Blaha, J. D.

Brown, C. U.

Gruen, T. A.

Kish, V. L.

Norman, Timothy L.

English

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Cedarville UniversityDigitalCommons@CedarvilleEngineering and Computer Science FacultyPresentations School of Engineering and Computer Science2-26-2001Circumferential Creep of Human Cortical BoneSuggests Threshold for Press - Fit StemsC. U. BrownV. L. KishTimothy L. NormanCedarville University, tnorman@cedarville.eduT. A. GruenJ. D. BlahaFollow this and additional works at: http://digitalcommons.cedarville.edu/engineering_and_computer_science_presentationsPart of the Biomedical Engineering and Bioengineering CommonsThis Conference Presentation is brought to you for free and open access byDigitalCommons@Cedarville, a service of the Centennial Library. It hasbeen accepted for inclusion in Engineering and Computer Science FacultyPresentations by an authorized administrator ofDigitalCommons@Cedarville. For more information, please contactdigitalcommons@cedarville.edu.Recommended CitationBrown, C. U.; Kish, V. L.; Norman, Timothy L.; Gruen, T. A.; and Blaha, J. D., "Circumferential Creep of Human Cortical BoneSuggests Threshold for Press - Fit Stems" (2001). Engineering and Computer Science Faculty Presentations. 115.http://digitalcommons.cedarville.edu/engineering_and_computer_science_presentations/115      Session 19 - Bone Mechanics II - Esplanade Ballroom 303-305, Mon 9:30 AM - 11:00 AM0110               47th Annual Meeting,  Orthopaedic Research Society,  February 25 - 28,  2001, San Francisco, CaliforniaCIRCUMFERENTIAL CREEP OF HUMAN CORTICAL BONE SUGGESTS THRESHOLD FOR PRESS-FIT STEMS*Brown, C U.; *Kish, V L.; +*Norman, T L.; *Gruen, T A.; *Blaha, J D.+*Musculoskeletal Research Center, West Virginia University, Morgantown, WV. PO Box 9196, HSCS, Morgantown, WV 26506-9196, 304-293-1072, Fax: 304-293-7070, tnorman@wvu.eduIntroduction: The degree of contact between a press-fit hip implant and theendosteal surface of the femoral canal is important for achieving initialfixation and clinical success where greater contact improves load transfer andlessens micromotion [1-5]. If the stem initially applies excessive stresses tothe surrounding bone, the bone may fracture [6]. Sufficient contact withoutexcessive stress must be achieved to maintain initial stability and allow newbone growth. However, the contact stresses at the time of implant insertionmay result in creep, time-dependent deformation under constant load [7].Creep due to hoop stress is an under-appreciated phenomenon that maythreaten initial stability by reducing the contact area for load transfer betweenbone and implant before new bone growth can occur [8].The objective of this study is to examine the time-dependent hoopresponse of femoral bone to an intramedullary radial load. The relationshipbetween hoop creep strain, creep rate and permanent hoop strain to hoopstress is examined to understand how femoral cortical bone deforms due tostresses that may occur as a result of a press-fit hip prosthesis.Methods: Ten fresh human femora were obtained from 8 male and 2 femalecadavers (mean age 39 ± 13 years). From these femora a total of 15 rightcircular cylinders 19 mm in length, with a 19 mm inner wall diameter and amean wall thickness of 2 ± 0.05 mm were wet machined. Four unidirectionalstrain gages were mounted on the outer surface of the specimen on theanterior, posterior, lateral and medial aspects; all gages were oriented tomeasure hoop strain. A test fixture applied internal pressure to the specimenwhile submerged in saline at 37°C [9]. A multi-phase loading protocol wasfollowed to observe the creep behavior [7]. The specimens were initiallyloaded to a pressure of 50 psi. The load was increased in 15 psi incrementsduring successive load phases. The internal pressure load was applied in lessthan 2 seconds and was held constant for 1 minute, released and the specimenunloaded for 2 minutes while strain was monitored (Figure 1). The creepstrain was calculated as the difference between the hoop strain measured at thebeginning and at the end of the 1-minute loading phase. The creep rate wasdetermined for each constant pressure phase by linear regression analysis ofcreep strain data during the last 40 seconds of the 1-minute load phase.Permanent strain was measured as the amount of strain remaining at the endof the 2-minute unloading phase.Results: Measured hoop strain values from the posterior cortex were usedfrom each specimen because this quadrant consistently displayed the greateststrain and was the location of the point of failure. To account for differencesin material properties among specimens, the hoop stress was normalized usinghoop stiffness determined at every load level during the tests. A linearregression of the creep strain data against normalized hoop stress was used tomodel the initial behavior (σH/EH<0.0005) (Figure 2). The line was offset,similar to the procedure to determine the yield strength of materials, to theupper bound of the initial data. The offset line passes through the strain dataafter the behavior has become nonlinear. An exponential fit was used to modelthe combined creep strain data for all specimens. The intersection of the linearoffset and exponential fit provides a value of normalized hoop stress, 0.00111,and creep strain, 189µε, at which the creep strain begins to increasenonlinearly. The increase in stress also caused a similar change in behavior forthe creep rate at 1.2µε/s and permanent strain at 93µε.Discussion: The creep strain, creep rate and permanent strain all initiallyexhibit linear behavior, until a particular stress level, or threshold, where itbegins to exhibit nonlinear exponential behavior. Creep strain is occurring atvery low levels of hoop stress, as low as 1.5 MPa. This is much less than thelongitudinal stress level, approximately 50 MPa, needed to cause creep strainfound by Fondrk et al. [7]. Jasty et al. [6] report a range of assembly hoopstrains between 100-2400µε for a press-fit prosthesis; the results of this studysuggest that hoop creep strain may occur at or below this level.The creep rate threshold determined in this study, 1.2µε/s, is less thanthe value of 10µε/s determined by Fondrk et al. [7]. Differences are mostlikely due to the anisotropy of bone. Our creep rate is similar to the creep rateof approximately 1µε/s that can be deduced from Pattin et al. [10] forlongitudinally oriented bovine bone specimens. To determine whether creeprates below, at or above our threshold will eventually lead to failure will bethe subject of long-term creep tests.This study, using a simplified loading environment, provides knowledgeof the extent and rate of hoop deformation. This deformation occurs at lowhoop stress levels and therefore may threaten initial fixation. The change incontact area between the implant and bone may reduce the contact stressesand result in a relaxing mechanism. The creep threshold evident in this studymay be useful as a design parameter and for evaluating the potential successof press-fit hip prostheses.Figure 1. The multi-phase cycles for a typical specimen.Figure 2. Creep strain accumulated during the 1-minute load phase for allspecimens.Acknowledgement: The authors would like to thank Dr. Stanley Wearden forhis statistical advice and Nina Clovis and Suzanne Smith for their labassistance.References: 1. Ring, Clin. Orthop., 1978. 2. Engh, Clin. Orthop., 1983. 3.Morscher, Clin. Orthop., 1983. 4. Robertson et al., Clin. Orthop., 1988. 5.Poss et al., Noncemented Total Hip Arthroplasty, 1988. 6. Jasty et al., J.Arthroplasty, 1993. 7. Fondrk et al., J. Biomech., 1988. 8. Blaha, The AdultHip, 1998. 9. Brown et al., Exp. Tech., 2000. 10. Pattin et al., BED ASME,1995.

Circumferential Creep of Human Cortical Bone Suggests Threshold for Press - Fit Stems

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Circumferential Creep of Human Cortical Bone Suggests Threshold for Press - Fit Stems

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