Dynamic Study TL BF 2003

COPYRIGHT © 2003 BY THE JOURNAL OF BONE AND JOINT SURGERY, INCORPORATED

A Dynamic Study of Thoracolumbar Burst Fractures

BY RUTH K. WILCOX, PHD, THOMAS O. BOERGER, FRCS, DAVID J. ALLEN, FRCS, DAVID C. BARTON, PHD, DAVID LIMB, BSC, FRCSED(ORTH), ROBERT A. DICKSON, DSC, AND RICHARD M. HALL, PHD

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

Background: The degree of canal stenosis following a thoracolumbar burst fracture is sometimes used as an indica-tion for decompressive surgery. This study was performed to test the hypothesis that the final resting positions of thebone fragments seen on computed tomography imaging are not representative of the dynamic canal occlusion andassociated 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 spinalcord. During impact, dynamic measurements were made with use of transducers to measure pressure in a syntheticspinal cord material, and a high-speed video camera filmed the inside of the spinal canal. A corresponding finite ele-ment 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 thespinal canal before being recoiled, by the action of the posterior longitudinal ligament and intervertebral disc attach-ments, to their final resting position. The pressure measurements in the synthetic spinal cord showed a peak in ca-nal pressure during impact. There was poor concordance between the extent of postimpact occlusion of the canal asseen on the computed tomography images and the maximum amount of occlusion that occurred at the moment of im-pact. The finite element model showed that the presence of the cord would reduce the maximum dynamic level of ca-nal occlusion at high fragment velocities. The cord would also provide an additional mechanism by which thefragment would be recoiled back toward the vertebral body.

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

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

nnually, more than 10,000 people in the United Statessustain a spinal cord injury1. About 15% of these inju-ries 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, andmany surgeons advocate decompressive surgery, even in pa-tients with otherwise clinically stable fractures, to prevent orreduce a neurological deficit4. Computed tomography demon-strates the degree of spinal stenosis, but there is doubt as towhether the position of the fragments seen on such scans rep-resents the true extent of the canal occlusion produced duringthe fracture process5. There is also evidence that, after a frac-ture, any ongoing compression of the spinal cord has no effectin patients with a stable neurological status and that some re-covery of function can be expected even if the patient istreated nonoperatively4,6.

The hypothesis for this study was that the final restingpositions of the bone fragments seen on computed tomogra-phy imaging are not representative of the dynamic fractureprocess. Implicit in this hypothesis is the assumption that thedamage to the spinal cord occurs at the moment of injury. Inprevious experimental studies, investigators have used indirectmeasurement techniques to determine the occlusion of thespinal canal after the removal of the spinal cord5,7. By using acombined experimental and computational approach, wewere able to investigate the dynamic fracture process by directmeasurement of the extent of canal occlusion and by simula-tion of spinal cord impact.

Materials and MethodsExperimental Model

horacolumbar spinal specimens from twenty-one-day-old male Holstein calves were retrieved from an abattoir

A

T

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and frozen at −20°C. After defrosting for twenty-four hours,the specimens were cut into three-vertebra segments, excessparavertebral muscle was removed, and the spinal cord wasextracted from the spinal canal. The ends of the specimenwere set in 80-mm-diameter polymethylmethacrylate endplates to produce flat, parallel surfaces. During casting, theends of the spinal canal were kept free of cement so that it waspossible 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 theimpact energy to be varied while the impact velocity wasmaintained. The specimens were housed between two stain-less-steel end plates, with the top plate guided to permit move-ment only in the impact direction.

High-Speed Video MeasurementsThe first series of tests was carried out with use of high-speedvideo imaging down the spinal canal during impact. A total oftwenty-one specimens were tested at impact energies ranging

Fig. 1-A

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

surements (Fig. 1-B).

