Pedicle Diameter Determined by Computed Tomography: Its Relevance to Pedicle Screw Fixation in the Lumbar Spine

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Intraoperative technical complications of pedicle screw fixation include screw cutout or maldirection and pedicle fracture. The aims of this study were 1) to use computed tomography to determine the average pedicle diameter; and 2) to compare these measurements with the outer diameter measurements of commonly used pedicle screws. The pedicle diameters of L2, L3, L4, L5, and S1 were measured in 154 adult patients (81 men, 73 women) who had low-back pain. The distance across the isthmus of the pedicle was measured with the distance mode on the axial computed tomographic image in the bone window setting. The lateral computed tomographic scout view was used to select the axial section through the midportion of the pedicle. To standardize data retrieval, the lower four motion segments were denoted as L2-S1, even when there were more or fewer than five lumbar vertebrae. The pedicle diameter at each level was measured and averaged for all patients and for men and women separately. The percentage of pedicles that measured less than 7 mm was determined at each level. The average pedicle diameters were 8.13 mm for L2, 8.7 mm for L3, 10.88 mm for L4, 14.54 mm for L5, and 18.37 mm for S1. Twenty percent of the L2 pedicle diameters, 15.6% of L3, and 1.9% of L4 were less than 7 mm; none of the L5 or S1 pedicles measured less than 7 mm. The outer diameters of the most commonly used pedicle screws range from 5 mm to 7 mm. Screw pitch, tooth profile, outer diameter, and depth of penetration affect implant strength. Choosing the largest diameter screw that can be accommodated by a pedicle increases resistance to pullout. The pedicles of L5 and S1 can safely accept 7-mm screws. A 7-mm screw can be used in 98% of L4 pedicles. Preoperative computed tomographic pedicle measurements should be obtained before instrumenting the L2 or L3 pedicles.

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