Surgical Management of Aneurysmal Bone Cysts of the Spine

James K. Liu, M.D.; Douglas L. Brockmeyer, M.D.; Andrew T. Dailey, M.D.; Meic H. Schmidt, M.D.

Disclosures

Neurosurg Focus. 2003;15(5) 

In This Article

Overview

Aneurysmal bone cysts primarily affect individuals who are between 10 and 20 years of age, with a slight preponderance of females.[2,15,43] The mean age at diagnosis varied from 14 to 16 years in several large studies.[15,41,43,57]

The thoracic (32%) and lumbar (34%) vertebrae are the most common sites for spinal aneurysmal bone cysts.[1,2,5,15,26,40,43] The posterior elements and the pedicles are usually affected first, and in 60 to 70% of cases the lesion extends into the VB. Continued expansion and bone destruction of the VB can ultimately lead to sudden collapse or angular deformity of the vertebral column and acute epidural compression of the spinal cord. In approximately 25 to 35% of cases, the lesion crosses over to an adjacent vertebra through surrounding soft tissues, but never through the intervertebral disc.[2,41] Associated scoliosis and kyphosis are also present in 10 to 15% of patients.[43,56]

The duration of symptoms prior to diagnosis is usually 4 to 8 months.[41] Local pain is the most common feature of this disease, and is often worse at night and with recumbency. This can be associated with swelling or a palpable mass that is often tender to the touch. Posterior masses are usually paravertebral, and anterior masses can sometimes mimic thyroid neoplasms. Neurological symptoms can be the initial presentation in 60 to 70% of cases. Early nerve root compression can cause paresthesias and radicular pain that can be referred to the chest wall, abdomen, or extremities. Epidural spinal cord compression by a rapidly expanding lesion or by VB collapse and kyphotic deformity can result in myelopathy, or sometimes acute paraparesis.[42,46,47] Acute spinal cord compression can occur in the absence of VB collapse if there is a break in the posterior cortex of the body that results in epidural extension of the lesion.[7] Rapid deterioration to complete paraplegia may occur if lesions are left untreated. Varying degrees of scoliosis may be caused by painful muscular spasm or wedge destruction of one half of the VB.[2]

Although aneurysmal bone cysts are benign, nonneoplastic lesions with no propensity to metastasize, their potential for rapid growth, considerable destruction of bone, and neurological involvement has led to aggressive therapy, despite reports of spontaneous regression or regression following simple biopsy sampling of the lesion.[49,51] Such regression appears to be a rare phenomenon, perhaps related to thrombosis and fibrosis of the aneurysmal bone cyst.[56]

The exact pathogenesis of aneurysmal bone cysts remains unknown. Some authors have suggested that the lesion is preceded by traumatic fractures or subperiosteal hematoma.[12,43] Others have suggested that the formation of aneurysmal bone cysts is the result of a reparative process. Although several theories have been postulated, these cysts are generally thought of as a secondary vascular phenomenon superimposed on a preexisting lesion, which presumably initiates a periosteal or intraosseous arteriovenous malformation.[1,3,20,27,31,32] The resultant hemodynamic forces generated by high-pressure vascular channels rapidly erode the osseous trabeculae into a cystic cavity. The associated reactive changes within the endosteum and periosteum incite accelerated osteoblastic and osteoclastic activities, which then rapidly remodel the bone while conforming to the hemodynamic forces, giving the lesion a ballooned, thin-shelled, and multiseptated soap-bubble appearance.[1,32,41]

The concept of aneurysmal bone cysts as a secondary phenomena occurring in a preexisting lesion is based on the fact that in approximately 20 to 30% of cases, an aneurysmal bone cyst is associated with an underlying skeletal lesion.[15,30,34] Some refer to these as secondary aneurysmal bone cysts: preexisting lesions have included giant cell tumor, osteoblastoma, hemangioma, chondroblastoma, nonossifying fibroma, chondromyxoid fibroma, fibrous dysplasia, telangiectatic osteosarcoma, and brown tumor of primary hyperparathyroidism.[4] Nevertheless, in most aneurysmal bone cysts, an underlying lesion is not encountered. This might be the result of sampling error or it could mean that the aneurysmal bone cyst destroyed all evidence of the preexisting lesion. It appears that primary aneurysmal bone cyst is an entity in itself, with unique clinical, neuroimaging, and prognostic behavior. The diagnosis of primary aneurysmal bone cyst should therefore be made when all other underlying lesions can be ex-cluded.[4]

