Subchondral lucencies and synovial disease: A case report and discussion

CLINICAL IMAGING 1990;14:333-340 333

SUBCHONDRAL LUCENCIES AND SYNOVIAL DISEASE: A CASE REPORT AND DISCUSSION

JAMES F. GRUDEN, MD, AND JEREMY J. KAYE, MD

The case ofa youngfemale with hip pain and acetabular subchondral lucencies on conventional radiographs is presented. Computed tomography (CT) demonstrated subchondral lucencies in the femoral head as well, and an arthrogram revealed synovia] irregularities. The differential diagnosis is discussed with an approach to subchondral lucencies. A synovectomy was performed on this patient with pigmented villonodular synovitis.

KEY WOKDS:

Arthritis, Hip, arthritis, Hip, computed tomography, Joints, neoplasms, Synovia1 membrane, neoplasms

PATIENT HISTORY AND PHYSICAL EXAMINATION

A 26 year-old otherwise healthy white woman presented with an acute exacerbation of left hip pain; intermittent pain had been present for 10 years, and the previous diagnosis had been of tendinitis or bursi- tis. Prior treatment had been with antiinflammatory agents with some symptom relief. She described her prior pain as dull, aching, and intermittent in nature; her pain had become sharp and stabbing recently,

From the Department of Radiology (J.F.G.), The New York Hospital-Cornell University Medical College and The Department of Radiology and Nuclear Medicine (J.J.K.), The Hospita1 for Spe- cial Surgerv, New York, New York.

Address requests for reprints to: James F. Gruden, Department of Radiology, The New York Hospital, 525 East 68th Street. New York, New York 10021.

Received June 20, 1990; accepted June 28, 1990. 0 1990 by Elsevier Science Publishing Co., Inc. 655 Avenue of the Americas, New York, NY 10010 0899/7071/90/$3.50

after minor trauma. Physical examination revealed full range of motion of the left hip with pain only at the extremes of joint excursion.

PATIENT EVALUATION:

Conventional radiographs of the left hip at the time of this presentation showed well-defined lucencies in the pelvis near the left acetabulum with thin sclerotic borders (Figure 1). The radiographic joint space was normal and no fractures were present. There were no soft-tissue calcifications, proliferative changes, osteopenia, or soft tissue masses about the hip evident on conventional radiographs, and the remaining osseous structures were normal in appearance.

A computer tomography (CT) scan of the pelvis had been performed several weeks earlier (Figure 2). This confirmed the presence of a normal joint space, but bony detail was poorly seen. An ovoid, low-density collection was identified just deep to the left gluteus maximus muscle. After intravenous contrast medium injection, a thick enhancing rim was demonstrated around the low-density m.ass. NO connection was shown between this solitary collection and the left hip joint or acetabulum. (The patient had been entirely unaware of the presence of this buttock mass.)

A repeat CT scan with images to show bony detail was performed at the time of this presentation (Figure 3). In addition to the known acetabular lucencies, this study revealed smal1 subchondral lesions in the left femoral head. The buttock mass was no longer demonstrable.

Laboratory studies included a normal complete blood count and normal blood chemistries. The uric

334 GRUDEN AND KAYE CLINICAL IMAGING VOL. 14, NO. 4

FIGURE 1. Anteroposterior radiograph of the left hip demonstrates well-defined lucencies in the acetabulum (arrows). Note the normal joint space, lack of productive changes, normal bone mineralization, and absente of calcifications.

acid was normal and tests for rheumatoid factor were negative. Aspiration of the joint was performed, and 5 cc of cloudy yellow fluid was obtained. An arthrogram done after aspiration injection of contrast demonstrated a lobular, irregular synovium, with disten- sion of the hip joint medially in the region of the lesser trochanter (Figure 4). NO communication between the hip joint and the buttock was identified. A summary of pertinent radiographic findings is given in Table 1.

COURSE

Smears of aspirated fluid were negative for bacteria, including both gram stains and stains for acid-fast bacilli. A synovia1 biopsy was performed and histo- logie findings were diagnostic of pigmented villonodular synovitis (PVNS). The patient subsequently un- derwent a synovectomy, and is doing wel1 at the time of this publication.

