What is callus?

Newly formed bone tissue is called callus. The term callus is derived from the Latin word "callus", which can be translated as "callus" or "thick skin". Callus is usually found after a broken bone and is used to heal and bridge the fracture in the bone. In such a case, the callus is also called "bone callus" or "fracture callus".

The callus is built up by so-called osteoblasts. Osteoblasts are the cells that are responsible for the formation of bone tissue. The callus formed by the osteoblasts eventually ossifies over time and thus ensures stable and usually complete healing and stability of the bone.

Read more on the subject at: Broken bone


After a bony fracture, a callus is formed by osteoblasts. In fracture healing, there is a choice between primary (direct) and secondary (indirect) fracture healing. The formation of a callus can usually only be found in secondary fracture healing. In primary fracture healing, the bone parts are usually still in direct contact with one another despite the fracture. In contrast to primary fracture healing, secondary fracture healing is characterized by the fact that the bones do not have fracture ends close to one another due to the fracture. This is also the main reason for the callus formation. By removing the pieces of bone from each other, a kind of bridging must occur. This is guaranteed by the callus.

First of all, scar tissue forms. The osteoblasts are then stimulated to form a soft callus. The soft callus can eventually solidify and thus stabilize the bone. The bone can again be slightly stressed through the formation of the callus and thus forms the basis for further remodeling measures within the bone and thus the final bone healing.

Phases of callus formation

Callus formation during fracture healing only occurs during secondary (indirect) fracture healing. In this case, the distance between the bone parts is too large, shifted slightly to difficult, or movements between the bone parts are possible.

Secondary fracture healing can be divided into five phases. First of all, the so-called "injury phase" takes place. This initially results in the destruction of the side of the bone parts facing the fracture. A hematoma forms, which in turn attracts inflammatory cells to the area of ​​the fracture. This phase is then referred to as the "inflammation phase". In this phase, in addition to the breakdown of the hematoma, bone-forming cells are also built up. The injury and inflammation phase take about the first four to six weeks after a fracture.

After four to six weeks, the inflammatory phase is followed by the granulation phase. In the granulation phase the inflammation has subsided and a soft callus develops. This consists largely of fibroblasts, collagen and sprouting capillaries. In the “callus hardening” that follows, this soft callus is finally hardened by mineralization of the newly formed tissue. The callus hardening phase should be completed after four months at the latest.

The last phase is called the "conversion phase". The load on the bone is possible again after the callus hardening, which leads to various remodeling measures within the bone. The nutrient supply of the newly built bone is established in this phase. After six months to two years, secondary bone healing is finally complete.

What is hypertrophic callus?

A hypertrophic callus is a callus formation that is very rapid and usually excessively strong. This can have various causes. Probably the most common cause of excessive callus formation after a fracture, however, is inadequate or inadequate immobilization of the fractured bone.

In contrast to atrophic callus, this type of callus formation shows a good blood supply and thus the function of building new bone tissue. By immobilizing the fracture gap, a hypertrophic callus can be avoided or reduced.

What is atrophic callus?

Atrophic callus is a reduced callus formation. The reduced callus formation is often caused by a greatly reduced blood flow in the area of ​​the bone fracture. The cause of the reduced blood flow is usually to be found in the fact that dead bone fragments collect in the fracture gap. The reduced blood flow prevents the bone-forming cells from building up, so that the fracture site remains unstable for a long time.

In the event of atrophic callus formation, surgery is often inevitable. The bone pieces are removed from the fracture site and the bone is stabilized with plates and nails if necessary.

When can I see callus in the X-ray?

In the course of secondary (indirect) fracture healing, the bone goes through various phases of healing. The first of these phases consists in the fact that the bones shorten slightly due to necrosis in the area of ​​the fracture and a widening of the fracture gap can thus be seen in the X-ray image. After about two weeks, the healing phase is complete.

The inflammation phase follows the injury phase described above. This usually takes another two to four weeks. After the inflammation phase, the granulation phase finally follows, in which a soft callus is formed. In most cases, the callus can be seen on the X-ray four to six weeks after the break.In contrast, if the callus is visible in the X-ray image, it can be concluded that the break was at least four weeks ago. Callus shows up on the x-ray as a somewhat thickened, usually less pigmented and indistinctly delimited expression between the break ends.

In the case of primary fracture healing, however, there is no callus formation, so that this cannot be seen in the X-ray image at any time.

