128 Clinical Features
with collateral ligament rupture, is not covered
in this chapter. Stress radiography for assessing
collateral ligament integrity may be indicated in
some cases of CR, where severe trauma results
in multi-ligament injury; a thorough discussion
of these injuries is beyond the scope of this text.
Measuring subluxation
For all of the described stress-imaging tech-
niques, the detection of subluxation is made
possible by assessing the cranial-caudal align-
ment of the tibia relative to the femur on
lateral views of the stifle after applying a
cranial tibial translational load. When palpable
instability caused by CR is present, altered
femorotibial alignment is usually obvious upon
subjective comparisons between stressed and
non-stressed images (Figure 18.1). In one of
the original descriptions for stress radiography,
the stress test was considered positive when a
vertical line running tangential to the femoral
condyles fell behind the caudal aspect of the
tibial plateau (de Roosteret al. 1998). Other
subjective findings considered to be indicative
of subluxation included cranial displacement
of the intercondylar eminence relative to the
lateral femoral condyle (Figure 18.1), and
caudal displacement of the popliteal sesamoid
relative to the tibial plateau.
Because the accuracy of subjective assess-
ments is likely dependent on the expertise of
the observer, numerous methods for the objec-
tive quantification of cranial tibial subluxation
have been described. For most methods, the
distance between a specified femoral landmark
and tibial landmark along a defined axis of
translation is calculated (de Roosteret al. 1998;
Plesmanet al. 2012). In the first description by
de Rooster and van Bree, cranial-caudal laxity
was best assessed by using the tibial plateau
as the axis of translation, and the caudal mar-
gins of the femoral and tibial condyles. The
distance from the caudal margin of the tib-
ial condyles to a line passing tangential to the
femoral condyles and perpendicular to the tib-
ial plateau is measured on stressed and non-
stressed radiographs, where the difference in
the distance defines the amount of laxity. Laxity
indices can also be made where the values are
normalized to a particular femoral dimension.
(A)(B)Figure 18.1 Neutral (A) and compressed (B) lateral
projection radiographs of a stifle with cruciate ligament
rupture. Note the change in position of a vertical line (red
dotted line) tangential to the caudal femoral condyles,
relative to the caudal margin of the tibial plateau. The
direction of change in position is also represented by the
black arrows. Note also the distal position of the popliteal
sesamoid (white arrow). Note the position of the
intercondylar eminence (∗), which is adjacent to the
cranial aspect of the femoral condyles on the compressed
view.In a recentex vivostudy defining landmarks for
measuring cranial tibial subluxation, the tibial
intercondylar eminence and the caudal margin
of the femoral intercondylar notch had the best
repeatability (Plesmanet al. 2012).
Any of the described measurements are
affected by radiographic positioning, where
even minor obliquity of the projection will sig-
nificantly alter the calculated alignment. Thus,
it is imperative that perfect lateral projections of
both the femur and tibia are obtained, regard-
less of the method used to calculate subluxa-
tion. This can be challenging for inexperienced
operators in some cases for several reasons,