Indentation instrument for the measurement of cartilage stiffness under arthroscopic control

Indentation instrument for the measurement cartilage stiffness under arthroscopic control T. Lyyra*g,

J. Jurvelin

ffi, P. Pitkken:,

U. V&Sinen*

of

and I. Kiviranta~

*Department of Anatomy, University of Kuopio, Kuopio, Finland; +Departments Clinical Physiology; ZMedical Technology and Qu-gery, Kuopio University Hospital, Kuopio, Finland; nM.E. Miiller Institute for Biomechanics, University Bern, Bern, Switzerland

of of

ABSTFtACT

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(~‘hnnge.s in the biomechanical prop&es of nrtirular rurtilrq are one oj the.jrst signs the tissue degeneralion. We have developed a small size indentalion instrument for the quantiJcntion oJcartilage stiffness under arthroscopir rontrol. Du,l-ing measurement, the indenter imposes n constant defiation on the cartilage and the maximal indenter /on-e, by which the cartilage resists the akjkmation, is used ns a meusure for cartilage sti@ess. The instrument has been tested in ~bor~to~ conditions with e~~i~~i~ and cadaver knee joint cmf.ilage samples. A &ear re~~t~i~n,,~hi~ was ~ol~n.~ ~~et~~een. ~ndent~~~ce and el~sta?n~ .~t~~~~~ess fr = ll.990, n = 14) as we& as betwe~ ~~ld~.t~~~~re and cartilage shear modulus obtained by a ref?renre deuire (I- = 0.879, of = 22). Also, the rmre&ion between fwo repealed mm.curemmts at the measurement sites. used to evaluate the rejnoducibility, was linear (r = 0.953, n = 161. Qunntitntiue deterlion of rartilage s@fkxs is possible with the in.rtrument.

Keywords:

Cartilage,

arthroscopy,

stiffness, indentatiou

Med. Eng. Phys., 1995, 1’01. 17, 395-399,July

INTRODUCTION

The articular cartilage in synovial joints is a highly specialized poroelastic connective tissue, which provides a lubricated surface for contacting bones and reduces the loads transmitted to the bone by deforming and increasing the area of contacting surfaces. Cartilage is composed of cartilage cells, the extracellular matrix and its interstitial fluid. Cartilage cells, chondrocytes, which synthesize matrix components, form only l-10% of the cartilage volume’. The matrix consists of proteoglycan (PC) macromolecules (15420% of the dry weigh?) and collagen fibres (about 50% of the dry weights). The compressive stiffness of cartilage is primarily determined by PGs’,‘, while the tensile properties are determined by collagen fibres”,‘. The first alterations of cartilage structure during cartilage degeneration include an increase in the amount of the interstitial water, a decreased content of PCs, and decreased aggregation of PGs’. The tensile strength of collagens may also diminish in arthritic cartilage ‘. If the content of PGs decreases or the content of water increases, cartilage becomes softer 4.0. Reduction in cartilage stiffCorrespondence to: ‘I’. f,yyra, Department of Anatomy, Kuopio. P.O. Box 1627, FIN-7021 1 Kuopio, Finland

Llnivrrsity

of‘

ness may thus be the first detectable sign of cartilage degeneration. Animal experiments have shown that changes in the mechanical properties of cartilage may be even more sensitive indicators in early degeneration than traditional histological measurementsiO. Detection of softening will reveal the onset of disease but, unfortunately, softening of the cartilage can be estimated in 71iuo only subjectively by palpatation of the articular surface with a blunt probe during arthrotomy or arthroscopy. Only a few devices have been used for the measurement of cartilage stiffness during clinical investi~tion”.‘~. In this study we describe a new indentation instrument for the quantitation of cartilage stiffness under arthroscopic control. With this instrument, objective measurements of cartilage stiffness can be made to diagnose the softening of articular cartilage at an early stage. With the arthroscopic instrument the cartilage alterations can be also followed-up, e.g., after surgical or medical treatments. We also believe that measurements of stiffness may be useful in further development of non-invasive cartilage imaging techniques, for example magnetic resonance imaging (MRI) of the joint.

