The Journal of Bone & Joint Surgery

The Journal of Bone & Joint Surgery, Volume 85, Issue 11

Scientific Articles | November 01, 2003

A Dynamic Study of Thoracolumbar Burst Fractures

Ruth K. Wilcox, P¹; Thomas O. Boerger, FRCS²; David J. Allen, FRCS²; David C. Barton, P¹; David Limb, BS²; Robert A. Dickson, DS²; Richard M. Hall, P¹

¹ School of Mechanical Engineering, University of Leeds, Leeds LS2 9JT, United Kingdom. E-mail address for R.K. Wilcox: r.k.wilcox@leeds.ac.uk
² Musculo-Skeletal Services, CSB, St. James's Hospital, Leeds LS9 7TF, United Kingdom

View Disclosures and Other Information

In support of their research or preparation of this manuscript, one or more of the authors received grants or outside funding from the Wish-bone Trust, Yorkshire Children's Spine Foundation, and the Engineering and Physical Sciences Research Council. None of the authors received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.

Investigation performed at the School of Mechanical Engineering, University of Leeds, and Musculo-Skeletal Services, St. James's Hospital, Leeds, United Kingdom

The Journal of Bone and Joint Surgery, Incorporated

J Bone Joint Surg Am, 2003 Nov 01;85(11):2184-2189

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Abstract

Background: The degree of canal stenosis following a thoracolumbar burst fracture is sometimes used as an indication for decompressive surgery. This study was performed to test the hypothesis that the final resting positions of the bone fragments seen on computed tomography imaging are not representative of the dynamic canal occlusion and associated neurological damage that occurs during the fracture event.

Methods: A drop-weight method was used to create burst fractures in bovine spinal segments devoid of a spinal cord. During impact, dynamic measurements were made with use of transducers to measure pressure in a synthetic spinal cord material, and a high-speed video camera filmed the inside of the spinal canal. A corresponding finite element model was created to determine the effect of the spinal cord on the dynamics of the bone fragment.

Results: The high-speed video clearly showed the fragments of bone being projected from the vertebral body into the spinal canal before being recoiled, by the action of the posterior longitudinal ligament and intervertebral disc attachments, to their final resting position. The pressure measurements in the synthetic spinal cord showed a peak in canal pressure during impact. There was poor concordance between the extent of postimpact occlusion of the canal as seen on the computed tomography images and the maximum amount of occlusion that occurred at the moment of impact. The finite element model showed that the presence of the cord would reduce the maximum dynamic level of canal occlusion at high fragment velocities. The cord would also provide an additional mechanism by which the fragment would be recoiled back toward the vertebral body.

Conclusions: A burst fracture is a dynamic event, with the maximum canal occlusion and maximum cord compression occurring at the moment of impact. These transient occurrences are poorly related to the final level of occlusion as demonstrated on computed tomography scans.

Clinical Relevance: In a thoracolumbar burst fracture, the final position of the fragments, as seen on computed tomography images at presentation, probably does not represent the maximum level of canal occlusion or peak cord pressure and therefore does not represent the probable damage to the cord tissue that occurred at the moment of impact.

Figures in this Article

Annually, more than 10,000 people in the United States sustain a spinal cord injury¹. About 15% of these injuries are burst fractures², which occur predominantly in younger patients, incurring a high financial and societal cost³.

During the fracture process, one or more fragments of the vertebral body are retropulsed into the spinal canal, and many surgeons advocate decompressive surgery, even in patients with otherwise clinically stable fractures, to prevent or reduce a neurological deficit⁴. Computed tomography demonstrates the degree of spinal stenosis, but there is doubt as to whether the position of the fragments seen on such scans represents the true extent of the canal occlusion produced during the fracture process⁵. There is also evidence that, after a fracture, any ongoing compression of the spinal cord has no effect in patients with a stable neurological status and that some recovery of function can be expected even if the patient is treated nonoperatively^4,6.

The hypothesis for this study was that the final resting positions of the bone fragments seen on computed tomography imaging are not representative of the dynamic fracture process. Implicit in this hypothesis is the assumption that the damage to the spinal cord occurs at the moment of injury. In previous experimental studies, investigators have used indirect measurement techniques to determine the occlusion of the spinal canal after the removal of the spinal cord^5,7. By using a combined experimental and computational approach, we were able to investigate the dynamic fracture process by direct measurement of the extent of canal occlusion and by simulation of spinal cord impact.

