Vertebral pedicle screws screws provide secure attachment for posterior spinal fixation devices. For correct and safe application of the pedicle screws, the most important parameter is the transverse diameter of pedicle isthmus, which can be
assessed
only by indirect measurements preoperatively.
The objects of this study were to define the correlation between CT measurements and direct cadaveric measurements of pedicle, to examine the pedicle morphology and to observe the pedicle cnanges with screw insertion.
Eight adults human cadaveric vertebrae were CT scanned ant then cut transversely for direct measurements which were compared with CT values of transverse diameters of the pedicle isthmus. In addition the angle of pedicle axis to the sagittal
plane
was
measured at each level. With cautious drilling and screw insertion, pedicle cnanges were observed and adequate screw size as related to the diameter of each pedicle was studied in 79 thoracic vertebrae.
Intimate positive correlation was found between CT measurements and direct measurements of cadaveric vertebrae (Actual value=1.07×CT value±0.006, r=0.893, P<0.0001). The narrowest transverse diameter of the pedicle isthmus was seen at T5
(5±0.5mm).
Pedicle fractures did not occurunless the screw size exceeded 86% of the outer cortical diameter. In fresh vertebrae, it is assumed that the elasticity and plasticity of the bone enables insertion of larger size screws. In the transverse plane,
posteromedial to anterolateral directed pedicle axis orientations were seen upto T6 (-2°/-1°).
In conclusion, measurement of pedicle diameter can be so accurately assessed with CT scan that clinically pedicle isthmus diameter can be predicted exactly and pedicle screw diameter smaller than 86% of measured pedicle diameter can be safely
used
in
thoracic spine of cadaver.
Considering bony elasticity and plasticity of living bone, further study with fresh vertebrae is necessary to get more accurate data about clinically applicable pedicle screw sizes.
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