Development and validation of a new transducer for intradiscal pressure measurement D.S. McNally, M.A. Adams and AX. Goodship Comparative Orthopaedic Research Unit, Department of Anatomy, University of Bristol, Park Row, Bristol BSl 5LS, UK Received February 1992, accepted June 1992

AJWRACT Potentially damaging tensilestressesin the annulusfibrosusare directly related to the hydrostaticpressure in the centreof an intervertebral disc: the designand development of a miniature strain gauge pressure transducer is described fir measuring such pressures. Static calibration tests in bulk liquid demonstrated that measurements ma& with the transducerwere of suficient accurq and stability for in vitro and in viva investigationsof spinal mechanics,and a study of the dynamic behaviourof the transducerdemonstratedthat it haa’a jequemy responsesuitablefor in vitro and in viva investigations. Tests within loa&d cadavericdiscs showed that the transducercould be used to make repeatable measurementswhich were free from significant artefacts,when the disc was subjectedtoforces of up to 4ooO N and when deformed in fill jlexionfextension. Keywords:

Pressure, stress, transducer, spine, intervertebral disc

INTRODUCTION The pressure within the nucleus pulposus of intervertebral discs (nuclear or intradiscal pressure) provides a direct measurement of the compressive force acting on the disc itself’, and indirectly permits estimates to be made of the forces acting on the apophyseal joints and in intervertebral ligaments. A simple pressure measurement device, the mercury-coupled Bourdon gauge, was used by Naylor and Snare2 in 1951 to measure pressure within the nucleus pulposus. With the advent of strain gauge pressure transducer measurement techniques, static and perfused saline-filled catheter systems have been used ‘,j. These techniq ues have three main disadvantages. Firstly, the intervertebral disc contains a proteoglycan gel of higher osmotic potential compared with the saline in the measurement catheter, which may cause the system to under-read the true measurement. Secondly, compliance of the measurement catheter or transducer causes the intrusion of nuclear material into the coupling catheter at high ressures’. If the nuclear material is sufficiently soPid to block the lumen of the coupling catheter, it will cause the system to underestimate the true pressure. Finally, the intervertebral disc itself has a very small compliance. Panjabi et aL3 showed that the introduction of 0.5 ml of fluid into the nucleus increased nuclear pressure by 6 kPa. Hence, perfused systems will cause large measurement artefacts. There have been a number of attempts to overcome these difficulties whilst retaining the basic Correspondence

and reprint requests to: Dr D.S. McNally

0 Butterworth-Heinemann 0141-5425/92/06495-04

liquid-coupled measurement system. Takashi et cd4 used silicone oil to eliminate osmotic interactions, and by removing air bubbles in a vacuum chamber, attempted to reduce the compliancy of the measurement system. Merriam et aL5 pre-pressurized the measurement system so that it a proximately balanced the pressure within the disc. TR is technique ensured minimal flow along the coupling catheter (in either direction), but could only provide a coarse estimate of the nuclear pressure under static conditions. Nachemson’ managed to avoid these problems by separating the coupling column from the disc with a deformable plastic membrane. This membrane also attenuated the pressure transmitted to the transducer, enabling a conventional blood pressure transducer to measure pressures which would otherwise greatly exceed its working range. This sealed system suffers from two ma’or drawbacks. The attenuation of the pressure by tI3e plastic membrane is dependent on the mechanical properties of the membrane, and the compliance of the liquid coupling catheter. It is therefore difficult to ensure re eatability in the construction of such systems. l% e low compliance of the sealed system means that even small changes in temperature will cause thermal ex ansion or contraction of the coupling liquid an x bring about large measurement artefacts. To avoid the problems associated with liquid-filled coupling lines, attempts have been made to place the transducer at the measurement site. For this to be achieved it was necessary to develop extremely small transducers. Okushima’ used a semi-conductor strain gauge membrane mounted on the end of a 0.8

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mm-diameter needle. Bowever, this transducer was very susceptible to variations of temperature7. More recently, metal film strain gauges have been em loyed’, but the transducer is comparatively large an B is mounted on a 3 mm-diameter needle. It was decided to design a transducer which was free from these disadvantages. All the problems associated with liquid coupling systems were avoided by using a small (1.3 mm-diameter) transducer which could be placed directly at the site of measurement without damaging or otherwise erturbing the disc tissues. Since the mechanical be Raviour of the disc, like all biological materials, varies greatly with applied load, the transducer was designed to operate over the full physiologically relevant range of O-5 MPa. The transducer was designed to respond to transient changes of intradiscal pressure, such as those which would be encountered during vigorous activity in life, without introducing measurement artefacts.

element and the needle, which might break this seal, was minimized by supporting the sensing element using cast and shaped epoxy formers. Fine stainless steel wires were used to connect the sensing element to components which completed the bridge and also provided temperature compensation. These were mounted within the body of an electrical connector. The connecting wires were protected by the needle and by a steel-braid-reinforced silicone rubber sheath. A header amplifier (based on a RS 308-315 module, R.S. Components, Corb , UK) was specially constructed. This unit also provi Bed a 1.0 V excitation signal for the bridge. The header amplifier permitted a variable offset voltage to be added to the transducer signal (to facilitate zeroing) and had a gain which could be varied between 20 and 50 dB.

