WO1991009568A1

WO1991009568A1 - Diagnostic electromechanical spectroscopy for detection of cartilage degeneration

Info

Publication number: WO1991009568A1
Application number: PCT/US1990/007634
Authority: WO
Inventors: Eliot Frank; Evan P. Salant; Alan J. Grodzinsky
Original assignee: Massachusetts Institute Of Technology
Priority date: 1989-12-22
Filing date: 1990-12-21
Publication date: 1991-07-11

Abstract

A surface probe is described for use in detecting the degree of degeneration in mammalian tissue. The probe comprises a means for applying to a surface of mammalian tissue a force which affects an electrokinetic parameter of the tissue. The probe further comprises a means for detecting an electrokinetic parameter. The effect on electrokinetic parameters is indicative of the amount of a charged species involved in tissue degeneration. A method is also described for comparing the detected electrokinetic parameters value to normal values for types of tissue examined.

Description

DIAGNOSTIC ELECTROMECHANICAL SPECTROSCOPY FOR DETECTION OF CARTILAGE DEGENERATION

Background of the Invention

Cartilage is a biological tissue which functions by providing a load-bearing, low-friction surface necessary for normal joint operation. The structure of this tissue is composed of a sparse population of cells and a large extracellular matrix (ECM). It is the ECM which allows cartilage to accomodate physiological mechanical loading of the joint. The ECM is composed principally of a hydrated collagen fibril network enmeshed in a gel of highly charged proteoglycan molecules. Proteoglycans are responsible for providing a high fixed charge density which contributes significantly to the overall compressive stiffness of cartilage necessary for normal joint function.

Osteoarthritis is a degenerative joint disease, affecting over 40 million Americans. During motion and weight bearing, osteoarthritis causes significant pain in disease affected individuals. One of the earliest events in osteoarthritis is a molecular level alteration of the cartilage extracellular matrix, and loss of highly charged proteoglycan macromolecules from the matrix. Since the ability of cartilage to withstand compressive loading is due mostly to the presence of proteoglycans, loss of these molecules makes the tissue softer and more susceptible to further wear and degradation. These molecular level changes often occur in very localized regions of cartilage along the joint surface, and occur nonuniformly with depth in the tissue.

The current technologies used for diagnosing such degenerative changes in cartilage matrix, such as x-rays, magnetic resonance imaging and visual inspection by arthroscopy, detect disease at relatively advanced, and probably irreversible, stages, unfortunately, detection of early degenerative changes, when therapeutic intervention might be most beneficial, is not presenty available. Thus, there is a need for the early quantitative assessment of degenerative changes in cartilage.

Summary of the Invention

One embodiment of this invention relates to a surface probe for use in detecting the degree of degeneration in mammalian tissue. The probe includes a means for applying to the surface of the mammalian tissue a force which affects an electrokinetic parameter. This parameter is indicative of the amount of charged species involved in degeneration of the mammalian tissue. The probe also includes a means for detecting the affected electrokinetic parameter.

Another embodiment of this invention relates to a method for detecting the degree of degeneration in mammalian tissue. The method includes application to the surface of the mammaliam tissue of a force which affects an electrokinetic parameter, indicative of the amount of charged species involved in the degeneration of mammalian tissue. The method also includes detection of the affected electrokinetic parameter. The detected electrokinetic parameter values are compared to a normal value for said tissues. This invention offers sensitivity and specificity for detection of changes in electrokinetic parameters of mammalian tissue due to degeneration.

Brief Description of the Drawings

Figure 1 is a schematic representation of a proteoglycan aggregate (PGA), illustrating the structural relationship of its components and stress of enzymatic degredation.

Figure 2 is an graphic representation of the decrease in detected electrical potential with time, after enzymatic removal of proteoglycans.

Figure 3 is a schematic view of one embodiment of a diagnostic surface probe according to this invention and cartilage.

Figure 4 is an exploded schematic view of one embodiment of a diagnostic surface probe.

