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chipping of the insert. Therefore a test set-up was developed where an almost slightly tiltedseating of an insert in a metal cup is caused by a soft particle to show the possibility ofdisturbed but nearly untilted seating and examine its effect on burst strength of the ceramic part.METHODS:From a technical point of view the soft tissue, e.g. fat tissue, muscle fibres, ligamentsetc., consists of a lot of different materials with differences in their mechanical properties. Dueto the fact that the stiffness of soft tissue is extreme low in comparison to metal alloy andceramics a direct load transfer by the soft tissue leading to point loading and a fracture of theceramic insert seemed to made less sense. Instead, the deformation while keeping a constantvolume, which is common to soft tissue due to its large content of water, seemed to have alarger effect. Therefore a soft material with volume constant deformation properties was chosenfor the in-vitro modelling. The soft tissue disturbing the taper-locking surfaces was simulated bypieces of silicone rubber. For the experiments a silicone rubber of a hardness of 30 (Shore A)was selected. Rubber of this hardness is strong enough to be easily cut in appropriate smallpieces of 1x1x2 mm 3 and placed between insert and metal cup taper. On the other hand thematerial is soft enough for representing soft tissue during mechanical loading of the insert.Figure 1: left: Ceramic insert with silicone particle enforcing a slightly tilted implant situation;right: magnified viewThe burst testing was done by realizing a load angle of 80° instead of 90° according to ISO7206-10. The shifting of the load angle was done to maximize the stresses in the rim of theceramic insert. Assuming a contact of insert and metal cup at the upper taper end under normalconditions and at the lower taper end in the area of the soft tissue contamination the line ofcontact between the parts is tilted around 10° degrees with respect to the front surface of theinsert. Shifting the load angle for the same 10° degrees leads to a loading perpendicular to thezone of contact, which maximizes the stresses in the parts.Figure 2: Test set-upRESULTS:The average of the burst loads is far below the values known from the standard burst testingaccording to ISO 7206-10. In some cases single values are below the minimum limit of 20 kNof the “FDA guidance document for the preparation of premarket notifications for ceramic BallHIP systems, draft from 10th January 1995”.Burst testing according to ISO 7206-10 leads to nearly symmetric fracture patterns. Incontrast to that, the fracture patterns of burst testing with soft particle disturbing the taper-lockshow a half-moon-like shape which is comparable to chip-off fractures which occurred intraoperatively.Figure 3: Fracture patternDISCUSSION:The burst tests with the tilted inserts showed a clear drop of the burst load compared to thecorrectly impacted inserts which underlines the significance of an untilted impaction. The largestandard deviation shows that even in case of impacting the insert in tilted state without fractureoccurrence the remaining risk of an in-vivo fracture later on may be on a very high level.file:///E|/ISTA2010-Abstracts.htm[12/7/2011 3:15:47 PM]