Pedicle Morphometry in the Upper Thoracic Spine Limits Safe Screw Placement in Older Patients

The Cleveland Clinic, Cleveland, Ohio, USA

PURPOSE: To characterize precise dimensions in upper thoracic pedicles of normal human vertebrae from an elderly population, and determine potential limits to safe screw placement at the T1 to T6 spinal levels.

METHODS: T1 to T 6 vertebrae were harvested from 18 human cadavers, age 62 - 85 at death. All subjects had died from disease unrelated to the musculoskeletal system, and all specimens were free of fracture, tumor, or deformity. Specimens were meticulously cleaned and individually examined to determine: a -medial/lateral width at the pedicle isthmus; b - cranial/caudal height at the isthmus; and c - coaxial distance from lamina to anterior vertebral cortex. Preliminary testing on unrelated specimens standardized the measurement method and confirmed reproducibility and validity of technique. Two independent observers performed repeated measures of each specimen using calipers. Mean values were recorded and compared level to level and side to side.

RESULTS: There were 11 female and 7 male subjects. There was no correlation between pedicle dimensions and sex or race. Dimension a determined pedicle screw selection; many pedicles were too small for the smallest screw available. Findings included a great reduction in pedicle diameter in the T 4 - 6 specimens. Mean diameters for each level:

· T1. a = 7.56 (3.8- 10.7 mm), 8% ‹ 4.5mm; c = 31.26 (26.7- 39.1)

· T2. a = 7.37 (4.3- 11.8 mm), 8% ‹ 4.5mm; c = 33.45 (29.5- 42.3)

· T3. a = 6.17 (3.8- 9.5 mm), 14% ‹ 4.5mm; c = 35.35 (30.5- 41.3)

· T4. a = 5.32 (2.8- 8.1 mm), 33% ‹ 4.5mm; c = 37.43 (32.6- 50.3)

· T5. a = 5.10 (1.9- 7.5 mm), 25% ‹ 4.5mm; c = 39.80 (33.6- 46.4)

· T6. a = 5.38 (3.1- 8.3 mm), 17% ‹ 4.5mm; c = 40.82 (32.5- 46.8)

CONCLUSION: Pedicle diameters in these elderly patients were small. Transpedicular screw placement is impossible in many of these vertebrae: 22% of T3- T6 pedicles could not physically accommodate a 4.5mm screw; 42% of these pedicles could not accept a 5.0mm screw. CT is necessary prior to upper thoracic screw fixation. Safe screw placement must consist of a "in-out-in" lateral vertebral body technique that avoids penetration of the medial pedicle wall.