218 Surgical Treatment
(A) (B)Pre-opPost-opFigure 27.1 Schematic drawing
depicting the reduction in tibial
plateau angle following a tibial
plateau leveling osteotomy and
the subsequent change in
individual tibiofemoral forces.
(A) The tibiofemoral forces are
broken down into a
perpendicular and a parallel
force relative to the tibial plateau
(blue arrows). The force parallel
to the tibial plateau represents
tibial femoral shear. (B) With
rotation of the tibial plateau note
the elimination of tibial femoral
shear. The resultant compressive
force remains unchanged and is
represented by the white arrow
in both A and B. Source:
Boudrieau 2009. Reproduced
with permission from John Wiley
& Sons, Inc.TPA. Interobserver variability in TPA measure-
ment has been shown to be relatively small
(standard deviation of 0.8◦) but significant,
whereas intraobserver variability and between
groups of observers was not statistically sig-
nificant (Cayloret al. 2001; Fettiget al. 2003).
Measurement variability in the TPA is reduced
when digital radiography and computer-
ized measurement systems are used (Unis
et al. 2010). While the stifle flexion angle has
been shown to not influence TPA measurement
(Aulakhet al. 2011), limb position does influence
radiographic TPA measurement. Cranial and
proximal positioning of the limb relative to the
X-ray beam results in overestimation, whereas
caudal and distal positioning results in under-
estimation of the TPA (Reifet al. 2004). Both
preoperative and intraoperative planning of the
osteotomy position improves the likelihood of a
more centered osteotomy, and reduces the risk
of tibial tuberosity fractures (Collinset al. 2014).
When planning techniques were compared,
measuring the distance from the insertion
point of the patella tendon to two points on the
proposed osteotomy line (perpendicular to the
cranial border of the tibial crest and at the joint
surface) proved the most accurate (Mossman
et al. 2015). An emphasis on osteotomy reduc-
tion at the expense of medial cortex alignment
minimizes angular and rotational deformi-
ties due to TPLO (Wheeleret al. 2003). Distal
positioning of the osteotomy (versus centering
on the proximal tibial long axis point divid-
ing the intercondylar tubercles) results in a
postoperative TPA greater than is expected. In
addition, undesired craniodistal translation of
the tibial plateau and tibial long axis shift is
noted (Kowaleskiet al. 2005).
In the original surgical technique as described
by Slocum, an alignment jig is secured to the
tibia with the goal of maintaining a fixed plane
during proximal segment rotation. The neces-
sity for a jig for TPLO has been questioned
by some authors. One study found no signifi-
cant differences in postsurgical TPA, tibial crest
thickness, varus–valgus malalignment, or tib-
ial torsion between TPLOs performed with or
without a jig (Bell & Ness 2007). The same
authors found that the use of a jig resulted in
a more distally positioned osteotomy. Another
group found no significant difference in post-
surgical TPA, fragment reduction, or proxi-
modistal osteotomy orientation when a jig was
not used, as long as the limb was held in 10–
15 ◦ of internal rotation and parallel to hori-
zontal while performing a vertically oriented
osteotomy (Schmerbachet al. 2007). A third
group showed that fibular penetration by screw
tip is more likely when a jig is not utilized
(Flynnet al. 2014). With the recent development
of a novel jig and saw guide (Marianoet al.
2016), current studies have shown improved