SPONK/SONK | Subchondral Insufficiency Fracture | Medial Femoral Condyle
MRI STAGING (MODIFIED KOSHINO)
Critical Must-Knows
- SPONK is now understood as subchondral insufficiency fracture rather than primary avascular necrosis
- Sudden onset medial knee pain in elderly female without trauma is the classic presentation
- MRI is essential for early diagnosis and lesion size measurement
- Lesions involving less than 40 percent of condylar width often heal with conservative care
- Large lesions or collapse greater than 2 mm usually require surgical intervention
Clinical Pearls
- "Differentiate from secondary osteonecrosis (steroids, alcohol, sickle cell)
- "Weight-bearing AP and Rosenberg views show early flattening of medial condyle
- "Lesion size on MRI (area and percentage of condyle) determines prognosis
- "High tibial osteotomy unloads the medial compartment in younger active patients
Clinical Imaging
Spontaneous Osteonecrosis of the Knee - Typical Appearance
Critical SONK Diagnostic and Decision Points
Demographics and Onset
Elderly female (greater than 60 years). Sudden onset severe medial knee pain without trauma. Night pain common. Differentiate from secondary ON which has risk factors (steroids, alcohol, haemoglobinopathy).
Location Specificity
Medial femoral condyle in greater than 90 percent. Weight-bearing surface. Lateral condyle or tibial plateau involvement suggests alternative diagnosis or secondary ON.
MRI Essential
Early diagnosis before X-ray changes. Quantify lesion size (area, percentage of condyle width, depth). Bone marrow oedema extent predicts healing potential.
Treatment Thresholds
Lesion size drives decision. Small lesions (less than 40 percent width, area less than 5 square centimetres) often heal conservatively. Large lesions or greater than 2 mm collapse require surgery.
Quick Decision Guide - SONK versus Secondary Osteonecrosis
| Feature | SPONK/SONK | Secondary ON | Key Discriminator |
|---|---|---|---|
| Demographics | Elderly female, no risk factors | Any age, steroid/alcohol/sickle history | History of systemic risk factors |
| Onset | Sudden severe pain, no trauma | Insidious or after risk exposure | Temporal relationship to steroids |
| Location | Medial femoral condyle only | Multiple sites, bilateral possible | Single site medial condyle favours SPONK |
| MRI pattern | Focal subchondral fracture line | Diffuse serpiginous lesions | Serpiginous pattern indicates secondary ON |
FRACTURESONK Pathophysiology Cascade
| F | Fatigue microfracture Subchondral bone fails under normal load in osteopenic elderly |
| R | Repetitive loading Daily weight-bearing propagates the insufficiency fracture |
| A | Avascular segment Fracture disrupts local blood supply to subchondral bone |
| C | Collapse progression Articular surface flattens under continued loading |
| T | Tibial subluxation Medial joint line narrowing and varus drift develop |
| U | Unloading required Protected weight bearing or osteotomy to offload |
| R | Repair attempt Fibrocartilage or fibrovascular tissue fills defect |
| E | End-stage OA Secondary degenerative changes if collapse persists |
| F | Fatigue microfracture Subchondral bone fails under normal load in osteopenic elderly | C | Collapse progression Articular surface flattens under continued loading | R | Repair attempt Fibrocartilage or fibrovascular tissue fills defect |
| R | Repetitive loading Daily weight-bearing propagates the insufficiency fracture | T | Tibial subluxation Medial joint line narrowing and varus drift develop | E | End-stage OA Secondary degenerative changes if collapse persists |
| A | Avascular segment Fracture disrupts local blood supply to subchondral bone | U | Unloading required Protected weight bearing or osteotomy to offload |
Hook:Subchondral insufficiency FRACTURE starts the SONK cascade - protect the bone early!
