CHONDROBLASTOMA
Rare Benign Epiphyseal Cartilage Tumor | Skeletally Immature | Chicken-Wire Calcification
COMMON LOCATIONS
Critical Must-Knows
- Epiphyseal location in skeletally immature patient is pathognomonic
- Chicken-wire calcification pattern on histology is diagnostic hallmark
- ABC formation occurs in 30-40% of cases creating mixed cystic-solid appearance
- Painful symptoms with activity-related pain and joint effusion common
- Curettage with adjuvants (phenol, cryotherapy) is standard treatment with 10-20% recurrence
Examiner's Pearls
- "Think chondroblastoma for any epiphyseal lesion in adolescent with pain
- "Chicken-wire calcification refers to calcified lacunar rim around chondroblasts
- "Giant cells present but not as numerous as giant cell tumor
- "Rare benign metastasis to lungs can occur (not malignant transformation)
Clinical Imaging
Imaging Gallery
Critical Chondroblastoma Exam Points
Epiphyseal Location
Epiphyseal location in open physis patient is the key diagnostic feature. After physeal closure, may extend into metaphysis. Most common sites: proximal humerus, distal femur, proximal tibia, proximal femur.
Histological Hallmark
Chicken-wire calcification pattern from calcified lacunar rims around chondroblasts. Mononuclear chondroblasts with oval nuclei plus osteoclast-type giant cells. S100 positive on immunohistochemistry.
ABC Association
Secondary ABC develops in 30-40% creating expansile cystic appearance. This can obscure underlying chondroblastoma diagnosis. Look for solid component with chicken-wire pattern.
Treatment Principles
Extended curettage with adjuvants is gold standard. Phenol, cryotherapy, or PMMA cementation reduce recurrence from 30% to 10-20%. Wide excision for recurrent or expendable bones (fibular head).
Epiphyseal Lesions: Differential Diagnosis
| Tumor | Age | Location | Key Feature | Giant Cells |
|---|---|---|---|---|
| Chondroblastoma | 10-25y (open physis) | Epiphysis | Chicken-wire calcification | Present (moderate) |
| Giant Cell Tumor | 20-40y (closed physis) | Epiphysis-metaphysis | Soap-bubble, eccentric | Numerous (predominant) |
| Chondrosarcoma (clear cell) | Over 30y | Epiphysis | Clear cytoplasm, low-grade | Absent |
| Infection (epiphyseal abscess) | Any age | Epiphysis | Fever, elevated inflammatory markers | Absent |
At a Glance
Chondroblastoma is a rare benign cartilage tumour (less than 1% of bone tumours) characterised by epiphyseal location in skeletally immature patients (ages 10-25 years). The key differential is that epiphyseal lesion + open physis = chondroblastoma, whereas closed physis suggests giant cell tumour. Most common sites are proximal humerus (20-25%), proximal femur, and around the knee (70% combined). Histological hallmark is "chicken-wire" calcification—calcified lacunar rims around mononuclear chondroblasts, with S100 positivity. Secondary ABC develops in 30-40%, creating mixed cystic-solid appearance. Treatment is extended curettage with adjuvants (phenol, cryotherapy, or PMMA), reducing recurrence from 30% to 10-20%. Rare "benign pulmonary metastases" can occur—these are not malignant transformation and are managed with observation.
EPIPHYSISChondroblastoma Key Features
Memory Hook:Think EPIPHYSIS - this is where chondroblastoma lives! Young patient, pain in epiphysis, chicken-wire on biopsy.
PHACTreatment Adjuvants to Reduce Recurrence
Memory Hook:PHAC the tumor! Use adjuvants to reduce recurrence from 30% down to 10-20%.
CGCHistological Triad of Chondroblastoma
Memory Hook:CGC - Chondroblasts, Giants, Chicken-wire. The classic histological triad!
