136 Clinical Features
(A)(B)Figure 19.1 Normal tendon anisotropy. (A) The tendon
fibers ‘drop out’ when the ultrasound beam is not parallel
to the tendon fibers. (B) The tendon fibers reappear when
the beam is parallel.
F TFPFigure 19.2 Normal cranial joint space of the stifle. The
synovium is the very thin, hyperechoic line just cranial to
the tibial cortex. F, femur; T, tibia; FP, fat pad.
the patellar tendon and has a coarser echotex-
ture in comparison to the tendon (Reedet al.
1995; Krameret al. 1999; Soleret al. 2007). The
collateral ligaments are similar in appearance,
although smaller, to the patellar tendon in
their echogenicity and echotexture; this is in
contradiction to other reports which state they
are not visible, and is likely due to the advance-
ments in technology and increasing resolution
capabilities (Krameret al. 1999; Samii & Long
2002; Soleret al. 2007). The medial collateral
ligament is seen along the medial joint surface,
at the most distal curve of the femoral condyle.
The lateral collateral ligament can be identified
along the lateral joint, caudal to the long digital
extensor tendon (LDE) with the tibial attach-
ment angling caudodistally toward the fibular
head (Vasseur & Arnoczky 1981). The LDE ten-
don is seen along the craniolateral joint space
superficial to the lateral meniscus and cranial to
the lateral collateral ligament. The thickness of
the LDE tendon will increase as the transducer
is moved distally, toward the musculotendi-
nous junction (Reed 1995). The stifle liga-
ments, in general, should maintain a constant
width.
The femoral condyles and tibial plateau are
defined as hyperechoic lines with clean distal
acoustic shadowing. The cartilage is a very thin,
hypoechoic line, superficial to the hyperechoic
cortical bone. Deep intra-articular structures are
the most difficult ligaments of the stifle joint to
examine. The cranial cruciate ligament (CrCL)
at its tibial attachment is best imaged from the
cranial skin surface in the sagittal plane with
the joint positioned in full flexion. In large dogs,
the CrCL can be seen in full extension, but in
small dogs the intercondylar space is too nar-
row, hindering visibility (Krameret al. 1999).
The CrCL appears as a hypoechoic structure
compared to the patellar tendon. It is lined by
the echogenic fat of the infra-patellar fat pad
and synovium, which is a discrete, thin, hyper-
echoic line deep to the infra-patellar fat pad.
The CrCL becomes hyperechoic when the trans-
ducer is perpendicular to the ligament fibers
(Gnudi & Bertoni 2001; Seonget al. 2005) (Fig-
ure 19.3). In one study, the hyperechoic fibers
became hypoechoic when the transducer was
angled 3◦ off from the proper perpendicular
orientation (i.e., drop-out or off-angle artifact)
(Reedet al. 1995; O’Connor & Graninger 2005).