- Email: [email protected]
Ex vivo measurement of lumbar intervertebral disc pressure using fibre-Bragg gratings
ARTICLE IN PRESS
Journal of Biomechanics 41 (2008) 221–225 www.elsevier.com/locate/jbiomech www.JBiomech.com
Short communication
Ex vivo measurement of lumbar intervertebral disc pressure using fibre-Bragg gratings Christopher R. Dennisona,, Peter M. Wilda, Peter W.G. Byrnesa, Amy Saaria, Eyal Itshayekb, Derek C. Wilsonb, Qingan A. Zhub, Marcel F.S. Dvorakb, Peter A. Criptonb,c, David R. Wilsonb a Department of Mechanical Engineering, University of Victoria, P.O. Box 3055, Victoria, B.C., Canada V8W 3P6 Division of Orthopaedic Engineering Research, Department of Orthopaedics, University of British Columbia, Vancouver, B.C., Canada V6T 1Z4 c Department of Mechanical Engineering, University of British Columbia, Vancouver, B.C., Canada V6T 1Z4
b
Accepted 12 July 2007
Abstract Methods were developed to measure intervertebral disc pressure using optical fibre-Bragg gratings (FBGs). The FBG sensor was calibrated for hydrostatic pressure in a purpose-built apparatus and the average sensitivity was determined to be 5.770.085 pm/MPa (mean7SD). The average coefficient of determination (r2) for the calibration data was 0.99, and the average hysteresis of the sensor was 2.13% of full scale. The FBG was used to measure intradiscal pressure response to compressive load in five lumbar functional spine units. The pressure measured by the FBG sensor varied linearly with applied compressive load with coefficients of determination ranging from 0.84 to 0.97. The FBG sensor’s sensitivity to compressive load ranged from 0.70270.043 kPa/N (mean7SD) in a L1–L2 specimen, to 1.0770.069 kPa/N in a L4–L5 specimen. These measurements agree with those of previous studies in lumbar spines. Two strain gauge pressure sensors were also used to measure intradiscal pressure response to compressive load. The measured pressure sensitivity to load ranged from 0.251 kPa/N (L4–L5) to 0.850 kPa/N (L2–L3). The average difference in pressure sensitivity to load between Sensors 1 and 2 was 12.9% of the value for Sensor 1, with a range from 1.1% to 20.4%, which suggests that disc pressure was not purely hydrostatic. This may have contributed to the difference between the responses of the FBG and strain gauge sensors. r 2007 Elsevier Ltd. All rights reserved. Keywords: Spine; Intervertebral disc; Low back pain; Pressure; Sensor; Nucleus pulposus; Disc degeneration
1. Introduction Understanding intervertebral disc mechanics is central to understanding disc injury and the etiology of disc degeneration (Nachemson, 1960). Pressure distributions in intervertebral discs are an important indicator of disc mechanics that have been measured both in vivo (Nachemson, 1966; Nachemson and Morris, 1964) and ex vivo (Cunningham et al., 1997; Fye et al., 1998; McNally and Adams, 1992; Nachemson, 1960; Wilke et al., 1996). Needle-mounted strain gauge pressure sensors have been widely used to measure disc pressure both ex vivo and Corresponding author. Tel.: +1 250 853 3198; fax: +1 250 721 6051.
