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1 Department of Radiology, Tri-Service General Hospital, National Defense
Medical Center, 325, Section 2, Cheng-Kung Rd., Neihu, Taipei 114, Taiwan,
Republic of China.
2 Department of Orthopaedic Surgery, Tri-Service General Hospital, National
Defense Medical Center, Neihu, Taipei 114, Taiwan, Republic of China.
3 Department of Radiology, School of Medicine, Taipei Medical University, 250
Wu-Hsing St., Taipei 110, Taiwan, Republic of China.
4 Department of Radiology, Taipei Medical University, Municipal Wan Fang
Hospital, 111 Hsing-Long Rd., Section 3, Taipei 116, Taiwan, Republic of
China.
5 Department of Radiology, Ohio State University Medical Center, 633 Means Hall,
1654 Upham Dr., Columbus, OH 43210.
6 Department of Radiology, Veterans Affairs Medical Center, 3350 La Jolla
Village Dr., San Diego, CA 92161.
Received January 13, 2003;
accepted after revision February 27, 2003.
Address correspondence to G.-S. Huang
(gsh5{at}seed.net.tw).
Abstract
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MATERIALS AND METHODS. A retrospective review of MR imaging of the knee was performed in 21 patients with histologically confirmed localized nodular synovitis. Surgical excision of the lesion was performed in all patients. The MR imaging appearances of the lesions were defined, and the clinical and surgical findings were reviewed.
RESULTS. All lesions presented as a solitary intraarticular mass. The mass originated from the synovial lining in different locations including the infrapatellar fat pad (n = 14), suprapatellar pouch (n = 5), and posterior aspect of the intercondylar notch (n = 2). A small (2.0–3.5 cm; mean diameter, 2.7 cm) ovoid mass (n = 13) was more common than a large (5.0–9.0 cm; mean diameter, 6.5 cm) polylobulated mass (n = 8). At surgery, a long pedicle attached the mass to the adjacent synovium in two patients, but this was observed on MR imaging in only one patient. The lesions showed intermediate (n = 15) or hyperintense (n = 6) signal intensity on T1-weighted images and heterogenously high (n = 13) or low (n = 8) signal intensity with variable circular foci of low signal intensity on T2-weighted images. On T2-weighted images, linear regions of high signal intensity within the mass were seen in seven lesions. Prominent enhancement of the lesion with IV contrast administration was shown in all patients who were given contrast material (n = 10). Knee pain, joint swelling, and a palpable mass were the most frequent clinical manifestations. An acutely painful knee was noted in one patient who presented with torsion of an infrapatellar pedicle. Five patients complained of locking of the knee, but at physical examination, restricted terminal knee extension was noted in nine patients.
CONCLUSION. Localized nodular synovitis of the knee predominantly involves the infrapatellar fat pad. It may produce symptoms related to mechanical derangement of the knee. Although there is no typical MR appearance for this lesion, many features help to differentiate it from pigmented villonodular synovitis.
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MR imaging was performed on 1.5-T scanners (Vista, Picker, Cleveland, OH; Signa, General Electric Medical Systems, Milwaukee, WI) with dedicated extremity coils. The examinations consisted principally of a combination of spin-echo T1-weighted (TR range/TE range, 450–650/15–20), spin-echo proton density–weighted (1800–2200/20–30) or fast spin-echo proton density–weighted (TR range/TE, 3000–3600/20), spin-echo T2-weighted (TR range/TE range, 1800–2200/80–90) or fast spin-echo T2-weighted (2500–3300/55–80), and fat-suppressed fast spin-echo T2-weighted (3000–3600/55–80) sequences. Variable combinations of sequences and imaging planes were used. In eight patients, an additional gradient-echo (TR/TE, 650/15; flip angle, 20°) sequence in the axial plane was performed. In 10 patients, fat-suppressed T1-weighted (TR range/TE range, 450–650/15–20) spin-echo imaging was performed after IV administration of gadolinium (0.1 mmol/kg of body weight). The field of view varied between 14 and 16 cm, the slice thickness ranged from 3 to 5 mm, and the interslice gap was from 0 to 1 mm. The number of acquisitions was either one or two. The imaging matrix ranged from 192 x 256 to 256 x 256.
