Advances in the Canine Cranial Cruciate Ligament, 2nd edition

94 Etiopathogenesis of Cruciate Ligament Rupture

SM

LFC

Figure 12.1 Arthroscopy image from a normal stifle
joint of a young female hound. In this lateral
compartment view, a white synovial lining lacking
vascular ingrowth or proliferation is evident. SM, synovial
membrane; LFC, lateral femoral condyle.

macrophages (type A) that are immunoreactive
and mainly phagocytic; and (ii) fibroblast-like
(type B) cells that produce collagens, fibronectin
and hyaluronan (Iwanagaet al. 2000). Collec-
tively, the synovial membrane provides many
key functions, including the secretion of syn-
ovial fluid to lubricate and nourish joint tissues,
the clearance of intra-articular debris, and the
regulation of immunological events.
In the normal joint, cruciate ligaments are
covered by a synovial reflection, so although
being intra-articular they are extrasynovial
(Figure 12.2) (Arnoczky & Marshall 1977).
The surrounding membrane contains many
small fenestrations that allow exchange of
synovial fluid and provide nutrients to the lig-
aments (Kobayashiet al. 2006). Abundant ves-
sels from the synovium and fat pad penetrate
the ligament as the predominant blood sup-
ply (Arnoczky and Marshall 1977; Kobayashi
et al. 2006). Compounds injected into the joint
space rapidly pass through the synovial mem-
brane and enter the capillary lumen of the
cruciate ligaments. However, intravenous injec-
tions do not reach the ligament perivascular
space (Kobayashi et al. 2006). The existence
of a blood–cruciate barrier function has been

CdCL

CrCL

Figure 12.2 Arthroscopic image of the intercondylar

notch from a normal stifle joint of a young female hound.

Both cruciate ligaments are visible with a thin overlying

synovial reflection. Several filamentous synovial

projections are present with the adjacent infrapatellar fat

pad. CrCL, cranial cruciate ligament; CdCL, caudal

cruciate ligament.

postulated to be analogous to the blood–

brain barrier (Kobayashiet al. 2006), and may

be important in the pathophysiology of CR.

Although debated, it is likely that joint inflam-

mation has an adverse effect on synovial bar-

rier function and metabolism of the cruciate

ligaments.

Synovitis

Synovitis is characterized by infiltration of the

synovial membrane with inflammatory cells,

resulting in vascularization and hyperplasia of

the synovium (Suttonet al. 2009). In dogs with

CR and osteoarthritis (OA), the cellular distri-

bution is typically a mononuclear infiltrate of B

and T lymphocytes, macrophages, and plasma

cells (Figure 12.3) (Suttonet al. 2009). Syn-

ovial fluid contains byproducts of this inflam-

matory response. Although total nucleated cell

counts are often normal or mildly increased

(<5000 cellsμl–1), this does not accurately reflect

the true magnitude of joint inflammation.

Many studies have demonstrated evidence

of immune dysregulation in the synovial