How long can you see callus?

The regression of the callus can take several months to years. The broken bone gains stability through the formation of the callus, so that the broken bone can gradually be loaded again. In the course of wound healing, the callus can also be referred to as "excess bone", which is broken down again over time. During wound healing, however, part of the callus is also converted into solid bone tissue, which is why parts of the callus remain as an integral part of the previously broken bone for life.

In the first few years this can usually still be seen clearly in the X-ray image. Over the years, all excess parts of the callus are finally completely broken down, so that after a while the original callus can hardly be separated from the bone tissue. In the area of ​​the former fracture, a slightly thickened bone, which was caused by the original callus formation, may under certain circumstances be visible for a lifetime on the X-ray.

What is callus distraction?

Callus distraction, also called callotasis, is the deliberate severing of a bone in order to increase its length in the further course.

The procedure for a callus distraction always follows the same principle. First, the bone to be treated is severed. This is followed by fixation of the bone so that the fracture ends of the bone are held at a certain distance from one another or are also pulled. Over a period of several weeks, a callus forms between the two ends of the fracture, which eventually solidifies and becomes stable bone material. The elongated bone grows together in this position.

The most common indication for callus distraction is bone malposition. For this reason, orthopedic surgeons are usually treated as specialists in this specialty. A typical example in which callus distraction can occur is the functionally relevant leg length difference. Callus distraction allows the short leg to be adapted to the length of the other leg by stable callus formation. A so-called ISKD nail is usually used for this operation. Callus distractions are rarely carried out during cosmetic surgery.

How can you accelerate / stimulate callus formation?

Direct callus formation is difficult to influence. However, various measures can be used to influence the phases before the start of callus formation. In the first four weeks after the fracture, it is crucial that many vessels sprout into the area of ​​the fracture ends. It is generally accepted that smoking hinders the formation of new blood vessels and thus prolongs it. In the first few weeks after a broken bone, you should avoid smoking cigarettes or using nicotine patches if possible.

In the phases of fracture healing, growth factors such as BMP-2 and BMP-3 play an important role. These growth factors have now been clinically approved in some cases, but should under no circumstances be taken without consulting a doctor, as the growth is completely uncontrolled.

Vitamin D and calcium are further factors for the rapid formation of a stable bone in the course of callus formation. These have essential functions during the building of a new bone.


Extracorporeal shock wave therapy, also known as ESWT for short, is mainly used today for the treatment of various “stone diseases”, such as kidney or gallstones. In addition, however, the extracorporeal shock wave therapy can also be used for purposes of bone fractures. In extracorporeal shock wave therapy, bones and other solid tissue components are specifically shattered with the help of shock waves, which can promote subsequent healing.

As a rule, after a bone fracture, callus automatically forms between the ends of the bones. In rare cases, however, the callus may not form, which is why the callus formation must be activated in a targeted manner in order to avoid possible pseudoarthroses and to enable bone healing. Shock wave therapy enables targeted fragmentation of the bone, which then initiates callus formation.

Read more on the subject at: Extracorporeal shock wave therapy

Callus pain

If pain occurs in the area of ​​the bone fracture, this can be an indication of a bone fracture healing disorder. A callus forms after a few weeks after the bone fracture. The callus forms an integral part of the bone in the following weeks. However, various complications can arise during callus formation.

If the callus remains unstable even after several weeks and does not develop into a solid bone component, this usually shows up as a painful and unstable fracture point in the affected person. Audible noises are also often described by those affected. Symptoms of this kind are caused by the fact that individual bone fragments are still moving in the fracture site because they were not solidified by the callus. If such symptoms occur, an X-ray examination should always be made to rule out a possible disruption of the healing process.

Read more on the subject at: Bone pain

Swelling of the callus

After a bone fracture, the fragments of the bone are connected within a few weeks by an initially unstable and then stable callus. Before the callus can form, however, not only blood but also tissue water collects at the break point. This leads to edema and associated swelling at the break point.

With the subsequent callus formation, this swelling gradually decreases and there should normally be no further swelling during the healing of the fracture. If there is renewed swelling in the area of ​​the broken bone, this should be examined by a doctor, as this could be an indication of a bone healing disorder. Possible inflammation could also be the cause of swelling in the area of ​​the broken bone and thus the callus.

Read more on the subject at: Inflammation in the bone