SIZEMRI Lesion Size Criteria
| S | Small (less than 40 percent) Width of medial condyle involved - high healing rate with conservative care |
| I | Intermediate (40-60 percent) Borderline - consider bisphosphonates and close monitoring |
| Z | Zone of oedema Extent of bone marrow oedema on STIR sequences predicts pain and healing time |
| E | Extent of collapse Greater than 2 mm subchondral collapse usually needs surgical intervention |
| S | Small (less than 40 percent) Width of medial condyle involved - high healing rate with conservative care | Z | Zone of oedema Extent of bone marrow oedema on STIR sequences predicts pain and healing time |
| I | Intermediate (40-60 percent) Borderline - consider bisphosphonates and close monitoring | E | Extent of collapse Greater than 2 mm subchondral collapse usually needs surgical intervention |
Hook:Measure SIZE on MRI before deciding conservative versus operative treatment!
STAGETreatment Selection by Stage
| S | Stage 1-2 small lesion Protected weight bearing, bisphosphonate, follow-up MRI at 3 months |
| T | Tibial osteotomy Younger active patient with varus alignment and preserved cartilage laterally |
| A | Arthroplasty (UKA) Older patient, large lesion, collapse, or failed conservative care |
| G | Global assessment Check lateral compartment, patellofemoral joint, patient age and activity |
| E | Expectant monitoring Serial radiographs and clinical review until pain settles or collapse occurs |
| S | Stage 1-2 small lesion Protected weight bearing, bisphosphonate, follow-up MRI at 3 months | G | Global assessment Check lateral compartment, patellofemoral joint, patient age and activity |
| T | Tibial osteotomy Younger active patient with varus alignment and preserved cartilage laterally | E | Expectant monitoring Serial radiographs and clinical review until pain settles or collapse occurs |
| A | Arthroplasty (UKA) Older patient, large lesion, collapse, or failed conservative care |
Hook:STAGE guides whether to protect, unload or replace the knee!
Overview and Epidemiology
Why This Matters
Spontaneous osteonecrosis of the knee (SPONK/SONK) is the most common form of osteonecrosis around the knee and must be distinguished from secondary osteonecrosis which carries systemic risk factors and often multifocal disease. The current understanding is that SONK begins as a subchondral insufficiency fracture in osteopenic bone rather than primary vascular occlusion. Early recognition on MRI before radiographic collapse allows a trial of protected weight bearing that can prevent progression in small lesions. Larger lesions or established collapse require unloading osteotomy or arthroplasty, making accurate lesion sizing and staging essential for surgical planning.
Epidemiology
- Peak age: 60-70 years, female predominance (2-3:1)
- Incidence: Underestimated, many cases labelled as OA or meniscus tear
- Location: Medial femoral condyle in greater than 90 percent of cases
- Bilateral: Less than 5 percent (unlike secondary ON)
Clinical Impact
- Sudden severe pain: Often mistaken for meniscus tear or flare of OA
- Night pain: Distinguishes from mechanical meniscus symptoms
- Rapid progression: Collapse can occur within weeks to months
- Functional loss: Antalgic gait, reduced walking distance, difficulty with stairs
Pathophysiology
Subchondral Insufficiency Fracture Hypothesis
Current evidence supports that SONK is initiated by a subchondral insufficiency fracture in osteopenic bone rather than primary avascular necrosis. Repetitive loading on weakened subchondral trabeculae produces microfracture. The fracture disrupts local blood supply, leading to focal osteonecrosis and subsequent articular collapse. This explains the typical single-site medial condyle location in elderly patients without systemic risk factors. Secondary osteonecrosis, by contrast, involves true vascular occlusion from fat emboli, coagulopathy or steroid-induced adipocyte hypertrophy and is often multifocal.
SONK versus Secondary Osteonecrosis - Key Pathophysiological Differences
| Feature | SPONK/SONK | Secondary ON | Clinical Relevance |
|---|---|---|---|
| Initiating event | Subchondral insufficiency fracture | Vascular occlusion (fat emboli, steroids) | History of risk factors points to secondary ON |
| Bone quality | Osteopenic elderly bone | Normal bone with superimposed insult | DEXA scan useful in SONK workup |
| Distribution | Single focus, medial condyle | Multiple foci, often bilateral | Screen hips and shoulders if secondary suspected |
| MRI appearance | Focal fracture line + oedema | Serpiginous geographic lesions | Pattern recognition avoids misdiagnosis |
Bone Remodelling Failure
Osteopenic subchondral bone fails under physiologic load. Reduced trabecular thickness and connectivity in elderly patients lower the threshold for microfracture. Continued weight bearing propagates the fracture line, isolating a segment of subchondral bone from its blood supply.
