in the audio frequencies of 20 Hz to 20 KHz.

14. A system for sensing the cure progress of PMMA cement as set forth in claim 2, wherein the applicator further includes a piston and the piston includes the distal tip of the sensor probe.

15. A surgical method, comprising: the steps of mixing the components of a bone cement and containing a quantity of the mixed cement within a surgical applicator during cure and monitoring the cure of the bone cement using a sensor assembly having a sensor probe which is in contact with the quantity of cement which monitors one or more of the mechanical resonance spectra and frequency peaks of the sensor probe in mechanical contact with the quantity of cement and a signal processing system which is in communication with the sensor probe to determine and communicate a state of cure of the cement based either the mechanical resonance spectra or the frequency peaks of the sensor probe.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The invention will be better understood and other features and advantages will become apparent by reading the detailed description of the invention, taken together with the drawings, wherein:

[0034] FIG. 1 is an illustration of the tester in use with an application syringe for the determination of the cement cure progress and showing a circuit diagram with the electronic components in accordance with the present invention;

[0035] FIG. 2 shows a photograph of a cement cure progress sensor test apparatus used to determine and characterize the variation of the resonance frequency shift as a function of time and temperature;

[0036] FIG. 3 is series of FFT spectra of the resonant frequency shift as a function of cure time and temperature (indicated in the legend);

[0037] FIG. 4 is a graph showing the measured frequency shift and temperature as a function of time for sample number 11 in a series of tests using the apparatus depicted in FIG. 2;

[0038] FIG. 5 is another graph showing the measured frequency shift and temperature as a function of time for sample number 12 in a series of tests using the apparatus depicted in FIG. 2;

[0039] FIG. 6 is another graph showing the measured frequency shift and temperature as a function of time for sample number 13 in a series of tests using the apparatus depicted in FIG. 2;

[0040] FIG. 7 shows a graph of all the data collected from all tests to illustrate the concept of an average frequency shift rate (Hz/sec) for 19 sets of data and how they have a consistent slope when only the first 400 seconds are considered. During this time, the temperature of the sample changes only about 1 deg C.; and

[0041] FIG. 8 shows a syringe style applicator in which the components of the cement are provided individually, subsequently exposed to each other and mixed, and finally monitored as a mixture to determine an appropriate state of cure.

DETAILED DESCRIPTION OF THE INVENTION

[0042] The present invention operates on the principle of the measurement of the resonance frequency of a mechanical structure in contact with the cement sample undergoing cure. As the cement cures, its viscosity changes and this affects the resonance frequency of the mechanical structure in contact with the cement. This mechanical structure is typically a probe tip that is in mechanical communication with a transducer that actuates vibrations on the probe and a transducer that senses those same vibrations. By monitoring the resonant frequency of the vibrations of the probe tip, one can ascertain in a quantitative way, the progress of the cure of the PMMA cement. FIG. 1 shows a voice coil actuator 101 that transmits mechanical oscillations through a piezoelectric transducer 102, through a probe shaft 103 that penetrates a syringe piston seal 104. The voice coil transducer 101 is mounted on the other side to the syringe handle 108 which resides inside the bore of a syringe body 196. The PMMA cement mixture that is being monitored is contained inside the syringe body 106 and the syringe piston seal 104, while in mechanical contact with the frequency probe shaft 103. Electrical wires connect the voice coil actuator 101 and piezoelectric transducer 102 to a signal processing system 115. The signal processing system 115 comprises an amplifier 111, which provides the excitation for the voice coil actuator 101, and the amplifier 111 is driven by a digital to analog converter (DAC) 110, that is controlled by a computer 109, which is also connected to an analog to digital converter (ADC) which receives the signal from the piezoelectric transducer 102. The ADC signal is captured and analyzed by software in the computer 109 where FFT's are performed on the data streams to generate a binary representation of the frequency spectrum so that the peak frequency position can be extracted.

[0043] FIG. 2 shows a photograph of the test apparatus with a voice coil actuator 101, a piezoelectric transducer 102, and a probe shaft 103 in contact with a PMMA cement sample 107 held in a small plastic sample holder fitted with drilled holes to allow for the probe tip 103 to be in contact with the cement, and for a temperature sensor 202 to be in contact with the cement. These items are mounted to a support frame 101 to position the sensor with respect to the PMMA sample. The tests all utilized 2.0 grams of monomer and 2.0 grams of polymer that has been mixed in a separate container and then transferred to the sample holder.

