Pedicle screw systems are now the commonest method of achieving posterior spinal fixation. Surgical planning in spinal surgery may include measuring pedicle size to guide screw size on WEBPACS. We performed a study to determine whether measuring pedicle size on CT is accurate and reproducible using the WEBPACS ruler tool.

A human cadaveric spine along with 5 geometrical shapes were scanned using a multislice spiral CT scanner with 1mm cuts. The objects and the pedicle diameters for lumbar and thoracic vertebrae in the axial plane were measured independently using the WEBPACS ruler tool by 2 observers (to the nearest 0.1mm). The geometrical shapes and pedicle size on the skeleton were then measured using Vernier callipers by an independent third observer. All measurements were repeated a week later.

The WEBPACS ruler on a CT scan is accurate to within 0.5-0.6mm of the true size of an object. The error for pedicle measurements is marginally higher (0.6-1.0mm) and this may reflect the fact that they are ill defined geometric shapes. Measuring pedicle size on CT for surgical planning may have implications for small pedicles when sizing them up for a good screw.