Fibrodysplasia ossificans progressiva

Fibrodysplasia ossificans progressiva (FOP) is a severely disabling autosomal dominant disorder of connective tissue characterized by congenital malformations of the great toes and progressive heterotopic ossification (HO) that forms qualitatively normal endochondral bone in characteristic extraskeletal sites.[1][2] The disease results from a gain-of-function mutation in the ACVR1/ALK2 gene encoding a bone morphogenetic protein (BMP) type I receptor; the classic mutation (c.617G>A; p.R206H) is present in all individuals with the classic phenotype.[1][3] This mutation confers a neofunction to ACVR1, rendering it aberrantly responsive to activin A as an agonist with BMP-like signaling that drives heterotopic bone formation.[3][4] Episodic inflammatory soft-tissue flare-ups progressively transform skeletal muscles, tendons, ligaments, and fascia into heterotopic bone, ultimately immobilizing the axial and appendicular skeleton.[1][5] In the maxillofacial region, HO of the masticatory muscles and mandibular depressors leads to progressive jaw restriction, which is a critical concern for dental specialists.[6][7] Imaging demonstrates both the characteristic skeletal anomalies and the progressive accumulation of heterotopic bone. Management is primarily preventive and supportive, with palovarotene now approved as the first targeted pharmacotherapy.[8][9]

The current accepted name is fibrodysplasia ossificans progressiva (FOP).[1][10] The older term myositis ossificans progressiva remains widely recognized and appears in historical literature, but is considered a misnomer because the disease affects not only muscle but also tendons, ligaments, fascia, and aponeuroses, and the early lesions are not primarily myopathic.[1] The abbreviation FOP is standard in clinical and research usage. Atypical presentations are classified as FOP-plus (classic features with additional atypical features) or FOP variants (major variations in one or both classic defining features).[1][10] The condition should not be confused with nonhereditary (acquired) myositis ossificans, which is a localized, trauma-related process without the genetic basis or progressive systemic course of FOP.[11]

• Prevalence: approximately 1 in 2,000,000 worldwide.[1]
• Demographics: no ethnic, racial, sex, or geographic predilection.[1]
• Inheritance: most cases are sporadic (de novo mutations); a small number show autosomal dominant germline transmission.[1]
• Age of onset: children appear normal at birth except for great toe malformations; episodic flare-ups and HO typically begin during the first decade of life.[1][5]
• Maxillofacial involvement: HO in the maxillofacial region is common, with TMJ disorders observed in all adult FOP patients in one cross-sectional study; onset of jaw restriction may be spontaneous or trauma-induced, with trauma-induced HO occurring at a significantly younger age.[6][12]

Congenital Features

• Bilateral hallux valgus malformations are present in over 97% of individuals with classic FOP and are often the first clinical sign, identifiable at birth. Malformations include monophalangism, shortened first metatarsals, and malformed first metatarsophalangeal joints.[1][10]
• Variable thumb malformations (hypoplasia, dysplastic phalanges) occur in approximately 50%.[10]

Flare-Ups and Heterotopic Ossification

Episodic painful soft-tissue swellings (flare-ups) are the hallmark postnatal manifestation. Flare-ups present with localized swelling, warmth, erythema, tenderness, and occasionally low-grade fever.[13] They may arise spontaneously or be precipitated by soft-tissue injury, intramuscular injections, viral infections, muscular stretching, falls, or fatigue.[1][5] Flare-ups typically begin in the head, neck, spine, and shoulders, then progress caudally and distally.[14] Each episode can transform soft tissue into heterotopic bone through endochondral ossification, progressively immobilizing joints.[1]

Maxillofacial Manifestations

• Restricted mouth opening is a major concern. A scoping review found that the location of maxillofacial HO determines the degree of limitation, with ossification of the masseter muscle producing the most severe restriction.[6]
• HO may involve the masticatory muscles (masseter, medial pterygoid), mandibular depressors (suprahyoid muscles), or the coronoid process region.[7][11][15]
• TMJ pseudo-ankylosis from HO of the medial pterygoid has been reported in young children after minor craniofacial trauma.[15]
• Dental pathology is highly prevalent: caries (DMFT ≥ 4) affects over 80% of FOP patients, chronic stomatitis becomes universal in adults, enamel hypoplasia increases with age, and Angle class II malocclusion develops in nearly half of adults.[12]
• Reduced salivary flow rate is observed in adults.[12]

Systemic Complications

• Scoliosis affects approximately 65% and may be rapidly progressive.[10]
• Conductive hearing loss is present in approximately 50%, usually from middle ear ossification.[10]
• Thoracic insufficiency syndrome from chest wall HO is the leading cause of death.[1][14]
• Most patients are wheelchair-bound by the end of the second decade.[1]

