Ligament and Tendon Enthesis: Anatomy and Mechanics - Structural Interfaces and Attachments in Biology - page 78

motion table. The laser sensor is attached to the rotary motion table and moves in a
circle as the table rotates around the stationary specimen, which is longitudinally
mounted along the table's center of rotation. Figure 4.9b shows an overhead view of
the path of the laser. As it moves, the sensor measures the distance to the specimen
surface and calculates values of a radius, r , the difference between the total and
measured radii of the system.
By plotting r along with the angular position of the laser system in polar
coordinates, an algorithm using Simpson's rule can be used to calculate the cross-
sectional area:

Area ¼ X

360 2

0 05 pðrðfÞþrðf þ 0 05 ÞÞ

: :

(4.1)

:

1440

f¼ 0 : 05

4.5.2 Measurement of Mechanical Properties of Tissue

The early development of a VDA system to determine ligament strain of the rabbit
FMTC during uniaxial tensile testing has yielded accurate, reproducible, and
automatic measurements [ 8 ]. With proper measurement of the cross-sectional
area of the ligament, the stress in the ligament could also be determined, and the
stress-strain relationship would represent the mechanical properties of the ligament
substance. A typical stress-strain curve is shown in Fig. 4.10 , illustrating a charac-
teristic shape and associated mechanical properties.
However, it should be noted that the tissue specimen must have uniform cross-
sectional area, an appropriate aspect ratio, proper alignment, and uniform stress in
order to obtain accurate stress-strain values. In the case of the MCL, obtaining a
uniform stress distribution is rather straightforward, as it has a relatively uniform
cross section. On the other hand, the complex anatomy of the ACL presents
difficulties which make simultaneous loading of all the collagen fibers of the entire
ligament nearly impossible [ 25 ]. Thus, for mechanical testing, it is usually
separated into medial and lateral bundles, and one bundle is chosen to be transected
and removed. The other bundle is then rotated for longitudinal alignment for tensile
testing.

4.5.3 Tissue Strain vs. Percent Elongation

To describe the change in length of ligaments and tendons in response to tensile
loading, both strain and percent elongation have been reported in the literature. The
difference in these parameters is that strain refers to the tissue substance, while