In the animal kingdom there also exists a non-pathological form of osteosclerosis, resulting in unusually solid bone structure with little to no marrow. It is often seen in aquatic vertebrates, especially those living in shallow waters, providing ballast as an adaptation for an aquatic lifestyle. It makes bones heavier, but also more fragile. In those animal groups osteosclerosis often occurs together with bone thickening (pachyostosis). This joint occurrence is called pachyosteosclerosis.

Osteosclerosis is a disorder that is characterized by abnormal hardening of bone and an elevation in bone density. It may predominantly affect the medullary portion and/or cortex of bone. Plain radiographs are a valuable tool for detecting and classifying osteosclerotic disorders. It can manifest in localized or generalized osteosclerosis. Localized osteosclerosis can be caused by Legg–Calvé–Perthes disease, sickle-cell disease and osteoarthritis among others. Osteosclerosis can be classified in accordance with the causative factor into acquired and hereditary.

The medication(s) listed below have been approved by the Food and Drug Administration (FDA) as orphan products for treatment of this condition. Learn more orphan products.

Approximately eight to 40 children are born in the United States each year with the malignant infantile type of osteopetrosis. One in every 100,000 to 500,000 individuals is born with this form of osteopetrosis. Higher rates have been found in Denmark and Costa Rica. Males and females are affected in equal numbers.

The adult type of osteopetrosis affects about 1,250 individuals in the United States. One in every 200,000 individuals is affected by the adult type of osteopetrosis. Higher rates have been found in Brazil. Males and females are affected in equal numbers.

The odds are greater in the Russian region of Mari El (1 of every 14,000 newborns) and much greater in Chuvashia (1 of every 3,500—4,000 newborns) due to genetic features of the Mari people and Chuvash people, respectively.

Pachyosteosclerosis is a combination of thickening (pachyostosis) and densification (osteosclerosis) of bones. It makes bones more heavy, but also more fragile. The condition often occurs in aquatic vertebrates, especially those living in shallow waters, creating ballast as an adaptation for maintaining neutral buoyancy and horizontal trim. It is in no way pathological. To resist roll, it frequently is found especially in ventral bones, whereas concentration near the lungs helps in maintaining trim.

Examples of animals showing pachyosteosclerosis are seacows (dugongs and manatees), the extinct Plesiosauria and Mesosauria and extinct aquatic sloths.

The only effective line of treatment for malignant infantile osteopetrosis is hematopoietic stem cell transplantation. It has been shown to provide long-term disease-free periods for a significant percentage of those treated; can impact both hematologic and skeletal abnormalities; and has been used successfully to reverse the associated skeletal abnormalities.

Radiographs of at least one case with malignant infantile osteopetrosis have demonstrated bone remodeling and recanalization of medullar canals following hematopoietic stem cell transplantation. This favorable radiographic response could be expected within one year following the procedure - nevertheless, primary graft failure can prove fatal.

Osteochondrodysplasia or skeletal dysplasia is a general term for a disorder of the development (dysplasia) of bone ("osteo") and cartilage ("chondro").

Osteochondrodysplasias are rare diseases. About 1 in 5,000 babies are born with some type of skeletal dysplasia.

Osteogenesis imperfecta is a rare condition in which bones break easily. There are multiple genetic mutations in different genes for collagen that may result in this condition. It can be treated with some drugs to promote bone growth, by surgically implanting metal rods in long bones to strengthen them, and through physical therapy and medical devices to improve mobility.

Melorheostosis is a medical developmental disorder and mesenchymal dysplasia in which the bony cortex widens and becomes hyperdense in a sclerotomal distribution. The condition begins in childhood and is characterized by thickening of the bones. Pain is a frequent symptom and the bone can have the appearance of dripping candle wax.

The differential diagnosis of malignant infantile osteopetrosis includes other genetic skeletal dysplasias that cause osteosclerosis. They are collectively known as osteosclerosing dysplasias. The differential diagnosis of genetic osteosclerosing dysplasias including infantile osteopetrosis has been tabulated and illustrated in literature citations.

