Synovial Tumors and Proliferative Diseases

The clinical diagnosis of synovial tumors and synovial proliferative processes is difficult, as symptoms and physical examination findings are often nonspecific. Advanced imaging modalities, such as MRI, high-resolution ultrasound, and computed tomography, can aid clinical decision-making by providing accurate diagnosis of such diseases in many cases. This article focuses on those specific imaging features of synovial tumors and proliferative processes that can provide accurate diagnosis and guide appropriate patient management.

Key points

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Clinical diagnosis of synovial tumors and proliferative processes remains difficult.
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Advanced imaging, particularly MRI, can provide accurate diagnosis of synovial tumors and proliferative processes.
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Most synovial tumors are benign but can be locally aggressive and cause significant morbidity.

Introduction

Accurate clinical diagnosis of synovial tumors and synovial proliferative diseases is difficult because symptoms and physical examination findings are often nonspecific. Advanced imaging modalities, such as MRI, high-resolution ultrasound, and computed tomography (CT), can aid clinical decision-making by providing accurate diagnosis of such diseases in many cases. Conventional radiography proves useful in the imaging evaluation of synovial processes not only serving as a baseline examination but also as a complement to advanced imaging. This article focuses on imaging features of synovial tumors and proliferative processes that can provide accurate diagnosis and guide appropriate patient management. In addition, technical and anatomic considerations pertinent to imaging the synovium are discussed.

Introduction

Accurate clinical diagnosis of synovial tumors and synovial proliferative diseases is difficult because symptoms and physical examination findings are often nonspecific. Advanced imaging modalities, such as MRI, high-resolution ultrasound, and computed tomography (CT), can aid clinical decision-making by providing accurate diagnosis of such diseases in many cases. Conventional radiography proves useful in the imaging evaluation of synovial processes not only serving as a baseline examination but also as a complement to advanced imaging. This article focuses on imaging features of synovial tumors and proliferative processes that can provide accurate diagnosis and guide appropriate patient management. In addition, technical and anatomic considerations pertinent to imaging the synovium are discussed.

Anatomic considerations

Synovial joints are composed of 2 layers, a thick superficial fibrous capsule and a thin inner layer, the synovial membrane. The fibrous capsule functions to stabilize the articulation between apposing bones, whereas the synovial membrane is composed of specialized mesenchymal tissue. This tissue lines intra-articular structures, including intra-articular ligaments, tendons, and the intracapsular periosteal surfaces not covered by hyaline cartilage. Synovial membranes also line tendon sheaths and bursae about the body.

Synovial membranes are also divided into 2 layers, the synovial intima and the subsynovial tissue. The synovial intima is composed of synoviocytes, macrophages, and fibroblasts and is one to 4 cell layers in thickness. Underneath the intima is the subsynovium that consists of adipocytes, macrophages, lymphatics, and loose connective tissue. The subsynovium contains a vascular and lymphatic network through which fluid enters a joint as an ultrafiltrate of plasma. It is this plasma plus the hyaluronic acid secreted by synovial cells that forms joint fluid and that nourishes and lubricates the articular surface. The synovial membrane also functions as a shock absorber and filter system for the joint.

The synovial membrane may give rise to a variety of tumors and tumorlike proliferative processes, the most common of which are giant cell tumor of the tendon sheath (GCTTS), pigmented villonodular synovitis (PVNS), localized nodular synovitis (LNS), synovial chondromatosis/osteochondromatosis, lipoma arborescens, synovial hemangioma, and fibroma of the tendon sheath. Synovial tumors and proliferative diseases can be part of either a systemic disease or a primary synovial process that can affect a single joint. Rarely, malignant synovial tumors can be present de novo, via malignant transformation from a benign synovial process, or from local extension.