Overview and Epidemiology
Clinical Significance
Chondroblastoma is a rare benign cartilage tumor accounting for less than 1% of all bone tumors. It is unique in arising from the epiphysis of long bones in skeletally immature patients (peak age 10-25 years). The characteristic histological feature is chicken-wire calcification - a lacy pattern of calcified lacunar rims surrounding chondroblasts. Despite being benign, chondroblastoma causes significant pain and can recur in 10-20% after curettage. Rare cases exhibit benign pulmonary metastasis (not malignant transformation) that may spontaneously regress.
Demographics
- Age: Peak 10-25 years (range 5-30)
- Sex: Male predominance 2:1
- Incidence: Under 1% of all bone tumors
- Skeletal maturity: 90% occur before physeal closure
Location Distribution
- Proximal humerus: 20-25% (most common)
- Distal femur: 20% (second most common)
- Proximal tibia: 15-20%
- Proximal femur: 10-15%
- Other sites: Calcaneus, talus, patella rare
Why Epiphyseal Location Matters
Differential Diagnosis by Skeletal Maturity
Epiphyseal lesion + OPEN physis = Think chondroblastoma (or infection)
Epiphyseal lesion + CLOSED physis = Think giant cell tumor (or clear cell chondrosarcoma)
The status of the growth plate is critical for narrowing differential diagnosis. Chondroblastoma arises when physis is open and may extend into metaphysis after physeal closure. Giant cell tumor typically presents after physeal closure (age 20-40).
Pathophysiology and Pathology
Histogenesis
Chondroblastoma arises from germinative cartilage cells in the epiphysis - primitive chondroblasts that normally form secondary centers of ossification. The exact trigger for neoplastic transformation is unknown. Recent molecular studies have identified recurrent H3F3B K36M mutations in 95% of chondroblastomas, suggesting this is a driver mutation.
Molecular Genetics
- H3F3B K36M mutation: Found in 95% of cases
- Histone H3.3 variant: Affects epigenetic regulation
- Diagnostic utility: Can help distinguish from mimics
- Not hereditary: Somatic mutation, not germline
Natural History
- Growth pattern: Slow progressive enlargement
- Joint involvement: May penetrate articular cartilage (5-10%)
- ABC formation: Secondary ABC develops in 30-40%
- Benign metastasis: Rare pulmonary implants (under 1%)
Gross Pathology
Curettage specimen reveals friable red-gray tissue with gritty consistency due to calcifications. The tumor is well-circumscribed within the epiphysis, sometimes with cystic hemorrhagic areas if secondary ABC is present.
Histology - The Diagnostic Triad
Microscopic Features
Mononuclear round to polygonal cells with distinct cell borders, pink cytoplasm, and oval nuclei with longitudinal nuclear grooves. Nuclei may have coffee-bean appearance. These are the neoplastic cells.
Osteoclast-type multinucleated giant cells scattered throughout. Less numerous than in giant cell tumor. Giant cells are reactive, not neoplastic.
Lacy network of calcified lacunar rims surrounding chondroblasts creating a chicken-wire pattern. This represents dystrophic calcification of pericellular matrix. Diagnostic hallmark on low power.
Cartilaginous matrix may be present focally. Chondroblasts embedded in hyaline or myxochondroid matrix. Calcification common.
Chicken-Wire Pattern Explained
The chicken-wire calcification pattern is created by calcification of the pericellular (lacunar) matrix around individual chondroblasts. On hematoxylin and eosin staining, this appears as a fine lacy network of purple-blue material outlining cells. At low magnification, it resembles chicken wire fencing. This pattern is PATHOGNOMONIC for chondroblastoma and distinguishes it from all other bone tumors including giant cell tumor.
