In Brief: PVNS is an uncommon joint
disease that usually afflicts the knee. It generally appears in
either a localized or widespread form, both of which are characterized
by overgrowth of the joints lining tissue (synovium). The
cause is unknown. While not malignant, local tissue overgrowth
within the joint can be difficult to control and may result in
chronic joint swelling and ultimately, joint damage and arthritis.
There is no effective non-surgical treatment, and in many cases
there are specific limitations and drawbacks to currently available
surgical treatment methods. PVNS of the knee can be a difficult
problem that requires both knee specialty care and excellent arthroscopic
surgical technique to manage optimally.
1 - This illustration depicts the knee joints
synovial lining membrane, diagrammatically. This tissue layer
lies just inside of the joints surrounding capsular
(envelope) ligament, and encloses the internal joint space.
The picture depicts the synovial membrane as if it were distended
with fluid, as often occurs in PVNS disease, thus accounting
for its expanded appearance.
Pigmented villonodular synovitis, or PVNS, is an uncommon but
not necessarily rare disease that affects the internal lining
membrane of the body's moving joints. Large joints are the most
commonly afflicted, with the most frequent site being the knee.
Joints where movement occurs between one or more bones always
have a surrounding envelope of ligament tissue (capsule) that
is lined internally by a thin, soft tissue membrane (colored
in blue, see FIGURE 1)comprised of synovial cells.
This synovial membrane secretes the lubricating (synovial) fluid
that keeps friction between its gliding, cartilaginous surfaces
of joints to an absolute minimum. A youthful, normally functioning
synovial joint generates less friction than one smooth piece of
ice gliding upon another!
For unknown reasons, some or all of the synovial
lining tissue of a joint occasionally undergoes a change and becomes
diseased, wherein the joint lining tissue becomes thick and overgrown
and accumulates a rust-colored, iron pigment known as hemosiderin.
Strange, foamy cells and large (so-called "giant") cells
with many nuclei also appear. The overgrowth of the joint lining
tissue can occur diffusely throughout a joint by
way of a generalized thickening of the entire lining membrane,
or a localized area of synovial membrane can overgrow
and form a discrete nodule (tissue mass) that remains attached
to the rest of the internal joint lining by way of a stalk. While
this disease process does involve abnormal tissue growth, it is
uniformly benign and has not been known to metastasize
as do malignant growths. PVNS can be considered a benign, "neoplastic"
(tumor growth) process, with some varieties being more aggressive
in their growth and thus harder to treat, and other varieties
being less aggressive in their growth and thus easier to treat.
Medical pathology textbooks generally describe two forms
of PVNS. The localized form is called "nodular" PVNS.
The non-localized form is referred to as "diffuse" PVNS.
While this author has never found an "in-between" variety
referenced in any textbook, I have seen and treated cases that
appeared to represent a mixed (diffuse and nodular) form
of the disease.
2a - This is a photograph (taken through the arthroscope)
of a nodule of localized PVNS tissue growing within a patients
knee joint. It was attached to the inner aspect of the medial
joint capsule. It became noticeable when it grew large enough
to begin putting pressure against the adjacent medial femoral
condyle. No other PVNS tissue was in this patients knee.
The localized (nodular) form of PVNS is generally
the least difficult to treat, as all of the abnormal tissue growth
is located in a single, excisable mass within the joint (see
FIGURE 2a). It is usually painless and often does not become
noticed until the tissue nodule reaches a size that causes swelling
or some type of internal joint impingement symptoms to the patient.
Obvious mechanical problems such as joint locking and snapping
can occasionally occur, simulating a torn cartilage or other structural
problem. On other occasions, the patient may simply feel a slowly
growing soft tissue mass within their knee joint and/or their
knee may become distended with excess synovial fluid (so-called
"water on the knee"). MRI scanning is the best non-invasive
means of identifying the presence of a synovial growth within
a joint. Treatment is usually rendered both simply and effectively
by way of either arthroscopic or "open"(access by way
of a traditional incision) resection of the abnormal tissue (see
2b - This photograph shows the same nodule of PVNS
tissue as seen in FIGURE 2a, following its surgical removal
from the joint. It measured approximately 3 cm in diameter,
being about the size of an almond shell. Histological evaluation
under the microscope confirmed that it was composed of PVNS
The diffuse form of pigmented villonodular synovitis is
more problematic. The synovial lining everywhere within
the large, complex multi-compartmental knee joint becomes overgrown,
sometimes taking on a rust-colored, "bearded" appearance
(see FIGURE 3). Every tiny nook and
cranny within the knee joint may be afflicted, making it impossible
to surgically eliminate every last bit of abnormal tissue without
literally destroying the knee and its surrounding capsule in the
process. For this reason and perhaps others, the diffuse form
of PVNS is much more likely to recur (relapse) following
attempted surgical resection than the nodular form. Almost nothing
is known about the mixed form, but in this author's opinion it
is probably best viewed as a variety of the diffuse form of the
disease, given that the nodules are small and interspersed amongst
diffuse synovial lining overgrowth.
3 - This arthroscopic photograph demonstrates the close-up
appearance of diffuse PVNS tissue lining the inside of an
afflicted patients knee joint. The synovial tissue is
dark orange and hypertrophic (overgrown), giving the internal
lining of the knee joint an "orange beard" appearance.
One thing that the diffuse form of PVNS has in
common with the nodular form is that it is often
misdiagnosed initially. The former may be even
harder to diagnose than the latter because it does not have a
discrete, nodular soft tissue mass as its calling card. Diffuse
PVNS is almost always an entirely painless disease, at least in
its early stages, and it most commonly presents itself as simple
knee swelling caused by an excessive accumulation of synovial
fluid within the joint. This excess fluid is secreted by the overgrown
joint lining tissue. Unless the quantity of fluid in the joint
becomes so great as to stretch the joint capsule and cause discomfort,
patients often regard this as more of an annoying curiosity than
a significant problem at first. Patients who present for medical
evaluation find that their knee x-rays are almost always normal
and that all of their blood tests are negative for diseases like
rheumatoid arthritis, lyme disease, lupus, etc. Clinicians
should always keep in mind the possibility of PVNS when presented
with a patient who is suffering from unexplained, recurrent fluid
accumulations in one of their joints, especially when the joint
fluid has a slightly more orange-brown color to it than normal.
Sometimes the delicate, overgrown joint lining tissue bleeds intermittently
and thus even red joint fluid may be found at times. The
presence of diffuse PVNS is verified by direct arthroscopic inspection
of the interior of the joint and taking a synovial tissue biopsy.
Microscopic tissue analysis is required to make a firm pathologic
diagnosis. Once this has been confirmed, treatment must be planned.