Tibial Tuberosity Advancement 229Figure 28.2 Schematic representation of stifle joint
flexion with respect to PTA. In full extension, the PTA is
90 ◦and in full flexion the PTA is< 90 ◦. There is a point
such that the PTA is 90◦, which is termed the ‘crossover’
point (see Figure 28.1C, Fs=0.0). At this point, there is
neither cranial nor caudal tibiofemoral shear force
present. The premise of the tibial tuberosity advancement
(TTA), according to Tepic, is to alter the geometry of the
proximal tibia such that the PTA is maintained at≤ 90 ◦
throughout the stifle joint range of motion during
weight-bearing; the TTA will change the PTA and move
the crossover point such that Fs=0.0 N when PTA= 90 ◦
when the stifle joint is in full extension. Further stifle joint
flexion ensures that Fs is always≤0.0NandPTA≤ 90 ◦.
The PTA is thus maintained at≤ 90 ◦throughout the stifle
joint range of motion during weight-bearing. Compare
with Figure 28.5. Source: Boudrieau 2009. Reproduced
with permission from John Wiley & Sons, Inc.
Mesfar 2007). A decrease in retropatellar pres-
sure after TTA also has been demonstrated
experimentally in the dog (Hoffmannet al.
2011). Theoretically, this diminished force can
protect the articular cartilage of both the
patella and the femur from subsequent damage.
Femorotibial contact pressure and location have
been evaluatedin vitrousing an experimental
model of a CrCL-deficient stifle joint, which
demonstrated an approximately 40% decrease
in contact area with an associated 100% increase
in peak pressure; furthermore, the positioning
of the peak pressure was found to shift cau-
dally (Kimet al. 2009). TTA appeared to restore
the normal femorotibial contact and pressure
(Kimet al. 2009), which may spare the menis-
cus from risk of trauma after TTA (Figure 28.4).
This study also suggested that because TTA
did not change the geometry of the joint, and
the pressure distributions essentially remained
unchanged, there may be less development of
osteoarthritis over time. All of these findings
could support clinical studies that implied an
absence of problems with the patellar tendon
and the joint surfaces after TTA (Hoffmannet al.
2006; Lafaveret al. 2007; Stein & Schmoekel
2008; Kimet al. 2009).
These studies have recently been challenged
by more recent evaluations performedin vivo
on clinical cases, where persistent instability
was observed in dogs after TTA despite report-
edly good postoperative function (Bottcher ̈ et al.
2013; Skinneret al. 2013). The reason for con-
tinued postoperative instability is debated, as
precise assessment of the amount of advance-
ment after surgery is lacking. A number of
errors inherent in the techniques for preop-
erative planning and intraoperative technical
failures will contribute to the final position of
the tibial advancement and the resultant PTA.
These include such items as the differing prox-
imal versus distal cage width and its final posi-
tion within the osteotomy and patella-based
versus distal tibial tuberosity-based advance-
ment. Furthermore, the appropriate PTA that
neutralizes tibiofemoral shear has not yet been
definitively documentedin vivo.
As the resultant PTA is crucial to the determi-
nation of the amount of TTA, it has been sug-
gested that the PTA is more accurately deter-
mined by the method of the common tan-
gent (PTACT) (Dennler et al. 2006; Schwandt
et al. 2006; Boudrieau 2009), as opposed to the
method using the TPA (PTATPA)(Boudrieau
2009; Hoffmannet al. 2011). The former method
has been proposed to be clinically more accu-
rate as it takes into account the anatomic rela-
tionship between the femoral condyles and tib-
ial plateau, as opposed to a static relationship
of the tibial plateau with the patellar tendon
(Boudrieau 2009) (Figure 28.5). Based on these
suppositions, PTACThas been recommended for
clinical use (2007 Veterinary Symposium – The