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Unicameral bone cyst

Case contributed by Rocio Soledad Garcia
Diagnosis certain

Presentation

Pain in the first and second extensor compartment of the hand.

Patient Data

Age: 25 years
Gender: Female

Hand radiographs

x-ray

There is a solitary radiolucent lesion occupying the proximal third of the 2nd metacarpal. No other bone lesions are observed. 

Wrist's MRI

mri

An intraosseous polylobulated cystic image at the level of the second metacarpal (cephalocaudal diameter of approximately 24.3 mm) projecting into the adjacent soft tissues. 
The image described corresponds to a single-chamber intraosseous cyst with a rupture of the bony cortex in the volar region with extravasation of the fluid into the soft tissues, acquiring a polylobulated appearance adjacent to the tendon structures. 
Bone edema is observed in the proximal epiphysis of the second metacarpal.
There is increased fluid in the distal radioulnar and prestyloid levels. 

Gross Patology

pathology

We present the intraoperative macroscopic image. A specimen was taken for anatomopathological study, which reported mucinous fluid content. The bone lesion was filled with bone substitute. 

Case Discussion

Benign, intramedullary, usually unilocular, fluid-filled, fibrous membrane-lined bone lesion. 

90% occur in long bones such as proximal humerus (50%) > proximal femur (25%) > proximal tibia and other long bones.  They arise in the metaphysis, adjacent to the physis. 

On imaging, radiographs are seen as a lytic and central lesion in the medullary cavity. The long axis parallels the length of the host bone. They are geographic lesions with thin, sclerotic margins, occasionally with pseudotrabeculations or internal septa. 
Slight circumferential expansion of the bone. 

Occasionally conditions of pathological fracture may lead to cortex rupture. May present "dropped fragment": bone-dependent fracture fragment that changes location with changes in position.

MRI: Confirm the cystic nature of the lesion. 
T1: low to intermediate signal intensity (S.I.).
Fluid-sensitive sequences: high S.I.
C+: low central S.I. + thin enhancing rim
The pathological fracture may complicate the MRI appearance.

Among the main differential diagnoses are aneurysmal bone cysts, fibrous dysplasia, and enchondroma. 

85% occur in the first 2 decades
They are asymptomatic, their finding is usually incidental. Symptoms are due to rupture associated with a pathological fracture. 
Spontaneous resolution with the maturity of the skeleton or fracture.

There is no consensus on the best treatment. But can be managed with intraosseous injections with various agents, alone or in combination, often multiple corticosteroids. The use of autogenous bone marrow. Bone graft substitutes (e.g. demineralized bone matrix, α-BSM).
α-BSM: resorbable calcium phosphate bone graft substitute. Trepanation percutaneous. Continuous drainage using percutaneously placed cannulated screws. 

Case contributed by Dra. Rocio Soledad Garcia and Dr. Tito Alfredo Atencia Rincón
 

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