DESIGN AND INSTRUMENT

CONSTRUCTION

OF THE

The instrument is composed of a measurement rod (length 150 mm, diameter 5 mm) joined to a handle (&~e la). The rod has an inclined (20”) flat end reference plate with a cylindrical planeended indenter in the centre of the inclined surface. The length of the indenter measured from the inclined surface is 300 pm and the diameter 1.3 mm. The indenter is joined to a bending beam inside the rod cover. The force applied to the indenter is measured through bending of the beam by a pair of strain gauge transducers. One strain gauge transducer is on the upper surface of the beam and the other on the lower surface of the beam (F&w~ I b,c.) . By the use of a Wheatstone bridge circuit, forces applied to the bending beam in the direction of the beam axis are eliminated (J&W-~ I d.). This eliminates also the temperature effects on the transducers, since alterations in temperature produce equivalent strains in both transducers. The pressing force by which the surgeon presses the flat inclined surface of the ins~ument against the cartilage surface is measured by another pair of strain gauges. The transducers are located on the upper and lower surface of the beam, at the proximal part of the rod, where the beam is in complete contact with the rod cover. In this part of the rod, the beam bends with the cover and the bending is related to the total force applied to the distal end of the instrument. The construction of the Wheatstone bridge circuit is similar to the strain gauges which measure the indenter force. The passive resistors of the Wheatstone bridge circuits and the balancing resistors are located inside the handle of the instrument. The instrument is made of stainless steel. The electrical components are coated to withstand water and detergents. Therefore the instrument can be washed and sterilized with steam (120°C). The instrument is connected to a power source and an amplifier unit. The power unit is electrically isolated from the Wheatstone bridge circuits

by a DC/DC-isolator. Isolated amplifiers are also used. The measurement unit meets the IEC standard (IEC 601) safety regulations for medical instruments (class BF) and the instrument can be used in the operating theatre. A PC computer system records and controls force signals during measurement and also processes the measured data. The system includes a 386SX computer, a 12-bit A/D-converter, a printer and an external monitor for the surgeon. Separate power units are used for the computer and external monitor to meet the safety requirements demanded of medical instruments. An interactive Turbo Pascal program has been developed for the sampling and analysis of the data. The sampling frequency of 50 Hz is used for both indenter force and pressing force signals. DESCRIPTION

Inderxer

w

IL “I 4aoK

(1)

where G is shear modulus, P indenter force, a indenter radius, o deformation and K a theoretical correction function due to the finite thickness of the cartilage. The correction function K(V, u/h) is, in addition to Poisson’s ratio, v, dependent on ratio h = cartilage the area-aspect (a/h, Distal

Cf

end of

d)

r aii&iz-= Articular

Figure 1 Schematic presentation of the instrument (a). Longitudinal section of the measurement rod showing localization of strain gauge transducers (b). Installation of the strain gauges (c). Instaliation of the Wheatstone bridge circuit (d)

396

MEASUREMENT

During arthroscopy the measurement rod is pushed with a trocar through the soft tissues into the knee joint. The reference plate of the instrument is adjusted via the video monitor and, using 10 N force, pressed perpendicular to the cartilage surface. This force confirms that the indenter is pressed into cartilage to a depth of 300 km (figure 2). The pressing force, displayed during the measurement on the computer monitor, is kept constant by the surgeon, while the indenter produces deformation perpendicular to the cartilage surface. A finite element model of the ins~ument shows that the beam and the cover bend similarly during measurement; therefore, the length of the indenter, and also deformation, is constant when measuring articular cartilage of variable stiffnesses. During arthroscopic examination of the joint only short time measurements can be made. After the instantaneous constant load application response of cartilage is to behave like an incompressible elastic solid, until the outflow of interstitial water startsr3. The formulation of shear modulus given by elastic14, visco-elastic’” and biphasic poroelastic13,“j models is the same, G=l

Measu~m~~r md I

OF THE

cartilage

ziz2io” (300 llm)