Immunohistochemistry
IHC Profile of Chondroblastoma
| Marker | Staining | Interpretation |
|---|---|---|
| S100 protein | Positive (strong) | Confirms chondroid differentiation |
| DOG1 | Positive | Specific for chondroblastoma |
| H3K36M | Positive (mutation-specific antibody) | Highly specific, aids diagnosis |
| Cytokeratin | Negative | Excludes epithelial tumors |
Secondary Aneurysmal Bone Cyst
30-40% of chondroblastomas develop secondary ABC component, creating blood-filled cystic spaces within the tumor. This can:
- Obscure the underlying chondroblastoma on imaging (appears purely cystic)
- Lead to rapid expansion and pathological fracture
- Make biopsy diagnosis challenging if solid component not sampled
Biopsy Sampling Error
When biopsying an epiphyseal lesion with ABC features, ensure multiple samples from solid areas are obtained. Sampling only the cystic/ABC component will miss the underlying chondroblastoma diagnosis. Review imaging to target solid nodules for biopsy needle placement.
Classification
WHO Classification (2020)
Chondroblastoma:
- Category: Benign cartilage tumor
- Behavior: Locally aggressive (intermediate)
- ICD-O Code: 9230/0
Key characteristics:
- Epiphyseal location (distinguishing feature)
- Contains mononuclear chondroblasts
- "Chicken-wire" calcification pattern
- H3K36M mutation (diagnostic marker)
Enneking Staging
Stage 1 (Latent):
- Intracapsular, well-demarcated
- Minimal symptoms, incidental finding
- Observation may be appropriate
Stage 2 (Active):
- Most common presentation
- Symptomatic, progressive growth
- Contained by natural barriers
- Curettage with adjuvant therapy
Stage 3 (Aggressive):
- Breaks through cortex or physis
- Extracompartmental extension
- May require en bloc resection
Key Distinguishing Feature
Chondroblastoma is the only benign cartilage tumor that arises in the epiphysis. This epiphyseal location with open physes in a young patient is virtually pathognomonic. Other cartilage tumors (enchondroma, osteochondroma) arise in metaphysis or diaphysis.
Clinical Presentation
Symptoms
- Pain: Dull aching pain, worse with activity (90%)
- Duration: Chronic pain over months to years
- Joint effusion: Reactive synovitis in adjacent joint (40%)
- Stiffness: Limited range of motion from pain and effusion
- Limp: If lower extremity involvement
Physical Examination
- Tenderness: Localized to epiphyseal region
- Effusion: Joint swelling from reactive synovitis
- ROM: Restricted by pain, not mechanical block
- Muscle atrophy: Quadriceps wasting if chronic knee pain
- Warmth: May be warm but not erythematous
Typical Clinical Scenarios
Common Presentations by Location
Adolescent athlete with shoulder pain worse with overhead activity. Limited abduction and forward flexion. May mimic rotator cuff tendinitis or impingement.
Adolescent with anterior knee pain and effusion. Pain with running or jumping. May be misdiagnosed as patellofemoral pain syndrome or Osgood-Schlatter disease.
Adolescent with knee pain localized to proximal tibia. Effusion common. May mimic meniscal tear or tibial spine injury.
Child or adolescent with hip pain and limp. Pain with internal rotation. May be confused with slipped capital femoral epiphysis (SCFE) or Perthes disease.
Clinical Red Flags
Think chondroblastoma when:
- Adolescent (10-25 years) with chronic epiphyseal pain
- Pain worse with activity, not relieved with rest
- Joint effusion out of proportion to trauma history
- X-ray shows epiphyseal lytic lesion with sclerotic rim
- MRI shows bone marrow edema extending beyond lesion
Do NOT dismiss as growing pains or overuse injury - persistent epiphyseal pain in adolescent warrants imaging.
Rare Presentations
Joint Penetration
In 5-10% of cases, tumor penetrates articular cartilage into joint space. Presents with mechanical symptoms, locking, catching. May mimic loose body or meniscal tear. Arthroscopy reveals cartilage defect with tumor tissue.
Benign Pulmonary Metastasis
Rare phenomenon (under 1%) where tumor cells implant in lungs without malignant transformation. Usually asymptomatic, discovered on chest imaging. May spontaneously regress. Not true malignancy - termed benign metastasizing chondroblastoma.
