The Role of Mechanobiology in the Attachment of Tendon to Bone - Structural Interfaces and Attachments in Biology - page 236

Blitz et al . used genetically modified mice with muscular defects to examine the
role of muscle loading on enthesis development [ 111 ]. Consistent with the studies
described above, this study demonstrated that while muscle loading was not
required for initiation of enthesis formation, it was necessary for the subsequent
growth and maturation of the enthesis.
11.5 Mechanobiology in Tendon-to-Bone Healing
Repair of ruptures at the tendon-to-bone insertion is a persistent problem in
orthopaedic surgery. Rotator cuff tears are a common injury to the upper extremity
and involve reattaching tendons to the humeral head [ 120 ]. ACL injuries are also
very common and surgical reconstruction techniques use tendon grafts as ligament
replacements that must heal in bone tunnels [ 121 ]. Tendon and ligament injuries
can be classified as acute or chronic ruptures. Acute injuries are generally the result
of extrinsic factors, while chronic injuries also involve intrinsic factors such as
tissue degeneration and a predisposition toward injury. While acute tendon-to-bone
ruptures usually have reasonable healing outcomes, the problem of healing in the
case of a chronic injury is confounded by significant degeneration of the transitional
tissue that extends to the tendon and underlying bone. Several studies have
indicated that nearly all Achilles tendon ruptures have histological evidence of
degeneration [ 122 ]. In contrast to acute trauma of a healthy tendon, this type of
injury can result from the compounded effects of aging and overuse, often from a
lifetime of repetitive motion. Increased loading from overuse disrupts the tissue
homeostasis, alters tissue composition, and results in functionally inferior tissue
mechanical properties leading to injury.
The role of mechanobiology in healing is less clear than the role of
mechanobiology in development. Insights from development indicate that
mechanical forces are critical for establishing a functional tissue. However,
these forces are precisely modulated; cellular responses to physical stress lead
to changes in tissue structure and function, ultimately determining tissue mechan-
ical properties. A stress-adaptation feedback loop then leads to further remodeling
of the local matrix. The same principles apply to healing, where outcomes are
affected by a myriad of chemical, biological, and mechanical factors. The healing
situation, however, is complicated because the ideal loading environment that
leads to optimal healing is not well defined. Furthermore, the immune response to
injury complicates the biological mechanotransduction outcomes. Optimization
of healing will require a more precise understanding of the mechanisms of cell
responses to varying loading conditions. This presents a challenge for orthopaedic
surgeons attempting to optimize post-operative healing through rehabilitation-
controlled loading.