Advances in the Canine Cranial Cruciate Ligament, 2nd edition

104 Etiopathogenesis of Cruciate Ligament Rupture

p = 0.08

p < 0.005

p < 0.05

#Stifle oligoarthritis

(n = 21)

300

1000

(A)

(B)

100

10

1

0.1

0.01

250

200

150

100

50

Relative TLR-2 Expression

(18S rRNA normalized)

TRAP Expression

(18S rRNA normalized)

0

Healthy

(n = 9)

Other arthritis

(n = 9)

#Stifle oligoarthritis

(n = 27)

Healthy

(n = 8)

Other arthritis

(n = 9)

Figure 13.3 (A) Relative expression of tartrate-resistant
acid phosphatase (TRAP), and (B) toll-like receptor 2
(TLR-2) in synovial fluid cells collected from dogs with
stifle (oligo) arthritis and cruciate ligament rupture,
healthy dogs, and dogs with other forms of arthritis
characterized by cartilage loss (osteoarthritis). Gene
expression was normalized to peripheral blood
mononuclear cells (PBMC) as an internal control using
the−ΔΔCtmethod. 18S rRNA was used as the
house-keeping gene. # indicates a significant difference
from internal PBMC controls (P<0.05). Boxes represent
median and the 25th and 75th percentiles. Whiskers
represent 10th and 90th percentiles. Outliers are also
plotted. Significant differences between groups are as
indicated. Source: Muiret al. 2007a. Reproduced with
permission from Elsevier.

Garneret al. 2013). Collectively, these observa-
tions suggest that macrophage activation and
neutrophil trafficking are important factors in
the development of chronic stifle synovitis in
dogs with CR.
Collagen in ruptured CrCL tissue appears
damaged, with loss of crimp in collagen
fibers (Hayashi et al. 2003). Interestingly,
recent research has shown that repeated sub-
rupture overload of ligament/tendon induces
nanoscale discrete plasticity damage to collagen

(A) 5 μm

(B) 500 nm

Figure 13.4 Sub-rupture loading of tendon and

ligament collagen creates strain-induced discrete

plasticity damage with kinked collagen fibrils (Vereset al.

2013). In anin vitrostudy, macrophage-like U937 cells

found on collagen fibrils with strain-induced discrete

plasticity were morphologically distinct from those found

on undamaged fibrils. The cells showed a greater

uniformity, being mostly of the ruffled type (∗in panel A).

Rather than maintaining a rounded shape, as on the

undamaged fibrils, many of the ruffled cells had acquired

a dome-shape, increasing their contact area with the

damaged fibrils (arrows in panel A). (B) Magnified view of

the boxed location in panel (A) showing structure of the

discrete plasticity fibril array that formed the substrate for

these cells. Source: Vereset al. 2015. Reproduced with

permission from John Wiley & Sons, Inc.

fibrils (Vereset al. 2013). U937 macrophage-like

cells recognize collagen fibrils with strain-

induced plasticity damage, bind to damaged

areas for collagen degradation, alter their func-

tional morphology (Figure 13.4), and secrete

matrix, which is hypothesized to be fibronectin

in vitro(Vereset al. 2015). Further studies are

needed to understand how macrophages and

fibroblasts interact with mechanically dam-

aged collagen when ligament homeostasis is

disturbed in dogs with CR.