E-mail address: [email protected] (C.R. Dennison). 0021-9290/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jbiomech.2007.07.015
in vivo (Hattori et al., 1981; Pospiech et al., 1999; Steffen et al., 1998). However, the utility of these sensors is limited because they do not characterize pressure profiles within the disc. They are also suspected of injuring the annular fibres of the disc and of altering the biomechanical response of soft tissues because they are rigid and over 1 mm in diameter (Cripton et al., 2001). Fibre-Bragg gratings (FBGs), which consist of an optical fibre with a Bragg grating inscribed into the fibre core, have excellent potential for biomechanical applications because they are small (125 mm diameter), mechanically compliant (allowing bending within the host structure to radii of 10 mm), immune to electromagnetic interference, and biocompatible. FBGs are commonly used to measure physical parameters such as strain (Black et al., 2002;
ARTICLE IN PRESS 222
C.R. Dennison et al. / Journal of Biomechanics 41 (2008) 221–225
Udd et al., 1996), temperature (Nunes et al., 2004; Yunqi et al., 2000), and pressure (Frank et al., 2003; Xu et al., 1993) and can also be configured to measure profiles of these parameters. The objectives of this study were: (a) to assess the linearity, sensitivity and hysteresis of a FBG sensor in pure hydrostatic compression, (b) to develop methods for implanting FBGs in the intervertebral disc, and (c) to determine the uncertainty in the FBG’s measurement of disc pressure in response to compressive load in functional spinal units and to compare its sensitivity to the sensitivity of needle-mounted strain gauge pressure sensors. 2. Materials and methods When an FBG is illuminated it reflects a single narrow spectrum of wavelengths centered at the Bragg wavelength. When hydrostatic pressure is applied to the FBG, pressure-induced strains within the fibre cause the Bragg wavelength to shift. These shifts vary linearly with applied pressure in the range 0–50 MPa (Xu et al., 1993). We calibrated FBG sensors (10 mm length, Blue Road Research, Gresham, OR) for hydrostatic pressure. In our calibration apparatus, pressure was generated in a glycerin-charged vessel with a manual pump (ENERPAC P141, Milwaukee, WI) and measured with a pressure sensor (OMEGA PX300-2KGV, Stamford CT, accuracy: 734 kPa). Two testing protocols were used to calibrate the sensors. In the first, pressure was increased incrementally from 1 MPa to 6.9 MPa. The Bragg wavelength was recorded at 0.5 MPa increments using an optical spectrum
analyzer (OSA; ANDO AQ6331, Tokyo, Japan) and the applied pressure was recorded from a digital readout (OMEGA DP25B-S, Stamford CT). In the second protocol, designed to quantify hysteresis, pressure was cycled from 1.4 MPa to 6.9 MPa to 1.4 MPa at 0.1 MPa/s. Pressures were not cycled below 1 MPa because we could not repeatably control the pump relief valve below this pressure. Each protocol was repeated three times, and the glycerin temperature was monitored to ensure that the calibration was isothermal. Sensor performance was assessed in five, two-vertebra human lumbar functional spinal units (Fig. 1(b)). Each specimen was stored at 20 1C in a sealed plastic container prior to testing, and allowed to thaw for 3 h to a temperature of 4 1C. Preconditioning loading cycles, with purely compressive loads ranging from 0 N to 2000 N to 0 N at a rate of 200 N/s, were applied with a materials testing machine (Instron 8874, Norwood, MA). The displacements of the disc and vertebrae were visually monitored to verify that the loads were purely compressive, and that bending or torsion of the specimen did not occur. The FBG sensor was inserted into the hub of a 25 gauge (0.51 mm outside diameter, 38 mm length) hypodermic needle. The needle was then passed through the annulus so that the tip was located approximately at the center of the nucleus pulposus. The FBG sensor was then advanced past the needle tip so that its entire length was exposed to the nucleus pulposus (Fig. 1(a)). The needle was left in the annulus to shield the fibreoptic cable from any traction that might be applied by the annulus (Fig. 1(b), location 3, also shown in Fig. 2). We then loaded each functional spinal unit from 0 N to 2000 N to 0 N at 10 N/s. After each test with the FBG sensor, we removed the FBG sensor and repeated the test with two strain gauge sensors (Sensors 1 and 2) (Model 060S; pressure range, 0–3.5 MPa; 2.45 mm diameter; Precision Measurement Co., Denton, Ann Arbor, USA) inserted into the center of the nucleus (Fig. 1(b), location 4).
Fig. 1. (a) FBG sensor inserted into 25 gauge (0.5 mm diameter, 38 mm length) hypodermic needle (1, hypodermic needle; 2, active sensing length of FBG (10 mm)), and inset, magnified image showing FBG sensor emerging from hypodermic needle; (b) FBG sensor and strain gauge sensor inserted into intervertebral disc (3, FBG Sensor; 4, strain gauge Sensor; 5, spine fixture including dental cement). (Taken by: Christopher R. Dennison.)
Fig. 2. Schematic cross-sectional diagram of FBG sensor insertion geometry (not to scale). Fibre optic cable passes through the needle hub and the hypodermic needle length (38 mm). The FBG sensor length (10 mm) is exposed to the nucleus pulposus while the hypodermic needle length shields fibre as it passes through the annulus and into the nucleus.
ARTICLE IN PRESS C.R. Dennison et al. / Journal of Biomechanics 41 (2008) 221–225 After all tests were completed, the specimens were frozen, dissected, and graded using the Thompson et al. (1990) and Galante (1967) grading scales.
