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 the mechanical strength of the screw and that of the nail. Our hypothesis is that screw strength might be im- proved by the removal of screw threads without chang- ing the screw’s diameter. This design could decrease the stress concentration effect of the threads and increase the core diameter at the screw–nail contact region. To test this hypothesis, two locking devices were specially manufactured and studied at our institute. One was a screw with partial threads at both ends, and the other was an unthreaded bolt (Fig. 1). These two devices were me- chanically tested, along with five commercially available tibial locking screws, and results were compared. Be- cause the metallurgy and microstructure may substan- tially affect the mechanical properties of implants (13,22), failure analysis of the fractured screws was car- ried out to investigate the mechanisms of fractures after the mechanical tests were completed. MATERIALS AND METHODS Structures of the Tested Screws The two specially designed devices were made from 316L stainless steel (Carpenter Technology, Reading, PA, U.S.A.; yield strength 786 MPa, tensile strength 1,000 MPa, elongation rate in 1.5 inches: 26%). Ten each of the two devices and the five types of commercially Accepted February 9, 2002. Address correspondence and reprint requests to Dr. Jinn Lin, De- partment of Orthopaedic Surgery, National Taiwan University Hospi- tal, No. 7, Chung Shan South Road, Taipei 100, Taiwan. No financial support of this project has occurred. The authors have received nothing of value. The both-ends-threaded screws and unthreaded bolts are not FDA approved and are not commercially available in the United States. The other five types of commercially available screws are FDA approved and commercially available in the United States. Journal of Orthopaedic Trauma Vol. 16, No. 10, pp. 701–708 © 2002 Lippincott Williams & Wilkins, Inc., Philadelphia 701