Cortical bone viscoelasticity and fixation strength of press-fit femoral stems: A finite element model

Many cementless implant designs rely upon a diaphyseal press-fit in conjunction with a porous coated implant surface to achieve primary or short term fixation, thereby constraining inteface micromotion to such a level that bone ingrowth and consequent secondary or long-term fixation, i.e., osseointegration, can occur Bone viscoelasticity, however has been found to affect stem primary stability; by reducing push-out load. In this investigation, an axisymmetric finite element model of a cylindrical stem and diaphyseal cortical bone section was created in order to parametrically evaluate the effect of bone viscoelasticity on stem push-out while controlling coefficient of friction (mu=0.15, 0.40, and 1.00) and stem-bone diametral inteiference (delta=0.01. 0.05, 0.10, and 0.50 mill). Based on results from a previous study, it was hypothesized that stem-bone inteiference (i.e., prcss-fit) would elicit a bone viscoelastic response which would reduce the initial fixation of the stem as measured by push-out load. Results indicate that for all examined combinations of /j, and 15, bone viscoelastic behavior reduced the push-out load by a range of 2.6-82.6% due to stress relaxation of the bone. It was found that the push-out load increased with mu for each value of delta, but minimal increases in the push-out load (2.9-4.9%) were observed as delta was increased beyond 0.10 turn. Within the range of variables reported for this study. it was concluded that bone viscoelastic behaviot; namely stress relaxation, has an asymptotic affect on stem contact pressure, which reduces stem push-out load. It was also found that higher levels of coefficient of friction are beneficial to primary fixation, and that an inteiference threshold exists beyond which no additional gains it) push-out load are achieved.