- Neuropathic infantile osteopetrosis

- Infantile osteopetrosis with renal tubular acidosis

- Infantile osteopetrosis with immunodeficiency

- IO with leukocyte adhesion deficiency syndrome (LAD-III)

- Intermediate osteopetrosis

- Autosomal dominant osteopetrosis (Albers-Schonberg)

- Pyknodysostosis (osteopetrosis acro-osteolytica)

- Osteopoikilosis (Buschke–Ollendorff syndrome)

- Osteopathia striata with cranial sclerosis

- Mixed sclerosing bone dysplasia

- Progressive diaphyseal dysplasia (Camurati–Engelmann disease)

- SOST-related sclerosing bone dysplasias

The disorder is progressive, with the ultimate severity of symptoms often depending on age of onset. In severe cases amputation has been performed when conservative measures such as physical therapy and regional anesthetics have been ineffective.

Idiopathic osteosclerosis is a condition which may be found around the roots of a tooth. It is usually painless and found during routine radiographs. It appears as a radiopaque (light area) around a tooth, usually a premolar or molar. There is no sign of inflammation of the tooth.

Condensing osteitis, sclerosing osteomyelitis, cementoblastoma, hypercementosis, Exostoses (tori).

Condensing osteitis may resemble idiopathic osteosclerosis, however, associated teeth are always nonvital in condensing osteitis.

A single copy of the abnormal gene from one parent is able to cause the disease; this is called autosomal dominance. A mutation in the DLX3 gene has been confirmed as the cause of TDO. The onset of TDO begins with a 4 base pair deletion on chromosome 17q21, causing a mutation, specifically frameshift, and the termination codon to be the cause of the lack of complete maturation of the tooth enamel; this mutation is also responsible for the osseous defects in the bone. DLX-3 is expressed in the placenta and is significantly important during embryonic development in regards to hard bone tissue which is present in the teeth, skull, and long bones such as in the arms and legs. During normal tooth development, DLX 3 shifts from predominant expression in the inner enamel epithelium; the outer layer does not express DLX 3. In TDO cases, the DLX-3 is present on the outer enamel epithelium and leads to the dental abnormalities seen in this disease. Improper expression of DLX-3 causes the tooth enamel to be thinner, which leads to attrition and is most often the cause of dental abscess seen in TDO persons.

During osseous, connective tissue, and dermal cell differentiation, DLX 3 in TDO is also responsible for upper cranial thickness, calvaria, osteosclerosis of the long bones, long narrow head (dolichocephaly), abnormally thin brittle nails, and premature closing of fibrous joints. Consequently, 95% of people with TDO that are 16 years old or younger show skeletal abnormalities before full maturation takes place. Lack of mastoid pneumatization by mastoid cells occurs in 82% of the cases and is rarely prevalent outside of TDO diagnosis. Mastoid pneumatization occurs at about 6 months of ages and acts to minimize pressure fluctuations in the Eustachian tubes of the ear. The mastoid lies posterior to the lower jawbone (mandible) and distal to the ear. The Eustachian tube connects the middle ear to the back of the nose, and acts to create a specific pressure in the ear canal that causes vibrations to the eardrum; if adequate pressure is not attained, muffled, dull hearing results. In addition to the mastoid pneumatization assisting the Eustachian tubes for normal hearing, lack of mastoid pneumatization causes inflammation of the ear, general irritation, and does not allow enough air in to assist with mucus flowing out. It is not completely understood why gene mutations occur, but it is known that genetic mutations that cause disease are acquired from either or both parents at fertilization.

Worth syndrome is caused by a mutation in the LRP5 gene, located on human chromosome 11q13.4. The disorder is inherited in an autosomal dominant fashion. This indicates that the defective gene responsible for a disorder is located on an autosome (chromosome 11 is an autosome), and only one copy of the defective gene is sufficient to cause the disorder, when inherited from a parent who has the disorder.

Worth syndrome, also known as benign form of Worth hyperostosis corticalis generalisata with torus platinus, autosomal dominant osteosclerosis, autosomal dominant endosteal hyperostosis or Worth disease, is a rare autosomal dominant congenital disorder that is caused by a mutation in the LRP5 gene. It is characterized by increased bone density and benign bony structures on the palate.

