Skeletal Radiol (1992) 21:326-329
Skeletal Radiology
Case report 738 Robert V. Bulas, M.D., Curtis W. Hayes, M.D., William F. Conway, M.D., Ph.D., and Thomas Loughran, M.D. Departments of Radiology and Orthopedics, The Medical College of Virginia Hospitals, Richmond, Virginia, USA
hnaging studies Fig. 1. A Lateral and B axial radiographs of the knee demonstrate increased soft-tissue density in the region of the infrapatellar fat pad, with no definite osseous abnormalities Fig. 2A, B. Tl-weighted sagittal (A) and axial (B) magnetic resonance (MR) images of the patella reveal an area of low signal intensity involving the inferior pole of the patella. Faintly seen within this region is a lower signal ring (7 mm in diameter) with an inter~ mediate signal interior (arrow). At the center is a tiny area of low signal. The adjacent infrapatellar fat pad demonstrates decreased signal intensity Fig. 3. Axial computed tomography (CT) section of the patella demonstrates a radiolucent lesion containing calcifications, surrounded by a rim of osteosclerosis
Address reprint requests to: R.V. Bulas, M.D., Department of Radiology, The Medical College of Virginia Hospitals, Richmond, VA 23298, USA 9 1992 International Skeletal Society
R.V. Bulas et al. : Case report 738 Clinical information
A 16-year-old white woman complained of infrapatellar pain in the left knee which began at the conclusion of her high school basketball season; the pain progressively worsened over the next 4 months. The past medical history was unremarkable. Physical examination revealed atrophy of the left quadriceps muscle, associated with decreased ability to extend the left knee actively. Full passive range of motion of the left knee was present. No point tenderness was noted.
327 Radiographs of the left knee were obtained. The patient was referred for magnetic resonance imaging (MRI) of the left knee with suspicion of patellar tendinitis ("jumpers's knee"). Radiographs of the left knee were interpreted as demonstrating increased soft-tissue density in the region of the infrapatellar fat pad, with no definite abnormalities of the osseous structures (Fig. 1). T1weighted sagittal and transverse MR images revealed an area of low signal intensity involving the inferior onehalf of the patella (Fig. 2). Faintly seen within this region was a 7-ram,
lower signal ring with an intermediate signal interior (arrows). At the center was a tiny area of low signal. Immediately inferior to the patella, Hoffa's fat pad showed an area of irregular decreased signal, suggesting inflammation or edema. The patellar tendon was intrinsically normal, although it was bowed slightly anteriorly. Computed tomography (CT) study was also carried out, confirming the presence of a radiolucent nidus, surrounded by sclerosis, and containing a tiny central calcification (Fig. 3). The patellar lesion was surgically excised.
328
Diagnosis: Osteoid osteoma of the patella
R.V. Bulas et al. : Case report 738
Pathological study
Histological evaluation of surgical specimen revealed findings consistent with osteoid osteoma of the patella (Fig. 4). The radiological differential diagnosis included osteoid osteoma, Brodie's abscess, and other primary patellar bone neoplasms, including chondroblastoma [2, 6].
Discussion Osteoid osteoma is a benign osteoid tumor that was first described by Jaffe in 1935 [5]. The lesion consists of a small, central, osteoid nidus, usually less than 10 mm in size, surrounded by a variable degree of reactive sclerosis. Adjacent to the lesion, a pronounced intraosseous and extraosseous inflammatory response is frequently present. This response is most marked when the lesion is located within the trabecular bone of the metaphysis or the epiphysis. The vast majority of osteoid osteomas occur in long bones of the lower extremities, usually in a cortical diaphyseal location, and involvement of the patella is extremely rare [2, 6, 8]. Osteoid osteomas present most frequently in males during the 2nd decade of life. The classic clinical presentation of osteoid osteoma consists of progressively increasing pain, which is usually worse at night and frequently relieved by salicylates. However, when in a juxtaarticular location, the clinical findings may be more indicative of an inflammatory synovitis or an injury to the soft tissues [1, 7, 9]. Thus, the clinical picture may be misleading, especially when occurring in the knee of young athletes, in whom chondromalacia, tendinitis, and internal derangement of the knee are so prevalent. Radiographically, the osteoid osteoma is classically described as a cortical, diaphyseal, radiolucent nidus, often containing calcification, surrounded by reactive osteosclerosis. When located in trabecular bone, the sclerotic reaction may be minimal, and the small nidus may not be detected on plain radiographs. In such cases, conventional tomogra-
