Flexion Distraction Injuries in the Thoracolumbar Spine: An In Vitro Study of the Relation Between Flexion Angle and the Motion Axis of Fracture

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1 Journal of Spinal Disorders & Techniques Vol. 15, No. 2, pp Lippincott Williams & Wilkins, Inc., Philadelphia Flexion Distraction Injuries in the Thoracolumbar Spine: An In Vitro Study of the Relation Between Flexion Angle and the Motion Axis of Fracture *Takeshi Hoshikawa, *Yasuhisa Tanaka, *Shoichi Kokubun, William W. Lu, Keith D. K. Luk, and John C. Y. Leong *Department of Orthopaedic Surgery, Tohoku University School of Medicine, Sendai, Japan; and Department of Orthopaedic Surgery, The University of Hong Kong, Hong Kong Summary: A new concept, the motion axis of fracture (MAF), which is defined as the transitional point from anterior compressive to posterior splitting failure on a lateral radiograph, has provided a true understanding of the mechanisms of flexion distraction injuries in clinical cases. This study was designed to produce in vitro injuries that have MAFs and to clarify the relation between the flexion angle and the MAF location. Adolescent porcine thoracolumbar spines were exposed to a vertical compressive load to failure at three different flexion angles and then examined radiographically. The MAF location was recorded as the distance from the anterior border to the MAF expressed as a percentage of the anteroposterior diameter of the vertebral body. All specimens showed similar injuries, with MAFs consisting of anterior compression fractures in the vertebral bodies and posterior disruptions. A significant negative correlation emerged between the flexion angle and the MAF location (r ; p < ). These results suggest that even a vertical compressive load contributes to the production of a flexion distraction injury with an MAF in the thoracolumbar spine. They also indicate that the flexion angle of the spine at which the vertical compressive load is applied is an important factor in determining the MAF location; that is, the larger the flexion angle, the more anterior the MAF. Key Words: Biomechanics Thoracolumbar spine Flexion distraction injury In vitro study. INTRODUCTION Denis three-column theory of thoracolumbar injuries is widely accepted (1). For flexion distraction injuries (seat belt type injuries), he described a failure of the posterior and middle columns under tensile forces generated by flexion with an axis in the anterior column, and a possible partial failure of the anterior column under compression. However, in most clinical cases, the axes appear to be Received May 10, 2001; accepted November 15, Address correspondence and reprint requests to Dr. T. Hoshikawa, Department of Orthopaedic Surgery, Tohoku University School of Medicine, 1 1, Seiryomachi, Aobaku, Sendai, , Japan. thoshikawa@hotmail.com Presented at the 19th Congress of the Hong Kong Orthopaedic Association, November 13 14, 1999, Hong Kong. located in the middle columns, because compressive failures of the anterior columns are more severe. Furthermore, many clinical cases can not be classified into any of the categories suggested by Denis. Kokubun et al. (2,3) and Tani et al. (4) proposed a new concept, the motion axis of fracture (MAF), which is defined as the transitional line from the anterior compressive to the posterior splitting failure. They used this concept to analyze clinical cases of thoracolumbar injuries with posterior splitting failure without translational dislocation; that is, flexion distraction injuries. The concept of MAF has not only improved our understanding of the mechanism of each flexion distraction injury but also provided a new and comprehensive classification scheme for flexion distraction injuries (2 4). The concept has also been informative for surgeons when they choose proper methods of surgical 139

