Pedicle and spinal canal parameters of the lower thoracic and lumbar vertebrae in the achondroplast population

STUDY DESIGN. Retrospective morphometric population study. OBJECTIVE. To characterize pedicle and spinal canal morphology of the achondroplastic lower thoracic and lumbar vertebrae and to suggest dimensions for improving pedicle screw selection and placement. SUMMARY OF BACKGROUND DATA. Although morphometric population studies exist for various races, to our knowledge, no such analysis has been made in achondroplastic patients. METHODS. With computer software, we measured pedicle parameters on the computed tomography images of 19 adult achondroplastic patients. RESULTS. Pedicle and chord lengths ranged from 9.5-12.5 mm and 29.5-36.4 mm, respectively. Transverse pedicle diameter increased from T9 (5.5 mm) to L5 (14.2 mm). Sagittal pedicle diameter declined from L1 (11.6 mm) to L5 (7.8 mm). Transverse angulation was greatest at L5 (15.7°) and smallest at T12 (1.1°). Pedicles were directed cranially at all levels, ranging from 3.8°-15.6°. Interpedicular distance and cross-sectional area were smallest at L4 (14.9 mm and 119 mm, respectively). Pedicle starting points diverged from T9 (13.6 mm) to L5 (19.2 mm). CONCLUSION. Achondroplastic pedicle morphology differs markedly from those of the normal spine: chord lengths are substantially shorter, pedicles are inclined cranially, pedicle starting points diverge progressively in the lumbar spine, and pedicle shape transitions from vertically to horizontally oriented ellipsoids along the lumbar spine. Consideration of this variation could maximize the effectiveness and safety of pedicle instrumentation.