Fig. 4. Photomicrographof the specimenobtained by surgicalcurettage demonstratesareas of woven bone lined by osteoblastswithina background of fibrovascularstroma (x 40)
phy, CT, and MRI are useful to delineate the nidus [3, 4, 10]. MRI of the knee was performed in this case, primarily to evaluate the patellar tendon. The case demonstrates both the value of MRI in the screening of patients with pain in the knee of obscure etiology and the limited ability of MRI to show bony changes. The diagnosis of osteoid osteoma was based on the MR appearance consisting of a central low signal intensity (representing calcification within the nidus), surrounded by an inner ring of intermediate signal intensity (representing the osteoid nidus itself). An outer ring of low signal intensity (representing reactive osteosclerosis was also noted. MRI also revealed intraosseous and adjacent soft-tissue signal changes consistent with inflammation or edema. In fact, the intramedullary and soft-tissue changes were so pronounced on MRI that they nearly obscured the underlying lesion. This appearance, which we have observed in several other cases of osteoid osteoma, may be suggestive of more aggressive pathological abnormalities, including those of infective and malignant neo-
plastic etiologies. Thus, it is necessary to be aware of the extent of the inflammatory process that accompanies this benign bone neoplasm, so that the entity is not dismissed from the differential diagnosis. Radiologists are accustomed to resolving bone lesions by their increased or decreased density due to changes in the cortical or trabecular bone pattern. With MRI, however, the intraosseous signal changes are generally due to a replacement of bone marrow fat by water or cellular infiltrate, and the alterations in bone density, so heavily relied upon in plain radiography, may not be readily apparent. This phenomenon is well demonstrated in the present case, as the secondary inflammatory reaction resulted in a decreased signal intensity of the bone marrow on the T1weighted images, nearly obscuring the sclerotic component that is so characteristic of osteoid osteoma on conventional radiography. Gradient echo images (performed according to our knee protocol at that time) were not helpful in this case. Conventional T2-weighted, spin echo images should, on the other hand, effectively
R.V. Bulas et al. : Case report 738 differentiate e d e m a f r o m sclerosis in such cases. Therefore, in cases o f obscure pain in the knee, conventional T2-weighted, spin echo images m a y be preferable to gradient echo images. Since M R I , even in the absence o f i n f l a m m a t o r y signal changes, is n o t optimal for resolving b o n e detail, the osteoid nidus m u s t be searched for carefully in order to m a k e the correct diagnosis. In summary, a case o f osteoid ost e o m a o f the patella in a 16-year-old w o m a n is presented. T h e i m p o r t a n c e o f including osteoid o s t e o m a in the differential diagnosis in y o u n g patients with articular complaints was discussed. The frequently pron o u n c e d intraosseous and extraosseous inflammatory components were stressed, as well the resultant difficulties in m a k i n g the clinical and c o n v e n t i o n a l r a d i o g r a p h i c diagnosis.
329 The utility and pitfalls o f M R I in defining b o t h the osteoid lesion and the a c c o m p a n y i n g i n f l a m m a t o r y changes were discussed. Thus, with attention to detail, M R I is a highly sensitive diagnostic test for b o t h meniscal disorders a n d extraarticular b o n y and soft-tissue abnormalities when evaluating patients with suspected disorders o f the knee.
References 1. Bussiere JL, Sauvezie B, Lopitaux R, Prin P, Valentin P, Rampon S (1976) Osteome osteoide avec synovite d'allure rhumatoide. Rev Rhum 43 (11):651 2. Ehara S, Khurana JS, Kattapuram SV, Rosenberg AE, E1-Khoury GY, Rosenthal DI (1989) Osteolytic lesions of the patella. AJR 153:/03 3. Firooznia H, Rafii M, Golimbu C (1985) Computed tomography of osteoid osteoma. CT 9:265
4. Glass RB, Poznanski AK, Fisher MR, Shkolnik A, Dias L (1986) MR imaging of osteoid osteoma. JCAT 10(6):1065 5. Jaffe HL (1935) Osteoid osteoma: a benign osteoblastic tumor composed of osteoid and atypical bone. Arch Surg 31 : 709 6. Linscheid RL, Dahlin DC (1966) Unusual lesions of the patella. J Bone Joint Surg [Am] 48(7): 1359 7.Micheli LJ, Jupiter J (1978) Osteoid osteoma as a cause of knee pain in the young athlete. Am J Sports Med 6(4) : 199 8. Planchon CA, Huard LC, Manlot G, Perez R (1984) Diagnostic and therapeutic value of radiologic and scintigraphic investigations. Illustration by a case of osteoid osteoma (in French). Ann Radiol 27(6) :495 9. Puddu G, Mariani P 0981) Osteoid osteoma of the Gerdy's tubercle in an athlete. Am J Sports Med 9(t) : 57 10. Yeager BA, Schiebler ML, Wertheim SB, Schmidt RG, Torg JS, Perasia PM, Dalinka MK (1987) MR imaging of osteoid osteoma of the talus. JCAT 11(5) : 916
