SYSTEMS AND METHODS FOR MONITORING THE CURE OF PMMA BONE CEMENT DURING SURGERY

Abstract

The invention comprises a system which determines the cure status of bone cement which is then used in making surgical decisions on the cement use and further comprises a system for aiding in securing an implant to a bone using a cured grout or bone cement. The system includes a device which comprises a frequency sensor joined to a circuit to monitor one or more frequency signals, and to an indicator that emits a signal in response to a current emitted to the circuit by the system while monitoring the state of cure. The invention also relates to surgical methods using the system.

Claims

1. A system for securing an implant to a bone during surgery, comprising an implant, a grout or bone cement comprising a composition that cures during the surgery, the grout being capable of securing the implant to the bone in a cured state, and a sensor which imparts an energy wave in the acoustic range to the cement and receives the energy wave from the cement and monitors a frequency change in the energy wave to determine a cure state of the cement and the sensor also being joined to a circuit and to an indicator that emits a signal in response to a current emitted to the circuit by the sensor.

2. A system for securing an implant to a bone as set forth in 1, wherein the signal is an audio signal, visual signal or haptic signal or signal to a robotic device.

3. A system for securing an implant to a bone as set forth in 1, wherein the grout or bone cement is an acrylate.

4. A system for securing an implant to a bone as set forth in 3, wherein the acrylate comprises methyl methacrylate or poly methyl methacrylate.

5. A system for securing an implant to a bone as set forth in 1, wherein the bone is a human bone.

6. A system for securing an implant to a bone as set forth in 1, wherein the implant is an intermedullary implant.

7. A method for securing an implant to a bone, comprising: the steps of surgically exposing a bone and selecting an implant, applying an adhesive, grout or bone cement to the implant or to the bone, the adhesive, grout or bone cement comprising a composition that cures during surgery, the adhesive being capable of securing the implant to the bone in a cured state, using a tester to determine a state of cure of the adhesive, the tester comprising a frequency sensor which transmits a signal having a frequency below 10 MHz and which receives a response of the signal propagated through the grout or bone cement, the frequency sensor being in electrical contact with a circuit which is capable of conducting a current in response to a change in the frequency of the received signal, and to an indicator that emits a signal in response to an increase in the current in the circuit.

8. A method for securing an implant to a bone as set forth in 7, wherein the signal is an audio signal.

9. A method for securing an implant to a bone as set forth in 7, wherein the signal is a visual signal.

10. A method for securing an implant to a bone as set forth in 7, wherein the signal is a haptic signal.

11. A method for securing an implant to a bone as set forth in 7, wherein the adhesive, grout or bone cement is an acrylate.

12. A method for securing an implant to a bone as set forth in 7, wherein the acrylate comprises methyl methacrylate or poly methyl methacrylate.

13. A method for securing an implant to a bone as set forth in 7, wherein the bone is a human bone.

14. A method for securing an implant to a bone set forth in 13, wherein the implant is an intermedullary implant.

15. A system for sensing the cure point of PMMA cement having a tester assembly having a circuit which conducts a current in response to a change in frequency of a received signal and a frequency sensor which includes a sensor probe which is a frequency in mechanical contact with the PMMA cement and which monitors a mechanical resonance spectra and frequency peaks of the tester assembly and which receives a response of the received signal propagated through the PMMA cement, the frequency sensor being in electrical contact with the circuit, and to an indicator that emits a signal in response to an increase in the current in the circuit.

16. A system for sensing the cure point of PMMA cement as set forth in claim 15 that utilizes a sensor having a first electromechanical transducer in mechanical series contact with a second electromechanical transducer in mechanical series contact with a mechanical probe with a distal tip in mechanical contact with the cement to be monitored.

17. A system for sensing the cure point of PMMA cement as set forth in claim 16, wherein the sensor is a first electromechanical transducer which acts as a mechanical excitation or transmitter of mechanical vibrations.

