Front Matter

Chapter 3 Musculoskeletal Structure and Physiology 67

acetabular ligament) originate and insert on the
same bone. Ligaments function primarily to
stabilize joints and direct and constrain joint
motion. Two basic forms of ligaments are
recognized. Capsular ligaments (e.g., collateral
ligaments of most joints) are collagen‐rich
thickenings of a joint capsule. Intra‐articular
ligaments (e.g., the round ligament of the
femoral head) cross joints within synovial
compartments. Intra‐articular ligaments are
surrounded with a thin epiligament that
merges  with the periosteum at both origin
and insertion.
Tendons and ligaments have similar histo­
logical structure and biochemical composition
(Figure  3.19). Both contain small numbers of
fibroblastic cells that are aligned with the direc­
tion of the collagen fibrils. The ECM is rich in
type I collagen and contains a variety of PGs
and variable quantities of elastin. The diameter
of collagen fibrils within tendons and liga­
ments is usually bimodally distributed (Frank
et  al., 1999). Larger diameter fibrils provide
higher tensile strength, while small diameter
fibrils increase the overall fibrillar surface area.
A bimodal distribution of fibril diameters leads
to increased density of fibril packing within a
given tendon cross‐sectional area and allows
the mechanical properties of a tendon to be fine
tuned through variations in interfibrillar

crosslinking as well as the composition of the

interfibrillar matrix.

Tendons and ligaments are highly aniso­

tropic and viscoelastic. They have high strength

and stiffness when loaded in tension but buckle

readily when loaded in compression. The

stress–strain relationships in tendons and

ligaments have been well characterized

(Figure  3.20). The initial elongation at the toe

region of the curve results from the straighten­

ing of fibrillar crimps. The linear elastic region

describes reversible elongation that occurs due

to direct fibrillar elongation or interfibrillar

shear; rapid loading causes fibrillar elongation,

while slow or sustained loading results in inter­

fibrillar shear and either stress relaxation or

creep. The yield point represents rupture of

interfibrillar bonds and permanent distortion

of collagen alpha‐helices. Macroscopic failure

of a tendon or ligament involves complete

mechanical disruption of the interfibrillar

matrix and radical interfibrillar shear with pull‐

out of individual fibers (Benjamin et al., 2008).

Entheses are specialized structures at the

attachment sites of tendons or ligaments upon

bone (Benjamin et  al., 2006; Shaw & Benjamin,

2007). Two basic forms of entheses are recog­

nized. Fibrous entheses occur where muscles

attach directly to diaphyseal bone. Fibrous

entheses are made up of collagen fibers

Figure 3.19 Histological appearance of canine cranial
cruciate ligament. Organized fascicles of collagen fibers
(C) are covered by a thin synovial epiligament (E).
Tendons have a similar structure and histological
appearance.

Stress

Microscopic damage

Rupture

To e

region

Strain

Straightening of fibrillar crimps

Figure 3.20 Stress–strain relationships of tendons and

ligaments. Initial elongation at the toe region reflects

straightening of the crimped collagen fibrils. Irreversible

deformation at the yield point occurs due to interfibrillar

shear. Failure involves loss of interfibrillar adhesion and

pullout of collagen fibers.