Osteoid Osteoma Mimicking Inflammatory Synovitis.

Osteoid osteoma is a common benign bone tumor of unknown etiology affecting the young adult with characteristic clinical and radiographic presentation in its common locations. However, when it arises in unusual intra-articular locations diagnosis may be misleading, often mimicking other entities leading to delayed management. We present a case series of three cases with intra-articular osteoid osteoma that were diagnosed with the help of correlative imaging.

Introduction

Osteoid osteoma is a benign osteoblastic bone tumor that occurs most frequently in men and boys between 7- and 25-year old commonly arising from cortico-diaphyseal and metaphyseal regions of long bones.[1] Most patients experience pain that worsens at night and is promptly relieved by the administration of salicylates.[2] However, intra-articular osteoid osteomas are less frequent (10%) and may present with non-specific symptoms that may mimic inflammatory synovitis.[3456] Radiological diagnosis may also be difficult in such cases due to less specific imaging findings; therefore, multimodality imaging may help in making accurate diagnosis.[47]

Case Report 1

A 20-year old male presented with complaint of pain for 8 months in left hip with limited range of motion. Pain usually becomes worse at night and was only relieved on medication. He had no previous history of any trauma. His X-ray hip joint was normal [Figure 1a]. The patient then underwent both magneto resonance imaging (MRI) and a 99mTechnetium methylene diphosphonate (99mTc-MDP) bone scan. A 99mTc-MDP bone scan showed increased tracer uptake in left hip joint with a tiny faint focus of increased activity on neck of femur [Figure 1b]. MRI showed significant joint effusion with abnormal focal signal intensity involving neck of left femur [Figure 1c]. At this point, no specific diagnosis could be established. Patient underwent exploratory surgery but it was of no help. Post-operation his symptoms worsened further. Six months post-operatively, he underwent F-18 Fluoride positron emission tomography-computed tomography (PET-CT) bone scan that showed focal increased tracer uptake corresponding to a tiny calcified nidus along the anterior cortex of neck of femur with mild uptake in left hip joint and joint effusion. A diagnosis of osteoid osteoma with reactive synovitis was made [Figure 2]. Then, patient was taken up for radiofrequency ablation for osteoid osteoma and became pain free from the 4th day [Figure 3].

Figure 1

(a) Normal X-ray hip joint, (b) bone scan showing increased tracer uptake in left hip joint with a tiny focus of increased activity on neck of femur, and (c) coronal MRI section showing significant joint effusion with abnormal signal intensities involving neck of left femur

Figure 2

(a) Whole body F-18 bone scan showing focal increased tracer uptake corresponding to a tiny calcified nidus along the anterior cortex of neck of femur with mild uptake in left hip joint and joint effusion, and (b) and (c) transaxial CT, fused PET-CT sections showing focal increased tracer uptake corresponding to a tiny calcified nidus

Figure 3

Radiofrequency ablation of osteoid osteoma

Case Report 2

A 16-year old male presented with severe pain and swelling of right elbow joint with restricted motion for past 4 months. The pain was not controlled by medication and was worse at night. Patient underwent both MRI and 99mTc-MDP bone scans. A 99mTc-MDP bone scan showed increased flow, blood pool, and tracer uptake in the region of right elbow [Figure 4]. MRI showed significant joint effusion and focus of low-signal intensity on T1 [Figure 5] in olecranon fossa of right humerus. A review of bone scan also revealed a tiny focal uptake in olecranon fossa [arrow, [Figure 4]. Thus, a diagnosis of osteoid osteoma was made. Patient then underwent surgical removal of osteoid osteoma and histopathology confirmed the diagnosis [Figure 4].