2 140 T. HOSHIKAWA ET AL. treatment. According to Kokubun et al. (2,3), if the MAF is located in the vertebral body, the injury can be managed with posterior compressive instruments, but if the MAF is posterior to the vertebral body, indicating a burst fracture, compression should not be applied to the posterior elements. Recently, the classification of thoracic and lumbar injuries proposed by Magerl et al. (5) has been universally accepted. Although the classification is comprehensive, their descriptions of flexion distraction injuries are insufficient in terms of compressive failure of the vertebral body accompanied by posterior splitting failure. In the classification, flexion distraction injuries are included in posterior element injury with distraction and classified into three types: injuries with vertebral body compression, those with transverse disruption of the disk, and transverse bicolumn fractures. Clearly, all the first injuries have compressive failures, and MAFs seem to lie close to the posterior walls of the vertebral bodies according to their descriptions. However, although some of the second injuries seem to have compressive failures and MAFs in the vertebral bodies, there is no description of them. In addition, they underestimated vertebral compressive failures in the transverse bicolumn fractures, as Denis did. To recognize and precisely describe the patterns of flexion distraction injuries, the concept of MAF is essential. As a step toward facilitating a more complete understanding of the mechanisms of flexion distraction injuries, we produced in vitro injuries with MAFs and investigated the relation between the flexion angle of the spine and the location of the MAF. MATERIALS AND METHODS Specimen Preparation We obtained thoracolumbar spines from 21 adolescent domestic pigs (age, 10 months; body weight, 80 kg). Pigs have 14 thoracic vertebrae. Each specimen had eight vertebrae and seven intervertebral disks, consisting of the caudal half of T11 to the cranial half of L4. We removed the muscles and ribs and left all other osteoligamentous structures and intervertebral disks intact. Radiographs were taken of each specimen to screen for any pre-existing abnormalities. We stored the specimens at 30 C in plastic bags and then thawed them at room temperature (about 20 C) before testing. We performed tests at room temperature and kept the specimens moist with physiologic saline at all times. We used fast-setting Epoxy (ARALDITE AW2104; Ciba Specialty Chemicals, Kwai Chung, Hong Kong) and anchoring screws inserted in the first two and last two vertebrae to embed the specimens, such that the centers of the T12 T13 and L2 L3 disks were aligned with the vertical axis of the testing machine. The multisegmental specimens consisted of five mobile disks and four vertebral bodies. A lateral radiograph was taken again of each specimen after embedding as a basis for measuring the MAF location. Testing Procedure For the tests, we used a biaxial hydraulic material testing machine and bending grips (MTS 858 Bionix and Model Spine Test Fixture, Minneapolis, MN, U.S.A.). We adjusted the cranial and caudal bending grips to align the caudal margins of the T12 and L3 bodies horizontally in the sagittal plane (Cobb angle of T12 L3 was equal to 0 ). We did not use the cranial margins of the vertebral bodies as bases because they were inclined anteriorly. We bent each specimen to a fixed flexion angle using the bending grips and exposed them to a vertical compressive load until overt disruption occurred in the posterior elements. We used three flexion angles: 20 (10 applied to the cranial end of the specimen, and 10 to the caudal end), 35 (17.5 at each end), and 50 (25 at each end). We randomly assigned seven specimens to each flexion angle group. Compression was applied at a rate of 0.05 mm/second using the deformation control mode of the testing machine. We recorded maximum loads at failure. Radiographs were taken of each specimen. We defined the MAF as the transitional point from anterior compressive to posterior splitting failure, as seen on the lateral radiograph (Fig. 1). When the transitional point was obscure, the MAF was defined as the point where the body height of the injured vertebra was the same as that before failure (4). We recorded the MAF location as the anterior border to the MAF distance expressed as a percentage of the anteroposterior diameter of the vertebral body (Fig. 1). Statistical Analyses We performed one-way analysis of variance and used the Scheffé test as a post hoc multiple comparison procedure to compare the average maximum loads at failure and MAF locations at the different flexion angles. We determined correlations of the maximum loads at failure and the MAF locations to the flexion angles using Pearson linear correlation analysis and linear regression analysis. We defined statistical significance as p All statistical analyses were performed using commercially available software (StatView; Abacus Concepts, Berkeley, CA, U.S.A.). RESULTS Our results indicate that the differences in maximum loads among the groups were significant (Table 1) and that

3 FLEXION\NDISTRACTION INJURIES IN THE THORACOLUMBAR SPINE 141 FIG. 2. The linear regression analysis results between the flexion angles of the specimens and the maximum loads at failure. We found a significant negative correlation. DISCUSSION FIG. 1. The motion axis of fracture (MAF) (4) is defined as the transitional point from anterior compressive to posterior splitting failure, as seen on the lateral radiograph. Its location is recorded as the anterior border to MAF distance expressed as a percentage of the anteroposterior diameter of the vertebral body (a/b 100%). In this specimen, the MAF was located at 31% from the anterior border of the vertebral body. the maximum loads correlated negatively to the flexion angles (Fig. 2). Radiographs obtained after failure showed that all specimens had similar injuries, with MAFs consisting of anterior compression fractures in the vertebral bodies and posterior disruptions through the growth plates (physes), facet joints, and interspinous and supraspinous ligaments (Fig. 1). Injured vertebrae were the caudal parts of T14 in 5 specimens, cranial parts of L1 in 15 specimens, and caudal parts of L1 in 1 specimen. The differences in MAF locations among the different flexion angle groups were significant (Table 1), and the MAF locations correlated negatively with the flexion angles (Fig. 3). The spine can be injured by a combination of axial force, shear force, bending moment, and axial torque (6). Most injuries in the sagittal plane without dislocation result from some combination of a bending moment (flexion extension) and an axial force (compression distraction). In the current study, the multisegmental specimens were exposed to vertical compressive loads in flexion. Theoretically, a compressive load P applied to a specimen under flexion can be separated into two components: a lateral load Q and an axial load S, as shown in Figure 4A. Load Q is a flexion bending moment and produces a compressive stress in the anterior elements and a tensile stress in the posterior elements. Load S is a compressive force and produces compressive stresses in both the anterior and posterior elements. If the tensile stress produced in the TABLE 1. Maximum load and the motion axis of fracture (MAF) at each flexion angle (mean ± SD) Flexion angle ( ) Specimen Maximum load (N) MAF (%) ± 1006* 49 ± ± 1258* 34 ± 4 # ± ± 5 # *p 0.036; p < ; p < 0.01; p < 0.004; p < , #p < (Scheffé test). FIG. 3. The linear regression analysis results between the flexion angles of the specimens and the MAF locations. We found a significant negative correlation.

