1 Introduction

Osteoid osteomas (OO) represent 10–12% of benign bone tumors, and approximately 5–12% of these are intraarticular. Around 13% of intraarticular OOs occur around the hip joint, with less than 1% of these located in the periacetabular region.^1,2 Thus, acetabular osteoid osteoma (AOO) is a rare diagnosis, and its presentation can mimic many other hip pathologies.

It has been reported that the intra and juxta-articular OOs of the hip can present with atypical symptoms. The delay in the diagnosis has been reported to be three times longer than that with classic extraarticular lesions.³ The most common presentation is groin pain that can mimic femoroacetabular impingement (FAI).⁴ The presentation spectrum includes muscle atrophy, limb length discrepancy, hip contractures, synovitis, and osteoarthritis. AOOs is difficult to appreciate on the plain radiographs due to occult reactive sclerosis and small nidus, adding to delay in diagnosis. Irreversible joint changes are reported with long-standing intraarticular OOs.⁵ Therefore, AOO is an important entity in the differential diagnosis of hip pain in adolescents and young adults.

The FAI is relatively common with femoral neck OOs. FAI is rarely reported with acetabular rim lesions but not with the other locations of the acetabulum. The reported methods for excision of AOOs include CT-guided percutaneous drilling, arthroscopic and open surgical excision. The use of intraoperative CT navigation for the percutaneous excision of difficult OOs has been reported in the literature.⁶ We present a case of AOO with symptomatic FAI treated with a combination of safe surgical dislocation (SSD), CT-navigated trans-articular excision, and osteochondroplasty in a young athlete.

2 Case report

An 18-year-old college-level athlete presented with complaints of left hip pain and limp for two years. The pain was insidious in onset, dull aching which aggravated with activity. Also, he had the typical night pain of OO. He had been taking oral NSAIDs regularly for pain relief. The hip's physical examination revealed mild tenderness over the anterior hip and subtle wasting of gluteal and thigh muscles. The range of motion of the left hip was terminally limited, and flexion-internal rotation was painful and 15° less than the opposite hip, typical of impingement. He had an antalgic gait and no limb length discrepancy.

His routine blood work-up in search of inflammatory and infective pathology was normal. Radiograph of the pelvis with both hips anteroposterior view showed subtle FAI signs with high cross-over sign and a CAM morphology (Fig. 1A). Definitive diagnosis has been delayed for one-year until further imaging was done due to persistent hip pain. MRI pelvis showed a lytic lesion with a 5mm nidus typical of OO in the superomedial aspect of the left acetabulum in the sub-articular region, with surrounding oedema and sclerosis. The hip joint showed mild effusion and synovitis and also a CAM morphology (Fig. 1D, E). Triphasic technetium bone scan showed a hot spot at the left acetabulum in delayed images favouring benign pathology (Fig. 1B, C).

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Fig. 1 (A) AP pelvis radiograph showing apparently normal-looking hips joints on both sides. (B,C) Bone scan showing the hot spot near the left acetabulum. (D) MRI axial view showing the osteoid osteoma with nidus. (E) MRI coronal view showing the nidus in the supero-medial aspect of left acetabulum with CAM morphology of neck in the left hip.

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3 Surgical technique

As this patient had an AOO in the sub-articular region and also had FAI morphology, we chose to address both the pathologies through SSD. We planned to use the AIRO intraoperative CT navigation to precisely localize the lesion and also to confirm complete excision of the nidus.

The patient was positioned in the lateral position. A standard technique of Ganz's SSD was used to dislocate the hip and the technique of CT-navigated excision of OOs was used to localize the lesion.⁶ After the surgical dislocation, a 4mm Steinmann pin was placed over the iliac crest, and the minimally invasive reflective array (MIRA) was attached to the pin. The preliminary registration scan of the acetabular region was done with AIRO-CT (Brainlab, Germany), with the hip in the dislocated position and the acquired data was transferred automatically to the computer navigation platform (Vectorvision; Brainlab) (Fig. 2A). The navigation tool with infrared markers was used to localize the lesion in the acetabulum (Fig. 2B). After localization, the lesion was de-roofed through acetabular cartilage using a 4mm high-speed burr. The lesion was curetted, and the nidus was sent for a histopathological examination. An immediate on-table post-excision CT scan showed complete excision of the lesion (Fig. 2E). After relocating the femoral head, a dynamic assessment of the hip revealed the classic anterolateral impingement of the labrum caused by the CAM bump. Hence, the femoral head's osteoplasty was done (Fig. 2 C, D).

