Measurement of effective pedicle diameter in the human spine.

Effective pedicle diameter (EPD), the maximal cancellous diameter of the spinal pedicle, demonstrates the maximal diameter available for transpedicular screw placement. The pedicles of 16 spines from T6 to L5 were measured directly with a graduated mean increase in the EPD ranging from 4.8 mm at T6 to 5.9 mm at L5. This direct pedicle measurement was significantly smaller than that of previously reported studies, which directly and radiographically measured pedicle outside width rather than inner diameter. Three specimens had little, if any, pedicular medullary cavity on direct measurement, although radiographic appearance of a medullary cavity existed. If EPD is significantly smaller than radiographic pedicle width measurements, safe transpedicular screw fixation may not be achieved. Preoperative planning must account for this so that transpedicular screws of correct diameter may be used and the complications of pedicular blowout fracture and neurologic impairment may be avoided.