Articular Collapse Cascade
Subchondral fracture leads to focal osteonecrosis. Loss of structural support causes flattening of the weight-bearing surface. Joint incongruity accelerates cartilage wear and secondary osteoarthritis. Varus malalignment develops, further increasing medial compartment load.
Classification and Staging
Modified Koshino MRI Staging for SONK
| Stage | Radiographic Findings | MRI Findings | Treatment Implications |
|---|---|---|---|
| Stage 1 | Normal or subtle sclerosis | Bone marrow oedema, possible linear fracture line | Protected weight bearing, high healing potential |
| Stage 2 | Subchondral radiolucency or flattening | Oedema + subchondral fracture line visible | Bisphosphonate trial, close radiographic follow-up |
| Stage 3 | Crescent sign, subchondral collapse | Fragmentation, fluid-filled cleft | Consider high tibial osteotomy or UKA |
| Stage 4 | Secondary OA, joint space loss | Full-thickness cartilage loss, osteophytes | UKA or TKA based on compartment involvement |
MRI staging is performed on coronal and sagittal T2-weighted and STIR sequences. The critical measurements are lesion width as percentage of condylar width, total lesion area, and depth of collapse.
Clinical Assessment
History
- Onset: Sudden severe medial knee pain, often after minor activity or at night
- No trauma: Patient frequently denies any injury
- Night pain: Wakes from sleep, distinguishes from mechanical symptoms
- Progression: Pain may improve slightly then worsen with collapse
- Risk factors: Ask specifically about steroids, alcohol, sickle cell, Gaucher disease
Examination
- Gait: Antalgic, varus thrust may be present if collapse advanced
- Swelling: Mild effusion, medial joint line tenderness
- ROM: Limited by pain, possible flexion contracture if chronic
- Stability: Usually stable unless secondary ligamentous laxity from collapse
- Alignment: Measure mechanical axis, varus deformity common in advanced cases
Differentiating SONK from Meniscus Tear or OA Flare
Key clinical discriminator: Sudden onset without trauma in an elderly patient with night pain should raise suspicion for SONK. Mechanical locking or catching suggests meniscus pathology. Gradual pain increase over months is more typical of primary osteoarthritis. Always obtain MRI in this demographic with unexplained medial knee pain.
Differential Diagnosis of Sudden Medial Knee Pain in Elderly
| Condition | Onset | Night Pain | Key Discriminating Feature |
|---|---|---|---|
| SPONK/SONK | Sudden, no trauma | Prominent | MRI shows subchondral fracture line and oedema |
| Medial meniscus tear | Twisting or squatting | Rare | MRI shows meniscal signal to articular surface |
| Medial OA flare | Gradual over weeks | Mild | Joint space narrowing, osteophytes on X-ray |
| Insufficiency fracture tibia | Sudden after minor fall | Moderate | MRI shows linear fracture in proximal tibia |
| Secondary ON | Variable, often insidious | Variable | Risk factor history, multifocal lesions on MRI |
Investigations
Imaging Protocol for Suspected SONK
Views: AP, Rosenberg (45-degree flexion PA), lateral, skyline
Look for: Subtle flattening of medial femoral condyle, subchondral sclerosis, early joint space narrowing. Normal X-ray does not exclude early SONK.
Clinical correlation: Compare with contralateral knee. Varus alignment greater than 5 degrees increases suspicion.
Sequences: Coronal and sagittal T1, T2, STIR, proton density
Key measurements: Lesion width (percentage of condyle), area in square centimetres, depth of collapse in millimetres, extent of bone marrow oedema.
Additional findings: Rule out meniscus tear, ligament injury, or secondary ON pattern (serpiginous lesions).
Indication: Planning high tibial osteotomy or UKA
Measurement: Mechanical axis deviation, medial proximal tibial angle, lateral distal femoral angle.
Decision impact: Determines whether corrective osteotomy can unload the lesion or whether arthroplasty is required.
