Biomaterial Database

The effects of different wire and screw combinations on the stiffness of a hybrid external fixator. 2000

L Yang, and M Saleeh, and S Nayagam

Division of Clinical Sciences, University of Sheffield, Northern General Hospital Trust, UK.

Hybrid external fixators that use tensioned wires in the metaphysis and screws in the diaphysis combine the advantages of both unilateral and circular fixators and provide considerable flexibility in frame construction. This study aims to determine the effects of wire and screw combinations on the mechanical characteristics of such systems. Acrylic rods were used to simulate a long bone with a transverse fracture gap. A standard hybrid fixator design was used to stabilize the fracture in accordance with modern concepts with wires being used on one side of the fracture and screws on the other side. Twelve different wire and screw combinations were tested in axial compression and four-point bending in two planes. All the fixators tested demonstrated stiffening under increasing axial load, the extent of which appeared to correlate inversely with the number of wires used. Increasing the number of wires by one increased the axial stiffness by 7-16 N/mm (average 11 N/mm). The bending stiffness was largely controlled by the number of wires and increased by 0.42-1.03 N m/mm (average 0.74 N m/mm) for each additional wire. The axial stiffness of fixators with three screws was 4-7 N/mm (average 6 N/mm) higher than those with two screws if the offset screw was not in use. When the offset screw was used no stiffness advantages were found for three screws over two screws. The use of an offset screw substantially increased axial stiffness (by 12-26 N/mm, average 20 N/mm) and is recommended for unstable fractures. The two-ring hybrid fixators with four wires in one ring and three screws in the other had stiffness characteristics similar to the conventional four-ring Ilizarov fixator. Knowledge gained regarding the effects of wire and screw combinations on overall stiffness will be helpful both in the construction of frames and in destabilization during fracture healing.

UI	MeSH Term	Description	Entries
D008422	Materials Testing	The testing of materials and devices, especially those used for PROSTHESES AND IMPLANTS; SUTURES; TISSUE ADHESIVES; etc., for hardness, strength, durability, safety, efficacy, and biocompatibility.	Biocompatibility Testing,Biocompatible Materials Testing,Hemocompatibility Testing,Testing, Biocompatible Materials,Testing, Hemocompatible Materials,Hemocompatibility Testings,Hemocompatible Materials Testing,Materials Testing, Biocompatible,Materials Testing, Hemocompatible,Testing, Biocompatibility,Testing, Hemocompatibility,Testing, Materials,Testings, Biocompatibility
D012044	Regression Analysis	Procedures for finding the mathematical function which best describes the relationship between a dependent variable and one or more independent variables. In linear regression (see LINEAR MODELS) the relationship is constrained to be a straight line and LEAST-SQUARES ANALYSIS is used to determine the best fit. In logistic regression (see LOGISTIC MODELS) the dependent variable is qualitative rather than continuously variable and LIKELIHOOD FUNCTIONS are used to find the best relationship. In multiple regression, the dependent variable is considered to depend on more than a single independent variable.	Regression Diagnostics,Statistical Regression,Analysis, Regression,Analyses, Regression,Diagnostics, Regression,Regression Analyses,Regression, Statistical,Regressions, Statistical,Statistical Regressions
D001863	Bone Screws	Specialized devices used in ORTHOPEDIC SURGERY to repair bone fractures.	Bone Screw,Screw, Bone,Screws, Bone
D001864	Bone Wires	Steel wires, often threaded through the skin, soft tissues, and bone, used to fix broken bones. Kirschner wires or apparatus also includes the application of traction to the healing bones through the wires.	Kirschner Wire,Kirschner Wires,Bone Wire,Wire, Bone,Wire, Kirschner,Wires, Bone,Wires, Kirschner
D004548	Elasticity	Resistance and recovery from distortion of shape.
D004867	Equipment Design	Methods and patterns of fabricating machines and related hardware.	Design, Equipment,Device Design,Medical Device Design,Design, Medical Device,Designs, Medical Device,Device Design, Medical,Device Designs, Medical,Medical Device Designs,Design, Device,Designs, Device,Designs, Equipment,Device Designs,Equipment Designs
D006801	Humans	Members of the species Homo sapiens.	Homo sapiens,Man (Taxonomy),Human,Man, Modern,Modern Man
D001696	Biomechanical Phenomena	The properties, processes, and behavior of biological systems under the action of mechanical forces.	Biomechanics,Kinematics,Biomechanic Phenomena,Mechanobiological Phenomena,Biomechanic,Biomechanic Phenomenas,Phenomena, Biomechanic,Phenomena, Biomechanical,Phenomena, Mechanobiological,Phenomenas, Biomechanic
D016267	External Fixators	External devices which hold wires or pins that are placed through one or both cortices of bone in order to hold the position of a fracture in proper alignment. These devices allow easy access to wounds, adjustment during the course of healing, and more functional use of the limbs involved.	Fixation Devices, External,Device, External Fixation,Devices, External Fixation,External Fixation Device,External Fixation Devices,External Fixator,Fixation Device, External,Fixator, External,Fixators, External
D016474	Weight-Bearing	The physical state of supporting an applied load. This often refers to the weight-bearing bones or joints that support the body's weight, especially those in the spine, hip, knee, and foot.	Load-Bearing,Axial Loading,Loadbearing,Weightbearing,Axial Loadings,Load Bearing,Weight Bearing