Osteoid osteoma is usually a diphyseal, bone-forming, benign lesion affecting the lower extremity in half of the cases ^[1]. The distal humerus involvement within the elbow joint is an uncommon site for an osteoid osteoma [1, 2]. The diagnosis is mostly delayed and often patients have long-standing symptoms and many undergo various tests or procedures before reaching the definitive diagnosis. Restricted elbow range of motion or flexion contracture is common presentation as noted in a large series of elbow osteoid osteoma cases ^[2]. There may be non-specific clinical features and the characteristic night pain relieved on taking non-steroidal anti-inflammatory drugs may or may not be noted in every case. The use of advance imaging modalities such as computerized tomography (CT) scan or magnetic resonance imaging (MRI) is warranted to identify the lesion, often missed on plain radiographs. Lesions in the juxta-articular or intra-articular location pose further diagnostic challenge due to many differentials for pain and limited movement to exclude ^[3]. The involvement of the coronoid or olecranon fossa and mimicking early elbow arthritis is uncommon presentation and should be considered as a rare cause of unexplained elbow arthritis in adolescents or young adults ^[4]. Besides it, cases showing minimal cortical sclerosis pose a diagnostic dilemma ^[5]. The subperiosteal location in the distal humerus in the intra-articular region is even more uncommon. High index of suspicion and identification of the lesion in the imaging is critical for the appropriate management and good functional outcome.

A 23-year-old male patient presented to us with complaints of atraumatic right elbow pain and restriction of movement for the past 6 and 3 months, respectively. The movement restriction, especially the flexion, was noticed gradually increasing over the past month. For pain, he used to take medicine on need basis from a local practitioner leading to transient relief. Pain, however, had no diurnal variation or association with activities. He was advised MRI of the elbow on account of recurrent pain, the reason for his visit to us. On examination, there was mild swelling around the elbow and anterior elbow tenderness. The active movement of extension was intact but active flexion was restricted and only ninety degree of passive elbow flexion was possible. There was no localized elevation of temperature and distal neurovascular  status was intact. There was neither any other joint involvement nor any systemic comorbidity associated. The radiographs of the elbow in initial period did not show any significant abnormality except a small fleck of radio-opacity noted anterior to the distal humerus (Fig. 1). MRI was done to better delineate the etiology and showed effusion in the elbow joint with edema in the bones around the distal humerus. The MRI images were suggestive of elbow arthropathy and considering monoarticular involvement and synovial hypertrophy, a provisional diagnosis of elbow tuberculosis was made. On closer inspection of images, however, a round defect was noted adjacent to the coronoid fossa on its medial aspect, as a clearly evident anomaly on both sagittal and axial images (Fig. 2 and 3a). A CT scan better delineated the lesion as a small bony lesion with an overlying thin cortical rim, abutting the coronoid fossa, and protruding anteriorly into the elbow joint (Fig. 3b and 4). Joint effusion and edema corresponding to the area in distal humerus were also noted. The lesion with a rim was considered as an osteoid osteoma and the confirmatory exploration and sample collection for biopsy was planned. The elbow was explored by an anterior approach with a 5-cm longitudinal incision centered over the coronoid fossa under tourniquet. The area near coronoid fossa revealed a protruding bony mass with thin rim of bone in the medial aspect of the fossa (Fig. 5a). The lesion was carefully excised en bloc from the underlying bone and was easily detached owing to its superficial location on the bone (Fig. 5b). The debridement of the adjacent bony and articular area and synovectomy was done and the joint was given a thorough lavage. The wound was then closed in layers followed by a protection elbow plaster slab. The post-operative period was uneventful and there was marked reduction in pain and gradual decrease in the swelling in coming days. The histopathology confirms the presence of an osteoid osteoma (Fig. 5c and d). The slab was removed after 2 weeks and the elbow range of motion exercises started. The patient had good wound healing (Fig. 6a) and full range of motion in flexion (Fig. 6b) and extension (Fig. 6c) with good radiological improvement in the follow-up at 4 months. No recurrence of the lesion or any complication associated were seen in the follow-up of further 9 months.

Pain and limited movement are common complaints and many of these cases with non-specific symptoms witness a delay in the definitive diagnosis. Many are misdiagnosed and treated for synovial disorder, enthesopathy, or sequela of old trauma ^[6]. Juxta- and intra-articular osteoid osteoma are rare lesions and may arise from any joint but common reported locations are hip, elbow, and talus. Intra-articular lesions, when subperiosteal, may encroach into the joint and present with features of mono-articular arthritis with swelling, pain, and limited joint movements. A case very similar to ours with a subperiosteal lesion having thin cortical shell in the coronoid fossa was reported ^[7]. Excision of the lesion resulted in marked clinical improvement. Osteoid osteoma has been described as a self-limiting disorder and spontaneous healing is also reported but cases with joint involvement require early treatment and are usually not candidate for conservative expectant management. Thermal ablation by percutaneous radiofrequency (RF) leads to thermal coagulation and induces localized necrosis. This can be an optional management method that is minimal invasive and works well in difficult locations ^[8]. Arthroscopy-based interventions such as excision, curettage, debridement, and synovectomy have also been frequently reported in recent times [5, 6, 7, 9]. In a recent systematic review, successful outcomes in the range of 93.8% along with zero post-operative complication and recurrence rate have been reported ^[10]. Major differentials include inflammatory arthritis, seronegative spondyloarthropathy, crystal deposition disorder, infective lesion like Brodie’s abscess, or tuberculosis in endemic regions ^[11]. Many cases undergo physical therapy and steroid injections for the elbow pain and consult only when conservative treatment fails [12, 13]. The presence of lesion is also reported in pediatric elbow (here olecranon fossa is common location) with pain and flexion-extension contracture in the presence of normal prono-supination ^[14]. Potential complications of childhood intra-articular osteoid osteoma-like skeletal abnormalities and joint movement problems necessitate appropriate diagnosis and treatment ^[15]. Acknowledgment of an osteoid osteoma in cases with long-standing elbow pain and stiffness as rare etiology should be done to avoid delay in the diagnosis and mismanagement.

This case underlines the importance of careful interpretation of radiological findings to arrive at the diagnosis in a case with unexplained painful restriction of movement. It also highlights that characteristic night pain responding to pain medication may not be found in all cases with osteoid osteoma. Excision leading to full recovery by open methods is a viable option in resource-limited areas where RF ablation or arthroscopic tools are not widely available.

Acknowledgment of the subperiosteal region, as the rare location for the osteoid osteoma, is important for appropriate diagnosis with the help of judicious use of clinical judgment and advanced imaging methods.