Cisto Ósseo Aneurismático

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Aneurysmal Bone Cyst

Aneurysmal bone cyst (AOC) belongs to the group of pseudotumor bone lesions. This group of diseases produces bone changes that mimic tumor lesions, from the point of view of radiographic imaging.

Aneurysmal Bone Cyst

The injuries that are part of this group are:

simple bone cyst.

aneurysmal bone cyst.

juxtacortical bone cyst (intraosseous ganglion).

metaphyseal fibrous defect (non-ossifying fibroma).

eosinophilic granuloma.

fibrous dysplasia (osteofibrodysplasia).

myositis ossificans.

brown tumor of hyperparathyroidism.

intraosseous epidermoid cyst.

giant cell reparative granuloma.

The aneurysmal bone cyst, also called multilocular hematic cyst, is a lesion of insufflative bone rarefaction filled with serosanguineous fluid, interspersed with spaces varying in size and separated by septa of connective tissue containing trabeculae of bone or osteoid tissue and ostoclastic giant cells (fig 1 ).

The origin and etiology of this process are still unknown, despite having been described by Jaffe and Lichtenstein since 1942. Cytogenetic studies suggest that there is a correlation between this lesion and chromosome 17 translocation phenomena.

The presence of osteoclast-type giant cells suggests that a process of localized bone reabsorption occurred, accompanied by accumulation of blood and septated either by connective tissue or by osteoid tissue with bone trabeculae.

These blood-filled cavities do not have blood supply that can be demonstrated by arteriography or intracystic contrast infusion and consequently do not have a pulsatile character. These pockets are not empty therefore they are not cysts nor do they represent any form of aneurysm. The term “aneurysmal bone cyst” is not appropriate for this condition.

It is therefore a benign lesion and according to Enneking it can be classified as active or aggressive benign. The presence of areas of fibrosis and reparative ossification is related to cyst regression or the result of a previous fracture (fig 2).

The stores occur in varying numbers and sizes, clumping together and causing erosion of the bone trabeculae, which expand and inflate the cortex. Histologically, blood gaps are observed separated from each other by connective septa and osteoclastic cells, without atypia.

However, this “phenomenon” of an aneurysmal bone cyst may appear alongside other tumor lesions such as osteoblastoma , chondroblastoma , chondromyxoid fibroma, giant cell tumor, teleangiectatic osteosarcoma, fibrous dysplasia and brown tumor of hyperparathyroidism , in addition to metastatic lesions secondary to thyroid or kidney neoplasia . These tumors with their characteristic histology may present isolated areas of the classic aneurysmal bone cyst. Therefore, small biopsy fragments can make accurate diagnosis difficult (fig 3).

The choice of the biopsy site must allow obtaining a representative sample of the heterogeneity of the lesion: A) COA ; B) TGC

It is observed that the lesion has areas of liquid content ( a-COA ) and solid areas ( b-TGC ).

The anamnesis and images of the lesion must be carefully analyzed, the biopsy site must be chosen that allows a sample to be taken from the different areas that appear heterogeneous on MRI, to allow for an accurate diagnosis.

The classic aneurysmal bone cyst has a homogeneous appearance, while the aforementioned tumor lesions, when accompanied by areas of aneurysmal bone cyst, necessarily become heterogeneous.

It is more frequent in the first three decades of life, with its peak incidence between 5 and 20 years of age, with a slight predominance in females.

The patient generally presents with mild pain at the site of the injury and when the affected bone is superficial, inflammatory signs such as increased volume and heat can be observed. Generally, the patient correlates the onset of symptoms with some trauma.

In evolution there may be a slow, progressive or rapidly expansive increase. It affects any bone, most frequently the lower limbs (tibia and femur representing 35% of cases) and vertebrae, including the sacrum and in the pelvis mainly the iliopubic branch. They can mimic joint symptoms when located in the epiphysis. Compromise in the spine can cause compressive neurological symptoms, although in most cases it affects the posterior structures.

GOALS

At the end of reading this chapter, the reader will be able to:

know the group of pseudo-tumor lesions;
characterize the typical aneurysmal bone cyst;
determine the imaging tests necessary to clarify the injury;
make the differential diagnosis;
choose the best treatment for each situation.

CONCEPTUAL SCHEME: COA

In the bone staging performed with scintigraphy, we found a single lesion with discrete uptake on the periphery of the lesion.