Figure 5 is a plan view of an interdigitating electrode pattern.

Figure 6 is a schematic diagram illustrating the passage of current from the surface electrodes into cartilage tissue and the resulting current density. Figure 7 is a cross-sectional view of cartilage and an electrode structure for applying current density to tissue and detecting mechanical stress.

Figure 8 is a cross-sectional view of cartilage and an electrode structure for applying mechanical displacement to the tissue for detecting electrical potential.

Figure 9 is a graphic representation of detected mechanical stress of cartilage in response to a surface application of current density and the effect of increasing current frequency.

Detailed Description of the Invention

The above features and other details of the surface probe system and methods of this invention will now be more particularly described with relevance to the accompanying drawings and pointed out in the claims. It will be understood that the particular embodiments of the invention are shown by way of illustration and not as limitations of the invention. The principle features of this invention can be employed in various embodiments without departing from the scope of the invention.

Cartilage and Connective Tissue

Articular cartilage is the dense connective tissue that functions as a load-bearing material in synovial joints. Cartilage from adult animals and humans is without vascular, nervous or lymphatic components. (Mankin, H.J. and K.D. Brandt, Osteoarthritis, Ed. Moskowitz, R.W., W.B. Saunders, Philadelphia, pps. 43-80 (1984)). The tissue's sparse population of cells derives its nutrition primarily from the synovial fluid (Mankin and Brandt, Ibid; McKibbin, B. and A. Maroudas, Adult Articulate Cartilage, Ed. Freeman MAR, Pittman Medical, Kent, U.K., pps 461-486 (1979)). The cells are responsible for the synthesis, maintenance and gradual turnover of an extracellular matrix (ECM) composed principally of hydrated collagen fibrils and highly charged proteoglycan molecules (Hay, E.D., Cell Biology of Extracellular Matrix, Plenum Press, N.Y. (1981)). The osmotically swollen ECM (20-30% of tissue wet weight) and the tissue's high water content (70-80% of wet weight) are responsible for the complex poroelastic behavior of cartilage that characterizes its normal response to transient (Mow et al., General

Biomechanics, 17: 377-394 (1984)) and cyclic (Lee et al. J. of Biomechanical Eng., 103; 280-292 (1981); Torzilli, P.A., Mechanics Res. Comm, 11: 75-82 (1984)) loads.

Electrokinetic Transduction

When cartilage is compressed, a mechanical-to- electrical transduction occurs resulting in measurable electrical potential (Bassett, C.A.L. and R.J. Pawluk, Science, 178; 982-983 (1972); Lotke et al., J. of Bone Joint Sur., 56A; 1040-1046 (1974); Grodzinsky et al., Nature, 275: 448-450 (1978)). Results involving transient stress relaxation (Grodzinsky et al., Ibid) and sinusoidal deformation of cartilage in uniaxial contained compression (Lee et al., Ibid; Grodzinsky et al.. Ibid) suggested that an electrokinetic effect called the streaming potential was the primary mechanism for this mechanical-to-electrical transduction in cartilage. The electrokinetic streaming potential is defined as follows: mechanical deformation of cartilage and other soft tissues containing charged, hydrated ECM causes a flow in interstitial fluid and entrained counterions relative to the fixed charge groups of the ECM (Anderson, J.C. and C. Eriksson, Nature, 218: 166-168 (1968); Grodzinsky et al., Ibid). Fluid convection of counterions tends to separate these ions from the oppositely charged molecules of the ECM. This charge separation produces an electric field colinear to the fluid flow and proportional to the fluid velocity at each position within the ECM. The open-circuit voltage thus produced is called the streaming potential. Amplitude and phase of the measured potential compares well to that predicted by a theroy combining sinusoidal electrokinetic transduction (Lee et al., Ibid; Grodzinsky, et al., Ibid) and cartilage fluid flow (Mow et al., Ibid), further confirming the streaming potential hypothesis.