Location and Distribution

HO in FOP follows a characteristic anatomic and temporal pattern: it begins in the dorsal, axial, cranial, and proximal regions and progresses to ventral, appendicular, caudal, and distal sites.[1][16] In the maxillofacial region, HO commonly involves the masticatory muscles and mandibular depressors. Osseous bridges may form between the hyoid bone and the mentum of the mandible through ossification of the suprahyoid musculature, or between the coronoid process and adjacent structures.[7][17] Ossification of the medial pterygoid muscle produces pseudo-ankylosis of the TMJ.[15]

Periphery and Shape

• Mature heterotopic bone demonstrates well-defined, corticated margins with a peripheral shell of cortical bone surrounding a medullary cavity, indistinguishable from normal bone.[1][2]
• Early or immature lesions may appear as ill-defined soft-tissue densities before progressive mineralization produces recognizable osseous morphology.[16][18]
• Osseous bridges between anatomic structures (e.g., hyoid-to-mandible, rib-to-pelvis) are characteristic of advanced disease.[7]

Internal Architecture

Mature HO exhibits normal-appearing trabecular bone with a medullary cavity, reflecting the endochondral ossification pathway.[2] The internal density is equivalent to that of the normotopic skeleton. Osteochondromas, which occur in approximately 90% of patients, appear as pedunculated (approximately 75%) or sessile (approximately 25%) bony projections arising from the cortex, most commonly at the proximal medial tibia.[10][19]

Effects on Teeth and Adjacent Structures

• Progressive jaw restriction from HO of the masticatory muscles and mandibular depressors limits interincisal opening, with adult patients showing a mean reduction of approximately 17 mm compared with controls.[12]
• Panoramic radiographs may reveal ectopic calcification or ossification adjacent to the coronoid process, ramus, or mandibular body.[17]
• The jaws themselves are not primarily affected by intraosseous lesions; rather, the pathology is extraskeletal, with HO forming in the surrounding soft tissues and secondarily restricting jaw function.[6]
• Cervical spine fusions (C2–C7) are present in approximately 80% and are visible on lateral radiographs or panoramic images that capture the cervical spine; these fusions result from intra-articular ankylosis of facet joints.[10][20]

Congenital Skeletal Anomalies

Radiographic identification of congenital skeletal anomalies is critical for early diagnosis:

• Great toe malformations: bilateral hallux valgus with monophalangism, shortened or malformed first metatarsals, and fusion of the first metatarsophalangeal joint.[1][10]
• Short broad femoral necks: present in approximately 70%.[10]
• Osteochondromas: proximal medial tibia is the characteristic location; usually bilateral and often asymptomatic.[19]
• Hip dysplasia: present in approximately 60% of individuals with acute hip pain.[10]

Developmental Arthropathy

CT analysis of 113 individuals with classic FOP demonstrated widespread developmental arthropathy throughout the axial and appendicular skeleton, including asymmetric joint space narrowing, subchondral sclerosis, and osteophyte formation, particularly in the hips, knees, and spine.[20] Costovertebral joints, intervertebral facet joints, and proximal tibiofibular joints frequently show partial or total intra-articular ankylosis, especially after age 13.[20]

Growth Pattern and Extent

HO is episodic and cumulative. Each flare-up may add new heterotopic bone, and the total body burden of HO increases progressively throughout life.[1][21] Low-dose whole-body CT is the preferred modality for quantifying total HO volume and is superior to DXA in sensitivity and reduced positional ambiguity.[21]

MRI

MRI is the preferred modality for evaluating early flare-ups before ossification is radiographically apparent.[18][22] Pre-osseous lesions demonstrate diffuse T2-hyperintense signal extending along affected muscles and fascial planes, reflecting edema and inflammatory infiltration.[22] MRI can detect preosseous lesions and help avoid unnecessary biopsy when characteristic skeletal malformations are present.[22] Dentate nucleus hyperintensity on brain MRI has been reported, suggesting that FOP may have CNS manifestations beyond the musculoskeletal system.[22]

Medical CT

CT is superior to plain radiography for evaluating the presence and volume of mature heterotopic bone.[18] Three-dimensional CT reconstructions are particularly valuable in the maxillofacial region for delineating the precise anatomic site of jaw ankylosis—distinguishing HO of the mandibular depressors from HO of the masticatory muscles or TMJ structures—which directly influences management decisions.[7] CT-only imaging performs comparably to PET/CT for detection and volumetric measurement of new HO lesions.[23]

Nuclear Medicine

18F-sodium fluoride PET/CT demonstrates increased tracer uptake at sites of active flare-ups and new HO formation.[23] Bone scintigraphy shows increased activity at flare-up sites, though its utility in determining disease progression requires further evaluation.[18]

FOP is distinguished from other conditions by the combination of congenital great toe malformations and progressive heterotopic endochondral ossification. When maxillofacial HO or jaw restriction is the presenting concern, the differential includes other causes of extraskeletal ossification and jaw ankylosis.