Figure 2 Indentation geometry for the arthroscopic measurement of cartilage stiffness. A constant deformation (300 pm) is produced in the cartilage with a cylindrical, plane-ended indenter and the force by which the tissue resists the deformation is an indicator of the cartilage stiffness

thickness). For calculation of the shear modulus from the instant response after load application. Poisson’s ratio v = 0.5 must be used’“,’ . Based on the isotropic model, the analogous form~~lation for Young’s modulus can also be derived. If the thickness of cartilage is known, equation (1) allows to determine the shear modulus with measurement. Normally the the arthroscopic actual cartilage thickness may not be available during arthroscopic evaluation. We have used the mean indenter force for 1 s after load application as an indicator for cartilage stiffness. If a significant stress-relaxation occurs within the first second of the measurement, the maximum indenter force at the beginning of the 1 s period is used. A small diameter indenter was used to minimize the effect of cartilage thickness on the indenter force (function K(U, u/h) in calculating shear modulus). In Figure 3 the indenter force is shown as a function of cartilage thickness, when Poisson’s ratio is 0.5 (instant response)‘” and the diameter of the indenter is 1.3 mm. The indenter force is virtually independent of the cartilage thickness, when cartilage is thicker than 2 mm.

I

Pressing force (N I Figure4 Determination of the pressing force with which the surgeon presses the instrument against the articular surface to produce the full-length contact of the indenter with cartilage. Indenter force (stiffness index) is presented as a function of pressing force for cartilages of different stiffness. After increasing the pressing force up to 10 N, the indenter force reaches a consrant value

(MPa) provided by the manufacturer (Teknikum, Vammala, Finland) have been tested. With a pressing force of 10 N, the measurements showed a highly linear relationship (Y = 0.990, n = 14) between the indenter force and elastomer stiffness ( Fi‘gzm

TEST

MEASUREMENTS

AND

Cadaver

The transducers were calibrated by loading the indenter with masses of O-1.2 kg. Calibration measurement indicated a highly linear response up to a force of 12 N for both transducers (rprr~s,sin= 0.998, n = 12; rindrn~~r T= 1,000, n = 12) * In or if er to find a suitable pressing force for measurements, cartilage samples of different stiffness were measured using pressing forces of O14 N. After application of a force of 10 N, the cartilage surface is in complete contact with the reference plate. Then the indenter compresses the cartilage by its total length (300 pm) and the indenter force reaches a constant value, highly independent of the pressing force ( F@LTP 4). testing

By attaching the instrument to a bench, elastomer samples 2 mm thick, with dynamic moduli E

f

I b

0-i II

5) .

RESULTS

Calibration

Elastomer

6MPa/

knee joint

measurement

Arthroscopic stiffness measurements were carried out using three human cadaver knees. The test sites (n = 22) on the femoral and tibia1 condyles, on the patellar groove of femur and on the patellar surface were localized with the landmarks recognized during arthroscopic evaluation of the joint surfaces. Three consecutive measurements were made at each measurement site. The measured indenter forces were averaged to indicate cartilage stiffness. After arthroscopic measurements, the knee joints were opened, indentation sites were marked with Indian ink and joints were cut into osteochondral samples (10 mm X 10 mm X 10 mm) for reference measurements. The results of the arthroscopic measurements were compared with the results obtained from the reference measurements carried out with a material testing device’“. With the reference device, one stress-relaxation indentation was made at each marked site using 835 p.m/s strain rate, 300 pm deformation and indenter of diam-

I

/

1

3

4

5

I 6

Cartilage thickness (mm) Figure 3 Normalized indenter force as a function of cartilage thickness with Poisson’s ratio v = 0.5 and the indenter of diameter 1.3 mm. Indenter force is normalized to be 1 with cartilage of infinite thickness. The presentation is based on the elastic model’+ of articular cartilage. The normal range of human knee cartilage thickness is usually from 2 mm to 4 mm

0

5

10

I’

Dynamic modulus (MPa! Figure 5 Correlation between dynamic modulus of elastomer (n = 14)

the measured samples given

stiffness values and hv the manufacturer

397

Instrument

for measurement

of cartilage

stifjness:

T. Lyyra

et al.

