Simple bone cyst

Basic radiographic appearance

Classic radiography is the starting point in the imaging diagnosis of a simple bone cyst. On standard radiographs, the pathology is visualized as a mildly expansile lytic lesion (expansile lytic lesion).

Most commonly, the lesion is spatially related to the growth plate (typical localization is the proximal humerus). The internal structure of the cyst is usually homogeneous; however, depending on the anatomical zone (e.g., in lesions of the femoral neck), multiple bone trabeculae may be identified within the lytic lesion, which must be taken into account when describing the radiograph.

Radiological signs of a pathological fracture

Since the cyst thins the cortical layer, the primary complication is a pathological fracture. At this point, the imaging appearance changes significantly, and highly specific symptoms appear:

• «Fallen fragment» sign (fallen fragment sign): Visualized on radiographs. A fragment of the fractured cortex literally falls to the bottom of the cystic cavity because it is filled with fluid, which does not impede the displacement of the fragment.
• «Rising bubble» sign (rising bubble sign): Occurs due to the accumulation of gas (vacuum phenomenon) that rises to the nondependent (superior) part of the cyst. On MRI, gas appears as a signal void (signal void) on T1- and T2-weighted images and produces a pronounced blooming artefact (blooming artefact) on gradient echo (GE) sequences.
• Fluid-fluid levels (fluid-fluid levels, FFLs): Hemorrhage into the cavity leads to sedimentation of blood shaped elements. On axial MRI images (proton density-weighted, FSE), the filling of the cavity with multiple horizontal fluid levels is clearly visualized, accompanied by pronounced pericystic edema.

Magnetic resonance imaging (MRI)

Magnetic resonance imaging provides comprehensive information about the contents of the lesion. In an uncomplicated state, a simple bone cyst demonstrates signal intensity that is completely typical for fluid.

For optimal visualization, coronal T1-weighted images (spin-echo) and axial fat-suppressed proton density-weighted fast spin echo (fat-suppressed proton density-weighted fast spin echo) are used. Following intravenous contrast administration, typical rim enhancement (rim enhancement) is noted.

Differential diagnosis

When identifying an expansile lytic lesion, a simple bone cyst must first be differentiated from two other pathologies:

1. Aneurysmal bone cyst (Aneurysmal Bone Cyst, ABC): Unlike a simple cyst, it can have not only a central, but also an eccentric or superficial localization. On MRI (T2-WI, fast spin echo), it appears as a multilocular lesion with the constant presence of fluid levels (sedimentation of blood elements) and intermediate signal on T1-WI.
2. Fibrous dysplasia (Fibrous Dysplasia, FD): Leads to the replacement of bone marrow with fibro-osseous tissue. Radiologically, it manifests as a geographic lesion with a diffuse ground-glass matrix (ground-glass matrix) and a thick sclerotic margin ('rind' sign — 'rind' sign). Purely lytic lesions are less common here and indicate cystic degeneration. Unlike a cyst, dysplasia is characterized by severe bone deformities (e.g., shepherd's crook deformity).

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Clinical and radiological presentation of an aneurysmal bone cyst (Aneurysmal Bone Cyst).
• Forms and associated syndromes of fibrous dysplasia (monostotic and polyostotic forms, McCune-Albright syndrome).
• Ground-glass matrix in CT diagnostics: physics of formation and differential series.
• Specific bone deformities in dysplasias (shepherd's crook deformity).

Other topics in the "Musculoskeletal System" section

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