3. Results The FBG calibration data had a mean coefficient of determination (r2) greater than 0.99 and ranged from 0.9984 to 0.9903. The mean sensitivity to pressure was 5.770.085 pm/MPa (mean7SD). The average hysteresis was 2.13% and ranged from 2.94% to 0.00%. The calibrations of strain gauge Sensors 1 and 2 were measured using our apparatus and found to be within 1.8% and 3%, respectively, of the manufacturer’s data. The manufacturer’s calibrations were used because this magnitude of error would have negligible effect on the interpretation and comparison of the measurements. FBG-measured disc pressure increased linearly with compressive load, with an average coefficient of determination (r2) of 0.93 (Table 1, Fig. 3). Disc pressure measurements made using the strain-gauge sensor had coefficients of determination always greater than 0.95 (Table 1, Fig. 3). Disc degeneration scores ranged from 2 to 3; major diameter ranged from 51 to 63 mm (one specimen was not available for measurement) (Table 1). 4. Discussion The observed linear response of disc pressure to applied compressive load is consistent with previous studies using different types of sensors. Cripton et al. (2001) reported coefficients of determination greater than 0.99 in a study of the cervical spine. Our average FBG-measured pressure sensitivity to load, CFBG, was 0.902 kPa/N. This falls within the range of values measured in previous studies of pressure in lumbar discs. These range from 0.840 kPa/N (0.67 MPa for 800 N of spine load at L2–L3, Nachemson, 1960) to 1.00 kPa/N (0.5 MPa for 500 N spine load at L2–L3, McNally and Adams, 1992) and 1.03 kPa/N
223
(0.82 MPa for 800 N spine load at L3–L4, Nachemson, 1960). The observed differences between the FBG and straingauge measured pressure sensitivities to load (CFBG and CSG, respectively) may be due to inhomogeneous size or composition of the nucleus material. Specimen number five had the least degenerated disc and also had the smallest difference between CFBG and CSG. The rest of the specimens had moderately degenerated discs, and also had larger differences between CFBG and CSG. Structural inhomogeneity may occur on a length scale comparable to the FBG sensor length (10 mm) and could therefore have deleterious effects on its pressure measurements. The inhomogeneous composition may not affect the strain gauge sensor to the same degree because its active sensing length is only 2 mm. It will be possible to test this hypothesis in the future by constructing FBG sensors with smaller active sensing lengths. Multi-sensor FBGs (i.e., several gratings on a single fibre) will allow us to assess the size and location of the functional nucleus by mapping pressure profiles across the disc. Pressure within the nucleus may not have been purely hydrostatic and this could be another contributing factor to the lack of agreement between CFBG and CSG. The average difference between CFBG and CSG was 75.7% of the value given by Sensor 1 and 103.4% of the value given by Sensor 2 (Table 1). However, the average difference in CSG between Sensors 1 and 2 was 12.9% of the value for Sensor 1 (Table 1), suggesting the lack of hydrostatic pressure is not the only cause of the difference in CFBG and CSG. Future work will attempt to identify the causes of the inconsistency. Pressure measurement with FBGs is less disruptive to spine biomechanics than existing techniques. There is potential to use the needle for fibre insertion only; the needle could be removed from the annulus, leaving only a bare fibre in situ. This could increase the utility of the sensor ex vivo by reducing its effect on the disc structures and increase its potential utility in vivo by
Table 1 Pressure sensitivity to load, coefficients of determination of the load vs. pressure results, and summary of specimen data showing level, age, sex, and linear regression results from spine pressure measurement tests Spine no.
Level
Sex–age (yrs.)
Disc gradeb
Major disc diameter (mm)
Fibre-Bragg grating sensor Pressure sensitivity to load, CFBG (kPa/N) (mean7SD)a
1 2 3 4 5
L4–L5 L4–L5 L4–L5 L2–L3 L1–L2 a
Male–70 Male–76 Female–75 Male–76 Male–46
3 3 3 3 2
One standard deviation. Grade results are for both Thompson and Galante. c Specimen not available for measurement. b
n/ac 59 63 51 55
1.0770.069 1.0470.071 0.84770.039 0.85170.095 0.70570.043
Strain Gauge sensor Coefficient of determination (r2) 0.94 0.93 0.97 0.84 0.95
Pressure senstivity to load, CSG (kPa/N) Sensor 1
Sensor 2
0.507 0.593 0.293 0.850 0.722
0.448 0.492 0.251 0.677 0.730
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Fig. 3. FBG sensor measurement of disc pressure as a function of compressive spine load (specimen 3, L4–L5). (a) The vertical error bars (70.12 MPa) show the uncertainty in pressure measurement based on the FBG calibrations and the mean sensitivity of the FBG sensors to pressure. (b) Strain gauge Sensor 1 measured disc pressure. (c) Strain gauge Sensor 2 measured disc pressure. Note: Both FBG and strain gauge results are relative to a zero reference pressure at 200 N.
reducing the risk and discomfort of disc pressure measurement. The FBG sensor system cost approximately $60,000 USD, most of which was the cost of the OSA. Custom equipment could be developed for a lower cost. The strain gauge system cost $15,000 USD.
