274 Surgical Treatment
(A) (B)Figure 32.2 An alternate theory, proposed by Tepic, suggests that the joint reaction force (magenta arrow) is
approximately parallel to the patella tendon, not the tibial long axis. In the cranial cruciate ligament-deficient stifle (A),
the joint reaction force can be resolved into a cranially directed tibiofemoral shear component and a joint-compressive
force (yellow arrows). By advancing the tibial tuberosity cranially, the patella tendon is perpendicular to the tibial plateau
during stance phase of gait (B). The joint reaction force, therefore, becomes perpendicular to the tibial plateau during
weight-bearing, thus can only be resolved into a joint-compressive force; cranial tibial thrust is eliminated. Source: Kim
et al. 2008. Reproduced with permission from John Wiley & Sons, Inc.
positioned at a standing angle (Montavonet al.
2002).Ex vivostudies demonstrated that stifle
stability after CrCL transection is attained by
TTA when the patellar tendon angle is 90± 9 ◦
(Apeltet al. 2007).
The proposed theoretical mechanism of
action of the TTA may also explain the mech-
anism of action for TPLO (Tepic et al. 2002;
Drygaset al. 2010; Sathyaet al. 2014). The Tepic
model takes into consideration both extensor
mechanism anatomy and the geometry of the
articulating surfaces of the stifle, and differs
from Slocum’s theory in that the direction of
the joint reaction force is dependent on the
inclination of the patellar tendon. By changing
the orientation of the tibial plateau relative to
the patellar tendon, TPLO may achieve a patel-
lar tendon angle of about 90◦in a different way
from the TTA (Drygaset al. 2010). These results
were confirmed in a clinical study reporting
the preoperative and postoperative TPA and
patellar tendon angles after TPLO (Sathyaet al.
2014).
Given that craniocaudal stability is directly
related to the patellar tendon angle and, there-
fore, the flexion angle of the stifle, it is possi-
ble that the degree of cranial tibial subluxation
may vary between different activities, or even
different breeds. When the stifle is loaded in
deep (90◦or greater) flexion, cranial tibial sub-
luxation does not occur, as the patellar tendon
angle also reaches 90◦(Kimet al. 2009b). Hence,
it could be argued that tibial osteotomies may
not benefit dogs with crouched postures, or dur-
ing activities such as stair-climbing. A recent
ex vivostudy demonstrated that TPLO did not
improve joint mechanics when CrCL-deficient
stifles were loaded in deep flexion (Kimet al.