Advances in the Canine Cranial Cruciate Ligament, 2nd edition

Biomechanics of the Cruciate Ligaments 15

(A)

(B)

Figure 2.2 Longitudinal section of intact cranial
cruciate ligament (CrCL) from a 1.5-year-old female
beagle viewed using bright light (A) and circularly
polarized light (B). Intact CrCL from young dogs has a
hierarchically organized structure. Birefringence of the
extracellular matrix collagen and the crimped structure
are clearly visible in polarized light as bright and dark
repeating bands within ligament fibers. Crimp is a
wave-like feature of ligament that exists at the level of the
fiber, and is the primary reason for ligament’s nonlinear
elasticity. Scale bar= 50 μm.

loading in any off-axis direction and/or with
some stifle flexion would result in different stiff-
ness and strength.
All ligaments are viscoelastic in their mechan-
ical behavior. That is, their stretch will increase

during sustained loads (creep), and tensile

load required to maintain a fixed position will

decrease with time (relaxation). While impor-

tant, these biomechanical characteristics are

often considered secondary. Consequently, test-

ing protocols ‘pre-condition’ specimens with

repeated loads and testing at a constant rate of

loading to minimize secondary effects, produc-

ing a standard force versus deformation curve

that can be used for biomechanical comparison.

Force–deformation curve

Structural properties are described using a

force–deformation curve, which can be divided

into three regions (Figure 2.3). The first is the

toe region, which is nonlinear and has a low ini-

tial stiffness; in this region, due to the crimped

nature of collagen fibers, small forces result in

a relatively large degree of lengthening. The

next region is the linear region, which displays

Ultimate Strength

Ultimate Elongation

Linear Slope

700

600

500

400

300

200

100

05 1 0

Deformation (mm)

Force (N)

15

Energy-to-failure

Figure 2.3 Typical force–deformation curve for

ligament during uniaxial tensile testing. The

force–deformation curve of ligament can be divided into

three regions. The toe region is the initial nonlinear part

of the curve, which is primarily due to the existence of

crimp at the level of the fiber. In this region, small forces

result in a large degree of lengthening because of the

crimped nature of the collagen fibers. The second is the

linear region, which occurs after crimp has been

maximally stretched. The linear slope of this region is

reflective of the material’s stiffness. The final region is the

point at which a ligament begins to rupture; the peak of

this part of the curve gives the material’s ultimate strength

and ultimate elongation values.