Grant, Research Tools and Instruments Grant, and Undergraduate Research Fellowship), the Canadian Institutes of Health Research (Interdisciplinary Capacity Enhancement Grant), the Canadian Arthritis Network (Network Scholar Award) and the Michael Smith Foundation for Health Research (Research Unit Award).
Conflict of interest The authors do not have any financial or personal relationships with other people or organizations that could inappropriately influence this work. Acknowledgments This work was supported in part by the Natural Sciences and Engineering Research Council of Canada (Discovery
References Black, C., Udd, E., Schulz, W., Kreger, S., Kunzler, M., Taylor, T., Lumsden, R., Heider, D., Zhicheng, Y., 2002. Using multi-axis fibre grating strain sensors to measure transverse strain and transverse strain gradients in composite materials with complex weave structures. Proceedings of SPIE 4694, 162–167. Cripton, P.A., Dumas, G.A., Nolte, L.-P., 2001. A minimally disruptive technique for measuring intervertebral disc pressure in vitro: application to the cervical spine. Journal of Biomechanics 34, 545–549.
ARTICLE IN PRESS C.R. Dennison et al. / Journal of Biomechanics 41 (2008) 221–225 Cunningham, B.W., Kotani, Y., McNulty, P.S., Cappuccino, A., McAfee, P.C., 1997. The effect of spinal destabilization and instrumentation on lumbar intradiscal pressure: an in vitro biomechanical analysis. Spine 22, 2655–2663. Frank, A., Bohnert, K.H., Bra¨ndle, H., Poulson, C.V., Pederson, J.E., Patscheider, J., 2003. Distributed feedback fibre laser sensor for hydrostatic pressure. IEEE Photonics Technology Letters 15, 1758–1760. Fye, M.A., Southern, E.P., Panjabi, M.M., Cholewicki, J., 1998. Quantitative discomanometry: technique and reproducibility in vitro. Journal of Spinal Disorders 11, 335–340. Galante, J.O., 1967. Tensile properties of the human lumbar annulus fibrosus. Acta Orthopaedica Scandinavica 100 (Suppl.), 1–91. Hattori, S., Oda, H., Kawai, S., Ube-Shi, 1981. Cervical intradiscal pressure in movements and traction of the cervical spine. Zeitschrift Fur Orthopadie 119, 568–569. McNally, D.S., Adams, M.A., 1992. Internal intervertebral disc mechanics as revealed by stress profilometry. Spine 17, 66–73. Nachemson, A.L., 1960. Lumber intradiscal pressure. Acta Orthopaedica Scandinavica 43 (Suppl.), 1–104. Nachemson, A.L., 1966. The load on lumbar disks in different positions of the body. Clinical Orthopaedics and Related Research 45, 107–122. Nachemson, A.L., Morris, J.M., 1964. In vivo measurements of intradiscal pressure. Journal of Bone and Joint Surgery—American Volume 46-A, 1077–1092.
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Nunes, L.C.S., Valente, L.C.G., Llerena, R.W.A., Braga, A.M.B., Triques, A.L.C., 2004. Simultaneous measurement of temperature and pressure using single fibre Bragg grating and fixed filter demodulation technique. Proceedings of SPIE 5622, 906–911. Pospiech, J., Stolke, D., Wilke, H.J., Claes, L.E., 1999. Intradiscal pressure recordings in the cervical spine. Neurosurgery 44, 379–384. Steffen, T., Baramki, H.G., Rubin, R., Antoniou, J., Aebi, M., 1998. Lumbar intradiscal pressure measured in the anterior and posterolateral annular regions during asymmetrical loading. Clinical Biomechanics 13, 495–505. Thompson, J.P., Pearce, R.H., et al., 1990. Preliminary evaluation of a scheme for grading the gross morphology of the human intervertebral disc. Spine 15 (5), 411–415. Udd, E., Nelson, D., Lawrence, C., Ferguson, B., 1996. Three axis strain and temperature fibre optic grating sensor. Proceedings of SPIE 2718, 104–109. Wilke, H.J., Wolf, S., Claes, L., Arand, M., Wiesend, A., 1996. Influence of varying muscle forces on lumbar intradiscal pressure: an in vitro study. Journal of Biomechanics 29, 549–555. Xu, M.G., Reekie, L., Chow, Y.T., Dakin, J.P., 1993. Optical in-fibre grating high pressure sensor. Electronics Letters 29, 398–399. Yunqi, L., Zhuanyun, G., Zhang, Y., Seng, K., Dong, C., Dong, X., 2000. Simultaneous pressure and temperature measurement with polymercoated fibre Bragg grating. Electronics Letters 36, 564–566.
