Biomechanics of the Cruciate Ligaments 17and unloading, stress–strain curves eventually
synchronize, which underscores the importance
for pre-conditioning before experimental liga-
ment biomechanical testing.
Biologic factors
There are a number of non-disease-related fac-
tors that affect ligament biomechanical prop-
erties, including age, body weight, pheno-
type, whether animals are gonadectomized,
and use/disuse (Laroset al.1971; Vasseuret al.
1985; Duvalet al. 1999).
In dogs, the effect of age on CrCL mechan-
ical properties has been of particular interest.
Skeletal maturity has been shown to increase
strength, stiffness and other mechanical proper-
ties of ligaments. In a rabbit model, stiffness and
ultimate strength dramatically increase from 6
to 12 months of age, followed by insignificant
changes from 1 to 4 years of age (Wooet al.
1990). However, significant weakening occurs
in the CrCL with age. Decreases in modulus,
ultimate stress and strain energy density have
been reported, particularly in dogs weighing
more than 15 kg (Vasseuret al. 1985).
It is recognized that excessive body condi-
tion is a risk factor for cruciate ligament rup-
ture (see also Chapter 14). Specific breeds of
dog are also at increased risk, such as the New-
foundland, Labrador Retriever, and Rottweiler,
while others, such as the Greyhound, are rela-
tively protected from the condition (Whitehair
et al. 1993; Duval et al. 1999), suggesting a
genetic component to disease predisposition
(Baird et al. 2014). Additionally, histologic,
metabolic, anatomic and immune cell popula-
tions have also been considered possible risk
factors (Wingfieldet al. 2000; Comerfordet al.
2005; Comerfordet al. 2006; Muiret al. 2007;
Ragetlyet al. 2011). Neutering of either gender
has been reported to increase the prevalence of
CrCL rupture (Slauterbecket al. 2004), although
recent studies have reported conflicting results
regarding both neuter status and gender predis-
position (Adamset al. 2011; Griersonet al. 2011)
(see also Chapter 14).
As is seen with bone, immobilization results
in significant decreases in ligament strength. A
study using a 9-week rabbit stifle immobiliza-
tion model showed that CrCL cross-sectional
area is significantly decreased and ultimate
strain significantly increased with immobiliza-
tion, although decreases in modulus were not
significant, and no changes in ultimate stress
were found (Newtonet al.1995). Pelvic limb
disuse also results in impaired ligament heal-
ing; the CrCL of male rats that underwent
hindlimb suspension for 3 weeks had signif-
icantly decreased ultimate strength, ultimate
stress, and elastic modulus versus controls
(Provenzanoet al. 2003). Relatively few stud-
ies have been performed studying the biome-
chanical effects of remobilization on previously
immobilized ligament, but over time material
properties appear to be restored before struc-
tural properties (Wooet al.1987).Sites of ligament rupture
In human beings and dogs, most CrCL/anterior
cruciate ligament (ACL) ruptures occur through
the body of the ligament. Similar to CrCL rup-
ture in dogs, most ACL ruptures in human
beings occur via a non-contact mechanism
(Ciminoet al. 2010). More recent evidence has
also suggested genetic predisposition for dis-
ease development (Rahimet al. 2014; Stepien-
Slodkowskaet al. 2016; Svobodaet al. 2016).
It has been shown in dogs with normal stifles
that tensile tests of the femur–CrCL–tibia com-
plex result in avulsion factures of the CrCL tib-
ial attachment site, as opposed to tears through
theCrCLbody(Kleinet al.1982). This sug-
gests that the mechanism resulting in CrCL rup-
ture in the majority of dogs is due to a patho-
logic process that results in the degradation of
CrCL material properties. Much work has been
focused on the specific histologic changes that
occur in ligaments that have ruptured, such
as the development of chondroid transforma-
tion of ligament fibroblasts, alterations to colla-
gen structure, changes in collagen fiber crimp,
and loss of ligament fibroblasts. However, the
cause and timeline of these changes and how
they relate to the progression of the cruciate lig-
ament rupture condition and development of
CrCL rupture remains unclear (Vasseuret al.
1985; Naramaet al. 1996; Hayashiet al. 2003).
It should be noted that avulsion fractures do
occur clinically in dogs, particularly immature
dogs. Such fractures are typically associated