Multiple Hereditary Exostoses

Multiple Hereditary Exostoses (MHE) is a rare bone disease that is characterized by growths of multiple osteochondromas—benign cartilage-capped bone tumors that grow outward from the growth plates of long bones. Osteochondromas can be associated with a reduction in skeletal growth, leading to short stature or limb length discrepancy, bony deformity, restricted joint movement, and nerve entrapment. 

Dr. Paley's lecture at the 2012 MHE Conference

Osteochondromas are believed to bud off the growth plates. The cartilaginous cap of the osteochondroma has the same structure as the growth plate and grows in the same fashion. Some osteochondromas tether the growth plate when they bud off, leading to limb deformity. This tethering decreases limb growth leading to diminished stature and asymmetric growth if one limb is more or less affected than the opposite limb. This will result in a limb length discrepancy (LLD). LLD is not uncommon in patients with MHE indicating that one side is often more tethered at the growth plate than the other side.

Osteochondromas are typically found in the long bones: the femur, tibia, fibula, and humerus are the most common sites of involvement. Since they arise off the growth plate, they lead to a progressive deformity that worsens over time, and can result in limb length discrepancies, pain and disability, joint subluxations (partial dislocations), restricted range of motion, and nerve entrapment. Joint deformities of the wrist and ankle are also observed and are caused by osteochondromas in-between adjacent bones (i.e. the radius and ulna, and tibia and fibula, respectively). Even though long bones are the most commonly affected, more aggressive forms can affect any bone in the body.

Osteochondromas can be large or small; in general, the larger the osteochondroma, the greater effect on altered growth of the affected growth plate. The size of the growth plate relative to the osteochondroma is also a factor: smaller osteochondromas on larger growth plates will have less of an effect than smaller osteochondromas on smaller growth plates. 

MHE is quite rare, occurring in 1 in 50,000 births. It is usually first diagnosed when parents note painless, hard nodules or bumps on the arms or legs of their child, thus prompting x-rays and visits to their pediatrician. A pediatric orthopedic consultation is usually then sought.

MHE is an autosomal dominant genetic disorder. This means a single copy of the affected gene can cause the disorder. Usually, the gene is inherited from the parents, but spontaneous mutations may also occur. MHE has been linked to mutations in the EXT 1, 2, and 3 genes, which are tumor-suppressing genes.

Although the osteochondromas are almost always of a benign nature, they can, on occasion, develop into a more malignant tumor, a chondrosarcoma; however, these malignancies are usually seen much later in life, as an adult, and are usually a result of rapid growth of a previous slow-growing lesion. They are also quite rare. The literature reports the rate of malignant transformation of a solitary osteochondroma to be 5%. 

Osteochondromas are usually described as appearing in one of two distinct shapes: pendunculated (narrow stalk) and sessile (broad stalk). There is a cartilage cap that is present on the top of the osteochondromas that are not usually visible in x-rays. A magnetic resonance imaging (MRI) may be necessary to view the cap. MRIs are usually only ordered for larger lesions that have shown rapid progressive growth.

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