First reported by Lichtenstein in 1938, fibrous dysplasia is a benign, non-hereditary skeletal condition marked by aberrant osteoblastic differentiation and maturation. The disease usually manifests during the first three decades of life, and the dysplastic changes start in early childhood. The most frequently affected areas of fibrous dysplasia are the long bones of the lower extremity and the craniofacial skeleton. It typically involves the diaphysis, sparing the epiphysis. Fibrous dysplasia can occur in several distinct clinical settings:

1. Monostotic fibrous dysplasia: The most common form (70–80%), affecting a single bone. These lesions are usually asymptomatic and are discovered incidentally on radiographs obtained for unrelated conditions [1, 2].
2. Polyostotic fibrous dysplasia: A less prevalent form, characterized by multiple foci involving several bones. It can be either monomeric or polymeric and is linked to more severe clinical symptoms, such as deformity, discomfort, and functional impairment [3, 4].

In addition, polyostotic fibrous dysplasia may be part of syndromic variants, namely:

• McCune-Albright syndrome: Characterized by a triad of endocrine hyperfunction (precocious puberty), café-au-lait macules, and polyostotic fibrous dysplasia ^[5].
• Mazabraud syndrome: A rare coexistence of polyostotic fibrous dysplasia with soft-tissue myxomas ^[6].

Radiologically, fibrous dysplasia appears as an expansile, lytic, intramedullary, diaphyseal, or metaphyseal lesion on plain radiographs. Computed tomography (CT) scans reveal the extent of the lesion and delineate the cortical boundary. Magnetic resonance imaging (MRI) scans suggest heterogeneous enhancement on contrast-enhanced images, showing a hypointense signal on T1-weighted images and a hyperintense or intermediate signal on T2-weighted images. MRI may also reveal evidence of soft-tissue expansion and/or cortical damage. For asymptomatic monostotic lesions, upper limb lesions, and cases without deformity, nonsurgical methods such as bi-annual follow-up are preferred. For symptomatic cases, treatment options include pharmacotherapy and surgery. Lesions causing pain but with no imminent risk of pathological fracture can be managed with bisphosphonates. Pain and pathological fractures in long bones often require surgery. Since fibrous dysplasia is a developmental malformation of bone, surgical treatment remains controversial. One treatment option is curettage of the lesion, bone grafting, and internal fixation. However, considering the size of the lesion, which is not expected to heal, curettage and bone grafting may not always be feasible. Internal fixation or splinting of the fracture without curettage and bone grafting can alleviate pain and promote healing of any pathological fracture.

A 24-year-old woman presented with a history of pain over the right arm for 3 years, which had worsened over the past 3 months. The pain was insidious in onset, gradually progressive, non-radiating, mild to moderate in intensity, intermittent, and associated with swelling for the past 2 months. The pain was aggravated by movement and lifting weight and relieved with rest and medication. The swelling was insidious in onset, gradually progressive, and diffuse over the diaphyseal region of the humerus. There was no history of trauma or constitutional symptoms. On examination, there was an ill-defined bony swelling and thickening involving the proximal and middle third of the right humerus, extending from 3 cm below the angle of the acromion to 7 cm above the olecranon tip. Flexion, abduction, and external rotation of the shoulder were painfully restricted. Her day-to-day activities were restricted. The range of motion at the elbow was full and free. There was no distal neurovascular deficit. Radiologically, there was a long expansile lytic lesion involving the middle and distal third of the diaphysis of the right humerus, with thinning of the cortex in the middle third, suggesting pathological fractures (Fig. 1).

CT scan of the humerus showed sparing of the proximal third of the humerus. MRI scan revealed an expansile intramedullary lesion involving almost the entire shaft of the right humerus, sparing the proximal metaphyseal region. The superior part of the lesion was more expansile, with marked thinning of the cortex and cortical breach at multiple places. It showed an intramedullary cystic component with a fluid–fluid level and thin peripheral enhancement. The inferior part of the lesion was mildly expansile with endosteal scalloping but an intact cortex. It showed intramedullary T1 iso- to hyperintense signal, T2 iso- to hypointense signal, short tau inversion recovery iso- to hyperintense signal, with heterogeneous post-contrast enhancement and internal non-enhancing areas. There was surrounding myofascial edema and enhancement, more so along the superior aspect of the lesion, suggesting the possibility of fibrous dysplasia with a simple bone cyst/cystic degeneration (Fig. 2).

She underwent a core needle biopsy of the expansile lesion for histopathological examination, which revealed hemorrhagic giant cells with fibroblastic proliferation and no signs of malignancy. Since malignant transformation was ruled out and in view of multiple lytic lesions in the humerus shaft, she was planned for intramedullary fixation rather than a surface implant. She underwent internal fixation of the right humerus with an intramedullary interlocking nail (size 7 × 180 mm). Initially, a 220 mm nail was inserted, but the distal nail tip was about to penetrate the olecranon fossa. A 200 mm nail was unavailable, so a 180 mm nail was used instead (Fig. 3).

Reamed medullary content was sent for histopathological examination, which showed anastomosing irregular bone trabeculae forming woven bone, suggestive of fibrous dysplasia (Fig. 4).

She was started on gentle shoulder range of motion exercises and pendulum exercises. She was informed about the future risk of fracture distal to the tip of the nail, as the tip can act as a stress riser. In case of a pathological fracture distal to the tip of the nail, fixation with a pre-contoured extra-articular distal humerus locking plate spanning the nail has been planned. Three months postoperatively, the patient had good pain relief and functional outcome, with healing of the pathological fracture (Fig. 5 and 6).

