Ligament and Tendon Enthesis: Anatomy and Mechanics - Structural Interfaces and Attachments in Biology - page 69

Fig. 4.2 Stained gauge lines
on test specimen and video
dimensional analyzer (VDA)
windows for tensile strain
determination (reproduced,
with permission, from [ 18 ])
Determining the biomechanical properties of ligaments and tendons as well as
their insertions is crucial to understanding the structure-function relationship for
purposes of homeostasis for injury prevention, repair, and healing. However, the
complex nature of insertion sites presents significant challenges when the properties
of the insertion site are separated from that of the tissue substance. Variation in
insertion type (direct or indirect), tissue geometry, matrix composition and distri-
bution, and orientation of collagen fibers makes it even more difficult to make
generalizations about properties of ligament and tendon entheses. For example, in
the supraspinous tendon, Thomopoulos and associates [ 10 ] found that collagen
fibers were significantly less organized at the bony insertion compared to the
muscle insertion.
An additional challenge in determining the biomechanical properties of the
ligament or tendon insertion site is the size of the test specimen. For ligaments
such as the MCL, it is necessary to prepare the test specimen with the femoral and
tibial bony attachments because the ligament substance is relatively short, so the
test specimen is that of a bone-ligament-bone complex [ 8 ]. An apparatus was
developed for uniaxial tensile testing using a VDA system to determine nonuniform
properties of the rabbit FMTC. The MCL is stained with several gauge lines for
tensile strain determination of the ligament midsubstance and insertion sites
(Fig. 4.2 ). This data and the output from a tensile load cell on a materials testing
machine produce a load-elongation diagram and stress-strain curve.
A typical load-elongation curve for the FMTC is shown in Fig. 4.3 , characterized
by a toe region, a linear region, and a failure region [ 11 , 12 ]. The nonlinearity of this
curve illustrates the ligament's function in guiding joint motion under low loading
conditions, while limiting excess motion and protecting the cartilage at higher
loads.
Using strain data obtained with the VDA system in the MCL, it was found that
the strain of the ligament substance is consistently smaller than the specific
deformations calculated for the bone-ligament-bone complex [ 8 ]. These results
suggest that the deformations near or at the ligament insertion sites to bone are
larger than in the midsubstance. This large variation of regional strain values along
the ligament substance highlights the possibility that larger deformations near
insertions may predispose these areas to higher incidence of tensile failure at low
stretch rates and stresses the need to characterize and understand the differences in
biomechanical properties between the ligament substance and insertion.