0, 0

2

4

Shear

6

modulus (MPa)

Figure 6 Correlation between the stiffness (in the form of indenter force) obtained from arthroscopic measurements and the shear modulus obtained from the reference measurements in human cadaver cartilage samples (1~ = 22). See text for details

eter 1.3 mm. During the measurement, the sample was immersed in Ringer’s solution. The force needed for the 300 l.i,rn deformation was recorded for 2 s with a 500 Hz sampling frequency. The maximum force was determined for the calculation of the shear modulus. After the test, cartilage thickness at the site of the indentation was measured with a penetrating needle techniquelg. The shear modulus of the cartilage was calculated according to elastic models of cartilage14. The linear correlation (r = 0.8’79) between the arthrostopic and reference measurements is shown in

Figure 6. Reproducibility of the arthroscopic measurements was evaluated by repeating the arthroscopic indentations at 16 measurement sites. The time interval between the first and the second measurement was 15 min. The correlation analysis showed a linear relationship for the measurements (r = 0.953, n = 16) (Figure 7). DISCUSSION We have developed an indentation instrument for objective measurement of cartilage stiffness during arthroscopy. The instrument is capable of measuring stiffness of joint cartilage reliably, it is washable, sterilizable and fulfills the electrical safety regulations for medical instruments. It is therefore suitable for clinical use. The measuring technique with our arthroscopic instrument is similar to traditional stress-relax-

0 0

2.5

5

Indenter

Figure 7

7.5

1 i.5

force (N)

Reproducibility of arthroscopic lation between two repeated measurements between the measurements was 15 min

398

IO

measurements. (n = 16).

The

Corretime

ation tests. During arthroscopic investigation, however, massive material testing devices, where the indenter is joined to a load framework and moved under the control of a displacement transducer cannot be used. Instead we have used the inclined flat surface in the distal end of the measurement rod as a reference plate. While the reference plate is in complete contact with the cartilage surface, the indenter imposes constant deformation of the tissue. The area of the reference plate, which is in contact with cartilage surface, is large compared to that of the indenter > 10). Only when very soft ( area,l,,,/areaind~ntrr cartilage is measured may the reference plate produce a small deformation of the cartilage when a pressing force of 10 N is used. This can contribute to the observation that the linear fit between the arthroscopic indenter force and the shear modulus measured with a material testing device using the same samples and geometry shows a positive offset (Figure 6). Immediately after a load is applied, cartilage behaves like an incompressible elastic solid and can be modelled by elastic models. These models can also be utilized for arthroscopic measurements if the cartilage thickness is determined, for example with MRI. Since the cartilage thickness is normally not known during arthroscopy, we use the mean indenter force measured for 1 s just after the load application as an indicator of the cartilage stiffness. The shear modulus determined from the short term response is dependent on the PG and collagen content of the cartilage20*2’~22. Therefore the instant indenter force reflects alterations in the tissue composition of the degenerated cartilage. We use an indenter with a small radius to minimize the effect of thickness on the indenter force. The indenter used in our measurements has a diameter of 1.3 mm. When cartilage thickness ranges from 2 mm to 4 mm, as normally is the case in an adult knee jointz3, the indenter force is virtually independent of the cartilage thickness. If cartilage has become thinner and is below 1 mm, our method may produce incorrectly high stiffness values. In this case, cartilage thickness must be determined and the indenter force must be corrected with the factor l/~ (equation 1). The instrument was tested with elastomer and cartilage samples. With the elastomer samples the measuring geometry and conditions (dry measurement, room temperature) were constant. The highly linear relationship between the measured indenter force and dynamic modulus of the elastomer samples as provided by the manufacturer shows that the instrument can separate materials of different stiffness. Cartilage measurements show that the arthroscopic indentations are reproducible and correlate well with the reference measurements. The stiffness measurements in cadaver knee joints show that the measurements can be carried out also under arthroscopic control.

