Biomedical Engineering Reference
In-Depth Information
The structure of the mature insertion shares some similar features to the growth
plates of developing bone [ 19 ]. The growth plate is commonly divided into regions
based on mineralization state and cell morphology [ 20 , 21 ]. The reserve zone
consists of small chondrocytes with little organization that are induced to prolifer-
ate in response to growth inducing stimuli. Cells of the proliferative zone are
organized into columns parallel to the long axis of the bone. Near the top of the
columns, cells become flattened in morphology while cells closer to the metaphysis
become hypertrophied and more spherical in morphology. In this hypertrophic
zone, cells become filled with calcium that begins to mineralize the matrix
surrounding the cells. The late hypertrophic zone is characterized by chondrocytes
undergoing cell death, interspersed with newly formed vascular channels that allow
osteoclasts and osteoblasts to remodel the mineralized matrix to form bone. The
fibrocartilage at the tendon enthesis also contains cells with a more rounded
chondrocyte-like morphology that are arranged into stacks perpendicular to and
spanning the mineralized interface [ 22 ]. Similar to the insertion, the growth plate
hypertrophic zone is characterized by graded variations of cell morphology, struc-
ture, and ECM composition, including mineralization.
The mature tendon-to-bone insertion has unique characteristics that contribute to
the mechanical performance of the interface. The transitional tissue within a tendon
enthesis utilizes a range of structural and compositional variations across a number
of length scales to effectively transfer muscle loads for joint motion. Strategies for
reducing stress concentrations at the insertion include a shallow tendon attachment
angle at the bone interface, shaping of transitional tissue morphology (i.e., splaying),
interdigitation of transitional tissue into bone, a compliant region, and functional
grading of the transitional tissue [ 12 , 23 - 25 ]. These factors alone or in concert are
all capable of reducing stress singularities at the tendon-to-bone interface.
11.2.2 The Mechanical Consequences of Morphological
Gradients
The structural and compositional variations described above have mechanical
consequences. The microstructural arrangement of collagen fibers across the
enthesis reduces stress and strain concentrations at the attachment [ 25 ]. Using a
finite element model, it was demonstrated that the pattern of fiber alignment at the
rotator cuff enthesis reduces stresses at the interface via a disorganized compliant
region. Whereas engineering practice would be to interpolate between the mechan-
ical properties of dissimilar materials such as tendon and bone, experimental
evidence indicates that natural insertions contain a region that is more compliant
than either the soft tissue or bone. The existence of this compliant region has been
verified in several orthopaedic tissues. Biomechanical tensile tests on rat
supraspinatus tendons indicated that the tissue near the insertion had a lower
stiffness than the tendon midsubstance [ 12 ]. Stouffer et al . measured increased
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