The Relation of Electrokinetic Transduction to Tissue Composition in Healthy and Diseased States

All ionized macromolecules can theoretically contribute to electrokinetic transduction, including those in the ECM. At physiological pH, collagen contains nearly equal numbers of ionized amino and carboxyl groups, and hence possesses little or no net fixed charge (Bowes, J.H. and R.H. Kenten, Biochemistry J., 43: 358-365 (1948); Li, S.T. and E. Katz, Biopolymers, 15: 1439-1460 (1976)). At pH values less than 5.0 or greater than 9.5 , collagen molecules will attain positive or negative net fixed charge, respectively, and collagen fibrils will swell laterally increasing their intrafibrillar water content (Glimcher, M.J. and S.M. Krane, Treatise on Collagen, Ed. Gould, B.S., Academic Press, N.Y. pps. 68-251 (1968) ; Nussbaum, J.H. and A.J. Grodzinsky, J. of Membrane Sci., 8; 193-219 (1981); Torzilli, P.A., J. of Ortho. Res., 3: 473-483 (1985)). Collagen can then contribute appreciably to electrokinetic effects provided that fluid flow through and around the individual fibrils of cartilage type II collagen (Grynpas et al., Biochim. Biophys. Acta, 626: 346-355 (1980)) is substantial. The proteoglycans (PG), whose structure and function have been extensively reviewed (Muir, I.H.M., The Joints and Synovial Fluid, Ed. Sokoloff, L., Academic Press, N.Y., pps. 28-94 (1980); Hascall, V.C. and G.K. Hascall, Cell Biology of Extracellular Matrix, Ed. Hay, E.D., Plenum Press, N.Y., pps. 39-64 (1981)) contribute the major source of fixed charge to electrokinetic interactions at physiological pH. As seen schematically in Figure 1, proteoglycan charge groups are located predominantly on the chondroitin sulfate (CS) and keratan sulfate (KS) glycoaminoglycan chains that are covalently linked to the protein core of the proteoglycan monomer (Hascall and Hascall, Ibid; Muir, Ibid). The chondroitin sulfate chain (mol. wt. 20,000) is a linear array of 20-60 disaccarides, each of which contains one ionized sulfate group (on the galactosamine moiety) and one carboxyl group, on the average. Keratan sulfate chains contain 5-10 disaccharides, each contributing one sulfate group (on the glucosamine moiety). The proteoglycan monomers, in turn, form large aggregates in cartilage (Muir, Ibid). The resulting fixed charge density of normal cartilage is as high as 0.1-0.2 mol 1^-1 of tissue fluid (Maroudas, A., Adult Articulate Cartilage, Ed. Freeman, M.A.R., Pittman Medical, Kent, U.K., pps 215-290 (1979). Proteoglycan charge groups are known to be responsible for the high Donnan osmatic swelling pressure (Maroudas, Ibid) that enables cartilage to electromechanically resist compressive loads in healthy cartilage.

Electrokinetic transduction in cartilage originates in the fixed charge groups of the proteoglycans of the tissues extracellular matrix. These charged groups are ionized under physiological conditions in vivo, and over a wide range of both pH and ion content in vivo (Maroudas, Ibid; Grodzinsky, A.J., CRC Critical Reviews Biomedical Eng., 9: 133-199 (1983)). Mobile electrolyte counterions within the tissue, together with these fixed charge groups, form electrical double layers whose properties are attracted by the local chemical environment (pH and ionic strength). Mechanical determination or fluid streaming within the tissue tends to separate the mobile ions from the fixed charge groups, giving rise to voltages called streaming potentials, as mentioned earlier. Conversely, an applied electric field can exert a force on the ionic space charge in the fluid phase, which will produce an electroosmotic convection of fluid. Applied fields also produce concomitant electrophoretic motion of the negatively charged solid matrix. These electroosmotic and electrophoretic effects can result in mechanical deformations and stress within the tissue. In other words, the net negative charge density of the proteoglycans matrix impacts to the surrounding fluid a net positive charge. The application of a current across the cartilage impacts an electrophoretic force on the proteoglycans and an oppositively directed force on the positively charged fluid. In confined area, such as cartilage, the resulting redistribution of fluid and solid generated by the uniform current induces a mechanical stress. This stress can be measured by a mechanical transducer.