18. A system for sensing the cure point of PMMA cement as set forth in claim 17, further including a second electromechanical transducer which also acts as a mechanical excitation receiver of vibrations.

19. A system for sensing the cure point of PMMA cement as set forth in claim 18, wherein the first and second electromechanical transducers are in mechanical contact with the PMMA cement and in series with a mechanical probe tip form an assembly with a self-resonant frequency spectrum and natural frequency peaks.

20. A system for sensing the cure point of PMMA cement as set forth in claim 15, wherein the degree of polymerization of the PMMA cement represents its state of cure and modifies the natural resonance of spectra of sensed by a sensor probe portion in contact with the cement.

21. A system for sensing the cure point of PMMA cement as set forth in claim 20, wherein the degree of polymerization modifies the resonance frequency of the sensor probe assembly by generating a new frequency.

22. A system for sensing the cure point of PMMA cement as set forth in claim 7, wherein the probe is in contact with the cement during cure.

23. A system for sensing the cure point of PMMA cement as set forth in claim 15, wherein the sensor comprises electromechanical transducers which are electromagnetically actuated voicecoil motors.

24. A system for sensing the cure point of PMMA cement as set forth in claim 15, wherein the sensor system comprises electromechanical transducers which are electrically actuated piezoelectric crystals.

25. A system for sensing the cure point of PMMA cement as set forth in claim 15, wherein the sensor comprises a plurality of electromechanical transducers in series which include an electromagnetically actuated voicecoil motor and an electrically actuated piezoelectric crystal transducers.

26. A system for sensing the cure point of PMMA cement as set forth in claim 25, wherein the sensor includes a dual voicecoil transducer or a dual PZT transducer.

27. A system for sensing the cure point of PMMA cement as set forth in claim 15, wherein the sensor uses a sweep signal can be in the audio frequencies of 20 Hz to 20 kHz or in the ultrasonic frequencies greater than 20 kHz depending on the regime of the natural resonance of the sensor probe assembly.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] FIG. 1 is an illustration of the tester in use with a total knee replacement and showing a circuit diagram with the electronic components in accordance with the present invention;

[0034] FIG. 2 is series of FFT spectra of the resonant frequency as a function of cure time and associated cement temperature of PMMA after mixing which demonstrates the cure profile of this sample;

[0035] FIG. 3 is a photograph of a prototype of the present invention; and

[0036] FIG. 4 shows the cement cure temperature and its first and second derivatives used to determine the exact time of the cure point.

DETAILED DESCRIPTION OF THE INVENTION

[0037] The standard means for stabilizing orthopedic implants used in total joint arthroplasty is grouting with an acrylic (poly methyl methacrylate, aka “PMMA”) material. In use, the PMMA is supplied in two parts: a powder monomer and a liquid catalyst. These two components are mixed during surgery and proceed from a liquid to a solid at varying rates depending on multiple factors such as temperature and humidity. The PMMA in its pliable state, known as the “dough state” is applied to the bone ends with the implant then being pushed onto the bone with the PMMA between the bone and implant. After hardening, the implant is considered fixed to the bone and motion is allowed. Motion prior to PMMA hardening can lead to lipid infiltration underneath the implant which breaks the bond between the PMMA and the implant leading to loosening and possible need for revision surgery.

[0038] The present state of the art for determining full curing of the PMMA intraoperatively is crude. This is ascertained either by direct palpation of the PMMA edge or by allowing the extra PMMA not used in implantation to harden. It is desirable to be able to provide a better, quantitative means to determine the full cure and to document that hardening of the PMMA has been achieved during surgery. More accurate determination would provide for decreased surgical time and better protection from PMMA implant bond breakage.

