lJJ~,-919(1’79’1MJ-0807
TECHNICAL
PRESSURE
NOTE
DISTRIBUTION UNDER AN INTERVERTEBRAL AN EXPERIMENTAL STUDY*
INTRODI
(TION
In order to fully understand how the vertebra load is transmitted from one vertebra to the next and its dependence on themechanical propertiesofdisc, it isessential to know the pressure distribution under the vertebra disc. In the young, by virtue of the physical characteristics of the nucleus, its structure is elastic and resilient. The nucleus functions as a perfect gel. distributing forces of compression and tension equally to all parts of the annulus. The annulus is flexible and elastic. With aging, because of the biochemical changes that occur in the nucleus, it becomes less elastic and more fibrous as a result of the increase in disorg.anized collagen and in polysaccharides, its gel characteristtcs become somewhat impaired, and forces are unevenly distributed to the annulus and the articular plates. The annulus is subjected to an abnormal compression and shows an evidence of degeneration. This process of abnormal compression related to degeneration is being further investigated.
DESCRIPTION Of AN INTERVERTEBRAL PRESSI RE TRANSDl (‘ER IIVPTI
A transducer which was capable of mapping the pressure distribution between a vertebral disc and its adjacent end plate had been developed. The design was a diaphragm-type pressure transducer with a diameter of 2.39 mm (0.094 in.) located in a slender box section which was 1.84mm (0.0725 in.) thick, 6.35 mm (0.25 in.) wide and 38 mm (1.5 in.) long. The diaphragm was 0.38 mm (0.015 in.) thick and was instrumented with a diaphragm type foil strain gage+ which had an overall diameter of 2.36 mm and was a complete Carm bridge. Figure 1 shows the underside of the IVPT with the backing removed. The IVPT was calibrated on a dead-weight tester which could apply a known hydrostatic pressure on the diaphragm. A custom-made holder was designed and fabricated to adapt the IV PT to the calibrator, as shown in Fig. 2. The response of the transducer was linear up to a pressure in excess of 4830 KPa (700 psi). It was also linear during unloading and there was minimal hysteresis. The pressures under an intervertebral disc were estimated to be well below the maximum calibration pressure. The unit was designed to be inserted into a lumbar disc just above the superior end plate so that it rested firmly on the end plate. Several of these units were needed to perform the
l Receiced 10 Ocrober 1978. f The strain gage was manufactured by BLH, Electronics
of Waltham, MA. The gage number is FAE S4-09-12-S6. Each arm has a resistance of 120R. 807
%mnJu
DISC-
mapping function. It was also necessary to control and measure the death of wnetration of each IVPT and to maintain its radial position below the disc during the application of load. To attain all of these aims. an IVPT holder was designed and fabricated. It is a partial circular ring which could accommodate up to 5 IVPT units and was equipped with positioning screws which held it firmly to a vertebral body during a loading cycle. The entire arrangement is shown in Fig. 3. Several feasibility tests were conducted with this IVPT, using unembalmed vertebral segments. Figure 4 shows the test set up in which 3 IVPT’s were used. The knurled set screws on the bottom of the holder prevented radial movement of the unit. PREPARATION
OF SPEC ItlESS
The testing procedure began with specimen preparation, during which a lumbar segment was X-rayed for pre-existing fractures and was stripped of all muscle. Care was taken to avoid damage to the bone and disc. The specimen was kept moist and at room temperature during this phase of the experiment. It was stored in a plastic bag at 1-2°C when not in use. The ends of the segment were potted in a pipe flange using a low melting alloy (Ostalloy. M. P. 47.3’C) in the same manner as that described by Hakim and King (1974). The cast ends provided a loading and a bearing surface suitable for testing in a universal loading machine. The disc was then carefully dissected to provide a tunnel into which the IVPT could be inserted. The depth was controlled so that the IVPT did not penetrate into the area of the nucleus pulposus. The holder was then attached to the vertebral segment and the IVPT’s were inserted into the disc. The entire assembly was covered by a plastic sheet before testing. IN
VITRO
TE4T PROt’EDl
RE ASD
RWI
Ln
A Richle testing machine was used to apply a quasi-static load on the specimen via a specially designed loading head. The head was adjustable in the mid-sagittal plane of the vertebra and was used to control the eccentricity of the applied load. The rate of loading was approximately 0.13 mm/set (0.005 in/set). This rate was selected because it did not alter the physical characteristics of the disc. The test was stopped when vertebral fracture occurred. A set of typical load/pressure curves are shown in Fig. 5. The IVPTs were located at the anterior and lateral edges of the vertebra, at a depth of approximately 10mm. The pressure was found to be a relatively linear function of load. Non-linearity was observed at the point of fracture when there was more rapid rise in pressure, especially for the right lateral IVPT. In this case fracture occurred at a load of
Technical Note
_ LATERAL LEFT __ ANTERIOR _._ LATERAL RIGHT
2 3 FORCE (NIX IO3 Fig. 5. Variation of discal pressure with applied load. Segment was loaded to fracture.
