Osteochondroma

Osteochondroma is a benign cartilage-capped bony neoplasm consisting of a projection of cortical and medullary bone continuous with the parent bone, covered by a hyaline cartilage cap.[1][2] The WHO classifies it among benign cartilaginous tumors of bone.[3] Although it is the most common benign bone tumor overall, osteochondroma is rare in the oral and maxillofacial region, where the mandibular condyle and coronoid process are the predominant sites.[2][4][5] The pathogenesis involves aberrant cartilaginous growth plate tissue that separates from the normal growth plate and proliferates autonomously in a subperiosteal location, recapitulating endochondral ossification.[6] Loss-of-function mutations in the heparan sulfate polymerase genes EXT1 or EXT2 underlie hereditary multiple osteochondromas (HMO) and have also been demonstrated as somatic biallelic events in solitary lesions.[7][8] The signature imaging finding is corticomedullary continuity between the lesion and the host bone. Surgical excision is the definitive treatment for symptomatic lesions, with an excellent prognosis and very low recurrence.

• Osteochondroma is the current WHO-accepted term.[3][9]
• The older term exostosis (or osteocartilaginous exostosis) was widely used historically but is no longer recommended for this neoplasm because it does not convey the essential cartilaginous component.[9]
• The hereditary syndrome of multiple lesions is now designated hereditary multiple osteochondromas (HMO), replacing the former terms hereditary multiple exostoses (HME), multiple osteochondromatosis, and diaphyseal aclasis.[9][10]
• In the maxillofacial literature, condylar osteochondroma is sometimes loosely termed "condylar hyperplasia" or "condylar tumor"; these are imprecise and should be avoided when the histologic diagnosis is established.[11]
• When a coronoid process osteochondroma forms a pseudoarticulation with the zygomatic arch, the eponym Jacob disease has been applied.[12]

Osteochondroma constitutes approximately 20%–50% of all benign bone tumors and 10%–15% of all bone tumors in the general skeleton.[1] Approximately 85% are solitary; the remaining 15% occur in the setting of HMO.[13] In the general skeleton, most present in children and adolescents, with a slight male predominance.[2][14]

Maxillofacial involvement is rare. The mandibular condyle is the most common craniofacial site, followed by the coronoid process; involvement of the mandibular body or other facial bones is exceedingly uncommon.[4][5][15] A literature review of coronoid osteochondromas identified only 38 histologically proven cases over 30 years, with a male-to-female ratio of approximately 2:1 and a median age at onset of about 29 years.[5] Condylar osteochondromas similarly show a slight male predominance, with most patients presenting in the second through fifth decades.[16][17]

Presentation and symptoms

Maxillofacial osteochondromas typically present as a slowly enlarging, painless, firm mass.[2] The clinical picture depends on the site of origin:

• Condylar osteochondroma — Common manifestations include progressive facial asymmetry, malocclusion (typically a contralateral open bite), mandibular deviation, limited or altered mandibular excursion, and temporomandibular joint (TMJ) dysfunction. Pain, when present, is usually mild and related to secondary joint derangement.[17][18]
• Coronoid osteochondroma — The hallmark is progressive limitation of mouth opening (trismus), often severe, accompanied by cheek fullness or zygomatic swelling when the lesion impinges on the zygomatic arch.[5]

Tempo and complications

Growth is slow and may parallel skeletal development, often ceasing after skeletal maturity.[14] Continued growth or new onset of pain after growth plate closure is a red flag for possible malignant transformation.[9][13] Condylar lesions may cause external auditory canal stenosis in rare cases.[17] Coronoid lesions may form a pseudojoint with the inner surface of the zygomatic arch.[5][12]

Osteochondroma can be mistaken clinically for TMJ internal derangement or condylar hyperplasia, delaying diagnosis.[16]

Location and distribution

In the jaws, osteochondroma arises almost exclusively from the mandibular condyle or the coronoid process.[4][5][18] Condylar lesions most commonly originate from the medial pole of the condyle, followed by the anterior-superior, posterior-superior, and lateral surfaces; a subset presents as a generally enlarged condyle.[17] Rare cases of endosteal (central) osteochondroma of the mandibular body have been reported.[15] Maxillary involvement is exceedingly rare.

Periphery and shape

The lesion projects from the bone surface as either a pedunculated (narrow stalk) or sessile (broad-based) exophytic mass.[1][13] Pedunculated forms characteristically have a mushroom-shaped or cauliflower-like contour, while sessile forms appear as a broad, dome-shaped enlargement of the affected process.[19] The periphery is well defined, with a corticated margin that merges seamlessly with the cortex of the parent bone.