MRI Measurement Pearl
Always measure lesion size on the coronal slice of maximum width. Calculate percentage of the medial condyle width. A lesion involving greater than 50 percent of the condyle width or with greater than 2 mm collapse has a high likelihood of progression and should be counselled toward surgical options early. Small lesions (less than 40 percent) with minimal oedema may be observed with protected weight bearing and repeat MRI at 8-12 weeks.
Management Algorithm
Conservative Management (Small Lesions, Early Stage)
Indications: Lesion width less than 40 percent of condyle, collapse less than 2 mm, patient compliant with protected weight bearing.
Protected Weight Bearing Protocol
Touch weight bearing with crutches or walker. Avoid full weight bearing to prevent propagation of subchondral fracture.
Partial weight bearing (50 percent) with gradual increase if pain allows. Repeat radiographs or MRI to assess healing.
Full weight bearing as tolerated once pain subsides and imaging shows resolution of oedema or stabilisation of lesion.
Bisphosphonate Role
Some centres use intravenous zoledronic acid or oral alendronate in early SONK to reduce bone turnover and support healing of the subchondral fracture. Evidence is limited to small series but the rationale is biologically plausible given the insufficiency fracture mechanism. Discuss with patient as off-label use.
Evidence Base
Original description of spontaneous osteonecrosis of the knee
- First description of 40 cases of spontaneous osteonecrosis affecting the medial femoral condyle in elderly patients
- All patients presented with sudden onset knee pain without trauma
- Radiographic progression from normal to subchondral collapse over months was documented
- Histology confirmed osteonecrosis in advanced cases
Subchondral insufficiency fracture as the cause of SONK
- Histological study of 14 resected femoral condyles with SONK showing subchondral fracture as the primary event
- No evidence of primary vascular occlusion in early lesions
- Fracture callus and granulation tissue consistent with reparative response to mechanical failure
- Supported the insufficiency fracture hypothesis over avascular necrosis theory
Unicompartmental arthroplasty for spontaneous osteonecrosis
- Prospective series of 33 medial UKAs performed for SONK with mean 10-year follow-up
- Implant survival 93 percent at 10 years with no revisions for progression of lateral disease
- Oxford Knee Scores comparable to UKA performed for primary osteoarthritis
- Key success factor was intact lateral compartment and ACL at time of surgery
Subchondral insufficiency fracture of the knee: review of current concepts and radiological differential diagnoses
- MRI features distinguish SONK from other causes of bone marrow oedema
- Lesion size and collapse depth on MRI are the main prognostic factors guiding treatment
Exam Viva Scenarios
Use these scenarios to practise clinical reasoning and management decisions
Scenario 1: Early SONK - Conservative Decision
"A 68-year-old woman presents with sudden onset severe medial right knee pain for 3 weeks. No history of trauma or steroid use. Pain wakes her at night. Weight-bearing radiographs are normal. MRI shows a 2.8 square centimetre lesion of the medial femoral condyle with bone marrow oedema but no subchondral collapse. The lesion involves 32 percent of the condylar width. What is your diagnosis and initial management?"
Scenario 2: Large Lesion with Collapse - Surgical Decision
"A 72-year-old active man presents with 4-month history of progressive medial knee pain. He recalls a minor twisting injury 5 months ago. Radiographs show 3 mm collapse of the medial femoral condyle with varus alignment of 7 degrees. MRI demonstrates a 6.2 square centimetre lesion involving 58 percent of the condylar width with a fluid-filled cleft. The lateral compartment and patellofemoral joint are preserved and he has an intact ACL. What surgical options would you discuss?"
