40 Structure and Function
TRANSLATIONSProximo-DistalROTATIONSMedio-LateralCranio-CaudalInternal–ExternalFlexion–ExtensionAdduction–AbductionFigure 5.1 Line drawing illustrating the six degrees-of-freedom of the femoro-tibial articulation. The femur and the tibia
rotate and translate about the three axes shown in the drawing. Copyright©Samantha J. Elmhurst at http://www.livingart.org.uk.
joint flexes (Mooreet al. 2016). The patella acts
as a pulley mechanism to improve the efficiency
of stifle extension, as it increases the lever arm
of the quadriceps mechanism by lengthening
the distance between the quadriceps muscle
force and the center of flexion–extension rota-
tion of the stifle. In doing so, a significant force
compressing the patella against the femoral
condyle is generated as the muscle contracts.
This retropatellar force is an important contrib-
utor to the stability of the patello-femoral joint,
and results in a force acting on the distal femur
in the cranial–caudal direction. The balance of
extensor and flexor moment acting at the stifle
joint contributes to dynamic stability (Tepicet al.
2002).
Unlike most other diarthrodial joints in the
dog, the bony congruency between the femoral
condyle and the tibial plateau adds little to
the stability of the stifle. Rather, primary and
secondary soft-tissue stabilizers provide stabil-
ity to the stifle. The CrCL acts as primary sta-
bilizer for both cranial–caudal translation and
internal–external rotation. The caudal cruciate
ligament (CaCL) has a primary role in lim-
iting caudal tibial translation, and also helps
to limit excessive internal–external rotation
(Arnockzky & Marshall 1977). The menisci act
as secondary stabilizers. The degree to which
they contribute to joint stability is dependent
on the condition of the primary stabilizers, in
particular the CrCL. In a CrCL-deficient sti-
fle the medial meniscus plays a role in pri-
mary stabilization, acting as a wedge opposing
femoral condyle translation and rotation (Pozzi
et al. 2006). The increased mobility of the lateral
meniscus does not allow it to act as a wedge
between the femur and the tibia, and may pro-
tect it from impingement and tearing. Other
passive stabilizers include the collateral liga-
ments and the joint capsule.
The dynamic stability of the stifle in the var-
ious daily activities is provided by a delicate
interplay between passive stabilizers and active
musculature. The stifle is controlled mostly by
two-joint muscles that cross either the hip and
the stifle, or the stifle and the hock. Contraction
of one of these muscles alone produces move-
ment of all the joints that the muscle crosses.
To isolate movement at a single joint, two-
joint muscles contract with other muscles, fre-
quently with a one-joint synergist. The quadri-
ceps and the hamstrings act as synergists in
the stifle and simultaneously contract during