Internal architecture

The hallmark radiographic feature is uninterrupted corticomedullary continuity between the lesion and the host bone — the cortex and medullary (cancellous) bone of the parent bone flow directly into the lesion without interruption.[1][9] The osseous component shows trabecular bone of normal density. The cartilage cap itself is radiolucent on conventional radiographs and may be invisible unless it contains foci of calcification or enchondral ossification, which appear as irregular stippled or ring-and-arc calcifications at the surface of the lesion.[1][20]

Effects on adjacent structures

• Condylar lesions frequently cause deviation of the mandible to the contralateral side, contralateral posterior open bite, and compensatory tilting of the occlusal plane. The tumor may form a pseudojoint beneath the articular eminence. Tooth displacement or alveolar changes are not a direct feature but may result from secondary skeletal asymmetry.[17]
• Coronoid lesions may impinge on the medial surface of the zygomatic arch, creating a pseudoarticulation and mechanically restricting mandibular opening.[5]
• Root resorption and displacement of the inferior alveolar canal are not characteristic features of osteochondroma, given its typical condylar/coronoid location.

Growth pattern and cortical involvement

Growth is exophytic and directed away from the nearest joint surface, reflecting the orientation of the aberrant growth plate cartilage.[6] The cortex of the parent bone is continuous with the lesion rather than destroyed. Cortical destruction, periosteal reaction, or aggressive features are absent in benign osteochondroma; their presence should raise concern for secondary chondrosarcoma.[1][21]

Variants and diagnostic pitfalls on dental radiographs

On panoramic radiographs, condylar osteochondromas may present with diverse appearances depending on the site of origin around the condyle, ranging from a discrete pedunculated projection to a diffusely enlarged condylar head that mimics condylar hyperplasia.[19] Superimposition of the skull base and petrous bone can obscure medially directed lesions on panoramic images. Cross-sectional imaging (CBCT or CT) is essential to confirm corticomedullary continuity, delineate the three-dimensional extent of the mass, and evaluate the relationship to the glenoid fossa, zygomatic arch, and middle cranial fossa.[5][22]

MRI

MRI is the preferred modality for evaluating the cartilage cap, which is the critical structure for assessing malignant potential.[23][24]

• The osseous stalk shows signal intensity identical to normal marrow on all sequences (T1 iso-/hyperintense, T2 variable), confirming medullary continuity.[2][25]
• The hyaline cartilage cap demonstrates high signal intensity on T2-weighted and fluid-sensitive sequences and intermediate signal on T1-weighted images. An intact low-signal perichondrium overlying the cap is a reassuring finding.[2][25]
• A cartilage cap thickness ≥2 cm in adults (≥3 cm in children) is highly suggestive of secondary chondrosarcoma; a standardized perpendicular measurement technique yields sensitivity and specificity approaching 100% and 95–98%, respectively.[23][24]
• Gadolinium enhancement within the cap or irregular, lobulated thickening raises concern for malignant transformation.[9][24]

MRI also detects complications such as bursa formation, neurovascular compression, and disc displacement in condylar lesions.[24]

Nuclear medicine

SPECT and SPECT/CT with technetium-99m MDP are used primarily to assess metabolic activity of the condyle when the clinical question is active condylar hyperplasia versus osteochondroma or other neoplasm.[22][26] Increased radiotracer uptake in the affected condyle relative to the contralateral side (>8–10% asymmetry) indicates active growth.[26][27] SPECT/CT provides high diagnostic accuracy (AUC ~0.89–0.93) for active condylar hyperplasia and can help guide the timing of surgical intervention.[26][27] In the context of HMO, FDG-PET may assist in screening for malignant transformation; an SUVmax ≥2 raises suspicion for chondrosarcomatous degeneration.[9]

Osteochondroma of the mandibular condyle or coronoid process must be distinguished from other causes of condylar enlargement, exophytic bony masses, or progressive facial asymmetry. The pathognomonic finding of corticomedullary continuity with a cartilage cap narrows the differential considerably, but this feature may be difficult to appreciate on panoramic radiographs alone.

The highest-risk diagnostic pitfall is misidentifying a secondary chondrosarcoma as a benign osteochondroma. Any osteochondroma demonstrating continued growth after skeletal maturity, new-onset pain, or a cartilage cap exceeding 2 cm warrants urgent cross-sectional imaging (preferably MRI) and biopsy to exclude malignant transformation.[1][9][23]

Diagnostic microscopic features

Osteochondroma recapitulates the architecture of a normal growth plate in three layers:[2][6][29]

• Fibrous perichondrium — the outermost layer of dense fibrous tissue continuous with the periosteum of the parent bone.
• Hyaline cartilage cap — composed of stratified zones of chondrocytes resembling a disorganized growth plate, with progressive maturation from a proliferative zone to hypertrophic chondrocytes undergoing endochondral ossification at the osteochondral junction. Irregular ossification and myxoid degeneration may be present.[2]
• Osseous stalk — mature lamellar and trabecular bone with fatty or hematopoietic marrow, continuous with the medullary cavity of the host bone.[1][30]

