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Osteoporotic spine fixation : a biomechanical investigation Tan, Juay Seng

Abstract

Intervertebral fusion of the aging spine is a common surgical procedure for a wide range of clinical problems including trauma, deformity and degeneration. Spinal instrumentation is often used to stabilize the spinal column and help facilitate the fusion. The main problems of using spinal instrumentation in elderly patients (many with osteoporosis) are loss of fixation due to loosening and adjacent segment effects. The objectives of this thesis were to compare existing techniques of osteoporotic spine fixation using in vitro biomechanical models and to investigate new surgical strategies, so as to provide information to the spine surgeon to decrease the incidence of implant loosening and reduce degenerative changes at the adjacent levels in elderly patients. Pedicle screw loosening is common in patients with poor bone quality. Augmentation of pedicle screws with laminar hooks, sublaminar wires and/or cement are techniques used clinically to minimize pedicle screw loosening. A novel testing configuration was developed to apply physiologic load in relevant magnitudes and directions to study pedicle screw motion in cadaveric vertebrae. Twenty-four lumbar vertebrae (L3-L5) were divided into three groups and instrumented bilaterally with pedicle screws. A model of loosened screws was created by overdrilling the pedicle screw trajectory. Two techniques of screw augmentation, using laminar hooks, sublaminar wires and/or calcium phosphate cement, were carried out within each group of specimens. Paired comparisons of screw motion magnitudes at the screw head and screw tip, and overall motion patterns were carried out. Screws augmented with cement exhibited primarily rotational motion patterns with minimal translational motion, and most closely resembled the motion pattern of screws in high density bone. All three augmentation techniques resulted in a 50% mean reduction in screw motion. No significant differences in motion magnitudes were found between the different augmentation techniques. The results suggested that augmentation of pedicle screws with calcium phosphate cement might provide enhanced fixation over laminar hook and sublaminar wire augmentation. Anterior interbody device subsidence is another clinical problem that leads to implant loosening. Ninety-six thoracolumbar cadaveric vertebrae were compressed to failure using various shaped indentors. The focus was primarily on the superior endplate. In forty-eight specimens, the purpose was to determine the effect of cage shape and cage size on cage-vertebra interface properties. In the remaining forty-eight specimens, the effects of pedicle screw insertion and cement augmentation of screws on cage-vertebra interface properties were determined for various shapes and sizes of devices. Failure load, failure strength and stiffness were compared. In the first part, larger sized devices (with 40% endplate coverage versus with 20%) resulted in 75% higher failure load. Clover-leaf shaped devices also resulted in at least 45% higher failure load and failure strength over kidney or elliptical shaped devices. Trabecular failure was found to occur in a semi-elliptical zone beneath the interbody device. In the second part, pedicle screw insertion disrupted the underlying trabecular bone and reduced cage-vertebral interface strength. Cement augmentation of pedicle screws structurally reinforced the underlying trabeculae, and resulted in an improvement in cage-vertebra interface strength. No differences were found between cement augmented anterior vertebral body screws and pedicle screws. There were also no differences in interface strength between indentor shapes following screw insertion, with and without cement augmentation. Larger sized interbody devices and cement augmentation of vertebral screws might reduce the incidence of interbody device subsidence in the osteoporotic spine. Cement augmentation of pedicle screws and extension of posterior instrumentation are two techniques to improve the stabilization of spinal fixation in the presence of osteoporosis. Using the traditional flexibility protocol and a new hybrid test technique, intersegmental range of motion within the fused segment and at adjacent levels following cement augmentation of pedicle screws and extension of posterior rods were compared using an in vitro biomechanical thoracolumbar model. Twelve T9-L3 segments were tested in a repeated measures fashion to determine the effects of posterior rod extension and cement augmentation. Intact flexibility tests under 5 Nm pure moments were first carried out in axial rotation, lateral bending, and flexion extension. The initial test configuration included a T11 corpectomy and reconstruction with an extendable cage, and pedicle screws instrumentation from T10-T12. Flexibility tests were carried out randomly on (1) the initial test configuration, (2) with additional posterior rod extension to L1 using standard rigid rods and (3) extension using flexible acetal rods. The T12 and L1 pedicle screws trajectory were overdrilled as before to simulate loosening in the osteoporotic spine. Following flexibility tests under these three fixation methods, the T12 and L1 screws were augmented with cement. The flexibility tests were repeated for the three conditions in randomized order again. Three-dimensional motion of each vertebra was measured using an optoelectronic camera system. Intersegmental ROM at the fusion level was normalized and compared under the same applied moment in the flexibility protocol. Vertebral body strains and intersegmental ROM at the adjacent segments were compared under the same overall T9-L3 ROM in the hybrid protocol. Using the flexibility protocol, cement augmentation resulted in better fixation at the fusion level than with posterior rod extension. Using the hybrid protocol, posterior rod extension reduced ROM at the adjacent level but resulted in increased ROM and strain at the remaining non-instrumented levels. The use of flexible rods resulted in lesser increase in ROM and strain at these remaining non-instrumented levels than with rigid rods. Flexible rod extension might provide a better alternative than with rigid rod extension, when extension rods are necessary to prevent adjacent level effect at the extension level. The findings in this thesis support the use of cement in the osteoporotic spine to (1) improve fixation strength at the pedicle screw-vertebra interface, (2) improve fixation strength at the interbody device-vertebra interface and (3) increase overall structural stiffness (decrease motion) at the fusion level. Larger interbody devices should be used in the osteoporotic spine to further enhance construct stiffness. Extension of posterior instrumentation with flexible rods might prevent adjacent level disease at the extension level, but it should be used with caution as it might aggravate the remaining non-instrumented levels and postpone the adjacent level diseases to these levels.

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