The highest-risk diagnostic pitfall is misdiagnosis of an early FOP flare-up as fibromatosis or sarcoma, leading to biopsy or surgical excision that triggers catastrophic HO at the operative site.[2][24] Examination of the great toes is mandatory in any child with unexplained soft-tissue swelling before proceeding to biopsy; confirmatory ACVR1 genetic testing from a peripheral blood sample poses minimal risk.[10][14]

Stages of Lesion Development

The histopathology of FOP lesions follows a sequential endochondral ossification pathway:[2][13]

• Early (pre-osseous) stage: lymphocytic infiltration and skeletal muscle cell degeneration, followed by highly vascular fibroproliferative tissue. These early fibromatous lesions are histologically indistinguishable from aggressive juvenile fibromatosis.[2][24]
• Intermediate stage: chondrogenesis with formation of a cartilaginous anlage, recapitulating normal endochondral bone development.[2][25]
• Mature stage: lamellar bone with marrow elements, qualitatively indistinguishable from normal bone.[1][2]

Immunohistochemistry and Molecular Correlation

BMP 2/4 immunostaining is positive in fibromatous cells of early FOP lesions but absent in histologically identical aggressive juvenile fibromatosis lesions, providing a potential discriminator when biopsy has been inadvertently performed.[24] S-100 antigen positivity can be detected before histologically differentiated osteochondral tissue appears.[2]

Radiologic-Pathologic Correlation

The progression from ill-defined soft-tissue density on imaging (corresponding to the fibroproliferative/chondrogenic stage) to well-defined corticated heterotopic bone (corresponding to mature lamellar bone with marrow) mirrors the sequential histopathologic stages. The endochondral nature of HO explains the characteristic mature bone morphology seen on radiographs and CT.[1][2]

Diagnosis and Workup

• Diagnosis is clinical, based on congenital great toe malformations and progressive HO.[1]
• Confirmatory genetic testing for ACVR1 mutations is performed on DNA from peripheral blood via routine venipuncture, which poses minimal risk of triggering HO.[10][14]
• Biopsy is contraindicated as it uniformly exacerbates HO at the operative site.[2][10]

Preventive Management

Prevention of flare-ups is the cornerstone of care:[10][14]

• Avoid intramuscular injections (substitute subcutaneous immunization when possible) and arterial punctures.
• Fall prevention through household safety modifications, ambulatory devices, and protective headgear.
• Avoid passive range-of-motion exercises, which are deleterious.[14]
• Respiratory infection prevention measures.

Dental Management

Dental care requires practitioners knowledgeable about FOP:[10][14]

• Preventive dental care from a young age with examinations every six months.[10]
• Avoid overstretching of the jaw during procedures; use caution with local anesthesia.[10]
• Consider prophylactic corticosteroids prior to dental and surgical procedures, particularly for submandibular or jaw flare-ups.[10]
• If general anesthesia is required, awake nasotracheal fiberoptic intubation by an anesthetist experienced with FOP is mandatory due to cervical spine fusions, jaw ankylosis, and risk of airway flare-ups.[10]
• Deliberate tooth extraction with careful planning may be performed when oral inflammation poses a greater risk of triggering HO than the procedure itself.[26]

Pharmacotherapy

• Palovarotene (Sohonos), a selective retinoic acid receptor gamma (RARγ) agonist, is FDA-approved for reduction of new HO volume in adults and pediatric patients (females ≥8 years, males ≥10 years) with FOP. The phase 3 MOVE trial demonstrated a 60% reduction in mean annualized new HO volume compared with natural history controls. Important safety concerns include premature physeal closure in skeletally immature patients.[8][9][27]
• Flare-up management: a brief course of high-dose corticosteroids within 24 hours of flare-up onset may reduce inflammation and edema; NSAIDs, COX-2 inhibitors, mast cell stabilizers, and leukotriene inhibitors are used for symptomatic relief.[1][10]
• Investigational agents include garetosmab (anti-activin A antibody), andecaliximab, fidrisertib, saracatinib, zilurgisertib, anakinra, and canakinumab.[14][28]

Prognosis

• Most patients are wheelchair-bound by the end of the second decade of life.[1]
• Median lifespan is approximately 40–56 years, with death most commonly from complications of thoracic insufficiency syndrome.[1][14]
• HO is episodic but cumulative and irreversible; no current therapy can reverse established