TRANSDUCER

The transducer was mounted in a water-filled cylinder. The application of a force to the piston resulted in a known increase in pressure within the cylinder. By using a Dartec computer-controlled materials tesing machine to appl forces to the piston, a variety of static or dynamical ry varying pressures could be generated. The linearity of the transducer was measured over the range O-2MPa. The maximum pressure was limited by the test equipment rather than the transducer. The temperature dependence of the transducer was measured over the range 19-55°C. A Biotec 601A pressure generator was used to assess the dynamic behaviour of the transducer over the frequency range l-50 Hz.

CONSTRUCTION

The sensing element was fabricated from a beryllium/copper (BeXu) plate 3 mm x 1.2 mm X 0.1 mm, the central 2.0 mm x 0.8mm portion of the plate being chemically etched to form a diaphragm with a thickness of 30pm. Both faces of the plate were coated with a 5pm film of SiO2 using vacuum evaporation. This coating serves to insulate and protect the Be/Cu plate. Two thin film chromium cermet strain gauge elements, together with gold contacts were evaporated onto the SiOg layer and were connected to form a half-bridge circuit, as shown schematically in Figure 1. The sensing element was mounted on the side of a 150 mm-long, 1.3 mm-diameter surgical steel needle (AISI type 304) as shown in Figure 2. The element was fitted into an accurately machined cut-out on the needle and sealed in place using a medical grade silicone elastomer. Movement between the sensing 2.Omm

TRANSDUCER

CALIBRATION

Calibration tests

Calibration results The calibration test results are summarized in Table 1. values compare favourably with transducers commonly employed for physiological measurements but over a much larger working pressure range. A linearity curve for the transducer is shown in Figure 3. The curve shows no marked deviations from the ideal linear plot up to 2 MPa. These

0.8mm

1.2 mm

EVALUATION OF THE TRANSDUCER INTERVERTEBRAL DISCS :L -

3.Omm

Figure

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Schematic

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Figure 2 ing needle

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Schematic

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transducer

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IN

Pressure transducers are designed to make measurements in a fluid medium, gas or liquid; it was therefore im ortant to validate ressure measurements made 1 y .the transducer wit! in the apparently Table 1 transducer

Calibration details of intradiscal pressure transducer. The was calibrated in a water bath at 37”C, exceptwherestated

Sensitivity Maximum

linearity

error

Maximum hysteresis error Temperature coefficient of zero Temperature coefficient of sensitivity Dynamic range flat to _t3dB

0.5 f0.2 +1.2 0.1 0.02 -0.03 O-50

mVN/kPa % (O-0.1 MPa) ‘% (O-Z.0 MPa at 20°C) % (-0.1-0.1 MPa) kPa deg-‘C O/od&C Hz

Intradiscal pressure measurement:D.S. McNal~ et al.

removed, ten times in succession, taking care to ensure that the orientation of the transducer membrane was constant. In this way, the repeatability of the pressure measurement was investigated at both compressive loads and also at four different orientations of the transducer (vertically up/down and laterally left/right). The sensitivity of the transducer to external forces transmitted down its mounting needle was investigated by applying, manually, a small bending moment to the needle. The required deflection of the needle to significantly affect the measurement of nuclear pressure was measured. Measurement of anisotropy

0

2

1 Applied

pressure

(MPa)

3 Typical linearity curve for the pressure transducer tested between 0 and 2 MPa Figure

non-fluid intervertebral disc. The potential of the transducer for in vivo measurements made it desirable to evaluate its performance in discs subjected to bending and compressive loading simulating vigorous activities in life. Specimen preparation Seven lumbar spines were collected at routine necropsies from subjects aged between 19 and 69 years and all grades of disc degeneration were represented. The s ines were stored in sealed plastic bags at - 17” 8 for up to three months, and then defrosted at 3°C for 12 h prior to dissection. Each spine was dissected into two motion segments consisting of two adjacent vertebrae and the intervening disc and ligaments. Motion segments were set in two cups of dental plaster to facilitate mounting between the jaws of a Dartec materials testing machine. A system of angle plates and rollers enabled the motion segment to be loaded in compression while wedged in various angles of flexion or extension. The maximum applied compressive force was 4000 N, and the angles varied between 12” of flexion and 8” of extension. All specimens were creep loaded at 1000 N for half an hour prior to mechanical testing to ensure that their hydration was within the normal diurnal rangeg. The pressure transducer was introduced into the centre of the disc down a 2mm-diameter plastic cannula which had previously been inserted using a 1.3 mm-diameter hypodermic needle. This method of introduction ensured the exact placement of the transducer without exposing it to damaging stresses and without significant disturbance of the disc itself. Repeatability of measurements The motion segment was loaded with a constant compressive load of either lOOON or 2000N. The transducer was introducted into the disc and then

For the concept of ‘pressure’ within the nucleus pulposus to have any validity, the material of the nucleus must behave as a fluid (i.e. be unable to support shear stress). Deviation from this ideal behaviour results in an anisotropic distribution of compressive stress. It was therefore important to ensure that the compressive stress measured by the transducer was indeed isotropic. This was achieved by introducing the transducer in each of the four orientations (up, down, left and right). Tests were repeated over the range of O-4000N compressive load and 12” flexion to 8” extension. Results of repeatability measurements The measurements of nuclear pressure were repeatable to + lo/oin any direction. It was also found that to cause the transducer to misread due to forces transmitted along the needle, a moment sufficient to bend the needle by 10” was re uired. This degree of bending is easily noticeable an 1 action to eliminate it can be taken. Results of measurement of anisotropy Comparison of the median values of the components signed-rank test, of axial stress, usin a Wilcoxon showed no evidence $