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    Cortex of the pedicle of the vertebral arch. Part II: microstructure
    (Amer Assoc Neurological Surgeons, 2007) Inceoglu, Serkan; Kilincer, Cumhur; Tami, Andrea; Mclain, Robert F.
    Object. Although the gross anatomy of the pedicle in the human spine has been investigated in great detail, knowledge of the microanatomy of trabecular and cortical structures of the pedicle is limited. An understanding of the mechanical properties and structure of the pedicle bone is essential for improving the quality of pedicle screw placement. To enhance this understanding, the authors examined human cadaveric lumbar vertebrae. Methods. In this study, the authors obtained seven human cadaveric,lumbar vertebrae. The lateral and medial cortices of these pedicle specimens were sectioned and embedded in polymethylmethacrylate. Cross-sectional slices of cortex were obtained from each specimen and imaged with the aid of a high-resolution light microscope. Assessments of osteonal orientation, determinations of relative dimensions, and histomorphometric studies were performed. Results. The cortex of the pedicle in each human lumbar vertebra had an osteonal structure with haversian canals laid down mainly in the anteroposterior (longitudinal) direction. The organization of osteons across the transverse cross-section was not homogeneous. The layer of lamellar bone that typically envelops cortical bone structures (such as in long bones) was not observed, and the lateral cortex was significantly thinner than the medial cortex (p < 0.05). Conclusions. The cortical bone surrounding the pedicle differed from bone in other anatomical regions such as the anterior vertebral body and femur. The osteonal orientation and lack of a lamellar sheath may account for the unique deformation characteristics of the pedicle cortex seen during pedicle screw placement.
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    Cortex of the pedicle of the vertebral arch.: Part 1
    (Amer Assoc Neurological Surgeons, 2007) Inceoglu, Serkan; Kilincer, Cumhur; Tami, Andrea; Mclain, Robert F.
    Object. Elastic deformation has been proposed as a mechanism by which vertebral pedicles can maintain pullout strength when conical screws are backed out from full insertion. The response to the insertion technique may influence both the extent of deformation and the risk of acute fracture during screw placement. The aim of this study was to determine the deformation characteristics of the lumbar pedicle cortex during screw placement. Methods. Lumbar pedicles with linear strain gauges attached at the lateral and medial cortices were instrumented using 7.5-mm pedicle screws with or without preconditioning by insertion and removal of 6.5-mm screws. The strains and elastic recoveries of the medial and lateral cortices were determined. Results. Mean medial wall strains tended to be lower than mean lateral wall strains when the 6.5-mm and 7.5-mm screw data were pooled (p = 0.07). After the screws had been removed, 71 to 79% of the deformation at the lateral cortex and 70 to 96% of the deformation at the medial cortex recovered. When inserted first, the 7.5-mm screw caused more plastic deformation at the cortex than it did when inserted after the 6.5-mm screw. Occasional idiosyncratic strain patterns were observed. No gross fracture was observed during screw placement. Conclusions. Screw insertion generated plastic deformation at the pedicle cortex even though the screw did not directly contact the cortex. The lateral and medial cortices responded differently to screw insertion. The technique of screw insertion affected the deformation behavior of the lumbar pedicles. With myriad options for screw selection and placement available, further study is needed before optimal placement parameters can be verified.