A popliteal giant synovial osteochondroma mimicking a parosteal osteosarcoma
© Toepfer et al.; licensee BioMed Central Ltd. 2013
Received: 13 February 2013
Accepted: 19 September 2013
Published: 25 September 2013
Both giant synovial osteochondroma and parosteal osteosarcoma are rare musculo-skeletal tumors, often localized in the vicinity of the knee. Misdiagnosis of a malignant bone tumor can entail fatal consequences. Etiology of giant synovial osteochondroma is widely unsolved but is believed to originate from synovial chondromatosis, a mostly benign metaplasia of the synovial membrane. Parosteal osteosarcoma is a low-grade surface osteosarcoma with a propensity of local recurrence and the potential of distant metastasis and therefore requiring a different therapeutical approach. We report the case of a popliteal giant osteochondroma mimicking a parosteal osteosarcoma. Relevant facts of this rare entity regarding pathogenesis, treatment, and differential diagnoses will be discussed.
For benign lesions of the bone, no reliable data regarding incidence rates are available. A large amount of benign bone tumors remains undiscovered due to an asymptomatic course. Many diagnoses of benign bone lesions are incidental findings. Malignant primary bone tumors are even less frequent, accounting for only 0.2% of all neoplasms . Classic, high-grade osteosarcoma is the most common primary malignant tumor of bone (35% of all bone sarcomas), parosteal osteosarcoma is far less frequent though, amounting to only 5% of all osteosarcomas. The second most common localization for parosteal osteosarcoma is the proximal tibia, after the distal metaphysis of the posterior femur. Most patients are young adults. A limitation of flexion of the affected knee due to a painless swelling may be the initial symptom. Complete excision is mandatory to avoid recurrence and progression to high-grade osteosarcoma .
Giant synovial osteochondroma is a very rare form of an osteochondromatous tumor and not to be confused with classic osteochondroma (osteo-cartilaginous exostosis), the most common bone tumor. Pathogenesis of this lesion is related to a synovial metaplasia in most cases. Descriptions are limited to case studies although its vicinity to large joints and tendons implies a high rate of mechanical impairment. Predilection site is the intra-articular, infrapatellar region of the knee. Besides an intra-articular form, which is often named giant synovial osteochondroma, an equally rare extra-synovial form does exist. It preferentially appears in the tendonsheath-rich areas of hand and foot or the knee and is also referred to as extraskeletal chondroma.
Differentiation between parosteal osteosarcoma and giant synovial osteochondroma proves to be difficult as localization, clinical course, and imaging can show remarkable similarities.
A 39-year-old Caucasian man presented with a painful swelling of the right popliteal space, which he first noticed 6 months ago. Patient’s history indicated no injury to the leg.
Physical examination revealed an approximately 6×4 cm hard and slightly tender mass palpable at the medial aspect of the popliteal space adjacent to the pes anserine. The mass was firm on palpation and immobile to the deep tissue. There were no inflammatory signs whatsoever. The functional testing of the knee joint showed a limited range of motion with a moderate flexion deficit of 120-0-0 °F/E. There was no pain during rotation and compression and the ligamentous structures showed sufficient stability. No signs of an intra-articular effusion were observed. Muscle strength and neurovascular examination of the leg were normal. No further, similar masses were noted during the general examination as well as no lymphadenopathy. The remainder of the physical examination and routine blood investigations, including uric acid levels, alkaline phosphatase, and inflammatory parameters (C-reactive protein, white blood cell count) were unremarkable.
On magnetic resonance (MR) images a homogeneous and lobulated yet relatively well-circumscribed juxtacortical mass was visualized. The lesion was located close to the bone and between the medial head of the gastrocnemicus muscle, which was slightly displaced dorsally, and the pes anserine.
Radiographically, maligancy could not be ruled out. Surgical biopsy was indicated to obtain a histologically verified diagnosis. Surgery was accomplished in a prone position and excisional biopsy with marginal resection of the lesion was performed. Macroscopically, a thin synovium-like membrane covering the bone-like lesion was discovered and resected. A second, smaller nodule attached to the cranial pole of the primary lesion was also excised in the process. Both specimens were sent for histopathologic analysis.
In this case, the patient remained free of symptoms and recurrence 2.5 years after surgery. Clinical examination and repeated imaging of the patient’s right knee is continuously performed every 6 months for a period of 5 years.
After clinical examination and imaging, a number of differential diagnoses for extra-osseous, heavily mineralized masses had to be taken into consideration. These included (giant synovial) osteochondroma, myositis ossificans, tumoral calcinosis, and parosteal osteosarcoma .
Giant synovial osteochondroma is a rare benign tumor that can be found mostly intra-articulary in the vicinity of the knee and hip joint. Besides trauma , an extra-synovial metaplasia of the outer layers of the capsule  is discussed as causes for this lesion. Yet in most cases, synovial chondromatosis is suspected to be the underlying reason. This arthropathy was first classified by Milgram in 1977 . Edeiken complemented the original classification in 1994 by adding giant synovial osteochondroma as final stage of synovial chondromatosis .
