Aneurysmal Bone Cysts of Spine: An Enigmatic Entity
Correspondence Address: Source of Support: None, Conflict of Interest: None DOI: 10.4103/0028-3886.293465
Source of Support: None, Conflict of Interest: None
Keywords: Aneurysmal bone cyst, pediatric, spine
An aneurysmal bone cyst (ABC) was first described by Jaffe and Lichtenstein in 1942. In 1950, Lichtenstein described the lesion as a localized tumor occurring in long bones and vertebrae, and characterized by “blow out distension” of the skeletal contour; the term “aneurysmal” is therefore partly descriptive. ABC is defined by World Health Organization as an expanding osteolytic lesion consisting of blood-filled spaces of variable size separated by connective tissue septa containing trabeculae of bone or osteoid tissue and osteoclast giant cells. They commonly occur during the first three decades of life and are slightly predominant in the females. It can occur virtually in any bone but most commonly affects femur and tibia. ABCs comprised approximately 1.4% of all bone tumors and 15% of all primary spine tumors.,
A 16-year-old girl presented with pain in upper back and paraparesis; she was found to have a bony tumor at D3 vertebral level with cord compression [Figure 1]a. She was operated by transthoracic route and tumor excision was done followed by fusion [Figure 1]b. She had no change in neurological status in immediate postoperative period, and she never attended the hospital after discharge as she lived outside our state.
A 15-year-old boy presented with low backache not responding to analgesics and on investigations was having an expansile multicystic tumor arising from posterior elements of L1 vertebra [Figure 2]a and [Figure 2]b. He was operated outside our hospital, and surgeon had encountered excessive bleeding so he abandoned the procedure. The patient then attended our center where he was operated by posterior approach, intraoperatively we did not face any major problem and as soon as we removed all the tumor the bleeding stopped. In postoperative period, pain resolved significantly and over next 3 weeks, it disappeared altogether. We are following the patient for last 5 years, and he is stable without any complaints. Postoperative magnetic resonance imaging (MRI) did not show any tumor [Figure 2]c.
An 18-year-old boy presented with rapid onset paraplegia with grade 1 power. He had pathologic collapse of D3 vertebra [Figure 3]a. He underwent transmanubrial approach and intraoperatively the tumor was found to be fleshy and vascular. Complete excision was done, and stabilization was achieved by anterior interbody fusion using bone graft and plating [Figure 3]b. He had gradual improvement in power and was able to walk with two person support, but power never normalized till last follow-up of eight months.
A young girl presented with back ache with paraparesis and MRI revealed D9 bony tumor with cord compression [Figure 4]a. She was operated by posterior approach, and she had improvement in power and was able to walk with minimal support. After 19 months, the patient had again deterioration of power; she was evaluated and was found to have cord compression at the same level [Figure 4]b. She was operated by transthoracic route and complete excision was done followed by instrumentation. She then improved in power and was independent and needed minimal assistance. She is in follow-up for last two years.
Etiology and pathogenesis
The true etiology of ABCs is unknown. Most investigators believe that ABCs are the result of a vascular malformation within the bone; however, the ultimate cause of the malformation is a topic of controversy. Three commonly proposed theories are as follows:
The natural history of ABC has been described as evolving through four radiological stages: initial, active, stabilization, and healing. In the initial phase, the lesion is characterized by a well-defined area of osteolysis with discrete elevation of periosteum. This is followed by a growth phase, in which the lesion grows rapidly with progressive destruction of bone and development of characteristic, “blown-out” radiological appearance. The growth phase is succeeded by the third and the most dangerous phase, which 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. The third phase is also marked by the presence of characteristic, “soap bubble appearance.” In the final healing phase seen in some ABCs, there is progressive calcification and ossification, with the lesion transformed into a dense bony mass.,
The true pathophysiology of ABCs is also unknown. Different theories about several vascular malformations exist; these include arteriovenous fistulas and venous blockage. The vascular lesions then cause increased pressure, expansion, erosion, and resorption of the surrounding bone. The malformation is also believed to cause local hemorrhage that initiates the formation of reactive osteolytic tissue.
The resultant hemodynamic forces generated by high-pressure vascular channels rapidly erode the osseous trabeculae into a cystic cavity giving the lesion a ballooned, thin-shelled, and multiseptated soap-bubble appearance.,,
Aneurysmal bone cysts have a characteristic gross appearance that has been described as a “blood-filled sponge,” referring to blood or serum-filled cavernous cysts separated by walls of woven bone and osteoclasts.,,,, A thin subperiosteal shell of new bone surrounds the structure and contains cystic blood-filled cavities. The tissue within the spaces shows brownish intertwining septa. The stroma contains proliferative fibroblasts, spindle cells, areas of osteoid formation, and an uneven distribution of multinucleated giant cells, multinuclear giant cells, and thin-walled blood vessels or a preponderance of fibrous tissue with enlarging vascular spaces. A solid variant of the ABC has also been described; the histologic findings are similar to the cystic lesions, but the solid variant has a solid gross appearance.
