Pedicle fixation in the thoracic spine. A morphometric study for placement of screws

There is a high risk of injury to the vascular-nervousstructures in performing pedicle fixation of thoracic spine.Nevertheless, the morphology of pedicles have been analysed only by a few authors and none of them haveinvestigated the safest technique for screw insertion. Thepurposes of the study were: 1) to analyse the morphologyof thoracic pedicles 2) to evaluate the more appropriateentry point and the optimal angle of insertion of thescrews.Material and Methods: We measured the transverse diameter (TD) of the pedicles in their cranial, middle and caudalpart of 99 thoracic vertebrae. In the sagittal plane weevaluated the distance from the centre of the pedicle to thebottom of the superior facet (P-F distance) and from thecentre of the pedicle to the superior margin of the transverse process (P-TP distance). Each vertebra was thenpositioned on a special frame horizontally and a photographof the vertebra was taken. We Then measured the pedicleorientation, the most appropriate entry point and angle ofinsertion of the screws. In a second part of the study, 122pedicles screws (diametre 5 mm) were implanted by 3orthopaedic surgeons in die 7 human cadavera. For thescrews placed on the left pedicles, the entry point used wasat the intersection between the superior margin of thetransvers process (SMTP) and the middle of the superiorfacet (entry point A): for the screws on the right pediclesthe entry point was between the SMTP and the external 2/3of the superior facet (entry point B).Results: . The mean TD of the pedicles was 6mm (range3-11 mm), the smallest diameters being found al T6 (mean4mm) and the largest at T12 (mean 8 mm). Overall the TDof the pedicles was less than 5mm in 28% of the pedicles.From T4 to T3. 59% of pedicles had a TD less than 5mm.From T4 to T9. The largest TD was at the middle part of thepedicle, while from TIO lo TI2 the largest diameter was inthe cranial part (p=0.003). The mean P-F distance and P-TPdistance were, respectively. 0.6mm (range -3;5 mm) and0.4mm (range -4;5mm). Greater variability was found inthe P-F distance compared with the P-TP distance. Pedicleangle in the horizontal plane was greatest at T4 (17") anddecreased progressively up lo T12 (2). The most appropriate screw orientation was, on average. 11°' anteromedially (19° at T4 and 8° at T12). Conclusion: . Pedicle from T9 to T12 show a meantransverse diameter which allow 3mm screws to be implanted safely. More Than half of the pedicles from T4 to T3show a TD smaller than 5mm: these pedicles .should bemeasured on CT scans before planning surgery. The distance V-TP appears to be a more reliable guide for the entrypoint of the screws in the sagittal plane .From the results ofour morphometric and cadaveric study we propose a newentry point placed at the external 2/3 of the superior facet.This entry point allows the screws to be directed anteromedially compared with the central entry point, thus reducing the risk of violating the anterolateral border of the vertebral body.