CASE

A 17-year-old male presented to his family physician with low back pain that had progressively worsened over the past few months. At presentation, he was starting to have weakness in both of his legs as well. The physical examination revealed mild soft tissue swelling along the upper lumbar spine and a mild decrease in strength and reflexes in the patient's lower extremities. The physician ordered radiographs of the lumbar spine (Figure 1). What does the radiograph show?


 

DISCUSSION

In Figure 1, an anteroposterior view of the lumbar spine, the spinous process appears to be missing at L2. This finding prompted further evaluation with MRI because the patient had no history of laminectomy. Figure 2 and Figure 3 are T2-weighted MRI scans that demonstrate a large mass, poste rior to the L2 vertebral body, which severely narrows the spinal canal. Fluid-fluid levels are also seen within the mass.

This young patient was found to have an aneurysmal bone cyst (ABC). These cysts are named for their radiographic appearance, which is aneurysmal or expansile.

ABCs are cystic, blood-containing cavities with thin walls and comprise approximately 1% of bone tumors. They are typically found in persons younger than 30 years; most are found by age 20 years. These tumors have been classified into two types: primary or secondary. Primary tumors appear to occur by themselves, whereas secondary ABCs are seen in conjunction with another bone lesion. Approximately 30% of ABCs are secondary and are typically associated with giant cell tumors, chondroblastomas, osteoblastomas, osteosarcomas, fibrous dysplasia, chondromyxoid fibromas, chondrosarcomas, or hemangioendotheliomas.1-3 ABCs can occur anywhere in the skeleton but are most often found in the tubular bones, flat bones, or posterior elements of the spine. Although they are benign, ABCs can be locally aggressive, causing pain, deformity, pathologic fractures, and neurologic symptoms.

Patients with an ABC typically present with pain, usually of acute onset. This pain may progressively worsen over approximately 6 to 12 weeks.3 There can be associated swelling, a palpable mass, pathologic fracture, radiculopathy, or headache, depending on location. Diagnosis can sometimes be made radiographically, but crosssectional imaging with CT or MRI may be helpful in evaluating the extent of invasion. Although the etiology of ABCs is unknown, many believe them to be secondary to a vascular malformation in bone, venous obstruction, or hemodynamic changes due to a concurrent bone lesion.4

Radiographs generally demonstrate an area of radiolucency with expanded, thinned cortex of bone. ABCs rarely involve the growth plate or articular surface. They usually have a balloonedout or soap bubble appearance. If the ABC is in a long tubular bone, it is typically found in the metaphysis. The differential diagnosis includes a simple bone cyst, giant cell tumor, enchondroma, brown tumor of hyperparathyroidism, osteoblastoma, telangiectatic osteosarcoma, and angiosarcoma.3 CT will typically show a lesion with internal septations and fluid-fluid levels, which represent layering of blood and serum. MRI will also demonstrate fluid-fluid levels. On MRI, ABCs are usually bright (high signal intensity) on T2 and fat-saturated sequences and intermediate or low signal intensity on T1-weighted sequences. If there is acute bleeding within the lesion, however, the T1 signal may be bright. Radionuclide bone scanning shows increased radiotracer uptake in the margin of the lesion with very little uptake centrally. Laboratory studies are usually not needed for diagnosis; however, alkaline phosphatase levels may be elevated.

Treatment of an ABC may include surgical curettage, wide resection, vascular embolization, radiation, cryotherapy, or hyperthermia.5 Wide resection is the most likely treatment to provide cure, but the invasiveness of this approach and the associated bleeding risks carry increased morbidity. Vascular embolization has demonstrated promise but has not been extensively used because ABCs are uncommon and because embolization has been available only for the last few decades. Embolization can be used to reduce bleeding before resection. Although prognosis is excellent, the most common complication is recurrence; this usually occurs within 1 year of treatment, with almost all recurrences being within 2 years. Patients should be followed regularly so that recurrence is detected early. JAAPA

REFERENCES

1. Knabe P, Petersilge C. Musculoskeletal imaging teaching files. Case seventy-eight—aneurysmal bone cyst. University Hospitals of Cleveland Department of Radiology Web site. http://www.uhrad.com/msiarc/msi078.htm. Accessed September 9, 2009.

2. Wheeless CR. Aneurysmal bone cyst. Wheeless' Textbook of Orthopaedics. Duke Orthopaedics. http://www.wheelessonline.com/ortho/aneurysmal_bone_cyst. Updated December 14, 2008. Accessed September 9, 2009.

3. Anand M, Wang E. Aneurysmal bone cyst. eMedicine. http://emedicine.medscape.com/article/386060. Updated April 3, 2007. Accessed September 9, 2009.

4. Eastwood B, McFarland M. Aneurysmal bone cyst. eMedicine. http://www.emedicine.medscape.com/article/1254784. Updated May 2, 2008. Accessed September 9, 2009.

5. Koci TM, Mehringer CM, Yamagata N, Chiang F. Aneurysmal bone cyst of the thoracic spine: evolution after particulate embolization. AJNR Am J Neuroradiol. 1995;16(4 suppl):857-860.