Imaging evaluation and technical considerations

Routine radiography should be a part of any imaging workup of a suspected synovial-based process and, at the very least, can serve as a baseline imaging examination for future follow-up. Radiographs are well suited to assess for mineralization, erosions, joint effusion, and joint space loss. CT can complement routine radiography and with its multiplanar ability can further characterize mineralization, subtle erosions, and joint fluid density. PET-CT does not currently play a significant role in the evaluation of synovial tumors and synovial proliferative processes. In the past decade or so, utilization of ultrasound has markedly increased for musculoskeletal evaluation, particular when there is a palpable soft tissue mass. Some synovial tumors can present as palpable masses, such as those in the hand where GCTTS commonly occur. Ultrasound in this setting is an appropriate imaging modality for evaluation.

Because of its excellent soft tissue contrast and multiplanar capability, MRI is the imaging modality of choice for the comprehensive evaluation of suspected synovial-based processes. Controversy remains, however, with regard to the optimal MRI technique to use for this indication, particularly whether or not to use intravenous gadolinium contrast.

On MRI, the fibrous joint capsule is normally seen as a thin low-signal intensity linear structure on both T1- and fluid-sensitive sequences. The vascular inner synovial membrane is, however, difficult to distinguish from this outer capsule. When visible the synovial membrane is intermediate in signal intensity on proton-density and T1-weighted sequences and high signal intensity on fluid-sensitive sequences, similar to joint fluid.

Although the synovium can definitely be evaluated well without the use of intravenous contrast, it is generally accepted that the use of contrast delineates the synovial membrane best. Thickening and nodularity in the form of synovial proliferation and hypertrophy reflect inflammation, best seen on T2-weighted sequences. However, thickened synovium in some cases cannot be distinguished from joint fluid on unenhanced MRI ; thus, MRI with intravenous gadolinium administration is often used.

Some investigators have recommended against the use of contrast for synovial-based processes. One reason for this is that the exact border between an effusion and the synovium can be difficult to distinguish in the presence of contrast material. This difficulty is due to the diffusion of contrast material into the joint over time resulting in the enhancement of the effusion. Moreover, in a study by Boegård and colleagues, synovial enhancement was present in joints of healthy, asymptomatic individuals; the synovial thickness in this study varied significantly in these asymptomatic individuals.

In addition to a combination of anatomy-specific (T1-weighted) and pathology-sensitive (T2-weighted or inversion recovery) sequences, gradient-echo (GRE) imaging sequences can be useful to evaluate those synovial processes in which hemosiderin deposition occurs because the paramagnetic effect of hemosiderin results in low signal intensity and signal decay, or blooming, more conspicuous on heavily T2*-weighted GRE sequences, which can be augmented further by higher magnetic field strength.

Benign synovial proliferative diseases

The benign synovial proliferative diseases comprise a group of pathologic processes that arise from the synovial membrane of joints, bursae, or tendons. Although the exact cause of such diseases remains unknown, current evidence supports a neoplastic cause to these processes.

The benign synovial proliferative processes are best categorized by location (intra-articular or extra-articular) and extent of disease (localized or diffuse). There is frequent overlap between these categories and subsequent confusion with the nomenclature of these entities. For example, when the disease is localized but intra-articular the term LNS is often used. The 3 most commonly encountered benign proliferative synovial diseases categorized in this manner include extra-articular localized (GCTTS), intra-articular diffuse (PVNS), and intra-articular localized (LNS).

Giant cell tumor of the tendon sheath

GCTTS typically presents as a slowly growing, painless soft tissue mass. It most often develops between the third and fifth decades and shows a female predominance. Most lesions are found in the hand and wrist. The foot and ankle are the second most common sites.

Although radiographs are normal in 20% of cases, they are important to obtain as a baseline examination. The underlying bone is usually normal but can show radiographic abnormalities, such as pressure erosions, which are reported in 15% to 32% of cases. In addition, in 50% to 70% of cases the radiographs show a soft tissue mass.