Fig. 1-B

Fig. 2

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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from 60 to 140 J. The method and its validation have been de-scribed in detail previously9. Briefly, each steel end plate con-tained a cutaway section in which a mirror was positioned at45° to the impact direction (Fig. 1-A). The specimen was posi-tioned so that the spinal canal was aligned between the twomirrors. Light was shone by means of the top mirror throughthe spinal canal and reflected by the bottom mirror into thelens (f/2.8D, AF 60 mm; Nikon, Melville, New York) of thehigh-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 spi-nal canal to be captured on video during the impact. Aftereach test, the images were downloaded to a personal computerand were subsequently analyzed with use of proprietary soft-ware (Image Pro Plus 3.0; Media Cybernetics, Silver Spring,Maryland). A custom-written algorithm was used to trace theoutline of the open canal, with the change in light intensity asthe canal was occluded taken into account. The cross-sectionalarea of the canal was then determined for each frame. Initialvalidation tests employing a second high-speed video camera

to film the outside of the specimen were performed to deter-mine if there was any lateral movement or buckling of thespecimen during impact. After impact, the nine specimensused in the validation tests were also imaged with computedtomography. The degree of canal occlusion was calculated asthe ratio of the postimpact canal area to that of the average ca-nal area of the two adjacent vertebrae.

Pressure MeasurementsA second series of experiments was carried out to determinethe increase in pressure in a synthetic spinal cord during thefracture process. A total of twenty-one tests were performed atimpact energies ranging from 20 to 140 J. A catheter-tip pres-sure transducer was custom manufactured (Gaeltec, Dunve-gan, Isle of Skye, United Kingdom) and was connected to adata acquisition system (LabVIEW 5.0; National Instruments,Austin, Texas). The frequency response of the system was de-termined, and the 3-dB cutoff was found to be in excess of 8kHz with a negligible ripple band below that frequency. Theimpact force was measured with a quartz load cell (Type 904B;

Fig. 3

Typical graph of canal occlusion versus time

as well as representative images at 0, 4, 10,

and 20 msec after impact.

Fig. 4-A

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.

Fig. 4-B

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Kistler Instrumente, Winterhur, Switzerland) housed in thelower end plate. The output was again fed into a personalcomputer by means of a charge amplifier. The 3-dB cutoff forthe load cell system was in excess of 50 kHz.

To measure the pressure, the inferior end of the spinalcanal was sealed with modeling clay and the transducer wassuspended vertically in the canal from the superior end of thespecimen. The sensing pad was aligned anteriorly at the cen-tral vertebra, and the transducer was held in place with ametal clip. There is a rapid deterioration in the mechanicalproperties of bovine spinal cord with freezing and time postmortem10 so a synthetic cord material, prepared from 9%(weight/weight) gelatin solution, was used for all tests. A pre-vious study showed that, at this concentration, the response ofgelatin under lateral impact is similar to that of a fresh spinalcord11. The solution was prepared with use of distilled waterand type-A gelatin from porcine skin (G2500; Sigma-Aldrich,Poole, Dorset, United Kingdom) and was poured into the ca-nal. Once the solution had gelled, the metal clip was removedso that the transducer stem was in contact only with the gela-tin (Fig. 1-B). This prevented vibrations from the surroundinghard tissue from being transmitted to the sensing pad throughthe transducer stem.

After testing, all of the specimens were imaged withcomputed tomography and the degree of canal occlusion wasdetermined as before.

Finite Element AnalysisThe effect of the spinal cord on the dynamics of the fragmentwere further investigated with use of computational simula-tion. A finite element model of a three-vertebra segment hadpreviously been constructed from the computed tomographyscans of a typical bovine specimen, and this was used to simu-late the impact on the specimen up to the moment of verte-bral body impact12. The geometry of the specimen at the timeof fracture was used to simulate fragment retropulsion by ap-

plying a velocity to the bone fragment in the posterior direc-tion (Fig. 2). The model included the spinal cord, dura mater,posterior elements, and posterior longitudinal ligament, whichhad become slack as a result of the compression of the verte-bral body. The material properties were derived from the liter-ature (Table I). Different fragment velocities ranging from 1 to20 m/sec were simulated to encompass the range that oc-curred in the experimental tests. Each simulation was repeatedwith the spinal cord and dura mater removed, and the differ-ence in fragment trajectories was determined.