All MR imaging studies were simultaneously evaluated by two experienced musculoskeletal radiologists in consensus because the number of patients was small. Each lesion was evaluated for site of origin, size, morphology, signal intensity, the absence or presence of associated synovitis, and the absence or presence of a joint effusion (small, moderate, or large). The presence of synovitis was defined as abnormal thickening of the synovial lining of the joint that was distinct from the lesion. The signal intensity of the lesion was determined by comparison with that of skeletal muscle. The degree of enhancement of the lesion on fat-suppressed T1-weighted fast spin-echo imaging was quantified (none, mild, or moderate). To complete the assessment, we also evaluated the integrity of the menisci, ligaments, and articular cartilage.
All lesions were completely excised by arthrotomy or arthroscopy within 2 weeks of the MR imaging examination. The surgical features were reviewed. Clinical follow-up for all 21 patients ranged from 5 months to 6 years. Follow-up MR imaging was performed in four patients and was evaluated using the same criteria as the preoperative studies.
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The duration of clinical follow-up ranged from 5 months to 6 years. Seventeen patients had immediate relief of their symptoms after surgery. Recurrence of pain occurred in four patients, but two of these patients had sustained an injury to the knee during the follow-up period. MR imaging that was performed on these four patients showed findings of internal derangement, with meniscal tears in all four and disruption of the anterior cruciate ligament in one. No localized nodular synovitis recurred in these patients.
MR Imaging Findings
On MR imaging, a solitary mass lesion in the knee joint was identified in
all patients. The mass lesion most commonly involved the infrapatellar fat pad
(Figs. 1A,
1B,
1C and
2A,
2B,
2C) and was noted in 14
patients (67%). The other sites of involvement included the suprapatellar
pouch (Fig. 3A,
3B) in five patients (24%) and
the posterior aspect of the intercondylar notch (Fig.
4A,
4B) in two patients (10%). The
two posterior lesions were attached to the posterior cruciate ligament. The
maximal diameter of the lesions ranged between 2.5 and 9.0 cm.
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Morphologically, the lesions appeared either ovoid (n = 13) or polylobulated (n = 8). Ovoid lesions were uniformly small, ranging from 2.0 to 3.5 cm (mean diameter, 2.7 cm) in maximal diameter and originated from different locations in the knee. Polylobulated lesions were relatively larger, ranging from 5.0 to 9.0 cm (mean diameter, 6.5 cm) in greatest diameter. All polylobulated lesions originated from the synovial lining of the infrapatellar fat pad. Because of their size, four of the eight polylobulated lesions extended into the anterior intercondylar notch (Fig. 1A, 1B, 1C). One lesion had a long pedicle attached to the synovial membrane in the lateral aspect of the suprapatellar pouch (Fig. 3B).
The lesions showed intermediate (n = 15) (Figs. 1A and 2A) or slightly high (n = 6) (Fig. 3A) signal intensity relative to that of skeletal muscle on the T1-weighted images. On the T2-weighted images, all lesions showed inhomogeneous signal intensity, with areas that were hyperintense (n = 13) (Figs. 1B and 3B) or hypointense (n = 8) (Fig. 4B). With fat suppression, all lesions showed inhomogeneous and relatively high signal intensity (Fig. 2B) on T2-weighted fast spin-echo images. Variable circular regions of low signal intensity (Fig. 1B), corresponding to the deposition of hemosiderin, were noted in 16 lesions, which appeared most conspicuous on T2-weighted and gradient-echo MR images. Linear or cleftlike regions of high signal intensity (Figs. 1B and 4B) were noted in seven lesions on T2-weighted images. Moderate enhancement within the lesions (Fig. 2C) was observed in all 10 patients who were administered IV gadolinium.
