Overview and definition
Hereditary multiple exostoses (HME) is a benign autosomal dominant skeletal disorder characterized by the formation of multiple osteochondromas—cartilage-capped bony outgrowths that project from the external surfaces of bones formed by endochondral ossification.[1][2] The condition results from heterozygous loss-of-function germline mutations in the EXT1 (8q24.11) or EXT2 (11p11.2) genes, which encode Golgi-resident glycosyltransferases essential for heparan sulfate biosynthesis.[2][3] Deficient heparan sulfate disrupts growth plate signaling—particularly bone morphogenetic protein (BMP), hedgehog, and Wnt/β-catenin pathways—leading to aberrant chondrocyte proliferation and ectopic osteochondroma formation at perichondrial sites.[4][5] The signature imaging finding is a bony excrescence demonstrating uninterrupted cortical and medullary continuity with the parent bone.[6][7] Osteochondromas are predominantly appendicular, arising at the metaphyses of long bones; craniofacial involvement is rare. Management is primarily surgical for symptomatic lesions, with lifelong surveillance for malignant transformation to secondary peripheral chondrosarcoma.[2][8]
Terminology and classification
The World Health Organization (WHO) currently designates the individual lesions as osteochondromas and the hereditary syndrome as multiple osteochondromas (MO), reflecting the cartilaginous origin of the growths.[2][8] In clinical practice, hereditary multiple exostoses (HME) and multiple hereditary exostoses (MHE) remain widely used, and the gene names retain the root "exostosin" (EXT1, EXT2).[2] Diaphyseal aclasis is an older synonym still encountered in some radiology and pathology texts.[9] Other historical terms include hereditary deforming chondrodysplasia, multiple osteochondromatosis, multiple cartilaginous exostosis, and Ehrenfried disease.[9] The term hereditary multiple osteochondromas (HMO) is used interchangeably with HME in the GeneReviews classification.[2]
Epidemiology
- Prevalence: approximately 1 in 50,000 individuals.[10][11]
- Inheritance: autosomal dominant with nearly complete penetrance.[2][11]
- Sex distribution: slight male predominance (male-to-female ratio ~1.5:1), though both sexes are affected.[11][12]
- Age of onset: median age at diagnosis is three years; approximately 96% of affected individuals manifest osteochondromas by age 12. Lesions grow during skeletal development and cease enlarging at skeletal maturity.[2]
- Anatomic distribution: the femur (~30%), tibia (~20%), and radius/ulna (~13% each) are most commonly involved. Flat bones (pelvis, scapula, ribs) are also affected. Skull, carpal, and tarsal bones are classically considered spared, although cranial base involvement (clivus) has been identified in nearly half of patients in one imaging series.[2][11][13]
- Craniofacial involvement: osteochondromas of the facial bones are rare. Reported maxillofacial sites include the mandibular condyle, coronoid process, mandibular ramus and body, maxillary sinus, and occipital bone.[9]
Clinical presentation
Presentation and symptoms
Most osteochondromas present in childhood as painless, firm, slowly enlarging bony masses near joints.[2][14] Systemic skeletal manifestations dominate the clinical picture and include shortened stature, limb-length discrepancy, forearm bowing with ulnar deviation, valgus deformities of the knee and ankle, and hip dysplasia.[2][15] Pain, when present, typically results from mechanical compression of adjacent tendons, nerves, or vessels.[1][2]
Craniofacial manifestations
Maxillofacial osteochondromas in HME are rare and most commonly involve the mandibular condyle or coronoid process.[9] Condylar lesions may produce progressive facial asymmetry, malocclusion, deviation on opening, and restricted mandibular range of motion.[16][17] Coronoid process involvement (Jacob disease) can cause progressive limitation of mouth opening due to impingement against the zygomatic arch.[9]
Complications and red flags
- Malignant transformation to secondary peripheral chondrosarcoma is the most serious complication, with a lifetime risk estimated at 2–5%. Axial sites (pelvis, scapula, ribs, spine) carry the highest risk. Warning signs include rapid growth of a previously stable lesion, new or worsening pain—especially after skeletal maturity—and a cartilage cap thicker than 2 cm.[2][8][18][19][20]
- Neurovascular compression, bursa formation, fracture through the stalk, and spinal cord compression from vertebral osteochondromas are additional recognized complications.[2][6]
Radiographic findings
Location and distribution
Osteochondromas characteristically arise from the juxtaphyseal (metaphyseal) regions of long bones, projecting away from the nearest joint.[2][6] In the jaws, the mandibular condyle is the most frequently reported site, followed by the coronoid process; involvement of the mandibular body, ramus, and symphysis is extremely rare.[9][21] Maxillary sinus osteochondromas have been reported occasionally.[9] In HME, lesions are typically bilateral and symmetric across the skeleton, though the number and distribution vary widely even within families.[2]
Periphery and shape
Osteochondromas present in two morphologic forms:
