Aneurysmal bone cysts are benign lesions that may develop in distinct parts of the skeleton. They consist of a blood-filled sac delimited to adjacent structures by a thin layer of osseous tissue.
Aneurysmal bone cysts (ABC) are benign lesions of the skeleton. Being blood-filled sacs that develop inside of the bones, they present pathological features of cysts. However, they also dispose of a thin osseous capsule and show rapid and locally destructive growth, a tendency towards formation of metastases and recurrence. Thus, they are sometimes referred to as benign skeletal tumors rather than mere cysts. A precise classification of this type of lesion is further complicated by the fact that little is known about their etiology and pathogenesis.
ABC are usually diagnosed in patients younger than 30 years, often even aged 20 years or less . While any part of the skeleton may be affected, metaphyses of long bones, particularly of the lower limbs most frequently develop ABC. ABC affecting the axial skeleton and flat bones account for a minor share of cases. With regards to possible triggers, hypotheses regarding genetic, traumatic or vascular origins have been proposed but not yet proven.
Diagnostics are based on radiographic images. Typically, dilated spots, less radiopaque then their immediate, osseous surroundings are visible upon radiographic examination. Their fluid content may be distinguished in magnetic resonance imaging screens. Such findings, encountered in young people presenting with skeletal symptoms, are suspicious for ABC, but may not be easily differentiated from certain neoplasms. Histopathological analysis of a biopsy sample may provide valuable information to this end.
Therapy is usually provided in form of a surgical intervention, curettage and implantation of a bone graft. Other therapeutic options are cryotherapy or sclerotherapy to obliterate the vascular malformation. It is important to completely eliminate ABC, otherwise there are high risks for recurrence. Indeed, most patients need to undergo several treatments until they can be cured.
While there are several hypotheses regarding possible causes of ABC, no consensus could be reached to this end. The most widely accepted theory assumes vascular malformations inside the bone to be the direct cause of ABC. However, these malformations may, in turn, result from a variety of triggers. Genetic and traumatic factors are often cited.
The hypothesis of gene disorders accounting for vascular malformations that ultimately provoke ABC is supported by the fact that ABC have been observed in sites that have previously not sustained any trauma or other known lesion. Such ABC are deemed primary ABC. And although benign, some primary ABC may indeed be true neoplasms. In this context, chromosomal anomalies and activation of oncogenes have been detected in many ABC samples without them being associated to any further bone lesion or tumor  .
Secondary ABC may develop after trauma or pathologic alterations of the bone. In fact, there is a high correlation between incidence of ABC and incidence of bone neoplasms. Between one fourth and one third of ABC patients also present bone tumors. Giant cell tumors are most frequently observed in ABC patients and appear similar in radiographic images. But virtually any type of tissue may serve as a starting point for tumor development and osteoblastoma, osteosarcoma, chondroblastoma, chondrosarcoma, distinct types of fibroma, hemangioendothelioma and metastases of other primary tumors have been co-diagnosed with ABC.
ABC are rare bone lesions and the annual incidence has been estimated to be 1.4 in 1,000,000 people . True values may exceed those estimates because ABC may be asymptomatic and may undergo spontaneous remission. Considering this limitation of available epidemiological data and if ABC are considered primary bone tumors, they account for up to 6% of their overall incidence. ABC presumably are, however, the second most common tumor in children . Concerning age cohorts, ABC incidence peaks in the second decade of life. More than 50% of all cases are registered in this group. Nearly 90% of ABC patients are younger than 20 years, but infants are rarely affected.
With regards to gender distribution, a slight preference for women has repeatedly been suggested.
Similar to etiology, pathogenesis of ABC is also poorly understood .
Some hypotheses have been adapted from those applicable to other vascular malformations, considering that this is the most widely accepted theory regarding etiology. In arteriovenous fistulas, for instance, blood pressure may increase in vessels not built to resist such high pressures. They may therefore expand, suffer lesions and undergo vessel wall remodelling. Pressure may be sufficiently high to cause atrophy of the adjacent tissues. Although this pathophysiological chain of events has not yet been observed in ABC, results of pressure measurements support this theory.
In one of the previous sections, activation of an oncogene has been mentioned as a possible cause of ABC . This condition yields over-expression of cadherin-11, a protein involved in cell-cell adhesion and over-expressed in certain types of cancer. Here, excess cadherin-11 seems to interfere with maturation of osteoblasts, renders them arrested in an earlier developmental stage. And although this theory may apply to the neoplastic sub-class of primary ABC, it is rather unlikely to be the trigger for secondary bone cysts.
Despite rather high rates of recurrence and even metastases, that require additional treatment, prognosis is generally excellent. The overall cure rate has been estimated to range between 90 and 95% .
Unfavorable prognostic factors due to association with higher risks of recurrence are open growth plates and consequently young age as well as situation in metaphyses of long bones, while the latter condition may apply to more than half of all ABC patients. Efforts have been undertaken to distinguish ABC stages and morphologic types, but such classifications have not yet reached general acceptance and neither have relations with higher risks of relapses.
In contrast, some studies associated lower risks of recurrence with certain therapeutic approaches. In detail, selective arterial embolization, intralesional excision and surgical resection have alternatively been recommended as first-line treatment due to a presumably lower rate of recurrence (see, for instance, ). These hypotheses have yet to be confirmed.