Materials and Methods

Materials and Methods | Results | Discussion | References

Experimental Model

Thoracolumbar spinal specimens from twenty-one-day-old male Holstein calves were retrieved from an abattoir and frozen at —20°C. After defrosting for twenty-four hours, the specimens were cut into three-vertebra segments, excess paravertebral muscle was removed, and the spinal cord was extracted from the spinal canal. The ends of the specimen were set in 80-mm-diameter polymethylmethacrylate end plates to produce flat, parallel surfaces. During casting, the ends of the spinal canal were kept free of cement so that it was possible to view the inside of the canal during testing.

Burst fractures were produced with use of the drop-weight method⁸, in which a 1 to 7-kg mass was dropped axially onto the specimen from a height of 2 m. This allowed the impact energy to be varied while the impact velocity was maintained. The specimens were housed between two stainless-steel end plates, with the top plate guided to permit movement only in the impact direction.

High-Speed Video Measurements

The first series of tests was carried out with use of high-speed video imaging down the spinal canal during impact. A total of twenty-one specimens were tested at impact energies ranging from 60 to 140 J. The method and its validation have been described in detail previously⁹. Briefly, each steel end plate contained a cutaway section in which a mirror was positioned at 45° to the impact direction (Fig. 1-A). The specimen was positioned so that the spinal canal was aligned between the two mirrors. Light was shone by means of the top mirror through the spinal canal and reflected by the bottom mirror into the lens (f/2.8D, AF 60 mm; Nikon, Melville, New York) of the high-speed camera (Kodak HS4540; Roper Scientific, MASD, San Diego, California) running at 4500 frames per second. This allowed silhouette images of the cross section of the spinal canal to be captured on video during the impact. After each test, the images were downloaded to a personal computer and were subsequently analyzed with use of proprietary software (Image Pro Plus 3.0; Media Cybernetics, Silver Spring, Maryland). A custom-written algorithm was used to trace the outline of the open canal, with the change in light intensity as the canal was occluded taken into account. The cross-sectional area of the canal was then determined for each frame. Initial validation tests employing a second high-speed video camera to film the outside of the specimen were performed to determine if there was any lateral movement or buckling of the specimen during impact. After impact, the nine specimens used in the validation tests were also imaged with computed tomography. The degree of canal occlusion was calculated as the ratio of the postimpact canal area to that of the average canal area of the two adjacent vertebrae.

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Experimental apparatus for drop-weight tests showing the setup for the high-speed video experiments (Fig. 1-A) and the pressure measurements (Fig. 1-B).

Pressure Measurements

A second series of experiments was carried out to determine the increase in pressure in a synthetic spinal cord during the fracture process. A total of twenty-one tests were performed at impact energies ranging from 20 to 140 J. A catheter-tip pressure transducer was custom manufactured (Gaeltec, Dunvegan, Isle of Skye, United Kingdom) and was connected to a data acquisition system (LabVIEW 5.0; National Instruments, Austin, Texas). The frequency response of the system was determined, and the 3-dB cutoff was found to be in excess of 8 kHz with a negligible ripple band below that frequency. The impact force was measured with a quartz load cell (Type 904B; Kistler Instrumente, Winterhur, Switzerland) housed in the lower end plate. The output was again fed into a personal computer by means of a charge amplifier. The 3-dB cutoff for the load cell system was in excess of 50 kHz.

To measure the pressure, the inferior end of the spinal canal was sealed with modeling clay and the transducer was suspended vertically in the canal from the superior end of the specimen. The sensing pad was aligned anteriorly at the central vertebra, and the transducer was held in place with a metal clip. There is a rapid deterioration in the mechanical properties of bovine spinal cord with freezing and time post mortem¹⁰ so a synthetic cord material, prepared from 9% (weight/weight) gelatin solution, was used for all tests. A previous study showed that, at this concentration, the response of gelatin under lateral impact is similar to that of a fresh spinal cord¹¹. The solution was prepared with use of distilled water and type-A gelatin from porcine skin (G2500; Sigma-Aldrich, Poole, Dorset, United Kingdom) and was poured into the canal. Once the solution had gelled, the metal clip was removed so that the transducer stem was in contact only with the gelatin (Fig. 1-B). This prevented vibrations from the surrounding hard tissue from being transmitted to the sensing pad through the transducer stem.