- Pedunculated (stalked): a narrow bony pedicle projects from the cortex, often directed away from the adjacent growth plate, with a bulbous or mushroom-shaped tip.[6][20]
- Sessile (broad-based): a broad-based bony protuberance with a wide attachment to the cortical surface.[2][20]
Both forms demonstrate well-defined, corticated margins. On panoramic radiographs, condylar osteochondromas may appear as irregular enlargement of the condylar head, a pedunculated projection from the condylar surface, or a lobulated mass arising from the medial or lateral pole.[17]
Internal architecture
The hallmark radiographic feature is uninterrupted continuity of the cortex and medullary (cancellous) bone from the parent bone into the lesion.[6][7] The bony stalk or base shows normal-appearing trabecular bone continuous with the host marrow cavity. The cartilage cap is radiolucent on plain radiographs and may contain stippled, ring-and-arc, or irregular calcifications, particularly in mature or large lesions.[6][22] In skeletally immature patients, the cartilage cap is thicker and more radiolucent; with maturation, progressive endochondral ossification reduces cap thickness.[23][24]
Effects on teeth and adjacent structures
Condylar osteochondromas may produce:
- Progressive mandibular deviation and facial asymmetry
- Contralateral open bite or ipsilateral premature occlusal contact
- Displacement of the condyle from the glenoid fossa
- Elongation of the ipsilateral mandibular ramus[16][17]
Osteochondromas of the mandibular body are exceedingly rare but may displace or resorb adjacent tooth roots.[21] Coronoid osteochondromas can impinge on the zygomatic arch, restricting mouth opening without direct dental effects.[9]
Cortical involvement and extent
The cortex of the parent bone flows smoothly into the cortex of the osteochondroma without periosteal reaction or cortical destruction—a key feature distinguishing this lesion from aggressive or malignant processes.[2][6] On cross-sectional imaging, the three-dimensional extent of the lesion, its relationship to the temporomandibular joint (TMJ) articular surface, and the precise site of origin (medial pole, lateral pole, anterior, or superior condylar surface) can be delineated, which is essential for surgical planning.[17][25] Cortical and medullary continuity is confirmed on axial and coronal sections, and the cartilage cap can be measured when visible.[19]
Growth pattern
Osteochondromas enlarge during active skeletal growth and stabilize at skeletal maturity.[2][23] Continued growth or enlargement of the cartilage cap after growth plate closure is a red flag for malignant transformation.[6][20]
Variants
In HME, individual osteochondromas may be sessile or pedunculated within the same patient. Rare endosteal (central) osteochondromas of the mandibular body have been reported.[21] Cranial base osteochondroma-like outgrowths along the clivus synchondroses represent a recently recognized variant in HME patients.[13]
Other imaging findings
MRI
MRI is the preferred modality for evaluating cartilage cap thickness, the most important imaging parameter for distinguishing benign osteochondroma from secondary chondrosarcoma.[19][26] The cartilage cap demonstrates intermediate signal on T1-weighted images and high signal on T2-weighted and STIR sequences.[27][28] A cap thickness ≥2 cm in adults (≥3 cm in children) is highly suggestive of malignant transformation, with reported sensitivity and specificity of 100% and 98%, respectively, using standardized measurement techniques.[19][20] MRI also depicts complications including bursa formation (fluid collection with peripheral enhancement), neurovascular compression, and soft-tissue extent.[26] Total-body MRI has been used as a screening tool in HME cohorts, identifying asymptomatic chondrosarcomas predominantly in flat bones.[29][30]
Medical CT
Medical CT provides superior cortical detail and reliably demonstrates the pathognomonic cortical and medullary continuity.[19][31] Cartilage cap thickness can be measured with high interobserver agreement and a concordance correlation coefficient of 0.92 compared with pathology.[32] CT is particularly useful for evaluating spinal osteochondromas, cranial base involvement, and complex anatomic relationships before surgery.[13][31]
Nuclear medicine
Technetium-99m bone scintigraphy may show increased uptake in actively growing osteochondromas; in skeletally mature patients, new or increasing uptake raises concern for malignant transformation.[8] FDG-PET has been used in the workup for chondrosarcomatous degeneration, with an SUVmax cutoff of 2 suggested, though low-grade chondrosarcomas may show lower uptake.[8] SPECT/CT and PET/CT combinations improve sensitivity and specificity for differentiating benign from malignant condylar lesions.[25]
Differential diagnosis
When multiple bony exostoses are identified on maxillofacial imaging in a patient with a known or suspected hereditary skeletal disorder, the differential centers on distinguishing HME from other conditions producing multiple osseous outgrowths or surface-based bony lesions, as well as differentiating individual osteochondromas from other condylar or jaw masses.
