18 Structure and Function
with obvious trauma and present with an acute
onset of severe lameness (Reinke 1982).
Synovitis and ligament biomechanics
The development of synovitis and joint degra-
dation has been clearly associated with CrCL
transection models that create joint instability
(Lipowitzet al. 1985). A relationship between
synovitis and joint instability in the canine cru-
ciate rupture arthropathy has been well estab-
lished over the past decade (see also Chap-
ter 12). One of the initial studies examining
the effect of synovitis on ligament biomechan-
ics was in a rabbit model, wherein the presence
of chronic synovitis significantly decreased lig-
ament strength. Histologically, these ligaments
showed a loss of normal fiber orientation, a dis-
organized cellular pattern, changes in the inter-
stitial matrix, and some infiltration of inflamma-
tory cells within the ligament body (Goldberg
et al. 1982). This work has led to the hypothesis
that synovitis precedes cruciate ligament rup-
ture in affected dogs, fitting with the clinical
observation that mid-body CrCL ruptures are
typical.
Another landmark study in this field evalu-
ated Wistar rats that are predisposed to spon-
taneous synovitis. In these animals, a correla-
tion was found between stifle synovitis and
chondroid metaplasia of the cruciate ligaments;
these changes were not age-related (Sasakiet al.
1998). Taken together, these data support the
hypothesis that stifle synovitis is a key factor
promoting joint degradation and diminished
ligament strength in dogs with cruciate liga-
ment rupture. Numerous studies undertaken
over the past decade, focused on understanding
the pathologic basis of canine cruciate ligament
rupture, have resulted in a body of work sup-
porting the hypothesis that osteoarthritis and
progressive cruciate ligament fiber rupture is
influenced by stifle synovitis (Muiret al. 2007;
Erneet al. 2009; Muiret al. 2011; Chuanget al.
2014; Littleet al. 2014). Further work to under-
stand the effects of synovitis on ligament struc-
tural properties is warranted.
Isometric points
With finite-sized areas of CrCL attachment, it
is not possible for all CrCL fibers to be isomet-
ric throughout flexion, and thus the CM and
CL bundles have differing biomechanical prop-
erties. Surgical protocols, however, require the
most isometric points to be used for joint sta-
bilization or ligament reconstruction. Stabiliza-
tion of the cruciate-deficient canine stifle occurs
primarily through either intra-articular or extra-
capsular techniques. Extracapsular stabilization
methods are still commonly employed today.
The general concept of these methods are to
align a suture outside the stifle joint capsule in
the direction of the previously existing CrCL.
Biomechanically speaking, the placement of
such a suture should not be considered so sim-
plistically. Ideally, sutures should be placed in
an isometric manner, which means that dur-
ing motion the suture attachment sites should
remain a constant distance apart, thus prevent-
ing laxity of the suture material and conse-
quent cranial translation of the joint. Numer-
ous studies have sought to identify isometric
points (Gu ́en ́egoet al. 2007; Roe et al.2008;
Hulseet al. 2010). It is important to note that
while these techniques attempt to use isomet-
ric points, such points do not exist biomechan-
ically in the extracapsular space of the canine
stifle.Biomechanics of grafts
The use of intra-articular grafts for the restora-
tion of joint stability has been well established
in human orthopaedics. However, in dogs with
cruciate ligament rupture the use of grafts in
CrCL-deficient stifles has fallen out of favor
in the clinical setting. Various allografts, auto-
grafts, and synthetic materials have been tested
in experimental animals, but none has shown to
be mechanically equivalent to an intact CrCL.
One challenge of using grafts to replace CrCLs
in dogs is the lack of strength they display over
time (Cabaudet al. 1980; Shinoet al. 1984; Curtis
et al. 1985; Vasseuret al.1985; McFarlandet al.
1986; van Renset al. 1986; Yoshiyaet al. 1986;
Johnsonet al. 1989; Thorsonet al. 1989). Unlike
experimental dogs used in these studies, client-
owned dogs affected with cruciate ligament
rupture typically exhibit moderate to severe
synovitis at the time of surgical treatment.
Therefore, without a disease-modifying treat-
ment, the use of grafts in these patients would
be even less likely to provide long-term stifle