After testing, all of the specimens were imaged with computed tomography and the degree of canal occlusion was determined as before.

Finite Element Analysis

The effect of the spinal cord on the dynamics of the fragment were further investigated with use of computational simulation. A finite element model of a three-vertebra segment had previously been constructed from the computed tomography scans of a typical bovine specimen, and this was used to simulate the impact on the specimen up to the moment of vertebral body impact¹². The geometry of the specimen at the time of fracture was used to simulate fragment retropulsion by applying a velocity to the bone fragment in the posterior direction (Fig. 2). The model included the spinal cord, dura mater, posterior elements, and posterior longitudinal ligament, which had become slack as a result of the compression of the vertebral body. The material properties were derived from the literature (Table I). Different fragment velocities ranging from 1 to 20 m/sec were simulated to encompass the range that occurred in the experimental tests. Each simulation was repeated with the spinal cord and dura mater removed, and the difference in fragment trajectories was determined.

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Finite element model at the initial point of fragment projection. Axial compression of the model had previously been simulated to the point of osseous fracture. PLL = posterior longitudinal ligament.

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TABLE I Mechanical Properties of Soft Tissues Used in Finite Element Study

Material	Properties	References
Posterior longitudinal ligament	E = 18.5 MPa (? < 11%)	16, 17
	E = 61.6 MPa (11% < ? < 34%)
	E = 46 MPa (? > 34%)
Dura mater	Anisotropic	18
	E = 142 MPa (circumferential/radial)
	E = 0.7 MPa (longitudinal)
Spinal cord	E = 1.3 MPa	19

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TABLE I Mechanical Properties of Soft Tissues Used in Finite Element Study

Material	Properties	References
Posterior longitudinal ligament	E = 18.5 MPa (? < 11%)	16, 17
	E = 61.6 MPa (11% < ? < 34%)
	E = 46 MPa (? > 34%)
Dura mater	Anisotropic	18
	E = 142 MPa (circumferential/radial)
	E = 0.7 MPa (longitudinal)
Spinal cord	E = 1.3 MPa	19

Results

Materials and Methods | Results | Discussion | References

High-Speed Video Measurements

In every case, the images obtained from the high-speed video clearly showed the vertebral bone fragment being projected into the spinal canal and then recoiling to the final resting position. The second camera showed no visible buckling of the specimen during impact and minimal lateral movement, giving a mean estimated error (and standard deviation) of 7% ± 4% for the calculated cross-sectional area of the spinal canal⁹. The relative cross-sectional area of the canal was plotted as a function of time (Fig. 3), and in every case the final level of canal occlusion was less than the maximum that occurred during impact. There was also an increase in maximum occlusion with an increase in impact energy (Fig. 4-A). Repeated tests at the same impact conditions showed similar levels of maximum occlusion, but there was greater variability in the final fragment position (Fig. 4-B).

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Typical graph of canal occlusion versus time as well as representative images at 0, 4, 10, and 20 msec after impact.

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Graphs of energy of impact versus maximum (Fig. 4-A) and final (Fig. 4-B) levels of canal occlusion as measured from the high-speed video. Error bars show the standard deviation.

The images of the nine specimens that were scanned with computed tomography showed that, in every case, a burst fracture had been produced, with lower impact energies (up to 60 J) producing predominantly Denis² type-C fractures and higher energies producing more type-A fractures. The fracture patterns were comparable with those seen in the human spine. Dissection of the central vertebra of each specimen also revealed a trapezoidal or wedge-shaped fragment typical of a burst fracture, with no obvious interference from the ring apophysis. The level of agreement between the canal occlusion measured on the postfracture computed tomography scans and the maximum dynamic occlusion seen on the high-speed video images was assessed with use of statistical methods proposed by Lin¹³. The concordance coefficient was 0.483 (95% confidence interval, 0.164 to 0.802), which was significantly different from the concordance coefficient between the occlusion measured with computed tomography and the final occlusion of 0.938 measured from the video (95% confidence interval, 0.793 to 0.982) (Figs. 5-A and 5-B).

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Mean-difference graphs of computed tomography-measured occlusion and maximum video-measured occlusion (Fig. 5-A) and computed tomography-measured occlusion and final video-measured occlusion (Fig. 5-B). The mean error and the 95% confidence interval are shown as dotted and dashed lines, respectively.