| Entity | Why it is considered | Best discriminator |
|---|---|---|
| Solitary osteochondroma | Single cartilage-capped bony projection, most common benign bone tumor of the condyle | Multiplicity of lesions, family history, and[2][6] EXT1/EXT2 mutation distinguish HME; solitary lesions lack the syndromic skeletal deformities[2] |
| Gardner syndrome (familial adenomatous polyposis) | Multiple osteomas of the jaws and skull with radiopaque, dense cortical bone masses | Osteomas lack a cartilage cap and medullary continuity; associated findings include intestinal polyposis, supernumerary teeth, and CHRPE[33][34][35] |
| Metachondromatosis | Autosomal dominant condition with coexisting osteochondromas and intraosseous enchondromas (PTPN11 mutation) | Lesions predominantly affect digits and point toward the nearest joint; no long-bone shortening, bowing, or joint deformity[2][8] |
| Condylar hyperplasia | Unilateral progressive mandibular asymmetry with enlarged condyle | Condylar hyperplasia shows diffuse condylar enlargement without a discrete exophytic mass or cartilage cap; histologically, it lacks the disorganized growth-plate architecture of osteochondroma[25][36] |
| Secondary peripheral chondrosarcoma | Enlarging, painful mass arising from a pre-existing osteochondroma, especially after skeletal maturity | Cartilage cap >2 cm, irregular or destroyed cortex, soft-tissue mass, and high T2 signal on MRI; histology shows mitotic figures and nuclear pleomorphism in high-grade lesions[2][19][37] |
The highest-risk diagnostic pitfall is failure to recognize malignant transformation of an osteochondroma into secondary peripheral chondrosarcoma, particularly in axial locations. Any osteochondroma demonstrating new pain, rapid growth after skeletal maturity, or a cartilage cap exceeding 2 cm warrants urgent cross-sectional imaging and multidisciplinary evaluation including biopsy.[2][19][37]
Histopathology
Microscopic features
Osteochondromas recapitulate the architecture of a normal growth plate.[23][38] The lesion consists of three layers:
- Outer fibrous perichondrium: dense connective tissue continuous with the periosteum of the parent bone
- Cartilage cap: hyaline cartilage with stratified zones of resting, proliferating, hypertrophic, and calcifying chondrocytes, though the organization is less orderly than in a normal physis[23][27]
- Bony stalk/base: mature lamellar and cancellous bone with a marrow cavity in direct continuity with the host bone medullary canal[6][38]
The cartilage cap undergoes progressive endochondral ossification, thinning with skeletal maturity.[23][24] In children, the cap is thicker and more cellular.