ACKNOWLEDGEMENTS The authors are grateful to Mr Jukka Laakkonen and Mr Aimo Tiihonen for their contribution during the design of the instrument. This work was supported by the Foundation of Finnish Inventions, North-Savo Fund of Finnish Cultural FounMagnus ~hrnrooth Foundation, The dation, Research Foundation of Rheumatic Diseases and The Instrumentarium Science Foundation.

REFERENCES 1. Hamrrman

D, Schubert M. Diarthrodial.joints, an essay. 1962; 33: 55-90. Stockwell RA. Riolog? OSCnrlilqq Glls. Gmbridge Llniversity Press, Gmbridge, 1979. Rollct 141, HandyJR, Sturgill BC. ~~hondroitin sulfate conctntratiorj and pr(~tein-p~~lysac~haride composition of articuiar cartilage in osteoarthritis. J Clilz Inztnf. 1963; 42: 853-9. Kcmpson GE, Muir H, Swanson SAV, Freeman h4AR. Grrclations between stiffness and chemical constituents of cartilage on the human femoral head. I~~~r~~rn ~~~~~~.s .-b-l/l. 1970: 215: 70-7. Armstrong (Xi, Mow \‘CI. ITariations in the intrinsic mechanical properties of human articular cartilage with age, degeneration and water content. .I RO?ZP ,&inl .Su~q, 198”; 64-A: 88-9-I. Harkness RD. Mechanical properties of collagenous tissws. In Hioloy~ o/ Collugm, BS Gould (ed). Raven Press, London. 247-310. Kempson GE. Muir H, Pollard C. Tuke M. The tensile propcrtics of the cartilage of’ human femoral condytes related to the content of- collagen and glyc(~s~jl~in~~glycarts. Ifiochim Bio#~s Actn, 1973; 297: 45672. McDevitt CA, Muir H. Biochemical changes in the cartilage of the kneejoint in experimental and natural osteoarthritis in the dog. ,/ Bonr,]oinl Sur,. 1976; 58-R 9&101. J~t~clin J, Kiviranta I, Tammi M, Helminen H.J. Softening of canine articular cartilage after ilnrnobili~dti~~1~ of the knee joint. Cli.rr Orlhop Rel RPL 1986; 207: 246-52. Lane JM, Chisena E, Balck J. Experimental knee instability: early mechanical property changes in articular car” Am,/

2. 3.

4.

5.

6. 7.

8. 9. 10.

tilag:f‘

Med,

in 21 rabbit

model.

Clint Or&ho&

1979;

140:

262~5.

11. Tkaczuk MD. Human cartilage stiffness: in viva studies. Clin Orthop, 1986; 206: 301-12. 12. Dashefsky JH. ~throscopic rn~~sl~r~rnent of chondromalacia of patella using microminiature pressure transducer. Adhroscofy, 1987; 3(2): 80-5. 13. Mak AF, I.ai WM, Mow VC. Biphasic indentation 01 articular cartilage: I. Theoretical analysis. ,I Rinm~ch 1987; 20: 703-14. 14. Hayes WC, Keer LM, Herrmann G, Mockros I.F. A mathematical analysis for indentation tests of articuiar cartilage.

,/ Riomwh

1972;

5: 541-5

1.

15. Parsons JR, Black J. The viscoelastic shrar behavior of normal rabbit ar&rlar cartilage. f Rirrmrrh 1977: 10: “I-9. 16. Mow VC:, Gibbs MC:, I,ai WM. Zhu WB, Athanasiou K4. Biphasic indentation of articular cartilage: II. A numerical algorithm and an cxpcrirnental 51udy. ,I Riom~k 1989: 22: 853-61. I’?. Jurvclin .J, Kivirarlta I, Arokoshi .J, Tammi M. I~rhninen Ii]. lrld~~rl~ti~~n study (ii- the biorr~~~ch~nic~i~ properties of articular cartilage in canine knee. I
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