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Synovial villiAn examination of these structures within the anterior compartment of the knee and metacarpo-phalangeal joints.

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Synovial Villi:
An Examination of These Structures Within
the Anterior Compartment of the Knee
and Metacarpo-Phalangeal Joints
with the attached patella, was removed through a
midline incision over the anterior aspect of the
knee joint. Ten surgical specimens were obtained
at the time of synovectomy for rheumatoid
synovitis, from both knee and metacarpo-phalangeal joints. Of those subjects without a history of
joint disease 2 were in the second decade,
2 in the third, 2 in the fifth, 1 in the sixth, 3 in
the seventh, 6 in the eighth and 2 in the ninth
decade, together with specimens from a 20-week
and a 36-week-old foetus.
All specimens were floated out in normal saline,
before microscopic examination using a X I 0 eyepiece and a low power objective (Reichert X1,
X2.5 or X5.0), with oblique side lighting. Photographic records were made, and the examination
was supplemented on occasion by examination
under a stereoscopic Olympus binocular microscope with a zoom objective. Structural form was
frequently greatly clarified by staining small areas
of the specimens with .05 per cent Evans blue
dye for about one minute.
Particular attention was paid to the following
areas: the anterior and posterior surfaces of the
suprapatellar bursa, the circumpatellar fold, and
the suprapatellar vascular pad.
Individual villi and localized areas of synovial
membrane were photographed and then removed
for the preparation of haematoxylin and eosin
stained sections. Van Giesen’s stain, and the
Gordon and Sweet silver impregnation method for
reticulin were used on selected sections. Specimens
were embedded in such a way as to ensure that
the plane of section was known.
of studying possible mechanisms responsible for the formation
and persistence of pathological joint effusions, the author was prompted to seek
descriptions of synovial villi in the literature. As no adequate descriptions of the
gross morphology of these structures was
found, the present study was made. It is
hoped that the descriptions which follow
will indicate that some correlation exists
between the form and function of the synovial villus. These descriptions have been
predominantly based on specimens taken
from the anterior compartment of the knee
joint, partly because concurrent studies of
joint effusions largely involve this joint and
partly because it was felt that it would be
useful to draw attention to the various
synovial structures within reach of the
synovial biopsy needle.
The Shorter Oxford English Dictionary
defines a villus as “a slender hair-like process or minute projection forming one of a
number closely set on a surface”. General
use has expanded the meaning of this term
to include tiny surface projections of quite
variable form; but it is considered to be
incorrect to classify as villi, unequivocally
macroscopic projections such as the
fat pads and synovial folds.
Thirty post-mortem specimens were obtained
from 20 subjects without a recorded history of
joint disease. The suprapatellar bursa, together
The general topography of the anterior
compartment of the knee joint is shown
in Fig. 1.
The main villous bearing areas were
From the Rheumatism Research Laboratories,
Wellcome Medical Research Institute, University
of Otago Medical School, Dunedin, New Zealund.
D. G. PALMER,M.D., M.R.C.P., M.R.A.C.P.:
Honorary Lecturer in Clinical Science and Clinical
Assistant to the Rheumatic Diseases Unit, Wellcome Medical Research Institute, Dunedin, New
AND RHEUMATISM,VOL. 10, No. 5 (October 1967)
Fig. 1.-Specimen,
anterior compartment
right knee joint, posterior aspect. The posterior wall of the suprapatellar recess has been
divided and opened out. The quadriceps tendon (Q.T.) lines in the midline above the
patella. The vascular suprapatellar (S.P.) and
circumpatellar pads (C.P.) are obvious. Also
shown is the lateral fold (L.F.), the medial
fold (M.F.) and below, the infrapatellar fat
pads (F.P.). (This specimen and the villi illustrated in Figs. 2-6 are from a 16 year old
found to be the lower half of the posterior
surface of the suprapatellar bursa and the
free borders and surfaces of the vascular
suprapatellar and circumpatellar pads.
