Microprocessor controlled arthrograph

MICROPROCESSOR CONTROLLED ARTHROGRAPH A Howet,

D. Thompson*

and V. Wrightt

use on a

ward or in a clinic, is simple to use and is readily accepted by patients. One test and its associated analysis, together with printed and plotted results, occupies little more than two minutes.

ABSTRACT 7hi.snew ar&vgraph based on a micnxompllte provides [email protected]~enxnts of variable amplitude, frequency and waveform. It is su.ciently small for Keywords:

Patient monitors,

arthrograph,

joint stiffness, rheumatoid

arthritis,

INTRODUCTION

control

study stiffness and articular gelling’, a modification of which was used by Such et al. to quantify stiffness of the knees in seventy normal subjects4. Thompsot? produced a new arthrograph which was capable of measuring stiffness of the knee over a given range of motion anywhere within the passive range of the joint; an idea which was adopted by Unsworth6 in a study of the metacarpophalangeal joints of young adults. All these arthrographs have been primarily research and not clinical tools, requiring the skills of a practised experimenter to obtain useful results, which still needed a tedious and time-consuming analysis in order to reveal information that would permit the comparison and monitoring of stiffness. to

Stiffness is a complaint of sufferers from many rheumatic and neuromuscular disorders; it is also a symptom in patients with diseases affecting the skin and subcutaneous tissues (such as systemic sclerosis). Normal subjects may complain of stiffness after unaccustomed activity, but there are also disorders of connective tissue characterized by increased mobility of joints. Patients with active rheumatoid arthritis commonly complain of waking in the morning with greatly increased stiffness which may persist for several hours. This mornin stiffness is so characteristic that it heads the list o B criteria proposed by the American Rheumatism Association for the diagnosis of rheumatoid arthritis. In both that disease and osteoarthrosis patients experience increased stiffness after resting in one position for some time and on getting up it takes some minutes to regain their usual mobility. This inactivity stiffness has been termed ‘articular gelling’. The problem for all these conditions is to devise quantitative methods for the measurement of stiffness and also to relate those findings to the symptoms of which the sufferers complain. BACKGROUND The mechanical devices most commonly applied to the measurement of stiffness are arthrographs in which an angular motion is imposed on the joint and the force resisting that motion is recorded. The resistive torque is plotted against displacement on an oscilloscope or X-Y plotter thereby giving a hysteresis loop from which a set of defining variables may be extracted. The first instrument of this kind was used by Wright and Johns’ in 1960 at the metacarpophalangeal (MCP) joint of the index finger. Originally a simple pendulum driven device, it was later modified to use a variable speed motor which permitted a wider range of frequencies and displacements. Long2 studied the rheology of hand control using a finger arthrograph based on that of Wright and Johns and demonstrated the importance of wrist position when measuring stiffness in the MCP joint. A knee arthrograph was developed tRheumatism Research Unit, University Department of Medicine, 36 Clarendon Road, Leeds, LS2 9PJ, UK ‘Faculty of Technology, Engineering Mechanics, The Open University, Milton Keynes, UK

Q 198.5 Butterworth & Co (Publishers) 0141-5425/85/020153-04 $03.00

microprocessor

In spite of the research activity of the past twenty five years, there has been no major therapeutic trial in which objective stiffness measurements have been used; due at least in part, to the difficulty of obtaining clinical data quickly and repeatably. To be of maximum use an arthrograph should be capable of operating with equal success in both research and clinical situations. For research purposes the investigator should be able to select one of a number of different waveforms of variable amplitude and frequency, but at the same time, the instrument should be sufficiently small and portable to enable it to be used on a hospital ward or clinic. It must be acceptable to both patients (by allowing them to relax despite possible debility) and clinicians (through being simple and quick to set up and use). Results should be available in an immediately useful form. Recent developments in microprocessor technology provide an opportunity to design and construct a device which will satisfy these criteria without being restrictively expensive. The subject of this paper is an arthrograph whose operation is entirely supervised by its own dedicated computer. For reasons of size and ease of patient relaxation it was decided to concentrate on a machine for measuring finger stiffness. This has the added advantage that since the metacarpophalangeal joint is often one bf the first to be involved in rheumatoid arthritis, it may be monitored before the joint becomes inflamed and painful. In order to eliminate torque arising from the weight of the finger the instrument was built to measure stiffness

Ltd J. Biomed Eng. 1985, Vol. 7, April

153

Arthrograph. A. Howe et al

in a horizontal plane. Stiffness in the MCP joints the first, middle and third fingers of either hand may be measured by simple adduction/abduction of the finger.

