Enostosis - Bone island

Commonly known as bone island

Mature cortical bone within the medulla
Due to failure of resorption during endochondral ossification
Asymptomatic

Features:
CT/Radiograph: 
- Spiculated margin - blends with the surrounding trabecular bone
- Dense sclerotic focus
MR:
- All sequences: Very low SI
Scintigraphy
- Typically no activity. Some may have increased activity

DDx:
- Dense osteoid osteoma
- Osteoblastoma

PVNS

Pigmented villonodular synovitis
- Unknown aetiology
- Synovial proliferation disorder
- intracellular Haemosiderin deposition (thus "Pigmented")

Monoarticular
Age 30-40s

Benign proliferation of synovium in the joint or in bursa/tendon sheath +/- hemorrhagic effusion
If occurs in the tendon sheath => then it is known as Giant Cell Tumor of the tendon sheath

Clinical features:
- Insidious onset
- Monoarticular arthropathy
- Swelling
- Stiffness
- Pain

Pathology:
- Hyperplastic synovium
- Lipid-laden foam cells, histiocytes, giant cells, haemosiderin deposits

Distribution:
- Knee > Hip > Ankle > Shoulder

Radiograph:
- Dense effusion
- Dense soft tissue mass (due to haemosiderin)
- Bone erosion
- Large subchondral cysts
- Preserved joint space
- Typically normal in the early stage

MRI:
- Joint effusion
- Focal / diffuse synovial thickening: Low signal T1w and T2w, and blooming artifact on GRE (due to hemosiderin deposition)

DDx:
- (repeated) Haemarthrosis such as Haemophilics
- Septic arthritis
- Articular amyloid deposition (similar in MRI appearance)

Rx:
- Synovectomy (~50% with local recurrence)

Bosniak classification for Renal Cyst

Applicable to US, CT or MR imaging.

	Features	Management
Bosniak I	Anechoic cyst Thin walled No septa, calcification, solid components No contrast enhancement	No intervention required	Class 1 Renal cyst
Bosniak II	Hairline-thin septa Fine calcification / Short thick calcification No contrast enhancement	No intervention required	Class 2 Renal Cyst Class 2. Notice the faint thin septa? Also a smaller simple cyst in the lower pole.
Bosniak IIF	Multiple hairline-thin septae +/- minimal thickening Minimal wall thickening Thick / Nodular calcification No soft tissue component	Needs follow-up
Bosniak III	Thick wall / septa, irregular / smooth	Needs surgical intervention	Class 3. Notice the thicker septa and larger size of the cyst? Class 3 Renal Cyst
Bosniak IV	Bosniak III + soft tissue component which enhances.	Clearly malignant