Journal of Biomechanics 41 (2008) 221–225 www.elsevier.com/locate/jbiomech www.JBiomech.com
Short communication
Ex vivo measurement of lumbar intervertebral disc pressure using fibre-Bragg gratings Christopher R. Dennisona,, Peter M. Wilda, Peter W.G. Byrnesa, Amy Saaria, Eyal Itshayekb, Derek C. Wilsonb, Qingan A. Zhub, Marcel F.S. Dvorakb, Peter A. Criptonb,c, David R. Wilsonb a Department of Mechanical Engineering, University of Victoria, P.O. Box 3055, Victoria, B.C., Canada V8W 3P6 Division of Orthopaedic Engineering Research, Department of Orthopaedics, University of British Columbia, Vancouver, B.C., Canada V6T 1Z4 c Department of Mechanical Engineering, University of British Columbia, Vancouver, B.C., Canada V6T 1Z4
b
Accepted 12 July 2007
Abstract Methods were developed to measure intervertebral disc pressure using optical fibre-Bragg gratings (FBGs). The FBG sensor was calibrated for hydrostatic pressure in a purpose-built apparatus and the average sensitivity was determined to be 5.770.085 pm/MPa (mean7SD). The average coefficient of determination (r2) for the calibration data was 0.99, and the average hysteresis of the sensor was 2.13% of full scale. The FBG was used to measure intradiscal pressure response to compressive load in five lumbar functional spine units. The pressure measured by the FBG sensor varied linearly with applied compressive load with coefficients of determination ranging from 0.84 to 0.97. The FBG sensor’s sensitivity to compressive load ranged from 0.70270.043 kPa/N (mean7SD) in a L1–L2 specimen, to 1.0770.069 kPa/N in a L4–L5 specimen. These measurements agree with those of previous studies in lumbar spines. Two strain gauge pressure sensors were also used to measure intradiscal pressure response to compressive load. The measured pressure sensitivity to load ranged from 0.251 kPa/N (L4–L5) to 0.850 kPa/N (L2–L3). The average difference in pressure sensitivity to load between Sensors 1 and 2 was 12.9% of the value for Sensor 1, with a range from 1.1% to 20.4%, which suggests that disc pressure was not purely hydrostatic. This may have contributed to the difference between the responses of the FBG and strain gauge sensors. r 2007 Elsevier Ltd. All rights reserved. Keywords: Spine; Intervertebral disc; Low back pain; Pressure; Sensor; Nucleus pulposus; Disc degeneration
1. Introduction Understanding intervertebral disc mechanics is central to understanding disc injury and the etiology of disc degeneration (Nachemson, 1960). Pressure distributions in intervertebral discs are an important indicator of disc mechanics that have been measured both in vivo (Nachemson, 1966; Nachemson and Morris, 1964) and ex vivo (Cunningham et al., 1997; Fye et al., 1998; McNally and Adams, 1992; Nachemson, 1960; Wilke et al., 1996). Needle-mounted strain gauge pressure sensors have been widely used to measure disc pressure both ex vivo and Corresponding author. Tel.: +1 250 853 3198; fax: +1 250 721 6051.
E-mail address: [email protected] (C.R. Dennison). 0021-9290/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jbiomech.2007.07.015
in vivo (Hattori et al., 1981; Pospiech et al., 1999; Steffen et al., 1998). However, the utility of these sensors is limited because they do not characterize pressure profiles within the disc. They are also suspected of injuring the annular fibres of the disc and of altering the biomechanical response of soft tissues because they are rigid and over 1 mm in diameter (Cripton et al., 2001). Fibre-Bragg gratings (FBGs), which consist of an optical fibre with a Bragg grating inscribed into the fibre core, have excellent potential for biomechanical applications because they are small (125 mm diameter), mechanically compliant (allowing bending within the host structure to radii of 10 mm), immune to electromagnetic interference, and biocompatible. FBGs are commonly used to measure physical parameters such as strain (Black et al., 2002;
ARTICLE IN PRESS 222
C.R. Dennison et al. / Journal of Biomechanics 41 (2008) 221–225
