Osteoid Osteoma of the Quadrilateral Plate of Acetabulum: a Case Report on an Unusual Cause of Hip Pain

Osteoid osteomas are rare and extremely painful benign bone tumours. They are usually seen in children in the growing age group and young adults. The patients typically complain of bony pain during the night time and usually relieved with rest and salicylates. Osteoid osteomas are commonly encountered in the limbs with proximal femur/hip region and tibial diaphysis being the usual locations. Pelvic locations are rare for osteoid osteoma and few cases have been published/reported in literature. A young female with long standing hip pain was diagnosed to have osteoid osteoma in an extremely rare location in her pelvic bone. After careful evaluation of the imaging, the lesion was found to be in the quadrilateral plate/medial wall of acetabulum. After through pre-surgical planning, she underwent the gold standard treatment for osteoid osteoma, i.e. computed tomography (CT)-guided radiofrequency ablation (RFA). Immediate post-procedure the patient reported complete relief of her symptoms and has been symptoms-free for the past 2 years of follow-up. A high level of clinical suspicion is warranted to keep osteoid osteoma as a differential diagnosis in a patient presenting with the relevant symptoms. They can be easily missed on radiographs and CT is a clincher for diagnosis and unusual anatomic sites can harbour the same. A carefully planned radiofrequency ablation under CT-guidance gives favourable and reliable results for treatment of osteoid osteomas.


Introduction
Osteoid osteoma is a rare benign bone tumour and constitutes 13% of all benign bone tumours encountered in orthopaedic oncology [1]. They are usually seen in the growing age group and mainly in adolescents [2]. Osteoid osteoma has a typical clinical presentation with pain which is significantly more during the night time [3], disturbing sleep, restricting daily routine activities and relived transiently on taking analgesics specifically NSAIDs. The common sites where osteoid osteomas are usually encountered are the proximal femoral metaphyseal region, tibial diaphyseal region and in the small bones of the hands and feet [4]. Occasionally in young children proximal femoral osteoid osteomas may cause referred pain to the knee region, thereby precluding diagnosis of osteoid osteoma for a significant duration of time. Osteoid osteomas are usually cortical lesions but they can be periosteal, juxta-cortical or intra-medullary in location in long bones/appendicular skeleton. The pelvic bone is a rare site for occurrence of osteoid osteoma [5,6] and very few cases in literature have been described of osteoid osteomas arising in the medial/ quadrilateral plate of acetabulum [6][7][8][9]. Osteoid osteomas are classified with their size being less than 2 cm with a central nidus and extensive peri-lesional sclerosis in cases which are detected or present late [10].
Osteoid osteomas are typically, a clinic-radiological diagnosis and definitive histopathological examination in the form of taking biopsy before planning definitive procedure is usually not required in cases where CT and MRI imaging This article is part of the Topical Collection on Surgery * Jagandeep Singh Virk jagandeepv1@gmail.com are conclusive for an osteoid osteoma along with typical clinical history. Our case is unique since it is a rare location where osteoid osteomas can be encountered and very few cases in literature have been described, in this area [6][7][8][9]. Keeping a high index of suspicion, good quality imaging for diagnosis and meticulous planning for treatment is essential in providing a good outcome. Nowadays, the gold standard treatment for treating osteoid osteomas is CT-guided radiofrequency ablation (RFA). This can be performed in a day-care setting under general anaesthesia for very young children and under spinal anaesthesia when osteoid osteomas are located in the lower limbs in adults. CT-guided RFA can be performed in a minimally invasive manner with accuracy and precision with reliable results. Planning the approach for carrying out the ablation in our case formed an essential component for technical success of the procedure due to proximity of the lesion to vital structures like the sciatic nerve posteriorly, femoral nerve and vessels anteriorly and the femoral head cartilage.

