Biomedical Engineering Reference
In-Depth Information
11.2 Structure-Function Relationships at the Attachment
of Tendon to Bone
The attachment of tendon to bone represents a fundamental engineering challenge.
Relative to tendon, bone is a stiff, brittle material with an elastic modulus near 20 GPa
in both tension and compression [
1
]. Conversely, tendon is relatively tough and
extensible with an elastic modulus of 450 MPa in tension, while buckling in compres-
sion [
2
]. The attachment of two dissimilar materials lends itself to stress singularities
at the interface and an increased risk of fracture. In order to overcome these
challenges, the tendon-to-bone insertion is uniquely adapted to provide a smooth
transfer of stress between two dissimilar tissues. To accomplish this, the insertion has
transitional tissue characterized by structural and compositional gradients over a
rangeoflengthscalesthatgiverisetogradedtissuemechanicalproperties.
The risk of elevated stresses leading to failure has important implications for
orthopaedic interfaces, which have high rates of rupture and tearing. Two examples
of interfaces prone to injury are the rotator cuff in the shoulder and the anterior cruciate
ligament (ACL) in the knee. Surgical repair of these tissues is particularly difficult
because the surgeon must overcome the challenge of attaching two materials with
vastly different mechanical properties. This contributes to documented rates of re-
rupture as high as 20% for minor rotator cuff tears and up to 94% for massive rotator
cuff tears [
3
,
4
]. Of note is that the functionally graded transitional tissue of uninjured
insertion is typically not recreated after surgical reattachment and healing [
5
-
7
].
11.2.1 Functionally Graded Morphology of the Mature
Tendon-to-Bone Insertion
From an anatomic perspective, two major types of attachments have been described
for tendon and bone [
8
]. The first type of attachment is termed “fibrous”; these
attachments insert into bone across a wide footprint, presumably to distribute loads
over a large area and reduce stresses. These insertions may include perforating
mineral fibers (“Sharpey's fibers”) that interdigitate into the underlying bone [
9
].
This type of attachment is found at the insertion of the deltoid tendon and at the tibial
insertion of the medial collateral ligament. The second type of attachment is termed
“fibrocartilaginous”; these attachments are found at the bony insertions of the rotator
cuff and Achilles tendons (Fig.
11.1
)[
8
,
10
]. Fibrocartilaginous attachments, which
contain a unique transitional tissue adapted to withstand a complex stress environ-
ment, will be the primary focus of this chapter.
Fibrocartilaginous insertions are typically characterized by the presence of
multiple distinct zones easily identified under the light microscope (Fig.
11.1
)[
8
].
The first zone is tendon proper, the middle zones are un-mineralized and
mineralized fibrocartilage, and the final zone is bone [
8
,
10
]. Each zone is
characterized by a unique profile of cell morphology and ECM composition.
Tendon consists of fibroblast cells with a matrix rich in type I collagen and the
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