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.
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 FIGURE 2b).
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.
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.
Treatment of PVNS
The most commonly performed treatment for diffuse, pigmented villonodular synovitis is a surgical synovectomy. Synovectomy simply means removal of the internal joint lining membrane. Before arthroscopic techniques were available, synovectomy was performed in an open fashion, with a large frontal incision literally opening the knee up like a book so that the majority of the joint lining tissue, which is in the front of the joint, could be stripped out. Opening up the rear of the knee to remove the synovial lining in the back of the joint was sometimes done as a more radical extension of the frontal synovectomy procedure. These open surgical procedures had serious drawbacks, in that they were quite painful and often produced major post-operative morbidity such as joint scarring and stiffness. In addition, as extensive as these procedures were, every last bit of abnormal synovial tissue was still not removed. As noted previously, to perform a genuinely complete surgical synovectomy, extreme damage to the joint would have to be done because the knee cartilages (menisci) would effectively have to be removed, along with portions of the internal (cruciate) ligaments. Only such radical surgery would allow access to all of the small nooks and crannies within the knee joint where PVNS tissue can hide. While not necessarily more effective than open synovectomy, in most cases a better approach is arthroscopic synovectomy (see FIGURE 4).
For patients interested in the technical aspects of arthroscopic knee synovectomy, reprints of my illustrated surgical technique paper, published in Master Techniques in Orthopedic Surgery,can be obtained by either writing or e-mailing our Knee and Shoulder Centers office. Simply stated, the objective of arthroscopic synovectomy is to remove as much abnormal joint lining tissue as is technically feasible without damaging the patient's knee in the process. Properly and meticulously performed, it is a very lengthy and technically difficult surgical procedure. It requires mastery of almost every knee access technique ever devised by arthroscopic surgeons. All of the major internal regions of the knee joint, in sequence, must be both visualized and accessed by surgical re-section instrumentation, all the while keeping the number of access (arthroscope portal) incisions to a minimum and avoiding unintended neurovascular injury or damage to other internal joint structures. The surgeon must be comfortable working in the posteromedial and posterolateral knee compartments (see FIGURE 5) and have the patience to follow the abnormal joint lining tissue down into every accessible fold and recess elsewhere within the knee joint. When PVNS disease afflicts the cruciate ligaments, the synovial lining membrane of these ligaments must be carefully and meticulously dissected away (both in the front and sometimes in back) without damaging the ligaments themselves (seeFIGURE 6).
Clinical experience has generally demonstrated that the more complete the degree of synovectomy achieved, the lower the recurrence rate of the disease. However, even with a consummately skilled surgeon using the most advanced arthroscopic techniques, it is still impossible to remove every last bit of PVNS tissue in the diffuse form of the disease. Current surgery is, therefore, limited to a thorough "debulking"process that falls short of total resection. The surgeon attempts to achieve the greatest possible debulking effect in order to minimize the recurrence rate,while still leaving the patient with a functional knee afterwards. Why the disease does not recur in all cases, following what we know to be less than a 100% synovectomy, is unknown. Diffuse PVNS is truly a strange disease that appears for uncertain reasons and may also completely exit a patient's life following treatment, even though by definition that treatment is known to have been incomplete. If there has been any common etiologic (causal) thread in many of the PVNS patients I have treated, it has been that they had some episode of knee trauma in their past that caused bleeding within their knee joint. Whether a brief exposure of a synovial joint to the iron-based hemoglobin pigment released by degrading red blood cells can actually cause PVNS has never been proven and at this point must be relegated to personal speculation.
Other means of treating pigmented villonodular synovitis include radiation beam therapy and radio isotope synovectomy. In the latter procedure a liquid, radioactive isotope is injected into the knee joint, subjecting the synovial lining to radiation treatment. This is an uncommon treatment that in the past has generally been reserved for difficult cases of PVNS that have recurred following a prior surgical synovectomy. Very few medical centers in the United States even offer such treatment, which is usually done under carefully controlled conditions in research protocols. Either form of radiation therapy can also be used as an adjuvant (additional or back-up) treatment following a surgical synovectomy.
If PVNS remains uncontrolled, over a period of years the patient's knee joint surfaces may gradually become destroyed, leading to a need for radical joint resection and replacement by prosthetic components ("total knee replacement" surgery). The initial radical resection stage of such a procedure allows relatively complete exposure inside the joint and thus an unusually complete surgical synovectomy. This often leads to a final cure of the disease, but the original knee joint has been lost forever.
In this authors experience, a recurrence of diffuse PVNS following
an initial synovectomy procedure is still worth treating with another
attempt at a comprehensive arthroscopic synovectomy,carried to the most
radical extent that is feasible. In some cases unusual measures such
as resecting the soft tissue from behind the posterior cruciate ligament
must be performed, as that is a site where nests of PVNS tissue may
form and even begin to reach outside the confines of the knee capsule.
Resection of such extra capsular (outside of the knee joint proper)
PVNS tissue with an arthroscope is an extremely tricky process, particularly
in the posterior regions of the knee joint where the major neuro vascular
structures that feed the lower leg and foot are located. Most scientific
articles and book chapters on PVNS do not even discuss the fact that
PVNS tissue can occasionally escape the confines of the joint cavity
to invade soft tissues outside of the capsule. While it is well known
that PVNS tissue can actually grow into the femur or tibia itself, apparently
by following vascular (blood vessel) channels into the bone, it is less
commonly appreciated that PVNS tissue can grow along small, trans-capsular
passage ways leading out of the knee joint cavity to form masses of
tissue in external regions. Two such regions are known as the lateral
popliteus recess and the medial gastrocnemius-semimembranosus bursa
(see FIGURE 7). Extra-capsular extensions
of PVNS disease are best detected pre-operatively by way of MRI scanning
with special, hemosiderin-weighted imaging techniques. If identified,
such extra-articular nests of PVNS tissue are usually best resected
by way of localized, open surgical exploration, leaving the intra-articular
PVNS tissue to be resected by arthroscopic means.
Resolution of the disease process is usually heralded by an elimination of the patient's prior recurrent knee effusions (excess joint fluid accumulations). Post-operative MRI scans are extremely difficult to read because of scar tissue artifact. If at some point excess knee joint fluid begins to return, this is suggestive, but not definitive proof of, recurrence. If long-lasting, the presence of recurrent fluid suggests the need for a follow-up MRI scan and an arthroscopic re-inspection of the joint. For recurrent disease, a combination of repeat synovectomy, possibly followed by irradiation or radioisotope, adjuvant synovectomy, can be considered. Individual consultation between the patient and the subspecialists at those few medical centers offering the latter technique is required to determine whether the patient is a potential candidate for this treatment.