Radiographically, it appears as a radiolucent insufflation lesion, preferably in the metaphyseal region of long bones (it can also occur in the epiphysis and diaphysis), with the presence of septa scattered throughout its content, with a “bullous” (or honeycomb) appearance, with thinning and expansion of the cortex, eccentric in 50% of cases or central location. They can also occur centrally in the cortical bone and in less than 8% of cases on the surface.

The radiographic appearance, however, is homogeneous. As the lesion progresses, a Codman’s triangle may form, giving a false impression of soft tissue invasion, which does not occur because the lesion always has a surface of connective tissue that circumscribes it (pseudo-capsule that delimits the area of injury to the compromised bone and adjacent tissues).

Magnetic resonance imaging, by performing cuts in different planes, often shows the presence of liquid levels, highlighting the numerous pockets separated by the connective septa. The diagnosis of an aneurysmal bone cyst on biopsy is accepted with greater ease when the MRI analysis of the entire lesion does not reveal any heterogeneous aspect. The presence of a heterogeneous structure on magnetic resonance imaging, in which the solid area presents contrast impregnation, implies the need to obtain a sample from this area for diagnosis, as this must be a case of association of an aneurysmal bone cyst with one of the aforementioned lesions.

The treatment of choice has been marginal resection or intralesional curettage, followed by filling the cavity with an autologous or homologous graft, when necessary. The cavity can also be filled with methylmethacrylate, although our preference is to use an autologous graft when possible, as it is a benign lesion. Some authors associate intralesional adjuvant treatment with the application of phenol, electrothermia or cryotherapy. In classic aneurysmal bone cysts, I do not see the point of this therapy, which, however, should be applied when the surgeon finds a “suspicious” area that was not detected on imaging. If the aforementioned benign tumors are involved, which may be accompanied by areas of aneurysmal bone cyst, local adjuvant therapy will be beneficial.

Some bone segments such as the ends of the fibula, clavicle, rib, distal third of the ulna, proximal radius, etc. can be resected, without the need for reconstruction.

In other situations, we may need segmental reconstructions with free or even vascularized bone grafts or joint reconstructions with prostheses in advanced cases with major joint involvement. In the spine, after resection of the lesion, arthrodesis may be necessary to avoid instability.

Radiotherapy should be avoided due to the risk of malignancy, however it is reserved for the evolutionary control of lesions that are difficult to access, such as the cervical spine, for example, or other situations in which surgical reintervention is not recommended.

Embolization as an isolated therapy is controversial. However, it can be used preoperatively to minimize bleeding during surgery. This practice is most often used in cases of difficult access, although its effectiveness is not always achieved. Infiltration with calcitonin has been reported with satisfactory results in isolated cases.

Recurrence may occur, as the phenomenon that caused the cyst is unknown and we cannot guarantee that surgery repaired it. The recurrence rate can reach thirty percent of cases.

Questions:

1- The aneurysmal bone cyst:

a- it is a tumoral lesion

b- it is a metastatic lesion

c- occurs alone or accompanies other bone injuries

d- it is a pseudo-aneurysm

2- Differential diagnoses of COA include:

a- Chondrosarcoma

b- TGC

c- Ewing sarcoma

d- cortical fibrous defect

3- According to Enneking’s classification, the COA is:

a- active benign lesion

b- latent benign lesion

c- low-grade malignant lesion

d- high-grade malignant lesion

4- In relation to the COA, it is correct to state:

a- occurs more frequently in elderly patients

b- presents osteoclast-type giant cells

c- should preferably be treated with wide resection

d- presents foci of calcification

5- The radiographic appearance of the COA is:

a- condensing bone lesion

b- heterogeneous bone lesion

c- homogeneous bone rarefaction lesion

d- bone lesion without precise limits.

6- The preferential treatment of the COA is:

a- intralesional curettage

b- segmental resection

c- segmental resection + endoprosthesis

d- Arthrodesis

7- The tumor lesions that most frequently present areas of aneurysmal bone cyst are:

a- tgc; chondrosarcoma; osteosarcoma and Ewing’s sarcoma

b- fibrous defect; tgc; adamantinoma and chordoma

c- osteoblastoma; chondroblastoma; chondromyxoid fibroma and tgc;

d- osteosarcoma; chondroblastoma; eosinophilic granuloma and lipoma

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