Use of Electrokinetic Phenomena to Detect Cartilage Degradation

A hallmark of degenerated cartilage is the loss of the highly charged proteoglycans which are responsible for the high fixed charge density of the tissue under physiological conditions. Figure 2 graphically illustrates the effect of enzymatic digestion on cartilage as a model system of cartilage degeneration (Frank et al., J. of Ortho. Res, 5: 497-508 (1987). Digestion with chondroitinase removes the highly charged chondroitin sulfate groups from the proteoglycans in the extracellular matrix. The loss of the charge density in the matrix results in significant decreases in the compressive stiffness and the streaming potintial of the tissue. Such enzymatic extractions have a proportionally larger effect on the streaming potential than the stiffness, since loss of all charge groups would ideally decrease the potential to zero but would not eliminate the contribution of the remaining solid content to the tissue's stiffness. Chondroitinase does result in a more dramatic decrease in streaming potential than mechanical stiffness. The relative change in streaming potential and stiffness would depend on the specific matrix attraction in question.

The detected mechanical stress produced by an applied current, or the electrical streaming potential produced by an applied mechanical displacement is used to infer the compositionally-dependent properties of cartilage. To accomplish this, a theoretical model that was previously developed for one dimensional electromechanical transduction in connective tissues (Frank and Grodzinsky, J. Biomechanics, 20(6): 629-639 (1987) is adapted for the three dimensional geometry (Sachs and Grodzinsky, Physicochemical Hydrodynamics, 1989) associated with the probe. This model is used to compute the electromechanical coupling coefficients, the equilibrium elastic moduli, hydraulic permeability, and electrical conductivity of cartilage (or other tissue) from the detected signals. Previous literature has demonstrated that these intrinsic mechanical and electromechanical properties of cartilage are directly dependent on the content and organizatiion of extracellular matrix molecules; the electromechanical coupling coefficients have proven to be the most sensitive indicators of cartilage degeneration (Frank et al., J. Orthop. Res., 5: 497-508 (1987).

The theoretical model relates the effects of macorscopic deformations on fluid flow and electrokinetic transduction in cartilage. It is convenient to average over molecular dimensions and consider macroscopically smoothed volumes. This macrocontinuum approach essentially combines lows for linear electrokinetic transduction in ionized media with the prinicples of the linear biphasic theory for cartilage (Mow et al., J. Biomech. Eng., 102; 73-84 (1980). The biphasic theory highlights the significance of fluid flow to the rheological behavior of cartilage.

A constitutive law for the total stress T_{i j} in a homogeneous, isotropic tissue valid for small strains ε_ij is.

(1)

where P_f is the fluid pressure, and the Lame constants

G and λ are functions of the ionic content of the cartilage.

For the case of incompressible fluid and solid constituents, continuity relates the local fluid and solid velocities, υ _s and υ _a - au/at, respectively

(Armstrong et al., vol pps (1984).

where Φ is the porosity and u is the displacement. The total area-averaged relative fluid velocity U, which is relevant to electrokinetic transduction, is defined by

(3)

Conservation of momentum takes the form

(4)

where inertial effects can e ignored.

Linear, macroscopic laws for electrokinetics of isotropic media (DeGroot and Mazur, cite (1969) are used to relate the relative fluid velocity U and the current density J in the tissue to the gradients in fluid pressure P- and electrical potential V,

(5)

where k₁₁ is the "short-circuit" hydraulic permeability, k₁₂ and k₂₁ are the electrokinetic coupling coefficients, and }. is the electrical conductivity. The coupling coefficients are equal by

Onsager reciprocity (k₁₂ = k ₂₁) , and can be expressed in terms of the charge density of ς-potential of th cartilage extracellular matrix using microscopic continuum models for cartilage (Eisenberg and Grodzinsky, cite (1985). For materials like cartilage in which the fixed charge density is negative, k₁₂ and k₂₁ are defined as negative.