Serial neuroimaging and histopathological correlative studies have been used to identify four stages in the natural history of aneurysmal bone cysts that may explain their pathogenesis.[23,26,41,57] The initial stage is marked by varying degrees of expansion and thinning of the cortical bone, which results in cystic changes. The growth phase is characterized by the expansile, soap-bubble appearance surrounded by progressively thinning eggshell cortex. The third and most dangerous phase involves a sudden, explosive increase in the size of the blood-filled cysts, which causes bone collapse and soft-tissue invasion. This usually marks the beginning of rapid neurological deterioration. In the final healing phase, seen only in some aneurysmal bone cysts, spontaneous ossification and shrinkage of the lesion occur.

On gross inspection, aneurysmal bone cysts have a multilocular, spongelike appearance consisting of blood-filled cavities separated by thin, fibrous septa.[2,15] They are expansile lesions that erode and destroy the surrounding bone, leaving an eggshell-thin rim of subperiosteal new bone that is continuous with the adjacent cortex. The core of the tumor consists of soft, fleshy, vascular tissue, in addition to a cystic trabeculation of the interior of the mass containing unclotted blood; the mass may invade adjacent soft tissue or surround the thecal sac. Bleeding appears to come from the soft tissue lining the cysts and may be profuse and difficult to control until all the lining has been removed.[26]

Histologically, the fibrous septa of aneurysmal bone cysts are made of fibroblasts, myofibroblasts, multinucleated osteoclast-like giant cells, hemosiderin deposits, blood vessels, and fields of osteoid and woven bone (Fig. 1).[1,2,13] The cavernous, blood-filled cysts are not true vascular channels because they lack an endothelial lining and the elastic tissue or smooth muscle that is found in the walls of normal blood vessels. Mitosis may be observed, which indicates proliferative activity. Aneurysmal bone cysts may expand by enlargement of the cavities or by proliferation of the basic tissue. The tumor has a well-differentiated, benign histological features, but can behave aggressively by invading into adjacent vertebrae and surrounding soft tissues, causing neurological compromise.[26,56]

Photomicrograph of tissue obtained at pathological examination showing fibroblasts and several multinucleated giant cells. There are variably sized, blood-filled cystic spaces in some areas of the lesion. H & E, original magnification × 400.

Neuroimaging studies usually reveal a characteristic lytic lesion and "ballooning" or expansion of bone with erosion and destruction of the cortex. A blown-out, egg-shell-thin layer of cortical or periosteal bone formation is usually present.[1,15,53] Pathological fracture with partial collapse of the VB is common; this may occasionally progress to complete collapse (vertebra plana).[42]

Preoperative CT scans are better for defining the location and extent of bone destruction and the integrity of the cortical bone, and for detecting calcium deposits (Fig. 2 left).[7] Sometimes, communicating cavities with fluid–fluid interfaces can be demonstrated. The CT modality is also useful for evaluating the pedicles and VBs to aid surgical planning for an instrumented fusion.

Neuroimages obtained in a 3-year-old girl who presented with a posterior neck mass associated with neck pain from an aneurysmal bone cyst at C-2. Left: Axial CT scan demonstrating a large, multicystic and expansile lesion of C-2, which replaced the posterior elements including the pedicles and left aspect of the VB. The odontoid process was normal. Center: Preoperative percutaneous embolization was performed. This angiogram obtained after injection of contrast material demonstrates filling of the multicystic lesion. Right: Plain x-ray film obtained after a C-2 laminectomy with resection of the aneurysmal bone cyst was performed, followed by an instrumented C1–3 fusion with rib allograft.

