Insufficiency fracture

Core concept

Fracture due to normal physiological load on abnormally weakened bone, i.e. a subtype of stress fracture occurring in abnormal bone.

Aetiology / risk factors

Bone is structurally weakened by:

• Generalised:
  • Generalized osteoporosis
  • Osteomalacia, vitamin D deficiency
  • Chronic inflammatory joint disease (e.g. RA, seronegative arthropathies)
  • Renal osteodystrophy
  • Paget disease (late phase, structurally weak bone)
• Local:
  • Post-radiation bone
  • Around prostheses (stress transfer / stress shielding)

Common locations

Classic sites (especially exam fodder):

• Pelvis & sacrum
  • Sacral insufficiency fractures (elderly osteoporotic women)
  • Pubic rami
• Spine
  • Vertebral insufficiency fractures
• Lower limb
  • Tibial plateau (subchondral collapse in osteoporotic knee)
  • Femoral neck and intertrochanteric region
    • Subtrochanteric region (can overlap with bisphosphonate-related stress fractures)
  • Foot & ankle
    • Calcaneus, talus, metatarsals in osteoporotic or neuropathic patients

Clinical features

• Insidious onset pain over affected region
• Often worsens with weight-bearing, improves with rest
• Frequently no recalled fall or accident
• Can mimic:
  • Metastatic disease (pelvis, sacrum, spine)
  • Osteomyelitis (tender long bone)
  • Degenerative spine disease

Imaging features

Radiograph

• Often normal or very subtle early
• Later:
  • Linear sclerosis or subtle lucent line
  • Mild cortical irregularity or depression (subchondral collapse)
  • Periosteal reaction / callus in long bones
• Examples:
  • Sacrum: vertical or oblique fracture lines in sacral ala, increased sclerosis adjacent to SI joints
  • Pubic rami: subtle nondisplaced fractures, sclerosis
  • Tibial plateau: subchondral radiolucent line with overlying depression or sclerosis

CT

• Better depiction of:
  • Cortical breaks and fracture lines
  • Subchondral collapse and articular surface involvement
  • Sclerosis pattern in sacral ala and pubic rami
• Useful to:
  • Confirm occult fractures when XR equivocal
  • Differentiate fracture from lytic metastasis in pelvis/spine (presence of fracture line, lack of aggressive bone destruction)

MRI

• Most sensitive for early insufficiency fractures.
• Typical pattern:
  • Fracture line: low signal on T1 and T2/STIR
  • Surrounding marrow oedema:
• Classic examples:
  • Sacral insufficiency fracture:
    • Bilateral vertical low-signal lines in sacral ala with surrounding oedema
    • Often sparing midline spinous elements
  • Subchondral insufficiency fracture of knee/hip:
    • Curvilinear low-signal subchondral band parallel to articular surface
    • Overlying cartilage may be intact or degenerated

Nuclear medicine (bone scan)

• Intense focal uptake at fracture site(s)
• Sacral insufficiency fractures:
  • Classic “Honda sign” / H-sign: bilateral vertical uptake in sacral ala + horizontal component across sacrum.
  • Can be incomplete or absent; absence doesn’t exclude the diagnosis.

Key differentials by location

Sacrum / pelvis

• Metastatic disease
  • Often more focal, mass-like, may involve anterior sacrum or soft tissue mass.
• Insufficiency fracture
  • Linear pattern, often bilateral, associated sclerosis, fracture line, no soft tissue mass.
• Osteitis condensans ilii
  • Triangular sclerosis on iliac side of SI joint, no fracture line, usually younger multiparous women, often asymptomatic or mild pain.
• Infection
  • More destructive change, soft tissue collections, systemic signs.

Spine (vertebral compression)

• Vertebral insufficiency fractures
  • Preserved posterior wall outline
  • Band-like low signal fracture line with surrounding oedema
  • No or minimal paraspinal / epidural soft tissue mass
• Malignant compression fracture
  • Involves posterior elements, convex posterior border, diffuse replacement of marrow, epidural / paraspinal soft tissue mass.

Long bones

• Fatigue fracture (younger overuse, normal bone)
• Metastasis / primary bone tumour (Pathological fracture)
  • Lytic/destructive lesion, periosteal reaction, soft tissue mass, lack of a simple fracture line pattern.
• Osteomyelitis
  • Cortical destruction, sinus tract, sequestrum; clinical/inflammatory markers help.

Pearls

• Classic case: elderly osteoporotic woman with atraumatic pelvic or low back pain → sacral ± pubic rami insufficiency fractures.
• Honda sign: H-shaped uptake on bone scan = sacral insufficiency fracture.
• MRI: low-signal fracture line with surrounding marrow oedema is key.
• Many insufficiency fractures (especially hip and vertebral) are also osteoporotic fragility fractures → wording matters in guidelines and exams.