Six patients had small joint effusions, and two patients had moderate-sized joint effusions. No appreciable joint effusion was present in the remaining 13 patients. Three patients had mild chondromalacia involving the medial femoral condyle and patella. No meniscal or ligament tear was identified. Generalized synovitis was not seen in any patient.
In the 14 patients with infrapatellar masses, the prospective diagnosis based on MR imaging was localized nodular synovitis versus intraarticular chondroma. In all other patients, the prospective diagnosis was localized nodular synovitis.
Every patient underwent an initial radiographic examination. In the 14 patients with lesions in the infrapatellar fat pad, the mass was evident in the lateral projection. One patient with a suprapatellar mass had positive radiographic findings of a well-circumscribed mass, but all other remaining patients had no obvious radiographic findings. The radiographic findings of localized nodular synovitis were nonspecific and included a masslike density that was well circumscribed and showed a conspicuous absence of matrix calcification or ossification. Similarly, two patients underwent CT that showed a well-circumscribed soft-tissue mass with no discernible matrix calcification or ossification in the infrapatellar fat pad and suprapatellar pouch, respectively.
Surgical Findings
All lesions were completely resected by either arthroscopy or open
arthrotomy, depending on the location of the localized nodular synovitis. All
were solitary, well-defined masses with smooth surfaces
(Fig. 1C). Both the oval and
polylobulated morphology correlated well with the MR imaging appearance.
Macroscopically, 15 lesions appeared yellow, four appeared brown, and two
appeared tan. The location of the origin of each lesion was confirmed at
surgery for all lesions. Two lesions had long pedicles, one involving the
infrapatellar fat pad and the other involving the lateral aspect of the
suprapatellar pouch. Torsion of one of the lesions was noted, but this
abnormality was not detected prospectively. No evidence of xanthochromic or
hemorrhagic effusion or synovitis was seen in any patient.
Mild chondromalacia with fibrillation of the articular cartilage in the medial femoral condyle and the patella was found in three patients, which was clearly shown on MR imaging. The menisci and cruciate ligaments appeared intact in all patients.
Histologic Findings
Histologically, the resected lesions showed features typical of localized
nodular synovitis consisting of well-defined soft-tissue masses with varying
amounts of histiocytic mononucleated giant cells, collagen strands, and
xanthomatous cells covered by a smooth lining of synovial tissue. The presence
of hemosiderin deposition was highly variable.
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The cause of localized nodular synovitis is unclear. Although Jaffe et al. [9] and Granowitz et al. [10] suggested that localized nodular synovitis is an inflammatory process, more recent studies have characterized the lesion as a benign neoplasm of the synovium [11, 12]. Traumatic, toxic, allergic, and genetic factors have also been reported to be responsible for development of this lesion [5]. In our series, only six patients had a discrete history of knee trauma before the onset of knee pain.
The infrapatellar fat pad was the most common site of involvement in our series, and this location is in agreement with the literature [5–8]. In our series, the suprapatellar pouch was the second most common site of involvement, although to our knowledge, involvement in this area has not been previously reported. Involvement of the intercondylar notch, seen in two patients in our study, is relatively uncommon, having been previously reported in five patients [1, 4, 13, 14]. When the cruciate ligaments are involved, localized nodular synovitis tends to more commonly affect the posterior cruciate ligament [4, 13, 14].
The clinical manifestations of localized nodular synovitis of the knee are nonspecific. The symptoms may include pain, swelling or fullness, joint-line tenderness, restricted knee motion, and a palpable mass. Reportedly, localized nodular synovitis may rarely present with locking of the knee, having been cited as the cause in four patients with lesions in the infrapatellar fat pad [5–7]. In our series, however, knee locking was not rare and was noted in five patients with large infrapatellar fat pad lesions. We noted that when the maximal diameter of the lesion in the infrapatellar pad exceeded 5 cm, it was likely to restrict terminal knee extension. Mechanical impingement may also stimulate the release of chemical substance P, which is rich in the synovial lining of the infrapatellar fat pad, inducing pain.
