|Year : 2016 | Volume
| Issue : 4 | Page : 291-294
Osteoblastoma of Patella: A Rare Case
S Krishna Kumari1, Prasad Uma1, HL Vasavi1, L Prasanna Kumar2
1 Department of Pathology, RIMS, Srikakulam, Andhra Pradesh, India
2 Department of Orthopaedic, RIMS, Srikakulam, Andhra Pradesh, India
|Date of Web Publication||23-Dec-2016|
Q. No. 49-3-3, Lalithanagar, Vishakapatnam, Andhra Pradesh - 530016
Source of Support: None, Conflict of Interest: None
Benign osteoblastoma of the patella is a rare tumor. Majority of the tumors occur in the spine, femur, and tibia. Osteoblastoma presents as a diagnostic dilemma due to clinical similarity with other benign bone forming tumors. Histopathological examination is the main stay of diagnosis and has good prognosis.
Keywords: Osteoblastoma, patella, rare
|How to cite this article:|
Kumari S K, Uma P, Vasavi H L, Kumar L P. Osteoblastoma of Patella: A Rare Case. J NTR Univ Health Sci 2016;5:291-4
| Introduction|| |
The patella is an infrequent location for the onset of benign or malignant bone tumors, as well as pseudotumoral lesions. There are few published articles in literature. They present clinically with severe pain in the anterior knee or an obvious growth of soft tissue. The most common tumor reported in the literature is giant cell tumor , followed by chondroblastoma. Among malignant tumors, the most common is the hemangioendothelioma, followed by lymphoma and osteosarcoma. Benign osteoblastoma arising from patella is rare;commonly occurs in spine, femur and tibia.
| Case Report|| |
A 16-year-old boy from Srikakulam district presented to the outpatient department of orthopedics in the year 2012 with a 6-year history of knee pain and swelling of the left knee, which was gradually increasing in size. The pain was worst at night. Six years back before the present swelling, the boy had taken treatment for an injury of the left knee which healed after therapy [Figure 1].
On physical examination, the patella was painful with significant swelling compared to the contralateral side. Articular range of motion was normal. Hemogram, C-reactive protein, and erythrocyte sedimentation rate were completely normal. Standard lateral and anteroposterior view of X-ray patella showed a well circumscribed lesion of 5 × 3× 3 cm size with mixed echogenicity. Both radiopaque and radiolucent areas were rimmed by sclerotic margins [Figure 2]. Computed tomography (CT) scan of the left knee confirmed the presence of fluid filled multiseptate cavities, suggesting the diagnosis of aneurysmal bone cyst/hemangioma. Fine needle aspiration cytology (FNAC) was performed; the aspiration showed only blood and blood cellular elements.
|Figure 2: X-ray lateral view - expansile cystic and lytic lesion of patella with rim of the sclerotic bone|
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Patellectomy was done with widely excised margins and the specimen was sent for histopathological examination. On gross examination, the specimen measured 5 × 5× 4 cm, and the cut section showed red granular surface with cystic spaces and tan white areas. It was gritty to cut and rimmed by sclerotic bone [Figure 3].
|Figure 3: Gross specimen - patella on cut section showing hemmorhagic and cystic areas with intervening bony septa|
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On microcopy, multiple sections showed anastamosing bony trabeculae irregularly arranged in a loose fibrovascular stroma. The trabeculae were lined by rim of osteoblasts, which were polygonal in shape with moderate eosinophilic cytoplasm, round-to-oval nuclei, and single nucleoli. Osteoid was seen [Figure 4]. In addition, there were large dilated vascular channels which had septae. In the septae, there was loosely arranged spindle cells, osteoclastic giant cells, areas of hemorrhage, and hemosiderin laden macrophages with adjacent foci of mature cartilage [Figure 5] and [Figure 6]. The diagnosis offered was osteoblastoma with secondary ABC changes.
|Figure 4: Small irregular bony trabeculae with osteoid and vascularised fibro connective tissue (Hematoxylin and eosin, 100×)|
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|Figure 6: Osteoclast giant cells with spindle cells in the septa (Hematoxylin and eosin, 400×)|
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The pain was completely relieved and the patient commenced normal exercise 48 hours after the operation. After 18 months, the follow up X-ray showed completely healed bone. On further follow up, the patient is fine till date without pain and recurrence of the lesion.
| Discussion|| |
Osteoblastoma is a rare benign tumor that accounts for less than 1% of all primary bone neoplasms. The neoplasm may be intramedullary, cortical, or subperiosteal in location. Sixty to sixty-five percent of the osteoblastomas are cortical and the remainder are medullary.  Osteoblastoma is usually diagnosed in young adults in the 2 nd to 4 th decade of life, and males are affected approximately twice as frequently as females. 
Jackson et al.  reported 35% percent of osteoblastomas in the spine, 10% in the femur, 5% in the tibia, and 9% in the bones of the foot and ankle in a series of 184 patients. Lucas et al.  reported 32% in the vertebral column, 12% in the femur, and 10% in the tibia.
Singh et al.  in their study analyzed 59 cases of lesions presenting in the patella. Of the 59 cases, 46% were non-neoplastic, 39% were benign, and 15% were malignant. The most common benign neoplasm was giant cell tumor (11 cases). Young patients are more likely to have a benign neoplasm. Lesions in patients less than 40 years of age included giant cell tumor, chondroblastoma, aneurysmal bone cyst (ABC), osteomyelitis, osteoid osteoma, and solitary bone cyst. In patients older than 40 years, the following were common lesions: intraosseous gout, metastasis, and intraosseous ganglion. Osteoblastoma arising from the patella is rare.
Osteoblastoma presents as a diagnostic dilemma and is difficult to differentiate from other benign bone forming tumors. The abovementioned lesions have to be differentiated from osteoblastoma and histopathological examination is essential to differentiate these various lesions. The salient features on histopathology, which differentiate osteoblastoma from other lesions, are:
In osteoblastoma, there are anastamosing trabeculae of osteoid and woven bone rimmed by single layer of benign osteoblasts. Loose fibrovascular stroma is seen between bony trabceulae with numerous osteoclasts. Intralesional hemorrhage and secondary ABC areas can be seen.  Sometimes cartilage is also seen.  In the present case, secondary ABC-like areas and cartilage was seen, which is uncommon. The tumor does not permeate into the adjacent bony trabaculae and the mitotic activity is low.
In aggressive osteoblastoma, the features are similar to benign osteoblastoma except for the morphology of osteoblasts, which are large and have epithelioid appearance. The osteoblasts are arranged in single cell layer and in aggregates, which are twice the size of normal osteoblasts. The cytoplasm is abundant eosinophilic with eccentric vesicular nuclei and prominent nucleoli. Mitosis and necrosis are not seen.
In osteoid osteoma, the microscopic features are similar to osteoblastoma. The only differentiating features is the size of the tumor which is less than 2 cm. Osteoblastoma such as osteosarcoma resembles osteoblastoma, however, permeation in to the surrounding host tissue is the diagnostic feature.
In aneurysmal bone cyst, there are large cystic spaces filled with blood and separated by fibrous septa, alternating with solid area. The cystic spaces and septa are lined by fibroblasts, myofibroblasts, histiocytes, but not endothelium; clusters of osteoclastic like multinucleated giant cells may be seen. The stroma is more cellular showing spindle cells. Reactive woven bone degenerated calcified and fibromyxoid tissue is sometimes seen.
Definitive diagnosis and clear treatment plan will prevent recurrence of osteoblastoma with good prognosis.
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Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]