The Role of Mechanobiology in the Attachment of Tendon to Bone - Structural Interfaces and Attachments in Biology - page 221

Fig. 11.1 Tendon attaches to bone across a functionally graded fibrocartilaginous transition site.
A schematic of the enthesis is shown in ( a ) and a toluidine blue-stained section from a rat
supraspinatus tendon-to-bone insertion is shown in ( b )
proteoglycans decorin and biglycan [ 2 , 11 ]. The tissue then transitions to a more
fibrocartilaginous morphology with rounder cell morphologies. The most abundant
collagens are types II and III, but collagen types I, IX, and X are also present along
with the proteoglycans decorin and aggrecan [ 11 - 15 ]. The mineralized fibro-
cartilage zone is characterized by the onset of mineralization, but it is distinct
from the underlying bone. The matrix is still characteristic of cartilage; rich in types
II and X collagen and aggrecan [ 12 - 16 ]. The final zone, the underlying bone,
consists of highly mineralized type I collagen.
While useful for making qualitative comparisons, this description of the insertion as
consisting of discrete “zones” is likely an oversimplification. Rather than discrete
zones with abrupt transitions, the insertion consists of a graded morphology that is
critical to transferring muscle forces from tendon to bone without large elevations in
stress at the interface. Instead of an abrupt transition at the interface between the un-
mineralized fibrocartilage and mineralized fibrocartilage regions, there is a gradual
increase in mineral content. This mineral gradient was first described in a rat rotator
cuff model as a nearly linear increase over a narrow region within the insertion [ 17 ]. A
graded interface has also been identified at the cartilage-bone interface [ 18 ], suggesting
that this strategy of using mineral gradients to dissipate stress at bone-soft tissue
interfaces is also a feature common to other types of orthopaedic interfaces.
Microstructural variation in collagen fiber alignment is also a feature of the
mature insertion. Using a rat rotator cuff model, collagen fiber alignment was
measured with quantitative polarized light microscopy [ 12 ]. This experiment
demonstrated that collagen fiber alignment varies across the insertion from well
aligned in tendon through an increasingly disordered region of fibrocartilage before
becoming again more aligned in the underlying bone.