title: Insertional torque and pullout strength of pedicle screws with or without repositioning: a porcine study.
abstract: PURPOSE. To evaluate the insertion torque and pullout strength of pedicle screws with or without repositioning. METHODS. 20 fresh porcine lumbar vertebrae of similar size were used. The entry point was at the site just lateral and distal to the superior facet joint of the vertebra, and to a depth of 35 mm. A 6.2-mm-diameter, 35-mm-long pedicle screw was inserted parallel to the superior end plate on one side as control. On the other side, an identical screw was first inserted 10º caudal to the superior end plate, and then repositioned parallel to the superior end plate. The insertional torque and pullout strength were measured. RESULTS. Three of the specimens were excluded owing to pedicle fractures during the pullout test. Repositioned pedicle screws were significantly weaker than controls in terms of the maximum insertional torque (3.20 ± 0.28 vs. 2.04 ± 0.28 Nm, 36% difference, p<0.01) and pullout strength (1664 ± 378 vs.1391 ± 295 N, p<0.01). CONCLUSION. Repositioning pedicle screws should be avoided, especially when the pedicle wall is breached. If repositioning is deemed necessary, augmentation with polymethyl methacrylate or a screw with a larger diameter should be considered.
authors.affiliation: Department of Orthopaedics and Traumatology, Penang Hospital, Penang, Malaysia.
authors.forename: Chun Ee
authors.initials: CE
authors.lastname: Tan
authors.affiliation: Department of Orthopaedics and Traumatology, The University of Hong Kong, Hong Kong.
authors.forename: Margaret Woon Man
authors.initials: MW
authors.lastname: Fok
authors.affiliation: Department of Orthopaedics and Traumatology, The University of Hong Kong, Hong Kong.
authors.forename: Keith Dip Kei
authors.initials: KD
authors.lastname: Luk
authors.affiliation: Department of Orthopaedics and Traumatology, The University of Hong Kong, Hong Kong.
authors.forename: Kenneth Man Chee
authors.initials: KM
authors.lastname: Cheung
mesh_terms: D000818:Animals; D001860:Bone Plates; D019544:Equipment Failure Analysis; D008159:Lumbar Vertebrae / Q000601:surgery*; D008422:Materials Testing; D008954:Models, Biological; D065289:Pedicle Screws; D013123:Spinal Fusion / Q000295:instrumentation*; D013552:Swine; D013718:Tensile Strength; D019415:Torque
keywords: bone screws; spine; torque