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.

These conditions nearly all present with an insidious onset of pain referred to the location of the bony damage. Some, notably Kienbock's disease of the wrist, may involve considerable swelling, and Legg-Calvé-Perthes disease of the hip causes the victim to limp. The spinal form, Scheuermann's disease, may cause bending, or kyphosis of the upper spine, giving a "hunch-back" appearance.

HME can cause pain to people of all ages. To children, this can be especially painful. During exercise, it can cause a significant amount of pain. Exostoses may be visible to naked eye from outside. Multiple deformities, as mentioned above, can be present. The Exotoses appear to slow their rate of growth when they reach a certain, variable mass.

It is characterized by the growth of cartilage-capped benign bone tumours around areas of active bone growth, particularly the metaphysis of the long bones. Typically five or six exostoses are found in upper and lower limbs. Most common locations are:

- Distal femur (70%)

- Proximal tibia (70%)

- Humerus (50%)

- Proximal fibula (30%)

HME can lead to the shortening and bowing of bones; affected individuals often have a short stature. Depending on their location the exostoses can cause the following problems: pain or numbness from nerve compression, vascular compromise, inequality of limb length, irritation of tendon and muscle, Madelung's deformity as well as a limited range of motion at the joints upon which they encroach. A person with HME has an increased risk of developing a rare form of bone cancer called chondrosarcoma as an adult. Problems may be had in later life and these could include weak bones and nerve damage. The reported rate of transformation ranges from as low as 0.57% to as high as 8.3% of people with HME.

Osteochondrosis is a family of orthopedic diseases of the joint that occur in children and adolescents and in rapidly growing animals, particularly pigs, horses, dogs, and broiler chickens. They are characterized by interruption of the blood supply of a bone, in particular to the epiphysis, followed by localized bony necrosis, and later, regrowth of the bone. This disorder is defined as a focal disturbance of endochondral ossification and is regarded as having a multifactorial cause, so no one thing accounts for all aspects of this disease.

The skeletal structures of individuals who have this disorder may have pronounced deformities. As reported by several medical doctors, the following features are commonly found in people who suffer from nail–patella syndrome:

Bones and joints

- Patellar involvement is present in approximately 90% of patients; however, patellar aplasia occurs in only 20%.

- In instances in which the patellae are smaller or luxated, the knees may be unstable.

- The elbows may have limited motion (e.g., limited pronation, supination, extension).

- Subluxation of the radial head may occur.

- Arthrodysplasia of the elbows is reported in approximately 90% of patients.

- General hyperextension of the joints can be present.

- Exostoses arising from the posterior aspect of the iliac bones ("iliac horns") are present in as many as 80% of patients; this finding is considered pathognomonic for the syndrome.

- Other reported bone changes include scoliosis, scapular hypoplasia, and the presence of cervical ribs.

Kidney issues may arise such as proteinuria and nephritis. Proteinuria is usually the first sign of renal involvement and either rapidly or years after suffering from asymptomatic proteinuria, renal failure occurs in around 5% of NPS patients. Hypothyroidism, irritable bowel syndrome, attention deficit hyperactivity disorder (ADHD), and thin tooth enamel are associated with NPS, but whether these are related or simply coincidences are unclear.

Rocker bottom foot, also known as congenital vertical talus, is an anomaly of the foot. It is characterized by a prominent calcaneus (heel bone) and a convex rounded bottom of the foot. It gets its name from the foot's resemblance to the bottom of a rocking chair.

It can be associated with Edwards' syndrome (trisomy 18), Patau syndrome (trisomy 13), Trisomy 9 and mutation in the gene HOXD10.

It can also be associated with Charcots foot.

An exostosis (plural: exostoses) is the formation of new bone on the surface of a bone. Exostoses can cause chronic pain ranging from mild to debilitatingly severe, depending on the shape, size, and location of the lesion. It is most commonly found in places like the ribs, where small bone growths form, but sometimes larger growths can grow on places like the ankles, knees, shoulders, elbows and hips. Very rarely are they on the skull.

They normally form on the bones of joints, and can grow upwards. For example, if an extra bone formed on the ankle, it might grow up to the shin.

Osteophytes are bone spurs that develop on the margins of joints secondary to external stimuli such as osteoarthritis. However, these are not always distinguished from exostoses in any definite way.

