Definition
The aneurysmatic bone cyst belongs to the category of benign bone tumours. It is a blood-filled cyst located in the bone, which is divided by septa into several individual cavities, i.e. chambers. An aneurysmatic bone cyst usually occurs at the age of 10-20 years, and is therefore a bone lesion in young people.
The majority of aneurysmatic bone cysts are definitely diagnosed before the age of 20. Both sexes are equally affected. An aneurysmatic bone cyst can generally develop on any bone in the human body.
However, predisposed sites are the thigh bone (lat. femur) and one of the two lower leg bones, namely the shin bone (lat. tibia).
In these two bones, the most common site of manifestation is the area of the metaphysis, i.e. the part between the bone shaft and the part of the bone forming the joint. In addition, an aneurysmatic bone cyst often occurs in the spinal column. The juvenile bone cyst is the most important differential diagnosis to aneurysmatic bone cysts. With the help of imaging techniques, however, a clear differentiation is possible.
Causes
The causes of an aneurysmatic bone cyst can be defined relatively clearly. There is a variant which is approximately 80% true and which is a primary idiopathic bone cyst. Idiopathic means that the cause of the cyst formation is not known or cannot be defined in detail. The second possibility for the cause of an aneurysmatic bone cyst is that it was secondary to other malignant bone lesions or that it occurred as a side effect of other diseases. In addition, there are no significant risk factors or sexual predispositions associated with the aneurysmal bone cyst that could have a causal influence.
Symptoms
The aneurysmatic bone cyst is a bone lesion with relatively little discomfort. Pain and swelling rarely occur. In exceptional cases, however, swelling can be so pronounced that it can simulate a tumour, i.e. a mass that is visible from the outside.
Often, patients do not even notice that they have an aneurysmatic bone cyst. It is often only recognised when a fracture of the affected bone occurs as a consequence or accompanying symptom. Due to the cyst, the bone is less stable and resistant, so that it can break under disproportionately low stress. Thus, the aneurysmatic bone cyst is often a secondary or accidental finding when x-rays or MRI images are taken for other indications. Since the aneurysmatic bone cyst is a benign bone tumour, there is no weight loss, night sweat or fever as would be expected with a malignant tumour.
Diagnosis
The diagnosis of an aneurysmatic bone cyst is made by means of imaging techniques. A clinical diagnosis is difficult to impossible, since there are no typical symptoms that would be sufficient for a clinical diagnosis alone. In addition, the aneurysmatic bone cyst manifests itself in a very variable manner in patients.
However, if there is evidence in the form of pain or swelling in predisposed bones or if there is a fracture that was probably provoked by a bone cyst, the first step is to take an x-ray in 2 planes. Here one can clearly see the bony lesion, which is usually located in the area of the metaphysis and can be clearly defined. One also speaks of an “osteolytic” lesion, i.e. a reduction or dissolution of the bony structure in the area of the cyst.
If the findings are still not completely clear after the x-ray, or if the findings cannot be distinguished from the differential diagnosis of a juvenile bone cyst, an MRI scan is made. The MRI shows a blood-filled bone lesion, which in contrast to the juvenile bone cyst is characteristically chambered, i.e. separated by a septum. A diagnosis can also be confirmed by an open biopsy.
An MRI image is not taken as part of the diagnosis of an aneurysmatic bone lesion until an x-ray has been taken. A characteristic feature of the aneurysmatic bone cyst in the MRI is that it is a bony, blood-filled lesion, which is chambered by septa. It is found on tubular bones, such as the thigh, mostly in the area of the metaphysis.
A typical phenomenon of aneurysmatic bone cysts in MRI is called “fluid-fluid-level”. This describes a so-called stratification phenomenon, which is caused by the sinking of components in the blood contained in the cyst. The stratification phenomenon can look like a further subdivision of the cyst that is already in a chamber, as the deposited blood components or sediments appear as lines. In the MRI image the differential diagnosis of a juvenile bone cyst can be excluded very well due to the existing septation, because the chambered bone cyst is a characteristic of an aneurysmatic bone cyst.
