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Assessment of strength and stability of pedicle screw fixation
241
Abstracts ASSESSMENTOFSTRENGTHANDSTABILITYOFPEDICLESC~ FIXATION Narayan Yoganandan, Frank A. Pintar, Dennis J. Maiman, Anthony Sancee, Jr. Department of Neurosurgery, Medical College of Wisconsin, Milwaukee, WI 63226 Marquette University, Milwaukee, WI and Veterans Administsation Medical Center, Milwaukee, WI The objective of this study was to determinethe alterations in the strength and stability imparted to the traumatized lumbar spine instrumented with transpedicular screws and spine plates. Seven fresh human cadaveric thoracolumbar spinal columns were prepared according to accepted techniques. To document the localized kinematics, retroreflective targets were inserted in the bony landmarks at every level of the spinal column: four in the vertebral body, one each in the facet articulation,and one at the tip of the spinous process at every level of the spine. Using an electrohydraulic testing device specimens were tested to failure (control run) under compression-flexion loading at a quasistatic rate of 2.5 mm/s. The specimen was reloaded, injury run, to deformations experienced in the control run. After instrumenting the injured spine with transpedicular screws and spine plates, a third cycle of loading (fixated run) was carried out. The load-deflection response indicated nonlinear characteristics. The stabilized spine had significantly higher initial stiffness compared to the injured spine (P < 0.001). However, the final stiffness was not significantly different between the two specimens. This stiffness, corresponding to the load shared by the anterior column may support the hypothesis that beyond a certain level of strain in the posterior column, the anterior column absorbs a majority of the external load. Therefore, it may be necessary to stabilize the anterior column to further incre.asethe strength of the injured spine. Localized kinematic data analysis indicated decreasing movements of the vertebral body, facet column, and the spinous process between the fixated levels. However, increased motions at levels proximal and distal to fixation observed in the present study may be further accentuated under in uivo situations, leading to hypermobility and degeneration of the spine.
THE ORGAN RESPONSE OF BONE TO THE REMOVAL OF FUNCTIONAL STIMULI Ted S. Gross and Clinton T. Rubin Musculo-Skeletal Research Laboratory 11794 Department of Orthopaedics, SUNY at Stony Brook, Stony Brook, NY If the functional environment of bone acts as the controlling mechanism to maintain skeletal mass, removal of this stimulus should engender an organ wide adaptive bone loss. Although previous work has demonstrated site specific bone loss in response to reduced mechanical stimulus, the extent and uniformity of the adaptive process has not been determined. In order to determine how bone, as an organ, adapts to the removal of functional stimuli, the right radius of three adult male turkeys were isolated from functional stimuli with parallel metaphyseal osteotomies. The musculature, vasculature, and innervation The radii were deprived of functional stimuli for a period of the diaphyseal shaft was left undisturbed. of eight weeks, after which the adaptive response of each isolated radius was contrasted with its intact, contralateral control. Three transverse sections were extracted at 1.5cm intervals spanning the middle 3cm of the diaphysis of the experimental and control radii. The sections were ground to 100 microns, microradiographed, and digitized to determine area1 properties. Eight weeks of isolation from functional stimuli precipitated a loss of bone tissue in all experimental cross sections, with an average (S.D.) loss of 13.8(6.3)% compared to the intact, contralateral control. Fluorescent labels and changes in area1 properties revealed that, similar to humans, the adult turkey radii responded to the removal of functional stimuli by activating quiescent cellular populations, which, in turn, reduced bone mass primarily by expansion of the endosteal envelope. Bone loss within each turkey was consistent in magnitude (17.3(5.0)%, 17.8(2.0)%, and 6.4(2.0)%, respectively), suggesting a drive toward a uniform adaptive response within each bone.
THE EFFECTS
OF COMPONENT DESIGN VARIATIONS ON MFXXANICAL OF PROPHYLACTIC KNEE BRACES
PERFOMANCE
B.J. Daley, J.L. Ralston, T.D. Brown, and R.A. Brand. Biomechanics Laboratory, Department of Orthopaedics University of Iowa, Iowa City, IA 52242. The use of a surrogate knee model, complete with muscle forces, is described in the testing of design variables of prophylactic knee braces. Components of the braces were varied as to width and thickness in a modular generic construct of a unilateral, dual-hinge brace. Although the load at failure of interchangable medial collateral ligament analogs was increased, and although the MCL strain relief increased with the brace in place, these values were not as large as the relief provided by the muscle forces alone. Inter-component variation demonstrated that the stiffer a component, the greater both its load at failure and its corresponding MCL strain relief. No single brace component could be identified as significantly increasing load at failure. Whether mechanically efficacious designs of braces will have widespread on-field use is questionable, because of an apparently unavoidable loss of speed and flexibilty due to increased mass.
