jot_v16_pp701-708

jot_v16_pp701-708 - Journal of Orthopaedic Trauma Vol. 16,...

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Mechanical Strength, Fatigue Life, and Failure Analysis of Two Prototypes and Five Conventional Tibial Locking Screws *Sheng-Mou Hou, †Jaw-Lin Wang, and *Jinn Lin *Department of Orthopaedic Surgery, National Taiwan University Hospital, and the †Institute of Biomedical Engineering, National Taiwan University, Taipei, Taiwan Objective: To investigate the effects of the design and micro- structure on the mechanical strength of tibial locking devices. Design and Methods: The mechanical strength of two proto- types of specially developed locking devices (a both-ends- threaded screw and an unthreaded bolt) was tested and com- pared with that of five types of commercially available tibial locking screws (Synthes, Howmedica, Richards, Osteo AG, and Zimmer) with similar dimensions. The devices were in- serted into a polyethylene tube and loaded at their midpoint by a materials testing machine to simulate a three-point bending test. Single-loading yielding strength and cyclic-loading fatigue life were then measured. Failure analysis of the fractured screws was performed to investigate the microstructure and potential causes of the fatigue fracture. Results: Test results showed that both yielding strength and fatigue life were closely related to the section modulus of the inner diameter of screws. Among the threaded screws, the both- ends-threaded screws had a higher yielding strength and longer fatigue life than the Osteo AG, Howmedica, Richards, and Zimmer screws. The unthreaded bolts had a lower yielding strength than Synthes screws, but they demonstrated the longest fatigue life among all. In failure analysis of broken screws, no metallurgical or manufacturing defects were found except for surface microimperfections. Conclusions: The implants investigated in this study are manu- factured with high-quality materials and manufacturing pro- cesses. The main cause of hardware failure was mechanical overloading. The five commercially used tibial locking screws had a relatively short fatigue life under high loading. Removing the screw threads might substantially increase the fatigue life of the locking devices. In unthreaded bolts, this increase might be tenfold to a hundredfold. Key Words: Tibial locking screws, Mechanical testing, Failure analysis. Closed interlocking nailing, with the advantages of minimal tissue injury and stable fracture fixation, is the most effective surgical treatment of tibial shaft fractures (1,12,15). However, this procedure may potentially be complicated by hardware failure around the nail aperture, especially for distal metaphyseal fractures, comminuted fractures, fractures with delayed union, and fractures treated with small nails (15,16,19,27). Implant failure may take the form of either single-load yielding or, more often, cyclic load fatigue fracture (16). Improving the design of the screws to increase their mechanical strength is warranted (14,18,19). Increasing the screw diameter to increase the mechanical strength, however, demands a larger screw hole of the nail and may jeop- ardize the nail’s strength (24). A dilemma exists between
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jot_v16_pp701-708 - Journal of Orthopaedic Trauma Vol. 16,...

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