What is Synostosis?

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  • Written By: Mary McMahon
  • Edited By: Kristen Osborne
  • Last Modified Date: 04 March 2020
  • Copyright Protected:
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A synostosis is a fusion of two neighboring bones. Sometimes, more than two bones can be involved, creating an aggregate synostosis where multiple bones are joined together when they should be separately articulated. In some cases, treatment may not be necessary, and in others, surgery is required to separate the bones and correct the problem. This can be performed by an orthopedic surgeon. After surgery, the site may need to be placed in a cast for stability during healing.

A common example is cranial synostosis, where one or more of the plates in the skull fuse together. While they are designed to do so, if this happens too early, the skull has no room to grow. Skull abnormalities can develop because the unfused bones will expand more, creating an irregularly shaped head. Patients can also have radio-ulnar synostosis, where the bones of the lower arm fuse, often near the elbow, limiting range of motion. Syndactyly, a condition where two fingers fuse together, is another example.


Usually synostosis is very visible. The patient may appear anatomically abnormal and can also report pain, stiffness, and a limited range of motion. If an X-ray is taken, it will clearly show the fused bones. The patient can be evaluated to determine if any steps need to be taken to address the problem. In surgery, the bones will be carefully separated and smoothed to discourage them from growing back together. With a complicated case, like one where the small bones of the hand or foot fuse together, the surgery can be very challenging to perform safely and accurately.

The causes of synostosis are not fully understood. Some people are born with fused bones, and other develop ossification between bones as they mature, sometimes as a result of a problem with ligaments and muscles surrounding the bone. Trauma is sometimes linked with abnormal fusion of bone and there are some genetic disorders where people are predisposed to ossification of body tissue, potentially leading to synostoses of some of the bones in the body.

Function after surgery will not be perfect. Patients may have pain and stiffness and could have a smaller range of motion than other people even after a synostosis is surgically corrected. Surgery will improve functionality over the fused bone, but it is not possible to restore the bones as though the fusion never occurred. People with mild impairments caused by abnormal fusion can discuss the risks and benefits of surgery to see if it is appropriate for their needs.



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