Investigations and Imaging
Imaging Gallery
Plain Radiography
X-rays are the first-line investigation and often suggest the diagnosis based on epiphyseal location and appearance.
Radiographic Features
Epiphyseal lesion in patient with open or recently closed physis. This location is diagnostic in the right age group. After physeal closure, tumor may extend into metaphysis.
Eccentric lytic lesion with well-defined geographic borders. Thin sclerotic rim common (50-70%). May have stippled calcifications within lesion (chicken-wire pattern not visible on X-ray).
Typically 2-5cm diameter. Smaller lesions may be entirely within epiphysis. Larger lesions expand epiphysis and may cross into metaphysis.
Usually absent unless secondary ABC causes expansion or pathological fracture. Solid periosteal reaction suggests aggressive behavior (rare malignant transformation).
Radiographic Pearl
Sclerotic rim is present in 50-70% of chondroblastomas and represents reactive bone formation. This distinguishes chondroblastoma from infection (no rim) and is less prominent than in chondroblastoma (thinner rim). If rim is very thick and irregular, consider clear cell chondrosarcoma in older patient.
MRI - Gold Standard for Extent Assessment
MRI is mandatory for surgical planning to assess:
- Extent of marrow involvement
- Cartilage penetration into joint
- Secondary ABC component
- Relationship to physis and neurovascular structures
MRI Characteristics
- T1: Low to intermediate signal (solid component)
- T2: High signal (cartilage) + very high signal (ABC cysts)
- STIR: Extensive bone marrow edema (extends beyond tumor)
- Enhancement: Solid portions enhance; cystic ABC areas have fluid-fluid levels
Key MRI Findings
- Epiphyseal centered: Confirms location
- Bone marrow edema: May extend throughout epiphysis and metaphysis (exaggerates true tumor size)
- Joint effusion: Reactive synovitis common
- ABC component: Fluid-fluid levels if secondary ABC present
MRI Overestimation of Tumor Size
Extensive bone marrow edema on MRI STIR sequences extends far beyond the actual tumor margins. This reactive edema is inflammatory response and not tumor infiltration. True tumor size is better assessed on T1-weighted images where only the solid lesion shows low signal. Do NOT mistake edema for tumor when planning surgery - curettage only the lytic cavity, not entire edema zone.
CT Scan
CT is useful for:
- Detecting calcifications within tumor (chicken-wire pattern as subtle stippled density)
- Assessing cortical breakthrough if ABC component causes expansion
- Preoperative planning for cortical window placement
Biopsy
Biopsy is required for definitive diagnosis before surgery. Open biopsy preferred over needle biopsy due to:
- Need for adequate tissue (avoid sampling only ABC component)
- Small epiphyseal lesions difficult to target with needle
- Risk of pathological fracture through biopsy tract
Biopsy Technique Considerations
| Approach | Indication | Advantage | Risk |
|---|---|---|---|
| CT-guided core needle | Large accessible lesion | Minimally invasive, outpatient | Sampling error if ABC component targeted |
| Open biopsy | Small epiphyseal lesion | Adequate tissue, direct visualization | Requires OR, pathological fracture risk |
| Excisional biopsy | Expendable bone (fibular head) | Diagnostic and therapeutic in one procedure | Not feasible for most locations |
Differential Diagnosis
Epiphyseal and Apophyseal Lesions: Key Differentiators
| Lesion | Age | Physis Status | Histology | Giant Cells |
|---|---|---|---|---|
| Chondroblastoma | 10-25y | Open | Chicken-wire calcification | Moderate |
| Giant Cell Tumor | 20-40y | Closed | Sheets of giant cells | Numerous |
| Clear Cell Chondrosarcoma | Over 30y | Closed | Clear cytoplasm, low-grade | Absent |
| Infection (epiphyseal abscess) | Any | Open or closed | Inflammatory cells, organisms | Absent |
| Langerhans Cell Histiocytosis | Under 20y | Usually metaphysis | Langerhans cells, eosinophils | Absent |
Chondroblastoma vs Giant Cell Tumor
Both are epiphyseal lesions with giant cells, but key differences:
Chondroblastoma:
- Age 10-25y (open physis)
- Chicken-wire calcification
- S100 positive
- Giant cells moderate in number
- 10-20% recurrence
Giant Cell Tumor:
- Age 20-40y (closed physis)
- No calcification pattern
- S100 negative
- Giant cells numerous (sheets)
- 20-50% recurrence, locally aggressive
Exam answer: "While both are epiphyseal lesions with giant cells, chondroblastoma occurs in younger patients before physeal closure and has characteristic chicken-wire calcification. Giant cell tumor presents after physeal closure with sheets of giant cells and no specific calcification pattern. S100 immunostain is positive in chondroblastoma, negative in GCT."
