TECHNICAL PAPERS: Bone/Orthopedics

Cortical Bone Viscoelasticity and Fixation Strength of Press-Fit Femoral Stems: An In-Vitro Model

[+] Author and Article Information
T. L. Norman1

Department of Engineering, Cedarville University, 251 N. Main Street, Cedarville, OH 45314tnorman@cedarville.edu

E. S. Ackerman

Programme d’Orthopedie Edouard-Samson, Hospital du Sacre-Coeur, 5400 boul. Gouin Quest, Bureau G-2000, Montreal (Quebec) H4J1C5, Canada

T. S. Smith

 DePuy, A Johnson and Johnson Co., 700 Orthopaedic Drive, Warsaw, IN 46581-0988

T. A. Gruen

 Zonal Concepts, 31153 Shaker Circle, Wesley Chapel, FL 33543

A. J. Yates

 UPMC Orthopedic Surgery, Shadyside Medical Bldg., Suite 415, 5200 Centre Avenue, Pittsburgh, PA

J. D. Blaha

Department of Orthopedics, University of Michigan, Ann Arbor, MI 48109

V. L. Kish

Department of Orthopedics, Musculoskeletal Research Center, West Virginia University, Morgantown, WV 26506-9196


To whom correspondence should be addressed.

J Biomech Eng 128(1), 13-17 (May 06, 2005) (5 pages) doi:10.1115/1.2133766 History: Received July 20, 2004; Revised May 06, 2005

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 24h 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 24h 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.

Copyright © 2006 by American Society of Mechanical Engineers
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Figure 4

Load ratio plotted against stem-bone interference. A significant quadratic relationship exists.

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Figure 5

Push-out loads for each coating at 0 and 24h. The asterisk indicates significant differences at P<0.05.

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Figure 1

Push-out test configuration

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Figure 2

Push-out load vs interference at t=0h

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Figure 3

Push-out load vs interference at t=24h



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