Advances in the Canine Cranial Cruciate Ligament, 2nd edition

14 Structure and Function

Size scale

fascicular

membrane

fibroblasts

Evidence:

crimp structure

(45–50 μ)

reticular

membrane

SEM

OM

EM

SEM

OM

x- r ay

EM

SEM

x- r ay

EM

x- r ay

EM

x- r ay

3.5 nm

1.5 nm 10–20 nm

64 nm

periodicity

3.5 nm staining

sites

TROPO-

COLLAGEN

SUB-

FIBRIL FIBRIL

FIBER

FIBER

BUNDLES

MICRO-

FIBRIL

50–500 nm 50–300 μ 100–500 μ

Figure 2.1 Diagram illustrating the hierarchical organization of ligaments and tendons. Ligament has a hierarchical
structure, starting at the level of individual fibroblasts. These cells are arranged to create a series of fibril structures,
which eventually results in the formation of fibers. Fiber bundles or fascicles then combine to create the ligament itself. It
should be noted that tendons also possess this basic hierarchical architecture. EM, electron microscopy; SEM, scanning
electron microscopy; OM, optical microscopy. Source: SEB Symposia XXXIV 1980 The mechanical properties of
biological tissues. Reproduced with permission from the Society of Experimental Biology.

recruitment of fibers to resist load. Crimped
fibers also help to provide resistance at extremes
of joint motion.
The strength of ligament and tendon relates
to the diameter of its composite fibrils; thicker
fibrils have a greater effect on tensile strength,
and are considered determinants thereof
(Ottani et al. 2001). In horses, the effect of
exercise on collagen fibril diameter has been
of great interest, particularly with regards
to superficial and deep digital flexor tendon
injury. These studies have shown that exercise
initially increases fibril diameter, but exercise-
induced microdamage results in a decrease in
fibril diameter (Cherdchuthamet al. 2001).
The CrCL is unique in architecture and func-
tion. It is a continuum of fibers from a dis-
tal cranial medial orientation on the lateral
femoral condyle to the cranial central inter-
spinous area of the tibial plateau. These fibers
are recruited differentially throughout stifle
flexion so that fibers are often defined by func-
tional bundles. Most commonly, two bundles
are described, the cranial medial (CM) bundle

and the caudal lateral (CL) bundle. In exten-

sion, both fiber bundles are tight and parallel,

whereas in flexion the structure twists when

the femoral attachment of the CrCL rolls back.

When this occurs, the CL bundle becomes slack,

while the CM bundle remains load-bearing.

The reduced number of CrCL fibers which

are load-bearing during flexion consequently

makes them more vulnerable to overstretch

damage.

Biomechanical properties

Because ligaments are designed to transfer load

from bone to bone in a longitudinal direc-

tion, uniaxial tensile tests are used to charac-

terize structural and material properties of liga-

ment using force–deformation and stress–strain

curves, respectively. Typically, all CrCL test-

ing is done with the stifle extended to recruit

fibers from both fiber bundles. This provides a

best case scenario for strength and stiffness and

comparable biomechanical properties. Stifle