Synostosis (plural: synostoses) is fusion of two bones. It can be normal in puberty, fusion of the epiphysis, or abnormal. When synostosis is abnormal it is a type of dysostosis.

Examples of synostoses include:

- craniosynostosis – an abnormal fusion of two or more cranial bones;

- radioulnar synostosis – the abnormal fusion of the radius and ulna bones of the forearm;

- tarsal coalition – a failure to separately form all seven bones of the tarsus (the hind part of the foot) resulting in an amalgamation of two bones; and

- syndactyly – the abnormal fusion of neighboring digits.

Synostosis within joints can cause ankylosis.

Radioulnar synostosis is one of the more common failures of separation of parts of the upper limb. There are two general types: one is characterized by fusion of the radius and ulna at their proximal borders and the other is fused distal to the proximal radial epiphysis. Most cases are sporadic, congenital (due to a defect in longitudinal segmentation at the 7th week of development) and less often post-traumatic, bilateral in 60%, and more common in males. Familial cases in association with autosomal dominant transmission appear to be concentrated in certain geographic regions, such as Sicily.

The condition frequently is not noted until late childhood, as function may be normal, especially in unilateral cases. Increased wrist motion may compensate for the absent forearm motion. It has been suggested that individuals whose forearms are fixed in greater amounts of pronation (over 60 degrees) face more problems with function than those with around 20 degrees of fixation. Pain is generally not a problem, unless radial head dislocation should occur.

Most examples of radioulnar synostosis are isolated (non-syndromic). Syndromes that may be accompanied by radioulnar synostosis include X chromosome polyploidy (e.g., XXXY) and other chromosome disorders (e.g., 4p- syndrome, Williams syndrome), acrofacial dysostosis, Antley–Bixler syndrome, genitopatellar syndrome, Greig cephalopolysyndactyly syndrome, hereditary multiple osteochondromas (hereditary multiple exostoses), limb-body wall complex, and Nievergelt syndrome.

Craniosynostosis (from cranio, cranium; + syn, together; + ostosis relating to bone) is a condition in which one or more of the fibrous sutures in an infant skull prematurely fuses by turning into bone (ossification). Craniosynostosis has following kinds: scaphocephaly, trigonocephaly, plagiocephaly, anterior plagiocephaly, posterior plagiocephaly, brachycephaly, oxycephaly, pansynostosis.

Your baby's skull has seven bones. Normally, these bones don't fuse until around age 2, giving your baby's brain time to grow. Joints called cranial sutures, made of strong, fibrous tissue, hold these bones together. In the front of your baby's skull, the sutures intersect in the large soft spot (fontanel) on the top of your baby's head. Normally, the sutures remain flexible until the bones fuse. The signs of craniosynostosis may not be noticeable at birth, but they become apparent during the first few months of your baby's life. The symptoms differs from types of synostosis. First of all there is Sagittal synostosis (scaphocephaly). Premature fusion of the suture at the top of the head (sagittal suture) forces the head to grow long and narrow, rather than wide. Scaphocephaly is the most common type of craniosynostosis. The other one is called Coronal synostosis (anterior plagiocephaly). Premature fusion of a coronal suture — one of the structures that run from each ear to the sagittal suture on top of the head — may force your baby's forehead to flatten on the affected side. It may also raise the eye socket and cause a deviated nose and slanted skull. The Bicoronal synostosis (brachycephaly). When both of the coronal sutures fuse prematurely, your baby may have a flat, elevated forehead and brow.

McGillivray syndrome is a very rare syndrome which is also known as a Craniosynostosis. It is characterized mainly by heart defects, skull and facial abnormalities and ambiguous genitalia. The symptoms of this syndrome are ventricular septal defect, patent ductus arteriosus, small jaw, undescended testes, and webbed fingers. Beside to these symptoms there are more symptoms which is related with bone structure and misshape.

