Presented at NACOB 98: North American Congress on Biomechanics Canadian Society for Biomechanics - American Society of Biomechanics University of Waterloo Waterloo, Ontario, Canada August 14-18, 1998

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A NOVEL TECHNIQUE FOR MEASURING IN VITRO PRESSURE
IN CERVICAL INTERVERTEBRAL DISCS

P.A. Cripton ^1,2 , G.A. Dumas ¹ , L.-P. Nolte ^1,2
1 Department of Mechanical Engineering, Queen's University, Kingston, Canada, K7L 3N6
² Müller Institute for Biomechanics, University of Bern, 3010 Bern, Switzerland

INTRODUCTION

Despite over 3 decades of experience in the lumbar spine and the probable role of disc pressure in the mechanics of cervical disc disease or herniation there is a paucity of basic cervical intervertebral disc (IVD) pressure data. One probable reason for this is the technical challenge associated with placing a pressure sensor in the nucleus pulposus: the uncinate processes prevent needle-mounted sensor insertion from lateral and the inferior protuberance of the antero-inferior rim of the superior vertebral body often prevents or complicates insertion from anterior (Fig.1).

Figure 1: At left the uncinate processes are visible on the lateral aspects of the superior endplates. At right the inferior protuberance is visible at the antero-inferior margin of the vertebral bodies.

The discs in this region have much smaller overall and nucleus cross-sections than those from other regions of the spine (Pooni et al., 1986) making placement in the nucleus difficult to accomplish. Investigators to date have used needle mounted pressure sensors (Pospiech et al., 1996),(Hattori et al., 1981). It is possible that needle mounted sensors alter the natural biomechanics of the segments in which they are implanted.

The objectives of this study were to develop a technique for measuring cervical disc pressure in vitro which altered the natural cervical disc and bony anatomy less than previously used techniques.

REVIEW AND THEORY

The pressures occurring in the nucleus pulposus of lumbar IVDs have been extensively studied in both in vivo (Nachemson, 1981) and in vitro (McNally and Adams, 1992) settings. In this region IVD pressures have been shown to increase linearly with applied compressive load and to be hydrostatic in healthy discs. Very few studies have been undertaken in the cervical spine. The in vivo study by Hattori et al.(1981) Focused on the effects of posture and degeneration on disc pressure while Pospiech et al.(1996) examined pressure in whole cervical spines subjected to small moments applied in vitro. No study to date has evaluated the basic behaviour of cervical IVD pressure in compression.

PROCEDURES

A stainless steel needle with an elliptical cross section, inner major and minor diameters of 2.45 mm and 1.10 mm respectively and a wall thickness of 0.3 mm was constructed and used to insert the transducer.

A miniature pressure sensor (model 060 S, Precision Measurement Company, Ann Arbor USA) was used. It is disc shaped and has one active measuring face, a diameter of 1.5 mm and a thickness of 0.3mm. The sensor was connected as the active arm in a Wheatstone bridge circuit. The system was calibrated using a pressure chamber after which output voltage from the Wheatstone bridge was a known linear function of the applied pressure

The needle was inserted from an antero-lateral direction, passing between the uncinate process and inferior protuberance (Fig. 2). When the technique was used with single functional spinal units (FSUs) the cranial-most endplate was used as a template from which the appropriate orientation and depth of insertion could be determined such that the needle tip embedded in the nucleus at the centre of the IVD. The sensor was inserted into the disc through the needle. A piece of wire with a groove machined in one end which fit onto the pressure sensor disc between the two wire leads was used to push the sensor into the nucleus. In the two final steps the needle was withdrawn over the insertion wire and sensor cable and then the insertion wire was withdrawn leaving the sensor embedded in the nucleus with only three 0.26 mm diameter flexible electrical cables passing through the annulus.

Figure 2: The sensor is mounted to the insertion wire ready to be inserted into the nucleus through the oval needle.

The opening made in the annulus by the needle was glued closed using a drop of "tissue glue" commonly used to close wounds (Histocryl, Braun, Melsungen, Germany). Sensor position was validated using radiographs taken from anterior, lateral and superior directions.

The technique was used to measure pressure in the cervical discs from two FSU preparations (one C3-4 and one C4-5). Disc pressures were recorded as the specimens were subjected to pure compression in a materials testing machine. The specimens were compressed, at a rate of 10 N/s under force control, to 800 N. A linear regression was performed on the resulting data to test the hypothesis that cervical disc pressure varies linearly with applied compression.

RESULTS

In both cases the post-test radiographs confirmed sensor placement within 1.0 mm of the geometric centre of the disc. In neither case was nucleus or other disc material extruded through the hole made during sensor insertion. The resulting pressure profiles were highly linear with coefficients of determination (r ² ) greater than 0.99 in both cases (Fig 3). The greatest pressures were recorded at the peak force of 800 N. Peak Pressures of 22.9 bar and 39.1 bar were recorded for the C4-5 and C3-4 specimens respectively.

Figure 3:Variation in disc pressure as a function of applied compression. The best linear fit to the data is also plotted for each case.

DISCUSSION

The technique presented represents an approach to measure IVD pressure which requires a very small intrusion to the nucleus and annulus volumes. Distortion of the pressure signals or specimen kinematic behaviour due to sensor insertion should be minimised using this approach. Three fine, flexible cables pass through the antero-lateral annulus compared to stiff circular needles, commonly greater than 1 mm in diameter, for needle mounted sensors. Placement of the sensor using the cranial most endplate of the FSU for depth and orientation guidance was accurate and repeatable in the two preliminary cases reported here.

The pressure signals varied linearly with applied axial compression. This behaviour is similar to that observed for lumbar discs(Nachemson, 1981). It is unclear, from this preliminary data, why the C3-4 specimen recorded pressures so much in excess of the C4-5 specimen. The specimens were from different donors but no difference in appearance or dimensions was large enough to explain this behaviour. It may be related to differences in disc grade. Disc grade for the specimens tested will be determined in the future. More specimens must be measured using this technique in order to provide a "baseline" of cervical disc pressure data.

REFERENCES

Hattori S. et al. Z.Orthop.,119,568-569,1981.

McNally D.S. and Adams M.A. Spine,17,66-73,1992.

Nachemson A. Spine,6,93-97,1981.

Pooni J.S. et al. Surgical-Radiologic Anatomy,8,175-182,1986.

Pospiech J. et al. Langenbecks Arch.Chir.,381,303-308,1996.