Synovial herniation pits, also known as Pitt pits or the descriptor fibrocystic changes at the anterosuperior femoral neck, are a common, usually incidental, finding on imaging.
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Epidemiology
Prevalence on x-rays of normal adults is ~5% (range 4-12%) 3.
Associations
- cam morphology FAI: occur in 5-33% of patients 2,3 but causation of synovial herniation pits by FAI has not been proven 9
Pathology
There is debate about the exact etiology of herniation pits. These lucencies represent a herniation of synovium or soft tissues into the bone through a cortical defect 1,3. They most commonly occur in the anterosuperior aspect of the femoral neck, just distal to the articular surface, although have also been reported to occur in the anteroinferior femoral neck 1-3. They are usually around 5 mm (range 1-15 mm) in diameter and more commonly unilateral than bilateral 2,7.
Radiographic features
Plain radiograph / CT
Synovial herniation pits are oval, round or "8-shaped" lucencies with sclerotic margins 1-3,7. On CT, an overlying cortical defect can be seen 5.
MRI
Well-defined peripheral low signal lesion with central low T1 and high T2 signal; T2 signal may heterogeneous. Intralesional fat is seen in some pits 5,7. Adjacent bone marrow edema may rarely be present 4,8.
Treatment and prognosis
They are most often incidental findings of no clinical significance 3. Synovial herniation pits are one of the skeletal “don’t touch” lesions. Synovial herniation pits can grow over time 6,7.
History and etymology
Synovial herniation pits were first described by Michael J Pitt, American radiologist, in 1982 1.
Differential diagnosis
Imaging differential diagnosis includes 4,5,7:
- osteoid osteoma
- intraosseous ganglion
- atypical skeletal metastasis
- chronic intraosseous abscess
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