Udd et al., 1996), temperature (Nunes et al., 2004; Yunqi et al., 2000), and pressure (Frank et al., 2003; Xu et al., 1993) and can also be configured to measure profiles of these parameters. The objectives of this study were: (a) to assess the linearity, sensitivity and hysteresis of a FBG sensor in pure hydrostatic compression, (b) to develop methods for implanting FBGs in the intervertebral disc, and (c) to determine the uncertainty in the FBG’s measurement of disc pressure in response to compressive load in functional spinal units and to compare its sensitivity to the sensitivity of needle-mounted strain gauge pressure sensors. 2. Materials and methods When an FBG is illuminated it reflects a single narrow spectrum of wavelengths centered at the Bragg wavelength. When hydrostatic pressure is applied to the FBG, pressure-induced strains within the fibre cause the Bragg wavelength to shift. These shifts vary linearly with applied pressure in the range 0–50 MPa (Xu et al., 1993). We calibrated FBG sensors (10 mm length, Blue Road Research, Gresham, OR) for hydrostatic pressure. In our calibration apparatus, pressure was generated in a glycerin-charged vessel with a manual pump (ENERPAC P141, Milwaukee, WI) and measured with a pressure sensor (OMEGA PX300-2KGV, Stamford CT, accuracy: 734 kPa). Two testing protocols were used to calibrate the sensors. In the first, pressure was increased incrementally from 1 MPa to 6.9 MPa. The Bragg wavelength was recorded at 0.5 MPa increments using an optical spectrum
analyzer (OSA; ANDO AQ6331, Tokyo, Japan) and the applied pressure was recorded from a digital readout (OMEGA DP25B-S, Stamford CT). In the second protocol, designed to quantify hysteresis, pressure was cycled from 1.4 MPa to 6.9 MPa to 1.4 MPa at 0.1 MPa/s. Pressures were not cycled below 1 MPa because we could not repeatably control the pump relief valve below this pressure. Each protocol was repeated three times, and the glycerin temperature was monitored to ensure that the calibration was isothermal. Sensor performance was assessed in five, two-vertebra human lumbar functional spinal units (Fig. 1(b)). Each specimen was stored at 20 1C in a sealed plastic container prior to testing, and allowed to thaw for 3 h to a temperature of 4 1C. Preconditioning loading cycles, with purely compressive loads ranging from 0 N to 2000 N to 0 N at a rate of 200 N/s, were applied with a materials testing machine (Instron 8874, Norwood, MA). The displacements of the disc and vertebrae were visually monitored to verify that the loads were purely compressive, and that bending or torsion of the specimen did not occur. The FBG sensor was inserted into the hub of a 25 gauge (0.51 mm outside diameter, 38 mm length) hypodermic needle. The needle was then passed through the annulus so that the tip was located approximately at the center of the nucleus pulposus. The FBG sensor was then advanced past the needle tip so that its entire length was exposed to the nucleus pulposus (Fig. 1(a)). The needle was left in the annulus to shield the fibreoptic cable from any traction that might be applied by the annulus (Fig. 1(b), location 3, also shown in Fig. 2). We then loaded each functional spinal unit from 0 N to 2000 N to 0 N at 10 N/s. After each test with the FBG sensor, we removed the FBG sensor and repeated the test with two strain gauge sensors (Sensors 1 and 2) (Model 060S; pressure range, 0–3.5 MPa; 2.45 mm diameter; Precision Measurement Co., Denton, Ann Arbor, USA) inserted into the center of the nucleus (Fig. 1(b), location 4).
Fig. 1. (a) FBG sensor inserted into 25 gauge (0.5 mm diameter, 38 mm length) hypodermic needle (1, hypodermic needle; 2, active sensing length of FBG (10 mm)), and inset, magnified image showing FBG sensor emerging from hypodermic needle; (b) FBG sensor and strain gauge sensor inserted into intervertebral disc (3, FBG Sensor; 4, strain gauge Sensor; 5, spine fixture including dental cement). (Taken by: Christopher R. Dennison.)
Fig. 2. Schematic cross-sectional diagram of FBG sensor insertion geometry (not to scale). Fibre optic cable passes through the needle hub and the hypodermic needle length (38 mm). The FBG sensor length (10 mm) is exposed to the nucleus pulposus while the hypodermic needle length shields fibre as it passes through the annulus and into the nucleus.
ARTICLE IN PRESS C.R. Dennison et al. / Journal of Biomechanics 41 (2008) 221–225 After all tests were completed, the specimens were frozen, dissected, and graded using the Thompson et al. (1990) and Galante (1967) grading scales.
