SIMPLE BONE CYST

T1 and STIR coronal and sagittal MR images in a 4 year old child showing a fairly well demarcated expansile septate cystic lesion in the proximal metaphyseal region of right humerus. It displays mild heterogeneity but is largely homogenous.

SIMPLE BONE CYST: 
-90% occur in long bones, most common in proximal humerus
-Metaphyseal expansile cystic lesion
-May contain "fallen fragment" which is  a fractured fragment of bone which moves in dependent part with change in patient position. It is usually seen on x-rays and CT.
-May show fluid-fluid level on CT and MRI
-"Rising bubble sign" : Bubble of gas in non-dependent part of lytic lesion suggestes pathologic fracture. It may be seen on CT and MRI.
-FDG active, may hence mimic metastasis on PET-CT

REFERENCE:
Manaster BJ, Roberts CC, Petersilge CA, Moore S, Hanrahan CJ, Crim J.Diagnostic Imaging Musculoskeletal: Non-traumatic disease. Amirsys.2-206, 2010

Focal dehiscnece of lamina papyracea

Above axial CT images show focal dehiscence / absence of lamina papyracea in the right orbit with herniation of orbital fat into the right ethmoid sinus. The medial rectus muscle on this side is slightly thickened and irregular although it does not herniate through the bony defect.
This was an incidental finding in the above young patient.

Dehiscence of lamina papyracea may be congenital or acquired( post traumatic/ post-surgical).
There may either be a focal bony defect or inward displacement of the the lamina papyracea.
The defect is usually small with focal herniation of orbital fat and/ medial rectus muscle into the ethmoid bulla.
However, there may be herniation of the optic nerve or the entire  globe.

IMPORTANCE: This defect provides a direct route for sinus surgery instruments which can cause orbital injuries like medial rectus muscle laceration, orbital hematoma, orbital fibrosis, perforation of the orbital wall, damage to globe etc. Hence, it should be highlighted to the otolaryngologist to avoid possible complications during FESS( functional endoscopic sinus surgery).
It may also cause mild diplopia.

REFERENCES:
Hoang JK, Eastwood JD, Tebbit CL, Glastonbury CM. Multiplanar sinus CT:A Systematic Approach to Imaging Before Functional Endoscopic Sinus Surgery. AJR; 194:W527-W536
Radiology picture of the day. Dr Ahmed Haroun.