Dear Sir,
A 12-year-old boy had a history of fall and sustained pathological fracture of proximal
humerus. He was further referred for MR imaging that showed a cystic lesion along
the long axis of the humerus in the proximal meta-diaphyseal region with no extension
across the epiphyseal plate. Lesion appeared heterogeneously hyperintense on T2-weighted
images with dependent blooming on GRE images indicating blood-fluid levels. Few internal
septa with multiple cortical breaks were demonstrated in the proximal aspect of the
lesion with periosteal elevation and collection. Linear T2-weighted hypointense foci
are seen within the lesion indicating fallen fracture fragments [[Figure 1]], [[Figure 2]], [[Figure 3]]. Unicameral bone cyst is described as a solitary, lytic metaphyseal lesion, which
does not cross the epiphyseal plate.[[1]] The lesion has well defined margins with narrow transition zone with the long axis
of the lesion parallel to the long axis of bone and is known to migrate towards diaphysis
during growth of child. It commonly occurs below 20 years of age and is commonly located
in proximal humerus and femur in skeletally immature individuals and lacks malignant
potential.[[2]] Radiological signs described in unicameral bone cyst are fallen fragment,[[3]] trapped door [[4]] and rising bubble signs.[[5]] Fallen fragment sign results from a pathologic fracture of the cyst wall, with
displacement of the fragment into the dependent portion of the cyst [[Figure 1]]. Trapped door sign is a variant of the fallen fragment sign described in literature
in which a periosteal hinge keeps the fragment from falling dependently and allows
it to change position with the patient[[Figure 3]]. The trapped door is seen due to hydrostatic pressure of the fluid contained within
the cyst. After sustaining a pathological fracture, presence of a gas bubble at the
most non-dependent portion of the lytic bone lesion implies the lesion is hollow.
On MRI, fluid level may be noted with cortical thinning, periosteal hematoma, septations,
and pathological fracture [[Figure 1]], [[Figure 2]], [[Figure 3]]. The periphery of the lesion and septa within may show enhancement on administration
of contrast. Unicameral bone cysts appear to be geographic lucent lesions on radiography
with a central location, cortical thinning, and gentle expansion. The bone cyst index
described by Kaelin and MacEwan [[6]] can be used as an easy method to assess the mechanical resistance of the cyst wall.
The bone cyst index is calculated by dividing the cyst area by the diameter of the
diaphysis squared [[Figure 4]]. Kaelin and MacEwan found that unicameral bone cyst with a bone cyst index of more
than 4 for the humerus and more than 3.5 for the femur were at high risk for fracture.
Ahn and Park [[7]] reported a second predictor for fracture is the bone cyst diameter and is expressed
as the percentage of bone occupied by the cyst in the transverse plane. Unicameral
bone cyst is considered to be at a high risk for fracture when the bone cyst diameter
reaches more than 85% in both the anteroposterior and lateral images [[Figure 4]]. A third predictor is the minimal cortical thickness with a value less than 2 mm
predicting a high risk of fracture [[Figure 4]].[[6]] Differential diagnoses for unicameral bone cyst include aneurysmal bone cyst, eosinophlic
granuloma, enchondroma, fibrous dysplasia, and intraosseous ganglia. MRI helps in
differentiating unicameral bone cyst from aneurysmal bone cyst.
Figure 1: Coronal MERGE (gradient) MR image of the shoulder joint demonstrating radiological
signs in a unicameral bony cyst involving the proximal humerus in a 12-year-old boy.
Note the meta-diaphyseal location of the cyst with no extension across the epiphyseal
plate. Linear hypointense focus is seen in the dependent position indicating “Fallen
fragment sign” (red arrow). Endosteal scalloping (pink arrow) and internal septation
(green arrow) is being demonstrated. Note the periosteal elevation with collection
demonstrating blood-fluid level (blue arrow)
Figure 2: Axial MERGE (gradient) MR image of the unicameral bony cyst demonstrating cortical
thinning (red arrow) and cortical break (green arrow). Note the blood-fluid level
within the cyst (blue arrow) and fluid collection/hematoma in the surrounding soft
tissues (pink arrow)
Figure 3: Sagittal T2-weighted MR image of the unicameral bone cyst in the proximal humerus
demonstrating a periosteal hinge that keeps the fractured bone fragment from falling
dependently into the cyst indicating “Trapped door sign” (blue arrow). Note the elevated
and thinned out periosteum (red arrow)
Figure 4: Sagittal T2-weighted MR image demonstrating the various indices predicting the risk
of pathological fracture in a unicameral bone cyst. Calculation of “Bone cyst index
(BCI)” first described by Kaelin and MacEwan[6] is demonstrated (in Red). BCI = [(a
+ b)/2 x c]/d2, which is obtained by dividing cyst area by the diameter of diaphysis squared. Calculation
of “Bone cyst diameter (BCD)” first described by Ahn and Park[7] is demonstrated (in
blue). BCD is expressed as the percentage of bone occupied by the cyst in transverse
plane and bone cyst diameter >85% is a significant predictor of fracture. Note the
thinned out cortex demonstrated (in yellow) with a minimal cortical thickness (MCT)
less than 2 mm predicting a high risk of pathological fracture
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