Subchondral insufficiency fracture of the femoral head
Overview
- Stress-related fracture of weakened subchondral bone in femoral head
- Typically due to mechanical overload on osteoporotic or osteopenic bone
- Distinct from avascular necrosis (AVN) - insufficiency rather than ischaemic mechanism
- High risk of rapid progression to irreversible collapse if untreated
- Often reversible with conservative management if detected early
Epidemiology
- Predominantly postmenopausal women (>60 years)
- Risk factors: osteoporosis, osteopenia, recent corticosteroid use, rheumatoid arthritis
- Unilateral > bilateral
- Increasing recognition due to widespread hip MRI for pain evaluation
Key radiological manifestations
Radiographs
- Early: often normal or subtle joint space narrowing
- Later:
- Subchondral lucency or sclerosis
- Flattening or deformity of superomedial femoral head
- Progressive collapse → secondary osteoarthritis
- Low sensitivity for early disease
MRI (modality of choice)
- Extensive bone marrow oedema pattern in femoral head ± neck
- T1WI: diffuse low signal
- T2WI/STIR: high signal oedema
- Diagnostic finding: subchondral linear low T1 signal paralleling articular surface (fracture line)
- Usually superomedial weight-bearing zone
- Fracture length >12–14 mm or thickness >4 mm → higher collapse risk
- Post-contrast: variable enhancement of oedematous marrow
- Absent "double-line" sign (unlike AVN)
- Late: crescent sign and flattening similar to AVN stage 3–4
CT
- Useful when MRI contraindicated
- Detects subtle subchondral fracture line or early collapse not visible on radiographs
- Shows cortical disruption or step-off
High-yield MRI signs
Subchondral low T1 line + massive oedema WITHOUT serpiginous double-line or focal segmental AVN pattern → strongly favours SIFFH over AVN.
Differential diagnosis
| Diagnosis | Key differentiators |
|---|---|
| Avascular necrosis (AVN/ONFH) | Double-line sign, segmental wedge-shaped lesion, risk factors (steroids, alcohol, sickle cell) |
| Transient bone marrow oedema syndrome (TBMOS) | Reversible oedema, no subchondral fracture line, often younger patients |
| Osteoarthritis | Joint space loss, osteophytes, subchondral cysts/sclerosis, no acute fracture |
| Rapidly progressive osteoarthritis (RPOA) | Severe joint destruction in <1 year, prominent chondrolysis |
| Septic arthritis | Large effusion, synovitis, systemic signs, periarticular enhancement |
| Labral tear / femoroacetabular impingement | Focal findings, no diffuse oedema or subchondral fracture |
Common pitfall
Early SIFFH frequently misdiagnosed as AVN → delays conservative treatment window. Always scrutinise coronal T1 images for the subchondral fracture line.
Imaging approach
- Initial: weight-bearing AP and lateral hip radiographs
- Persistent pain with normal or equivocal radiographs → MRI without contrast
- Include coronal and sagittal T1 and fluid-sensitive sequences
- Measure fracture line length and thickness for prognostic stratification
- Follow-up MRI at 3–6 months to assess resolution versus progression
- CT rarely required unless planning intervention or MRI unavailable
Pearls
- Superomedial location + osteoporosis + acute or subacute pain → consider SIFFH first
- Fracture line involving >50% of femoral head diameter → near-certain collapse
- Conservative success high if no collapse at presentation (protected weight-bearing 6–12 weeks)
- Progression to collapse can be rapid (weeks to months) → flag urgency in report
- Analogous entity in knee: spontaneous osteonecrosis of the knee (SONK), now reclassified as subchondral insufficiency fracture
Reporting tip
Explicitly state "subchondral insufficiency fracture with extensive marrow oedema; recommend protected weight-bearing and urgent orthopaedic referral to prevent collapse."