Abstracts ASSESSMENTOFSTRENGTHANDSTABILITYOFPEDICLESC~ FIXATION Narayan Yoganandan, Frank A. Pintar, Dennis J. Maiman, Anthony Sancee, Jr. Department of Neurosurgery, Medical College of Wisconsin, Milwaukee, WI 63226 Marquette University, Milwaukee, WI and Veterans Administsation Medical Center, Milwaukee, WI The objective of this study was to determinethe alterations in the strength and stability imparted to the traumatized lumbar spine instrumented with transpedicular screws and spine plates. Seven fresh human cadaveric thoracolumbar spinal columns were prepared according to accepted techniques. To document the localized kinematics, retroreflective targets were inserted in the bony landmarks at every level of the spinal column: four in the vertebral body, one each in the facet articulation,and one at the tip of the spinous process at every level of the spine. Using an electrohydraulic testing device specimens were tested to failure (control run) under compression-flexion loading at a quasistatic rate of 2.5 mm/s. The specimen was reloaded, injury run, to deformations experienced in the control run. After instrumenting the injured spine with transpedicular screws and spine plates, a third cycle of loading (fixated run) was carried out. The load-deflection response indicated nonlinear characteristics. The stabilized spine had significantly higher initial stiffness compared to the injured spine (P < 0.001). However, the final stiffness was not significantly different between the two specimens. This stiffness, corresponding to the load shared by the anterior column may support the hypothesis that beyond a certain level of strain in the posterior column, the anterior column absorbs a majority of the external load. Therefore, it may be necessary to stabilize the anterior column to further incre.asethe strength of the injured spine. Localized kinematic data analysis indicated decreasing movements of the vertebral body, facet column, and the spinous process between the fixated levels. However, increased motions at levels proximal and distal to fixation observed in the present study may be further accentuated under in uivo situations, leading to hypermobility and degeneration of the spine.
THE ORGAN RESPONSE OF BONE TO THE REMOVAL OF FUNCTIONAL STIMULI Ted S. Gross and Clinton T. Rubin Musculo-Skeletal Research Laboratory 11794 Department of Orthopaedics, SUNY at Stony Brook, Stony Brook, NY If the functional environment of bone acts as the controlling mechanism to maintain skeletal mass, removal of this stimulus should engender an organ wide adaptive bone loss. Although previous work has demonstrated site specific bone loss in response to reduced mechanical stimulus, the extent and uniformity of the adaptive process has not been determined. In order to determine how bone, as an organ, adapts to the removal of functional stimuli, the right radius of three adult male turkeys were isolated from functional stimuli with parallel metaphyseal osteotomies. The musculature, vasculature, and innervation The radii were deprived of functional stimuli for a period of the diaphyseal shaft was left undisturbed. of eight weeks, after which the adaptive response of each isolated radius was contrasted with its intact, contralateral control. Three transverse sections were extracted at 1.5cm intervals spanning the middle 3cm of the diaphysis of the experimental and control radii. The sections were ground to 100 microns, microradiographed, and digitized to determine area1 properties. Eight weeks of isolation from functional stimuli precipitated a loss of bone tissue in all experimental cross sections, with an average (S.D.) loss of 13.8(6.3)% compared to the intact, contralateral control. Fluorescent labels and changes in area1 properties revealed that, similar to humans, the adult turkey radii responded to the removal of functional stimuli by activating quiescent cellular populations, which, in turn, reduced bone mass primarily by expansion of the endosteal envelope. Bone loss within each turkey was consistent in magnitude (17.3(5.0)%, 17.8(2.0)%, and 6.4(2.0)%, respectively), suggesting a drive toward a uniform adaptive response within each bone.
THE EFFECTS
OF COMPONENT DESIGN VARIATIONS ON MFXXANICAL OF PROPHYLACTIC KNEE BRACES
PERFOMANCE
B.J. Daley, J.L. Ralston, T.D. Brown, and R.A. Brand. Biomechanics Laboratory, Department of Orthopaedics University of Iowa, Iowa City, IA 52242. The use of a surrogate knee model, complete with muscle forces, is described in the testing of design variables of prophylactic knee braces. Components of the braces were varied as to width and thickness in a modular generic construct of a unilateral, dual-hinge brace. Although the load at failure of interchangable medial collateral ligament analogs was increased, and although the MCL strain relief increased with the brace in place, these values were not as large as the relief provided by the muscle forces alone. Inter-component variation demonstrated that the stiffer a component, the greater both its load at failure and its corresponding MCL strain relief. No single brace component could be identified as significantly increasing load at failure. Whether mechanically efficacious designs of braces will have widespread on-field use is questionable, because of an apparently unavoidable loss of speed and flexibilty due to increased mass.