Management Algorithm
Extended Curettage with Adjuvants
Gold standard treatment for chondroblastoma. Goal is to remove tumor while preserving joint and growth plate.
Surgical Technique
- Review MRI to identify true tumor margins (not edema)
- Plan cortical window away from articular surface
- Protect adjacent physis if still open
- Identify neurovascular structures at risk
- Create cortical window (1-2cm) using drill and osteotome
- Position window to access all tumor while preserving subchondral bone
- Save bone window for later replacement if possible
- Thorough curettage with sharp curettes of all sizes
- Remove all gross tumor tissue and membrane
- Extend to subchondral bone but do NOT penetrate articular cartilage
- Inspect cavity walls under direct visualization
- High-speed burr: Remove additional 1-2mm of cavity wall
- Phenol: Apply 5% phenol-soaked gauze for 2 minutes, then copious saline lavage
- OR Cryotherapy: Liquid nitrogen freeze-thaw cycles (2-3 cycles)
- OR Argon beam: Thermal ablation of cavity surface
- Bone grafting: Morselized allograft or autograft
- OR PMMA cementation: Methylmethacrylate (generates heat, kills cells)
- Fill cavity completely to restore structural integrity
- Replace cortical window or use cement as bone substitute
Why Adjuvants Reduce Recurrence
Extended curettage with adjuvants reduces recurrence from 30% (simple curettage) to 10-20%. Mechanisms:
- High-speed burr: Mechanically removes microscopic tumor on cavity wall
- Phenol: Chemical cauterization kills surface cells (penetrates 1-2mm)
- Cryotherapy: Freeze-thaw cycles rupture cell membranes
- PMMA: Exothermic polymerization generates heat (60-80°C) killing cells
Exam answer: "I would perform extended curettage with adjuvant treatment to reduce recurrence. After thorough curettage with curettes, I would use high-speed burr to remove an additional 1-2mm of cavity wall, then apply phenol for 2 minutes followed by copious saline lavage. This reduces recurrence from 30% to approximately 10-20%."
Protecting the Physis
In patients with open growth plate, avoid damage to physis during curettage:
- Stay within epiphysis, do NOT cross physis into metaphysis
- Use imaging guidance or fluoroscopy to confirm margins
- If tumor extends across physis (rare), accept some residual tumor to preserve growth
- Growth arrest from physeal damage causes limb length discrepancy
In proximal femur chondroblastoma, risk of avascular necrosis if disrupting blood supply. Consider wide excision with femoral head replacement in older adolescent near skeletal maturity.
This completes the curettage technique discussion.
