Experimental Observation of Pedicle Screws in Postoperative CT scan - Stainless steel vs. Titanium / 대한정형외과학회잡지

Vertebral pedicle screws have been widely used for secure posterior spinal fixation. When postoperative CT scan films were made, the blurring of pedicle screws were observed and we could not figure nut the exact diameter of screw and canal encroachment. There is no information in the difference hetween actual diameter and measured diameter of pedcile screws in CT yet. In this study, we try to find out partial voiume averaging artifact of pedicle screws made of stainless steel and titanium. Partial volume averaging artifact occurs when the shape of an object changes within the thickness of the CT slice or when a relatively small object is only partially included within the slice. Four pig cadaveric spinal column including 6 vertebrae each were prepared and CT scans were performed after insertion of C-D screws(stainless steel) to the left and TSRH screws(titanium) to the right pedicle. Another CT scans were performed after insertion of C-D screws to the right and TSRH screws to the left pedicle. The third CT scans were made after removal of all the pedicle screws. Actual C-D screw diameter was 6mm and CT scanned diameter was 8. I 6+/-0.66mm. Actual TSRH diameter was 6.5 mm and CT scanned diameter was 6.59+/-0.34mm. In conclusion, stainless steel has more partial volume averaging artifact than titanium. Safety margin of pedicle screw made of stainless steel is about 2mm and that of titanium is about 1 mm.

Accuracy of CT measurements in Pig Spine / 대한정형외과학회잡지

At present the newer computerized tomography scanners provide a means for direct measurement of the size of normal and abnormal body structures from monitor. In orthopedics and rheumatology, CT is very important and useful means to make correct diagnosis and treatment modalities. In spite of its increasing importance in orthopedic field, reports about accuracy of CT are not common. We reviewed several reports that described about anatomical measurement using CT or similar experiments to our study. But there are few statements that have directly compared CT and caliper measurements in spine or other bones. So, authors performed experiments with 36 vertebrae of 8 pig spines to know the difference of CT measurement from actual measurement. The results were as follow: 1. In CT, bony portions including diameter of body and both pedicles were overestimated about 5% than actual ones. 2. In CT, AP and lateral diameters of spinal canal were 6~7% smaller than actual measurement and canal area, estimated by the products of AP and lateral diameter of spinal canal, is about 13% smaller than actual size. 3. It is the result of cortical overestimation around canal that spinal canal area was estimated less than actual size. 4. Main cause of this differences was partial volume effect that developed during CT scanning. In conclusion, CT scanner is the useful means and gives many useful informations to diagnose the spinal disease, but the physicians must understand mechanical, physical and mathematical limitations of CT and discrepancy of measurement data.