Keywords: mixing technique, fatigue strength, bone cement, stress concentration, theory of critical distances, porosity, implant loosening, biomaterials, porous materials, pore-free materials
The effect of mixing technique on fatigue of bone cement when stress concentrations are present
Mixing techniques used to reduce porosity in bone cement demonstrate large improvements in fatigue behaviour in test specimens, but do not appear to reduce the rate of loosening of implants in vivo. We examined data showing the effect of notches on fatigue strength and found, surprisingly, that notches with low stress-concentration factors (Kt < 2.4) had no effect compared to unnotched specimens. Using the theory of critical distances (TCD) we found the characteristic fatigue strength of the porous material to be similar to the measured fatigue strength of pore-free material; the characteristic length (L) was similar to the average pore size. This allowed us to define a number of different cases, depending on the relative dimensions of the notch and the pores. We concluded that techniques which reduce porosity can be expected to improve fatigue life only in certain cases, which are frequently encountered in test specimens but only rarely occur in clinical situations.