DISCUSSION

Well-defined lucencies closely related to joint space have been termed “subchondral cysts,” “subarticular pseudocysts” and “geodes” by various authors (1-3).

Resnick, et al., prefer the term “geode” because these lesions are not true cysts by pathologie criteria as they lack an epithelial lining (3). In addition, simple fluid may or may not be the most abundant material in the lesion; blood, synovia1 tissue, myxoid ele- ments, or synovia1 fluid may be present alone or in combination. Al1 wil1 appear less dense than bone radiographically. In addition, some radiolucent lesions are thought to represent merely focal osteoporosis (3). “Subchondral lucency” is the best descriptive term, without attempting to deduce the nature of the lesion by conventional radiographs.

Subchondral lucencies about the hip are most often seen in association with osteoarthritis (OA), where they are most often noted in the acetabulum (so-called “Egger’s cysts”). It is thought that mechani- cal damage to the articular cartilage allows synovia1 fluid intrusion into the subchondral bone and contin- ued use of the joint forces this fluid further into the cancellous bone, where expansion can occur (4). Bony traumatic contusion with hematoma formation is another theory to explain these lucencies (5). Simi- lar radiolucencies can be noted in calcium pyrophos- phate dihydrate deposition disease (CPPD), where subchondral lucencies are often more extensive than in OA (3).

The patient presented here had a normal radiographic joint space and no evidente of proliferative changes, making these more common causes of subchondral lucencies untenable. Our discussion wil1 focus, therefore, on subchondral lucencies in the ab- sence of osteoarthritis.

In the absente of mechanica1 damage to the articular cartilage, subchondral lucencies should point to a pathologie condition of the synovium as the under-

FIGURE 2. Axial contrast-enhanced CT scan. Low density collection with thick enhancing rim and a suggestion of internal separation is present deep to the left gluteus maximus muscle (arrow).

O(:TOBER-I)ECEMBER 1990

fluid or diseased synovium directly into the subchon- dra1 bone. Freiberger and Breimer raised the possibil- ity that expansion of the process occurs more readily in the deeper cancellous bone, accounting for the distribution of the lesions (7).

In patients with PVNS, different frequenties of osseous change have been reported; erosions and subchondral lucencies are more common in joints with a tight capsule, such as the hip, than in joints with looser capsules, such as the knee (6). The kneet with several bursae that are not confined by a tight capsule, allows for joint decompression into these bursae. This relationship, with relatively more synovia1 cysts and fewer subchondral lucencies at the knee, also has been reported previously in rheumatoid arthritis (9). Thus it is likely that in many of the synovia1 proliferative diseases, the amount of increased articular pressure and the tightness of the capsule are important determinants of the radiographic tïndings. In joints with tighter capsules, such as the hip, bony changes may occur earlier and be identified more frequently than in a joint such as the knee, where decompression by synovia1 cyst formation is more common. This

FIGURE 4. Frog-lateral radiograph from an arthrogram. Note the irregularity of the synovium and the distended capsule projecting medially. These non-specific findings can be found in a variety of synovia1 disorders. NO contrast fills the radiolucencies in the acetabulum.

FIGURE 3. Axial CT scan with bone algorithm shows cys- tic lesions in the left acetabulum (Figure 3A) as wel1 as in the femoral head (Figure 3B).

lying etiology. CT scanning has been helpful to con- firm the presence of lesions on both sides of a joint, as in our cases, which clarifies the diagnostic category as a synovia1 disease rather than a purely bony process (6). Lucencies on only one side of a joint may be due to synovia1 disease or primary bony disease (e.g., an epiphyseal lesion).