i
_ ___ -._
0
1.0
5
LATERAL ANTERIOR LATERAL
LEFT RIGHT
1.5
FORCE (N) XIO3 Fig. 6. Variation of discal pressure with applied load for another specimen.
3261 N (728 Ib)and the corresponding pressures were: lateral left, 2241 KPa (324 psi); anterior, 2344 KPa (340 psi); and lateral right, 2779 KPa (403 psi). Figure 6 is another set of curves for a.second vertebral segment. This illustrates that there is a difference in pressure around the disc. This shows that it is nossible to measure the nressures under the disc and this data’is being used in a finite element model to simulate surgical conditions, such as a laminectomy. Bioenyineering Center.
.
Wayne State University, 418 Healrh Sciences Building, Detroit. MI 48202, U.S.A.
H. S. RANU R. A. DENTON A. I. KING
REFERENCES
Hakim, N. S. and King, A. I. (1974) The load bearing role of articular facets during static loading - spinal column. Advances in Bioengineeriny (Edited by Brighton, J. A. and Goldstein, S.). ASME, New York.
809
Fig. 3. Holder adapter used to calibrate thz IVPT by means of a dead iveiyht rester
810
Fig. 4. IVPT in use around a lumbar spinal segment.
812
TECHNICAL
PRESSURE
NOTE
DISTRIBUTION UNDER AN INTERVERTEBRAL AN EXPERIMENTAL STUDY*
INTRODI
(TION
In order to fully understand how the vertebra load is transmitted from one vertebra to the next and its dependence on themechanical propertiesofdisc, it isessential to know the pressure distribution under the vertebra disc. In the young, by virtue of the physical characteristics of the nucleus, its structure is elastic and resilient. The nucleus functions as a perfect gel. distributing forces of compression and tension equally to all parts of the annulus. The annulus is flexible and elastic. With aging, because of the biochemical changes that occur in the nucleus, it becomes less elastic and more fibrous as a result of the increase in disorg.anized collagen and in polysaccharides, its gel characteristtcs become somewhat impaired, and forces are unevenly distributed to the annulus and the articular plates. The annulus is subjected to an abnormal compression and shows an evidence of degeneration. This process of abnormal compression related to degeneration is being further investigated.