The Development and Form of Normal
These observations were based largely
on the specimens obtained from the 2 foetuses, 2 16-year old youths, a 23-year-old
and a 26-year-old male. The numbers of
villi present in the specimens from the 4
young adults varied from sparse to prolific
in the lower portion of the posterior wall
of thp suprapatellar recess; but appear-
Fig. 2.-Vascular
leaf-shaped villi. The
avascular elongated processes arising from
these villi are secondary villi. (Evans blue dye,
oblique light, XSO).
ances were similar in the other areas under
1. The leaf-shaped villus (Figs. 2 and
3). 'These structures have a narrow base
and expand into a flattened lanceolate
form enclosing a vascular network. No villi
were noted in a 20-week-old foetus, but a
single villus found in the specimen from
a 36-week-old foetus was a disproportionately large example of this form (length
3 mm.). I n the specimens obtained from
the young adults this form of villus was
found arising as part of a variably developed villous covering over the lower part
of the posterior wall of the supra-patellar
recess, and from the supra- and circumpatellar pads; b u t was not entirely restricted to these areas.
2. The fan-shaped illu us (Fig. 4.) I n the
young adult specimens, vascular villi of
this shape were found to form the inner
Fig. 3.-Histological
section of a villus
within the group illustrated in Fig. 2. The
section has included an avascular apical secondary villus. (Haematoxylin and eosin,
free border of the circumpatellar pad. An
almost continuous series of such villi produced a scalloped edge to this structure,
directed inwards towards the patella. In
a well-developed posterior wall, i.e., juxtafemoral villous complex, such villi varied
from delicate folds containing a single
capillary loop to structures having an attached base up to 3 or 4 mm. long and
containing a complex arcading system of
vessels. The longer-based examples of this
form have been classified as folds. It was
apparent that villi of this nature had not
developed in the immature synovial membrane and, though often recognizable in
specimens from elderly persons, had undergone much degeneration.
3. Villous folds. This category was limited to include only minute vascular synovial folds. Villous folds were quite consistently found in all the adult knee joint
specimens examined. The juxta-patellar
Fig. 4.-A vascular fan-shaped villus. One
of a series arising from the circumpatellar pad
and directed towards the patella. (Oblique
light, X80).
surface of the circumpatellar pad in particular was usually folded so as to resemble
the bellows of a concertina. A transverse
section through this pad showed, on microscopical examination, a series of parallel
tongue-like projections of synovium, directed into the circumpatellar recess (Figs.
5 and 6 ) . Less consistently, the surface of
the suprapatellar pad was sometimes
thrown into a series of overlapping vascular folds. Villous folds from the lower
posterior part of the suprapatellar pouch
are shown in Figs. 7 and 8.
Abnormal Villi
1 . Degenerate uilli. It was apparent that
beyond the fifth decade, there was a decrease in the well-formed vascular villi of
the type considered above, with a marked
increase in other types of villous processes.
a) Infiltrated uilli. Leaf -shaped villi were
Fie. 5.-A transverse section through the
circukpatellar pad. A substantial fibro;s fold
the more
deeply placed vascular folds in this particular
section. (Haematoxylin and eosin, X36).
transverse section through the
folds illustrated in Fig, 7 . (Haematoxylill and eosin, 168).
Fig. 6.-A higher power photomicrograph
of two of the tongue-like vascular folds seen
in Fia. 5. (Haematoxylin and eosin, X168).
seen only occasionally in the joints from
aged persons. Instead, club-shaped villi
were found, which varied in appearance
from vascular structures expanded with
areolar tissue to large fat-filled villi 4 4
mm. in length. It was considered that these
villi arose as a result of connective tissue
increases within leaf-shaped villi.
b) Attenuated villi. Arising individually
from the synovium as slender filiform
strands, and made up of little more than
a pair of small blood vessels, these villi
were considered to be the attenuated remnants Of leaf-shaped
c) Pseudovilli (Figs. 9 and 10). Villi of
this form largely accounted for the apparent increase of synovial processes with
age in the knee joints of the elderly. These
were formed as the result of multiple irregular tears deep into the various delicate synovial folds and villous processes.
The disproportionate length of some of
these processes was felt to be the result of
tears extending parallel to the vascular
arcades after the initial rupture at the free
surface. As discussed below, there was also
some evidence of active Iongitudinal
Fig. 8.-A
Fig. 9.-Tears extending into the superior
free border of the suprapatellar pad. Note the
superficial network of vascular loops. (Evans
blue dye, oblique light, XSO).
Fig. 10.-Tag villi on the posterior aspect
of the quadriceps expansion. (Evans blue dye,
oblique light, X80).
Fig. 11.-Rheumatoid
villi. No vascular
network is visible beneath the granular opaque
surface. (Oblique light, XSO).