Case Presentation
A female, 28 years of age, presented to the orthopaedic oncology department with complaints of severe pain in her left hip and groin region for the past more than 1 year. The pain was typically more during the night time and extremely debilitating to the point that she had difficulty in performing even her daily routine activities. The patient was bed-bound for most times and had developed a significant limp while walking because of the pain. The pain was transiently relieved with non-steroidal anti-inflammatory drugs (NSAIDs) but the pain returned back once the effects of the NSAIDs weaned off and on exertion or doing simple routine household activities.
There was no history of any prior trauma, long-term fever, weight loss or medications for any other illness in the past. There was no history of smoking, alcohol intake or substance abuse in the past. Also, no history of steroid intake in any form was present. There was no family history of similar complaints priorly. On examination, tenderness was elicited over the left hip and gluteal region and all hip movements were painful and restricted due to the pain. There was no limb-length discrepancy and distal neurovascular status was normal for the left lower limb.
Routine laboratory tests (complete blood cell count, erythrocyte sedimentation rate and uric acid measurements) were performed and were within normal limits. Patient already had some imaging investigations of the affected region got done from other centres. Plain radiographs of the pelvis with bilateral hip regions did not show any abnormality. On magnetic resonance imaging (MRI), there was a lesion in the medial wall/quadrilateral plate of the left acetabulum which was hypointense on the T1-weighted image and hyperintense on proton density fat-saturated images. T2-weighted fat suppressed axial image showed a small round hypointense focus with surrounding hyperintense signal suggestive of oedema surrounding most likely a neoplastic lesion (Fig. 1a). Possibilities of a cartilage-based lesion/enchondroma and osteoid osteoma were provided. A non-contrast computed tomography (NCCT) scan of the pelvis with both hip joints was got done to further delineate the possibility of an osteoid osteoma, since visualization of the nidus and surrounding sclerosis is better on the same. CT revealed a well-defined lucent lesion (measuring ~ 7 × 7 mm) with a central sclerotic dot and surrounding sclerosis in the posterosuperior aspect of the medial wall of left acetabulum ( Fig. 1b and c). These features were suggestive of intracortical, intraarticular osteoid osteoma in the medial wall of the left acetabulum. Since there was no history of fever or any constitutional symptoms, a differential diagnosis of infection was not kept and hence bone scan or labelled scintigraphy which helps to differentiate between the two conditions was not deemed necessary and hence not done. Also, labelled scintigraphy has also been of use to distinguish the two but has been reported to be non-specific [11].
Following the diagnosis, the patient was planned for surgical intervention to relieve the patient of her symptoms since conservative medical treatment in the form of analgesic medications had already been tried and did not give much relief to the patient. The patient was planned for a CT-guided RFA procedure from a posterolateral/oblique approach. The approach was planned carefully after anatomical detailing due to rarity of the location of the lesion. The direct posterior approach risked injury to the sciatic nerve, lateral approach risked damage to the joint cartilage and anterior approach was not feasible due to risk of injury to the femoral nerve and vessels (Fig. 2). The procedure was performed in the CT scan room itself under CT-guidance and under spinal anaesthesia. After standard painting and draping, the entry point over the skin was localized under CT-guidance. A stab incision was given and blunt dissection up to the bone was performed with a long-tip haemostat curved artery forceps. Jamshidi (J) needle 8G was introduced to reach up to the nidus of the lesion and then the trocar was removed from the cannula of the J-needle. The position of the J-needle cannula was confirmed under CT-guidance. The radiofrequency ablation probe, Soloist™ Straight Needle Electrode (Boston Scientific), was introduced through the cannula of the J-needle and the probe connected to the radiofrequency generator. The cannula of the J-needle was retracted by 5 cm to prevent heat transmission from the probe tip to the metal cannula and thereby preventing overheating of the surrounding tissues. After final confirmation of the intra-nidal position of the radiofrequency ablation probe under CT-guidance, two cycles of ablation were carried out using impedance as procedural end-point.
Following the procedure, the patient reported pain relief from the very next day and all symptoms including limping were completely resolved after 1 month and the patient has remained asymptomatic for the past 2 years of follow-up with no signs of recurrence or arthritis of the hip joint.

