12

Synovitis Precedes Stifle

Instability Associated with

Cruciate Ligament Rupture

Jason A. Bleedorn

Introduction

Joint inflammation is an expected and central
feature of cruciate ligament rupture (CR) in
dogs. Inflammation involves all joint tissues
and, in particular, the synovium. Synovitis leads
to joint pain and promotes joint degradation.
However, the underlying trigger for the devel-
opment of synovitis, and its contribution to dis-
ease progression, remain unclear. This chap-
ter will describe the characteristics of synovial
inflammation in dogs with CR and its impact on
cruciate ligament and joint degeneration.
Knowledge of joint inflammation in dogs
with CR has grown substantially during the
past decade. CR represents the end-stage of a
complex disease condition and, in some cases,
a long course of disease. Many of the investiga-
tions in the field have focused on the end-stage
of the disease, representing complete mechan-
ical or organ failure. However, clinical, radio-
graphic and arthroscopic evidence of stifle syn-
ovitis and arthritis can be found early in the
course of the disease (Bleedorn et al. 2011).
Recent studies have used the contralateral stifle
joint as a model for studying incipient CR. The
use of arthroscopy in dogs has also increased
awareness of profound changes in the syn-
ovial membrane and allows for objective assess-

ment. These are important steps for advancing

the understanding of early changes in the joint

before the development of stifle instability asso-

ciated with complete CR.

Normal synovium

The synovial membrane is a specialized col-

lagenous tissue lining the interior and all struc-

tures within a synovial joint, except articular

cartilage. It can be identified macroscopically as

a thin (0.5–5 mm thickness), translucent, con-

nective tissue layer with numerous villous pro-

jections (Figure 12.1) (Iwanagaet al. 2000). It

is similar to vascular or peritoneal endothe-

lium, but lacks a continuous basement mem-

brane (Iwanagaet al. 2000). The synovium has

two distinct layers: an inner synovial intima

containing a rich vascular, nervous, and lym-

phatic network, and an outer, supportive layer

that is continuous with the fibrous joint capsule

(Suttonet al. 2009). Sensory nerve endings are

abundant and located within or adjacent to the

fibrous capsular layer (Schenket al. 1996). Syn-

oviocytes are loosely arranged within the mem-

brane and have an integral function in main-

taining joint health (Iwanagaet al. 2000). Two

main morphologic and functional types exist: (i)

Advances in the Canine Cranial Cruciate Ligament, Second Edition. Edited by Peter Muir. © 2018 ACVS Foundation.
This Work is a co-publication between the American College of Veterinary Surgeons Foundation and Wiley-Blackwell.

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