The observation of a pedicle is relevant because torsion of this pedicle can produce acute knee pain, which was seen in one of our patients. However, this condition can be difficult to diagnose unless images are carefully inspected. Of two patients with long pedicles in our study, only one pedicle was detected with confidence on the MR images.
The MR imaging appearance of intraarticular localized nodular synovitis is variable. Typically, intraarticular localized nodular synovitis appears either as a well-defined, small ovoid lesion or as a large polylobulated soft-tissue mass with iso- or hyperintense signal intensity relative to skeletal muscle on T1-weighted images and variable signal intensity on T2-weighted images. Circular regions of intermixed low signal intensity corresponded to regions of high hemosiderin concentration, and the conspicuity of this pattern increased on gradient-echo images. In addition, an internal cleftlike or linear high-signal-intensity region of the lesion in T2-weighted images was identified in seven lesions. To our knowledge, this observation has not been previously reported. We speculate that this finding may relate to tissue necrosis. Enhancement of localized nodular synovitis is presumably related to the presence of numerous proliferative capillaries in the collagenous stroma [13].
Although intraarticular localized nodular synovitis shares similar histologic characteristics with pigmented villonodular synovitis, these entities have been considered to represent different manifestations of synovial proliferation [9, 11, 15–18]. Characteristic features of pigmented villonodular synovitis, not found in localized nodular synovitis, are the presence of diffuse frondlike projections of synovium and an abundance of hemosiderin deposition, which serve as distinguishing observations. Furthermore, the macroscopic appearances of pigmented villonodular synovitis and localized nodular synovitis also reflect the differing amounts of hemosiderin deposition [11, 15–18]. Another important distinguishing feature between pigmented villonodular synovitis and localized nodular synovitis is in their growth. As pigmented villonodular synovitis becomes more involved, the synovial masses constrict the joint, whereas localized nodular synovitis tends to grow outward, becoming pedunculated.
The differential diagnoses of a mass in the infrapatellar fat pad include several pathologic processes. Hoffa's disease is an entity characterized by inflammation and fibrosis of the infrapatellar fat body. The ill-defined margin of the lesion, often associated with edema, is characteristic of the posttraumatic process. Chondroma or osteochondroma of the infrapatellar fat pad has a signal intensity pattern consistent with either cartilage or bone marrow and lacks the deposition of hemosiderin. Other lesions such as a tophus from gout and focal arthrofibrosis do not typically have the same characteristics as localized nodular synovitis.
Surgical intervention is the best therapeutic choice for patients with localized nodular synovitis. Complete excision of the lesion usually is accomplished by either arthroscopy or open arthrotomy, and the decision for which procedure to perform depends on the location and size of the lesion. Recurrence is rare unless the lesion is not excised completely, and to our knowledge, recurrence has been described only twice in the literature [6, 11]. In our series, we have seen no recurrence during a 6-year follow-up period.
This study had several limitations. Because of the length of time used as a selection parameter, there was some variation in the imaging protocols performed in our patient population. Second, this was a retrospective study that selected for localized nodular synovitis on the basis of histology. Although we noted a variety of MR appearances for localized nodular synovitis, it is difficult to speculate whether a particular lesion will itself undergo changes in its appearance as it evolves. Last, only four patients have undergone imaging since their operation, although none has had symptoms related to localized nodular synovitis.
In summary, intraarticular localized nodular synovitis most commonly involves the infrapatellar fat pad. The clinical presentation is variable but may mimic mechanical derangement of the knee. MR imaging can facilitate preoperative diagnosis and provide information important for surgical planning. Many features help to distinguish localized nodular synovitis from pigmented villonodular synovitis, including the appearance of a smooth surface, involvement of a small region of synovium, small tumoral volume of hemosiderin, and the absence of a hemorrhagic joint effusion.
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