Facet orientation in the thoracolumbar spine: Three-dimensional anatomic and biomechanical analysis

Study Design. Thoracolumbar facet orientations were measured and analyzed. Objectives. To establish a comprehensive database for facet orientation in the thoracolumbar vertebrae and to determine the normal human condition. Summary of Background Data. Most studies on facet orientation have based their conclusions on two-dimensional measurements, in small samples or isolated vertebrae. The amount of normal asymmetry in facet orientation is poorly addressed. Methods. Transverse and longitudinal facet angles were measured directly from 240 human vertebral columns (males/females, blacks/whites). The specimens' osteologic material is part of the Hamann-Todd Osteological Collection housed at the Cleveland Museum of Natural History (Cleveland, OH). A total of 4,080 vertebrae (T1-L5) from the vertebral columns of individuals 20 to 80 years of age were measured, using a Microscribe three-dimensional apparatus (Immersion Co., San Jose, CA). Data were recorded directly on computer software. Statistical analysis included paired t tests and analysis of variance. Results. Facet orientation is independent of gender, age, and ethnic group. Asymmetry in facet orientation is found in the thorax. All thoracolumbar facets are positioned in an oblique plane. In the transverse plane, all facets from T1 to T11 are positioned with an anterior inclination of approximately 25° to 30° from the frontal plane. The facets of T12-L2 are oriented closer to the midsagittat plane of the vertebral body (mean range, 25.89°-33.87°), while the facets of L3-L5 are oriented away from that plane (mean range, 40.40°-56.30°). Facet transverse orientation at the thoracolumbar junction is highly variable (∼80% with ∼101° and ∼20% with 35°). All facets are oriented more vertically from T1 (∼150°) to L5 (∼170°). The facet sagittal orientations of the lumbar zygoapophyseal joints are not equivalent. Conclusions. Asymmetry in facet orientation is a normal characteristic in the thorax.