An aneurysmal bone cyst (ABC) is a benign osteolytic tumour composed of blood-filled cavities separated by septa containing fibroblasts, osteoid tissue, and multinucleated giant cells ^[1]. Although considered reactive, molecular studies suggest a neoplastic origin in primary lesions ^[1,2]. ABC mostly affects children and young adults and most commonly involves the metaphysis of long bones ^[3]. Treatment includes extensive intralesional curettage with or without bone grafting, cement augmentation, cryotherapy, selective arterial embolisation, sclerotherapy, and en bloc resection ^[4,5,6]. Recurrence rates, particularly in aggressive lesions, following curettage range between 15% and 30% ^[7,8]. The proximal humerus presents specific challenges due to its biomechanical importance in shoulder girdle mobility and limb function. Extensive lesions with cortical thinning or impending fracture require aggressive management and structural reconstruction of anatomy to restore function ^[9]. We present a case of a large proximal humerus ABC managed with en bloc resection, fibular strut grafting, and PHILOS fixation.

A 28-year-old female presented with pain and progressive swelling over the left shoulder for 5 months, associated with difficulty performing overhead activities. Examination revealed localised warmth and prominent superficial veins. Distal neurovascular status was intact (Fig. 1).

Figure 1: Pre-operative clinical photograph. (a) Anterior aspect (b) Lateral aspect pre-operative radiograph. (c) Anteroposterior view (d) Lateral view.

Plain radiographs showed a well-defined expansile lytic lesion with cortical thinning in the metaphyseal region. Computed tomography confirmed cortical expansion without breach. Magnetic resonance imaging (MRI) demonstrated multiple internal septations and characteristic fluid–fluid levels highly suggestive of ABC. The lesion was classified as Capanna Type II ^[7] (Fig. 2).

Figure 2: Pre-operative computed tomography (CT) scan. (a) Coronal CT image (b) sagittal CT image. (c) Three-dimensional CT reconstruction. Magnetic resonance imaging (MRI) of the right shoulder (d) coronal MRI image (e) sagittal MRI image (f) axial MRI image.

A biopsy performed elsewhere was confirmatory of ABC. The patient underwent two sessions of selective arterial embolisation without clinical or radiological improvement. Pre-operative embolisation was repeated to reduce intraoperative blood loss, as ABCs are highly vascular lesions ^[6] (Fig. 3).

Figure 3: (a) Pre-embolization angiographic image (b) post-embolization angiographic image.

Through a standard deltopectoral approach, en bloc excision of the lesion was performed up to healthy bleeding bone margins. A 15-cm ipsilateral non-vascularised fibular strut graft was harvested (Fig. 4).

Figure 4: Intraoperative images. (a) Intraoperative photograph of the deltopectoral approach. (b) Complete curettage and removal of the lesion. (c) PHILOS plate fixation with fibular strut graft support. (d) Gross specimen of the lesion. (e and f) Intraoperative C-arm images.

The graft was inserted to reconstruct the defect and restore structural integrity. Fixation was achieved using a PHILOS plate under fluoroscopic guidance, providing angular stability and anatomical alignment ^[10] (Fig. 5).

Figure 5: Post-operative radiograph. (a) Anteroposterior view (b) lateral view.

Histopathological examination confirmed ABC. Postoperatively, protected mobilisation was initiated early, followed by progressive range-of-motion exercises based on radiographic healing (Fig. 6).

Figure 6: Follow-up radiographs at 1, 3 and 6 months showing maintained fixation of the proximal humerus with PHILOS plate and fibular strut graft, with no evidence of recurrence of Aneurysmal Bone Cyst. (a),(c),(e) Anteroposterior view (b),(d),(f) Lateral view(c, d) Clinical photographs demonstrating satisfactory functional range of motion of the right shoulder at 6 months postoperatively

ABC is a locally aggressive lesion with significant recurrence potential if inadequately treated ^[8]. While intralesional curettage remains common, extensive lesions with cortical compromise may require en bloc resection to minimise recurrence risk ^[4]. Selective arterial embolisation reduces intraoperative blood loss and serves as adjunctive or primary treatment in select cases ^[6]. However, in large lesions unresponsive to embolisation, definitive surgical excision remains the mainstay of management. Structural reconstruction following proximal humerus resection is essential to maintain shoulder biomechanics. Non-vascularised fibular strut grafting provides immediate mechanical support and promotes biological incorporation ^[11]. The PHILOS plate offers angular stability, particularly in metaphyseal bone with a compromised cortex, allowing early mobilisation and favourable functional recovery ^[10]. Donor site complications following fibular harvest are reported to be lower when proper technique is used ^[12]. In our case, the combination of oncologic clearance and stable biomechanical reconstruction resulted in graft incorporation, union, and restoration of full shoulder function without recurrence during follow-up. Limitations of the study include the single-case nature and limited follow-up duration.

Large proximal humerus ABCs need to be addressed with both oncologic and reconstructive challenges. Pre-operative embolisation followed by en bloc resection, non-vascularised fibular strut grafting, and PHILOS fixation provides reliable graft incorporation, mechanical stability, and satisfactory functional outcomes. Long-term follow-up is essential to monitor for recurrence.

Large aneurysmal bone cysts of the proximal humerus that fail non-operative measures such as selective arterial embolisation may require en bloc excision for adequate local control. Reconstruction using a non-vascularised fibular strut graft combined with PHILOS plate fixation provides stable biological reconstruction, facilitates early mobilisation, and can result in satisfactory functional and radiological outcomes.