Brodie's abscess is a term used to describe a form of subacute osteomyelitis, most frequently developing in the lower extremity. Localized symptoms of pain, swelling and tenderness might be present, while systemic signs are absent. The diagnosis is difficult to make solely on clinical and radiographic criteria, as both benign and malignant bone tumors can have a similar macroscopic appearance.
The insidious and often minimal development of symptoms in Brodie's abscess, an uncommon type of subacute osteomyelitis, frequently leads to a delayed diagnosis   . Defined as a well-demarcated intramedullary lytic abscess that does not cause bone enlargement, Brodie's abscess is principally diagnosed in the population under 25 years of age with a slight predilection toward male gender   . Previous trauma seems to be an important risk factor in the pathogenesis  . Staphylococcus aureus is the main causative agent, but a number of gram-positive and gram-negative bacteria have been described in the literature as well, including Klebsiella pneumoniae, Pseudomonas aeruginosa, and coagulase-negative Staphylococci  . The metaphysis of the lower extremities, particularly the tibia, is the predominant location of this lesion    . The clinical presentation is centered around localized symptoms, such as pain, tenderness, and swelling of the part of the extremity overlying the abscess, while systemic signs (fever, chills, constitutional complaints, etc.) are absent in a vast number of patients   . In addition, pain and tenderness might last for months or even years before the diagnosis is made, which could be attributed to the subacute course of the disease, but also to a missed diagnosis .
Brodie's abscess can be hard to recognize. For this reason, a comprehensive workup is necessary for patients who present with symptoms suggesting a localized disease of the skeletal system, as neoplastic processes (both benign and malignant) are potential diagnoses . Obtaining a detailed patient history is the first step, during which the course and progression of symptoms, as well as a history of recent trauma, should be assessed. After conducting a thorough physical examination and inspection of the affected extremity, imaging studies must be employed to establish an initial diagnosis. Plain radiography is a useful initial method for the evaluation of the skeleton, and typical findings that point toward Brodie's abscess are the presence of a well-defined soft-tissue mass that causes a geographical pattern of bone lysis, marginal sclerosis, and the absence of a periosteal reaction or bone enlargement  . Furthermore, magnetic resonance imaging (MRI) is used for further diagnostic clarification, and the "penumbra sign" (the central part of the abscess shows a low T1 signal intensity and a high T2 signal intensity, whereas the outer layer is always hypointense) is considered to be a highly specific, although not pathognomonic, feature of Brodie's abscess  . In addition to imaging procedures, microbiological testing in the form of blood cultures (or cultivation of the material obtained from the abscess itself) is also recommended, but studies report that between 20-80% of cultures yield negative results   .