title: Fully Threaded Versus Partially Threaded Screws: Determining Shear in Cancellous Bone Fixation.
abstract: Many researchers have studied and compared various forms of intraosseous fixation. No studies have examined the effects of shear through stiffness and failure strength of a fully threaded versus a partially threaded screw. Our hypothesis was that the fully threaded lag screw technique would provide greater shear strength and resistance. Thirty-six synthetic sawbone blocks were used to test screw fixation. In group 1 (n = 9), 2 blocks were fixed together using a fully threaded 4.0-mm stainless steel cancellous bone screw and the lag technique. In group 2 (n = 8), 2 blocks were fixed together using the standard manufacturer-recommended method for inserting 4.0-mm partially threaded stainless steel cancellous bone screws. The constructs were then mechanically tested. Shear was applied by compressing each construct at an axial displacement rate of 0.5 mm/s until failure. The fully threaded screw had a significantly greater (p = .026) initial stiffness (106.4 ± 15.8 N/mm) than the partially threaded screw (80.1 ± 27.5 N/mm). The yield load and displacement for the fully threaded group (429.4 ± 11.7 N and 7.2 ± 0.35 mm) were 64% and 67% greater than those for the partially threaded screw group (261.4 ± 26.1 N and 4.3 ± 1.03 mm), respectively. The results of the present study have demonstrated the importance of a full-thread construct to prevent shear and to decrease strain at the fracture. The confirmation of our hypothesis questions the future need and use of partially threaded screws for cancellous bone fixation.
authors.affiliation: Chief Resident, Foot and Ankle Division, Department of Orthopaedic Surgery, John Peter Smith Hospital, Fort Worth, TX. Electronic address: michael.w.downey@gmail.com.
authors.forename: Michael W
authors.initials: MW
authors.lastname: Downey
authors.affiliation: Associate Professor, Department of Orthopaedic Surgery, Bone and Joint Institute, University of North Texas Health Science Center, Fort Worth, TX; and Department of Materials Science and Engineering, University of North Texas, Denton, TX.
authors.forename: Victor
authors.initials: V
authors.lastname: Kosmopoulos
authors.affiliation: Attending Surgeon, Department of Orthopaedic Surgery, John Peter Smith Hospital; and Department of Orthopaedic Surgery, Bone and Joint Institute, University of North Texas Health Science Center, Fort Worth, TX.
authors.forename: Brian B
authors.initials: BB
authors.lastname: Carpenter
mesh_terms: D001696:Biomechanical Phenomena; D001863:Bone Screws; D005593:Fracture Fixation, Internal / Q000295:instrumentation*; D050723:Fractures, Bone / Q000503:physiopathology / Q000601:surgery*; D006801:Humans; D008953:Models, Anatomic; D033081:Shear Strength
keywords: biomechanics; cancellous bone screw; holding strength; lag technique; screw failure; shear strength