COMPARISON BETWEEN CONICAL/CYLINDRICAL SCREW

COMPARISON AvantagesDrawbacks Conical screw - Risk of screw loosing if the screw is slightly unscrewed - This screw has not to be back out. - A correct insertion depth has to be respected - Good mechanical performance - Good mechanical strength compared to cylindrical screw Cylindrical screw - Good bone anchorage - This screw can be easily adjusted into the bone ( it can be screwed and unscrewed) - More constraints in the screw - Less mechanical strength compared to conical screw CHOICE DEPENDING ON APPLICATION CERTAIN SCEWS CAN BE ALSO HYBRIDE (conical part and cylindrical part)

ARTICLE # 1 Comparison of the in vitro holding strengths of conical and cylindrical pedicle screws in a fully inserted setting and backed out 180 degrees. Lill CA, Schlegel U, Wahl D, Schneider E. AO Research Institute, Davos, Switzerland. Previous investigations have suggested that conical and cylindrical pedicle screws have comparable holding strengths. So far, the remaining performance in screws turned back or loose as a result of other reasons has not been determined. Twenty-four cadaveric spines from 6- to 8-week-old calves were examined. After bone mineral density was determined, four pedicle screws (two conical and two cylindrical screws) were inserted. The screws were fully inserted and half of them turned back 180 degrees. Twenty-four axial pullout and 24 cyclic loading tests with subsequent pullout tests were conducted. The pullout strengths of conical screws turned back 180 degrees are significantly smaller (1.8 kN) than those of cylindrical screws (4.3 kN). After cyclic loading, the displacement of conical screws is significantly greater (6.9 mm) than that of cylindrical screws (4.7 mm). Pedicle screws, especially conical ones, need to be placed to a correct depth, and they should not have to be backed out. PMID: [PubMed - indexed for MEDLINE] Lill CASchlegel UWahl DSchneider E

ARTICLE # 2 Increase of pullout strength of spinal pedicle screws with conical core: biomechanical tests and finite element analyses. Hsu CC, Chao CK, Wang JL, Hou SM, Tsai YT, Lin J. Department of Mechanical Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan. Screw loosening can threaten pedicle screw fixation of the spine. Conical screws can improve the bending strength, but studies of their pullout strength as compared with that of cylindrical screws have shown wide variation. In the present study, polyurethane foam with two different densities (0.32 and 0.16 gm/cm3) was used to compare the pullout strength and stripping torque among three kinds of pedicle screws with different degrees of core tapering. Three- dimensional finite element models were also developed to compare the structural performance of these screws and to predict their pullout strength. In the mechanical tests, pullout strength was consistently higher in the higher density foam and was closely related to screw insertion torque (r=0.87 and 0.81 for the high and low density foam, respectively) and stripping torque (r=0.92 and 0.78, respectively). Conical core screws with effective foam compaction had significantly higher pullout strength and insertion torque than cylindrical core screws (p<0.05). The results of finite element analyses were closely related to those of the mechanical tests in both situations with or without foam compaction. This study led to three conclusions: polyurethane foam bone yielded consistent experimental results; screws with a conical core could significantly increase pullout strength and insertion torque over cylindrical; and finite element models could reliably reflect the results of mechanical tests. PMID: [PubMed - indexed for MEDLINE] Hsu CCChao CKWang JLHou SMTsai YTLin J