3. Results The FBG calibration data had a mean coefficient of determination (r2) greater than 0.99 and ranged from 0.9984 to 0.9903. The mean sensitivity to pressure was 5.770.085 pm/MPa (mean7SD). The average hysteresis was 2.13% and ranged from 2.94% to 0.00%. The calibrations of strain gauge Sensors 1 and 2 were measured using our apparatus and found to be within 1.8% and 3%, respectively, of the manufacturer’s data. The manufacturer’s calibrations were used because this magnitude of error would have negligible effect on the interpretation and comparison of the measurements. FBG-measured disc pressure increased linearly with compressive load, with an average coefficient of determination (r2) of 0.93 (Table 1, Fig. 3). Disc pressure measurements made using the strain-gauge sensor had coefficients of determination always greater than 0.95 (Table 1, Fig. 3). Disc degeneration scores ranged from 2 to 3; major diameter ranged from 51 to 63 mm (one specimen was not available for measurement) (Table 1). 4. Discussion The observed linear response of disc pressure to applied compressive load is consistent with previous studies using different types of sensors. Cripton et al. (2001) reported coefficients of determination greater than 0.99 in a study of the cervical spine. Our average FBG-measured pressure sensitivity to load, CFBG, was 0.902 kPa/N. This falls within the range of values measured in previous studies of pressure in lumbar discs. These range from 0.840 kPa/N (0.67 MPa for 800 N of spine load at L2–L3, Nachemson, 1960) to 1.00 kPa/N (0.5 MPa for 500 N spine load at L2–L3, McNally and Adams, 1992) and 1.03 kPa/N
223
(0.82 MPa for 800 N spine load at L3–L4, Nachemson, 1960). The observed differences between the FBG and straingauge measured pressure sensitivities to load (CFBG and CSG, respectively) may be due to inhomogeneous size or composition of the nucleus material. Specimen number five had the least degenerated disc and also had the smallest difference between CFBG and CSG. The rest of the specimens had moderately degenerated discs, and also had larger differences between CFBG and CSG. Structural inhomogeneity may occur on a length scale comparable to the FBG sensor length (10 mm) and could therefore have deleterious effects on its pressure measurements. The inhomogeneous composition may not affect the strain gauge sensor to the same degree because its active sensing length is only 2 mm. It will be possible to test this hypothesis in the future by constructing FBG sensors with smaller active sensing lengths. Multi-sensor FBGs (i.e., several gratings on a single fibre) will allow us to assess the size and location of the functional nucleus by mapping pressure profiles across the disc. Pressure within the nucleus may not have been purely hydrostatic and this could be another contributing factor to the lack of agreement between CFBG and CSG. The average difference between CFBG and CSG was 75.7% of the value given by Sensor 1 and 103.4% of the value given by Sensor 2 (Table 1). However, the average difference in CSG between Sensors 1 and 2 was 12.9% of the value for Sensor 1 (Table 1), suggesting the lack of hydrostatic pressure is not the only cause of the difference in CFBG and CSG. Future work will attempt to identify the causes of the inconsistency. Pressure measurement with FBGs is less disruptive to spine biomechanics than existing techniques. There is potential to use the needle for fibre insertion only; the needle could be removed from the annulus, leaving only a bare fibre in situ. This could increase the utility of the sensor ex vivo by reducing its effect on the disc structures and increase its potential utility in vivo by
Table 1 Pressure sensitivity to load, coefficients of determination of the load vs. pressure results, and summary of specimen data showing level, age, sex, and linear regression results from spine pressure measurement tests Spine no.
Level
Sex–age (yrs.)
Disc gradeb
Major disc diameter (mm)
Fibre-Bragg grating sensor Pressure sensitivity to load, CFBG (kPa/N) (mean7SD)a
1 2 3 4 5
L4–L5 L4–L5 L4–L5 L2–L3 L1–L2 a
Male–70 Male–76 Female–75 Male–76 Male–46
3 3 3 3 2
One standard deviation. Grade results are for both Thompson and Galante. c Specimen not available for measurement. b
n/ac 59 63 51 55
1.0770.069 1.0470.071 0.84770.039 0.85170.095 0.70570.043
Strain Gauge sensor Coefficient of determination (r2) 0.94 0.93 0.97 0.84 0.95
Pressure senstivity to load, CSG (kPa/N) Sensor 1
Sensor 2
0.507 0.593 0.293 0.850 0.722
0.448 0.492 0.251 0.677 0.730
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Fig. 3. FBG sensor measurement of disc pressure as a function of compressive spine load (specimen 3, L4–L5). (a) The vertical error bars (70.12 MPa) show the uncertainty in pressure measurement based on the FBG calibrations and the mean sensitivity of the FBG sensors to pressure. (b) Strain gauge Sensor 1 measured disc pressure. (c) Strain gauge Sensor 2 measured disc pressure. Note: Both FBG and strain gauge results are relative to a zero reference pressure at 200 N.
reducing the risk and discomfort of disc pressure measurement. The FBG sensor system cost approximately $60,000 USD, most of which was the cost of the OSA. Custom equipment could be developed for a lower cost. The strain gauge system cost $15,000 USD.
