The Challenge of Attaching Dissimilar Materials - Structural Interfaces and Attachments in Biology - page 10

Fig. 1.8 ( a ) Schematic of implant in a femur. ( b ) Representative slices of the implanted stem
surrounded by bone cement. ( c ) Cement mantle cracks generated from “sharp-edge” corners. ( d )
Debonding along the metal-bone cement interface (adapted, with permission, from [ 13 ])
Consider, for example, the attachment of stems to the femur for total hip
arthroplasty. Stress shielding can result in bone loss if stresses are poorly distributed
along the bone-metal interface. In the case of a noncemented stem, this problem
can be alleviated if the fixation is implemented along the entire length of the stem
surface [ 11 ]. The use of hydroxyapatite coatings may enhance bone ingrowth and
reduce bone resorption, without a detrimental effect on the stress distribution. One
of the solutions proposed to alleviate the problem of attaching metal to bone
employed a graded interface between the metal and bone materials as is exemplified
by cemented hips (Fig. 1.8 ). The cement (polymethylmethacrylate) is first applied
to the hip to avoid a direct linking between the bone cement and metal of the
artificial hip. The tendency of the metal-cement interface to debond is alleviated
through the use of a silane-coupling coating of metal that enhances the hydrolytic
stability [ 12 ]. The metal-to-cement bond can further be improved using an addi-
tional silica oxide interlayer that adheres to the oxide on the surface of metal [ 13 ].
In the experiments reported in this study, femur stems covered with a silica/silane
interlayer coating were cemented into artificial femur bones and subject to standard