ResultsHigh-Speed Video Measurements

n every case, the images obtained from the high-speedvideo clearly showed the vertebral bone fragment being

projected into the spinal canal and then recoiling to the finalresting position. The second camera showed no visible buck-ling of the specimen during impact and minimal lateral move-ment, giving a mean estimated error (and standard deviation)of 7% ± 4% for the calculated cross-sectional area of the spinalcanal9. The relative cross-sectional area of the canal was plot-ted as a function of time (Fig. 3), and in every case the finallevel of canal occlusion was less than the maximum that oc-curred during impact. There was also an increase in maxi-mum occlusion with an increase in impact energy (Fig. 4-A).Repeated tests at the same impact conditions showed similarlevels of maximum occlusion, but there was greater variabilityin the final fragment position (Fig. 4-B).

The images of the nine specimens that were scannedwith computed tomography showed that, in every case, aburst fracture had been produced, with lower impact energies(up to 60 J) producing predominantly Denis2 type-C fracturesand higher energies producing more type-A fractures. Thefracture patterns were comparable with those seen in the hu-man spine. Dissection of the central vertebra of each specimenalso revealed a trapezoidal or wedge-shaped fragment typical

I

Fig. 5-A

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.

Fig. 5-B

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of a burst fracture, with no obvious interference from the ringapophysis. The level of agreement between the canal occlusionmeasured on the postfracture computed tomography scansand the maximum dynamic occlusion seen on the high-speedvideo images was assessed with use of statistical methods pro-posed by Lin13. The concordance coefficient was 0.483 (95%confidence interval, 0.164 to 0.802), which was significantlydifferent from the concordance coefficient between the occlu-sion measured with computed tomography and the final oc-clusion of 0.938 measured from the video (95% confidenceinterval, 0.793 to 0.982) (Figs. 5-A and 5-B).

Canal PressureIn all of the tests, the pressure was seen to rise to a maximumand return to zero. The mean total length of the pulse was 42 ±15 msec, with no dependence on the impact energy (Pearsoncorrelation = −0.01). There was a rise in the mean maximumpressure with increasing impact energy (r = 0.659, p = 0.001).There was a high correlation between the maximum pressureand the peak load measured on the bottom load cell (r =0.937, p < 0.001). The computed tomography images showedthat all impacts of >20 J had produced burst fractures, butthere was not a significant correlation, with the numbersavailable, between the extent of occlusion seen on the imageand the impact energy, peak pressure, or peak load (Pearsoncorrelation coefficient = 0.414 ± 0.062, 0.349 ± 0.121, and0.364 ± 0.105, respectively).

Finite Element AnalysisThe finite element analysis showed that, at high fragment ve-locities, the presence of the spinal cord and dura mater made aconsiderable difference to the fragment propagation, with

these tissues acting both to reduce the maximum displace-ment and to increase the level of recoil (Table II).

Discussionhe calf spine has been shown to exhibit mechanical re-sponses similar to those of the human spine under a range

of loading conditions14, and it has been used previously tomodel the burst fracture process15. In the current study, bovinespecimens were chosen to reduce interspecimen variabilitysince they could be harvested from animals with a narrow agerange. The fracture patterns that were produced correspondedwell with those observed in clinical practice, and the model wastherefore considered to be representative of the human spine.The impact energies were within the range of those used in ca-daveric studies5,7,8, although a comparison of equivalent frac-tures suggests that slightly higher energies were required for thebovine specimens because of the higher density of bovine bone.The low level of concordance between the amount of canal oc-clusion measured on the postfracture computed tomographyscans and the maximum occlusion demonstrated by the videoat impact indicates that the computed tomography image didnot represent the maximum dynamic canal occlusion andtherefore cannot be used to determine the events that occurredduring the fracture. At higher-impact energies, the entire poste-rior section of the vertebral body became detached during thefracture process. In some cases, the lack of constraint on thefragment allowed it to rotate and become lodged in the canal. Inother cases, alignment with the vertebral body was maintainedand the fragment was recoiled back into the vertebral body. Itwas therefore not possible to predict solely from the final posi-tion of the fragment what degree of canal occlusion had oc-curred dynamically.