4 142 T. HOSHIKAWA ET AL. FIG. 4. A: A multisegmental specimen was subjected to a vertical compressive load in flexion. The compressive load P can be separated into a lateral load Q and an axial load S. Load Q produces a flexion bending moment and load S is a compressive force. B: A specimen in a larger flexion angle. The same amount of vertical compressive loading produces a larger bending moment and a smaller compressive force (P = P', Q < Q', S > S'). posterior elements by load Q exceeds the compressive stress produced by load S, and if the resultant tensile stress then exceeds the tensile strength of the material of the specimen, a flexion distraction injury occurs. Thus, under appropriate conditions, a vertical compressive load alone produces a flexion distraction injury in the thoracolumbar spine. We believe this may also occur in humans. The preset flexion angle of the spine at which the spine is exposed to the vertical compressive load is an important factor in determining the MAF location. To our knowledge, no previous biomechanical study has investigated the relation between the flexion angle and the injury pattern of flexion distraction injuries in the thoracolumbar spine. In the current study, the locations of the MAFs were more anterior for larger flexion angles and more posterior for smaller flexion angles (Fig. 3). This finding is explained as follows: When the flexion angle becomes larger, the vertical compressive load (P'), assumed to be constant, produces a larger bending moment (Q') and a smaller compressive force (S') (Fig. 4B), which produce a larger tensile stress and a smaller compressive stress, respectively, in the posterior elements. The resulting tensile stress in the posterior elements is larger for the larger flexion angle. Therefore, when the flexion angle is larger, a smaller vertical compressive load is required to cause tensile failure of the posterior elements. This was substantiated by the negative correlation found between the maximum load at failure and the flexion angle in this study (Fig. 2). The smaller vertical compressive load required to cause tensile failure of the posterior elements produces an even smaller compressive force, thereby producing a smaller compressive stress in the posterior elements. The tensile stress due to the bending moment at failure, therefore, is also smaller. Because the compressive stress due to the bending moment is nearly equal to the tensile stress in magnitude, the resultant compressive stress in the anterior elements, in combination with the compressive stress due to the smaller compressive force, is smaller. As a result, the area of compressive failure is smaller in the vertebral body. Thus, the locations of the MAFs are more anterior for larger flexion angles. The specimens in the current study were subjected to quasi-static loading, which did not simulate an in vivo situation. Some studies (7,8) have indicated that the loading rate affects the failure site of the spine. Dynamic loading may produce different patterns of injury and change the MAF location. However, the current study was designed to compare the relative MAF locations at different flexion angles under identical loading conditions, which can be achieved satisfactorily only by quasi-static loading. Although the porcine spine is widely used as a model for in vitro mechanical studies, its anatomic structures are different from those of the human spine (e.g., the epiphyseal bone plates and growth plates under them) (9). However, the porcine model allows the control of age, weight, physical activity, and genetic differences, which would be impossible with human specimens. In the current study, although the splitting failure passed through the growth plates in all specimens, we believe that the use of the porcine spine was adequate to investigate the mechanisms of flexion distraction injuries in the thoracolumbar spine. Some investigators (10 12) produced in vitro flexion distraction injuries in human specimens. Cantilever bars attached to the cranial ends of the specimens were impacted from the posterior to produce a combination of a bending moment and a distractive force. Judging from the radiographs and descriptions provided, MAFs seemed to be located in the anterior halves of the vertebral bodies. In contrast, Kokubun et al. (2,3) and Tani et al. (4) reported that most patients with flexion distraction injuries had MAFs located in the posterior halves of the vertebral bodies. The authors speculate that to produce practical flexion distraction injuries with the MAFs in the posterior halves of the vertebral bodies, the angle between the specimen and the impact loading must be changed. That is, the specimen should be inclined to the posterior or the direction of the loading should be turned downward. CONCLUSION When evaluating a spinal injury, the clinician must consider the mechanism of injury. This study provides information that even a vertical compressive load produces a flexion distraction injury in the thoracolumbar spine, and that the flexion angle of the spine at which the vertical

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