The macroscopical and radiologic appearance of the lesion, either lobulated or smooth, depends on the potential genesis. A conglomerate of multiple small synovial chondromas is supposedly able to create one single coherent, lobulated cartilaginous giant osteochondroma. Otherwise, the continuous growth of a singular synovial chondroma is discussed to lead to a smooth and unruffled circumscribed tumor. In both scenarios, the cartilaginous structure of the synovial giant osteochondroma can be replaced completely by bone through enchondral ossification . Nutritive supply of these progressively growing lesions is presumably coming from residual connections to the synovial membrane or by diffusion from the synovial fluid .
A review of the literature shows that studies to this entity are limited to case reports [4, 5, 7–12]. Besides an intra-articular form, an equally rare extra-synovial form does exist. It preferentially appears in the tendon sheath-rich areas of hand and foot and the knee. Thus, it is important to point out that a single, giant synovial osteochondroma, as a massive form of synovial chondromatosis, can potentially arise from any synovial tissue. Common synonyms used for giant synovial osteochondroma are extra-skeletal chondroma, ossifiying chondroma , intracapsular or para-articular chondroma , and extra-osseous osteochondroma . Rarely, a malignant transformation from synovial chondromatosis into a synovial chondrosarcoma has been reported [14, 15].
The differentiation between an extra-/para-articular or intra-articular giant synovial osteochondroma has to be established by the localization seen on preoperative imaging and encountered during surgery. It has to be noted that synovial covering of a chondromatous mass may derive from the tendon-sheaths or synovium of the popliteal bursae as well.
Myositis ossificans is a benign, self-limiting, solitary, ossifying soft-tissue disease typically occurring within the muscles of the upper and lower extremities . It is commonly associated with soft tissue trauma, although in almost one-third of the cases there is no history of injury ; the pathogenesis of this lesion is unknown as well. The term heterotopic ossification is used as a synonym. Symptoms start with a painful swelling and local irritation with hyperthermia and diffuse edema . The appearance of myositis ossificans on images varies, depending on the stage of development. Plain radiographs are initially negative. After 2-4 months the mass develops its typical feather-like feature, consisting of thin bony trabeculae. The heterotopic bone is usually more dense at the periphery of the mass, creating sharply defined borders. With ongoing maturation the mass becomes more ossified and resembles reparative bone callus. At the end, the lesion has the features of mature bone. Histologic proof of the diagnosis is given by the zoning phenomenon, where ossification and maturation progress from the periphery to the center . This provides an important differentiation to (parosteal) osteosarcoma, where maturation of the lesion proceeds vice versa and presents a reverse zoning phenomenon. Surgical trauma might cause recurrence and marginal excision is only indicated after the lesion has matured.
Tumoral calcinosis is a very rare benign soft tissue calcification. Lesions consist of massive calcium deposits mostly occurring at the para-articular soft tissue . The disease preferably affects the black population from birth until 20 years of age. In most cases local pain is the leading symptom . Often the lesion is localized at multiple sites; large joints such as shoulder, hip, and knee joints are more often affected. Imaging shows a grape-like cluster of rounded calcified masses, separated by radiolucent septae . As described for calcifying tendinitis, the histological differentiation is clear. Active foreign body reaction, including the presence of macrophages and giant cells, is bordered by granular calcified material. The etiology of this disease remains unclear [19, 20]. Several metabolic diseases (in particular secondary hyperparathyroidism through chronic renal failure) accompanied by an imbalance of the calcium/phosphate homeostasis are being held responsible for the occurrence of this lesion . Still, blood test may show normo-calcemic parameters . Causal treatment of systemic disorders must come before surgery, though surgical excision is indicated with symptomatic lesions.
This is why parosteal osteosarcoma remains hard to differentiate from benign entities [25–27]. However, new findings in immunohistochemistry with determination of MDM2 and CDK4 markers show promising results in distinguishing parosteal osteosarcoma from benign clinical and histological mimics .
Assessing dignity of extra-osseous, mineralized masses by imaging alone can provide some difficulty. Benignant lesions in the vicinity of the knee can show remarkable similarities to malignancies and vice versa. For a histological clarification of uncertain lesions, a biopsy is indicated, even if the symptoms are minor. In differential diagnostics between osteochondroma and parosteal osteosarcoma, histological analysis is not yet routinely able to provide diagnostic certainty in all cases. Therefore, surgical therapy with complete excision and narrow follow-ups are strongly recommended until further developments in immuno-histochemical analyses give absolute certainty.
Written informed consent was obtained from the patient for publication of this case report and any accompanying images. A copy of the written consent is available for review by the Editor-in-Chief of this journal.
We state, that all authors have seen and agreed with the contents of the manuscript as submitted and indicate that the work has not been submitted or published elsewhere in whole or in part.
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