Canal Pressure

In all of the tests, the pressure was seen to rise to a maximum and return to zero. The mean total length of the pulse was 42 ± 15 msec, with no dependence on the impact energy (Pearson correlation = —0.01). There was a rise in the mean maximum pressure with increasing impact energy (r = 0.659, p = 0.001). There was a high correlation between the maximum pressure and the peak load measured on the bottom load cell (r = 0.937, p < 0.001). The computed tomography images showed that all impacts of >20 J had produced burst fractures, but there was not a significant correlation, with the numbers available, between the extent of occlusion seen on the image and the impact energy, peak pressure, or peak load (Pearson correlation coefficient = 0.414 ± 0.062, 0.349 ± 0.121, and 0.364 ± 0.105, respectively).

Finite Element Analysis

The finite element analysis showed that, at high fragment velocities, the presence of the spinal cord and dura mater made a considerable difference to the fragment propagation, with these tissues acting both to reduce the maximum displacement and to increase the level of recoil (Table II).

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TABLE II Difference in Simulated Maximum Lateral Fragment Displacement with and without the Spinal Cord

	Maximum Fragment Displacement (mm)		Difference Between Displacements (% of canal diameter)
Speed (m/sec)	With Cord and Dura	Without Cord and Dura
1	3.7	4.2	3.6
3	4.3	6.1	11.9
5	4.8	6.9	14.2
10	5.4	8.0	17.6

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TABLE II Difference in Simulated Maximum Lateral Fragment Displacement with and without the Spinal Cord

	Maximum Fragment Displacement (mm)		Difference Between Displacements (% of canal diameter)
Speed (m/sec)	With Cord and Dura	Without Cord and Dura
1	3.7	4.2	3.6
3	4.3	6.1	11.9
5	4.8	6.9	14.2
10	5.4	8.0	17.6

Discussion

Materials and Methods | Results | Discussion | References

The calf spine has been shown to exhibit mechanical responses similar to those of the human spine under a range of loading conditions¹⁴, and it has been used previously to model the burst fracture process¹⁵. In the current study, bovine specimens were chosen to reduce interspecimen variability since they could be harvested from animals with a narrow age range. The fracture patterns that were produced corresponded well with those observed in clinical practice, and the model was therefore considered to be representative of the human spine. The impact energies were within the range of those used in cadaveric studies^5,7,8, although a comparison of equivalent fractures suggests that slightly higher energies were required for the bovine specimens because of the higher density of bovine bone. The low level of concordance between the amount of canal occlusion measured on the postfracture computed tomography scans and the maximum occlusion demonstrated by the video at impact indicates that the computed tomography image did not represent the maximum dynamic canal occlusion and therefore cannot be used to determine the events that occurred during the fracture. At higher-impact energies, the entire posterior section of the vertebral body became detached during the fracture process. In some cases, the lack of constraint on the fragment allowed it to rotate and become lodged in the canal. In other cases, alignment with the vertebral body was maintained and the fragment was recoiled back into the vertebral body. It was therefore not possible to predict solely from the final position of the fragment what degree of canal occlusion had occurred dynamically.

The high level of correlation between the pressure readings and impact conditions (r = 0.937 for peak impact load) demonstrates that the pressure produced in the spinal canal depends on the magnitude of the impact. The duration of the pressure pulse was similar to that seen on the load cell below the specimen and was considerably longer than that of the fragment motion across the canal observed on the high-speed video. This indicates that the pressure rise is the summation of the stress wave generated by the impact on the specimen and that produced locally by the fragment impact. However it was caused, the maximum pressure generated in the spinal canal is likely to be a good indicator of the level of spinal cord damage and hence the neurological deficit. The low correlation between maximum pressure and the degree of occlusion measured on the computed tomography scan (r = 0.349) indicates that a computed tomography measurement of occlusion after a fracture cannot be used to determine the dynamic rise in spinal canal pressure during the impact and thus cannot be used to indirectly measure the extent of spinal cord damage.

Since the spinal cord and dura mater could not be left in place during the high-speed video experiments, a finite element model was used to simulate their effect on the fragment dynamics. The model showed that at high fragment velocities, as would occur at 140-J impacts, the presence of the cord and dura made a considerable difference with regard to the amount of canal occlusion that occurred. The cord and dura appeared to act in a manner similar to that of the posterior longitudinal ligament in reflecting the fragment back toward the vertebral body. Hence, the overall pattern of the fragment dynamics is the same with or without the cord, with the fragment being projected into the spinal canal before recoiling back toward the vertebral body.

