Biomedical Engineering Reference
In-Depth Information
between the natural and postsurgical attachments likely explain the mechanical
inferiority of the latter tissue. These differences also motivate further study of the
mechanisms of load transfer between the natural tendon-to-bone insertion site.
Four strategies have been identified as contributing to the effectiveness of the
natural tendon-to-bone insertion site [ 18 ]. They include: (1) a shallow attachment
angle at the insertion of transitional tissue and bone, (2) shaping of gross tissue
morphology of the transitional tissue, (3) interdigitation of bone with the transi-
tional tissue, and (4) functional grading of transitional tissue between tendon and
bone. These mechanisms are further discussed in the next section of this chapter and
in Chaps. 3 and 11.
In one study, experimentally measured collagen fiber orientations and mineral
concentration data were used in micromechanically based models to monitor the
stiffness distribution along the insertion site. This distribution complied with
the above-mentioned drop in the stiffness, the lower stiffness region being closest
to the tendon [ 15 ]. The same model predicted an abrupt increase in the stiffness of
the insertion between the mineral percolation threshold and the bone. The rationale
for a decrease in the stiffness remained unclear, however. Subsequent studies were
necessary to demonstrate that the compliant region optimizes distribution of
stresses to reduce stress concentrations [ 19 ]. Using an example of an idealized
rotator cuff insertion site, it was demonstrated that a biomimetically inspired
compliant and functionally graded interface between tendon and bone can lead to
a drastic reduction and even elimination of stress concentrations [ 19 ].
1.3 Prevention of Fracture at the Interface of Dissimilar
Materials: Engineering and Biological Solutions
The interface between dissimilar materials presents a number of challenging
problems to medical doctors, researchers, and engineers alike. In theory, a stress
singularity at the corner of such an interface (Fig. 1.9 ) could cause the onset of
cracking at a stress level that would be benign to each material considered individ-
ually [ 20 ]. The effect of material property mismatches was demonstrated for the
interface between bone-like isotropic and tendon-like orthotropic materials [ 18 ].
The contributions of functional grading, local and gross morphology, and interdig-
itation were studied and discussed in detail in [ 18 ]. The order of the singularity was
shown to be a function of the property mismatch, and to approach a constant order
in the case of a relatively stiff isotropic material attaching to an orthotropic
material. The singularity could be avoided only in the case of a relatively compliant
isotropic material, the observation being interesting for potential applications, but
irrelevant for the tendon-to-bone attachment.
While the previous results refer to fracture along the interface between dissimilar
materials, an improvement in the stress distribution and fracture characteristics
of the attachment may be possible using an interfacial
layer. The beneficial
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