Fig. 12.-Photomicrograph of the specimen
shown in Fig. 11, which shows the villous
surface to be of fibrinoid material and blood
vessels to be absent. (Haematoxylin and eosin,
x 200 ).
Villous processes which have been referred to as “tag” villi were noted on all the
synovial surfaces studied, after such surfaces had been stained with Evans blue
dye. They appeared as minute degenerate
sheets, strips or strands of synovial membrane. It was thought probable that these
structures arose as the result of the synovial
membrane being stripped from the underlying supporting tissues.
2 Proliferative villi (Fig. 11). a) Hyperplastic uilli. In specimens of synovial membrane obtained from the 10 joints affected
by rheumatoid arthritis the villous structure
was found to be distinctive. It is probable
that these villi were derived both from preexisting synovial villi and by spontaneous
growth, as numerous rheumatoid villi may
be found in situations where normal villi
are not commonly seen.
The characteristic villi of rheumatoid
arthritis varied very much in shape; but
were always bulky and dense, and had
nothing of the delicate form normal villi
possess. They were either elongated and
pedunculated or squat and sessile. Adhesions often formed bridges between them
and vessels might Or might not be Seen
coursing over their surfaces. They arose
singularly or formed a complicated Key2 has reproduced drawings by Hammar3 of villi projecting from a synovial
branching vegets‘1t’ion.
It seemed that the vasculature of such surface. Key indicated that villi could be
villi gradually became imbedded in an in- classified according to the type of undercreasing matrix of granulation tissue and lying connective tissue to which they were
in villi which, histologically, had under- attached, i.e., that areolar, fibrous and fatty
gone partial fibrinoid degeneration; no villi could be recognised, and that they
superficial vascular network was visible by might be pedunculated or sessile and could
have secondary villi projecting from them.
oblique light observation.
6 ) Secondary villi (Figs. 2 and 3). He also drew attention to the distinction
These minute villi, made clearly visible by between membranous folds and localized
Evans blue staining of the specimen, arose villous processes. Sigurdson described
as small projections from the surfaces of tapering and blunted villi and observed
all forms of parent villi and membranes. that these projecting villi were connected
They appeared either as short stubble-like to the joint capsule by a pedicule or stalk.
During the present study it appeared
processes, or as lengthy strands. Histological examination showed these to consist that a classification relating the type of
of cords of ovoid cells enclosed in a net- connective tissue core to that found in the
work of reticulin. The finer of these struc- subjacent synovium was not always infaltures had no vascular core; but the more lible (areolar villi are often found arising
bulky enclosed one or more capillaries.
from the fat pads), and that a morphological classification based on histological
A Morphological Classification of Synovial
studies failed particularly to distinguish
pedunculated villi from membranous or vilThe following systematic classification of lous folds. Neither of these methods of
villi is based on those forms which have classification is of particular help in the
been described in this paper:
problem of correlating morphology with
Normal oilli: (Villi presumed to be normal the probability of some normal function.
It was felt that in describing the external
functional appendages of the synovium )
form of synovial villi, a practical basis for
1) Leaf-shaped villi
2 ) Fan-shaped villi
their classification and information as to
3 ) Villous folds
their probable life history would be proAbnormal villi and pseudooilli. (Villi provided.
duced by or damaged by disease)
Sigurdson4 and Gardner and Gray5 have
1 ) Degenerate villi
illustrations of structures in
a ) Infiltrated villi
b ) Attenuated villi
human foetal joints described as villi,
c ) Pseudovilli (membranous villi and though these structures could equally
tag villi)
well be synovial folds, rather than, as is
2 ) Proliferative villi
implied, finger-like projections. The presa ) Hyperplastic villi
ent study has included the observations
b ) Secondary villi
that fully developed leaf-shaped villi may
be present in the human foetal knee at 36
The external morphology of the synovial weeks. Villous folds could well develop by
villus seems not to have been studied in a concertina action across the membrane;
detail. Interest in these structures is, how- but normal pedunculated villi no doubt deever, of long standing. Their original de- velop as organized synovial outgrowths.
scription is attributed to Lushka, 1 and
The present observations also confirmed
a previous account by Bennett, Waine and
Bauer6 that degenerative changes could
appear within the knee by the third decade.