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. ABCs 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.
Aneurysmal bone cysts are generally considered rare, accounting for only 1–6% of all primary bony tumors. A group from Austria reported an annual incidence of 0.14 ABCs per 100,000 people; however, the true incidence is difficult to calculate because of the existence of spontaneous regression and clinically silent cases. A biopsy-proven incidence study from the Netherlands showed that ABCs were the second most common tumor or tumor-like lesion found in children. ABCs present in patients with a wide age range from childhood onward; however, the majority of patients are young adults. These cysts develop in long bones and the vertebral column, as well as in the flat bones. Lesions in the long bones vertebrae are well-documented and fairly common. They account for three-quarters of all cases. In the spine, they have a predilection for the lumbar spine, followed by equal occurrence in the thoracic and cervical spine. In the spine, they frequently involve the posterior elements, especially the lamina; however, the lesion can involve all aspects of the vertebrae, including the vertebral body (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 25–35% of cases, the lesion crosses over to an adjacent vertebra through surrounding soft tissues, but never through the intervertebral disc., Associated scoliosis and kyphosis are also present in 10–15% of patients.,
Patients usually present with pain, a mass, swelling, a pathologic fracture, or a combination of these symptoms in the affected area. The symptoms are usually present for several weeks to months (usually 4–8 months) before the diagnosis is made, and the patient may also have a history of a rapidly enlarging mass. Neurologic symptoms associated with ABCs may develop secondary to pressure or tenting of the nerve over the lesion, typically in the spine.
Pathologic fracture occurs in about 8% of ABCs, but the occurrence rate may be as high as 21% in ABCs that have spinal involvement. Varying degrees of scoliosis may be caused by painful muscular spasm or wedge destruction of one-half of the VB. Other findings may include the following: deformity, decreased range of motion, weakness, stiffness, reactive torticollis, bruit over the affected area, warmth over the affected area.,,,,
Radiographic findings usually consist of an eccentric or, less commonly, a central or subperiosteal lesion that appears cystic or lytic. Images may show the expansion of the surrounding bone with a blown-out, ballooned, or soap-bubble appearance. Some views may show an eggshell-appearing bony rim surrounding the lesion. One may see the cystic spaces and rarely, partially ossified septa.
Computed tomography scanning
The same characteristics are demonstrated as on plain radiographs; however, computed tomography (CT) scans also show internal septation (i.e., calcified rim, eggshell appearance), which may be completely or partially intact. Fluid-fluid levels can also be seen; these are most often found in the ABC, but they are not exclusive to it. The fluid-fluid levels are caused by the separation of cellular material and serum within the cystic spaces. To see these levels best, the patient must remain in the position in which they are imaged for at least 10 min to obtain enough separation of the materials of different attenuation. The CT modality is also useful for evaluating the pedicles and VBs to aid surgical planning for an instrumented fusion.
Magnetic resonance imaging
Findings are similar to those of CT scans, but MRIs can more specifically reveal blood within the lesion, as well as expansion into the soft tissues.
The degree of compression of neural elements is also better visualized., In cases in which the ABC extends through the paravertebral soft tissues to involve adjacent vertebrae, the intervertebral discs are intact and not involved by the lesion., ABCs have a heterogeneous appearance on both T1- and T2-weighted MR images. MRI reveals a well-differentiated expansile mass with multiple internal septations.
In some cases, a hyper vascular area around the ABC is shown. An intense diffuse area of persistent contrast accumulation may be visualized without main afferent or efferent vessels observed. Selective angiography can sometimes delineate the blood supply to the hyper vascular lesion, along with pathological circulation and occasional arteriovenous shunts. It may be helpful to plan selective arterial embolization as the primary treatment or as a preoperative method to help control intraoperative blood loss.
The optimal treatment of ABCs of the spine remains a subject of controversy in the literature. Because of their unique anatomical structure and function, there are special considerations when managing ABCs of the spine. One must take into account the age of the patient, the surgical accessibility of the lesion, necessity to minimize intraoperative blood loss, the presence of neurological compression, the presence of a pathological fracture and deformity, and potential postoperative instability after complete resection.,,
Although various modalities are used to treat ABCs,, Boriani and colleagues devised a treatment paradigm based on the degree of diagnostic certainty. If the diagnosis is nearly certain on the basis of clinical and radiographic findings, selective arterial embolization is recommended, followed by complete curettage or en bloc resection (especially for lesions limited to the posterior elements). In cases, when diagnosis is uncertain, a needle biopsy can yield nonspecific tissue given the large hemorrhagic components of these tumors. Therefore, open biopsy is preferred, followed by curettage if the tissue is diagnostic for an ABC. Vertebroplasty may also be used to reinforce the bony defects after curettage. Anterior or posterior reconstructions should be performed in adults after resection or curettage, depending on the amount of bone removed. However, in children, some authors advocate bracing before skeletal maturity, followed by delayed arthrodesis, whereas others recommend aggressive resection and arthrodesis.
