15.1 DXI Hematological and Vascular Bone Disease

Why This Topic Matters on DXI

Diagnostic Imaging (DXI) items on NBCE Part III test whether you can recognize disease patterns on radiographs and correlate them with pathophysiology, not simply memorize picture captions. Hematologic and vascular bone disorders are a recurring high-yield cluster because they produce reproducible, testable radiographic signs: crescent signs, serpiginous calcified rims, H-shaped vertebrae, and epiphyseal overgrowth. This section builds the diagnostic reasoning chain from vascular insult to bone death to the imaging findings you will be asked to identify.

Avascular Necrosis (Osteonecrosis): The Core Mechanism

Avascular necrosis (AVN), also called osteonecrosis or ischemic necrosis, is death of bone and marrow elements from interruption of blood supply. The mechanism matters for boards because every downstream imaging finding follows logically from it: infarcted trabecular bone cannot remodel or repair itself, so the dead segment retains its original density (or becomes relatively denser as surrounding living bone becomes osteopenic from disuse and hyperemia), while the articular surface eventually collapses because dead subchondral bone cannot support mechanical load.

Common causes tested on DXI include corticosteroid use, alcoholism, trauma (femoral neck fracture, hip dislocation), sickle cell disease, Gaucher disease, systemic lupus erythematosus, dysbarism (caisson disease), radiation, and idiopathic cases. The femoral head is the single most commonly affected and most frequently tested site because its retinacular blood supply is an end-arterial system with minimal collateral flow.

Ficat-Arlet Staging: The Imaging Progression Every Candidate Must Know

The crescent sign — a thin subchondral radiolucent line paralleling the articular surface — represents a subchondral insufficiency fracture and is the classic radiographic marker separating reversible (Stage II) from structurally compromised (Stage III) disease. On MRI, the double-line sign on T2-weighted images (an inner high-signal line of granulation tissue bordered by an outer low-signal line of sclerotic reactive bone) is pathognomonic for osteonecrosis and can be present before any plain-film abnormality — MRI is the most sensitive modality for early AVN, often positive when radiographs remain normal (Stage I).

High-Yield AVN Sites and Eponyms

Sickle Cell Disease: A Systemic Vaso-occlusive Model

Sickle cell disease deserves its own emphasis because it produces nearly every finding tested under "hematologic bone disease" through a single mechanism: chronic vaso-occlusion from sickled erythrocytes plus compensatory marrow hyperplasia.

Marrow hyperplasia findings (from chronic hemolytic anemia driving red marrow expansion):

• Widened diploic space of the skull with thinned outer table
• "Hair-on-end" appearance of the calvarium (radiating trabecular striations, shared with thalassemia)
• Generalized osteopenia with coarsened, accentuated trabeculae

Vaso-occlusive/infarction findings:

• "H-shaped" or "fish" vertebrae — central endplate depression from infarction of the vertically-oriented central end-arteries that supply the vertebral body endplates (the peripheral endplate has a separate collateral supply and is spared), producing a step-like biconcave deformity distinct from the smooth biconcavity of osteoporosis
• Bone infarcts in the diaphyses and metaphyses of long bones, seen as serpiginous, sclerotic-margined lucent areas
• Dactylitis ("hand-foot syndrome") — painful symmetric swelling of the hands and feet in infants/toddlers from infarction of the small tubular bone marrow, sometimes with periosteal reaction
• AVN of the femoral and humeral heads, often bilateral and occurring at a younger age than idiopathic AVN
• Growth plate ischemia producing "Lincoln log" or step-off vertebrae and limb-length discrepancies

Sickle cell patients also carry an elevated risk of Salmonella osteomyelitis (versus Staphylococcus aureus in the general population) — a fact frequently paired with imaging questions in this cluster. Distinguishing an infarct from osteomyelitis on plain film alone can be extremely difficult; MRI and clinical correlation (fever, elevated inflammatory markers) are usually required.

Hemophilic Arthropathy

Recurrent hemarthrosis in hemophilia A (factor VIII deficiency) or B (factor IX deficiency) triggers a cycle of synovial iron deposition, synovial hypertrophy, and chondrocyte-mediated cartilage destruction that produces a characteristic arthropathy most often in the knee, elbow, and ankle.

Radiographic findings:

• Joint effusion / hemarthrosis (earliest finding)
• Epiphyseal overgrowth — hyperemic synovium accelerates local epiphyseal growth, producing a "ballooned" epiphysis larger than expected for age
• Squaring of the patella
• Widening of the intercondylar notch of the femur (a highly specific sign)
• Juxta-articular osteoporosis
• Subchondral cysts and joint space narrowing in later stages
• Ankylosis in end-stage disease

The Arnold-Hilgartner classification (Stages I-V) tracks this progression from soft-tissue swelling only (Stage I) through epiphyseal overgrowth (Stage II), early joint space narrowing and subchondral irregularity (Stage III), cartilage destruction with joint space loss (Stage IV), to complete joint disorganization/fibrous ankylosis (Stage V). Because hemophilic arthropathy predominantly affects children and young men and produces epiphyseal overgrowth plus joint space narrowing, it is a classic distractor pairing against juvenile idiopathic arthritis on exam vignettes — the clinical history of a bleeding disorder is the differentiator.

Bone Infarct Imaging Pattern

A bone infarct (medullary osteonecrosis, distinct from subchondral/epiphyseal AVN) presents as a serpiginous, well-defined lucent-to-sclerotic lesion in the metaphysis or diaphysis, often described as resembling "smoke rising from a chimney" or having a calcified "rind." Common causes beyond sickle cell include long-term corticosteroid therapy, alcoholism, Gaucher disease, pancreatitis, and dysbarism/caisson disease in divers and aviators. Unlike epiphyseal AVN, medullary infarcts rarely progress to joint collapse because they do not directly involve the weight-bearing subchondral surface, but they remain an important differential for a lytic-appearing lesion that should not be mistaken for a bone tumor — the serpentine, well-marginated sclerotic border is the key distinguishing feature from an aggressive lesion.