[0044] FIG. 3 shows a series of frequency spectra collected by excitation of the voice coil transducer and measuring the signal generated by the piezoelectric transducer. This signal is digitized using an ADC and the digital representation of the time series signal is converted to the frequency domain using an FFT in software. The FFT spectra are plotted and show that the peak resonant frequency of the probe tip in mechanical contact with the cement sample varies from about 150 Hz at the start to about 220 Hz at the end of the test. During the first approximately 400 seconds of the test duration, the cement goes from a runny liquid state to a thick viscous dough state, after which the dough is to firm to use and lacks stickiness. During this curing period of 400 seconds, the temperature of the cement only changes about 1? C. The small change in temperature, clearly, is not a reliable indicator of the state of cure of the cement, whereas the peak resonant frequency changes by about 16 Hz and is easily discernable. If we plot the peak resonance frequency as a function of time as shown by the circle markers in FIGS. 4, 5, and 6, we can see that during the first 400 seconds, the change in frequency versus time is linear in behavior and a linear curve fit for the first 400 seconds gives the slope indicated: 0.037 Hz/sec for FIG. 4, 0.0433 Hz/sec for FIG. 5, and 0.0458 Hz/sec for FIG. 6. FIGS. 4, 5, and 6 also shows that the first 400 seconds, the bulk cement temperature as monitored using an embedded negative temperature coefficient (NTC) 10 k ohm thermistor shows that the temperature only rises about 1? C. Furthermore, we can see that the resonance frequency continues to vary linearly beyond the usable cement cure time of 400 seconds until the frequency peak shifts at an accelerated rate until it changes no more (not shown) when the sample has fully cured and is hard. The lack of change of resonance frequency can also be used as a positive indicator of the cement reaching the final cure state.

[0045] FIG. 7 shows the data from 19 separate tests all plotted on the same graph and the average frequency shift versus time slope has been calculated and shown as having a value of 0.0462 Hz/sec+/?0.00624 Hz/sec or a 13.5% standard deviation. This shows that for all mixtures and tests, the progress of the PMMA cure can be tracked using this slope and when a change in frequency of about 16 Hz has been detected, the sample has reached the point of non-usability and must be discarded. So detecting a 4 Hz shift for example, would indicate that the cement has reached 25% of its usable life while it is awaiting inside the applicator syringe. Similarly, detecting a 12 Hz frequency shift would indicate that the PMMA cement has progressed about 75% through its usable life as it awaits use in the applicator syringe. Now, for the first time, a simple and quantitative means of monitoring the in-situ progress of the PMMA cement cure inside the applicator syringe or mixing chamber is possible. FIG. 1 illustrates a prototype of the invention with a syringe handle and piston that contains the voice coil actuator and piezoelectric transducer sensors for the real-time and in-situ monitoring of the PMMA cure progress inside a disposable applicator syringe. FIG. 8 shows an applicator 200 in which the components are provided in separate compartments 210, but which can be ruptured by a plunger 214 and feed through a spiral mixing container 220 to agitate and combine the components. In this mixing compartment 220, the state of the mixture is monitored to determine the appropriate state for use, and the applicator includes an applicator nozzle 222 through which the mixture is feed for use. This applicator provides a closed system such as to confine noxious fumes and to reduce the mess and trouble that can be associated with the individual components and with the adhesive mixture. The applicator system can be provided in a sterilized or sterilizable state.

Theory and Analysis

[0046] PMMA undergoes an exothermic reaction during cure. During the phase transition of PMMA from liquid to solid (curing) the exothermic reaction and this generates heat that causes the bulk temperature of the cement to increase. This temperature increase however, is not as easily monitored as the shift in resonance frequency of the mechanical structure formed by the transducer voice coil/piezoelectric transducer and probe tip which is in contact with the cement. As the cement cures, its viscosity or stiffness under mechanical strain increases. This mechanical property modifies the resonance frequency of the vibrating probe tip in such a way that is easily measured and distinguished using a FFT to determine the resonance frequency peak.

[0047] One theory as to this shift follows: In its simplest form, the resonance structure of the probe tip can be thought of as a simple cantilever beam is given by the equation:

[00001]$f=\frac{\mathrm{Kn}}{2?}\sqrt{\frac{\mathrm{EIg}}{{\mathrm{wl}}^4}}$

Where Kn is the mode of vibration, E is Young's modulus, l is the area moment of inertia, g is the gravitational acceleration, w is the beam width, and l is the beam length. Having the tip of the beam in contact with the viscous cement will essentially be affecting the beam's apparent Youngs modulus and this in turn, modifies the resonant frequency. We can see that a stiffer cement will increase the Youngs modulus and cause the resonant frequency to increase with increasing viscosity or stiffness of the dough state. We can also see why it is so sensitive because the viscosity is applied to the tip of the cantilever beam so that has an effect that is proportional to the 4.sup.th power of the length of the beam. There are other resonance structures that may be even more sensitive or can be designed to be more advantageous (such as discs with torsional springs or other 3D mechanical resonant structures such as a tuning fork). These other embodiments are just variants of the main technique taught here: that the change in the viscosity of the PMMA cement can be detected and monitored by the change in the resonant frequency of a structure in mechanical contact with the cement sample. Regardless of this theoretical underpinning, it has been shown empirically, and statistical significance that the present invention works.

[0048] Although the present invention has been described based upon the above embodiments and the data produced by measurement of the performance of the resulting invention that has been reduced to practice, it is apparent to those skilled in the art that certain modifications, variations, and alternative constructions would be apparent, while remaining within the spirit and scope of the invention. In order to determine the metes and bounds of the invention, reference should be made to the following claims.