Diagnostic Radiology/Musculoskeletal Imaging/Tumors Basic/Hereditary Multiple Exostoses

Most common of osteochondrodysplasias characterized by formation of multiple osteochondromas.

Pathophysiology
Osteochondroma is continuous with underlying medullary and cortical bone.

A hyaline cartilage cap involutes after reaching skeletal maturity.

The solitary form is the only benign neoplasm associated with radiation.

Prevalence
1:50,000-1:100,000; 1:1,000 on Guam/Mariana Islands

Genetics

 * Autosomal dominant (incomplete penetrance in females); 3 distinct loci on chromosomes 8,11, 19
 * 2/3 of affected individuals have a positive family history

Age
Forms shortly after birth; virtually all patients discovered by 12 years of age; M:F = 1.5:1.0

Physical Appearance
Short stature (40%) due to development of exostoses at the expense of longitudinal bone growth

Location
Any portion of the skeleton preformed in cartilage may be involved.

Multiple and usually bilateral; knee (70-98%), humerus (50-98%), scapula and rib (40%), elbow (35-40%), hip (30-90%), wrist (30-60%), ankle (25-54%), hand (20-30%), foot (10-25%), pelvis (5-15%), vertebra (7%)

Site
Metaphyses of long bones near epiphyseal plate (distance to epiphyseal line increases with growth)_

Disproportionate shortening of an extremity (50%)


 * Upper Extermity
 * Pseudo-Madelung deformity
 * Ulnar shortening and longer bowed radius
 * Ulnar tilt of distal radial articular surface
 * Ulnar deviation of hand
 * Dislocation of radial head
 * Radioulnar synostosis
 * Shortening of 4th and 5th metacarpals
 * Supranumerary fingers/toes
 * Lower Extremity
 * Coax valga (25%)
 * Genu valgus (20-40%)
 * Valgus deformity of ankle = tibiotalar tilt (45-54%)
 * Undertubulation with widened metadiaphyseal junction:
 * Erlenmeyer flask deformity of distal femur

CT
Wavy pelvis sign = small sessile lesion create undulating cortical contour



Fig. 1 Lateral and AP views of forearm demonstrate exostoses arising from the distal radius and proximal ulna. The radial head and the radiocapitellar joint are dysplastic. Pseudo-Madelung deformity is present with ulnar tilt of distal radius articular surface and ulnar subluxation of hand.



Fig. 2 AP shoulder on the same patient demonstrates exostoses of the proximal humerus and scapula body. Soft tissue masses in the axilla were considered overlapping shadows.



Fig. 3 Views of the femur demonstrate multiple sessile osteochondromas. The proximal femur involvement gives the femoral neck a widened appearance.

Clinical Significance
If not discovered incidentally by radiograph, multiple exostoses may create symptoms depending on its size and location. Impingement of nerves and tendons or inflammation of overlying bursae may contribute to pain. The most concerning potential cause of pain, however, is malignant sarcomatous transformation, which occurs in 10-20%!

Treatment and Followup

 * There is no universally accepted standard for follow-up of multiple exostoses. Most consider annual routine clinical exam in children as the standard, with radiographs if symptomatic. Difficult regions to assess clinically, such as the pelvis, may warrant radiographic studies routinely.
 * The cartilage cap is a useful part of the neoplasm to focus on in a radiographic study:
 * Benign < 1.5 cm or 2.0 cm.
 * Malignant > 1.5 cm or 2.0 cm.
 * However, it is common for patients with osteochondromas to develop a bursa over the exostosis. It can be extremely difficult to distinguish this bursa from a cartilage cap and vice versa.
 * Keep in mind that in a skeletally immature patient, growth due to young age will thicken the cartilage cap and simulate malignancy.
 * Follow-up is typically done routinely by radiographs: increasing destruction or change in appearance of the cartilage mantle is worrisome for malignancy.
 * Ultrasound is useful to evaluating cap thickness and overlying bursae.
 * MRI T2-weighted images are the most useful for evaluating the cartilaginous cap.
 * Bone scan is generally nonspecific, but may identify sites that are more active and merit closer attention.