Theory and Analysis

[0039] PMMA undergoes an exothermic reaction during cure. During the phase transition of PMMA from liquid to solid (curing) the exothermic reaction has a thermal curve shown in FIG. 4. PMMA cure temperature is represented by Reference Temperature (T.sub.cure .sub.Point) which is determined in a calculation between the maximum temperature and the ambient temperature. This cure point has been standardized for Orthopedic PMMA as referenced in the ASTM Designation: F451 - 16, “Standard Specification for Acrylic Bone Cement of the Joint in Preparation of Finishing the Surgery.”

[0040] A great deal of work has been done to analyze the polymerization rates of polymers in order to optimize molding conditions and manufacturing methods. This body of work relates to the present invention as it informs the cure process and the thermodynamic analysis of the reaction as it proceeds. It is therefore useful to review this work for an understanding of the very complex polymerization process and for the desired quantification for cure rate as it relates to the present invention.

[0041] Lionetto, F.; Maffezzoli A., report in “Relaxations during the post cure of unsaturated polyester networks by ultrasonic wave propagation, dynamic mechanical analysis and dielectric analysis.” J. Polym. Sci. Polym. Phys. 2005, 43, 596-602 on the frequency and temperature dependence of two relations related to 1) the glass-transition temperature of a partially cured sample and 2) the glass-transition temperature for a fully cured sample over 6 decades of frequency in post-cure polyester networks. Specifically, these authors subjected cured samples to longitudinal ultrasonic waves at 2 MHz transmitted from a piezoelectric transmitter to a piezoelectric receiver through a sample in the form of a thin disk 30 mm in diameter and 2 mm in thickness. The samples were low molecular weight isopthlatic unsaturated polyester resins formed as the polymerization product of polyester oligomers dissolved in a reactive solvent (styrene). Their interest ultimately related to further knowledge of the isothermal post-cure behavior of sheet molding and bulk molding compounds, but the analysis involved the thermodynamic characterization of the polycondensation/addition reaction. The authors identified 2 phase transformations relating to gelation and vitrification. As part of this work, the authors used low frequency dynamic mechanical analysis of thin sample strips of 40 mm × 9 mm × 1.5 mm at 1 Hz and at 1% strain during heating from 30° to 200 ° at 2°/minute, to verify the results of their longitudinal wave ultrasound investigation. The authors concluded that the peaks observed were the result of a first low temperature relaxation attributed to the sample and further to additional cross-linking due to a kinetic controlled reaction and a second peak resulted from a change to a diffusion controlled cross-linking reaction and corresponding to the maximum glass transition temperature illustrating the full cure, and from the empirical data they were able to calculate the free energy of the system G. They also conclude that the ultrasonic wave propagation is related to the viscoelastic behavior which is described by the complex modulus and can be obtained from measurements of c and a and that when the sample dimension normal to the acoustic wave is large in comparison with the wavelength, the wave propagation is governed by the complex bulk longitudinal modulus (L″)

[00001]$L*=L^{\prime }+\text{i}{L}^{″}$

[0042] The real and imaginary components can be calculated as follows:

[00002]$L^{\prime }=\frac{\text{ρ}{\text{c}}^2\left[1-{\left(\frac{\text{αλ}}{2\text{π}}\right)}^2\right]}{{\left[1+{\left(\frac{\text{αλ}}{2\text{π}}\right)}^2\right]}^2}\text{and}{L}^{″}=\frac{\text{2ρ}{\text{c}}^2\left(\frac{\text{αλ}}{2\text{π}}\right)}{{\left[1+{\left(\frac{\text{αλ}}{2\text{π}}\right)}^2\right]}^2}$

[0043] For the studied systems, the term aλ/2π always remains lower than 0.05; therefore L′ and L″ can be calculated as follows:

[00003]$L^{\prime }=\text{ρ}\text{c2}{L}^{″}=2\text{ρ}\frac{{\text{C}}^3\text{α}}{\text{ω}}$

where w is the angular frequency and ρ is the density of the resin, which is assumed to be constant and equal to 1100 kg/m.sup.3 because, in comparison with the velocity change during crosslinking (more than 70% of the initial value), the density change (ca. 5%) can be neglected with a good approximation.