Some surgeons operate on patients when computed tomography images demonstrate canal compromise of more than a fixed amount—usually 40% or 50%. This amount is chosen by surgeons on the basis of anecdotal evidence rather than controlled clinical studies⁴. Both the experimental and the computational results of our study showed that the final level of occlusion does not represent the greatest occlusion that occurs during impact. Furthermore, the results of the high-speed video tests showed that, at higher levels of occlusion, the final position of the fragment was poorly correlated with the maximum level of impingement. Any neurological damage is likely to occur at the point of maximum canal occlusion, which also corresponds with the maximum pressure generated in the cord. Hence, spinal cord damage probably is not accurately represented by the final resting positions of the bone fragments seen on postinjury imaging of the spinal canal.

References

Materials and Methods | Results | Discussion | References

Go BK, DeViro MJ, Richards JS. The epidemiology of spinal cord injury. In: Stover SL, DeLisa JA, Whiteneck GG, editors. Spinal cord injury: clinical outcomes from the model systems. Gaithersburg, MD: Aspen; 1995. p 21-5.21 1995

Denis F. The three column spine and its significance in the classification of acute thoracolumbar spinal injuries. Spine.1983;8: 817-31.8817 1983 [PubMed][CrossRef]

DeVivo MJ. Causes and costs of spinal cord injury in the United States. Spinal Cord.1997;35: 809-13.35809 1997 [PubMed][CrossRef]

Boerger TO, Limb D, Dickson RA. Does `canal clearance' affect neurological outcome after thoracolumbar burst fractures? J Bone Joint Surg Br.2000;82: 629-35.82629 2000 [PubMed][CrossRef]

Chang DG, Tencer AF, Ching RP, Treece B, Senft D, Anderson PA. Geometric changes in the cervical spinal canal during impact. Spine.1994;19: 973-80.19973 1994 [PubMed][CrossRef]

Limb D, Shaw DL, Dickson RA. Neurological injury in thoracolumbar burst fractures. J Bone Joint Surg Br.1995;77: 774-7.77774 1995

Panjabi MM, Kifune M, Wen L, Arand M, Oxland TR, Lin RM, Yoon WS, Vasavada A. Dynamic canal encroachment during thoracolumbar burst fractures. J Spinal Disord.1995;8: 39-48.839 1995 [PubMed]

Willen J, Lindahl S, Irstam L, Aldman B, Nordwall A. The thoracolumbar crush fracture. An experimental study on instant axial dynamic loading: the resulting fracture type and its stability. Spine.1984;9: 624-31.9624 1984 [PubMed][CrossRef]

Wilcox RK, Boerger TO, Hall RM, Barton DC, Limb D, Dickson RA. Measurement of canal occlusion during the thoracolumbar burst fracture process. J Biomech.2002;35: 381-4.35381 2002 [PubMed][CrossRef]

Oakland RJ, Wilcox RK, Hall RM, Barton DC. The mechanical response of spinal cord to uniaxial loading. Trans Orthop Res Soc.2002;27: 783.27783 2002

Wilcox RK, Boerger TO, Hall RM, Barton DC.Canal pressure measurements and video recording of thoracolumbar burst fractures. Presented as a poster exhibit at the Annual Meeting of the American Academy of Orthopaedic Surgeons; 2001 Feb 28-Mar 4; San Francisco, CA. 2001

Wilcox R, Allen D, Barton D, Hall R, Limb D, Dickson R.An investigation of the burst fracture mechanism using a combined experimental and finite element approach. Read at the Fourth World Congress of Biomechanics and American Society of Biomechanics; 2002 Aug 4-9; Calgary, Alberta, Canada. 2002

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Topics

fracture ; finite element analysis ; spinal canal ; spinal cord ; burst fracture

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Ruth K. Wilcox, Thomas O. Boerger, David J. Allen, David C. Barton, David Limb, Robert A. Dickson, Richard M. Hall; A Dynamic Study of Thoracolumbar Burst Fractures. The Journal of Bone & Joint Surgery. 2003 Nov;85(11):2184-2189.

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