of

INSTRUMENT The arthrograph is in one case which contains the drive assembly, interface and calibration circuitry, arm support and transducers. The associated computer hardware consists of a Comart Cl00 Communicator with twin floppy disk drives, an IO Technology 12 bit DAC board, a visual display unit and an Epson printer. The drive assembly is based on a PML G9M4 DC servo-motor and amplifier. When these are used in conjunction with position and tachometer feedback it is possible to move the motor armature in phase with an electrical waveform fed into the amplifier (Figure I). This obviates the need for a mechanical device such as a scotch yoke converter or gearbox and so reduces mechanical backlash in the system. It also allows the leading and trailing edges of square or step waves to be more accurately reproduced. The finger holder is a split plastic tube into which the finger is placed (Figure 2); the finger holder and torque transducer are mounted directly onto the shaft of the motor. When positioned correctly the finger may be secured by tightening the tube onto the proximal phalangeal joint by means of a Velcro strip. Torque is measured with a strain-gauged aluminium cantilever arm; position feedback and position monitoring are provided by a single precision plastic potentiometer, also mounted directly on the motor shaft. The instrument is calibrated by the application of known weights and angles. COMPUTER

PROGRAM

The control program is written in PASCAL and C. There are two automatic test procedures, one of which may be used simply by pressing a single key: this runs the test, analyses the data and produces a

Figure

2

The finger arthrograph

with subject hand in place

copy of the results on the printer. The second automatic test is similar in function but includes other frequently used preset values. There is also a manual mode that is entirely menu driven. The operator may select a test waveform (sine, square, triangular or step), frequency (0.01 - 3.0 Hz), amplitude and offset. The computer will not allow combinations of parameters which would be likely to cause patient distress or to introduce inertial effects into the results. When the test is started the arthograph first searches for the equilibrium position of the joint. Two positions, at which the torques necessary to maintain a constant rate of strain reach a threshold value are measured and the equilibrium position is defined as the midpoint between them. Tests are carried out relative to this datum, thereby allowing both an easy comparison of data from different subjects and a reduction in errors arising from inaccurate positioning of the hand. The chosen test waveform is passed to the servo-amplifier by means of a digital-to-analogue converter. The instantaneous position and resistance to motion are collected from the transducers and monitored by the computer. Each successive loop is compared with the previous one until the results are repeatable, at which point the test is stopped. The operator is offered the choice of repeating the test, choosing a new test or analysing the stored data. The area and mean slope of the loop are calculated and resented, together with a copy of the loop itsel P, on the printer. The test data are saved on a floppy disk for further analysis if required.

TACHOMETER

METHOD

KEYBOARD Figure

154

1

Schematic diagram of finger arthrograph

J. Biomed Eng. 1985, Vol. 7, April

OF OPERATION

In practice the joint most commonly tested is that of the middle finger. The hand is placed on the arthrograph so that the centre of rotation of the MCP joint coincides with that of the motor; there is no power to the motor at this time and the finger may be moved freely in order to aid alignment. Support is provided below adjacent MCP joints and the arm rests on a ‘bean-bag’ in such a manner as to allow the wrist to be straight. Both the arm and the metacarpal bone of the finger being tested are aligned parallel to the long axis of the arthrograph; when in place they are stabilized in this position by

Arthrograph: A. Howe et al

which was obtained from a 22-year-old normal male. The loop has two principal characteristics: its mean slope and its area; the latter corresponding to the energy dissipated per cycle. In order to remove the dependence that the area otherwise has upon the mean slope of the loop and to standardize the results from different tests, the area of the loop was normalized by expressing it as a percentage of the area of a triangle fitted as shown in Figure 4. REPRODUCIBILITY

DISPLACEMENT

A:

B,C:

Mean Slope of Central Region Mean Outslde

Figure

Typical

3

Hysteresis

hysteresis

Slopr

of

Regions

loop

0.08 -

: 44%

Mean slope : 0.0105 Nmldegree

<

(6, 0.05

The stiffness of the middle fingers of the right hand of three subjects was measured in the arthrograph using the standard test described above (4O amplitude, 0.2 Hz); each subject was tested a total of nine times. After the third and sixth tests the hand was removed from the arthrograph and then replaced. Coefficients of variation for the mean slope and energy dissipation were calculated for each set of three values and for the combined results, Table I, Subject 3 was unable to relax completely in the machine, and this is reflected in the higher coeffkients of variation. The overall correlations are generally greater than the individual ones, possibly the result of a slightly different positioning of the hand after its replacement in the arthrograph; there may also be a stress-relaxation’ or thixotropic effects caused by the continual movement of the joint.

