Revista Brasileira de Ortopedia
Print version ISSN 0102-3616
ROSA, Rodrigo César et al. Biomechanical analysis of the variables related to the pullout strength of screws in the vertebral fixation system. Rev. bras. ortop. [online]. 2008, vol.43, n.7, pp. 293-299. ISSN 0102-3616. http://dx.doi.org/10.1590/S0102-36162008000700005.
OBJECTIVE: To observe the relevance of the pilot hole in the different manners of preparation: dull-tipped probe, cutting-tip probe, and burr, for the purpose of evaluating the level of pullout strength of screws with different diameters. METHODS: Screws of 5, 6, and 7 mm were inserted in the bovine bone test samples. The pilot hole was made with the dull-tipped probe, the cutting-tip probe, and burr. The perforation diameter was smaller, equal, and larger than the inner diameter of the screw. Mechanical pullout assays were performed in the three diameters for each manner of preparation of the pilot hole after inserting the implant. The mechanical assays were performed in an Emic®, Software Tesc 3.13 universal machine with load cell of 2000 N, load application speed of 2 mm/min, pre-load of 5 N, and accommodation time of 10 seconds. The property evaluated in the mechanical assays was the maximum pullout strength. RESULTS: In the group with 5 mm and 6 mm screws, and increase was observed in the pullout strength when the pilot hole diameter was smaller than the inner diameter of the screw in all manners of preparation (dull-tipped probe, cutting-tip probe, and burr). No statistical difference was seen in the seven millimeter screw group for all perforation diameters and manners of preparing the pilot hole. CONCLUSION: The pilot hole diameter has an influence upon the pullout strength of the screws in the vertebral fixation system. The making of a pilot hole with diameter larger than the inner diameter of the screw reduces the pullout strength of the implant, irrespective of the manner of preparation of the pilot hole.
Keywords : Spine; Bone screws; Biomechanics; Orthopedic procedures; Cattle.