In primary synovia1 disease, the bony lesions may result from erosion by abnormal synovium, either through tiny tracks in the articular cartilage, along vascular foramina, or at the joint margins where no articular cartilage is present. The increased intra-articular pressure caused by abnormal proliferative synovium may also contribute to the process, forcing


TABLE 1. Summary of Radiographic Findings

1. Monoarticular disease of the Ieft hip.

2. Well-defined subchondral lucencies on both sides of the left hip joint, seen in the femoral head only on a CT scan.

3. Normal joint space and bone mineralization; no periarticular calcifications.

4. Irregular synovium on arthrography with a distended hip capsule.

5. Prior fluid-density collection with an enhancing rim deep to the left gluteus maximus.

may also explain the relatively greater frequency of joint space narrowing in the hip with respect to the knee in synovia1 disorders (10). This differente in the radiographic manifestations of the same disease process has been observed with PVNS (8); no compa- rable studies exist for the other synovia1 processes.

DIAGNOSTIC CONSIDERATIONS:

Synovia1 diseases can be classified into the arthritides, neoplasia/metaplasia, and infiltrative disorders. Al1 have in common the ability to cause increased intra-articular pressure. In addition, the abnormal synovium may directly invade bone. Many specific entities can be differentiated on clinical and/ or laboratory grounds. Associated findings such as calcification, periarticular osteoporosis, the status of the joint space, and the number and distribution of involved joint(s) are also of import.

A. Arthritides

1. Infectieus causes would have to be indolent in our patient, given her long history.

a. Tuberculosis is usually monoarticular and has a predilection for the hip or knee (11). Marked synovia1 proliferation results in marginal erosions and/or formation of subchondral lucencies. The cartilage itself is usually spared until late in the course of the disease, and the radiographic joint space is, therefore, preserved for a long period of time. Calcifications are occasionally present. A striking feature is the presence of marked periarticular osteoporosis from a combination of disuse and inflammatory hyperemia. However, osteopenia is not always found and may depend in part on the degree of disability and joint use.

TB is protean in its manifestations; and is difficult to absolutely exclude in our patient. Stains of joint aspirates and even cultures of aspirated fluid are no-

toriously unreliable in the diagnosis. Synovia1 biopsy is often necessary to make or exclude the diagnosis.

b. Other chronic infectieus arthritides, such as Brucellosis and various fungal infections could ac- count for findings similar to those of tuberculosis, but al1 are quite rare.

2. Jnflammatory arthritis in which pannus formation and/or increased pressure within the joint is present also may be present with subchondral lucencies. The most common of these are discussed.

a. The pannus formation of rheumatoid arthritis (RA) is exuberant, leading to direct invasion of the subchondral and subsequently cancellous bone, either through tracks in the articular cartilage or at its margins. Classic RA is polyarticular disease, and isolated large joint involvement would be uncom- mon. The “robust” form of RA, however, may affect a solitary large joint. It is thought that in this variant of the disease, which is more common in men, contin- ued use of the joint prevents the classic periarticular osteoporosis from developing. Subchondral lucencies are often seen, usually in association with a narrowed joint space. As in any synovia1 proliferative disease, the specific joint involved, whether “tight” or “loose,” influences the frequency of bony changes. It is known, for example, that in knees affected by RA, extra-articular synovia1 cyst formation is more commonly noted than bony changes (9). In our patient, the negative test for rheumatoid factor and the monoarticular disease makes RA unlikely.

b. Gout is almost always diagnosed clinically be- fore radiographic findings are evident. Bony lesions may arise from pressure erosion due to tophaceous deposits or from hyperplastic inflammed synovium directly invading the subchondral and cancellous bone; increased intra articular pressure may also play a role in formation of subchondral lucencies. Gout has a predilection for the metatarsophalangeal joint of the great toe, and often affects multiple joints. Clin- ically, gout was excluded in our patient by a normal uric acid.

c. Other inflammatory arthritides such as ankylos- ing spondylitis affect the hip; these were excluded on the basis of clinical findings and spine radiographs and wil1 not be discussed further.

3. MisceJJaneous arthritides include hemophilia. a. Hemophilia is manifested by joint effusions

(which may be dense due to hemosiderin deposition in the synovium) in association with subchondral lucencies which can be quite large. The joint space in this polyarticular process is typically narrowed, especially in long-standing cases. Subchondral lucencies in patients with hemophilia formed by sev-

OC’I-OBER-DECEMBER 1990 SUBCHONDRAL L1JCENCIE.S 337

era1 processes. With the bleeding diathesis, minor trauma may lead to bony contusion, visible as lucencies within the bone. In addition, repetitive bleeding into the joint acts as an irritant to the synovium, with subsequent proliferation of this tissue. Increased pressure in the joint may also play a role. Hemophilia was clinically excluded in our patient, but should be considered in the appropriate clinical setting.