[0044] The authors also studied dielectric results and concluded two peaks can be observed in the dielectric loss factor, where similarly to dynamic mechanical and ultrasonic results, the first peak can be attributed to devitrification and cure reactivation. The further increase in the crosslinking density leads to a reduction in the molecular mobility and dielectric losses. As previously observed, the residual reactivity decreases with increasing iso-thermal cure of the first cure step because of an increase in the degree of cure of the networks developed in the isothermal cure. These authors subsequently plotted the logarithm of the frequency (logf) versus the reciprocal of the temperature of the dynamic mechanical, dielectric and ultrasonic loss factor peaks. Thus, Lionetto and Maffezzoli were able to show a good correlation of the frequency of the experimentally derived data with the Williams-Landel-Ferry equation derived from free volume theory interpretation of the glass-rubber transition

[00004]$\log \frac{f}{f_0}=\frac{-C_1*\left(T-T_0\right)}{C2+\left(T-T_0\right)}$

where fo and To are the reference frequency and temperature and are taken to be 1 Hz and T measured at 1 Hz, respectively. The constants C and C.sub.2 have values of 22 and 111.9 K, respectively, and have been obtained by an onlinear fitting procedure.

[0045] In a later publication, Monitoring the Cure State of Thermosetting Resins by Ultrasound Francesca Lionetto and Alfonso Maffezzoli Materials 2013, 6, 3783-3804; doi:10.3390/ma6093783 report extensively on the theory of sound wave propagation in polymers as follows:

[0046] Ultrasonic waves are mechanical vibrations (in the region of 20 kHz-100 MHz), which propagate through very small displacements of atoms and chain segments around their equilibrium positions. In the case of polymers, the forces acting along chain segments and between molecular chains, create displacements into neighboring zones, thus, creating stress waves through the material. Several kinds of ultrasonic waves may propagate through solids, namely longitudinal waves, shear waves, Rayleigh waves (or surface acoustic waves), and Lamb waves (or plate waves). In longitudinal waves, the material is subjected to alternate local compression and expansions and the motion of the particles of material transmitting these waves is in the same direction as the propagation of the wave. In shear waves the solid is locally subjected to shearing forces and the particle motion is perpendicular to the direction of the propagation of the wave. Since gases and liquids are practically incapable of transmitting shear, for ultrasonic cure monitoring, longitudinal waves are normally preferred to shear waves, which present a very high level of attenuation in liquids and soft gel samples. However, shear waves can be used to follow the curing process after the gelation stage. Ultrasonic waves are characterized by a wavelength, amplitude of displacement, and velocity of propagation. In most applications, ultrasonic waves are generated with a transducer, which converts electrical energy into ultrasonic waves. The same transducer (or a second one) will convert the ultrasonic wave back to an electrical signal for further analysis. The acoustic characteristics of a material are determined by two parameters, the ultrasonic velocity, c; and the ultrasonic attenuation coefficient, a. The first is the velocity of propagation of elastic waves, which is calculated from the measured “time of flight”, that is the time taken by the sound to propagate through the sample. The speed of sound in a homogenous medium is directly related to both elastic modulus and density; thus, changes in either elasticity or density will affect pulse transit time through a sample of a given thickness. The attenuation is a measure of dissipative energy, converted to heat, as the wave propagates through the material.

