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					   Joint Structure                             1

1. Fibrous (synarthosis) i.e. sutures in

2. Cartilaginous (amphiarthrosis) i.e.

3. Synovial (diatrhosis) i.e. limbs

   Joint Structure                                                Joint Histology

                                                          • The articular cartilage (c) does not contain
                                                            nerves (pain receptors), blood or lymphatic
                                                          • The articular cartilage is firmly attached to the
                                                            subchondral bone (asterisks).
                                                          • The articular cartilage has little capacity for
                                                          • Nutrients for cartilage are provided by synovial
Banks JB, Applied Veterinary Histology, William Wilkins
Synovial Fossae (arrows):

            –   Bilateral depression with no cartilage
            –   Not clearly visible at birth
            –   Lubrication?
            –   Ooops!! (inexperienced pathologists may mistake
                these structures as a lesion)
•   Early degenerative change
•   Loss of proteoglycans
•   Degradation of collagen fibers
•   Increase water content
•   Dull yellowish appearance

                                     normal   fibrillation

•   Fibrillation is an early degenerative change of
    the articular cartilage.

•   The causes of fibrillation are not specific and
    include trauma, irritation and chemical injury
    induced by enzymes released by leukocytes
    during inflammation.

•   Fibrillation is biochemically characterized by
    loss of proteoglycans, degradation of collagen
    fibers and i
    fib                     t       t t f
               d increase water content of

•   Grossly,
    Grossly fibrillated cartilage appears dull and
    has a yellowish discoloration as shown in this
    picture (arrowheads).
     Fibrillation / HE Section

•   Microscopically, fibrillation is
    characterized by vertical fissures in
    cartilage (c). Note the fissure delineated
    by the arrows in this illustration.

•   These cartilaginous fissures become filled
    with synovial fluid and eventually with
    g                   (g)
    granulation tissue (g).

•   As fibrillation progresses, the fissures
    expand through the entire thickness of the
    cartilage until the subchondral bone is
         h d d            d to        i l fluid.
    reached and exposed t synovial fl id

•   If the injury is severe, the surrounding
    cartilage becomes loose and detaches
    from the subchondral bone Pieces of
    detached cartilage floating in the synovial
    space are called “joint mice.”

Eburnation is a degenerative condition
characterized by the loss of the entire articular
cartilage and exposure of the subchondral
bone in one or more areas of the joint

With mechanical friction the subchondral
becomes polished and takes an ivory-like
appearance, hence the name eburnation
(ivory-like). The subchondral bone thickens,
creating a condition known as osteosclerosis

Clapping the affected joint with a knife or any
 th       t l bj t     ld        l    b
other metal object would reveal an abnormal   l
sound (bone to metal) rather than the dull
sound (metal on cartilage). This technique is
routinely used during a postmortem
      i ti to      fi     b     ti
examination t confirm eburnation.
Eburnation / Dog

                              g                  p                                 joint appears
This is a femoral head showing eburnation. The superficial bone at the site of the j      pp
thicken, polished and smooth.

This macerated specimen was obtained from a case of chronic degenerative joint disease in a
  g         p y p
dog with “hip dysplasia.”
Cartilage Fibrillation and loss
of attachment to the
   b h d l bone
subchondral b

Dog / Osteochondrosis

An important degenerative change in
articular cartilage associated with
fibrillation is the loss of attachment to the
subchondral bone Pieces of
degenerating cartilage are released into
the joint cavity forming the so-called joint

This is a picture of the humeral head of a
dog with chronic degenerative joint
disease affecting the shoulder. The dog
had lameness that started as a subtle
gait abnormality. The dog was

Note the fibrillation and deep fissures
involving the articular cartilage of the
humerus (arrowheads).
Joint Mice / Shoulder Dog

Joint mice are viable pieces of detached
articular cartilage free floating in the synovial
cavity. These pieces of detached cartilage
are nourished by the synovial fluid and
remain viable and continue to grow for long
                                  g           g
periods of time.
Joint mice are a common sequel to
degenerative j
  g                         , particularly
              joint diseases, p          y
This is a picture of the shoulder joint of a
dog with chronic degenerative joint disease.
Note the four pieces of free cartilage in the
synovial space (arrowheads). Also note a
large piece of degenerating articular cartilage
in the head of the humerus (asterisk) which
appears to be near detachment.