B. Jnfiltrative Disorders of the Synovium

1. Amyloidosis may cause bone lesions rarely, most often involving multiple joints, especially in the upper extremity (12). Radiographic findings range from soft tissue swelling alone to bony erosive changes from amyloid masses. Amyloid infiltration of the synovium can also occur, leading to subchon- dra1 lucencies by the usual mechanisms. Amyloid bone disease is often associated with hemodialysis in the chronic renal failure population; bone changes are usually noted several years after hemodialysis is begun.

C. Neoplasm or Neoplasm-Like Conditions of the Synovium (AlJ Monoarticular Disease Processes)

1. Synovial sarcoma is a malignant tumor of syno- via1 origin. Usually this process is manifested as a soft tissue mass in a periarticular location, not actually within the joint. Multiple punctate calcifications may be noted within the soft tissue mass. Bony changes, when present, consist of cortical destruction due to invasion rather than erosion, and this tumor is usually easily distinguished radiographically from the benign synovia1 diseases.

2. Pigmented villonodular synovitis (PVNS) is an idiopathic disorder of the synovium, thought to be either reactive (13) or to be a benign neoplastic process (14). The disease may be focal or diffuse within the joint. Tendon sheaths or joints may be involved. Most PVNS involving synovia1 joints is of the diffuse variety, and is a monoarticular disease, with 80% of reported cases involving the knee and most of the remainder affecting the hip (15). Affected patients are usually in the 2nd or 3rd decade with no significant differente in incidence between men and women. Patients usually present with chronic pain.

The abnormal synovium and increased intra-articular pressure cause formation of subchondral lucencies, the frequency of which depends on the joint involved. In PVNS, the tight hip joint cannot decom- press as wel1 as the more capacious knee, and erosive

FIGURE 5. Anteroposterior radiograph of the left hip in a different middle-aged male patient with PVNS. Note the subchondral lucencies and erosions of the femoral neck, normal joint space, lack of productive changes, absente of calcifications, and normal bone density.

changes are thus more common in the hip (Figure 5). In the hip, about 80% of cases have bony erosions, whereas only approximately 20% of cases have PVNS in the knee demonstrate these changes (8). In our patient, the fleeting fluid collection deep to the glu- teal muscles may have represented a bursal extension that developed as a means of decompressing the joint. In the absente of bony changes on plain radiographs, only prominent soft tissue swelling or evidente of a joint effusion may be seen (Figure 6).

The classic radiographic findings of PVNS were described by Freiberger and Breimer (7), and consist of 1) monoarticular disease, 2) normal joint space (at least in the knee), 3) normal bone mineralization, 4) nodular soft tissue swelling, 5) lack of calcification in the soft tissue masses, 6) absente or systemic disease, and 7) chronicity. Recently a case with calcification in dystrophic tissue in PVNS was reported (16), but this is the exception rather than the rule. New imaging techniques may help to differentiate several processes. PVNS, for example, may show areas of signal void on al1 pulse sequences on magnetic reso- nance imaging (MRI) especially FLASH or gradient- echo protocols (Figure 7). This is due to the hemosiderin deposited within the subsynovial tissue of the lesion. The MRI findings in PVNS were recently described by Steinbach, et al. (17).


A

FIGURE 6. Anteroposterior and lateral views of the knee in a patient with PVNS. Note prominent soft tissue swelling consistent with synovitis. NO bony lesions are present, and the joint space is maintained.

