Chondroblastoma

Chondroblastoma is a rare benign cartilage producing tumour that arises in the epiphysis or apophysis of a skeletally immature bone. It makes up under 1% of bone tumours, and about 5% of benign bone tumours.

It occurs mostly between 10 and 25 years of age, with a male to female ratio of about 2 to 1. Common sites are the proximal humerus, distal and proximal femur (including the greater trochanter), proximal tibia and the bones of the foot (talus, calcaneus). The temporal bone and the pelvis are less often affected.

Complications are local pain and joint effusion, joint stiffness, muscle atrophy, pathological fracture, growth disturbance, secondary aneurysmal bone cyst change (in about 20%), recurrence, and rare lung metastases (under 1%) that are often slow growing and treated by resection.

Pathophysiology.

More than 90% of tumours carry an H3F3B mutation (lysine 36 to methionine, H3K36M), which alters histone methylation, and this can be detected by immunohistochemistry. The tumour consists of chondroblasts with grooved (coffee bean) nuclei, eosinophilic chondroid matrix, giant cells, and delicate pericellular calcification ("chicken wire"). Growth near the physis may disturb bone growth, and epiphyseal location explains the joint symptoms.

Pain near a joint for months (usually mild and aching, progressive), joint swelling or effusion, tenderness, restricted motion, a limp, and muscle atrophy. Pain may be referred (hip lesions can present with knee pain). Pathological fracture is uncommon. Constitutional symptoms are absent.

Examine the joint line, effusion, range, muscle bulk, gait and leg length.

• Radiographs: a well defined, round or oval lytic lesion in the epiphysis or apophysis, 1 to 4 cm, with a thin sclerotic rim, with stippled matrix calcification in 30 to 50%, and sometimes periosteal reaction in the adjacent metaphysis.
• MRI: lobulated lesion with intermediate to low signal on T1 and heterogeneous signal on T2, with extensive bone marrow and soft tissue oedema, which may mimic infection or a more aggressive tumour. It shows physeal crossing and secondary aneurysmal bone cyst components (fluid levels).
• CT: shows matrix calcification and the cortical shell.
• Biopsy (when features are atypical): chondroblasts, chondroid matrix, giant cells; H3K36M immunostaining is highly specific.
• Chest imaging when the tumour recurs or is aggressive.

Differential diagnosis.

• Giant cell tumour of bone (after physeal closure) and clear cell chondrosarcoma.
• Aneurysmal bone cyst and osteomyelitis (Brodie abscess).
• Langerhans cell histiocytosis and intraosseous ganglion.
• Osteoid osteoma and avascular necrosis.

Surgery is needed for nearly all lesions, as they grow and destroy the joint surface.

Surgery.

• Extended intralesional curettage using a high speed burr, with a window large enough to inspect the whole cavity, with an adjuvant (phenol, cryotherapy, argon beam coagulation or electrocautery) and filling with bone graft, bone substitute or cement. Recurrence is 10 to 20% (up to 40% in some locations such as the proximal femur and the temporal bone).
• A lesion near the physis needs care to protect the growth plate; a lesion under the joint surface may need subchondral support with graft to prevent collapse.
• En bloc resection for recurrence or expendable bone. Joint reconstruction (arthroplasty) is rare.
• Radiofrequency ablation can be used for small, selected lesions (under 2 cm) away from the joint surface in specialist hands.
• Pulmonary metastases: surgical resection, with systemic therapy not routinely used.

Operative antibiotic prophylaxis. Cefazolin 2 g intravenously (3 g when weight is 120 kg or over; 30 mg/kg to a maximum of 2 g in children) within 60 minutes before incision as a single dose (repeat after 4 hours or blood loss over 1500 mL); with severe penicillin allergy, vancomycin 15 mg/kg intravenously starting 90 to 120 minutes before incision as a single dose, with levels and creatinine checked in prolonged cases.

Thromboprophylaxis. Enoxaparin 40 mg subcutaneously once daily after lower limb surgery with reduced mobility in adolescents and adults at risk (older than 14 years, obesity, previous thrombosis, oestrogen use), started 6 to 12 hours after surgery and continued for 10 to 14 days (20 mg once daily at creatinine clearance under 30 mL/min; adjust by anti-Xa level in obesity and renal impairment; check the platelet count on days 5 to 14 for heparin induced thrombocytopenia). Use mechanical prophylaxis in other patients.

Analgesia.

• Paracetamol 1 g orally every 6 hours as needed for up to 5 to 7 days (maximum 4 g daily; 3 g daily when weight is under 50 kg, with chronic alcohol use or hepatic impairment).
• Ibuprofen 400 mg orally every 8 hours with food for up to 5 days (maximum 1200 mg daily); avoid with eGFR under 30 mL/min/1.73 m2, peptic ulcer, heart failure or anticoagulant use.
• Children: paracetamol 15 mg/kg orally every 4 to 6 hours (maximum 75 mg/kg daily and not exceeding 4 g daily) and ibuprofen 10 mg/kg orally every 6 to 8 hours with food (maximum 40 mg/kg daily and not exceeding 2400 mg daily), for up to 5 days.

Monitoring. Clinical and radiographic review at 6 weeks, 3 months, 6 months and 12 months, then every year for at least 3 to 5 years (most recurrences appear in the first 2 years). MRI when recurrence is suspected. Growth and alignment until skeletal maturity. Chest radiograph or CT yearly after a recurrence.

Referral and escalation. Refer every suspected chondroblastoma to a bone tumour service for biopsy and treatment. Urgent referral for rapid growth, pathological fracture, joint surface collapse or suspected lung disease.

Special groups.

• Skeletally immature patients: protect the physis and monitor for growth disturbance and angular deformity.
• Proximal femur and temporal bone: higher recurrence; treat in a specialist centre.
• Pregnancy: avoid NSAIDs in the third trimester; defer non-urgent surgery.
• Kidney impairment: avoid NSAIDs.