Surgical Technique
Extended Curettage Technique
Standard approach:
- Cortical window: Create adequate access (larger than lesion diameter)
- Intralesional curettage: Remove all visible tumor with curettes
- High-speed burr: Extend margins 1-2 mm into normal bone
- Adjuvant therapy: Apply local adjuvant
- Bone void filling: Bone graft or cement
Adjuvant options:
- Phenol (89% pure solution, 3 min)
- Cryotherapy (liquid nitrogen)
- High-speed burr (mechanical)
- Argon beam coagulation
Bone Void Management
Options for filling defect:
Bone cement (PMMA):
- Mechanical support
- Allows early weight-bearing
- Exothermic reaction adds adjuvant effect
- Easier surveillance for recurrence
Bone graft:
- Autograft or allograft
- Biological reconstruction
- Preferred in young patients
- May obscure recurrence on imaging
Extended Curettage Reduces Recurrence
Extended curettage with adjuvant therapy reduces recurrence from 30% (simple curettage) to 10-15%. The combination of mechanical extension (high-speed burr) plus local adjuvant (phenol or cryotherapy) plus bone cement provides the best local control while preserving joint function and physeal integrity.
Complications
Treatment-Related Complications
Surgical Complications After Curettage
| Complication | Incidence | Risk Factors | Management |
|---|---|---|---|
| Recurrence | 10-20% (with adjuvants), 30% (without) | Incomplete curettage, no adjuvant, ABC component | Repeat curettage or wide excision for multiple recurrences |
| Pathological fracture | 2-5% | Large subchondral defect, early weight-bearing | Non-weight-bearing, internal fixation if displaced |
| Articular cartilage damage | 5-10% | Subchondral curettage too aggressive | Cartilage repair, accept degenerative arthritis risk |
| Growth disturbance | Under 5% | Physeal injury during curettage in young patient | Limb length monitoring, epiphysiodesis or lengthening if needed |
| Joint stiffness | 10-15% | Prolonged immobilization, adhesions | Aggressive physiotherapy, manipulation under anesthesia |
| Infection | 2-3% | Standard surgical site infection risk | Antibiotics, debridement if deep infection |
Managing Recurrence
Recurrence after curettage occurs in 10-20% despite adjuvants. Typical timeline: 1-3 years post-op. Management:
- Confirm recurrence with MRI and biopsy (distinguish from bone graft resorption)
- First recurrence: Repeat extended curettage with adjuvants - success rate 70-80%
- Second recurrence: Consider wide excision if feasible (expendable bone)
- Multiple recurrences: En bloc resection or accept disease and manage symptoms
Key point: Multiple recurrences do NOT indicate malignant transformation. Chondroblastoma is benign but can be locally persistent. Avoid radiation therapy due to malignant transformation risk.
Postoperative Care
Immediate Postoperative Care
Hospital stay: Usually 1-2 days
Pain management:
- Multimodal analgesia
- Transition to oral medications day 1
- Avoid NSAIDs initially (theoretical bone healing concern)
Wound care:
- Sutures/staples removed 10-14 days
- Keep wound dry until healed
- Watch for hematoma, drainage
Weight-Bearing and Activity
Weight-bearing protocols:
- Touch weight-bearing for 4-6 weeks (lower extremity)
- Progressive weight-bearing based on bone healing
- Full weight-bearing by 8-12 weeks typically
Activity restrictions:
- Upper extremity: sling 2-4 weeks, then ROM exercises
- Avoid impact activities for 3-6 months
- Return to sports when imaging confirms healing
Surveillance for Recurrence
Recurrence typically occurs within 2 years. Follow-up schedule: radiographs every 3 months for year 1, every 6 months for year 2, then annually for 5 years. CT or MRI if plain films suspicious. Most recurrences present with return of pain before imaging changes.