[0047] In this paper, the authors use sound velocity and attenuation as a measure of the cure and in using sound waves in their experiment conclude that it is necessary to maintain precise sample dimensions to determine an accurate assessment of the sound velocity and attenuation of the signal (for example in dynamic mechanical analysis (DMA) which looks at hysteresis or viscolelastic characteristics in responses to determine modulus as it relates to the final cure state. Their characterizations follow:

[0048] The propagation of longitudinal elastic waves can, in fact, be tracked even in liquid mixtures of monomers or oligomers. When the sample dimension normal to the direction of the acoustic wave propagation is large compared to the wavelength, the measurement of ultrasonic velocity and attenuation may be used to calculate the storage (L′) and loss (L″) components of the longitudinal modulus from the following expressions:

[00005]$L^{\prime }=\frac{\text{ρ}{\text{c}}^2\left[1-{\left(\frac{\text{αλ}}{2\text{π}}\right)}^2\right]}{{\left[1+{\left(\frac{\text{αλ}}{2\text{π}}\right)}^2\right]}^2}\text{and}{L}^{″}=\frac{\text{2ρ}{\text{c}}^2\left(\frac{\text{αλ}}{2\text{π}}\right)}{{\left[1+{\left(\frac{\text{αλ}}{2\text{π}}\right)}^2\right]}^2}$

where p is the material density; c the ultrasonic velocity; the ultrasonic attenuation coefficient; and λ is the wavelength of propagating waves, obtained from the ratio of velocity to frequency f, (λ= c/f). In linear elasticity, the longitudinal modulus is one of the elastic moduli available to describe isotropic homogeneous materials. L′ and L″ are related to the bulk (K′ and K″) and shear (G′ and G″) moduli from the equations:

[00006]${\text{L}}^{\prime }={\text{K}}^{\prime }+4/3{\text{G}}^{\prime }\left(2\right){\text{L}}^{″}={\text{K}}^{″}+4/3{\text{G}}^{″}$

[0049] Note that these relationships are valid for plane strain conditions, where L′ corresponds to the elastic modulus for specimens where the change in dimensions take place only in one direction, that is when deformations in the other two directions are constrained so that the dimensions remain unchanged. These conditions occur in specimens or structures where two dimensions are much larger than the third. When a λ/2π << 1, i.e., when the extent of attenuation per wavelength is small, as in most practical applications, the following simplified formulas can be used to calculate the two components of the complex longitudinal modulus:

[00007]$\begin{array}{l}{L}^{\prime }=\text{ρ}\text{c 2 ;}{\text{L}}^{″}=2\text{ρ}\text{c 3}\text{a}/\text{ω}\left(4\right)\text{where}\text{ω}\text{is the angular frequency}\\ \left(\text{ω}=2\text{π}\text{f}\right).\end{array}$

[0050] A second set of authors, Dunne, Xu, Makem, and Orr at the Medical Polymer Research Institute and the School of Mechanical & Aerospace Engineering both at the University of Belfast, UK reported on the theory of pulse-echo ultrasonic characterizations in their paper “Ultrasonic characterization of the mechanical properties and polymerization reaction of acrylic based bone cement” published 2006 at DOl: 10.1243/09544119JEIM168. Dunne et al. reported that broadband ultrasonic attenuation is related to the viscoelastic properties of a material with specific interest in PMMA bone cement. These studies used a single transducer to generate an ultrasonic wave pulse at 2.25 MHz and to study the time of flight to determine the attenuation of the wave as a result in the change in density of the sample cement during cure.

[0051] The following PhD thesis which has a great deal of relevant general information from work on characterization of the glass transitions of fully cured epoxy resins relating to the use of sound waves in DMA analysis of polymers and the underlying theories of thermodynamics relating to the polymerization processes. “Ultrasound Technique for the Dynamic Mechanical Analysis (DMA) of Polymers” vorgelegt von B.Eng MSc Jarlath Mc Hugh aus Longford, Irland von der Fakultät III - Prozesswissenschaften der Technischen Universität Berlin zur Erlangung des akademischen Grades Doktor der Ingenieurwissenschaften, http://citeseerx.ist.psu.edu/viewdoc/download?rep=rep1&type =pdf&doi=10.1.1.218.5019. This author investigated the influence of sound waves in the frequency range of 0.1 to 50 Hz using conventional DMA with a dual cantilever set-up at varying temperatures and used that to compare the use of the Williams Landel Ferry (WLF) equation to predict the influence of measurement frequency on the ultrasound results of his ultrasound propagation studies at a frequency of 4 MHz. relevant portions of McHugh’s analysis follows:

