= benign cartilaginous growth in medullary cavity; bones preformed in cartilage are affected (NOT skull)

Frequency: 3–17% of biopsied primary bone tumors

• 2^nd most common cartilage-containing tumor!

Etiology: continued growth of residual benign rests of cartilage displaced from the growth plate

Age: 10–30 years; M÷F = 1÷1

Histo: lobules of pure hyaline cartilage

• usually asymptomatic, painless swelling
• pain → suspect pathologic fracture

Location: (usually solitary; multiple = enchondromatosis)

1. in 40% small tubular bones of hand (most frequent tumor here), distal + mid aspects of metacarpals, proximal / middle phalanges
   • Most common benign tumor of the hand!
2. proximal femur, proximal humerus, tibia, radius, ulna, foot, rib (3%)

Site: central within medullary canal + metaphyseal; epiphysis only affected after closure of growth plate

• oval / round area of geographic destruction with lobulated contour + fine marginal line
• chondroid matrix:
  • ground-glass appearance
  • dystrophic calcifications within small cartilage nodules / fragments of lamellar bone:
    • pinhead, flocculent, stippled “rings and arcs” pattern
• bulbous expansion of bone with thinning of cortex in small tubular bones of phalanx, rib, fibula:
  • cortical endosteal scalloping
  • NO cortical breakthrough / periosteal reaction
• Madelung deformity = bowing deformities of limb, discrepant length

MR:

• low- to intermediate SI on T1WI
• high SI on T2WI ← high water content of extracellular matrix
• low-signal intensity matrix calcifications
• normal fat marrow interspersed between cartilage nodules
• peripheral enhancement pattern

CEMR:

• peripheral nodular + septal enhancement ← avascular tumor

Cx:

1. Pathologic fracture → pain
   • may be better characterized on CT
2. Malignant degeneration in long-bone enchondromas in 15–20% (severe new pain in an adult patient, interval growth at imaging, loss of marginal definition, cortical disruption, local periosteal reaction)

DDx:

1. Epidermoid inclusion cyst (phalangeal tuft, history of trauma, more lucent)
2. Unicameral bone cyst (rare in hands, more radiolucent)
3. Giant cell tumor of tendon sheath (commonly erodes bone, soft-tissue mass outside bone)
4. Fibrous dysplasia (rare in hand, mostly polyostotic)
   

   Enchondroma versus Chondrosarcoma in Appendicular Skeleton

   Enchondroma Intramedullary Chondrosarcoma Mean age and sex ratios 40 years; M÷F = 2÷3 50 years; M÷F = 11÷9 Palpable mass 28% 82% Pain 40% (fracture associated) 95% (prolonged + increasing) Lesion location hands, feet axial skeleton Radiographic Features Site diaphysis metaphysis, epiphysis Lesion size <5 cm >5–6 cm Soft-tissue extension (MR) 3% 76% Deep endosteal scalloping >⅔ of cortical thickness 90% Periosteal reaction (X-ray) 3% 47% Cortical destruction (CT) 8% 88% Pathologic fracture (X-ray) 5% 27% Endosteal scalloping >⅔ of length of lesion 79% Cortical remodeling (X-ray) 15% 47% Cortical thickening (X-ray) 17% 47% Matrix mineralization (CT) 100% 94% Small hyperintense T1 foci 65% 35% Bone scintigraphy (↑ uptake c/w anterior iliac crest) 21% 82%

5. Bone infarct
6. Chondrosarcoma (exceedingly rare in phalanges, metacarpals, metatarsals)