Aneurysmal Bone Cyst of the Proximal Femur in a Pediatric Patient: A Case Report

Aneurysmal bone cysts (ABCs) are rare, benign but locally aggressive, hemorrhagic, expansile bone lesions most often occurring in the metaphysis of long bones in children and adolescents. They occur at an estimated rate of 0.14 per 100,000 per year. Radiographically, ABCs are eccentric, radiolucent, expansile lesions with cystic cavities causing cortical thinning and bulging. Primary ABCs are associated with USP6 gene rearrangements, while secondary ABCs can arise in association with other bone lesions. Biopsy is essential to differentiate ABCs from telangiectatic osteosarcoma. Standard treatments include curettage with grafting or substitutes, and wide resection in select cases to prevent recurrence; stabilization is indicated in the setting of fragility or pathologic fracture. This report presents a pediatric case of an ABC of the proximal femur with a pathologic fracture, highlighting evaluation and management considerations for acute limb pain and gait disturbance in children.

Clinical evaluation of pediatric lower extremity pain begins with history and physical examination, including assessment of gait, range of motion, signs of infection, deformity, and neurovascular status, followed by basic screening labs and radiographs as indicated [1]. Hip pathology can present as referred knee pain, and failure to evaluate both joints may delay diagnosis [2]. Many young, well-appearing children with acute gait disturbance have normal radiographs (96%), and most show spontaneous resolution on follow-up (81%) [3]. Asymptomatic benign bone tumors of the extremities are relatively common in children, with a prevalence of 18.9% on longitudinal radiographic review [4]. Imaging guidelines recommend a radiograph of the entire bone containing a suspected lesion and prompt referral to a surgical specialist or oncologist; MRI is useful to further characterize lesions and guide management decisions [5].

Case presentation: A six-year-old male developed progressive left leg pain beginning in July 2022. Initial knee and lower leg radiographs obtained by his pediatrician were normal. Over months, the pain progressed to a limp with inability to bear weight, particularly noted during football activity. By October 2022, he could not ambulate and returned for evaluation. Repeat radiographs of the hip showed an ill-defined lytic lesion with pseudotrabeculation involving the proximal left femoral diaphysis extending toward the femoral neck, with a lucency in the inferior neck consistent with a likely pathologic fracture. He was transferred to a tertiary center for further workup and orthopedic consultation. Laboratory studies (CBC, BMP) were within normal limits. Physical examination revealed no gross deformity; intact motor function and sensation; strength 5/5 for knee extension and 4/5 for knee flexion; pain localized to the mid-anterior left thigh; and inability to perform a straight leg raise. MRI demonstrated a multiloculated expansile lesion of the left femoral neck with areas of layering hemorrhage and a pathologic fracture. Pediatric orthopedic surgery performed a biopsy showing benign-appearing cystic tissue, followed by open reduction and internal fixation with pins and a plate for fracture stabilization. Curettage of the lesion was performed, and the cavity was filled with an injectable inductive graft and crushed cancellous bone. Postoperatively, physical and occupational therapy instructed ambulation with a walker, and hematology/oncology followed the case. Permanent pathology confirmed an aneurysmal bone cyst.

- A pediatric patient with progressive limb pain and inability to ambulate was found to have an aneurysmal bone cyst of the proximal femur with an associated pathologic fracture. - Imaging progression: normal initial knee/leg radiographs; subsequent hip radiographs identified an ill-defined lytic lesion with features suggestive of pathologic fracture; MRI confirmed a multiloculated expansile lesion with hemorrhagic components. - Management included biopsy, curettage with grafting, and open reduction internal fixation for stabilization, with final pathology confirming ABC. - Literature data points: ABC incidence approximately 0.14 per 100,000 per year; among young children presenting with acute gait disturbance, 96% have normal radiographs and 81% show spontaneous resolution on follow-up; prevalence of asymptomatic benign extremity bone tumors is 18.9% in children.

This case underscores the importance of thorough evaluation of pediatric patients presenting with lower extremity pain and gait disturbance, including attention to referred pain from the hip to the knee. Despite the high likelihood of benign, self-limited causes and often normal initial imaging in young children, focal pain, inability to ambulate, and localized tenderness warrant prompt imaging of the symptomatic region and adjacent joints. Radiographs may identify suspicious osseous lesions requiring urgent specialist referral and further characterization with MRI. Definitive diagnosis of ABC relies on histopathology to differentiate it from malignant mimics such as telangiectatic osteosarcoma. Management integrating curettage with bone grafting and stabilization is effective in cases complicated by pathologic fracture. Early recognition and appropriate referral can prevent delays in treatment and complications.

In children presenting with significant limb pain, focal tenderness, or inability to ambulate, prompt radiographic evaluation and consideration of adjacent joint pathology are essential. When imaging reveals a suspicious lesion, early orthopedic consultation is recommended. This case highlights an aneurysmal bone cyst of the proximal femur presenting with a pathologic fracture and demonstrates successful management with biopsy, curettage, grafting, and internal fixation. Clinicians should maintain a broad differential for gait disturbances in children and apply imaging and referral guidelines to facilitate timely diagnosis and treatment.