Friction in hip-joint prostheses and its influence on the fixation of the artificial head.

The head of an implanted hip joint endoprosthesis is exposed to torques, which are transferred during gait due to the friction between the head and the cup prosthesis. In prostheses with ceramic ball heads, which are widely used now, and in which the head is fixed onto the stem by conical clamping, these torques could possibly affect the connection. In this study, torques transferred from the cup to the head are compared to the torques which are required to loosen the head from the metallic spigot. The results show that for the investigated head and taper types and sizes, under normal conditions the connection is safe with respect to undesired rotation. However, it is shown that for polluted sliding surfaces the fixation strength could possibly be exceeded.

Fatigue of bone cement with simulated stem interface porosity.

Cracks in bone cement have been observed in carefully examined post-mortem preparations of cemented stems. These cracks were probably caused by fatigue, and frequently appeared to initiate at pores. Ubiquitous porosity, occurring preferentially at, or near, the stem, is most likely caused by polymerization shrinkage. Preparation of air-free cement has only a marginal influence on the interface porosity, but pre-heating the stem in order to reverse the direction of polymerization can reduce or eliminate it. To estimate the impact of interface porosity on the fatigue strength of bone cement, test plates for this study were cast in a steel mold without release foils, and with one side of the mold warmer. Sample plates so prepared from chilled, partial vacuum-mixed PALACOS, have one face essentially pore-free and the other porous, the extent and morphology of the porosity being very similar to that observed on the stem-cement interface. Four-point bending fatigue strength, determined after 60 d conditioning in Ringer's solution at 37 degrees C, was only 20 MPa (at 10(6) cycles, with the porous side under tension) compared to 30 MPa for conventionally prepared, pore-free material. This corresponds to a 10-100 fold reduction in cycles to failure in the range of stresses predicted to occur in vivo.

Fatigue behavior of direct post-and-core-restored premolars.

Evaluation of long-term mechanical behavior of new types of restorations in clinical trials is time-consuming. A partial alternative can be found in experimental fatigue-testing, which simulates accelerated mechanical deterioration. The aim of this study was to determine the feasibility of using fatigue-testing of a complex dental restoration and to evaluate the mechanical fatigue behavior of premolar teeth restored with a titanium alloy post and an amalgam or composite core. Eighty-seven human upper premolar teeth were decoronated, embedded, and restored with a prefabricated post of 1 mm diameter. The teeth were randomly assigned to one of two groups corresponding with a core build-up of amalgam or chemically-cured core composite, respectively. Five to 21 days after restoration, the specimens were subjected to cyclic loading (frequency, 5 Hz), at an angle of 45 degrees to the long axis of the tooth. The boundary technique was used for determination of the mean fatigue strengths of the restorations at 10(4), 10(5), and 10(6) cycles, simulating up to 1-3 years of clinical functioning. Mean fatigue strength was expressed in percentage of initial strength: For 10(4), 10(5), and 10(6) cycles, the results were 66%, 58%, and 52%, respectively, for the amalgam and 62%, 62%, and 53% for the composite group. It is concluded that fatigue-testing of more complex systems is possible, if a suitable testing method is selected. The restorations showed a comparable strength reduction after 10(6) cycles of about 50% of their initial strength. The composite core build-up showed a behavior less predictable than that of the amalgam, which might be attributed to handling parameters.

Numerical investigations of the influence of implant shape on stress distribution in the jaw bone.

The stress distribution generated in the surrounding jaw bone was calculated and compared for different types of dental implants (cylindrical, conical, stepped, screw-shaped, hollow cylindrical) by means of the finite-element method. Both a fixed bond and a pure contact without friction between implant and bone were considered as interface conditions. The results demonstrate that different implant shapes lead to significant variations in stress distributions in the bone. In particular, implant surfaces with very small radii of curvature (conical) or geometric discontinuities (stepped) imply distinctly higher stresses than smoother shapes (cylindrical, screw-shaped). Moreover, a fixed bond between implant and bone in the medullary region (as may be obtained with a bioactive coating) will be advantageous for the stress delivered to bone, since it produces a more uniform stress distribution than does a pure contact.

[Contact surfaces, compressive forces and pressure distribution in osteosynthesis. Model studies using pressure-sensitive foil]. / Kontaktflächen, Anpresskräfte und Druckverteilung bei Osteosynthesen. Modelluntersuchungen mit Druckmessfolien.

Previous pressure gauges have been unable to measure exactly the stress over the cross-section of fractures or osteotomies. This first became possible with the pressure sensitive foil. Known operative procedures were examined with this foil using a self-compressing plate and a synthetic tube model. Contact area, compressive force, pressure and pressure distribution were measured simultaneously and analogously. A straight plate produces extremely little force and pressure on the gap. Both are limited to about a quarter of the cross-section just below the plate. The consequence is no sufficient stability. Also, using a prebent plate results in inhomogeneous pressure distribution with high values, but a distinct decrease of the stress on the opposite side of the plate, especially by compression applied at the end of the plate. But compression from near the fracture side gives the highest values and a most homogeneous force distribution. In this case the force recorded 1700 N, the mean pressure 5,2 MPa and the contact 90% of the cross-section. The length thrust of the plate ends approximately at a torque of screw tightening at 1,0 Nm. Using torques up to 2,5 Nm, force and pressure increase isometrically. Higher torques have no influence. Converging screws result in smaller contact, force and pressure, because they produce a vector force away from the gap and a smaller length thrust.

Failure of ceramic hip endoprostheses by slow crack growth--lifetime prediction.

The slow crack propagation velocity v, which depends on the stress intensity factor K, has been measured for three different high-density aluminas. Assuming a load history for the prosthesis which approximates conditions during walking, the lifetimes of the components of a total hip endoprosthesis were calculated as a function of the initial flaw size. Different geometries for the preexisting cracks, as well as varying physiological parameters, were considered. It is shown that the lifetime is very dependent on the quality of the material and that it is reduced by increased body weight, walking speed, and varus positioning. Comparison of failure behavior in different parts of the femoral component showed that flaws in the stem must be one order of magnitude smaller than those in the neck to achieve comparable lifetimes.

[Brush abrasion with 27 commercial composites and their surface changes in the scanning electron microscope]. / Bürstenabrasion mit 27 handelsüblichen Composites und ihre Oberflächenveränderung im Rasterelektronenmikroskop.

Twenty-seven commercially available composites and three amalgams were examined under practical test conditions in terms of their abrasion behavior during brushing. Differences in wear of more than the factor 10 were established. In addition, quite different surface alterations are produced which can be explained by the special abrasion mechanism.