Discussion
The acetabulum is a rare site for osteoid osteoma [5,12,13]. Very few cases have been reported in literature till now describing an osteoid osteoma in the medial wall of acetabulum/quadrilateral plate of acetabulum [6, 7, 8 and 9]. Due to rarity of the site, these lesions are frequently missed on plain radiographs. Plain radiographs are not the ideal modality to diagnose osteoid osteomas especially in the intra-medullary location of appendicular skeleton and in complex anatomic sites like the pelvis and spine [14]. This also explains why many times these lesions have a delayed diagnosis. Good quality CT scans of the involved region in a bone window makes the lesion clearly visible and helps define the nidus of the lesion. Relationship of the lesion to the surrounding soft tissue structures, nerves and vessels can also be studied on CT scan, although better delineated on an MRI scan.
Osteoid osteomas are characterized by localized pain, typically more during the night time. Analgesic and antiinflammatory medications like aspirin are justified for palliative non-surgical treatment for osteoid osteomas which do provide symptomatic relief to the patients. In cases not responding to medications, surgical excision/ablation is the option which should be done to prevent developmental complications like growth disturbances, especially when these lesions are encountered in young children [15].
Surgical treatment for osteoid osteomas encountered in the pelvis and especially in the medial wall of acetabulum is a surgical challenge for any surgeon. The access to the lesion in such a location is technically challenging and limited by the presence of vital structures like the sciatic nerve posteriorly, femoral nerve and vessels anteriorly and possibility of damage to the triradiate and femoral head cartilage. Although various surgical approaches to osteoid osteoma in the medial wall of acetabulum have been described, including open surgery such as a safe surgical dislocation of the hip [7], anterior intra-pelvic approach [8] and lesser invasive procedures like arthroscopic-guided [6] and CT-guided ablation of the lesion [9], optimal management for acetabular osteoid osteoma has not been established.
Open procedures involve giving large surgical incisions and removing a large chunk of the bone containing the lesion or even burring of the lesion, do tend to give the patients a significant immediate morbidity in terms of post-op wound care, restricted weight bearing for a significant amount of time and even pathological fractures in few cases [16][17][18]. Most of the times the lesion localization is done based on CT imaging planning and intra-operatively fluoroscopic guidance is used to identify the lesion with the possibility of one missing the lesion occasionally [16][17][18]. Minimally invasive methods using arthroscopic excision and ablation under CT-guidance are considered to be the treatment of choice for such small lesions [19,20]. Arthroscopic excision is limited to intra-articular osteoid osteomas [19], whereas CT-guided procedures can be carried out for all possible locations with excellent anatomic delineation of the lesion and intra-operative monitoring of the surgical steps of the procedure. Recently, Efthymiadis et al. investigated in a systematic review and proportional meta-analysis the ideal minimally invasive method for treatment of hip osteoid osteoma. They concluded that radiofrequency ablation (RFA) and arthroscopic excision could be the most effective treatment options for osteoid osteoma [21].
The method employed for treatment of these lesions is also guided by the available infrastructure at the hospital/ centre where the patient is presenting, surgeon experience and expertise and economic factors. We at our centre have a 128-slice CT scan and availability of a RFA generator. Ablation through a standardized radiofrequency generator rather than an electrosurgical generator does prevent any chances of thermal injury or heat transmission to surrounding tissues [22]. The procedure of choice for these lesions at our centre is CT-guided RFA using the Boston-Scientific RF300® system which uses impendence as procedural end-point thus ensuring completeness of ablation and prevention of any thermal transmission or injury to the surrounding tissues.
Meticulous pre-procedure planning was performed with the radiology team and possible safest access to the lesion was planned. A posterolateral approach in the prone position of the patient was planned which spared the sciatic nerve and also prevented damage to the femoral head cartilage. As a protocol once the intra-nidal positioning of the RFA probe was ensured, two cycles of ablation were carried out. The RFA system used by us does not work on the principle of attaining a specific temperature set point and maintaining that temperature for a specified duration. It works on the principle of impedance and resistance which automatically cuts off the system current to the affected area once tissue ablation in the desired zone is achieved.
Our patient reported complete resolution of pain from the very next day and full-weight bearing was initiated. Patient did not report recurrence of the pain thereafter and followup radiographs at 6 months, 1 and 2 years did not show any evidence of arthritis of the hip joint. The patient was able to resume back all her daily routine activities just after 3 months of the procedure.

Conclusion
In conclusion, we successfully treated medial wall/quadrilateral plate acetabular osteoid osteoma without any symptom recurrence using CT-guided radiofrequency ablation. In addition, we preserved the sciatic nerve and triradiate and femoral cartilage by careful pre-operative planning.
A CT-guided procedure should be considered the treatment of choice for osteoid osteoma of the medial wall of acetabulum since it is a minimally invasive, effective, safe, less morbid and better alternative to en bloc resection/open procedures even in this difficult location.