With regard to cross-sectional imaging, both ultrasound and MRI can accurately evaluate local extent of the lesion and involvement of adjacent structures. Sonographically, GCTTS demonstrates a well-defined solid, hypoechoic, vascular mass that abuts a tendon ( Fig. 1 ). Broad contact of the mass with the tendon favors GCTTS, although the extent of contact with tendon is variable. Because GCTTS arises from the tenosynovium and not the tendon itself, the mass typically does not move when the tendon moves, such as with flexion/extension dynamic imaging. Ultrasound is well suited to perform such dynamic imaging maneuvers.

On MRI, GCTTS demonstrates a well-defined mass that is intimately associated with a tendon. Signal characteristics on MRI are variable and nonspecific, with T1 hypointense to isointense to muscle and T2 intermediate intensity ( Fig. 2 ). Magnetic susceptibility artifact related to the hemosiderin deposition can be present on GRE sequences. Avid enhancement after gadolinium contrast administration is reported in most cases.

Surgical resection of GCTTS is usually curative with a reported low rate of local recurrence.

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Hand and wrist most common
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Well-defined mass that contacts a tendon
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Mass that does not move with the tendon with dynamic ultrasound
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Variable MRI signal characteristics
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Low rate of local recurrence after surgical resection

Key points for GCTTS

Pigmented villonodular synovitis

PVNS is characterized by the development of villi with nodular thickening of the synovial membrane and deposition of hemosiderin-laden macrophages. Although the cause remains unknown, current theories suggest a combination of a reactive inflammatory process and chromosomal translocation resulting in neoplastic proliferation of the synovium. PVNS is overall a rare disease, has equal male and female prevalence, and is most commonly seen in those aged between 20 and 50 years.

PVNS most commonly occurs in the knee (75%), hip (15%), ankle, and shoulder. Involvement of more than one joint is rare, and diffuse involvement of an affected joint is the common presentation. Diffuse involvement presents typically with a joint effusion, limited range of motion, and tenderness. Joint fluid aspiration reveals an elevated cholesterol content, with hemarthrosis in 75% of cases.

Conventional radiographs serve useful as a baseline examination but are often nondiagnostic. They may reveal joint effusion or soft tissue swelling, without calcifications ( Fig. 3 ). Advanced PVNS may demonstrate joint space narrowing and periarticular erosions.

MRI is the advanced imaging modality of choice for the diagnosis, evaluation, and surgical planning of PVNS. Characteristic MRI features of PVNS allow an accurate diagnosis in 83% to 95% of cases. Magnetic resonance (MR) features include synovial hypertrophy and irregularity with diffuse low signal nodularity (see Fig. 3 ). GRE imaging demonstrates characteristic blooming artifact related to the paramagnetic effects of iron in hemosiderin deposition ( Fig. 4 ). On short tau inversion recovery (STIR) images, the lesions tend to be intermediate in signal intensity ( Fig. 5 ). The hemosiderin deposition produces frondlike projections along the synovium, with enhancement after gadolinium administration. Diffuse disease can extend beyond the joint capsule.

Although a benign disease, diffuse PVNS can be locally aggressive and result in joint damage. Nonoperative management includes physical therapy and corticosteroid injection. External beam radiation therapy has been described for nonresectable lesions or nonoperative candidates. More recently, it was shown that an oral small molecule, PLX3397, that inhibits certain kinases was well tolerated and demonstrated a profound activity response in advanced PVNS cases.

Surgical treatment is controversial and depends on the extent of disease. The diffuse nature of the disease makes complete surgical resection difficult, with recurrence rates reported between 8% and 56%. Arthroscopic excision with complete synovectomy has been proposed by some investigators as the surgical treatment of choice. However, incomplete arthroscopic excision is a concern, with arthroscopic recurrence rates up to 25% for diffuse intra-articular disease. For this reason, some investigators advocate open arthrotomy excision with complete synovectomy to decrease the risk of incomplete excision. The addition of radiation therapy after incomplete surgical resection has been advocated by some, with recurrence rates in these cases similar to rates of patients who undergo complete synovectomy alone.