The term synostosis (plural: synostoses) refers to the fusion of bones usually at cartilaginous or fibro-osseous connections. Synostoses occur physiologically, as asymptomatic anatomical variants or might be abnormal and cause clinical symptoms as a functional loss. The latter is clinically significant and might need treatment.

Physiological synostoses occur as a result of ossification of synchondroses or syndesmoses with advancing age and include the following:

• physeal closure of primary and secondary ossification centers

• synostoses of the sacral vertebra

• synostoses of pubic, ischial and iliac bones (acetabulum, ischiopubic)

• synostoses of the sternal body segments

• synostosis of the mandibular synchondrosis

Synostoses can exist as anatomical variants that are rarely associated with symptoms and include the following:

• symphalangism

• block vertebra

• lumbosacral transitional vertebra (Castellvi type III and IV)

• manubriosternal and/or sternoxiphoidal joint fusion

Abnormal synostoses can be congenital or acquired, can occur as complications following trauma or surgery, or intended postsurgical outcomes. They include the following clinical conditions ^1-5:

• premature physeal closure

• craniosynostosis

• Klippel Feil syndrome

• radioulnar synostosis

• tibiofibular synostosis

• humeroradial synostosis

• osseous carpal coalition or tarsal coalition

• multiple synostoses syndrome

• spondylocarpotarsal synostosis syndrome

• complete joint ankyloses

• osseous surgical spinal fusion

• adjacent level ossification

Epidemiology

Epidemiological data depend on the location or type of the synostosis and on whether they are congenital or acquired and seem to vary significantly between different populations.

The reported incidence of congenital cervical vertebral synostoses varies between 0.5 and 6.25% ⁴, the frequency of occipitocervical synostoses is less common ranging between 0.08% and 3% ⁵. Craniosynostosis has a reported incidence of 0.4-05% ^6,7. Both congenital radioulnar and tibiofibular synostosis are rare conditions ^8-11. Carpal and tarsal coalitions are again more common with an estimated frequency of 0.1% to 8% depending on the population in carpal coalitions ¹² and about 1% of tarsal coalitions ^13,14, where synostoses make up about one-third of talocalcaneal coalitions ¹⁴.

Acquired synostoses can occur as rare complications of forearm or ankle fractures. Posttraumatic radioulnar synostosis has been reported in up to 2% ² and distal tibiofibular synostosis in up to 5% after transsyndesmotic or suprasyndesmotic ankle fractures (AO type 44-B or 44-C) ¹⁵.

Associations

Synostoses might be associated with the following conditions ^2,3:

• trauma

  • fractures

  • cranial trauma

• surgery

• heterotopic ossifications

• inflammatory arthritis

• infection (osteomyelitis, septic arthritis, tuberculosis)

Risk factors

Predisposing factors for acquired synostoses include trauma and treatment-related factors such as ^2,3:

• fractures of two adjacent bones

  • comminuted fractures and/or fractures at the same level

  • open fractures with significant soft tissue injury

  • bone fragments on the interosseous membrane

  • associated cranial trauma

• prolonged trauma-to-surgery interval

• prolonged immobilization

• delayed rehabilitation

Diagnosis

The diagnosis of synostosis can be easily confirmed with imaging such as plain radiographs or CT.

Clinical presentation

Also depending on the type and location synostoses might be asymptomatic or can present with deformity and functional loss or decreased range of motion ^2,3. Incomplete synostosis can present with pain ².

Pathology

Etiology

Synostoses can be congenital or acquired. Some of the congenital synostoses have been linked to genetic disorders ⁴.

Location

Synostoses tend to occur at cartilaginous or fibro-osseous connections between two adjacent bones.

Subtypes

Synostoses can be classified based on their location.

Radiographic features

Plain radiograph

Plain radiographs can easily confirm and depict the synostosis as an osseous fusion between adjacent bones.

Ultrasound

Ultrasound might show a cortical bridge but is unable to assess the deep located bony structures.

CT

CT can precisely demonstrate the location and extent of the synostosis by complete fusion of trabecular and cortical bones.

MRI

Complete synostosis is demonstrated by osseous fusion and a continuous bridge of the cortical and cancellous bone as well as the bone marrow signal. In addition, MRI can depict bone marrow edema or soft tissue edema indicating incomplete synostosis.

Nuclear medicine

A bone scan can show the maturation and activity of the synostosis.

Radiology report

The radiological report should include the description of the following:

• an osseous bridge between two adjacent bones

• associated bone marrow edema

• associated soft tissue edema or soft tissue abnormalities

Treatment and prognosis

Management and prognosis of synostoses will depend on the location, type and extent and associated clinical symptoms ^2,3. Any physiological synostoses and asymptomatic variants will not require treatment whereas craniosynostosis or posttraumatic radioulnar synostosis might require surgery ². The development of distal tibiofibular synostosis after ankle surgery on the other hand has been shown to not significantly alter the surgical outcome and therefore will probably also not require further treatment ³.

History and etymology

Spinal vertebral fusions have been already described by Greek historians ¹⁵. The first tarsal coalition has been described by Georges-Louis Leclerc, Comte de Buffon in 1769 ¹³. The first proximal radioulnar synostosis was reported by the Dutch anatomist Eduard Sandifort in 1793 ^8,9. Craniosynostosis were first precisely categorized by Rudolph Virchow in 1851 ¹⁶ and the first talonavicular synostosis was reported by RJ Anderson in 1879 ¹⁷.

See also

• multiple synostoses syndrome

• diaphyseal tibiofibular synostosis

• proximal radio-ulnar synostosis