Fibrous dysplasia is a benign condition characterized by fibrous tissue replacement of the skeletal system. It is associated with a mutation in the GNAS gene on chromosome 20, which activates the α-subunit of the Gs stimulatory protein and causes dysregulated proliferation of bone marrow stromal cells, producing deformed bone. The “receptor-activated nuclear factor kappa-β ligand” has been seen to be elevated in human bone marrow stromal cells that are transduced with mutated GαS. Denosumab treatment has been demonstrated to reduce the pace of growth of the lesion in patients with aggressive fibrous dysplasia ^[7]. Histologically, the tissue is primarily made up of mature, dense collagenous tissue with regularly spaced fibrous bone trabeculae embedded within ^[8]. Woven bone is characterized by its thin, anastomosing bone trabeculae that form an interlacing network without any specific orientation. In a classic lesion, the collagenous tissue metaplasia produces bone, and the trabeculae are rimmed by a noticeable absence of osteoblasts (Lichtenstein). The traditional “Chinese letter” or “alphabet soup” histologic pattern is most frequently seen in long bone lesions where the stroma has low cellularity with curved, irregular trabeculae of woven bone arranged in a discontinuous manner ^[9]. Giant cells are rare and are seen in regions of hemosiderin accumulation and degeneration. Radiologically, fibrous dysplasia appears as an expansile, lytic, intramedullary, diaphyseal, or metaphyseal lesion on plain radiographs. The margins are well-defined, with mild endosteal scalloping with or without sclerotic borders. Its distinctive “ground glass” look results from the typical trabecular woven bone being replaced with a homogenous fibro-osseous matrix. It can be associated with pathological fractures, which may show a periosteal reaction, though this is usually absent. Pathological fractures can cause deformities and distortion throughout the lesion. CT scans reveal the extent of the lesion and delineate the cortical boundary of the lesion. MRI scans suggest heterogeneous enhancement on contrast-enhanced images; a hypointense signal is seen on T1-weighted images and a hyperintense or intermediate signal is seen on T2-weighted imaging. MRI also reveals evidence of soft-tissue expansion and/or cortical damage. Kinnunen et al. ^[10], in their study, concluded that since the signal intensity in fibrous dysplasia is highly variable, MRI alone cannot be used to diagnose the condition. Fibrodysplastic bone can eventually undergo either a benign or a malignant transformation. Although it can happen in any bone with fibrous dysplasia, transformation into aneurysmal bone cysts has been seen most frequently in the skull. Many pre-existing benign bone tumors can potentially develop aneurysmal bone cysts. When aneurysmal bone cysts develop in fibrous dysplasia bone, the already fragile and dysplastic bone degrades into a growing, blood-filled cyst. Usually, the bone cyst grows far more quickly than fibrous dysplasia would, which increases bone pain and fracture risk ^[4]. In a study of 50 patients with humeral lesions, Majoor et al. reviewed fracture risk factors and surgical intervention reasons. Of the lesions, they found that 54% had at least one fracture. Initially, nonoperative treatment was the approach for each fracture. But in the end, eight individuals needed at least one surgical intervention. The presence of cystic degeneration, axial length >30 mm, and circumferential cortical involvement >50% were linked to a higher risk of fracture and could be regarded as indicators for the preventive treatment of humeral lesions ^[11]. For certain fractures of the upper extremities, standard closed reduction is frequently suitable. However, as remodeling and correction of residual angulation usually do not occur as quickly and reliably in fibrous dysplasia as they would in normal bone, the fractures should not be allowed to heal with residual angulation. Therefore, internal fixation for fractures of the upper extremities should be considered. Due to the pathological nature of the bone, which makes it more prone to recurrent fracture, weight-sharing devices in the form of an intramedullary nail or elastic nail are preferred over weight-bearing devices such as plates and screws, as there is a high risk of screw back-out, implant failure, and the plate ends can act as stress risers, leading to fractures. In young individuals, where a lesion is in the humerus with a narrow medullary canal, intramedullary nails have been used ^[12]. Intralesional curettage and bone grafting have shown suboptimal outcomes, including fractures, graft resorption, and the need for revision surgery [13,14]. However, in cases of monostotic mono-focal lesions with minimal risk of fracture, such as in lesions of the upper extremities, it can be considered a modality. Internal fixation of the pathological fracture in the bone alone has been effective in lowering pain and further fracture risk ^[4]. However, it can be used in conjunction with curettage and grafting. In our case, internal fixation of the lesion was performed alone because bone grafts undergo resorption, the lesions were multifocal, sufficient and suitable autograft was unavailable, and allograft was not accessible at our center. We noticed during regular follow-ups that stabilizing the lesion with intramedullary fixation led to callus formation, and healing of the fracture, and the patient became pain-free with a good functional outcome.

Fibrous dysplasia in long bones presenting with pathological fracture can cause pain and restriction of movements. Stabilizing the lesion with an intramedullary implant without curettage and bone grafting helps in fracture healing, alleviates pain, and restores the functional range of motion.

Fibrous dysplasia with a multifocal lesion and pathological fracture of the humerus in a young female can be treated with internal fixation of the lesion without curettage. This approach leads to fracture healing with a pain-free arm and good functional outcome, avoiding long surgical procedures for curettage and bone grafting, and preventing donor site morbidity.