Finally, conservation of current states that the current density J has zero divergence for all frequencies of interest

/ (6)

Two additional relations, important to specifying all quantities in three dimensions, are obtained by taking the curl of Equation (5), / (7)

and

(8)

Equations (1) to (8) constitute the general three-dimensional theory of electromechanical transduction. This theory can be solved for the specific geometry of the probe, and applied to either the case of current-generated mechanical stress or displacement-generated electrical streaming potentials.

Operation of the Surface Probe and System

The present invention measures electromechanical properties specific to tissues containing charged macromolecules by applying to the surface of the tissue an excitation (either electrical or mechanical) while simultaneously detecting the electrokinetic response (either mechanical or electrical, respectively). Specifically, the probe measures electromechanical properties by one of two alternative methods: 1) applying an electric current via surface electrodes and measuring the resulting current generated stress via a force transducer; or 2) applying a mechanical stress via a force transducer and measuring the mechanically generated streaming potential via surface electrodes. The excitation is time varying and also space varying along the surface. The tissue electrokinetic response to the excitation is also time varying and space varying along the surface and proportional to the amount of charged macromolecues within the tissue. Critical to the design and function of the probe is the electric current density in the tissue. The applied current density drives electroosmosis of the fluid and electrophoresis of the solid within the tissue, resulting in a buildup in pressure which in turn is measured as the current generated stress. The distribution of current within the tissue is a function of effective wavelength (X-2X electrode separation for the geometry) . Simply stated, if the stimulating electrodes are relatively close together (λ<<δ), the current field can only penetrate to a depth within the tissue approximately equal to the electrode separation. On the other hand, for widely spaced electrodes (λ>>δ), the current density will penetrate to the full thicknesses of the tissue. Thus by varying the wavelength, one can effectively probe varying depths into the cartilage.

As illustrated in Figure 3, the surface probe is constructed as a self-contained unit with electrodes on the open face of the surface which is applied to the cartilage. In addition, the probe is multilayered and consists of individual layers of insulation and a mechanical transducer. The open face of the probe with surface electrodes is applied to the articular surface of the cartilage.

The practical limitations for probe dimensions relate to designing it for passage through an arthroscopic probe. Of course, the dimensions of the probe employed in other applications will depend on the particular use. The constraints on the quantity of applied current density lies between the amount of current sufficient to provide a response but not enough to damage the cartilage.

Figure 4 illustrates the individual components of the surface probe in relationship to one another. The surface electrodes are bonded by adhesive to a ground plane which can consist of a Mylar® shield with its lower surface metalized. This, in turn, is bonded by adhesive to one or more layers of 1 ml thick mylar insulation. The restraints which affect the choice of electrodes include ease of fabrication and the need to minimize or eliminate deleterious chemical reaction products associated with the passage of electric current. Thin, chlorided silver strips, 1 mil thick, are easily fabricated into an interdigitated array for the excitory electrodes as diagrammed in Figure 5. These chlorided silver strips can also be used for sensor electrodes. The mechanical transducer which can consist of Kynar PVDF piezo film (52 μm thick) is bonded by adhesive to the last layer of insulation. Finally, to the piezoelectric film is bonded an array of silver chloride strips as sensing electrodes. A commercially avialable piezo film which has an appropriate dynamic range of 10^-5 to 10⁺⁸ N/m² and a broad band frequency response from dc to 1 GHz may be used. in addition, a patterned array of this film can be fabricated with a strip width as small as 25 μm and inter-strip spacimg of 100 μm. Since cartilage thickness will vary, a series of arrays can be fashioned to probe short, middle and long wave responses. The components and construction of this embodiment of the surface probe meet the experimental parameters which define that there is no vertical motion of the cartilages solid matrix at the cartilage-probe interface and no relative normal fluid flow at the cartilage-probe interface. A microcomputer system is used to control the absorbers and to take data in real time from the mechanical sensors and the electrode array.