Skeletal Radiology
Case report 738 Robert V. Bulas, M.D., Curtis W. Hayes, M.D., William F. Conway, M.D., Ph.D., and Thomas Loughran, M.D. Departments of Radiology and Orthopedics, The Medical College of Virginia Hospitals, Richmond, Virginia, USA
hnaging studies Fig. 1. A Lateral and B axial radiographs of the knee demonstrate increased soft-tissue density in the region of the infrapatellar fat pad, with no definite osseous abnormalities Fig. 2A, B. Tl-weighted sagittal (A) and axial (B) magnetic resonance (MR) images of the patella reveal an area of low signal intensity involving the inferior pole of the patella. Faintly seen within this region is a lower signal ring (7 mm in diameter) with an inter~ mediate signal interior (arrow). At the center is a tiny area of low signal. The adjacent infrapatellar fat pad demonstrates decreased signal intensity Fig. 3. Axial computed tomography (CT) section of the patella demonstrates a radiolucent lesion containing calcifications, surrounded by a rim of osteosclerosis
Address reprint requests to: R.V. Bulas, M.D., Department of Radiology, The Medical College of Virginia Hospitals, Richmond, VA 23298, USA 9 1992 International Skeletal Society
R.V. Bulas et al. : Case report 738 Clinical information
A 16-year-old white woman complained of infrapatellar pain in the left knee which began at the conclusion of her high school basketball season; the pain progressively worsened over the next 4 months. The past medical history was unremarkable. Physical examination revealed atrophy of the left quadriceps muscle, associated with decreased ability to extend the left knee actively. Full passive range of motion of the left knee was present. No point tenderness was noted.
327 Radiographs of the left knee were obtained. The patient was referred for magnetic resonance imaging (MRI) of the left knee with suspicion of patellar tendinitis ("jumpers's knee"). Radiographs of the left knee were interpreted as demonstrating increased soft-tissue density in the region of the infrapatellar fat pad, with no definite abnormalities of the osseous structures (Fig. 1). T1weighted sagittal and transverse MR images revealed an area of low signal intensity involving the inferior onehalf of the patella (Fig. 2). Faintly seen within this region was a 7-ram,
lower signal ring with an intermediate signal interior (arrows). At the center was a tiny area of low signal. Immediately inferior to the patella, Hoffa's fat pad showed an area of irregular decreased signal, suggesting inflammation or edema. The patellar tendon was intrinsically normal, although it was bowed slightly anteriorly. Computed tomography (CT) study was also carried out, confirming the presence of a radiolucent nidus, surrounded by sclerosis, and containing a tiny central calcification (Fig. 3). The patellar lesion was surgically excised.
328
Diagnosis: Osteoid osteoma of the patella
R.V. Bulas et al. : Case report 738
Pathological study
Histological evaluation of surgical specimen revealed findings consistent with osteoid osteoma of the patella (Fig. 4). The radiological differential diagnosis included osteoid osteoma, Brodie's abscess, and other primary patellar bone neoplasms, including chondroblastoma [2, 6].
Discussion Osteoid osteoma is a benign osteoid tumor that was first described by Jaffe in 1935 [5]. The lesion consists of a small, central, osteoid nidus, usually less than 10 mm in size, surrounded by a variable degree of reactive sclerosis. Adjacent to the lesion, a pronounced intraosseous and extraosseous inflammatory response is frequently present. This response is most marked when the lesion is located within the trabecular bone of the metaphysis or the epiphysis. The vast majority of osteoid osteomas occur in long bones of the lower extremities, usually in a cortical diaphyseal location, and involvement of the patella is extremely rare [2, 6, 8]. Osteoid osteomas present most frequently in males during the 2nd decade of life. The classic clinical presentation of osteoid osteoma consists of progressively increasing pain, which is usually worse at night and frequently relieved by salicylates. However, when in a juxtaarticular location, the clinical findings may be more indicative of an inflammatory synovitis or an injury to the soft tissues [1, 7, 9]. Thus, the clinical picture may be misleading, especially when occurring in the knee of young athletes, in whom chondromalacia, tendinitis, and internal derangement of the knee are so prevalent. Radiographically, the osteoid osteoma is classically described as a cortical, diaphyseal, radiolucent nidus, often containing calcification, surrounded by reactive osteosclerosis. When located in trabecular bone, the sclerotic reaction may be minimal, and the small nidus may not be detected on plain radiographs. In such cases, conventional tomogra-