The MR imaging modality provides the most optimal evaluation and delineation of the contents of and the full extent of bone and soft-tissue involvement by the aneurysmal bone cyst (Fig. 3A and B). The degree of compression of neural elements is also better visualized.[7,43] In cases in which the aneurysmal bone cyst extends through the paravertebral soft tissues to involve adjacent vertebrae, the intervertebral discs are intact and not involved by the lesion.[15,41]

Neuroimages obtained in a 7-year-old girl who presented with back pain, abdominal pain, and myelopathy from a T-6 aneurysmal bone cyst. A and B: Sagittal (A) and axial (B) T2-weighted MR images demonstrating the characteristic heterogeneous, soap-bubble appearance of aneurysmal bone cysts with multiple fluid–fluid levels. There is spinal cord compression from epidural expansion of the mass. C: Axial CT scan revealing extensive bone destruction of the posterior elements, the right pedicle, and VB. The patient underwent gross-total resection of the aneurysmal bone cyst via a posterior transpedicular approach. Postoperatively, the patient was placed in an external orthosis and followed closely with serial imaging. D: Sagittal T2-weighted MR image obtained 2 months postresection demonstrating marked kyphotic deformity with spinal cord compression. The patient experienced increased back pain and mild lower-extremi-ty weakness, and therefore underwent spinal reconstruction and stabilization with thoracic pedicle screws and rods and an interbody rib allograft at T-6. E and F: Postoperative plain x-ray films of the spinal construct, lateral view (E) and anteroposterior view (F). The patient achieved a full recovery of motor function and was able to walk.

Aneurysmal bone cysts have a heterogeneous appearance on both T-1 and T2-weighted MR images. The presence of multiloculated cysts containing fluid–fluid interfaces on T2-weighted images is very characteristic and highly suggestive of the diagnosis of aneurysmal bone cyst (Fig. 3A and B). The hypointense component is on the dependent portion of the fluid level within the cyst and the hyperintense component is on the nondependent side. Because these cysts contain blood from different stages of their evolution, the darker dependent portions of the cyst likely represent sediments of cellular and proteinaceous elements of unclotted blood.[7] Although the presence of fluid–fluid interfaces is highly suggestive of the diagnosis of aneurysmal bone cyst, they have also been described in telangiectatic osteosarcoma, unicameral bone cyst, chondroblastoma, giant cell tumor, and osteoblastoma. To differentiate aneurysmal bone cyst from other masses containing fluid–fluid interfaces, administration of intravenous Gd demonstrates smooth enhancement of the internal septations within aneurysmal bone cysts. Theoretically, the septations in aneurysmal bone cysts are thin and smooth, whereas those in other masses, most of them neoplastic in nature, would be nodular.[6] The hypervascularity of the complex septa network accounts for the soap-bubble appearance of the lesion on enhanced MR images.

Radioisotope bone scans can show increased uptake in some cases.[15] Selective angiography can sometimes delineate the blood supply to the hypervascular lesion, along with pathological circulation and occasional arteriovenous shunts.[1] Identification of arterial feeding vessels to the lesion may be useful for preoperative embolization. Visualization of the lesion can also be performed by direct percutaneous injection of a radiopaque substance into the aneurysmal bone cyst (Fig. 2 center). This is usually done in conjunction with percutaneous embolization procedures.

Aneurysmal bone cysts of the spine must be differentiated from giant cell tumor, hemangioma, fibrous dysplasia, osteosarcoma, and metastatic lesions. Giant cell tumor is uncommon in patients younger than 20 years of age, and it occurs mostly in the sacrum. Unlike in aneurysmal bone cysts, there are no cavernous vascular spaces. On pathological examination, the giant cells are bigger and more numerous in giant cell tumor. Vertebral hemangioma is differentiated by its characteristic trabeculated, honeycomb appearance on CT scans. It usually involves only one VB and does not expand and balloon out like an aneurysmal bone cyst. Fibrous dysplasia of the vertebra affects the same age group as aneurysmal bone cysts, usually includes more than one vertebra, grows slowly until the patient's skeletal growth ceases, and is often painless. Osteosarcoma and metastatic lesions, on the other hand, are painful tumors found in older patients, and do not produce bone expansion and ballooning.[1]

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