Cortical bone viscoelasticity and fixation strength of press-fit femoral stems: an in-vitro model.
Cementless total hip femoral components rely on press-fit for initial stability and bone healing and remodeling for secondary fixation. However, the determinants of satisfactory press-fit are not well understood. In previous studies, human cortical bone loaded circumferentially to simulate press-fit exhibited viscoelastic, or time dependent, behavior. The effect of bone viscoelastic behavior on the initial stability of press-fit stems is not known. Therefore, in the current study, push-out loads of cylindrical stems press-fit into reamed cadaver diaphyseal femoral specimens were measured immediately after assembly and 24 h with stem-bone diametral interference and stem surface treatment as independent variables. It was hypothesized that stem-bone interference would result in a viscoelastic response of bone that would decrease push-out load thereby impairing initial press-fit stability. Results showed that push-out load significantly decreased over a 24 h period due to bone viscoelasticity. It was also found that high and low push-out loads occurred at relatively small amounts of stem-bone interference, but a relationship between stem-bone interference and push-out load could not be determined due to variability among specimens. On the basis of this model, it was concluded that press-fit fixation can occur at relatively low levels of diametral interference and that stem-bone interference elicits viscoelastic response that reduces stem stability over time. From a clinical perspective, these results suggest that there could be large variations in initial press-fit fixation among patients.
Norman
T L
TL
Department of Engineering, Cedarville University, 251 N. Main Street, Cedarville, OH 45314, USA. tnorman@cedarville.edu
Ackerman
E S
ES
Smith
T S
TS
Gruen
T A
TA
Yates
A J
AJ
Blaha
J D
JD
Kish
V L
VL
D000328:Adult; D000368:Aged; D003198:Computer Simulation; D004548:Elasticity; D019544:Equipment Failure Analysis / Q000379:methods*; D005269:Femur / Q000503:physiopathology* / Q000601:surgery*; D020342:Finite Element Analysis; D017276:Friction; D006622:Hip Prosthesis; D006801:Humans; D066298:In Vitro Techniques; D008875:Middle Aged; D008954:Models, Biological; D011312:Pressure; D019919:Prosthesis Implantation / Q000379:methods*; D013314:Stress, Mechanical; D014783:Viscosity