Outcomes and Prognosis
Long-Term Prognosis
Excellent overall prognosis for chondroblastoma. With appropriate treatment:
- 80-90% cured with single curettage and adjuvants
- Recurrences manageable with repeat curettage or excision
- Functional outcomes excellent in most cases
- Risk of degenerative arthritis low (under 10%) if articular surface preserved
- Malignant transformation extremely rare (under 1%)
Key Prognostic Messages
For patients and families:
- Chondroblastoma is benign - not cancer
- Surgery is curative in 80-90% with single procedure
- Even if recurrence occurs, repeat surgery is effective
- Long-term function and return to sports expected
- Lifetime risk of arthritis is low if joint surface protected
- No need for lifelong surveillance after 5 years disease-free
Evidence Base and Key Studies
Natural History and Treatment Outcomes
- Retrospective review of 76 chondroblastomas treated with curettage
- Recurrence rate 21% with simple curettage, 12% with adjuvant use
- Proximal femur location had highest recurrence (35%)
- Functional outcomes excellent in 88% at 5-year follow-up
- No cases of malignant transformation observed
Histopathological Features and Diagnostic Criteria
- Classic description of chondroblastoma histology from 44 cases
- Chicken-wire calcification pattern described as pathognomonic
- Giant cells present in all cases but variable in number
- S100 positivity confirms chondroid differentiation
- Secondary ABC formation in 30% of cases
H3F3B K36M Mutation in Chondroblastoma
- H3F3B K36M mutation identified in 95% of chondroblastomas
- Mutation specific for chondroblastoma (not in GCT or chondrosarcoma)
- Affects histone H3.3 methylation and epigenetic regulation
- Can be detected by immunohistochemistry (H3K36M antibody)
- Useful diagnostic tool for difficult cases
Adjuvant Therapies to Reduce Recurrence
- Comparison of curettage alone vs curettage with adjuvants in 50 cases
- Recurrence 28% with curettage alone vs 10% with phenol adjuvant
- High-speed burr plus phenol further reduced recurrence to 8%
- PMMA cementation had lowest recurrence (5%) but thermal necrosis risk
- No difference in functional outcomes between adjuvant types
MCQ Practice Points
Exam Pearl
Q: What is the classic location and age distribution for chondroblastoma?
A: Chondroblastoma is an epiphyseal tumor (arises in secondary ossification center) occurring in skeletally immature patients (10-25 years, M greater than F). Most common locations: Proximal humerus, proximal tibia, distal femur, proximal femur. The epiphyseal location in a young patient with open/recently closed physes is pathognomonic. This contrasts with giant cell tumor (metaphyseal/epiphyseal in skeletally mature patients).
Exam Pearl
Q: What are the characteristic imaging features of chondroblastoma?
A: Radiographs: Well-defined, geographic lytic lesion in the epiphysis; Sclerotic rim (narrow zone of transition); Internal matrix calcification (40-60%, "chicken-wire" pattern); Size usually 3-6 cm. MRI: Low-intermediate signal T1, heterogeneous T2; Marked surrounding bone marrow edema (disproportionate to lesion size - classic feature); May extend across physis. CT best demonstrates matrix calcification.
Exam Pearl
Q: What is the histological hallmark of chondroblastoma?
A: Chondroblasts: Round/polygonal cells with well-defined cytoplasmic borders, grooved or "coffee bean" nuclei. Chicken-wire calcification: Fine calcification surrounding individual cells (pericellular). Chondroid matrix: Immature cartilaginous matrix between cells. Giant cells: Scattered osteoclast-like multinucleated giant cells present but not as prominent as in GCT. Immunohistochemistry: S-100 positive, SOX9 positive; H3K36M mutation present in 95% of cases.
Exam Pearl
Q: What is the standard treatment for chondroblastoma?
A: Standard treatment is intralesional curettage with extended techniques (high-speed burr, electrocautery, phenol or hydrogen peroxide) followed by bone grafting or cement. Local recurrence rate: 10-20% (higher in skeletally immature patients with open physes). Cryotherapy should be used cautiously near physis/articular cartilage. Radiofrequency ablation is an option for small lesions. Rare malignant transformation or pulmonary metastases can occur (1-2%).
Exam Pearl
Q: How do you differentiate chondroblastoma from other epiphyseal lesions?
A: Chondroblastoma: Young patient (open physes), epiphyseal, sclerotic margin, matrix calcification, extensive edema. Giant cell tumor (GCT): Skeletally mature patient, extends from metaphysis into epiphysis, no sclerotic margin, no matrix, less edema. Clear cell chondrosarcoma: Older patient (30-60 years), proximal femur common, may have aggressive features. Langerhans cell histiocytosis: Younger children, may be epiphyseal but often diaphyseal, no matrix. Infection (Brodie abscess): Metaphyseal location more common, periosteal reaction.