These authors and others7 observed, but
could not account for, the frequent increase
and prominence of “villous” processes with
increasing age. The term hypertrophy, as
used by most authors in referring to this
phenomenon, is for the most part inappropriate, as the underlying process is predominantly one of degeneration, with tears
appearing in pre-existing synovial structures. Though the initial impression on examining the synovial membrane from an
aged joint is usually one of increased villous number and size, closer examination
has revealed attenuated, infiltrated and
torn villous remnants, which are devoid of
an organized vasculature and could well
be physiologically functionless. Any increase in bulk is usually the result of connective tissue infiltration, predominantly
fat. Only the presence of the opalescent
thread-like secondary villi could be regarded as resulting from continued growth.
Meyers regarded intracapsular fringes of
processes and tags as being mostly attrition products.
Any attempt to correlate villous form
with likely function remains speculative.
It would seem reasonable to suppose that
a rich blood supply, combined with a large
surface area, does parallel the potentiality
of the membrane for absorption and secretion. On occasion the appearances of the
circumpatellar and suprapatellar regions
were almost gill-like in form, being made
up of superimposed longitudinal folds of
synovial membrane. However, Clarks did
not find any specific localization of absorptive function to the synovial villi using
intra-articular injections of trypan blue.
The various forms of normal pedunculated
villi within the knee do vary numerically
from joint to joint and in general have
seemed to be of less importance than the
more consistent villous folds.
Some comments have been made concerning the vascular architecture of synovial villi. Reference should be made to the
study of the vascular anatomy of the
synovial membrane reported by Ferri,l”
whose illustrations of villous vascular patterns are well worth studying.
The financial support for this work was provided
by the Golden Kiwi Medical Research Distribution
Committee. My thanks go to Dr T.C. Highton
for his help and constructive criticism.
The synovial villus is normally an extremely vascular structure of leaf-shape, fanshape or folded form.
With age degenerative changes occur, consisting of infiltration, attenuation, traumatic
reduplication and longitudinal growth. The increase in “villous” processes with age is
mainly due to the formation of pseudovilii. Normal villous structures become damaged
and degenerate.
In rheumatoid arthritis the abnormal gross morphology is distinctive and can be
correlated with the better-known histological changes.
Le villo synovial es usualmente un structura extremente vascular con un conformation
de folio o de flabello o plicate.
Alterationes degeneratori occurre con le avantiamento del etate. Istos consiste de
infiltration, attenuation, reduplication traumatic, e crescentia longitudinal. Le augment0
in le processos “villose” con le avantiamento del etate del subject0 es le resultato del
formation de pseudovillos. Normal structuras villose deveni lesionate e suffre degeneration.
In casos d.e arthritis rheumatoide, le anormalitates macromorphologic es distincte e
pote esser correlationate con le melio cognoscite alterationes histologic.
1. Lushka, H.: Zur Entwicklungsgeschichte der
Gelenke. Arch. Anat. u. Physiol, 1885, p.
481. Quoted by Kling, 1938.
2. Key, J. A.: The synovial membrane of joints
and bursae. Special Cytology, Ed. 2, 1084.
New York, Paul B. Hoebler, 1932.
3. Hanimar, J. A.: Ueber den fieneren bau der
gelenke. I. Die Gelenkenmembran. Arch.
Mikr. Anat. 43:266, 1894.
4. Sigurdson, L. A.: The structure and function
of articular synovial membranes. J. Bone
Jt. Surg. 12:603, 1930.
5. Gardner, D. .I. and Gray, E.: Prenatal development of the human knee and superior
tibio-fibular joints. Amer. J. Anat. 86:235,
6. Bennett, G. A., Waine, H., and Bauer, W.:
Changes in the knee joint at various ages.
Ed. 1, 57. New York, The Commonwealth
Fund, 1942.
Parker, F., Keefer, C. S., Myers, W. K. and
Irwin, R. I.: Histological changes in the
knee joint with advancing age. Arch Path.
17:516, 1934.
Meyer, A. W.: The minuter anatomy of attrition lesions. J. Bone Jt. Surg. 13:341,
Clark, Le G.: An experimental study of the
nature of the synovial membranes of joints.
J. Anat. 63:152, 1928.
Ferri, E.: Ulterio osservazioni istobiologiche a
contrast0 di fase sullo strato sinoviale.
Ateneo Parmense 35:1, 1964.
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structure, examination, compartment, villian, joint, knee, within, synovial, anterior, metacarpal, phalangeal
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