Working Diagnosis:
Pathologic patella fracture secondary to aneurysmal bone cyst

Treatment:
The patient was immobilized in a long-leg non-weight-bearing cast and referred to orthopedic oncology. He underwent incisional biopsy of the patella and curettage augmented by laser beam, allograft bone grafting, and internal fixation. He remained in a hinged-knee braced locked in extension for six weeks, with progressive extension and strengthening. The patient has had an excellent outcome.

Outcome:
The patient returned to full activity approximately four months post-operatively. Hardware was removed approximately nine months after internal fixation. Follow-up radiographs revealed healed patella fracture with no evidence of aneurysmal bone cyst recurrence. He has returned to full activity.

Author's Comments:
Aneurysmal bone cysts account for 1-6% of all primary tumors, and less than 1% of ABCs occur in the patella. These tumors typically occur in the 2nd decade of life. Although benign, ABCs can be aggressive and may result in anterior knee pain and pathologic fracture. The mainstay of treatment for ABCs is curettage. The recurrence rate following curettage of an ABC has been reported as 2-25%. In this case, the curettage augmented by laser beam, allograft bone grafting, and internal fixation, resulted in complete fracture healing with no recurrence of the aneurysmal bone cyst one year post-operatively.

Editor's Comments:
The author presents an interesting case of a patellar aneurysmal bone cyst, especially since most lower extremity ABCs occur at the femur, tibia, or fibula. Common presenting symptoms include pain, localized swelling or palpable mass, and pathologic fractures. Recurrence is rare after surgical excision, especially after 2 years.

References:
1) Balke, M. Uncommon cause for anterior knee pain - Aggressive aneurismal bone cyst of the patella. Sports Medicine, Arthroscopy, Rehabilitation, Therapy & Technology. 2010; 2(9).
2) Chen L. Imaging features and review literature of aneusymal bone cyst. Chin J Radiol. 2005: 30:269-275
3) Marudanayagam A. Secondary aneurismal bone cyst of the patella: A case report. Iowa Orthopedic Journal. 2006; 26: 144-146.
4) Nydick, J. An 11-year-old boy with a patella fracture. Clin Orthop Relat Res. 2009; 467: 3365-3370
5)Singh, J. Tumour and tumour-like lesions of the patella –a multicentre experience. Eur Radiol. 2009; 19:701-712
6) Somasekhar Reddy N. Primary aneurismal cyst of the patella. Indian J Orthop. 2006; Apr-Jun: 43(2): 216-217.

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