T

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

Speed (m/sec)

Maximum Fragment Displacement (mm) Difference Between Displacements

(% of canal diameter)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

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

Material Properties References

Posterior longitudinal ligament E = 18.5 MPa (ε < 11%)E = 61.6 MPa (11% < ε < 34%)E = 46 MPa (ε > 34%)

16, 17

Dura mater AnisotropicE = 142 MPa (circumferential/radial)E = 0.7 MPa (longitudinal)

18

Spinal cord E = 1.3 MPa 19

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The high level of correlation between the pressure read-ings and impact conditions (r = 0.937 for peak impact load)demonstrates that the pressure produced in the spinal canaldepends on the magnitude of the impact. The duration of thepressure pulse was similar to that seen on the load cell belowthe specimen and was considerably longer than that of thefragment motion across the canal observed on the high-speedvideo. This indicates that the pressure rise is the summation ofthe stress wave generated by the impact on the specimen andthat produced locally by the fragment impact. However it wascaused, the maximum pressure generated in the spinal canal islikely to be a good indicator of the level of spinal cord damageand hence the neurological deficit. The low correlation be-tween maximum pressure and the degree of occlusion mea-sured on the computed tomography scan (r = 0.349) indicatesthat a computed tomography measurement of occlusion aftera fracture cannot be used to determine the dynamic rise inspinal canal pressure during the impact and thus cannot beused to indirectly measure the extent of spinal cord damage.

Since the spinal cord and dura mater could not be left inplace during the high-speed video experiments, a finite elementmodel was used to simulate their effect on the fragment dynam-ics. The model showed that at high fragment velocities, as wouldoccur at 140-J impacts, the presence of the cord and dura made aconsiderable difference with regard to the amount of canal oc-clusion that occurred. The cord and dura appeared to act in amanner similar to that of the posterior longitudinal ligament inreflecting the fragment back toward the vertebral body. Hence,the overall pattern of the fragment dynamics is the same with orwithout the cord, with the fragment being projected into the spi-nal canal before recoiling back toward the vertebral body.

Some surgeons operate on patients when computed to-mography images demonstrate canal compromise of more thana fixed amount—usually 40% or 50%. This amount is chosen

by surgeons on the basis of anecdotal evidence rather than con-trolled clinical studies4. Both the experimental and the compu-tational results of our study showed that the final level ofocclusion does not represent the greatest occlusion that occursduring impact. Furthermore, the results of the high-speed videotests showed that, at higher levels of occlusion, the final positionof the fragment was poorly correlated with the maximum levelof impingement. Any neurological damage is likely to occur atthe point of maximum canal occlusion, which also correspondswith the maximum pressure generated in the cord. Hence, spi-nal cord damage probably is not accurately represented by thefinal resting positions of the bone fragments seen on postinjuryimaging of the spinal canal. �

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Ruth K. Wilcox, PhDDavid C. Barton, PhDRichard M. Hall, PhDSchool of Mechanical Engineering, University of Leeds, Leeds LS2 9JT, United Kingdom. E-mail address for R.K. Wilcox: [email protected]

Thomas O. Boerger, FRCSDavid J. Allen, FRCSDavid Limb, BSc, FRCSEd(Orth)Robert A. Dickson, DScMusculo-Skeletal Services, CSB, St. James’s Hospital, Leeds LS9 7TF, United Kingdom

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.