Osteochondroma



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.

Epidemiology
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.


Clinical presentation
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 sketetal maturity has been reached is suspicious for malignant degeneration.

Pathology
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

Location
They most commonly arise from appendicular skeleton, especially around the knee

lower limb : 50% of all cases
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
Radiographic features
Plain film
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
CT demonstrates the same findings as on plain film, but is better able to demonstrate medullary continuity and the cartilage cap.

Ultrasound
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
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 else where, 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.

Bone scan
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 eclasis) 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.

Differential diagnosis
Imaging differential considerations include

hands : bizarre parosteal osteochondromatous proliferation (BPOP)
humerus : supracondylar spur : projects towards to the elbow joint
malunited fracture