Medical therapy/nonsurgical treatment
Selective arterial embolization has shown much promise in small studies. However, relatively few cases have been treated with this therapy because ABCs are rare and because selective arterial embolization has been available only since the 1980s. Selective arterial embolization has the advantage of being able to reach difficult locations, being able to save joint function when subchondral bone destruction is present, and making the complications that are associated with invasive surgery (e.g., bleeding) less likely to occur. Selective arterial embolization may be performed within 48 h before surgery to reduce the amount of hemorrhage.
Some of the literature suggests that selective arterial embolization can be a primary treatment if the following conditions are met; certain diagnosis of ABC, technical feasibility and safety, stability; no evidence of pathologic fracture or impeding fracture and no neurologic involvement.
Embolization may be considered as the primary therapy in patients with a recurrent lesion after previous surgeries or in patients who cannot tolerate surgery, but only if pathological fracture, spinal deformity, instability, and neurological compromise are absent. The long-term effect of embolization results in involution of the soft-tissue component, sclerosis, and ossification. This mineralization usually becomes apparent after 3 months to 2 years.,,, Presumably, the occlusion of feeding arteries dampens the hemodynamic forces that underlie the destructive bone remodeling and promotes spontaneous reossification.
There is controversy regarding the role of radiation therapy. Although some authors have reported favorable results with radiation as the primary treatment,, this modality should not be the first line of treatment for spinal ABCs. Capanna, et al. suggested that there was no benefit from the addition of radiation to curettage or to partial resection and curettage. Some opponents of its use argue that the risks of radiation therapy are not justified, given the benign behavior of ABCs and the high rate of cure with complete excision.,,
The mechanism by which radiotherapy acts on ABC is not clear. The vascular lesions undergo progressive thickening of the vascular wall and luminal thrombosis. The mesenchymal tissue undergoes fibrosis, deposition of fibrinous exudates and necrosis.
Indications for radiation therapy are limited, and it remains an adjuvant therapy for patients with inoperable lesions, aggressive recurrent disease, or medical conditions that place them at high risk during surgery. Some authors believe that embolization therapy should be attempted before the use of radiation therapy.,
Unusually, ABC can be treated with intralesional curettage; more commonly ABC is treated by intralesional excision. The difference between curettage and excision is that excision involves wide unroofing of the lesion through a cortical window by careful abrasion of all the surfaces with a high-speed burr. Several reports also advocate en bloc resection for the spine.,,,,,, Although the en bloc resection is technically challenging, this technique minimizes the risk of recurrence. Intralesional curettage with or without bone grafting has been described.,,
Complete resection is the treatment of choice with ABCs of the spine, especially in patients who present with a neurological deficit. Total excision, en bloc if possible, provides the highest rate of cure, with an excellent prognosis.,,,, Subtotal excision is associated with a high incidence of recurrence, which is seen within 6–12 months. Early surgical intervention with total excision of all affected bone is recommended for immediate decompression. Total excision must include the entire cyst wall, all abnormal tissues that feel spongy, and bone surfaces that are lined with fragile and hyper vascular membranes.,,,,,,,,,,, Excision must include the entire cyst wall because partial excision is related to a higher risk of recurrence. Intraoperative bleeding generally subsides when this layer is removed. Because the posterior elements are almost always involved, a posterior approach should be considered initially. With a posterior exposure, any tumor involvement of the pedicles with extension into the anterolateral aspect of the VB is surgically accessible with a unilateral or bilateral transpedicular approach. If there is extensive anterior VB involvement and resection and decompression from a posterior approach are inadequate, then a separate anterior approach should be used, either in the same operation or at a later date.,,
The collapse of the VB can produce kyphotic deformity and resultant neurological compromise. Extensive tumor resection can also result in postoperative iatrogenic instability. If instability and/or deformity already exists, or if the amount of bone resection is expected to result in instability, then simultaneous reconstruction and instrumented stabilization should be planned.,,,,,,,,
Total excision with or without instrumentation is the optimal approach for local control of the tumor and it prevents recurrence. Recurrence is reported in 10–44% of the cases, and usually rare when the tumor is excised completely. Ninety percent of recurrences occur within two years. Thus, posttreatment follow-up should be at least 24 months.,, Hay etal. reported that there were no recurrences when total excision was performed, and a 25% recurrence rate after partial excision. Total excision of large tumors results in bony instability, and instrumentation is necessary to maintain structural integrity. It is also reported that careful preoperative planning is important for management of postexcision spinal instability.
Aneurysmal bone cyst is not a tumor but in view of local destructive nature, they are classified as tumors. They are classified as primary or secondary on basis of absence and presence of associated lesions. Usually, young age is effected (teenage, early twenties). The best treatment is complete surgical excision and radiation should not be used as routine.
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Conflicts of interest
There are no conflicts of interest.
[Figure 1], [Figure 2], [Figure 3], [Figure 4]