When used in the phrases "cartilaginous exostosis" or "osteocartilaginous exostosis", the term is considered synonymous with osteochondroma. Some sources consider the two terms to mean the same thing even without qualifiers, but this interpretation is not universal.

Osteopenia is a condition in which bone mineral density is lower than normal. It is considered by many doctors to be a precursor to osteoporosis. However, not every person diagnosed with osteopenia will develop osteoporosis. More specifically, osteopenia is defined as a bone mineral density T-score between −1.0 and −2.5.

Painful fat herniation, described as painful feet due to fat herniations through the thin fascial layers of the weight-bearing parts of the heel is rare, piezogenic pedal papules or Piezogenic papules are very common. These papules can also be found in wrist areas and commonly found in connective tissue disorders, especially Ehlers Danlos Syndrome. However, due to their preponderence amongst humans, their presence does not automatically mean the existence of Ehlers-Danlos syndrome and in most cases they are of little significance.

A buccal exostosis (also termed alveolar exostosis), is an exostosis (bone prominence) on the buccal surface (cheek side) of the alveolar ridge of the maxilla or mandible. Some consider them a variation of normal anatomy rather than a disease.

Navicular syndrome, often called navicular disease, is a syndrome of lameness problems in horses. It most commonly describes an inflammation or degeneration of the navicular bone and its surrounding tissues, usually on the front feet. It can lead to significant and even disabling lameness.

Subungual exostoses are bony projections which arise from the dorsal surface of the distal phalanx, most commonly of the hallux.

Nail–patella syndrome (NPS) (also known as "HOOD syndrome") is a genetic disorder that results in small, poorly developed nails and kneecaps, but can also affect many other areas of the body, such as the elbows, chest, and hips. The name "nail–patella" can be very misleading because the syndrome often affects many other areas of the body, including even the production of certain proteins. Those affected by NPS may have one or more affected areas of the body, and its severity varies depending on the individual. It is also referred to as iliac horn syndrome, hereditary onychoosteodysplasia (HOOD syndrome), Fong disease or Turner–Kieser syndrome.

Diagnosis of NPS can be made at birth, but is common for it to remain undiagnosed for several generations. While there is no cure available for NPS, treatment is available and recommended.

Formation on the lower jaw occurs much less commonly than on the upper jaw. They are painless and have no risk of turning into cancer, although they may slowly expand over years, or sometimes more rapidly.

Osteophytes, commonly referred to as bone spurs are bony projections that form along joint margins. They should not be confused with enthesophytes, which are bony projections that form at the attachment of a tendon or ligament. Osteophytes are not always distinguished from exostoses in any definite way, although in many cases there are a number of differences. Osteophytes are typically intra-articular (within the joint capsule).

Osteophytes form because of the increase in a damaged joint's surface area. This is most common from the onset of arthritis. Osteophytes usually limit joint movement and typically cause pain.

Osteophytes form naturally on the back of the spine as a person ages and are a sign of degeneration in the spine. In this case, the spurs are not the source of back pains, but instead are the common symptom of a deeper problem. However, bone spurs on the spine can impinge on nerves that leave the spine for other parts of the body. This impingement can cause pain in both upper and lower limbs and a numbness or tingling sensations in the hands and feet because the nerves are supplying sensation to their dermatomes.

Spurs can also appear on the feet, either along toes or the heel, as well as on the hands. In extreme cases, bone spurs have grown along a person's entire skeletal structure: along the knees, hips, shoulders, ribs, arms and ankles. Such cases are only exhibited with multiple exostoses.

Osteophytes on the fingers or toes are known as Heberden's nodes (if on the distal interphalangeal joint) or Bouchard's nodes (if on the proximal interphalangeal joints).

Osteophytes may also be the end result of certain disease processes. Osteomyelitis, a bone infection, may leave the adjacent bone with a spur formation. Charcot foot, the neuropathic breakdown of the feet seen primarily in diabetics, can also leave bone spurs that may then become symptomatic.

The Geist classification divides the accessory navicular bones into three types.

- Type 1: An os tibiale externum is a 2–3 mm sesamoid bone in the distal posterior tibialis tendon. Usually asymptomatic.