Guidelines, Registries & Global Practice
Global Epidemiology
- SONK incidence increasing with ageing population and greater MRI access worldwide
- Medial condyle predominance consistent across all ethnic groups and regions
- Female predominance 2-3:1 in every published series from Europe, Asia, North America
- Under-diagnosis common in low-resource settings where MRI is not readily available
Practice Variation by Resource Setting
- High-resource: Early MRI, lesion size quantification, bisphosphonate protocols, UKA or osteotomy in specialist centres
- Limited-resource: Diagnosis often delayed until radiographic collapse, protected weight bearing with crutches, TKA when advanced
- Universal principle: Lesion size and collapse depth remain the critical decision factors regardless of geography
- Surgery availability: UKA and osteotomy concentrated in high-volume arthroplasty centres globally
Society and Reference Guidance (Side by Side)
| Source | Diagnosis Emphasis | Conservative Care | Surgical Threshold |
|---|---|---|---|
| AAOS / Knee Society (US) | MRI for all unexplained medial knee pain in elderly | Protected weight bearing 6-12 weeks, consider bisphosphonates | Greater than 40 percent lesion or greater than 2 mm collapse |
| BOA / BASK (UK) | Clinical suspicion plus MRI staging | Non-operative first line for small lesions | Individualised, lesion size and patient factors |
| EFORT / European consensus | Differentiate SONK from secondary ON early | Weight-bearing restriction, metabolic workup | Osteotomy in younger, UKA/TKA in older |
| APOA / Asian registries | High incidence in elderly Asian females noted | Similar conservative protocols | UKA popular due to smaller stature and lower demand |
Registry and Evidence Note
There is no dedicated SONK registry. Data are derived from national joint registries (NJR, AJRR, AOANJRR) reporting outcomes of UKA and TKA performed for osteonecrosis, and from small prospective series on conservative care and osteotomy. The evidence base remains level 3-4. Key principle across all guidelines: measure lesion size accurately on MRI and base treatment on percentage of condyle involved and depth of collapse.
Controversies & Areas of Uncertainty
Bisphosphonate efficacy
Small series suggest zoledronic acid or alendronate may accelerate healing of subchondral fractures in SONK, but no high-quality randomised trials exist. Use remains off-label and centre-dependent.
Optimal protected weight-bearing duration
Six to twelve weeks is conventional, but exact duration and degree of unloading lack high-level evidence. Serial MRI at 8-12 weeks is increasingly used to guide progression.
Osteotomy versus UKA in borderline cases
Patients aged 60-70 with medium-sized lesions and mild collapse present a decision dilemma. Some surgeons favour joint preservation with osteotomy, others prefer the reliability of UKA. Shared decision-making with patient activity goals is essential.
Role of core decompression or grafting
Limited evidence supports core decompression or bone grafting for early SONK. Most centres reserve these for rare cases in very young patients or as part of research protocols.
SPONTANEOUS OSTEONECROSIS OF THE KNEE
Clinical summary
Key Demographics and Presentation
- •Elderly female (greater than 60 years), sudden medial knee pain without trauma
- •Night pain is characteristic, distinguishes from meniscus tear or OA flare
- •Greater than 90 percent involve the medial femoral condyle weight-bearing surface
- •No systemic risk factors (steroids, alcohol, sickle) - these suggest secondary ON
MRI Staging and Sizing
- •Stage 1: Normal X-ray, bone marrow oedema only - high healing potential
- •Stage 2-3: Subchondral lucency or collapse - measure width percentage and depth
- •Small lesion (less than 40 percent width, less than 3.5 cm2) - conservative trial
- •Large lesion (greater than 50 percent) or greater than 2 mm collapse - surgical candidate
Differential Diagnosis
- •Secondary ON: multifocal, serpiginous lesions, risk factor history
- •Medial meniscus tear: mechanical symptoms, twisting injury, MRI meniscal signal
- •Insufficiency fracture tibia: linear fracture line in proximal tibia on MRI
- •Primary OA: gradual onset, joint space narrowing and osteophytes on X-ray
Treatment Thresholds
- •Conservative: touch weight bearing 6-12 weeks, consider bisphosphonate
- •High tibial osteotomy: younger active patient, correctable varus, preserved lateral compartment
- •UKA: older patient, large lesion or collapse, intact ACL and lateral cartilage
- •TKA: multicompartment disease, ligament deficiency, inflammatory arthritis
Prognosis and Follow-up
- •Small lesions: greater than 80 percent heal with conservative care
- •Large lesions with collapse: high progression risk, counsel early for surgery
- •UKA survival in SONK: 90-93 percent at 10 years when selection criteria met
- •Serial MRI at 8-12 weeks for conservatively managed patients to detect collapse