Ligament and Tendon Enthesis: Anatomy and Mechanics - Structural Interfaces and Attachments in Biology - page 79

Fig. 4.10 Typical stress-strain curve showing the mechanical properties of the MCL

percent elongation describes the entire bone-ligament-bone complex. Strain is
defined as:

l l 0
l 0

e ¼ (4.2)

where l o represents the initial distance between gauge markers on the tissue
substance and l represents the final distance. In contrast, percent elongation is
equal to the crosshead displacement of the material testing machine divided by
the estimated original length of the tissue specimen [ 8 ]. Previously, researchers did
not distinguish between these two values, often reporting percent elongation as
strain with values well outside the true limits of ligaments and tendons. For
example, Noyes and associates erroneously reported the average strains of the
ACL at failure to be 50-60% [ 26 , 27 ].
Using the previously described methods and technology for biomechanical
testing, large percent elongation values based on clamp-to-clamp measurements
of ligaments and tendons (on the order of 20-50% at failure) with much smaller
strains (less than 20%) could be demonstrated. Noyes et al. [ 28 ] studied the strain
distribution of human ACL subbundles using load-to-failure testing, finding the
percent elongation to be 25-30%. However, in measuring local surface strain, they
found a large spatial variation in strain, with the highest strain at the insertion sites
and strain as low as 7% in the midsubstance tissue. Studies in our research center
have produced analogous results, as the strains at failure for the MCL substance in
dogs, swine, and rabbit were found to be 14, 12, and 7%, while percent elongation
of the FMTC was 21, 30, and 16% [ 8 ]. Lam et al. [ 29 ] also achieved similar results
in the rabbit MCL, finding the largest strain rates at the femoral insertion of the