McGillivray syndrome is a birth defect in which one or more of the joints between the bones of your baby's skull close prematurely, before your baby's brain is fully formed. When your baby has craniosynostosis, his or her brain cannot grow in its natural shape and the head is misshapen. It can affect one or more of the joints in your baby's skull. In some cases, craniosynostosis is associated with an underlying brain abnormality that prevents the brain from growing properly. Treating McGillivray usually involves surgery to separate the fused bones. If there is no underlying brain abnormality, the surgery allows baby’s brain to grow and develop in adequate space.

The word pansynostosis is also Greek derived and can be translated as 'all one bone', indicating that all of the sutures are closed. In general practice, the term is used to describe the children with three or more cranial sutures closed.

Pansynostosis can present in several ways. The appearance can be the same as that seen with primary microcephaly: a markedly small head, but with normal proportions. However, pansynostosis can also appear as a Kleeblattschädel (cloverleaf skull), which presents with bulging of the different bones of the cranial vault. The condition is associated with thanatophoric dwarfism.

Oxycephaly, also known as turricephaly and high-head syndrome, is a type of cephalic disorder. This is a term sometimes used to describe the premature closure of the coronal suture plus any other suture, like the lambdoid suture,

The cranium consists of three main sections including the base of the cranium (occipital bone), the face (frontal bone), and the top (parietal bones) and sides (temporal bone) of the head. Most of the bones of the cranium are permanently set into place prior to birth. However, the temporal and parietal bones are separated by sutures, which remain open, allowing the head to slightly change in shape during childbirth. The cranial sutures eventually close within the first couple of years following birth, after the brain has finished growing.

In individuals with SCS, the coronal suture separating the frontal bones from the parietal bones, closes prematurely (craniosynostosis), occasionally even before birth. If the coronal suture closes asymmetrically or unilaterally, then the face and forehead will form unevenly, from side-to-side. People with SCS have pointy, tower-like heads because their brain is growing faster than their skull, resulting in increased intracranial pressure (ICP) and causing the top of the head and/or forehead to bulge out to allow for brain growth. The face appears uneven, particularly in the areas of the eyes and cheeks, and the forehead appears wide and tall.

Because of the abnormal forehead, there is less space for the normal facial features to develop. This results in shallow eye sockets and flat cheekbones. The shallow eye sockets make the eyes more prominent or bulging and cause the eyes to be more separated than normal (hypertelorism). The underdeveloped eye sockets, cheekbones, and lower jaw cause the face to appear flat. Furthermore, the minor downward slant of the eyes along with the drooping eyelids (ptosis) adds to the overall unevenness of the face.

Saethre–Chotzen syndrome (SCS), also known as Acrocephalosyndactyly type III is a rare congenital disorder associated with craniosynostosis (premature closure of one or more of the sutures between the bones of the skull). This affects the shape of the head and face, resulting in a cone-shaped head and an asymmetrical face. Individuals with SCS also have droopy eyelids (ptosis), widely spaced eyes (hypertelorism), and minor birth defects of the hands and feet (syndactyly). In addition, individuals with more severe cases of SCS may have mild to moderate mental retardation or learning disabilities. Depending on the level of severity, some individuals with SCS may require some form of medical or surgical intervention. Most individuals with SCS live fairly normal lives, regardless of whether medical treatment is needed or not.

Radioulnar synostosis is a rare condition where there is an abnormal connection between the radius and ulna bones of the forearm. This can be present at birth (congenital), when it is a result of a failure of the bones to form separately, or following an injury (post-traumatic).

It typically causes restricted movement of the forearm, in particular rotation (pronation and supination), though is not usually painful unless it causes subluxation of the radial head. It can be associated with dislocation of the radial head which leads to limited elbow extension.

Trigonocephaly (Greek: 'trigonon' = triangle, 'kephale' = head) is a congenital condition of premature fusion of the metopic suture (Greek: 'metopon' = forehead) leading to a triangular shaped forehead. The merging of the two frontal bones leads to transverse growth restriction and parallel growth expansion. It may occur syndromic involving other abnormalities or isolated.