Skeletal disturbances such as localized warmth, pain, swelling and in some cases even pathologic fractures are the most common symptoms of ABC . Pain and swelling often limit the patient's range of motion, render them rather stiff. Because these symptoms are often observed in the course of several weeks, swelling may aggravate and a hard mass may become palpable. Deformities may become visible. If a pathologic fracture has not been diagnosed upon initial presentation, it may occur during this time as well. It has been estimated that less than 10% of all patients present pathologic fractures when first consulting their physician with ABC, but this share may subsequently rise significantly.
In more than half of all ABC patients, the above mentioned symptoms affect the long bones, frequently their metaphyses, in close proximity to growth plates, and only rarely epiphyses or apophyses . Mostly, ABC affect the lower limbs and here their distal parts, mainly tibia and fibula. About one out of five patients presents with ABC in their upper limbs and about the same share with such lesions affecting the axial skeleton, particularly spine and sacrum. Jaw, sphenoid bone and other bones contributing to paranasal sinus may present ABC, too.
Depending on the precise location of ABC and potentially compressed adjacent structures, e.g., vessels and nerves, additional symptoms may develop. The latter is most commonly observed in spinal ABC.
As is the case with several other skeletal disorders, diagnostic imaging is the mainstay of workup. Radiolucent areas may be visible on X-ray and computed tomography images, while magnetic resonance is more commonly applied to define the cysts fluid filling. Although such findings have been suggested to be pathognomonic for ABC  , potentially severe differential diagnoses may be overlooked if no histopathological analysis of tissue samples is carried out. The latter is indicated if any doubt towards the nature of radiographic alterations remains and may be very helpful to rule out giant cell tumors and telangiectatic osteosarcoma, that both present similar in imaging diagnostics. Other benign or malign neoplasms may also most easily be ruled out in a histopathologic approach. The fact that ABC are often associated with bone neoplasms further stresses this point. Some experts recommend to realize such analysis routinely.
If biopsy samples are obtained, it is important to cover the whole lesion. Otherwise, pathological features present only in certain parts of the lesion may not be detected. A complete resection may even precede biopsy taking, in which case the resected material may be thoroughly examined. ABC typically show a fibrous stroma and interspersed blood-filled lagoons, giving it the appearance of a blood-soaked sponge. In some cases, these "sponges" are more solid than blood-soaked. Fibroblast, fibrocytes and osteoblasts contributing to osteoid formation dominate the cellular picture. Multi-nucleated giant cells may be present and often surround the lagoons of blood. ABC possess a thin, osseous capsule. While mitotic figures may be visible, they should not appear abnormal.
Of note, laboratory of blood samples are sometimes ordered. They do generally not reveal much more than increased levels of alkaline phosphatase.
Treatment depends on size, symptoms and development of ABC. In fact, small ABC located in non-weight-bearing bones that are not causing any symptoms and don't seem to grow may not require any therapy, but should be monitored regularly. If any change towards a more aggressive ABC is observed, surgical treatment is recommended, but such cysts may also undergo spontaneous remission.
The majority of ABC requires surgical treatment. Surgical curretage is the therapy of choice . If at all possible, the lesion should be removed completely. Remaining bone defects are filled with bone grafts or synthetic materials and this procedure is particularly important to confer stability to weight-bearing bones. To further reduce the risk of recurrence, cryotherapy or sclerotherapy may be employed post-surgically.
Selective arterial embolization has recently been proposed as an alternative treatment option, but should not be applied to instable bones .
If secondary ABC are diagnosed, the underlying disease should be accordingly treated.
Preventive measures for ABC cannot be recommended. Patients with a medical history, however, may benefit from regular analyses of blood samples and imaging screens to detect possible recurrences early. Such monitoring is recommended for several years after initial diagnosis.
The term aneurysmal bone cyst (ABC) is a descriptive denomination of benign bone lesions of unknown cause. An ABC is a blood-filled sac inside any bone that is covered by a thin osseous layer. It may cause focal tissue destruction and be rather resistant to therapy, but the majority of ABC patients, usually adolescents, can be eventually be fully cured.
As has been mentioned above, causes of ABC are not known. There are distinct theories regarding a genetic or traumatic triggers and possibly both have a right to exist: While some ABC are detected in previously healthy bones, others may develop after the respective bone sustained a traumatic lesion. Some ABC are associated with bone tumors, but the causal relationship between both is not yet clear.
Warmth, pain and swelling are the most frequently reported symptoms. Since ABC most often develop in the long bones of the limbs, notably the lower limbs, these symptoms are usually experienced in these parts of the body. They may persist for weeks until a definite diagnosis can be made. Over the course of time, swelling may increase, a hard mass may become palpable and the affected bones may deform.
Although diagnostic imaging such as X-ray examinations and computed tomography scans usually allow for a largely reliable diagnosis, histopathological analysis of the altered tissue is strongly recommended to rule out more severe diseases such as benign and malign bone tumors.
Instead of obtaining a small, maybe not representative tissue sample for analysis, histopathology is often combined with treatment: The recommended therapy for ABC is surgical resection and the tissue that is excised in this procedure may subsequently be assessed for typical features of this lesion.
Most ABC are treated surgically. Under general anesthesia, the bone cyst is accessed, opened and removed by scraping it out with a curette. The remaining bone defect is filled with either a bone graft or synthetic material.
It may be difficult to eliminate the complete lesion during one surgical intervention and therefore, the risk of recurrence is high. Most patients need to undergo repeated curettage until they can be fully cured. Supplemental therapy, e.g., cryotherapy or sclerotherapy, may be applied to reduce the risk of relapses.
Very small, asymptomatic and non-growing ABC may just be monitored. Spontaneous remission is possible.