3. Primary synovia1 chrondomatosis is the main differential diagnostic consideration with PVNS in this patient. In primary synovia1 chondromatosis, there is metaplasia of the subsynovial tissue, with cartilage formation. Cartilaginous loose bodies may break off and float freely in the joint. This disease, like PVNS, affects young patients in their 2nd and 3rd decade and may involve synovia1 joints or tendon sheaths. The diffuse form usually involves the hip or knee, is chronic, and is monoarticular. Bone mineralization is normal in this condition, and nodular carti- lagenous masses are present within the joint capsule. In nearly two-thirds of the cases, calcification or ossification of the cartilage fragments is noted on conventional radiographs (Figure 8). This allows it to be distinguished from PVNS; if no calcific densities are present, the radiographic findings are virtually iden- tical to those of PVNS (Figure 9). Bony changes consist of intra-articular erosions due to the presence of cartilaginous bodies packed within the joint. A recent report described bony erosions in 30% of patients, al1 in the hip or shoulder; no cases of synovia1 chondro-

B

matosis involving the knee demonstrated bony erosions (18). Elevated intra-articular pressure may also lead to the formation of subchondral lucencies. On MRI, signal void seen on al1 pulse sequences is due to the presence of ossification or calcification of carti-

TABLE 2. Evaluation of Subchondral Lucencies

DISTRIBUTION One side of joint: Synovia1 or bony process Both sides of joint: Synovia1 process

DEGENERATIVE (PROLIFERATIVE) CHANGES Present: Egger’s cysts (osteoarthritis) Absent: Synovia1 disease

JOINT SPACE STATUS Narrowed: Arthritis or other synovia1 process especially in

“tight” joints (see text)

Normal: Synovia1 disease (nonarthritic-infiltrative or tumorous)

CALCIFICATIONS Present: Synovia1 osteochondromatosis Absent: PVNS

Synovia1 chondromatosis Other synovia1 disease

OCTOBER-DECIXMBEK 1Wl SUBCHONJ)KAI, I.lJCENCIES 339

B

FIGURE 7. MRI findings in PVNS. (A) A sagittal MR sec- tion (TR 2000, TE = 80) on a patient with PVNS of the knee. There is a large posteriorly-located mass near the joint space. Note the heterogenous signal of this mass, including areas of signal void (confirmed on other pulse sequences) consistent with the presence of hemosiderin. (B) Axial MR scan (TR = 750, TE-30, FA-28”) in another patient with PVNS of the knee shows lobulated mass posteriorly with signal void; this is also consistent with the presence of hemosiderin in the tumor.

lage nodules. Correlation with plain films should allow distinction of this entity from PVNS when sig- na1 void is seen on al1 MR sequences, as PVNS wil1 not show calcifïcation.

Synovia1 biopsy was performed in this case; this is usually necessary to establish the diagnoses of PVNS, synovia1 chondromatosis, or tuberculeus arthritis. Most of the other synovia1 diseases in the differential diagnosis are differentiated on the basis of clinical and laboratory findings.

SUMMARY

Subchondral lucencies, which are relatively common, may be differentiated by a logica1 approach (Table 2). Subchondral lucencies in the absente of productive changes are usually indicative of synovia1 disease. While the differential diagnosis of these diseases is rather extensive, it can be significantly simplified by considering clinical, laboratory, and radiographic findings. The bony changes of synovia1 processes vary in severity. In tight joints, such as the hip, bony

FIGURE 8. Anteroposterior radiograph of the right hip. Note the typical appearance of primary synovia1 chondromatosis, with innumerable calcified or ossified bodies within the joint. In this case, no erosions or subchondral lucencies present.


FIGURE 9. Noncalcified synovia1 chondromatosis. (A) Anteroposterior radiograph of the left hip demonstrates no abnormality. (B) Arthrogram demonstrates innumerable round and ovoid filling defects consistent with noncalcified cartilaginous bodies in the joint.

changes are more frequently noted. Associated radiographic findings of importante in patients with syno- via1 diseases include status of mineralization, type of joint involved, thecharacteristics of associated soft tissue masses, and the presence or absente of calcification, status of the joint space, and degree of proliferative changes. Imaging with CT and MRI can be helpful. Familiarity with the various synovia1 diseases and their conventional radiographic and other imaging technique findings is important in order to adequately assess subchondral lucencies, whether they are called “cysts, ” “geodes” or “pseudocysts.”

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Subchondral lucencies and synovial disease: A case report and discussion