Table

I Egulor

Reproducibility

1 8

(deg

-0.08b

Hysteresis loop obtained from a 22-year-old normal Figure 4 male using the standard test procedure (4’ amplitude, 0.2 Hz frequency relative to equilibrium position)

means of a strap. In order that the test may be conducted automatically it is important that the subject is completely relaxed; more than 90% of subjects could achieve this when the frequency of the test was in the region of 0.2 Hz. ANALYSIS OF THE

DATA

A typical hysteresis loop is shown in Figure 3. The loop may be divided into three distinct regions; two outside areas giving information on the range and laxity of the joint and a central region which represents the more interesting stiffness measure,ments. A standard test amplitude of 4” either side of the equilibrium position was adopted; this gives a hysteresis loop similar to that shown in Figure 4,

data: mean slopes

Subject

Test

I disploce&

la

CoetXcient

of variation

number A

B

C

1 2

0.0103 0.0100

0.0089 0.0092

0.0161 0.0 149

(133) 4 5 6 (4-6) 7 8 9 (7-9)

0.0104 0.0098 0.0099 0.0100

0.0093 0.0090 0.009 1 0.009 I

0.0146 0.0157 0.0151 0.0158

0.0102 0.0100 0.0101

0.0100 0.0100 0.0101

0.0167 0.0166 0.0156

Overall

Table

1b

Reproducibility

C (%I

2.0

2.3

5.2

1.0

0.6

2.4

1.0

0.6

3.7

1.9

5.0

4.6

Coefficient of variation

A

B

C

1 2

28 29

39 35

47 48

(133, 4 5

29 31 29

33 35 37

(4G 7 8

30 28 30

(799)

30

Overall

B (%I

data: energy dissipation

Subject

Test number

A @I

* (%I

B (%I

C (%I

49 40 44

2.0

8.5

2.1

35 35 35

45 48 44

3.3

3.2

6.1

36

42

3.9

1.6

6.8

3.4

4.6

6.7

J. Biomed Eng. 1985, Vol. 7. April

155

Arthrografdx A. Howe et al

Arthritis and Rheumatism

CONCLUSIONS The arthrograph has shown itself capable of producing accurate and rapid measurements of joint stiffness. In the automatic mode a complete test takes less than two minutes and requires minimal operator attention; for this reason it would appear well suited to clinical use and its ability to apply tests of different waveform and frequency is particularly valuable in research, for instance in the development of a rheological model for the MCP joint. The short duration of the test makes it possible to investigate a large number of joints within a short period and suggests other applications: the quantification of joint stiffness in normal individuals and in patients with different rheumatic diseases; an analysis of pathology within the joint; an evaluation of the relief of stiffness by intraarticular therapy, by surgery, and by systemic drug therapy; and the correlation of pharmacokinetic drug profiles with a quantitative assessment of stiffness. ACKNOWLEDGEMENTS The authors gratefully

156

J.

Biomed

acknowledge

Eng. 1985, Vol. 7, April

support

of the

Council.

REFERENCES Wright, V. and Johns, R.J. Observations on the measurement of joint stiffness. Arthritis Rhmm, 1960, 3, 328-340 Long, C., Thomas, D. and Crochetiere, W.J. Visco-elastic factors in hand control. Excerpt Med Znt. Congress series, 1964, 107, 440-445 Goddard, R, Dowson, D., Longfield, M.D. and Wright, V. A study of articular gelling. In: Lubrication and Wear in Human Joints, (Ed. V. Wright), Sector, London, 1969, 134-141 Such, C.H., Unsworth, A, Wright, V. and Dowson, D. Quantitative study of stiffness in the knee joint Ann km Dis., 1975, 34, 286-291 Thompson, D., Wright, V. and Dowson, D. A new form of knee arthrograph for the study of stiffness, Eng. Med, 1978, 70, 84-92 Unsworth, A., Bey, P.M.A. and Haslock, I. Stiffness in metacarpophalanged joints of young adults. Clin Phys. PhysioL Meas., 1981, 2, 123-133 Unsworth, A., Yung, P. and Haslock, I. Measurement of stiffness in the metacarpophalangeal joint the arthrograph. Clin Phys. PhysioL Meas., 1982, 3, 273-281 Walsh, E.G., Lakie, M., Wright, G.W. and Tsemetzis, Sk Measurement of muscle tone using printed motors as torque generators. Eng. Med., 1980, 9, 167-171