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Fig. 2 (A) Intraoperative patient positioning on AIRO-CT with MIRA fixed to iliac crest (note that the hip is dislocated). (B) Accurate localization of lesion with navigation tool in all three planes. (C) Intraoperative picture of the left proximal femur showing the CAM bump. (D) Intraoperative picture of the left proximal femur after removing the CAM bump and recreating the head-neck offset. (D) Post-excision CT scan confirming complete excision of the nidus.

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The patient had complete pain relief typical of OO. Histopathology confirmed the diagnosis of OO. The patient was symptom-free at a 4-years follow-up and resumed his sports activity. Radiographs at final follow-up showed no evidence of FAI, avascular necrosis (AVN), or osteoarthritis (Fig. 3).

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Fig. 3 (A) AP, (B) frog-leg lateral and (C) Dunn lateral view at 4-years follow-up showing no evidence of FAI, AVN or arthritis.

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4 Discussion

The AOOs are rare, present with atypical symptoms, and delayed diagnosis is a common clinical scenario. Clinical and imaging findings of intraarticular OOs are significantly different from the classic presentation of extra-articular OOs.³ Plain radiographs show only subtle signs of OOs, unlike the typical extra-articular locations. Missed diagnosis in up to 21% of cases has been reported even with MRI. CT remains the gold standard for the diagnosis of intraarticular OOs. Approximately 50% of the intraarticular hip OOs are associated with radiographic arthritic changes at diagnosis.⁷ Thus, early diagnosis and aggressive surgical treatment are indicated for all intraarticular OOs.

Morphological changes in a joint with the intraarticular OOs is a well-reported complication.⁵ Pertaining to hip joint - impingement, hip subluxation, cartilage, and labral lesions have been reported. FAI is common with femoral head and neck lesions. Though unusual, it has been reported with acetabular rim lesions too. The Table 1 shows the literature on AOO with FAI.^8–10 Marwan et al. reported a 31-year-old male with AOO who underwent hip arthroscopy to manage the tumour after failing to respond to medications and CT-guided radiofrequency ablation. They noted a loss of sphericity of the femoral head, but no treatment was done for the FAI.⁸ Newhouse et al., in 2020 reported a technical note and a case AOO with FAI treated with cartilage-preserving arthroscopic-assisted radiofrequency ablation in a young adult.⁹ Denker et al., in 2006 reported a case of AOO of the acetabular rim with FAI treated with hip arthroscopic removal of a rim–based lesion and concomitant FAI correction.¹⁰ Thus, while treating intraarticular OOs of the hip, the primary pathology (osteoid osteoma), as well as secondary morphological changes, need to be addressed for complete pain relief. Therefore, more invasive procedures are required for intraarticular hip OOs than the classic extra-articular OOs, where a percutaneous technique is a gold standard.

Over the past two decades, CT-guided percutaneous methods and hip arthroscopy have superseded open resection. However, associated intraarticular pathological changes like FAI and labral lesions cannot be fully addressed with percutaneous techniques. Also, histological confirmation of the diagnosis is not possible with percutaneous ablation. There are reports of arthroscopic resection of AOO with good results. We have no experience with this technique and we believe that it could technically be challenging to identify a small nidus and access it deep inside the acetabular cavity.

Percutaneous techniques were opted out in this case due to the sub-articular location and the co-existing FAI. The advantage of AIRO CT navigation and real-time three-dimensional imaging for the excision of OOs of difficult sites with a 100% success rate has already been reported⁶. In this particular case, localization of the lesion would have been impossible without intra-operative CT navigation. The precise localization helped in the successful excision of the lesion with minimal damage to articular cartilage. As the radiation is concerned, we believe that navigation significantly reduces the radiation as there was only one pre-operative CT scan for registration and one post-op limited cuts scan for confirmation of excision. SSD facilitated direct access to the lesion and also addressed the associated FAI. Though SSD has been used for the excision of intraarticular hip OOs in the literature, ours is the first case to utilize SSD combined with real-time intraoperative CT navigation to treat AOO with FAI.

5 Conclusion

Acetabular osteoid osteomas with FAI are rare. These can be treated successfully with SSD and intraoperative real-time CT navigation. This approach helps address both the pathologies in a single sitting and has the add-on advantage of minimal radiation.

Contributorship details

K Venkatadass - Conceptualisation of the technique and surgeon who operated, reviewing the manuscript.

V Durga Prasad - Manuscript preparation, figures, literature review, artwork.

Nasser Mohammed Mansor Al Ahmadi – Literature review, manuscript preparation.

S Rajasekaran – technique description, Reviewing and editing the manuscript.

Funding

This research received no specific grant from any funding agency in public, commercial, or not-for-profit sectors.

Declaration of conflicting interests

All the Authors declares that there is no conflict of interest.

Patient consent

An informed consent for the publication was obtained from the parent/guardian.