Differential Diagnosis for a 12-year-old with Left Thigh Pain and Inability to Bear Weight

Given the presentation of a 12-year-old with left thigh pain, inability to bear weight, and an X-ray showing a femur shaft fracture, the differential diagnosis can be organized into the following categories:

• Single Most Likely Diagnosis

  • Trauma: The most straightforward explanation for a femur shaft fracture in a child is trauma, which could be due to a fall, sports injury, or any significant impact on the leg. The fracture itself is a direct consequence of the force applied to the bone exceeding its strength.

• Other Likely Diagnoses

  • Pathological Fracture: This occurs when a bone breaks due to a pre-existing condition that weakened the bone structure, such as osteogenesis imperfecta, bone cysts, or tumors. The presence of a fracture with minimal trauma or a fracture in an unusual location could suggest a pathological cause.
  • Low Vitamin D Deficiency (Rickets): While less common in areas with abundant sunlight and fortified foods, vitamin D deficiency can lead to softening of the bones (osteomalacia in adults, rickets in children), increasing the risk of fractures.
  • Carbonic Anhydrase II Deficiency: A rare genetic disorder that affects the metabolism of bone, leading to osteopetrosis (an increase in bone density) and renal tubular acidosis. It could potentially increase the risk of fractures due to altered bone structure.

• Do Not Miss Diagnoses

  • Child Abuse: In any case of unexplained injury, especially fractures in non-ambulatory children or fractures with inconsistent histories, child abuse must be considered. It's crucial to investigate the circumstances surrounding the injury.
  • Bone Tumors: Although rare, primary bone tumors (such as osteosarcoma or Ewing's sarcoma) can weaken bones and lead to pathological fractures. Early diagnosis is critical for effective treatment.
  • Osteogenesis Imperfecta: A genetic disorder characterized by fragile bones, often due to defects in the genes that codify for type I collagen. It can lead to multiple fractures with minimal trauma.

• Rare Diagnoses

  • Idiopathic Juvenile Osteoporosis: A rare condition of unknown cause that leads to bone fragility and fractures in children, without any identifiable underlying disease.
  • Fibrous Dysplasia: A condition where normal bone and marrow are replaced with fibrous tissue, leading to weakened bones and potential for fractures.

Each of these diagnoses requires careful consideration of the patient's history, physical examination, and additional diagnostic tests as needed. The presence of a femur shaft fracture in a child warrants a thorough investigation to determine the underlying cause and to provide appropriate management.