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Osteoid osteoma (OO) is a benign bone lesion,
representing almost 10% of all benign skeletal lesions (1).
It is most frequently seen in young patients in their 20s and 30s,
with a peak age in the early twenties (1,2).
A male predominance has already been established,
with a reported male-to-female ratio of 4:1 in one large patient series (3).
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OOs can involve any bone of the skeletal system,
but is mostly reported in long bones of the lower extremity.
The proximal femur is the most common location followed by the tibia,
posterior elements of the spine,
and the humerus.
OO is found in the diaphysis or the metaphysis of the proximal end of the bone more often than the distal end (4).
A nidus surrounded by reactive sclerotic bone is the main feature of this lesion.
(figure 1)
Classically,
based on radiographic localization of the nidus,
OOs may be categorized through three types: cortical ,
medullary and subperosteal.
(5)
Cortical osteoid osteoma is the paradigmatic type of this lesion consisting of a small central nidus,
usually radiolucent,
associated with a surrounded sclerosis.
Intramedullary type is relatively rare,
just few cases have been reported in the literature.
This latter type shows little or no reactive bone formation surrounding the nidus thus,
the sclerotic aspect does not appear.
Finally,
the third type of OO is the subperosteal that most frequently occurs in the intra-articular portion of the bones and may be difficult to detect.
However,
some authors consider that all OOs originate in a subperiosteal location,
and through osseous remodelation there is a migration of the lesion into an endosteal position (6).
This theory is supported by the predominantly surface location of intra-articular lesions,
where there is a lack of functional periosteum (7).
Intra and juxta- articular represent approximately 13% of OOs,
the commonest site being the hip.(8) In this respect,
the less frequency intra-articular localization may also present a diagnostic challenge because the classical radiological and clinical findings usually present in extra-articular localization may not be present.
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This pictorial review depicts the wide range of appearances of intra-articular OO in the hip with emphasis on MRI.