Figure 5 illustrates an interdigitating pattern of positive and negative surface electrodes. When the electrodes are being used to impose a current driven mechanical stress in cartilage, a current gradient is created in the tissue.

Figure 6 is a diagrammatic representation of current being applied to a cross-section of cartilage by the surface electrode and the resulting current density lines. As the current is increased in amplitude from the surface electrodes, the current density through the tissue is increased proportionately. From Figure 6, this can be visualized as longer and thicker arrows. Conversely, as the applied current is decreased, the current density arrows would be smaller and shorter. With regard to frequency of current, the inverse is true. A higher frequency results in smaller and shorter arrows, as well as, less penetration of the current through the tissue. A lower frequency causes greater penetration of the current through the tissue.

The surface probe employs surface electrical and mechanical signals that are periodic in both space and time. Spatially, the functions are approximately 2-dimensional, varying with depth and with one surface dimension, and independent of the orthogonal surface dimension (e.g., a standing wave of imposed potential or displacement). By varying the imposed temporal frequency and the spatial wavelength of the input stimulus along the surface, it is possible to "tune" to specific depth within the tissue and thereby characterize the extent of focal degeneration even though only surface transducers and detectors are used.

A spatially varying electrical excitation is applied to the surface of the cartilage and the resulting mechanical stress is measured simultaneously at the surface (Figure 7). The experimental parameters are defined such that there is no vertical motion u_z of the solid matrix and no relative normal fluid flow U_z at the upper cartilage surface (z=0) within the extent of the electrode structure. The flat, multilayered sensor probe is used to measure the spatially periodic stress generated at the surface by the applied current. This probe satisfies the displacement and fluid boundary conditions above, and also support the electrode array. These constraints are met by bonding a patterned array made from a piezoelectric film onto an insulating substrate (Figure 4). The silver chloride array will then be bonded on top of the piezoelectric film. A commercially avialable film which has an appropirate dynamic range of 10^-5 to 10⁺⁸ N/m² and a board band frequency response from dc to 1 GHz. In addition, a patterned array of this film can be fabricated with a strip width as small as 25 μm and inter-strip spacing of 100 μm. Since cartilage thickness will vary, a series of arrays can be fashioned to probe short, middle and long wave responses. In another embodiment, an array of piezoelectric or electromechanical transducers is constructed to impose the desired displacement on the cartilage surface (Figure 8). Each transducer has its own drive so that a spatially varying displacement can be imposed. In this scheme, a grid of potential sensing AgCl electrodes can be attached to the sides of the transducers in contact with the cartilage.

A microcomputer system is used to control the transducer and to take data in real time from the mechanical sensors and the electrode array. Since the measured response will be at the same (temporal) frequency as the excitation, phase-lock detection and Fourier decomposition at the excitation frequency will be used to find the amplitude and phase of the response (during use). An additional advantage of using an imposed spatial wavelength is the increased sensitivity obtained by measuring the response at the same wavelength. Differential amplification of the measured response between the "peak" and "valley of the imposed spatial periodic waveform should greatly enhance th signal-to-noise ratio.

The measured streaming potential and current generated stress will first be related to intrinsic, macroscopic material properties of cartilage. Linear electrokinetic transduction at each position in the tissue can be described the non-equilibrium thermodynamic coupling laws that relate gradients in fluid pressure P_f and potential V to relative fluid velocity U and current density J,

VΦ

where k₁₁ is the short-circuit hydraulic permeability, k₂₂ is the electrical conductance and k₁₂ , k₂₁ are the electrokinetic coupling coefficients (equal to each other by Onsager reciprocity). The important intrinsic material properties that characterize the macroscopic behavior of cartilage are: a) "pure mechanical" parameters: k₁₁: equilibrium elastic moduli (e.g., lame coefficients) b) "pure electrical" parameters: k₂₂ c) electromechanical coupling coefficients: k₁₂ = k₂₁

An appropriate and sufficient combination of amplitude and phase measurements (e.g., streaming potential, current generated stress and conductance) can be used to compute all the above material properties.