Collagen types I, II, and X are expressed in a pattern similar to, but less organized than, normal growth plate cartilage.[29]

Distinguishing osteochondroma from condylar hyperplasia

Compared with condylar hyperplasia, osteochondroma shows a significantly thicker cartilage cap, a higher proportion of bone formation, and greater PCNA-positive (proliferating) cell counts.[11] EXT1 immunoexpression is also higher in osteochondroma.[11]

Malignant transformation

Histologic features suggesting secondary chondrosarcoma include markedly thickened cartilage cap (>2 cm), hypercellularity, nuclear atypia, myxoid change, and permeation of host bone trabeculae.[21] Most secondary chondrosarcomas arising in osteochondroma are low grade (grade 1).[21][31]

Molecular findings

Solitary osteochondromas harbor somatic biallelic inactivation of EXT1, resulting in deficient heparan sulfate synthesis.[7][8] In HMO, a germline heterozygous loss-of-function mutation in EXT1 (chromosome 8q24) or EXT2 (chromosome 11p11) is complemented by a somatic second hit.[7][10] EXT1 mutations account for 55%–75% of HMO cases and are associated with greater disease severity.[32] Cytogenetic abnormalities including aneuploidy and loss of heterozygosity in the cartilage cap confirm the neoplastic nature of the lesion.[8][29]

Workup and referral

Patients with clinical suspicion of condylar or coronoid osteochondroma should undergo cross-sectional imaging (CBCT or CT) to confirm corticomedullary continuity and delineate the extent of the mass.[5][22] MRI is indicated when the cartilage cap must be evaluated or malignant transformation is a concern.[23][24] SPECT or SPECT/CT may be considered to assess active condylar growth when the differential includes condylar hyperplasia.[26] Referral to an oral and maxillofacial surgeon is appropriate for all suspected cases.

Definitive treatment

Surgical excision is the standard treatment for symptomatic osteochondroma.[13][33] Options include:

• Local resection (conservative condylectomy) — excision of the tumor mass while preserving the condylar head; appropriate for pedunculated lesions amenable to complete removal with clear margins.[4]
• Total condylectomy — indicated for large or sessile lesions where the condyle cannot be preserved; reconstruction may involve costochondral graft, vertical ramus osteotomy, or total joint replacement.[33]
• Coronoidectomy — the preferred approach for coronoid osteochondroma, typically via an intraoral route.[5]

Concomitant orthognathic surgery (e.g., sagittal split osteotomy, Le Fort I) may be required to correct secondary dentofacial deformity.[33] A multidisciplinary approach involving oral and maxillofacial surgery, orthodontics, and prosthodontics optimizes functional and occlusal outcomes.[34]

Recurrence and prognosis

Recurrence after complete excision is very low. A systematic review of 439 surgically treated condylar osteochondromas reported a recurrence rate of only 0.22%.[33] In the general skeleton, local recurrence is less than 2% when resection margins are free of tumor, including the cartilage cap and perichondrium.[13][25]

Malignant transformation

Malignant transformation to secondary chondrosarcoma occurs in approximately 1% of solitary osteochondromas and 2%–5% of patients with HMO.[1][9][10] In the jaws, sarcomatous transformation is exceedingly rare but has been reported in the coronoid process.[12] Red flags include continued lesion growth after skeletal maturity, new-onset pain, and cartilage cap thickness >2 cm.[9][13] Lifelong clinical and radiographic surveillance is recommended for patients with HMO.[31]

Hereditary multiple osteochondromas (HMO)

HMO is an autosomal dominant disorder with ~98% penetrance, caused by heterozygous loss-of-function mutations in EXT1 or EXT2.[10][35] The median age of diagnosis is 3 years, and nearly all affected individuals are diagnosed by age 12.[10] Although osteochondromas in HMO predominantly affect the metaphyses of long bones, craniofacial involvement can occur and should be considered in any patient with known HMO who develops jaw symptoms or facial asymmetry.

The lifetime risk of malignant transformation in HMO is estimated at 2%–10%, substantially higher than for solitary lesions.[9][10] The pelvis and scapula are the most common sites of malignant change.[9] Lifelong clinical surveillance with imaging of symptomatic or growing lesions is recommended.[9][31]

Pediatric considerations

In skeletally immature patients, osteochondromas may continue to grow; surgical intervention is generally deferred until after skeletal maturity unless the lesion causes significant functional impairment, because premature excision carries a higher risk of recurrence.[14] A cartilage cap thickness threshold of 3 cm (rather than 2 cm) is used in children to account for the normally thicker cap during active growth.[13][24]