Radiologic-pathologic correlation
The pathognomonic cortical and medullary continuity seen on imaging directly reflects the histologic architecture: the cortex of the parent bone merges seamlessly into the cortex of the osteochondroma, and the trabecular bone of the stalk is continuous with the host marrow.[6] Cartilage cap thickness measured on MRI or CT correlates well with pathologic measurement and is the most reliable parameter for distinguishing benign osteochondroma from chondrosarcoma.[19][37]
Molecular findings affecting classification
HME results from heterozygous loss-of-function mutations in EXT1 or EXT2.[2][39] EXT1 mutations account for 55–75% of cases and are associated with a more severe phenotype (greater number of osteochondromas and more pronounced skeletal deformities) compared with EXT2 mutations.[39][40] Osteochondroma formation requires a somatic "second hit" further reducing heparan sulfate production at the site of tumor induction.[4][41] Biallelic inactivation of EXT1 has been demonstrated in the cartilage cap cells of both hereditary and sporadic osteochondromas.[41][42] Approximately 15% of clinically diagnosed HME patients lack identifiable EXT1 or EXT2 mutations, suggesting additional loci or mechanisms.[39]
Management and prognosis
Workup and referral
Diagnosis is established by clinical and radiographic identification of multiple osteochondromas, supported by family history.[2][7] Genetic testing for EXT1 and EXT2 mutations confirms the diagnosis and enables cascade screening of at-risk family members.[2][39] Patients should be referred to an orthopedic oncology or musculoskeletal tumor center for comprehensive evaluation and long-term management.[43]
Treatment
- Asymptomatic osteochondromas require no intervention.[2]
- Symptomatic lesions (pain, mechanical impingement, cosmetic deformity) are treated with surgical excision, which must include the cartilage cap and overlying perichondrium to minimize recurrence. Local recurrence after complete excision is uncommon.[2][8][20]
- Maxillofacial osteochondromas: condylar or coronoid lesions causing malocclusion, asymmetry, or restricted opening are managed by surgical excision, which may be performed via an intraoral, preauricular, or endoscope-assisted approach depending on lesion location.[9][16]
- Skeletal deformities (forearm bowing, angular malalignment, limb-length discrepancy) may require corrective osteotomies, hemiepiphysiodesis, or ulnar-lengthening procedures.[2][8]
- Chondrosarcomatous transformation is treated by en-bloc surgical resection with tumor-free margins, ideally at a bone tumor referral center. Adjuvant chemotherapy and radiotherapy have limited roles in chondrosarcoma.[2][12][43]
Surveillance
- Clinical assessment for new symptoms, deformity progression, and neurologic manifestations should be performed periodically, though no universally accepted surveillance timeline exists.[2]
- For osteochondromas involving the pelvis or scapula—sites with the highest malignant transformation risk—imaging every 2–3 years is considered reasonable; MRI is the preferred modality.[2]
- Total-body MRI has been used for screening in adult HME cohorts and can detect asymptomatic chondrosarcomas, though long-term cost-effectiveness data are lacking.[29][30]
- Patients and families should be educated about red flags for malignant transformation: new or worsening pain, rapid growth of a lesion after skeletal maturity, and palpable enlargement.[2][8]
Prognosis
HME does not affect life expectancy in the absence of malignant transformation.[12] The lifetime risk of chondrosarcoma is estimated at 2–5%, with the pelvis and scapula as the most common sites of malignant change.[8][18] Most secondary chondrosarcomas are low-grade (grade 1) and carry a favorable prognosis after adequate surgical resection.[29][43] Functional outcomes are influenced by the burden of skeletal deformities; patients may undergo multiple surgeries throughout life.[10][15]
Special considerations
Genotype-phenotype correlation
EXT1 mutations produce a more severe phenotype than EXT2 mutations, with a greater number of osteochondromas, more pronounced skeletal deformities, and potentially higher malignant transformation risk.[39][40] This distinction may guide counseling regarding disease burden and surveillance intensity.
Pediatric considerations
Nearly all patients are diagnosed in childhood (median age 3 years).[2] Resection of periphyseal osteochondromas in growing children carries a theoretical risk of growth plate injury and subsequent growth disturbance; conservative management may be appropriate for transiently symptomatic lesions in skeletally immature patients.[2] Screening spine MRI in childhood has been recommended by some experts to identify spinal osteochondromas that could cause cord compression, though evidence supporting routine screening in asymptomatic children is limited.[2]
Cranial base involvement
A retrospective imaging study identified osteochondroma-like outgrowths in the clivus in nearly half of 50 consecutive HME patients, a previously unrecognized site of involvement.[13] This finding suggests that the craniofacial skeleton is not entirely spared in HME and may warrant attention on cross-sectional imaging that includes the skull base.