Ligament and Tendon Enthesis: Anatomy and Mechanics - Structural Interfaces and Attachments in Biology - page 73

Fig. 4.6 Histogram of modes of failure of human femur-ACL-tibia complex (FATC) tested from
the younger, middle, and older donors studied (reproduced, with permission, from [ 9 ])

alterations with age, it appears that sufficient organelles remain in a few cells of
each layer to maintain viability.
Biomechanical studies of soft tissue-bone junctions in cadaver specimens from
donors over than 50 years of age have shown significant reductions in the structural
properties of the femur-ACL-tibia complex (FATC). In our research center, we
have found that the failure mode changed with age. In younger donors, tibial
avulsion was much more common, whereas in older donors, there were more
midsubstance failures (Fig. 4.6 ). This shift in failure mode was accompanied with
a decrease in linear stiffness, ultimate load, and energy absorbed at failure for
the older group, suggesting that there is a more rapid deterioration of the ACL
substance with increasing age [ 9 ].
It should be noted that this unusually fast degradation of the ACL is unique to
this ligament, as the FMTC showed insignificant change in stiffness or ultimate
load with age in the rabbit model [ 14 ]. This illustrates the uniqueness of each
ligament in its growth, development, and aging. Investigators should thus be
cautious when extrapolating age-related changes from one ligament to another.

4.4 Stress and Motion-Dependent Homeostasis

4.4.1 Immobilization

Even short periods of immobilization have deleterious effects on tendons,
ligaments, and bone, as well as the insertion sites. Osteoclasts at the bony insertion
have been found to resorb bone and sever the deep fibrous attachment, severely
weakening the insertion and rendering it susceptible to failure by avulsion, as it