Figure 4

(a) and (b) anterior and posterior bone scan showing increased tracer uptake in region of right elbow, and (c) magnified view of elbow joint showing tiny focal uptake (arrow) in olecranon fossa

Figure 5

(a) and (b) MRI showing focus of low signal intensity on T1 and STIR images

Case Report 3

A 23-year old female presented with pain for 1 year in right hip joint, her pain became worse at night and was relieved with medication. She underwent an MRI 6 month back, which showed joint effusion and marrow edema in neck and trochanteric region of right femur. Possibility of infective/inflammatory pathology was considered. She was treated conservatively but was not relieved of pain. She underwent a repeat MRI after 6 months, which showed no significant interval change between two scans. For further evaluation, a bone scan was done that showed increased blood pool and 99mTc-MDP increased tracer uptake in proximal right femur noted [Figure 6]. A careful review of previous MRI showed a tiny focal signal abnormality in medial cortex of neck of femur [Figure 7]. A diagnosis of osteoid osteoma was suspected. For further evaluation, computed tomography (CT) was performed that showed small cortical lytic lesion with calcified nidus involving medial cortex of right femur [Figure 8a]. The patient then underwent radiofrequency ablation for osteoid osteoma and became pain free [Figure 8b] on 2nd day of procedure.

Figure 6

(a)–(d) Bone scan showing increased blood pool (arrow) and increased tracer uptake in proximal right femur

Figure 7

(a) and (b) Coronal and transaxial MRI sections showing a focal signal abnormality in the medial cortex of neck of femur (arrow).

Figure 8

(a) Transaxial CT section showing small cortical lytic lesion with calcified nidus involving medial cortex of right femur, (b) radio frequency ablation of osteoid osteoma.

Discussion

Intra-articular osteoid osteoma that occurs within or near a joint is rare and accounts for 10% of all osteoid osteomas.[34]I t mainly arises in the hip joint, other joints such as the ankle, elbow, wrist, and knee are less commonly affected.[8]

Due to the rarity and non-specificity of its clinical presentation such as pain, joint tenderness, joint effusion even mono-arthritis, diagnosis may be difficult and is mistaken for other etiologies such as inflammatory or infectious arthritis, aseptic osteonecrosis of the femoral head, fatigue fracture, radicular syndrome, or even pigmented villonodular synovitis.[4568]

Standard radiographs are not very useful due to the absence of any perilesional sclerosis or periosteal reaction.[3] On MRI, the osteoid osteoma has low to intermediate signal intensity on T1-weighted images and variable signal intensity on T2-weighted images, depending on the amount of mineralization. Edema in adjacent bone marrow and soft tissue and joint effusion also may be seen that shows high contrast enhancement after gadolinium administration. However, precise localization of the osteoma may not be easily detectable as in 35% of the cases, as it is often hidden by the associated perilesional edema surrounding the lesion, while in 50% of the cases, the osteoid osteoma has an atypical presentation, which may lead to misdiagnosis.[7]

Bone scintigraphy and CT-scanning are the initial investigations of choice for the diagnosis of osteoid osteoma.[7910] Bone scintigraphy is highly sensitive but has a lower specificity than CT-scan particularly in the case of intra-articular location because bone sclerosis around the nidus which cannot be early detected since there is a less intense uptake due to the associated synovial reaction.[11] As osteoid osteoma evolves, signal intensity of peripheral bone sclerosis decreases, while the nidus can be more easily detectable on scintigraphy. Recently, hybrid imaging using single photon emission computed tomography-computed tomography (SPECT-CT) and F-18 Fluoride PET-CT provided the advantages of both the modalities in a single imaging study, thus providing accurate data regarding the size and location of the lesion.

In our cases, the F-18 Fluoride PET-CT scan helped in making the diagnosis in the first case, while in second and third cases, it was the review of both 99mTc-MDP bone scan and MRI simultaneously, which helped us in clinching the diagnosis.

Conclusion

Osteoid osteoma should be kept as a differential diagnosis for atypical mono-articular synovitis, especially in the adolescent age group. High index suspicion, careful clinical history, and careful review of correlative imaging are necessary not to miss a diagnosis of intra-articular osteoid osteoma. The F-18 Fluoride PET-CT bone scan can be considered as investigation of choice for osteoid osteoma, because of its higher sensitivity than the 99mTc-MDP bone scan and inherent diagnostic CT in PET/CT equipment.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.