The Challenge of Attaching Dissimilar Materials - Structural Interfaces and Attachments in Biology - page 11

torsional fatigue loading. A comparison between coated and uncoated (control)
specimens demonstrated a significant improvement of the bond strength available
using a silica/silane coating. Thus, introduction of a coating between metal and
bone may improve contact and reduce loosening in hip arthroplasty. Notably, while
the dental interface discussed in Sect. 2.1 was graded, satisfactory results in the
present case were achieved by merely introducing an interface, without a direct
intentional grading. Ways that an interlayer might be beneficial in the context of
alleviating stress singularities are discussed in Chap. 3.
1.2.2.3 Tendon-to-Bone Attachment
As an example case of a clinically relevant attachment, the mechanics of the
tendon-to-bone insertion will be explored in the remainder of this chapter. Later
chapters will review the current surgical approaches for repairing tendon to bone,
and the lackluster clinical outcomes that have been achieved to date (Chap. 12 will
review the repair of the rotator cuff to the humeral head and Chap. 13 will review
anterior cruciate ligament reconstruction, which requires tendon graft healing in a
bone tunnel). Chapter 3 will discuss the mechanics of this attachment in greater
detail. The mechanical properties of tendon and bone are vastly different; bone has
a modulus on the order of 20 GPa and tendon has an axial modulus on the order of
450 MPa and a transverse modulus on the order of 45 MPa. This material mismatch
presents significant challenges for stress transfer between the two tissues. These
issues are directly relevant to surgical considerations as well as for potential
engineering biomimetic designs. Surgical and rehabilitation considerations as
well as future biologic approaches are discussed in Chaps. 11, 12, and 13. Several
subsequent chapters then address the potential for biomimetic designs for enhanced
tendon-to-bone repair. Chapter 14 reviews functionally graded approaches for
interface tissue engineering. Chapter 15 presents approaches for synthesizing
fibrous tissues for attachment of tendons and ligaments to bone. Finally, Chap. 16
discusses fabrication of layered scaffolds for the formation of tendon/ligament- and
cartilage-to-bone insertions. The methods discussed in these chapters include
designs based on the optimization of cell types, development of scaffolds, and the
use of exogenous factors (including soluble growth factors and bioreactors) to
successfully regenerate both tendons/ligaments and their interfaces with bone.
The transfer of the load between tendon and bone has been considered in a
number of studies (e.g., [ 14 - 16 ]). The studies were motivated by the observation
that once damaged, a resilient tendon-to-bone insertion is not regenerated after
healing. This is reflected in the extraordinary high re-tear rate after surgical repair
of rotator cuffs, ranging from 20% to 94% (with the range dependent on the extent
of the initial injury and the age of the patient) [ 17 ]. While the uninjured tendon-to-
bone insertion site is characterized by variations in properties from tendon to bone
that are highly anisotropic, the scar tissue forming at healed attachments is isotro-
pic. Furthermore, the compliant region that is present at the uninjured attachment
site, as discussed below, is not regenerated during healing. These differences