Osteophytes are bone
outgrowths derived from
chondrification of fibrous tissue.
It is a non-specific lesion which
i f     d in
is found i many d            ti
and inflammatory conditions of

Osteoph te formation starts
early as three days after injury
but is only detected grossly
around two weeks and
radiographically in five weeks.

Osteophytes are mainly found
in the junctions of cartilage with
periosteum or along the
insertions of synovial capsules
to the bone.

   e process of os eop y e
The p ocess o osteophyte
formation is known as
Osteop ytes

If the source of articular injury ceases,
osteophytes no longer grow but remain
forever as multiple periarticular spurs of
bone. These growths cause variable
degree of joint deformity.

This is the acetabulum of a cow with
chronic joint disease. Note the massive
numbers of osteophytes along the margins
and within the joint

Osteophytes are sometimes seen as
                 g         joints of old
incidental findings in the j

•   Bone outgrowths at the junction of
    cartilage with periostium

•   Chondrofication and ossification of
    fibrous tissue

•   Joint deformity

A pannus is a focal aggregate of granulation
tissue that starts at the synovial membrane
and extends into the articular cartilage

Pannus formation is typically associated to
chronic synovial injury and synovits. Release
of proteases from macrophages induces
degradation of cartilage and chondromalacia.
deg adat o o ca t age a d c o d o a ac a
It is most commonly seen in immune-
mediated arthritis.

In the bottom picture note the formation of
        p                       g
several pannus structures arising from the
synovial membrane.
       Synovial Villous Hyperplasia

Synovial villous hyperplasia is a non-specific
change that results from persistent low-grade
irritation of the synovial membrane. This
synovial change is frequently accompanied by
excessive production of synovial fluid
(synovial effusion).

Synovial villous hyperplasia is often found in
chronic degenerative joint diseases. The
synovial membrane is superficially covered by
countless villous projections which are best
demonstrated by immersing the joint in water
as illustrated in this picture.

Note th     l t               f th
N t the velvety appearance of the synoviali l
membrane with severe villous hyperplasia.
The cartilage appears essentially normal.

The histological view of synovial villous
hyperplasia is illustrated in the next slide.
             Synovial Villous

A histological section of a synovial
membrane showing severe villous
hyperplasia. Note the large number of
exophytic projections (asterisks) arising
from the synovial membrane. Synovial villi
are lined by hypertrophic synoviocytes.

There are two types of synoviocytes.
Synoviocytes type A are phagocytic, while
synoviocytes type B are responsible for
the production of the synovial fluid. Both
types of synoviocytes proliferate rapidly in
response to injury (reactive hyperplasia).
   th    h b      h       h
If there has been hemorrhage,
synoviocytes type B phagocytize red blood
cells and accumulate hemosiderin and the
synovial membrane turns yellow
(pigmented synovitis).
                   Joint Response to Injury

End-stage joint is a generic term used to describe several chronic articular changes
in the same joint. End-stage joints show various degrees of fibrillation, eburnation,
osteophytes, deformation, villous hypertrophy, pannus, capsule fibrosis and occasionally
          (fixation, immobility).
ankylosis (fixation immobility)

                  • Fibrillation               • Eburnation

                 • Osteophytes                 • Villous hyperplasia

                   Joint i
                 • J i t mice                    Capsular fibrosis
                                               • C    l   fib   i

                 • Pannus formation

                                   End Stage Joint
        End Stage Joint

Note the large number of ostrophytes
arising from the articular cartilage in
this joint. Some osteophytes,
particularly those in the synovial
p          y               y
junction are notably large (asterisk).

The synovial membrane is hyperemic
and thickened suggesting synovial
villous hyperplasia (S).

At the synovial-cartilage junction
there are few small pannus formations

There are also some adhesions
between cartilage and synovial
membrane (arrowheads).

Although not clearly shown in this
picture, the articular capsule was
fibrotic and thickened.