- Type 2: Triangular or heart-shaped ossicle measuring up to 12 mm, which represents a secondary ossification center connected to the navicular tuberosity by a 1–2 mm layer of fibrocartilage or hyaline cartilage. Portions of the posterior tibialis tendon sometimes insert onto the accessory ossicle, which can cause dysfunction, and therefore, symptoms.

- Type 3: A cornuate navicular bone represents an enlarged navicular tuberosity, which may represent a fused Type 2 accessory bone. Occasionally symptomatic due to bunion formation.

An accessory navicular bone is an accessory bone of the foot that occasionally develops abnormally in front of the ankle towards the inside of the foot. This bone may be present in approximately 2-21% of the general population and is usually asymptomatic. When it is symptomatic, surgery may be necessary.

Surgery can be performed at any age because it does not alter any other bones.

Symptoms of an accessory navicular bone may include plantar fasciitis, bunions and heel spurs.

They tend to be painful due to the pressure applied to the nail bed and plate. They can involve destruction of the nail bed. These lesions are not true osteochondromas, rather it is a reactive cartilage metaplasia. The reason it occurs on the dorsal aspect is because the periosteum is loose dorsally but very tightly adherent volarly.

Subungal malignant melanomas can also occur in canines.

They are distinct from subungual osteochondroma.

Heel pain is very common in horses with navicular syndrome. Lameness may begin as mild and intermittent, and progress to severe. This may be due to strain and inflammation of the ligaments supporting the navicular bone, reduced blood flow and increased pressure within the hoof, damage to the navicular bursa or DDF tendon, or from cartilage erosion.

Affected horses display a "tiptoe" gait - trying to walk on the toes due to heel pain. They may stumble frequently. The lameness may switch from one leg to another, and may not be consistent. Lameness usually occurs in both front feet, although one foot may be more sore than the other.

Lameness is usually mild (1–2 on a scale of 5). It can be made worse when the horse is worked on a hard surface or on a circle.

After several months of pain, the feet may begin to change shape, especially the foot that has been experiencing the most pain, which tends to become more upright and narrow.

Osteochondromatosis is a condition involving a proliferation of osteochondromas.

Types include:

- Hereditary multiple exostoses

- Synovial osteochondromatosis

Like osteoporosis, osteopenia occurs more frequently in post-menopausal women as a result of the loss of estrogen. It can also be exacerbated by lifestyle factors such as lack of exercise, excess consumption of alcohol, smoking or prolonged use of glucocorticoid medications. It can also be a result of exposure to radiation.

Osteopenia occurs more frequently in participants in non-weight-bearing sports like bicycling or swimming than in participants in weight-bearing sports like powerlifting and running, since bone-loading exercise tends to protect or possibly increase bone mineral density.

In particular, the condition is often noted in young female athletes. It is one of the three major components of female athlete triad syndrome, along with amenorrhea and disordered eating. Female athletes tend to have lower body weight, lower fat percentage, and higher incidence of asthma than their less active peers. A chronic negative energy balance can suppress estrogen levels and decrease bone mineral density.

It is also a sign of normal aging, in contrast to osteoporosis which is present in pathologic aging.

Osteopenia is also a common effect of coeliac disease, even among patients who are otherwise asymptomatic.

Limited normal functions and movements are caused by osteochondromas growing slowly and inwardly. The majority of osteochondromas are symptomless and are found incidentally. Each individual with osteochondroma may experience symptoms differently and most of the time individuals will experience no symptoms at all. Some of the most common symptoms are a hard immobile painless palpable mass, adjacent muscle soreness, and pressure or irritation with heavy exercising.

Major symptoms arise when complications such as fractures, bone deformity or mechanical joint problems occur. If the occurrence of an osteochondroma is near a nerve or a blood vessel, the affected limb can experience numbness, weakness, loss of pulse or color change. Periodic changes in the blood flow can also take place. Approximately 20% of patients experiencing nerve compression commonly acknowledge vascular compression, arterial thrombosis, aneurysm, and pseudoaneurysm. Formation of pseudoaneurysm and venous thrombosis lead to claudication, pain, acute ischemia, and symptoms of phlebitis. If the tumor is found under a tendon, it can cause pain during movement causing restriction of joint motion. Pain can also occur due to bursal inflammation, swelling or fracture at the base of the tumor stalk. Some of the clinical signs and symptoms of malignant osteochondroma are pain, swelling, and mass enlargement.