The definitive characterization of a lithic cortical nodule,
generally smaller than 1.0-2.0 cm,
with variable central nidus mineralization associated with sclerosis,
thickening and inflammatory changes of the surrounding bone is sufficient for the diagnosis (21,
30),
especially when located in typical sites (12,
13).
The OO nidus shows variable signal intensity on MRI with a target-like appearance,
since the non-mineralized vascular stroma has intermediate or high signal intensity on T2 and may present intense gadolinium enhancement,
while the mineralized portion presents low signal intensity on all sequences (31-35).
Although the accuracy of MRI in OO diagnosis is lower than CT according to many studies (36-38),
with the improvement of the spatial resolution in the current devices,
the use of volumetric sequences and the radiologist's learning,
this diagnosis can be easily suspected,
besides that,
the use of paramagnetic contrast media may also help (39,
40).
Thus,
MRI is an option rather than CT,
especially in pediatric population,
in an attempt to avoid the use of ionizing radiation.
Pitfalls,
differential diagnoses and OO-mimicking lesions:
In general,
greater size,
medullary location,
less surrounding osteosclerosis,
periosteal reaction and bone marrow edema may help to recognize OO-mimicking lesions (23).
1.
Multicentric OO (fig 2): it is a rare condition,
possibly overlooked,
defined as the presence of more than one nidus,
which can cause diagnostic and therapeutic difficulty since all nidus need to be detected and treated.
Most often the nidus are close to each other (12,
42,
43,
44).
2.
Intra-articular OO (fig 3,
4 and 5): the most common location is the hip (fig 3),
other joints such as ankle,
elbow (fig 4),
knee and wrist are more rarely affected (45).
Clinical symptoms are atypical,
present as arthritis,
most often without nocturnal worsening and with little improvement after NSAIDs treatment,
easily confused with inflammatory diseases or pyroarthritis,
scintigraphy also presents synovitis pattern (16,
46).
The radiological aspect is similar to OO in other sites,
however,
cortical thickening is reduced or absent,
due to the lower periosteal apposition at the joint (12,
21).
The subchondral OO is rarer and may be confused with chondromalacia (Figure 5) due to the reactional changes in the subchondral bone.
3.
Fracture / stress reaction (Figure 6,
7 and 8): in young patients who practice physical activity,
this differential diagnosis may be problematic,
since,
like OO they frequently affect the femoral neck region (fig 6) and tibia diaphysis (fig 7).
Depending on the location of OO,
subchondral fractures might also have a similar presentation (Fig.
8).
In the case of stress fracture,
periosteal reaction,
fracture line,
bone marrow edema and periosteal reaction can be visualized.
In OO,
although there may be edema and periosteal reaction,
the unequivocal nidus characterization and absence of a cortical fracture make the diagnosis very likely (47).
If there is any doubt,
CT is the best tool and the follow-up may also help,
since the fracture / stress reaction will consolidate or regress.
4.
Crystal deposition disease (fig 9 and 10): Virtually can occur in any site,
such as tendon,
ligament,
fibrocartilage or joint capsule,
when there is intraosseous migration determining cortical remodeling and bone marrow edema (fig 9 and 10),
may be a problematic differential diagnosis.
Age should be taken into account in differentiation,
since OO affects mainly younger patients and the deposition of microcrystals a larger age group (48),
although there is overlap between the two age entities around the 4th decade of life,
especially regarding hydroxyapatite deposition.
5.
Osteomyelitis / intracortical abscess (Fig.
11): A small cortical abscess with bone sequestrum inside can easily simulate the mineralized nidus of osteoid osteoma and vice versa,
especially on plain radiographs.
However,
some features allow for its differentiation in sectional studies.
The margin of the abscess is usually irregular and the sequestrum position eccentric,
whereas OO is the opposite,
margins are regular,
and nidus mineralization is most often central.
Moreover,
the abscess does not show gadolinium enhancement in its central portion,
while the OO shows enhancement of the nidus,
except for the mineralized portion (21).
The penumbra sign (fig.
11),
characterized by the high signal intensity halo on T1 around the lesion,
although non-specific,
favors the possibility of infectious diseases (49).
6.
Glomus tumor (Fig.
12): OO of the distal phalanx often presents atypical clinical picture,
with little painful or even painless,
single-digit clubbing and diffuse thickening of the nail bed with a high T2 signal intensity and gadolinium enhancement,
which can lead to the erroneous diagnosis of glomus tumor,
especially if MRI is the only available exam (50-52).
However,
the glomus tumor is a well-defined nodule in the nail bed,
with no thickening of the rest of the nail bed or matrix and may present remodeling of the dorsal cortical of the distal phalanx (53) (Fig.
12).
Besides that,
it is worth remembering that single-digit clubbing is relatively rare and the possibility of primary bone neoplasm should always be investigated,
being the most common neoplasm enchondroma and OO in this setting (50-52).
7.
Aggressive bone lesion: In most cases,
it has a very different imaging pattern than OO,
characterized by a replacement of bone marrow with a marked low T1 signal intensity,
presenting a generally well-demarcated transition with preserved bone marrow and may also show cortical rupture and extracortical involvement.
On the other hand,
the pattern of edema that occurs in OO is characterized by a gray,
hazy and ill-defined T1 intermediate signal intensity,
with no substitution of bone marrow (Fig.
13).
8.
Osteoblastoma (Figure 14): Although some authors consider OO and osteoblastoma as spectra of the same pathology,
most papers and WHO classify these tumors as separate entities (54).
The two lesions,
although very similar,
present important clinical and radiological differences: Osteoblastoma is larger,
typically greater than 2.0 cm,
less painful,
has less response to salicylates,
progressive growth,
malignant potential,
may be associated with other tumors,
has less inflammatory changes and reactive sclerosis (fig.
14) (55,56).
9.
Chondroblastoma (Figure 15): A rare and painful benign bone neoplasm,
generally smaller than 4.0 cm,
which has a predilection for the epiphyses or apophyses of the immature skeleton,
most prevalent in the femur followed by the humerus and tibia,
predominating in males.
The lesion is lithic,
central or eccentric intramedullary,
with well-defined limits,
with a thin halo of sclerosis,
high T2 signal intensity,
sometimes with lobulated contours and gadolinium enhancement,
central chondroid-pattern calcifications may be present in about 30-40% of the cases and associated inflammatory changes,
possibly with synovitis and surrounding soft tissue edema (4,
21).
The epiphyseal and medullary localization,
lobulated contours,
chondral-like calcifications and larger dimensions may aid in the differentiation with OO,
which is usually smaller,
cortical and diaphyseal,
however,
small and mineralized chondroblastomas may be indistinguishable (21).
10.
Miscellaneous: The OO can manifest in a similar way to other pathologies besides than those mentioned,
such as contusional bone marrow edema (fig.
16),
impingements (fig.
17),
enthesitis (fig 18),
compensatory hypertrophy of the pedicle,
intracortical hemangioma,
osteochondroses,
cortical desmoid,
fibrous dysplasia,
eosinophilic granuloma,
among others (6,
12,
21).