Clinical History
19-year-old male patient with hereditary multiple exostoses, admitted for resection of a left lower limb growing and painful exostoses. The patient had no acute chest complaints.
Imaging Findings
Figure 1: Chest radiograph showing multiple bony thoracic sessile & pedunculated osteochondromas. Note the cortical & medullary space continuity of all the lesions. Note also the associated abnormal dysplastic bony appearance as seen prominently at the right posterior 5th rib. The underlying inferior aspect of the posterior 6th rib is scalloped by the effect of adjacent rib exostoses.
Figure 2 (axial T1-weighted MRI image) & Figure 3 (axial T2-weighted MRI image with fat saturation):
Theses selected MRI images show a dysplastic left proximal femur with a sizable anterolateral osteochondroma. The most worrisome feature is the thickness of the overlying cartilaginous cap that easily exceeds 1.5 cm. Note the normal signal intensity of cartilage on MRI, being hypointense on T1-weighted & hyperintense on T2-weighted sequences.
Discussion
Hereditary multiple exostoses (HME), also known as multiple familial osteochondromatosis, is a rare autosomal dominant disease, with incomplete penetrance. 2/3 of the affected patients have a positive family history. The main feature of this disease is the finding of multiple exostoses. The hallmark of a single exostosis is the cortical & medullary space continuity with the bone. As with what is seen in a single osteochondroma, those lesions could be sessile or pedunculated. The most common location would be around the knee. Almost any other bone can be involved in HME as well.
It should be noted that this disease is associated with bony growth abnormalities & dysplasias. In fact, HME is the most common form of the osteochondrodysplasias. Patients present with short stature in 40% of the cases. Changes at the distal radius & ulna could lead to a pseudo-Madelung deformity & those at the proximal femur could lead to an Erlenmeyer flask deformity.
Complications are similar to those associated with a solitary osteochondroma, although the rate is higher. Complications include fractures, vascular injuries, neurological compromise, bursa formation & malignant transformation. The risk of malignant transformation into a chondrosarcoma is about less than 5% in HME, in contrast to 1% in a solitary osteochondroma. New onset pain in an osteochondroma should always alert the treating physician. A cartilaginous cap thickness exceeding 1.5cm on MRI examination should introduce the possibility of malignant transformation.
Take Home Points:
* HME is associated with bony growth abnormalities & dysplasias.
* HME has about < 5% risk of malignant transformation of an osteochondroma in contrast to the risk of only about 1% in a solitary osteochondroma.
* The most important imaging feature pointing towards malignant transformation of an osteochondroma is an MRI cartilaginous cap thickness exceeding 1.5cm.
Differential Diagnosis List
Hereditary multiple exostoses (HME)
Multiple enchondromatosis (such as Ollier´s Disease or Maffucci Syndrome)
Polyostotic fibrous dysplasia (such as McCune-Albright Syndrome)
Final Diagnosis
Hereditary multiple exostoses (HME)
