Poster # Posterior Ligamentous Tension During Flexion of Human Cadaver Cervical Spine Specimens

Transcription

1 Poster # Posterior Ligamentous Tension During Flexion of Human Cadaver Cervical Spine Specimens Robert K. Hurford, Jr., MD, PhD (Chesterfield, MO), Richard M. Ozuna, MD (Boston, MA) INTRODUCTION: Cervical spine flexion and extension radiographs have been used to test the dynamic stability of the cervical spine for over forty years. Biomechanically, the posterior ligamentous structures of the subaxial cervical spine provide stability in flexion. Although the function of these posterior ligaments has been well characterized, the amount of flexion necessary to adequately stress the posterior ligaments is less clear. In this study, we assessed relative strain in the ligamentum flavum in human cadaver cervical spine specimens during flexion. All levels in the subaxial cervical spine from C2-C3 through C7-T1 were tested. METHODS: Nine human cadaver specimens were obtained from donors age 44 to 94. Five specimens were preparations from C1 to T3, and four specimens were preparations from C1 to T1. Left sided laminotomies were made to expose the underlying ligamentum flavum. Individual specimens were then potted at both ends and placed into a rig designed to apply pure flexion and extension moments. The neutral position was determined by balancing 2 lbs. of extension moment with 2 lbs. of flexion moment (Figure 1). In the neutral position, a small longitudinal incision was made in the ligamentum flavum and an AIFP force probe (Microstrain; Burlington, Vermont) was inserted. AIFP force probe measurements were made at neutral and then successive amounts of flexion produced by the application of manual traction to the flexion moment. At each point, a C arm image was taken. The amount of flexion in the subaxial cervical spine was determined by the Cobb method as the angle between the inferior endplate of C2 and the superior endplate of T1. RESULTS: Figure 2 shows sequential radiographic images from neutral through maximum flexion for one specimen. The levels tested in the subaxial cervical spine included C2-C3, C3-C4, C4-C5, C5-C6, C6-C7, and C7-T1. In neutral, all specimens were lordotic, and the degree of lordosis at neutral was set to 0 degrees of flexion. At subsequent points, the amount of flexion in the subaxial cervical spine was determined in reference to neutral. With the AIFP force probe, the relative strain in the ligamentum flavum during flexion at each level was tested for the nine different specimens, and the results of two representative specimens are presented in Figures 3 and

2 Figure 3 shows the strain curves for the ligamentum flavum from a 44 yo female specimen, and Figure 4 illustrates the strain curves from a 66 yo female specimen. Both graphs include standard deviations. These graphs show that the majority of strain generated in the ligamentum flavum develops during the last 10 to 15 degrees of flexion. In fact, for all nine specimens, most of the measured strain in the ligamentum flavum developed during the final 10 to 15 degrees of flexion. The total amount of flexion seen in the different specimens ranged from 46 to 64 degrees. In four of the nine specimens tested, a significant amount of tension, defined as greater than 20 percent of maximum strain, was measured at all levels by 40 degrees of flexion. In the other five specimens, 50 degrees of flexion was necessary to produce significant tension in the ligamentum flavum at all levels. CONCLUSIONS: The results obtained in these experiments show that measured strain in the ligamentum flavum during flexion of the subaxial cervical spine increases in a nonlinear manner. A rapid increase in strain occurred during the last 10 to 15 degrees of flexion. Flexion and extension views are commonly used to evaluate clinical stability of the cervical spine after trauma, previous surgery, or in the presence of pathological lesions. These views have been incorporated into cervical spine clearance guidelines published by the Eastern Association for the Surgery of Trauma (2000 Update) and the Congress of Neurological Surgeons. Both of these protocols state that 30 degrees of flexion is necessary for an adequate flexion view. However, 30 degrees of flexion was chosen as an arbitrary value to ensure some motion of the cervical spine. In all nine specimens we tested, 30 degrees of subaxial flexion did not produce significant tension in the ligamentum flavum. We suggest that at least 40 degrees of flexion in the subaxial cervical spine, measured from C2 to T1, is necessary to produce significant tension in the posterior ligamentous structures. 50 degrees of subaxial flexion is preferable because five of our nine specimens did not develop significant tension in the ligamentum flavum before 50 degrees of flexion. These results are applicable both to flexion radiographs and to dynamic fluoroscopic evaluation of the cervical spine. 264

3 Figure 1. Cervical spine testing rig. Human cadaver cervical spine specimen in neutral position in cervical spine testing rig. 265

4 Figure 2. Sequential radiographic images of a cervical spine specimen. A, neutral. B, 13 degrees of flexion between the inferior endplate of C2 and the superior endplate of T1 measured by the Cobb method. C, 26 degrees of flexion. D, 37 degrees of flexion. E, 47 degrees of flexion. F, 56 degrees of flexion. Also note that the force probe can be visualized at the C5-C6 interspace. 266

5 100% Figure 3 Specimen #1 Strain Curves Percentage of Maximum Strain in Ligamentum Flavum 80% 60% 40% 20% 0% % Degrees of Flexion of Subaxial Cervical Spine from Neutral C2-C3 C3-C4 C4-C5 C5-C6 C6-C7 C7-T1 100% Figure 4 Specimen #2 Strain Curves Percentage of Maximum Strain in Ligamentum Flavum 80% 60% 40% 20% 0% % Degrees of Flexion of Subaxial Cervical Spine from Neutral C2-C3 C3-C4 C4-C5 C5-C6 C6-C7 C7-T1 267