title: A finite element analysis of two novel screw designs for scaphoid waist fractures.
abstract: The scaphoid is the most often fractured carpal bone. Scaphoid fracture repair with a headless compression screw allows for early functional recovery. The rotational stability of a single screw may be limited, having a potential negative impact on the healing process. Two novel screws have been designed to provide improved rotational stability compared to the existing ones. Using a computational finite element model of a scaphoid osteotomy, we compared the efficacy of one simple screw and the two new screws in restricting inter-fragmentary motion (IFM) in three functional positions of the wrist and as a function of inter-fragmentary compression force. The in-plane IFM was primary rotational and was better restricted by the new screws compared to the conventional one when the inter-fragmentary compression force was below 15-20 N, but provided no clear benefit in total flexion independently of the compression force. To better understand the differences in the non-compressed case, we analyzed the acting moments and investigated the effects of the bending and torsional screw stiffness on IFM. By efficiently restricting the inter-fragmentary shear, the new screws may be clinically advantageous when the inter-fragmentary compression force is partially or completely lost and may provide further benefits toward earlier and better healing of transverse waist fractures of the scaphoid.
authors.affiliation: AO Research Institute Davos, Clavadelerstrasse 8, 7270 Davos, Switzerland; Institute of Lightweight Design and Structural Biomechanics, Vienna University of Technology, Gußhausstraße 27-29, A-1040 Vienna, Austria. Electronic address: peter.varga@aofoundation.org.
authors.forename: Peter
authors.initials: P
authors.lastname: Varga
authors.affiliation: Institute of Lightweight Design and Structural Biomechanics, Vienna University of Technology, Gußhausstraße 27-29, A-1040 Vienna, Austria; Institute for Surgical Technology & Biomechanics, Universität Bern, Stauffacherstrasse 78, CH-3014 Bern, Switzerland.
authors.forename: Philippe K
authors.initials: PK
authors.lastname: Zysset
authors.affiliation: Department of Trauma Surgery, Medical University of Vienna, Austria.
authors.forename: Philip
authors.initials: P
authors.lastname: Schefzig
authors.affiliation: Center for Medical Physics and Biomedical Engineering, Medical University of Vienna, Währinger Gürtel 18-20, A-1090 Vienna, Austria.
authors.forename: Ewald
authors.initials: E
authors.lastname: Unger
authors.affiliation: Center for Medical Physics and Biomedical Engineering, Medical University of Vienna, Währinger Gürtel 18-20, A-1090 Vienna, Austria.
authors.forename: Winfried
authors.initials: W
authors.lastname: Mayr
authors.affiliation: Department of Trauma Surgery, Medical University of Vienna, Austria.
authors.forename: Jochen
authors.initials: J
authors.lastname: Erhart
mesh_terms: D001863:Bone Screws; D019245:Compressive Strength; D005260:Female; D020342:Finite Element Analysis; D050723:Fractures, Bone / Q000000981:diagnostic imaging / Q000601:surgery*; D006801:Humans; D008422:Materials Testing; D011474:Prosthesis Design; D012399:Rotation; D021361:Scaphoid Bone / Q000293:injuries*; D055114:X-Ray Microtomography
keywords: Biomechanics; Finite element method; Fracture healing; Scaphoid; Screw osteosynthesis