What is callus?
Callus is the name given to newly formed bone tissue. 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 Kncohen fracture and is used to heal and bridge the fracture in the bone.
In such cases, callus is also called “bone callus” or “fracture callus”. The callus is built up by so-called osteoblasts. Osteoblasts are thus the cells responsible for the formation of bone tissue. The callus formed by the osteoblasts eventually ossifies over time, thus ensuring stable and usually complete healing and stability of the bone.
Function
After a bony fracture, a callus is formed by osteoblasts. During fracture healing, a distinction is made between primary (direct) and secondary (indirect) fracture healing. The formation of a callus is usually only found in secondary fracture healing.
In primary fracture healing, the bone parts are usually still in direct contact with each other despite the fracture. Secondary fracture healing, in contrast to primary fracture healing, is characterized by the fact that the fracture does not cause the bones to have any near fracture ends. This is also the main reason for callus formation.
The removal of the bone fragments from each other must result in a kind of bridging. This is ensured by the callus. First, scar tissue is formed.
Then the osteoblasts are stimulated to form a soft callus. The soft callus can finally harden and thus stabilize the bone. By forming the callus, the bone can be lightly loaded again and thus provides the basis for further remodelling measures within the bone and thus for 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, slightly to severely displaced, or movement between the bone parts is possible. Secondary fracture healing can be divided into five phases.
First, the so-called “injury phase” takes place. The first stage involves destruction of the side of the bone parts facing the fracture. This leads to the formation of a hematoma, which in turn attracts inflammatory cells into the area of the fracture.
This phase is then called the “inflammatory phase”. In addition to the breakdown of the hematoma, bone-forming cells are also built up in this phase. The injury and inflammation phase takes 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 is formed. This consists largely of fibroblasts, collagen and capillaries.
In the following “callus hardening” 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 “reconstruction phase”.
After callus hardening, the bone can be loaded again, which leads to various remodelling measures within the bone. The nutrient supply of the newly rebuilt bone is established in this phase. After six months to two years, secondary bone healing is finally complete.
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