[0052] Two macroscopic phenomena gelation and vitrification usually mark the progress of this polymerization process. The point at which growth and branching of polymer chains leads to a transition from a liquid state to a rubbery state is called gelation. At or about this transition an abrupt change in viscosity or complex modulus will be observed. As the reaction continues it typically slows as the glass transition temperature nears the set mould temperature. This gradual cessation of the reaction marks the transition from the rubbery to the glassy state of the curing material. The resin eventually solidifies unless further reaction is triggered by increasing the cure temperature. This transition is referred to as vitrification. From the perspective of a material processor it is important to follow the progression of the reaction and if possible identify the two phenomena, for example on the variation of a complex modulus curve,

[0053] Using the Williams Landel Ferry (WLF) equation it is possible to predict the influence of measurement frequency on the ultrasound results. f Determination of complex longitudinal and shear modulus as well as tan(δ) from sound velocity and attenuation measurements. Expression of acoustic parameters in terms of their representative moduli or tan(δ) is necessary to compare ultrasound measurements with conventional DMA at measurement frequencies between 0.1 and 33 Hz. All acoustic parameters are evaluated as a function of frequency and temperature using a specifically developed software package based on Fast Fourier Transformation FFT principles. Using this analysis technique, factors such as the influence of frequency on the measured sound velocity and attenuation are considered. Signal amplitude losses at material boundaries, for example due to reflection are also accounted for by employing a two sample measuring technique.

[0054] This technique was employed to measure the variations in ultrasound parameters that are related to changes in viscoelastic properties of epoxy polymers as a result of the curing reaction. Only isothermal cure is monitored over a range of temperatures using transducers operating in transmission mode at a middle frequency of 4 MHz. Interpretation of ultrasound results is possible when qualified information about the chemical reaction as well as the resulting changes in viscoelastic properties in terms of complex modulus is available. Two macroscopic phenomena gelation and vitrification usually mark the progress of this polymerization process. The point at which growth and branching of polymer chains leads to a transition from a liquid state to a rubbery state is called gelation. At or about this transition an abrupt change in viscosity or complex modulus will be observed. As the reaction continues it typically slows as the glass transition temperature nears the set mould temperature. This gradual cessation of the reaction marks the transition from the rubbery to the glassy state of the sample. Experiments are performed using a Differential Scanning Calorimeter DSC and a rheometer which are employed to obtain information relating to the degree of cure or progression of the polymerization reaction and the resulting changes in viscoelastic properties and state transitions. The analysis techniques are based on different physical operating principles making a direct comparison very difficult. The Arrhenius relation that is valid for all techniques is employed to compare results. By combining results it is possible to determine the sensitivity of the ultrasound parameters to changes in the viscoelastic properties of the epoxy during the curing reaction and specifically to the material transformations at gelation and vitrification.

[0055] The present invention is different from but related to the analysis set forth in the cited literature, which provides mathematical evidence of the empirical basis of the present invention, which is described below with reference to the drawings. The present invention uses a tester to monitor the state of cure of the PMMA after initial mixing and prior to full cure as is illustrated in FIG. 4. This is accomplished by insertion of the probe member of a tester having a frequency sensor comprising a sound wave transmitter and receiver sensor, such as an acoustic transducer which acts as a transmitter of an acoustic wave and an ultrasound and/or acoustic piezoelectric transducer which acts as a receiver of the response wave to monitor one or more frequency changes in the transmitted wave in response to changes in the curing cement. The system of the present invention also includes a guide to provide access to the cement actually used to stabilize the bone construct (i.e., “in situ”). The variance between the transmitted sound wave and the received sound wave is monitored to register and alert a user when a condition has been reached, such as the defined frequency response.

