Mechanical aspects of degree of cement bonding and implant wedge effect.
BACKGROUND
The degree of bonding between the femoral stem and cement in total hip replacement remains controversial. Our objective was to determine the wedge effect by debonding and stem taper angle on the structural behavior of axisymmetric stem-cement-bone cylinder models.


METHODS
Stainless steel tapered plugs with a rough (i.e. bonded) or smooth (i.e. debonded) surface finish were used to emulate the femoral stem. Three different stem taper angles (5 degrees , 7.5 degrees , 10 degrees ) were used for the debonded constructs. Non-tapered and tapered (7.5 degrees ) aluminum cylindrical shells were used to emulate the diaphyseal and metaphyseal segments of the femur. The cement-aluminum cylinder interface was designed to have a shear strength that simulated bone-cement interfaces ( approximately 8MPa). The test involved applying axial compression at a rate of 0.02mm/s until failure. Six specimens were tested for each combination of the variables. Finite element analysis was used to enhance the understanding of the wedge effect.


RESULTS
The debonded stems sustained about twice as much load as the bonded stem, regardless of taper angle. The metaphyseal model carried 35-50% greater loads than the diaphyseal models and the stem taper produced significant differences. Based on the finite element analysis, failure was most probably by shear at the cement-bone interface.


CONCLUSIONS
Our results in this simplified model suggest that smooth (i.e. debonded) stems have greater failure loads and will incur less slippage or shear failure at the cement-bone interface than rough (i.e. bonded) stems.
Yoon
Yong-San
YS
Mechanical Engineering, KAIST, Science Town, Daejeon 305-701, Republic of Korea. ysyoon@kaist.edu
Oxland
Thomas R
TR
Hodgson
Antony J
AJ
Duncan
Clive P
CP
Masri
Bassam A
BA
Choi
Donok
D
D000268:Adhesiveness; D019644:Arthroplasty, Replacement, Hip / Q000295:instrumentation* / Q000379:methods*; D001843:Bone Cements; D002484:Cementation / Q000379:methods*; D003198:Computer Simulation; D055119:Elastic Modulus; D005270:Femur Head / Q000503:physiopathology* / Q000601:surgery*; D006622:Hip Prosthesis; D008954:Models, Biological; D013314:Stress, Mechanical
D001843:Bone Cements