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Multiple Cartilaginous Exostoses (Hereditary Exostosis)

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- Discussion:
    - an osteochondroma that results from a dysplasia of peripheral growth plate;
           - as its name implies, multiple hereditary exostosis is an inherited condition which produces multiple exostoses;
    - occurs in 1 out of 50,000 people;
    - typically inherited as an autosomal dominant trait, w/ 96% penentrance;
           - 10% may have no family history;
           - EXT1 on 8q24.1 and EXT2 on 11p13 are the two genes most strongly associated with MHE  
           - ref: EXT1 regulates chondrocyte proliferation and differentiation during endochondral bone development.  
    - average age of diagnosis is 3 yrs;
    - cartilaginous exostoses arise from metaphyses, point away from epiphysis, and appear to extend down diaphysis during growth;
    - they increase in size & number w/ growth, but may become latent at maturity;
    - osteochondromas are seen in several sites;
    - in over 90% of cases distal tibia, proximal tibia, proximal femur, and proximal humerus are involved.
           - will also involve iliac crests, scapulae, and ribs;
    - sarcomatous degeneration:
           - chondrosarcoma may develop in 1-2% of patients > 21 yrs of age;
           - lesions at risk are those occurring near pelvis, scapula, proximal humerus, proximal femur, & spine have increased risk of malignant transformation;
           - change in size of the exostosis or onset of pain in an affected adult is cause for concern and investigation.
           - monitoring pts via annual bone scans has been recommended, but its efficacy remains unproven.

- Radiographic Distribution of Lesions:
    - scapula: 40%
    - proximal humerus: 50%
    - ribs: 40%
    - distal radius: 30%
    - distal ulna: 30%
    - hands: 20-30%
    - ilium: 15%
    - mid femur 30%
    - distal femur: 70%
    - proximal tibia 70%
    - proximal fibula 30%
    - distal tibal and fibula: 20-25%
    - feet: 10-15%

- Clinical Presentation:
    - despite the high frequency of radiographic findings, a much smaller percentage of patients will demonstrate clinical lesions;
          - 75% of patients will have at least one clinically evident lesion;
          - 50% will demonstrate a forearm deformity;
          - 45% will demonstrate an ankle deformity;
          - 20% will demonstrate knee deformity;
    - clinical problems include:
          - fracture;
          - pressure of exostosis on surrounding soft tissues;
          - neurovascular compromise;
          - stature:
                 - 40% will demonstrate short stature;
                 - individuals may be small, but are not considered dwarfs;
          - leg length inequality may be severe enough to require equalization procedures in half of the individuals;

- Ulna in MED:
    - shortening is the major problem.
          - distal ulna contributes more to total bone length than does distal radius and therefore is more prone to shortening from a distal exostosis;
          - over 50% of patients will be affected;
    - diff dx: multiple enchondromatosis:
    - effects of ulnar shortening include;
          - radial bowing and angulatory deformity;
          - ulnar deviation of the wrist;
          - ulnar translation;
          - radial head dislocations
                - more common w/ severe ulnar shortening;
                - may lead to loss of pronation and increased ulnar variance;
                - ulnar varience is measured as the difference between the radial and  ulnar physis;

    - treatment:
          - probably most patients with forearm involvement do not require treatment despite severe deformity;
                 - references:
                         - Evaluation of the Forearm in Untreated Adult Subjects with Multiple Hereditary Osteochondromatosis
                         - Forearm Deformities in Hereditary Multiple Exostosis: Clinical and Functional Results at Maturity
          - excision of a distal osteochondroma gives unpredictable results;
          - radial head should never by excised in a growing patient;
          - stapling of radial hemiepiphysis may improve wrist alignment as well as improving supination and pronation;
          - ulnar lengthening:


- Knee Joint in MED:
       - leg length inequality may require equalization procedures in 50%;
       - fibular involvement by osteochondroma may cause: genu valgum;

- Ankle in MED:

    - valgus ankle deformity;
    - diastasis of the ankle; 
    - surgical indications:
          - pain from trauma or from prominent masses;
          - ankle pain assoc w/ deformity
          - w/ tibiotalar valgus > 15 deg, hemiepiphyseal stapling is indicated;
    - surgical treatment:
          - fibular lengthening + hemiepiphyseal stapling when valgus coexists w/ leg length discrepancy

Multiple hereditary osteochondromata.

Management of deformities of the forearm in multiple hereditary osteochondromas.

Synovial osteochondromatosis: a histopathological study of thirty cases.

Deformities of the forearm in patients who have multiple cartilaginous exostosis.

The natural history of hereditary multiple exostoses.

Knee deformities in multiple hereditary exostoses. A longitudinal radiographic study.

Hereditary Multiple Exostoses: One Center’s Experience and Review of Etiology.  

Severity of disease and risk of malignant change in hereditary multiple exostoses. A genotype-phenotype study.

Involvement of the Spine in Patients with Multiple Hereditary Exostoses