DESCRIPTION Of AN INTERVERTEBRAL PRESSI RE TRANSDl (‘ER IIVPTI
A transducer which was capable of mapping the pressure distribution between a vertebral disc and its adjacent end plate had been developed. The design was a diaphragm-type pressure transducer with a diameter of 2.39 mm (0.094 in.) located in a slender box section which was 1.84mm (0.0725 in.) thick, 6.35 mm (0.25 in.) wide and 38 mm (1.5 in.) long. The diaphragm was 0.38 mm (0.015 in.) thick and was instrumented with a diaphragm type foil strain gage+ which had an overall diameter of 2.36 mm and was a complete Carm bridge. Figure 1 shows the underside of the IVPT with the backing removed. The IVPT was calibrated on a dead-weight tester which could apply a known hydrostatic pressure on the diaphragm. A custom-made holder was designed and fabricated to adapt the IV PT to the calibrator, as shown in Fig. 2. The response of the transducer was linear up to a pressure in excess of 4830 KPa (700 psi). It was also linear during unloading and there was minimal hysteresis. The pressures under an intervertebral disc were estimated to be well below the maximum calibration pressure. The unit was designed to be inserted into a lumbar disc just above the superior end plate so that it rested firmly on the end plate. Several of these units were needed to perform the
l Receiced 10 Ocrober 1978. f The strain gage was manufactured by BLH, Electronics
of Waltham, MA. The gage number is FAE S4-09-12-S6. Each arm has a resistance of 120R. 807
%mnJu
DISC-
mapping function. It was also necessary to control and measure the death of wnetration of each IVPT and to maintain its radial position below the disc during the application of load. To attain all of these aims. an IVPT holder was designed and fabricated. It is a partial circular ring which could accommodate up to 5 IVPT units and was equipped with positioning screws which held it firmly to a vertebral body during a loading cycle. The entire arrangement is shown in Fig. 3. Several feasibility tests were conducted with this IVPT, using unembalmed vertebral segments. Figure 4 shows the test set up in which 3 IVPT’s were used. The knurled set screws on the bottom of the holder prevented radial movement of the unit. PREPARATION
OF SPEC ItlESS
The testing procedure began with specimen preparation, during which a lumbar segment was X-rayed for pre-existing fractures and was stripped of all muscle. Care was taken to avoid damage to the bone and disc. The specimen was kept moist and at room temperature during this phase of the experiment. It was stored in a plastic bag at 1-2°C when not in use. The ends of the segment were potted in a pipe flange using a low melting alloy (Ostalloy. M. P. 47.3’C) in the same manner as that described by Hakim and King (1974). The cast ends provided a loading and a bearing surface suitable for testing in a universal loading machine. The disc was then carefully dissected to provide a tunnel into which the IVPT could be inserted. The depth was controlled so that the IVPT did not penetrate into the area of the nucleus pulposus. The holder was then attached to the vertebral segment and the IVPT’s were inserted into the disc. The entire assembly was covered by a plastic sheet before testing. IN
VITRO
TE4T PROt’EDl
RE ASD
RWI
Ln
A Richle testing machine was used to apply a quasi-static load on the specimen via a specially designed loading head. The head was adjustable in the mid-sagittal plane of the vertebra and was used to control the eccentricity of the applied load. The rate of loading was approximately 0.13 mm/set (0.005 in/set). This rate was selected because it did not alter the physical characteristics of the disc. The test was stopped when vertebral fracture occurred. A set of typical load/pressure curves are shown in Fig. 5. The IVPTs were located at the anterior and lateral edges of the vertebra, at a depth of approximately 10mm. The pressure was found to be a relatively linear function of load. Non-linearity was observed at the point of fracture when there was more rapid rise in pressure, especially for the right lateral IVPT. In this case fracture occurred at a load of
Technical Note
_ LATERAL LEFT __ ANTERIOR _._ LATERAL RIGHT
2 3 FORCE (NIX IO3 Fig. 5. Variation of discal pressure with applied load. Segment was loaded to fracture.
i
_ ___ -._
0
1.0
5
LATERAL ANTERIOR LATERAL
LEFT RIGHT
1.5
FORCE (N) XIO3 Fig. 6. Variation of discal pressure with applied load for another specimen.
3261 N (728 Ib)and the corresponding pressures were: lateral left, 2241 KPa (324 psi); anterior, 2344 KPa (340 psi); and lateral right, 2779 KPa (403 psi). Figure 6 is another set of curves for a.second vertebral segment. This illustrates that there is a difference in pressure around the disc. This shows that it is nossible to measure the nressures under the disc and this data’is being used in a finite element model to simulate surgical conditions, such as a laminectomy. Bioenyineering Center.
.
Wayne State University, 418 Healrh Sciences Building, Detroit. MI 48202, U.S.A.
H. S. RANU R. A. DENTON A. I. KING
REFERENCES
Hakim, N. S. and King, A. I. (1974) The load bearing role of articular facets during static loading - spinal column. Advances in Bioengineeriny (Edited by Brighton, J. A. and Goldstein, S.). ASME, New York.
809
Fig. 3. Holder adapter used to calibrate thz IVPT by means of a dead iveiyht rester
810
Fig. 4. IVPT in use around a lumbar spinal segment.
812