[0056] A controller, such as a CPU, loaded with analytic software, acts to analyze the frequency response and to determine a pre-set condition as a “desired dough state” in which the cement is ready for application to the bone and/or implant. In this instance, the tester probe is preferably in direct contact or optionally “coupled” to the cement after the initial mixing and during cure. When mixing occurs in a vacuum, in order to avoid venting dangerous fumes within the OR, the mixing container includes a probe tip that is in contact with the cement, such as being borne on a mixing paddle or otherwise inserted into the low pressure mixing chamber.

[0057] In a second or alternative use of the tester, the probe is inserted into a cement surface in vivo to determine the state of cure of the applied cement so that the procedure can resume at the appropriate state of cure for the cement as it actually resides on the implant or within the bone. In this instance, the controller can be programmed to alert the user immediately prior to full cure to avoid the probe adhering to the in vivo cement.

[0058] FIG. 1 shows an illustration of a device of the present invention to monitor the cure state of bone cement in place in a total knee replacement implant seated in a knee. The acoustic resonant frequency sensing system consists of a sensor probe shaft 103, in mechanical contact with a piezoelectric (or other type) electromechanical transducer 102 acting as a mechanical vibration receiver, which is in series, in mechanical contact with a second electromechanical transducer (such as a voice coil) 101 acting as a mechanical vibration transmitter. These three components 101, 102, 103 together, comprise the sensor probe assembly, and the probe shaft has a distal end or tip that is in mechanical contact with the cement to be monitored at the probe contact point 100. It may be the mechanical hardness or degree of firmness in the cement at the probe contact point 100 that modifies the mechanical resonance frequency of the sensor probe assembly 116. The cement mantle 108 forms a mechanical bond between the inner bone 104, tibial plateau 107 and the metal implant 106. The inner bone 104 is contained inside the outer bone 105 wall. The sensor probe assembly 116 is in electrical communication with the signal processing system 115 via electrical wires 117 which connect the voicecoil transmitter actuator 101 to the amplifier 111, and the piezoelectric acoustic receiver 102 to the signal conditioner 112. A computer 109, under software control and via a digital to analog converter (DAC) 110, generates an excitation frequency sweep of sinusoidal waves from approximately 100 Hz to 20 kHz over a duration of about 1 second and this signal is amplified in an audio power amplifier 111 which then produces the signal to drive the actuator 101. Mechanical vibrations in the voicecoil actuator 101 then travel through the piezoelectric acoustic receiver 102, which in turn, is transmitted mechanically and in series with the frequency probe shaft 103, and finally to the probe contact point 100. The mechanical vibrations in the sensor probe assembly 116 produce a unique characteristic resonance spectrum obtained from the FFT of the signal from the piezoelectric receiver 102 that is then transmitted to a signal conditioner 112 and is digitized using an analog to digital converter (ADC). The ADC signal is recorded and analyzed by the computer 109 running under software that can perform real time FFT’s. The resulting resonance frequency spectra are then displayed on a screen/display 114.

[0059] FIG. 3 shows the resulting acoustic resonant frequency spectra of the sensor probe versus cement cure time and associated cement temperature. The baseline frequency peak 201 at about 2.7 kHz represents the peak frequency associated with soft or uncured cement. As the cement hardens, there are a series of progressions of broad spectral features that rise and shift from a lower frequency of about 200 Hz up to 1 kHz as a progression of cure time 203. At about the 16.sup.th trace (denoted in the legend by the arrow 205) the temperature of the cement suddenly reaches 62° C. only seconds later and this is accompanied by a sudden generation of a new frequency peak 202 at about 1.3 kHz. Note that a temperature inflection point based cement cure method shown in FIG. 4 also shows that the cement has cured at this point through changes in the rate of change of the temperature plus an absolute change greater than a preset value as described elsewhere [Funk et al. 2021]. This peak frequency generated and is sustained and the shape of the FFT spectra does not revert to the pre-cured state where the base peak 201 was located at 2.7 kHz. It is one theory that due to the polymerization process and free energy and/or rheological changes, the cement provides a better acoustic impedance match for the mechanical probe tip and thereby has more authority to mechanically modify the natural resonance frequency of the probe assembly structure. It is conceivable that a special mechanical assembly can be designed such that it exploits a unique resonance mode that may make the determination of cement cure point even more unambiguous. For example, it may be possible to make the assembly small and stiff to drive the natural resonance frequency of the probe well into the ultrasonic regime in order to use smaller piezoelectric transducers.