Fig. 4. Photomicrographof the specimenobtained by surgicalcurettage demonstratesareas of woven bone lined by osteoblastswithina background of fibrovascularstroma (x 40)
phy, CT, and MRI are useful to delineate the nidus [3, 4, 10]. MRI of the knee was performed in this case, primarily to evaluate the patellar tendon. The case demonstrates both the value of MRI in the screening of patients with pain in the knee of obscure etiology and the limited ability of MRI to show bony changes. The diagnosis of osteoid osteoma was based on the MR appearance consisting of a central low signal intensity (representing calcification within the nidus), surrounded by an inner ring of intermediate signal intensity (representing the osteoid nidus itself). An outer ring of low signal intensity (representing reactive osteosclerosis was also noted. MRI also revealed intraosseous and adjacent soft-tissue signal changes consistent with inflammation or edema. In fact, the intramedullary and soft-tissue changes were so pronounced on MRI that they nearly obscured the underlying lesion. This appearance, which we have observed in several other cases of osteoid osteoma, may be suggestive of more aggressive pathological abnormalities, including those of infective and malignant neo-
plastic etiologies. Thus, it is necessary to be aware of the extent of the inflammatory process that accompanies this benign bone neoplasm, so that the entity is not dismissed from the differential diagnosis. Radiologists are accustomed to resolving bone lesions by their increased or decreased density due to changes in the cortical or trabecular bone pattern. With MRI, however, the intraosseous signal changes are generally due to a replacement of bone marrow fat by water or cellular infiltrate, and the alterations in bone density, so heavily relied upon in plain radiography, may not be readily apparent. This phenomenon is well demonstrated in the present case, as the secondary inflammatory reaction resulted in a decreased signal intensity of the bone marrow on the T1weighted images, nearly obscuring the sclerotic component that is so characteristic of osteoid osteoma on conventional radiography. Gradient echo images (performed according to our knee protocol at that time) were not helpful in this case. Conventional T2-weighted, spin echo images should, on the other hand, effectively
R.V. Bulas et al. : Case report 738 differentiate e d e m a f r o m sclerosis in such cases. Therefore, in cases o f obscure pain in the knee, conventional T2-weighted, spin echo images m a y be preferable to gradient echo images. Since M R I , even in the absence o f i n f l a m m a t o r y signal changes, is n o t optimal for resolving b o n e detail, the osteoid nidus m u s t be searched for carefully in order to m a k e the correct diagnosis. In summary, a case o f osteoid ost e o m a o f the patella in a 16-year-old w o m a n is presented. T h e i m p o r t a n c e o f including osteoid o s t e o m a in the differential diagnosis in y o u n g patients with articular complaints was discussed. The frequently pron o u n c e d intraosseous and extraosseous inflammatory components were stressed, as well the resultant difficulties in m a k i n g the clinical and c o n v e n t i o n a l r a d i o g r a p h i c diagnosis.
329 The utility and pitfalls o f M R I in defining b o t h the osteoid lesion and the a c c o m p a n y i n g i n f l a m m a t o r y changes were discussed. Thus, with attention to detail, M R I is a highly sensitive diagnostic test for b o t h meniscal disorders a n d extraarticular b o n y and soft-tissue abnormalities when evaluating patients with suspected disorders o f the knee.
References 1. Bussiere JL, Sauvezie B, Lopitaux R, Prin P, Valentin P, Rampon S (1976) Osteome osteoide avec synovite d'allure rhumatoide. Rev Rhum 43 (11):651 2. Ehara S, Khurana JS, Kattapuram SV, Rosenberg AE, E1-Khoury GY, Rosenthal DI (1989) Osteolytic lesions of the patella. AJR 153:/03 3. Firooznia H, Rafii M, Golimbu C (1985) Computed tomography of osteoid osteoma. CT 9:265
4. Glass RB, Poznanski AK, Fisher MR, Shkolnik A, Dias L (1986) MR imaging of osteoid osteoma. JCAT 10(6):1065 5. Jaffe HL (1935) Osteoid osteoma: a benign osteoblastic tumor composed of osteoid and atypical bone. Arch Surg 31 : 709 6. Linscheid RL, Dahlin DC (1966) Unusual lesions of the patella. J Bone Joint Surg [Am] 48(7): 1359 7.Micheli LJ, Jupiter J (1978) Osteoid osteoma as a cause of knee pain in the young athlete. Am J Sports Med 6(4) : 199 8. Planchon CA, Huard LC, Manlot G, Perez R (1984) Diagnostic and therapeutic value of radiologic and scintigraphic investigations. Illustration by a case of osteoid osteoma (in French). Ann Radiol 27(6) :495 9. Puddu G, Mariani P 0981) Osteoid osteoma of the Gerdy's tubercle in an athlete. Am J Sports Med 9(t) : 57 10. Yeager BA, Schiebler ML, Wertheim SB, Schmidt RG, Torg JS, Perasia PM, Dalinka MK (1987) MR imaging of osteoid osteoma of the talus. JCAT 11(5) : 916