Common signs of Say–Meyer syndrome are trigonocephaly as well as head and neck symptoms. The head and neck symptoms come in the form of craniosynostosis affecting the metopic suture (the dense connective tissue structure that divides the two halves of the skull in children which usually fuse together by the age of six). Symptoms of Say–Meyer syndrome other than developmental delay and short stature include

- Intellectual disability.

- Low-set ears/posteriorly rotated ears

- Intellectual deficit as well as learning disability

- Intrauterine growth retardation (poor growth of a baby while it is in the mother's womb)

- Posterior fontanel

- Premature synostosis of the lambdoid suture (the fusion of the bones to the joint is premature)

- Narrow forehead

- Trigonocephaly (a frontal bone anomaly that is characterized by a premature fusion of the bones which gives the forehead a triangular shape)

- Hypotelorism or hypertelorism (reduced or increased width between the eyes)

- Craniosynostosis (when one or more seam-like junctions between two bones fuses by turning into bone. This changes the growth pattern of the skull)

- Low birth weight and height

The affected patients sometimes show a highly arched palate, clinodactyly (a defect in which toes or fingers are positioned abnormally) and ventricular septal defect (a heart defect that allows blood to pass directly from left to the right ventricle which is caused by an opening in the septum). Overall, Say–Meyer syndrome impairs growth, motor function, and mental state.

Many people with this disorder have a premature fusion of skull bones along the coronal suture. Not every case has had craniosynostosis however. Other parts of the skull may be malformed as well. This will usually cause an abnormally shaped head, wide-set eyes, low set ears and flattened cheekbones in these patients. About 5 percent of affected individuals have an enlarged head (macrocephaly). There may also be associated hearing loss in 10-33% of cases and it is important for affected individuals to have hearing tests to check on the possibility of a problem. They can lose about 33-100% of hearing.

Most people with this condition have normal intellect, but developmental delay and learning disabilities are possible. The signs and symptoms of Muenke syndrome vary among affected people, and some findings overlap with those seen in other craniosynostosis syndromes. Between 6 percent and 7 percent of people with the gene mutation associated with Muenke syndrome do not have any of the characteristic features of the disorder.

Many of the characteristic facial features (among other) of Jackson–Weiss syndrome result from the premature fusion of the skull bones. The following are some of the more common, such as:

- Preaxial foot polydactyl

- Tarsal synostosis

- Frontal bossing

- Proptosis

The bones of children are very malleable in infancy. This will generally mean that, despite the presence of a coalition, the bones can deform enough to allow painless walking until the child's skeleton has matured enough. 'Skeletal maturing' means that bone is laid down in the tissue that forms the immature bone shape gradually until adult bone is achieved at about the age of seventeen years in the feet. Other body parts reach skeletal maturity at different times. The onset of symptoms related to a tarsal coalition usually occurs at about nine to seventeen years of age, with a peak incidence occurring at ten to fourteen years of age. Symptoms may start suddenly one day and persist, and can include pain (may be quite severe), lack of endurance for activity, fatigue, muscle spasms and cramps, an inability to rotate the foot, or antalgic gait.

Congenital radioulnar synostosis is rare, with approximately 350 cases reported in journals, and it typically affects both sides (bilateral) and can be associated with other skeletal problems such as hip and knee abnormalities, finger abnormalities (syndactyly or clinodactyly), or Madelung's deformity. It is sometimes part of known genetic syndromes such as triple X-Y (XXXY), Apert's, William's, or Holt-Oram. It has been reported to run in families typically following an autosomal dominant inheritance pattern which means children of an affected parent have a 50% chance of having the condition. When associated wth amegakaryocytic thrombocytopenia this inheritance has been found to be caused by mutations to the "HOXA11" gene.

Diagnosis can be characterized by typical facial and cranial deformities.

