Biomedical Engineering Reference
In-Depth Information
Regardless of graft choice, successful ACL reconstruction relies on incorporation of
the graft into the bone tunnel. All graft choices except patellar tendon grafts require
tendon-to-bone healing within one or both bone tunnels.
13.2 Biology of Normal Tendon-to-Bone Attachments
The attachment of tendon to bone represents a unique biologic and biomechanical
challenge. There is a significant difference between the stiffness of the tendon and
bone, which creates a high level of stress at their interface. In native tendon-to-bone
attachments, this mismatch is dampened by the presence of a unique zone of
transitional tissue at the enthesis. This transitional zone acts to transfer stress
between tendon and bone by graduated change in the structure, composition, and
mechanical properties [ 12 ].
Two types of entheses exist: direct and indirect insertions. Direct insertions
(fibrocartilagenous entheses) have tendon fibers that pass directly into the cortex
over a small surface area. The superficial fibers insert into the periosteum and deep
fibers attach to bone at right angles or tangentially [ 13 ]. The microscopic structure
of direct insertions consists of four zones with gradual transitions between each
zone (Fig. 13.1 ). These zones include: tendon, uncalcified fibrocartilage, calcified
Fig. 13.1 Photomicrographs showing the ( a ) Safranin O staining, ( b ) Hematoxylin & Eosin
staining, and ( c ) polarized microscopic image of the direct tendon-to-bone insertion. Note the
gradual transition of the four zones at the direct tendon-to-bone insertion. Magnification 20; B
bone; CFC calcified fibrocartilage; UCFC uncalcified fibrocartilage; T tendon (reproduced, with
permission, from ref. [ 13 ])
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