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Structural Characteristics of the Pedicle and Its Role in Screw Stability

Hirano, Toru, MD*; Hasegawa, Kazuhiro, MD*; Takahashi, Hideaki E., MD*; Uchiyama, Seiji, MD*; Hara, Toshiaki, PhD; Washio, Toshikatsu, MSc; Sugiura, Toru, MSc; Yokaichiya, Motoo, MSc; Ikeda, Masayuki, MSc

Biomechanics
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Study Design. Cross-sectional regional bone mineral density of the pedicle was measured by peripheral quantitative computed tomography. Biomechanical tests were performed to clarify the role of the pedicle in screw stability.

Objectives. To identify the structural characteristics of the pedicle that supports pedicle screw stability and the differences in these characteristics between normal and osteoporotic vertebrae.

Summary of Background Data. The pedicle screw is an essential component of many systems used to align the spine. The contribution of the pedicle to screw stability, however, has not been fully investigated.

Methods. Trabecular, subcortical, and cortical bone mineral density and the area of the pedicle were measured by peripheral quantitative computed tomography. Bone mineral density also was recalculated in four circumferential layers. These parameters were compared between normal and osteoporotic individuals. The relative contribution of the pedicle to screw stability was evaluated by caudocephalad and pull-out loading in a vertebra with or without its body.

Results. Inner trabecular, middle subcortical, and outer cortical bone mineral density and cortical bone area in the pedicle were significantly lower in osteoporotic vertebrae than those in normal vertebrae. In the pedicle, bone mineral density increased close to the outer layer. Bone mineral density not as thick even in the outer layer in osteoporotic subjects. Approximately 80% of the caudocephalad stiffness and 60% of the pullout strength of the pedicle screw depended on the pedicle rather than on the vertebral body.

Conclusion. Screw stability depends on the structural characteristics of the pedicle. The pedicle was denser in the subcortical bone, in which the threads of the screw engage, than in trabecular bone. In osteoporosis, bone mineral density was not as dense even in the outer layer, and the cortex was thinner than normal. A larger screw would not enhance screw stability and may break the thin cortex in osteoporotic vertebrae.

From the *Department of Orthopaedic Surgery, Niigata University School of Medicine, and the †Department of Mechanical Engineering, Niigata University, Asahimachi-dori, Niigata City, Niigata, Japan.

Acknowledgment date: August 5, 1996.

First revision date: January 21, 1997.

Acceptance date: April 16, 1997.

Device status category: 11.

Address reprint requests to: Kazuhiro Hasegawa, MD, PhD; Department of Orthopaedic Surgery; Niigata Cancer Center Hospital; 2-15-3 Kawagishi-cho; Niigata City; Niigata 951; Japan.

© Lippincott-Raven Publishers.