Cortex of the pedicle of the vertebral arch. Part II: Microstructure.

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.