Mechanical Strength, Fatigue Life, and Failure Analysis of Two Prototypes and Five Conventional Tibial Locking Screws

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Abstract
To investigate the effects of the design and microstructure on the mechanical strength of tibial locking devices. The mechanical strength of two prototypes of specially developed locking devices (a both-ends-threaded screw and an unthreaded bolt) was tested and compared with that of five types of commercially available tibial locking screws (Synthes, Howmedica, Richards, Osteo AG, and Zimmer) with similar dimensions. The devices were inserted 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. 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. The implants investigated in this study are manufactured with high-quality materials and manufacturing processes. 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.