A layered model that accounts for variation of tissue properties with depth has also been formulated for the case of uniaxial compression (Frank, et al., "Advanced in Bioengineering," Am. Soc. of Mech. Eng., pps. 5-6, Nov. 19-22, 1988). Such variations are important for surface detection of non-uniform cartilage degeneration. Therefore, the surface periodic model is extended in an analogous manner to incorporate material nonuniformities. This further extension is valuable for the use of variable wavelength and frequence stimuli to detect and localize cartilage degeneration. Therefore, the approach of comparing normal cartilage to various stages of degeneration is made via a comparison of these intrinsic parameters. Normal values from a statically significant, crosssectional sampling will be determined for each type of tissue, under both in vivo and in vitro conditions.

It is important to note that the "pure mechanical" parameters (elastic moduli, permeability k₁₁) and "pure electrical" parameters (k₂₂) are far less sensitive to degradative loss of proteoglycan molecules than the electrokinetic parameters k₁₂ , k₂₁. Since k₁₂, k₂₁ are proportional to matrix charge density, loss of all proteoglycans will decrease k_12, k₂₁ from their normal value to zero. In contract, k₁₁, k₂₂ and the equilibrium moduli are known to change by only a factor of 2 (A.J. Grodzinsky, Ibid (1983); Eisenberg, S.R. and A.J. Grodzinsky, J. of Ortho. Res, 3: 148-159 (1985)). Hence, the "dynamic range" of k₁₂, k₂₁ in terms of molecular composition is far greater.

Therefore, the electromechanical spectroscopy approach proposed here focuses on k₁₂ and k₂₁. This approach is a far more sensitive indicator of early cartilage degeneration than purely biomedical of electrical tests (e.g., mechanical indentation or electric spectroscopy).

A principal use for this invention is its use as a diagnostic probe to assess the chemical, electrical and mechanical properties of cartilage. The purpose of the probe is to diagnose the viability of cartilage during arthroscopic examination of an articular joint. This will aid detection of the early onset of joint disease, such as, which occurs in osteoarthritis. In addition to the application of the surface probe in detecting cartilage degeneration, other biological applications exist in tissue repair and replacement. For example, the need for transplantation of articular cartilage, menisci, ligaments, muscle, and other tissues has necessitated development of suitable preservation and storage methods

(Eisenberg, S.R. and A.J. Grodzinsky, "The Swelling of Articular Cartilage: Electromechanochemical Forces," Trans. 1984 Orthop. Res. Soc. Atlanta, GA, Feb 7-9, p.31 (1984; Grodzinsky, A.J., Proc. 37th Conf. on Med. Biol., Los Angeles, CA, p. 342,Sept 17-19, 1984;

Kavesh, N.G. et al., Proc. Bioelec. Repair and Growth Soc, Kyoto, Japan, p. 9, Nov. 5-8, 1984; Gray, M.L., et al., Proc. Bioelec. Repair and Growth Soc, Kyoto, Japan, p. 46, Nov. 5-8, 1984). The electromechanical surface probe will provide a nondestructive means for characterizing the ability of transplant tissues to maintain their functional and molecular integrity following (cryo) presentation and storage, and as a means for monitoring the continued viability of the tissues after transplant.

The surface probe will also have non-biological applications to materials in which electrical fields can induce deformation, such as, with polyelectrolytic gels and membranes.