Australian Context
Tertiary Referral Centers
- Peter MacCallum Cancer Centre (Victoria) for complex cases requiring wide excision
- Royal Children's Hospital (Victoria) for pediatric chondroblastoma management
- Queensland Children's Hospital (Queensland) pediatric bone tumor unit
- Sydney Children's Hospital (NSW) musculoskeletal tumor service
Pathology Services
- Victorian Bone Pathology (Victoria) for expert MSK pathology review
- SA Pathology (SA) bone tumor panel with H3K36M testing
- NSW Health Pathology for immunohistochemistry
- Second opinion pathology recommended for all epiphyseal lesions
Medicolegal Considerations
Documentation for epiphyseal lesions in adolescents:
- Record growth plate status (open vs closed) on imaging reports
- Document differential diagnosis includes chondroblastoma, GCT, infection
- Biopsy required before surgery - never treat epiphyseal lesion without histology
- Informed consent: recurrence risk 10-20%, growth plate injury risk, arthritis risk
- Surgical plan must protect physis if still open
Consent discussion for curettage:
- Risk of recurrence (10-20%) requiring repeat surgery
- Risk of growth disturbance if physis damaged (under 5%)
- Risk of arthritis from articular surface damage (5-10%)
- Phenol risks (skin burn if leakage)
- Alternative of wide excision for recurrent cases (only if expendable bone)
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
"How do you differentiate chondroblastoma from giant cell tumor?"
"Describe your surgical technique for extended curettage of a chondroblastoma in the proximal tibia of a 16-year-old."
"What is your follow-up protocol after curettage of a chondroblastoma, and how do you detect recurrence?"
CHONDROBLASTOMA
High-Yield Exam Summary
Key Facts
- •Rare benign epiphyseal cartilage tumor (under 1% of bone tumors)
- •Peak age 10-25 years (open physis), male 2:1
- •Most common sites: proximal humerus (20-25%), around knee (70% combined)
- •Epiphyseal location before physeal closure is pathognomonic
Histology Triad (CGC)
- •Chondroblasts - mononuclear cells with oval grooved nuclei
- •Giant cells - osteoclast-type, fewer than in GCT
- •Chicken-wire calcification - calcified lacunar rims (pathognomonic)
- •S100 positive, H3K36M mutation in 95%
Clinical Presentation
- •Activity-related pain for months, joint effusion common
- •Epiphyseal tenderness, restricted ROM from pain
- •X-ray: eccentric lytic with thin sclerotic rim
- •MRI: extensive marrow edema (overestimates tumor size)
Differential Diagnosis
- •Giant cell tumor: age 20-40y, closed physis, sheets of giant cells
- •Clear cell chondrosarcoma: age over 30y, malignant, no chicken-wire
- •Infection: fever, elevated CRP/ESR, no calcification pattern
- •Chondromyxoid fibroma: metaphyseal, lobulated architecture
Treatment
- •Extended curettage with adjuvants (gold standard)
- •Adjuvants: phenol, high-speed burr, cryotherapy, or PMMA
- •Reduce recurrence from 30% to 10-20%
- •Wide excision for recurrent or expendable bones (fibular head)
Complications and Outcomes
- •Recurrence 10-20% with adjuvants (1-3 years post-op)
- •Secondary ABC in 30-40% (makes curettage more difficult)
- •Malignant transformation extremely rare (under 1%)
- •Benign pulmonary metastasis (under 1%) - observe, often regresses
Surgical Pearls
- •Protect physis in young patients (avoid growth arrest)
- •Preserve subchondral bone (prevent articular collapse)
- •Biopsy solid component if ABC present (avoid sampling error)
- •PMMA for large subchondral defects (structural support plus thermal kill)