[0060] FIG. 3 shows a photograph of a prototype of the present invention. Here, the components of the probe sensor assembly are shown as the voicecoil actuator 101, the piezoelectric acoustic receiver 102, the frequency probe shaft 103, and the signal processing system 115. The prototype shown here is mounted in a fixture comprised of aluminum beams to hold the probe assembly steady and secure against a sample of PMMA cement held in place while its temperature is monitored using a thermistor embedded in the cement. It is interesting to note that indication of the cement cure point by the present invention matches the cure point predicted by means of simultaneously sensing the temperature rate of change illustrated in FIG. 4 using the apparatus 301 in another invention by Funk et al [2021].

[0061] The following section describes the mixing procedure and the conditions for use of the cement and for traditional judgement as to the state of cure, and proposes a further embodiment of the invention for use in determining an appropriate state of cure for application in the bone or on the implant prior to implantation.

[0062] The behavior of cement is strongly temperature sensitive; increasing the cement’s temperature will speed up the curing of the cement, however it will result in an earlier dough-time and significantly reduced available working time.

[0063] When mixing cement, all of the powdered pre-polymer is mixed with all of the liquid monomer to ensure that the cement components are homogenous and that the cement fully polymerizes or cures. While specific surgical procedures may influence the decision when the cement is ready to apply, in general it is ready to apply to the metal implant just before the cement has reached its dough state (i.e. while the cement is still tacky) to aid adherence to the implant. The cement should not be excessively runny (or have excessive flow from the nozzle when using a syringe). Also, voids in the cement can reduce the strength of the cured cement; mixing in a vacuum system reduces the inclusion of air voids with the added benefit of reducing exposure to monomer fumes for OR staff, but the use of vacuum during mixing can also influence the setting time. In accordance with the invention, this point is determined by the use of the wave tester system of the present invention having the probe in contact with the cement following mixing, such as in the mixing bowl, on a mixing paddle, or within an applicator holding the cement prior to application on the implant or bone. Also, a syringe style holder may be designed to contain the curing cement and having a dedicated probe situated for optimal frequency analysis based on the work mentioned herein, such as having a longitudinal penetration into the curing cement sample.

[0064] Implant surfaces should be clean and dry and clean gloves should be used when cementing since the mixed cement in its working phase is incompatible with water and so aqueous liquids, such as saline or blood, will reduce its bonding strength, and lipids can also reduce the mechanical strength of the cement. The use of a haemostatic agent (e.g. a solution containing H202) can help reduce biological debris trapped within the bone-cement interface and is known to help improve the cement bond. Washing bone surfaces, for example with pulsatile lavage helps remove loose bone, blood, fat, and marrow also exposes the porous bone and helps to achieve penetration of bone cement so as to result in a stronger interlock at the bone-cement interface. The bone and implant should also be dried prior to applying the cement by using an absorbent pad or a dry sponge, by suction, or by a carbon dioxide jet. In areas of dense or sclerotic bone, drilling keying holes in the bone may assist in creating a greater degree of cement interdigitation.

[0065] Cement penetration into the bone can be improved using various techniques. The cement can be syringed under pressure, by or a nozzle can be seated directly onto the cut bone surface and the cement can be syringed into the cancellous bone structure. A peripheral seal can help retard later ingress of osteolytic micro-particulate debris into the bone, particularly at the periphery of the bone implant site.

[0066] 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.