Exemplification

The femoropatellar groove region of the distal femur of adult bovine knee joint was surgically removed and prepared for testing. The joint surface contained regions of reasonably flat cartilage approximately 1-3 cm in area. During the process, the cartilage surface of the femoropatellar groove was kept free of blood by rinsing with phosphate buffered saline. For testing, the joint was mounted in a jig. The cartilage surface was equilibrated in buffered saline containing protease inhibitors to prevent natural degeneration and loss of matrix macromoleculeε that would otherwise occur gradually during testing over a several hour period. (The inhibitor solution includes 0.1 M 6-aminohexanvic acid. 011 M disodium EDTA, 0.001 M benzamidine hydrochloride, 0.001 M phenylmethylsulphanyl fluoride and 0.01M

N-ethylmateimide). These conditions simulate an in vivo environment.

After the joint was mounted and equilibrated with buffer solution, the surface probe was positioned on the cartilage surface. First, a series of increasing current densities was applied to the cartilage surface at three different frequencies, 0.05, 0.25 and 1.00 Hz. Secondly, the resultant stress was measured after each application of current. As seen in Figure 9, as the applied current was increased, the resultant mechanical stress also increased. The effect of increasing frequency had the converse effect and decreased the resulting mechanical stress.

Claims

1. A surface probe for use in detecting the degree of degeneration in mammalian tissue, comprising: a) means for applying to a surface of the mammalian tissue a force which affects an electrokinetic parameter indicative of the amount of charged specie involved in tissue degeneration; and b) means for detecting said electrokinetic parameter.

2. A surface probe of Claim 1 wherein said means for applying comprise electrodes for passing an electrical current through said tissue and said means for detecting comprise a mechanical stress detector.

3. A surface probe of Claim 2 wherein said electrodes for passing an electric current through said tissue comprise an interditating grid of conductive material.

4. A surface probe of Claim 3 wherein the conductive material of said electrodes comprise a metal.

5. A surface probe of Claim 4 wherein the conductive metal of said electrodes comprises silver chloride.

6. A surface probe of Claim 5 wherein the silver chloride electrode receives input from a time varying current source.

7. A surface probe of Claim 5 wherein the means for detecting said electrokinetic parameter comprises a piezoelectric polymeric film.

8. A surface probe of Claim 7 wherein the means for detecting said electrokinetic parameter transmits current to a microcomputer system for data acquisition and processing.

9. A surface probe of Claim 1 wherein said means for applying comprise means for applying a mechanical stress to said tissue and said means for detecting comprise a detector for mechanically- generated electrical potential.

10. A surface probe of Claim 7 further comprising a means for electrically isolating the means for affecting said electrokinetic parameter from the means for detecting said electrokinetic parameters.

11. A method for detecting the degree of degeneration in mammalian tissues, comprising: a) applying to a surface of the mammalian tissue a force which affects an electrokinetic parameter indicative of the amount of charged species involved in tissue degeneration of said mammalian tissue; b) detecting said electrokinetic parameter; and c) comparing the detected electrokinetic parameter values to a normal value for said tissue.

12. A method of Claim 11 wherein the application of force is passed into said tissue at varying depths or thickness by using electrical currents of variable temporal frequency and a variable imposed spatial wavelength.

13. A method of Claim 12 wherein the application of force which affects an electrokinetic parameter and the detection of the said electrokinetic parameter can occur sequentially or simultaneously in time.

14. A method of Claim 11 wherein the mammalian tissue is cartilage tissue.

15. A planar, multilayered, surface probe for diagnosing degenerative changes in cartilage tissue, comprising: a) surface electrodes connected to an electrical current generator for application of electrical current to a cartilage surface; and b) a first insulating sheet with its lower surface metalized for electrical grounding, and said first insulating sheet bonded to the electrode surface; and c) a second insulating sheet bonded to the metallized side of the first insulating sheet; and d) a piezoelectric polymeric film for transduction of mechanical stress to electrical current, bonded to the second insulating sheet; and e) electrodes for transmitting electrical current generated by the piezoelectric polymeric film to a detector.

16. A system for diagnosing degenerative changes in cartilage tissue, comprising: a) a surface probe of Claim 15; b) a microcomputer for controlling electro-mechanical output and measuring electro-mechanical input; and c) connectors for transmitting electrical current between the microcomputer of elememt (b) and the surface probe of element (a).