Is it safer to place pedicle screws in the lower thoracic spine than in the upper lumbar spine?

STUDY DESIGN

An anatomic study of 100 patients comparing the pedicle isthmic width of the lower thoracic spine and the upper lumbar spine using magnetic resonance imaging.

OBJECTIVES

To compare the lower thoracic pedicles and upper lumbar pedicles in nondeformity patients as a surrogate measure of safety of pedicle screw use.

SUMMARY OF BACKGROUND DATA

Pedicle isthmic width is the significant limiting factor in the safety and proper placement of transpedicular screws. The presumption in the past has been that the lumbar pedicles are larger than the thoracic pedicles. Few publications in the English-language literature specifically evaluate the association between the pedicle isthmic widths of the lower thoracic and upper lumbar.

METHODS

The study evaluates 100 patients, without coronal spinal deformities. MRIs were obtained of the pedicles from T10 to L2 and subsequently measured using the axial T2-weighted views. Lower thoracic and upper lumbar pedicle isthmus, the narrowest section of pedicle, was investigated and compared. The "medial pedicle to medial rib corridor" at T10-T12 was defined and measured as part of the methodology of the study. Statistical analysis included one-way analysis of variance with post hoc least significant difference pairwise comparisons.

RESULTS

The smallest pedicle isthmic width was at L1 (mean +/- SD, 6.0 +/- 1.6 mm), while T12 (mean +/- SD, 7.6 +/- 1.5 mm) had the largest pedicle width. Although smaller in diameter than T12, both T10 (mean +/- SD, 6.2 +/- 1.2 mm) and T11 (mean +/- SD, 7.5 +/- 1.6 mm) had larger pedicle width than L1 (P < 0.01). Pedicle widths were larger in males compared with females (P < 0.05).

CONCLUSIONS

The results show that the lower thoracic pedicles are larger than the upper lumbar pedicles. This may make it safer to place screws in the lower thoracic spine than in the upper lumbar spine. Upper lumbar may be so small (<5 mm) to preclude safe conventional screw placement.