Amelogenesis imperfecta hypomaturation type with taurodontism are often confused. Amelogenesis imperfecta of the hypomaturation type with taurodontism (AIHHT) has no hair or bone changes which helps to differentiate between TDO cases and AIHHT. Polymerase chain reaction also known as PCR is used to amply pieces of DNA and observed for the 141 base pair allele as a result of a deletion of four nucleotides in exon 3 of the DLX-3 gene. Additionally, the current research shows that there is heavy reliance on the physical characteristics in the differentiation of TDO verses AIHHT and the severity and prevalence of their expression. For instance, taurodontism is severely expressed in TDO, but mildly expressed in AIHHT. Currently, researchers are trying to identify the reason for the alteration in the DLX-3 and DLX-7 genes that are responsible for AIHHT versus TDO.

Condensing osteitis is a periapical inflammatory disease that results from a reaction to a dental related infection. This causes more bone production rather than bone destruction in the area (most common site is near the root apices of premolars and molars). The lesion appears as a radiopacity in the periapical area hence the sclerotic reaction. The sclerotic reaction results from good patient immunity and a low degree of virulence of the offending bacteria. The associated tooth may be carious or contains a large restoration, and is usually associated with a non-vital tooth.

Infection of periapical tissues of a high immunity host by organisms of low virulence which leads to a localized bony reaction to a low grade inflammatory stimulus.

Erdheim–Chester disease is associated with high mortality rates. In 2005, the survival rate was below 50% at three years from diagnosis. More recent reports of patients treated with Interferon therapy describe an overall 5-year survival of 68%. Long term survival is currently even more promising, although this impression is not reflected in the recent literature.

Approximately 500 cases have been reported in the literature to date. ECD affects predominantly adults, with a mean age of 53 years.

Lipodystrophy can be caused by metabolic abnormalities due to genetic issues. These are often characterized by insulin resistance and are associated with metabolic syndrome.

The primary sign of myelofibrosis is reactive bone marrow fibrosis, but it is often accompanied by:

- Abdominal fullness related to an enlarged spleen (splenomegaly).

- Bone pain

- Bruising and easy bleeding due to inadequate numbers of platelets

- Cachexia (loss of appetite, weight loss, and fatigue)

- Enlargement of both the liver and spleen

- Fatigue

- Gout and high uric acid levels

- Increased susceptibility to infection, such as pneumonia

- Pallor and shortness of breath due to anemia

- In rarer cases, a raised red blood cell volume

- Cutaneous myelofibrosis is a rare skin condition characterized by dermal and subcutaneous nodules.

Lipodystrophy is a disorder in which the body is unable to produce fat. The medical condition is characterized by abnormal or degenerative conditions of the body's adipose tissue. ("Lipo" is Greek for "fat", and "dystrophy" is Greek for "abnormal or degenerative condition".) A more specific term, "lipoatrophy", is used when describing the loss of fat from one area (usually the face). This condition is also characterized by a lack of circulating leptin which may lead to osteosclerosis.

The one known curative treatment is allogeneic stem cell transplantation, but this approach involves significant risks.

Other treatment options are largely supportive, and do not alter the course of the disorder (with the possible exception of ruxolitinib, as discussed below). These options may include regular folic acid, allopurinol or blood transfusions. Dexamethasone, alpha-interferon and hydroxyurea (also known as hydroxycarbamide) may play a role.

Lenalidomide and thalidomide may be used in its treatment, though peripheral neuropathy is a common troublesome side-effect.

Frequent blood transfusions may also be required. If the patient is diabetic and is taking a sulfonylurea, this should be stopped periodically to rule out drug-induced thrombocytopenia.

Splenectomy is sometimes considered as a treatment option for patients with myelofibrosis in whom massive splenomegaly is contributing to anaemia because of hypersplenism, particularly if they have a heavy requirement for blood transfusions. However, splenectomy in the presence of massive splenomegaly is a high-risk procedure, with a mortality risk as high as 3% in some studies.

In November 2011, the FDA approved ruxolitinib (Jakafi) as a treatment for intermediate or high-risk myelofibrosis. Ruxolitinib serves as an inhibitor of JAK 1 and 2.

The "New England Journal of Medicine" (NEJM) published results from two Phase III studies of ruxolitinib. These data showed that the treatment significantly reduced spleen volume, improved symptoms of myelofibrosis, and was associated with improved overall survival compared to placebo.