Observatory signs of trigonocephaly are:

- a triangular shaped forehead seen from top view leading to a smaller anterior cranial fossa

- a visible and palpable midline ridge

- hypotelorism inducing ethmoidal hypoplasia

Imaging techniques (3D-CT, Röntgenography, MRI) show:

- epicanthal folds in limited cases

- teardrop shaped orbits angulated towards the midline of the forehead ('surprised coon' sign) in severe cases

- a contrast difference between a röntgenograph of a normal and a trigonocephalic skull

- anterior curving of the metopic suture seen from lateral view of the cranium on a röntgenograph

- a normal cephalic index (maximum cranium width / maximum cranium length) however, there is bitemporal shortening and biparietal broadening

The neuropsychological development is not always affected. These effects are only visible in a small percentage of children with trigonocephaly or other suture synostoses. Neuropsychological signs are:

- problems in behaviour, speech and language

- mental retardation

- neurodevelopmental delays such as ADHD (Attention Deficit Hyperactivity Disorder), ODD (Oppositional Defiant Disorder), ASD (Autism Spectrum Disorder) and CD (Conduct Disorder). Many of these delays become evident at school age.

Muenke syndrome, also known as FGFR3-related craniosynostosis, is a human specific condition characterized by the premature closure of certain bones of the skull during development, which affects the shape of the head and face. First described by Maximilian Muenke, the syndrome occurs in about 1 in 30,000 newborns. This condition accounts for an estimated 8 percent of all cases of craniosynostosis.

Tarsal coalition (also known as peroneal spastic flatfoot, calcaneonavicular bar, talocalcaneal bar, tarsal synostosis, or tarsal dysostosis) is an abnormal connecting bridge of tissue between two normally-separate tarsal bones. The term 'coalition' means a coming together of two or more entities to merge into one mass. The tissue connecting the bones, often referred to as a "bar", may be composed of fibrous or osseous tissue. The two most common types of tarsal coalitions are calcaneo-navicular and talo-calcaneal, comprising 90% of all tarsal coalitions. There are other bone coalition combinations possible, but they are very rare. Symptoms tend to occur in the same location, regardless of the location of coalition: on the lateral foot, just anterior and below the lateral malleolus. This area is called the sinus tarsi.

A dysostosis is a disorder of the development of bone, in particular affecting ossification.

Examples include craniofacial dysostosis, Klippel–Feil syndrome, and Rubinstein–Taybi syndrome.

It is one of the two categories of constitutional disorders of bone (the other being osteochondrodysplasia).

When the disorder involves the joint between two bones, the term "synostosis" is often used.

Jackson–Weiss syndrome (JWS) is a genetic disorder characterized by foot abnormalities and the premature fusion of certain bones of the skull (craniosynostosis), which prevents further growth of the skull and affects the shape of the head and face. This genetic disorder can also sometimes cause intellectual disability and crossed eyes as well, it was characterized in 1976.

The growth retardation dates from the intrauterine period (development in the uterus.) The long-term developmental growth and outcome is not known, but the early childhood development is known, which is said to be moderately delayed. Craniosynostosis is usually rare among the X-Linked Intellectual Disability Syndromes, but when it is present, it affects the metopic structure (forehead).

Tsukuhara syndrome is an infrequently occurring skeletal dysplasia characterised by a caudal synostosis of the vertebra at birth.

Hypotelorism is a medical condition in which there is an abnormally decreased distance between two organs or bodily parts, usually pertaining to eyes (orbits), also known as orbital hypotelorism.

It is characterized by a nearly symmetrical presence of a spoon hand (classical type) or, more frequently, an oligodactylous hand. Individuals with this syndrome present the following symptoms: carpal, metacarpal and digital synostoses, disorganization of carpal bones, numeric reduction of digital rays and toe syndactyly. Additionally, other symptoms may include radioulnar synostosis, brachymesomelia, radius head dislocation, metatarsal synostoses and numeric reduction of rays.

Cenani–Lenz syndactylism, also known as Cenani–Lenz syndrome or Cenani–syndactylism, is an autosomal recessive congenital malformation syndrome involving both upper and lower extremities.