272 Surgical Treatment
predicting clinical function is likely a reflection
of the discrepancy between measures ofjoint
laxity versus measures of dynamic stability.
Laxity tests measure the maximum displace-
ment of the joint in response to an applied
external load, in the absence of muscle forces.
Simple laxity elicited by palpation cannot
simulate the complexity, directions and rate of
application of muscular forces produced at the
stifle during movement. Even the tibial com-
pression test, which attempts to simulate
weight-bearing, fails to replicate significant
loads transmitted across the stifle, such as a
quadriceps force. For this reason, it would be
more correct to use the term ‘laxity’ when static
stability is evaluated, and consider dynamic
stability only if stifle kinematics can be eval-
uated in the whole range of motion during
different gait patternsin vivo.
Single-plane fluoroscopy is frequently used
for the accurate non-invasive kinematic evalu-
ation of human knees, and the technique has
been validated for dog stifles (Joneset al. 2014).
While biplanar fluoroscopy is considered the
most accurate modality for determining joint
kinematics, the lateral single plane modality
was accurate to within 1.28 mm for translations
and 1.58◦for rotations (Joneset al. 2014). This
technique allows for evaluation during locomo-
tionin vivoin clinical cases, and has since been
used to determine normal dog stifle kinemat-
ics during treadmill walking and trotting, stair
climbing, and sitting (Kimet al. 2015).
Biomechanics of tibial osteotomies
In 1983 Slocum described the cranial tibial
thrust as a tibiofemoral shear force occurring
during weight-bearing (Slocum & Devine 1983).
Slocum also presented a theoretical model
which proposed that the magnitude of cranial
tibial thrust was dependent on the degree of
the caudal slope of the tibial plateau (Slocum
& Slocum 1993). The compressive forces of
weight-bearing, assumed to be parallel to the
axis of the tibia, can be resolved into a cra-
nially directed component responsible for cra-
nial tibial translation, and a joint-compressive
force (Figure 32.1). In the normal stifle the cra-
nial shear component is opposed by the CrCL
and by muscle forces. In the CrCL-deficient
stifle, the uncontrolled cranial shear force
results in cranial tibial subluxation.
Based on his model, Slocum proposed that
the tibial plateau angle (TPA) was a major
factor in stifle biomechanics influencing the
magnitude of cranial shear force. In an attempt
to dynamically decrease the uncontrolled
shear force in the CrCL-deficient stifle, Slocum
described in 1984 the closing cranial wedge
ostectomy (CCWO) for the treatment of CR
in dogs (Slocum & Devine 1984). This tech-
nique was intended to eliminate cranial tibial
thrust during the weight-bearing phase of the
stride by reducing the TPA. Its mechanism has
been recently investigated in anex vivostudy
evaluating the size of the wedge ostectomy
necessary to neutralize the tibial thrust (Apelt
et al. 2010). An ostectomy with a wedge angle
equal to TPA +5◦resulted in a stable stifle at
a TPA of approximately 6◦. Other factors that
influence the postoperative TPA after CCWO
include the position of the ostectomy and the
cortical alignment after reduction (Baileyet al.
2007). These variables may be responsible for
the discrepancies in TPA reported after CCWO
(Maciaset al. 2002).
A large CCWO can shorten the tibia cranially
and alter the femoro-patellar joint, lowering the
patella relative to the femur and leading to
hyperextension of the stifle joint (Corr & Brown
2007). Kinematic gait analysis after the CCWO
procedure has shown an increase in extension
during the swing phase of the stifle and talocru-
ral joints, and an increase in limb extension at
paw contact, but no changes occurred in the sti-
fle and talocrural joint angles at the stance phase
(Leeet al. 2007).
The goal of TPLO is also to neutralize cra-
nial tibial thrust and prevent tibial subluxation
(Slocum & Slocum 1993). Biomechanical studies
have demonstrated that after tibial plateau rota-
tion, the tibiofemoral shear force shifts from cra-
nial to caudal when the limb is loaded (Warzee
et al. 2001; Reifet al. 2002). Thus, it has been
postulated that joint stability is dependent on
the caudal cruciate ligament (CaCL) neutraliz-
ing caudal tibial translation after TPLO (Warzee
et al. 2001; Reifet al. 2002). TPLO has a signif-
icant effect on CrCL strain, which decreases as
the tibial plateau is rotated (Hayneset al. 2015).
This protective effect of TPLO on CrCL strain
is the basis for recommending TPLO in dogs