Simon Zabler a , Tatjana Rack b , Alexander Rack c , Katja Nelson d Fatigue induced deformation of taper connections in dental titanium implants The present study deals with in-situ microgap measure- ments in the internal taper connections of dental implants. Using X-ray phase contrast microtomography, the connect- ing interface between implant and abutment is probed non- destructively in three dimensions. Interference fringes across the conical interface occur due to the presence of microgaps, their intensity being a measure of the gap’s width. Thus, for each point on the interface, interferences are extracted from the volumetric image in terms of normal projection maps which are, at selected points, compared to forward simulations to quantify the local gap width. Four designs of dental implants are tested in the \as-received" state as well as after application of cyclic extra-axial load. Results show different degrees of microgap opening by cyclic deformation according to the implants’ design as well as a great amount of detail on the actual interface, i. e. fretting scars, grooves and wear debris. Keywords: Microtomography; X-ray imaging; Phase-con- trast; Dental implants; Fatigue 1. Introduction Dental screw implants made of titanium and titanium alloys have been used for many decades for the partial and full restoration of the upper and lower jaw [1]. Under the daily exercise of chewing these implants have to be biocompati- ble and withstand corrosion as well as fatigue forces [2 – 5]. Dental implants can either be designed as one-piece or two-piece devices [6]. The latter comprises the hollow screw implant and the abutment. Two-piece implants have both medical and mechanical advantages over a one-piece design [7]: I. After inserting the implant it is covered easily with tissue to allow for safe healing before the abutment is added a few weeks later, and II. The plug-socket connection between implant and abutment is fastened with an addi- tional abutment screw which acts as a preloaded spring holding the two pieces together. The tightening force of the abutment screw is set to remain below its yield when random chewing forces apply [8, 9]. Thanks to this design, the bending fatigue strength of two-piece implants has been shown to be superior to any bulk one-piece design [6, 10, 11]. Concerning the geometry of the implant – abutment connection (IAC) a variety of designs has been tested [2, 3, 12]. While the traditional concept is based on a horizon- tal butt joint connection, a significant part of modern dental implants feature taper connections of varying taper ratio and angle [10, 13]. This step towards tapered IACs was mostly motivated by mechanical considerations. In me- chanical engineering butt joints are mainly used for axially loaded connections. Consequently, technical failures such as screw loosening and/or fracture occur when extra-axial bending moments are applied, as is the case in vivo during chewing [8, 9]. Compared to butt joints, taper connections have been found to perform much better in bending fatigue, because the moments mainly turn into elastic deformation at the IAC and do not become a lever for straining the abut- ment screw [6, 7]. Consequently, implant and screw frac- tures can be avoided [14 – 16]. However, due to geometric imperfections both butt joints and taper joints contain hol- low spaces inside the IAC which have direct contact with the oral cavity. Risk of bacterial infiltration (through the IAC and into the inner implant cavities) and of crevice cor- rosion are the consequences [4, 17 – 19]. For the strains of bacteria present in the oral cavity, a few micrometre wide gap is considered sufficiently \open" for infiltration [20 – 22]. Despite the commonly acknowledged existence of such microgaps, relatively little is known about fretting and cyclic plastic deformation, in other words the possible enlargement of initially small microgaps to larger cavities [5, 23 – 25]. For tapered IACs the geometry, i. e. the taper angle, the joint diameter and length, have been shown to play an important role for the implant’s fatigue properties suggesting that the latter can be judged from the time-evo- lution of microgaps during fatigue [3, 12, 26, 27]. By means of high-resolution radiography it has been de- monstrated that microgaps exist in conical IACs under load [28]. These microgaps result from incongruent fit of implant and abutment. Independent of this misfit, taper connections give a certain mobility to the assembly which can be considered advantageous as long as the resulting strains along the IAC are purely elastic and do not accu- mulate to fatigue damage in terms of cyclic deformation. By means of quantitative phase-contrast radiography the width of microgaps has been determined in different taper connections from 30 lm down to ca. 0.1 lm spaces de- pending on the desgin and the applied force vector [29]. Cyclic fatigue has been shown to enlarge, in other words, to \open up" these microgaps permanently [30]. This work is an extension to these radiographic observations: by applying phase contrast X-ray microtomography (XMT), the entire conical interface across the IAC is S. Zabler et al.: Fatigue induced deformation of taper connections in dental titanium implants Int. J. Mat. Res. (formerly Z. Metallkd.) 103 (2012) 2 207 a University of Würzburg, Department of Physics and Astronomy – LRM, Germany b Charité University Medicine, Department of Oral- and Maxillofacial Surgery, Germany c European Synchrotron Radiation Facility, Grenoble, France d University Medicine Freiburg, Department of Oral- and Maxillofacial Surgery, Germany  2012 Carl Hanser Verlag, Munich, Germany www.ijmr.de Not for use in internet or intranet sites. Not for electronic distribution.