PubMed · 8607465
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R J Nikolai. 1996. Comments on "Equilibrium Clarified".. https://pubmed.ncbi.nlm.nih.gov/8607465/
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STUDY DESIGN: A cadaveric cervical spine specimen fixed between a fiberglass torso and a plastic skull was used as a model to determine the effect of halo structural parameters on motion at a lesion simulated at C5-C6. In a second part, nine commercially available halo devices were compared. OBJECTIVES: To define the contributions of the various components of the halo apparatus to reducing motion in an injured cervical spine and to compare the stability offered by a sample of commercially available halo devices. Controversy exists concerning the ability of the halo apparatus to stabilize the injured cervical spine. SUMMARY OF BACKGROUND DATA: The halo apparatus has been shown to be the most effective nonsurgical method for stabilizing the fractured spine. Nonetheless, several clinical studies have demonstrated that unacceptably large motions can occur at the injured spinal segment stabilized with a halo apparatus. METHODS: Each cadaveric cervical spine was mounted onto a fiberglass torso and a rigid plastic skull was attached to the base of the occiput. A posterior ligamentous lesion was created between C5 and C6. The halo ring was fitted to the skull and a vest to the torso. Loads were applied to the skull in flexion, extension, and lateral bending, and relative angulation between C5 and C6 was measured with electroinclinometers. In the first part, the effect of parameters such as vest tightness, vest-thorax friction, vest deformation, and connecting bar rigidity on spinal angulation were measured using one vest. In the second part, the stability offered by each of nine commercially available halo devices was compared. RESULTS: Increasing chest strap tightness and decreasing vest deformation reduced angulation at the spinal lesion. Once connecting bar joints were tightened to 25% of their recommended torque, increased tightening or adding additional bars had no effect on rigidity. Although specific vests permitted significantly greater motion in specific directions, no vest allowed greater angulation consistently in all loading planes. CONCLUSIONS: Increasing vest tightness, decreasing the deformability of the vest, and ensuring a good fit can reduce motion in the fractured spine. Most commercially available halo vests provide similar mechanical stability to the injured cervical spine.
STUDY DESIGN: The three-dimensional (3-D) interrelations in the correction of the spine and rib cage produced by the Boston brace were analyzed in a group of adolescents with idiopathic scoliosis. OBJECTIVES: To investigate the coupling movements between the spine and rib cage initiated by brace wear (i.e., the displacements of the spine that take place in other directions than the ones generated by brace pressures on the thorax). SUMMARY OF BACKGROUND DATA: The effects of thoraco-lumbo-sacral orthosis in the frontal plane have been well documented, but they have never been studied in terms of 3-D coupled movements between the spine and rib cage. METHODS: The spine and rib cage of 36 adolescents with idiopathic scoliosis with and without their Boston brace were reconstructed in 3-D using a stereo-radiographic technique. Several geometric indices were evaluated on the trunk, and the relative motions of the spine and rib cage resulting from brace wearing were compared by means of Student t tests, Pearson correlation matrices, and linear regressions. RESULTS: Rib cage transverse plane translations resulting from brace pressures are related to those of the spine. Coupled movements between the spine and rib cage were found to alter substantially the expected 3-D correction of the trunk. Significant anterior displacements of the thorax were observed and were statistically associated with lateral displacements of the spine and with an increase of spinal thoracic curvatures in the frontal and sagittal planes. CONCLUSION: Brace loads are not applied in an optimal way to correct the 3-D deformities associated with thoracic idiopathic scoliosis. Loads applied on the posterior rib hump should be reequilibrated to reduce anterior displacement of the trunk.
STUDY DESIGN: This study was designed to evaluate the biomechanical performance of 5 different cross-link brands to determine which design characteristics are biomechanically desirable. METHODS: The Cotrel-Dubousset, Isola, Puno Winter Byrd, Rogozinski, and Texas Scottish Rite Hospital systems were assembled to vertebral models according to the manufacturer's specifications. Three constructs were tested for each brand of instrumentation: without cross-links, with one cross-link, and with two cross-links. Four modes of loading: axial, torsional, flexion-extension, and lateral-flexion were used. Load-displacement curves were plotted. The stiffness was calculated from the slope of these curves. OBJECTIVES: Five different rigid pedicle screw systems were tested to determine: 1) what are the characteristics of cross-link design that are most effective in limiting torsional motion; 2) whether two cross-links are more effective than one; and 3) whether cross-linkage increases the construct stiffness in lateral bending. SUMMARY OF BACKGROUND DATA: Cross-linkage has been shown to increase the torsional stiffness of rod and screw constructs. Increased construct stiffness has been correlated with higher fusion rates. RESULTS: Increases in axial, flexion-extension, or lateral-flexion stiffness, with the addition of one or two cross-links, were not statistically significant. In torsional loading, increases in stiffness within brands were statistically significant in every case. The average increase was 44% with one added cross-link and 26% with two. The magnitude of the increase in torsional stiffness was compared with the cross-sectional area of the respective cross-link. Greater stiffness correlated with larger cross-sectional area (r = 0.81 for one cross-link, and r = 0.60 for two). CONCLUSION: The use of cross-linkage in spinal fusion increases torsional stiffness in pedicle screw and hook constructs. This study 1) confirmed the effectiveness of cross-linkage in limiting torsional motion and showed the superiority of two cross-links to one cross-link in limiting torsional motion, 2) showed that increase of torsional stiffness of a cross-linked construct is proportional to the cross-sectional area of the cross-link, and 3) demonstrated that cross-links do not increases stiffness in the lateral flexion mode.