Grant, Research Tools and Instruments Grant, and Undergraduate Research Fellowship), the Canadian Institutes of Health Research (Interdisciplinary Capacity Enhancement Grant), the Canadian Arthritis Network (Network Scholar Award) and the Michael Smith Foundation for Health Research (Research Unit Award).
Conflict of interest The authors do not have any financial or personal relationships with other people or organizations that could inappropriately influence this work. Acknowledgments This work was supported in part by the Natural Sciences and Engineering Research Council of Canada (Discovery
References Black, C., Udd, E., Schulz, W., Kreger, S., Kunzler, M., Taylor, T., Lumsden, R., Heider, D., Zhicheng, Y., 2002. Using multi-axis fibre grating strain sensors to measure transverse strain and transverse strain gradients in composite materials with complex weave structures. Proceedings of SPIE 4694, 162–167. Cripton, P.A., Dumas, G.A., Nolte, L.-P., 2001. A minimally disruptive technique for measuring intervertebral disc pressure in vitro: application to the cervical spine. Journal of Biomechanics 34, 545–549.
ARTICLE IN PRESS C.R. Dennison et al. / Journal of Biomechanics 41 (2008) 221–225 Cunningham, B.W., Kotani, Y., McNulty, P.S., Cappuccino, A., McAfee, P.C., 1997. The effect of spinal destabilization and instrumentation on lumbar intradiscal pressure: an in vitro biomechanical analysis. Spine 22, 2655–2663. Frank, A., Bohnert, K.H., Bra¨ndle, H., Poulson, C.V., Pederson, J.E., Patscheider, J., 2003. Distributed feedback fibre laser sensor for hydrostatic pressure. IEEE Photonics Technology Letters 15, 1758–1760. Fye, M.A., Southern, E.P., Panjabi, M.M., Cholewicki, J., 1998. Quantitative discomanometry: technique and reproducibility in vitro. Journal of Spinal Disorders 11, 335–340. Galante, J.O., 1967. Tensile properties of the human lumbar annulus fibrosus. Acta Orthopaedica Scandinavica 100 (Suppl.), 1–91. Hattori, S., Oda, H., Kawai, S., Ube-Shi, 1981. Cervical intradiscal pressure in movements and traction of the cervical spine. Zeitschrift Fur Orthopadie 119, 568–569. McNally, D.S., Adams, M.A., 1992. Internal intervertebral disc mechanics as revealed by stress profilometry. Spine 17, 66–73. Nachemson, A.L., 1960. Lumber intradiscal pressure. Acta Orthopaedica Scandinavica 43 (Suppl.), 1–104. Nachemson, A.L., 1966. The load on lumbar disks in different positions of the body. Clinical Orthopaedics and Related Research 45, 107–122. Nachemson, A.L., Morris, J.M., 1964. In vivo measurements of intradiscal pressure. Journal of Bone and Joint Surgery—American Volume 46-A, 1077–1092.
225
Nunes, L.C.S., Valente, L.C.G., Llerena, R.W.A., Braga, A.M.B., Triques, A.L.C., 2004. Simultaneous measurement of temperature and pressure using single fibre Bragg grating and fixed filter demodulation technique. Proceedings of SPIE 5622, 906–911. Pospiech, J., Stolke, D., Wilke, H.J., Claes, L.E., 1999. Intradiscal pressure recordings in the cervical spine. Neurosurgery 44, 379–384. Steffen, T., Baramki, H.G., Rubin, R., Antoniou, J., Aebi, M., 1998. Lumbar intradiscal pressure measured in the anterior and posterolateral annular regions during asymmetrical loading. Clinical Biomechanics 13, 495–505. Thompson, J.P., Pearce, R.H., et al., 1990. Preliminary evaluation of a scheme for grading the gross morphology of the human intervertebral disc. Spine 15 (5), 411–415. Udd, E., Nelson, D., Lawrence, C., Ferguson, B., 1996. Three axis strain and temperature fibre optic grating sensor. Proceedings of SPIE 2718, 104–109. Wilke, H.J., Wolf, S., Claes, L., Arand, M., Wiesend, A., 1996. Influence of varying muscle forces on lumbar intradiscal pressure: an in vitro study. Journal of Biomechanics 29, 549–555. Xu, M.G., Reekie, L., Chow, Y.T., Dakin, J.P., 1993. Optical in-fibre grating high pressure sensor. Electronics Letters 29, 398–399. Yunqi, L., Zhuanyun, G., Zhang, Y., Seng, K., Dong, C., Dong, X., 2000. Simultaneous pressure and temperature measurement with polymercoated fibre Bragg grating. Electronics Letters 36, 564–566.