Pedicle morphometry in the upper thoracic spine: limits to safe screw placement in older patients

An anatomic study of pedicle dimensions and orientation was performed for upper thoracic vertebrae from elderly human subjects.

To quantify dimensions of thoracic pedicles, and to determine the potential for safe transpedicular screw fixation in the upper thoracic spine.

Clinical and anatomic reports support thoracic pedicle fixation as a safe, effective alternative to hook fixation in both normal and osteoporotic bone. Much available data, however, pertains to young, robust patients, thoracolumbar segments, and mixed placement techniques.

For this study, T1-T6 vertebrae from 18 human cadavers were separated into individual vertebrae. Examiners measured each vertebra to determine medial-lateral pedicle width, cranial-caudal pedicle height, and coaxial depth from lamina to anterior vertebral cortex. Mean values were derived from repeated measures compared by level and side.

Dimension a varied with individual and level, but not between the left and right pedicles. Pedicle diameter uniformly diminished as specimens proceeded caudally from T2. Findings showed that 25% of T1 pedicles, 17% of T2 pedicles, and 42% of T3 pedicles were narrower than 5.5 mm. At T4 61% of pedicles were too small, at T5 67% were too small, and at T6 75% were too small to accept a 5.5-mm screw. Dimension b remained relatively constant. Pedicles became increasingly narrow and oblong in the T4-T6 cross sections. Dimension c increased consistently from T1 to T6. Safe screw lengths ranged from 30 mm at T1 and T2, to 35 mm at T4 -T5, to 40 mm at T5 and T6.

Even the largest patients had some pedicles that could not accommodate the smallest standard pedicle screw, and more than one half of the pedicles average patients were too small. Transpedicular screw placement is not safe in these patients. Proper placement must avoid penetration of the medial pedicle wall.