Osteochondromas are usually thought of as benign bone tumour although they are more correctly thought of as developmental anomalies. This not withstanding, they are very common and account for 10 - 15% of all bone tumours and 20 - 50% of all benign bone tumours 3.
Osteochondromas develop during childhood (period of skeletal growth) but once formed remain for the rest of the individual's life. They can present at any age and are most frequently found incidentally.
They are usually sporadic, but can be part of:
- hereditary multiple exostoses (HME) ; also known as diaphyseal aclasis
- Trevor disease ; also known as dysplasia epiphysealis hemimelica
Malignant transformation occurs in the cartilage cap, and is uncommon in sporadic solitary osteochondromas (~ 1 %), whereas in the setting of hereditary multiple exostosis the rate is much higher (5 - 25%) 5.
Symptomatic presentation is either due to mechanical effects of the lesion, fracture, or malignant transformation.
Mechanical symptoms include:
- impingement upon nearby structures
- nerve compromise
- vascular compromise
- reactive myositis
- palpable lump
- bursal formation and bursitis
Fractures occur typically through the neck of pedunculated lesions.
Ongoing growth and or pain after skeletal maturity has been reached is suspicious for malignant degeneration.
Osteochondromas are essentially a part of the growth plate which separates and continues growing independently, without an associated epiphysis, usually away from the nearby joint. The medullary cavity is continuous with the parent bone, and they are capped by hyaline cartilage. Osteochondromas can be congenital or occur as a result of previous trauma to the growth plate, including previous irradiation 3-4.
They most commonly arise from appendicular skeleton, especially around the knee 3.
- lower limb : 50% of all cases 3
- femur (especially distal) : most common : 30%
- tibia (especially proximal) : 15 - 20%
- less common locations : feet, scapula
- upper limb
- humerus : 10 - 20%
- less common locations : hands, pelvis
- spine : the posterior elements of spine are an uncommon, but not rare, site for these tumours
An osteochondroma can be either sessile or pedunculated, and is seen in the metaphyseal region typically projecting away from the epiphysis. There is often associated broadening of the metaphysis from which it arises. The cartilage cap is variable in appearance. It may be thin and difficult to identify, or thick with rings and arcs calcification and irregular subchondral bone.
New cortical irregularity or continued growth after skeletal maturity has been reached, as well as frankly aggressive features (e.g. bony destruction, large soft tissue component, metastases) are all worrying for malignant transformation.
CT demonstrates the same findings as on plain film, but is better able to demonstrate medullary continuity and the cartilage cap.
Ultrasound is able to demonstrate the cartilage cap very accurately as a hypoechoic region bounded by bone on its deep surface and muscle / fat superficially.
MRI is best at assessing cartilage thickness (and thus assessing for malignant transformation), presence of oedema in bone or adjacent soft tissues and visualising neurovascular structures in the vicinity.
The cartilage cap of osteochondromas appears the same as cartilage elsewhere, with intermediate to low signal on T1 and high signal on T2 weighted images.
A cartilage cap of over 1.5cm in thickness is suspicious for malignant degeneration.
During growth osteochondromas demonstrate increased uptake, but with time they become no more active than normal bone. Presence of increased activity in adulthood should raise the possibility of a complication (fracture, malignancy).
Treatment and prognosis
As they represent normal growth of a growth plate in an abnormal direction, they often stop growing with the rest of the skeleton 3.
In most instances no treatment is required. The osteochondroma can be locally resected for cosmetic or mechanical reasons.
If malignant transformation occurs ( ≈ 1% in solitary osteochondromas and ≈ 5 - 25% with diaphyseal aclasis) then the resultant chondrosarcoma is usually of low grade (67 - 85% of cases), and surgery is usually curative (70 - 90%) 3,5. Limb sparing wide local excision usually suffices.
Local recurrence is seen in both benign and malignant lesions, due to spillage of cartilage cells into the resection bed. Rates are estimated at 2% and up to 15% respectively 3.
Imaging differential considerations include
- hands : bizarre parosteal osteochondromatous proliferation (BPOP)
- humerus : supracondylar spur : projects towards the elbow joint
- malunited fracture
- 1. Dähnert W. Radiology Review Manual. Lippincott Williams & Wilkins. (2007) ISBN:0781766206. Read it at Google Books - Find it at Amazon
- 2. Eisenberg RL, Johnson NM. Comprehensive radiographic pathology. Mosby Inc. (2007) ISBN:0323036244. Read it at Google Books - Find it at Amazon
- 3. Murphey MD, Choi JJ, Kransdorf MJ et-al. Imaging of osteochondroma: variants and complications with radiologic-pathologic correlation. Radiographics. 20 (5): 1407-34. Radiographics (full text) - Pubmed citation
- 4. Chew FS, Weissleder R. Radiation-induced osteochondroma. AJR Am J Roentgenol. 1991;157 (4): 792. AJR Am J Roentgenol (citation) - Pubmed citation
- 5. Lee JK, Yao L, Wirth CR. MR imaging of solitary osteochondromas: report of eight cases. AJR Am J Roentgenol. 1987;149 (3): 557-60. AJR Am J Roentgenol (abstract) - Pubmed citation
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