Pycnodysostosis is a disease of the osteopetrosis type. It is characterized by the constant build-up of bone material without old bone parts being broken down. In contrast to the other syndromes of this type, the disease remains limited to the bones.


What is pycnodysostosis?

Pycnodysostosis was first described in 1962. It is a very rare hereditary bone disease characterized by the constant increase in bone mass. There are no data on the exact prevalence of this condition. However, the frequency is assumed to be less than one in a million. Overall, only a few hundred cases of this disease have been observed to date. See topbbacolleges for Definitions of Intertrigo.

The key symptom is increased bone fragility, because the inability of the osteoclasts to function means that no remodeling processes can take place in the bone. The bones are therefore constantly being built up without the old bone substance being broken down. As a result, a very high bone density develops. However, this is associated with extreme brittleness of the bones.

Pycnodysostosis thus belongs to the group of osteopetrosis. In contrast to the classic osteopetrosis, which has been described again and again over the past centuries, pycnodysostosis usually does not show anemia and no cranial nerve damage. Therefore, it can be easily distinguished from other osteopetrosis. In the past, osteopetrosis diseases were also referred to as marble bone disease.

This designation indicates that the bones are very dense and strong, but also extremely fragile and brittle. The reason for this condition is the lack of degradation processes by the osteoclasts. New substance is constantly added to the old bone substance without the remodeling process, which leads to a loss of elasticity in the bones.


Pycnodysostosis is caused by a mutation of a gene on chromosome 1. It is the gene CTSK, which is responsible for coding the enzyme cathepsin K. Cathepsin K stimulates the bone-degrading cells (osteoclasts) to break down bone substance as part of the remodeling process. Due to a lack of cathepsin K, this process cannot take place here. New bone substance is constantly being built up. The disease is inherited in an autosomal recessive manner.

Symptoms, Ailments & Signs

Pycnodysostosis manifests itself primarily in osteosclerosis. In contrast to osteoporosis, osteosclerosis is characterized by constant bone formation, which leads to high bone density. However, the bones are fragile and prone to spontaneous fractures. At the same time, the patients exhibit moderate short stature. The body size can reach up to 1.50 meters.

Unexplained acroosteolysis occurs at the terminal phalanges. This means that the terminal phalanges of the finger and toe bones show signs of disintegration. Furthermore, the clavicle appears dysplastic. In addition, characteristic cranial anomalies occur, which show up in an enlarged cranial volume, in interlocking bones between the individual cranial plates and the persistence of the large fontanel.

In addition, defective and abnormally shaped teeth (pointed and conical) can occur, which are also badly positioned. Tooth eruption is often delayed. Brittle and irregularly shaped nails sometimes occur. Anemia and cranial nerve damage are usually not part of the symptom complex of pycnodysostosis. In very rare cases, however, anemia, an enlarged liver, sleep apnea, breathing difficulties and general haematological disorders can occur. Mental development is normal.

Diagnosis & course of disease

Although pycnodysostosis is a rare disease, it can also be easily distinguished from other osteopetrosis. The gene defect is defined as a mutation in the CTSK gene on chromosome 1. A family history can clarify whether cases of this disease have already occurred in close relatives.

In terms of differential diagnosis, pycnodysostosis must be differentiated from other forms of osteopetrosis, osteoporosis, cranial dysplasia and idiopathic acroosteolysis. Pycnodysostosis can be confirmed by x-ray studies of the skull and skeleton. A genetic test should bring complete certainty.


Due to pycnodysostosis, those affected suffer from relatively high bone density. However, despite the increased bone density, the bones are fragile and break easily, so patients with pycnodysostosis are more likely to suffer from fractures and other injuries. Patients are therefore usually dependent on increased precautionary measures to prevent serious injuries.

However, short stature is not uncommon. In some cases, pycnodysostosis can also negatively affect the teeth, causing them to break off easily or have an unnatural shape. Most of those affected also suffer from brittle nails due to the disease, which also have an irregular shape. Some internal organs may be enlarged by the disease, and breathing difficulties are not uncommon.

However, pycnodysostosis has no negative impact on the mental development of the patient. A causal treatment of pycnodysostosis is usually not possible. The affected person is dependent on regular checks and examinations so that nerves are not pinched. In the worst case, this can lead to paralysis of the entire body. The life expectancy of the patient is usually not affected by the pycnodysostosis.

When should you go to the doctor?

Pycnodysostosis must always be treated by a doctor. It is not possible to treat the disease with self-help methods. The doctor should be consulted for pycnodysostosis if the person concerned suffers from various bone problems. This can lead to short stature or deformities. In this case, it is primarily the parents who have to observe the child and, if necessary, inform the doctor. Furthermore, deformations of the teeth or nails can indicate pycnodysostosis and must also be examined by a doctor. This is the only way to avoid further complications.

Pycnodysostosis is diagnosed and treated by a pediatrician or general practitioner. There are no particular complications. However, since the disease can also affect the internal organs, these should be examined regularly.

Treatment & Therapy

Due to the hereditary nature of the disease, causal therapy is not possible. Treatment can only be symptomatic. This requires interdisciplinary cooperation between physicians. Constant orthopedic monitoring of the patient is important. Broken bones, which are very difficult to heal, have to be treated again and again.

The treatment concept also includes constant monitoring of the statics of the spine. The spinal column tends to develop spondylolisthesis. Spondylolisthesis is an instability of the spine, which is characterized by the fact that its upper section can slide forward with the gliding vertebra. However, this cannot be determined so easily, since this change is not immediately visible and also does not necessarily lead to complaints.

However, there is a risk of pinching nerves, which can eventually cause severe pain and symptoms of paralysis. The prognosis of pycnodysostosis is favorable. Although it cannot be cured, there is no progressive deterioration in the course of the disease.


Pycnodysostosis is a hereditary disease with an autosomal recessive mode of inheritance. In affected families, genetic counseling can help prevent the disease from being passed on to offspring. First of all, a genetic test should be carried out.

If both parents carry this gene, there is a 25 percent chance for the offspring to inherit the disease. In people who are already ill, prophylaxis against further bone fractures and changes in the spine consists above all in constant medical monitoring.


Since the treatment of pycnodysostosis can only be symptomatic and the disease is incurable, there is no aftercare of the disease in the true sense. Nevertheless, regular visits to the doctor and follow-up care for some treatments are essential. Bone fractures caused by pycnodysostosis require regular medical monitoring.

Because the disease makes fractures more difficult to heal and complications more likely, they should be more intense than ordinary fractures. Regular monitoring for spondylolisthesis is important, since this frequently occurs in people suffering from pycnodysostosis. If the patient has already had such a slipped vertebra, follow-up care must also be taken here.

Orthopedic rehabilitation measures, accompanied by a doctor or orthopedist, are recommended after successful treatment of the spondylolisthesis and freedom from pain in the patient. They can usually be used after about twelve weeks. These measures usually consist of physiotherapy exercises, relaxation exercises and physical activity, which can also be combined with other measures such as psychotherapy.

These measures are intended to strengthen the abdominal and back muscles. A general orthopedic check-up of the patient is also necessary, since pycnodysostosis is not a progressive disease and can generally be treated well, but side effects can occur again and again.