76 Etiopathogenesis of Cruciate Ligament Rupture
increased laxity and eventual joint degenera-
tion. However, a recent study which examined
the morphometric characteristics of the pelvic
limbs of Labrador Retrievers with and without
CR (Mostafaet al. 2009) found ICN stenosis not
to contribute to the risk of CR, though ligament
changes were not examined in this study.
The contribution of other aspects of femoral
conformation such as femoral varus and antev-
ersion angles have been described (Ragetlyet al.
2011). The latter authors found that an increased
femoral anteversion angle (FAA) (Figure 9.5)
and body fat composition was greater in predis-
posed pelvic limbs from Labrador Retrievers. In
a recent study the measurement of FAA, using
Figure 9.5 The femoral anteversion angle (FAA) equals
the value of the tangent of ‘a’ divided by ‘b’. The long
axis of the femoral shaft is drawn on the lateral (A) and
craniocaudal (B) femoral views between two points
centered in the proximal femoral shaft. These two points
were identified by bisecting the two femoral cortices,
respectively, at one-fourth (25% line) and one-half (50%
line) of the total femoral length. (A) The distance ‘a’ from
the center of the femoral head to the extended axis of the
femoral shaft was measured on the lateral view of the
femur. (B) The distance ‘b’ from the center of the femoral
head to the extended axis of the femoral shaft was
measured on the craniocaudal view of the femur. Source:
Ragetlyet al. 2011. Reproduced with permission from
John Wiley & Sons, Inc.
computed tomography (CT), has been found
to be more accurate than using a single lateral
radiograph (Mostafaet al. 2014).Tibia
The stifle joint is subjected to external ground
reaction forces at weight-bearing and inter-
nal forces generated by muscular contraction.
These forces not only compress the femoral and
tibial articular surfaces, but also generate a cra-
nially directed shear force in the tibia known as
cranial tibial thrust (CTT). CTT is present in the
stifle as the tibial plateau is not perpendicular to
a line drawn between the center of motion of the
stifle and hock joints, but is directed caudodis-
tally (Slocum & Devine 1983). Numerous stud-
ies have been conducted to evaluate the associ-
ation of the tibial plateau angle (TPA) and CR
(Read & Robins 1982; Maciaset al. 2002; Duerr
et al. 2007). Although anatomic differences in
the shape of the proximal tibia have been doc-
umented in dogs with CR, its role in the rup-
ture mechanism is unclear (Wilkeet al. 2002;
Guastellaet al. 2008). The mean TPA in dogs
varies between 23◦and 25◦, while a wide range
of TPA has been reported (13–34◦)innormal
dogs and can vary according to breed (Aert-
senset al. 2015). A study conducted by Wilke
and others revealed that the functional TPA is
approximately parallel to the ground in most
dogs (Wilkeet al. 2002). Although pathologi-
cal increases in TPA (> 55 ◦) have been corre-
lated with CR (Read & Robins 1982; Maciaset al.
2002), whether a general association between
TPA and CR exists in dogs remains controver-
sial. A recent study found that an excessive
TPA may contribute to CR, as demonstrated in
experimental Beagles where TPAs> 40 ◦resulted
in chondroid changes in the ligament ECM
which could lead eventually to non-contact lig-
ament degeneration (Ichinoheet al. 2015).
Studies have shown that the TPAs are not
significantly different in Labrador Retrievers
with and without CR, and also in a comparison
between Greyhounds and Labrador Retrievers
(Wilkeet al. 2002; Reif & Probst 2003). Interest-
ingly, German Shepherd dogs have an increased
TPAwhen compared to dogs of high-risk breeds
such as the Rottweiler (Guastellaet al. 2008),
buthavealowriskofCR.Thetrueeffect