Morphological Risk Factors for Cruciate Ligament Rupture 77of TPA on CrCL stressesin vivois currently
unknown, as muscular force, body size, obesity,
rapid weight gain, relative inactivity, and exer-
cise can all modify the amount of stress sus-
tained by the CrCL, in addition to the TPA (Col-
borneet al. 2005). The TPA was more likely to be
predictive of CR in predisposed pelvic limbs, in
combination with morphometric characteristics
such as the FAA (as described above) and body
fat composition (Ragetlyet al. 2011). Muscular
imbalance may also be a risk factor in the devel-
opment of CR in at-risk breeds, where atrophy
of the quadriceps muscle may lead to an over-
dominance of the gastrocnemius increasing cra-
nial tibial thrust in predisposed pelvic limbs
(Mostafaet al. 2010).
Development of the tibial tuberosity and
the shape (convexity) of tibial condyles may
also be relevant in the pathogenesis of CR
(Guerreroet al. 2007). Smaller tibial tuberosity
widths are thought to increase CTT and pro-
mote CrCL degeneration, thus leading to rup-
ture in a younger population of dogs (Inauen
et al. 2009). In a multivariate analysis performed
by Mostafa and others in Labrador Retrievers,
cranial angulation of the proximal portion of the
tibia, excessive steepness of the tibial plateau,
and distal femoral torsion appeared more likely
to be associated with CR than femoral angula-
tion, tibial torsion, intercondylar notch stenosis
and increased inclination of the patellar tendon
(Mostafaet al. 2009).
In human beings, controversy also exists over
whether posterior tibial slope is an anatomic
risk factor for ACL injuries. Meister and others
failed to demonstrate any relationship between
non-contact ACL injuries and the posterior tibia
slope (Meisteret al. 1998). However, in more
recent studies, tibial slope has been found to be
a risk factor for ACL rupture (Sonnery-Cottet
et al. 2011; Sturnicket al. 2015).
Patella/quadriceps mechanism
and Q-angle
Conformational abnormalities of the distal
femur, such as genu varum and malalign-
ment of the quadriceps mechanism associated
with medial patellar luxation (MPL), can cause
increased internal stress on the CrCL (Duval
et al. 1999). In human beings, the quadriceps
(Q-angle) may contribute to an increased risk of
ACLinjury (Shambaughet al. 1991), although its
role in non-contact ACL injuries is unclear. The
Q-angle is the angle formed by a line directed
from the insertion of the m. rectus femoris on
the ilium to the middle to the trochlear groove
and a second line from the tibial tuberosity to
the trochlear groove mirroring the course of the
patella tendon. In normal dogs, the Q-angle is
reported to be 10.5◦medially and can increase
with different grades of MPL (Mortariet al.
2009), increasing up to 36.6◦for a grade III MPL.
The average Q-angle in dogs with isolated CR
was reported to be 19.3◦in this study, because
of loss of CrCL restraint on internal rotation of
the tibia (Kaiseret al. 2001). Ragetly and others
did not find any effect of Q-angle in their mul-
tivariate approach to predict CR risk in predis-
posed Labrador Retriever pelvic limbs (Ragetly
et al. 2011).
The angles between the patellar tendon
and the tibial plateau have also been mea-
sured in humans (Nisell 1985) and in healthy
dogs (Dennler et al. 2006) as a function of
stifle joint flexion. Dogs with partially torn
CrCLs, anatomically, have marginally larger
angles between the patellar tendon and tibial
plateau compared with joints with intact CrCLs
(Schwandtet al. 2006).Fabella
A recent study found that 70% of West High-
land White Terriers had a mediodistal posi-
tion of their medial fabella, compared to other
small- and large-breed dogs. This finding was
not associated with MPL or CR, and is consid-
ered to be a normal anatomic finding for this
breed (Storket al. 2009).Conclusion
Stifle joint morphology, in particular the distal
femoral ICN, femoral anteversion, the TPA and
muscular imbalance, may play a contributing
role in CrCL degeneration and CR in the dog.
The ICN has been shown to be smaller in dog
breeds with an increased predisposition to CR,
resulting in an altered ECM turnover. Morpho-
metric characteristics of the anatomy of the stifle