Exostosis/Osteochondroma of the Proximal Diametaphyseal Region of the Medial Tibia
An exostosis or osteochondroma of the proximal diametaphyseal region of the medial tibia is a benign cartilage-capped bony outgrowth that develops from the surface of the bone, containing a marrow cavity continuous with the underlying bone. 1
Definition and Classification
- Osteochondroma (exostosis) is the most common benign bone tumor, comprising approximately 35% of all benign bone tumors 2
- It can occur as a solitary lesion (85% of cases) or as multiple lesions (15% of cases) in the context of hereditary multiple osteochondromas (HMO), an autosomal dominant disorder 3, 1
- The tumor typically forms at the metaphyseal region of long bones, with the proximal tibia being a common location 4
Anatomical Features
- The lesion consists of a cartilage cap covering a bony projection that extends from the surface of the bone 1
- In the proximal tibia, it typically develops on the medial aspect and grows away from the adjacent joint 4
- The marrow cavity of the exostosis is continuous with that of the underlying bone 4
- When located in the proximal tibia, it can affect the normal growth and development of the adjacent fibula 2
Clinical Presentation
- Most osteochondromas are asymptomatic and discovered incidentally 2
- When symptomatic, they may present as:
- A painless, slow-growing mass or swelling in the proximal medial tibia 3
- Pain due to mechanical irritation of surrounding tissues 3
- Mechanical limitations of joint movement 1
- Vascular complications such as compression of adjacent blood vessels 5
- Neurological symptoms due to compression of nearby nerves, such as the deep peroneal nerve 2
Diagnostic Evaluation
- Radiographs are the initial imaging modality of choice, showing a bony outgrowth from the proximal tibia 6
- CT can better define the bony components and continuity with the parent bone 6
- MRI is essential to evaluate:
- The cartilage cap thickness (important for assessing malignant potential)
- Relationship to surrounding structures
- Any associated complications 7
- Biopsy may be necessary for definitive diagnosis in atypical cases 7
Complications
- Fracture through the stalk of the osteochondroma 3
- Vascular complications including popliteal vein thrombosis or arterial compression 5
- Neurological symptoms from nerve compression 2
- Deformity of the tibia or adjacent fibula 2
- Bursa formation over the osteochondroma 3
- Malignant transformation to chondrosarcoma (rare, occurs in <1% of solitary lesions and up to 5-6% in HMO) 6, 3
Management
- Asymptomatic lesions generally require observation only 3
- Surgical excision is indicated for:
- Complete surgical resection with free margins is the treatment of choice to prevent recurrence 3
- Recurrence rate is less than 2% if complete resection is achieved 3
Warning Signs of Malignant Transformation
- Cartilage cap thickness >2 cm in adults or >3 cm in children 3
- New onset of pain in a previously asymptomatic lesion 3
- Continued or accelerated growth after skeletal maturity 3
- Rapid increase in size 6
Prognosis
- Excellent prognosis for benign osteochondromas with appropriate management 3
- Local recurrence is rare (<2%) after complete surgical excision 3
- Regular follow-up is recommended for lesions that are not surgically removed, particularly in patients with hereditary multiple osteochondromas who have a higher risk of malignant transformation 6
Genetic Considerations
- Hereditary multiple osteochondromas are caused by mutations in either EXT1 (chromosome 8q24.11-q24.13) or EXT2 (chromosome 11p11-12) genes 1
- These genes are involved in the biosynthesis of heparin sulfate proteoglycans which play important roles in normal epiphyseal growth plate development 1
- Even solitary, non-hereditary osteochondromas may show biallelic inactivation of the EXT1 locus 1