When a bone is fractured, a callus forms as the fracture heals. This tissue ossifies over time and provides full restoration of function and stability. However, under certain conditions, fracture healing may be pathologic and may involve various complications.
What is the callus?
The term callus is derived from the Latin word callus (“callus,” “thick skin“). This term stands for newly formed bone tissue after a fracture. Scar tissue initially forms at the fracture site, bridging the fracture gap. Gradually, the callus ossifies and forms new bone tissue. The terms bone callus or ”’fracture callus”’ are often used synonymously. In bone healing, a distinction is made between a primary and a secondary healing process. Only secondary bone formation results in callus formation, which can be visualized radiographically after several days to weeks. Depending on the phase of bone healing, different forms of callus are distinguished: callus made of pure connective tissue is called myelogenous, periosteal or endosteal callus, depending on the type of forming connective tissue. If this solidifies due to the incorporation of calcium, it is a provisional callus or intermediate callus. Shortly before complete healing, bony callus forms, which is modeled and degraded over time.
Anatomy and structure
Depending on the phase of bone healing, callus forms from different tissues. Fibrocartilaginous callus consists of tight connective and cartilaginous tissue and provisionally connects the fracture ends. This tissue is transformed into woven bone during endochondral ossification. Unlike lamellar bone, this is an immature form of bone in which the collagen fibers of the bone matrix do not run in any particular direction but crisscross. Only in the final stage of the healing process are the fibers of the bone matrix aligned in parallel, resulting in a load-bearing lamellar bone. The callus, which was initially cartilaginous and connective tissue-like, is completely ossified at this point.
Function and tasks
A distinction is made between primary and secondary bone healing. Primary bone healing proceeds via Havers’ canals. These are channels in the bone cortex that contain blood vessels and nerve fibers. The function of the Haversian canals is to supply the bone with nutrients and to transmit stimuli. If the width of the fracture gap is less than one millimeter and the outer periosteum is still intact, capillary-rich connective tissue can grow into the fracture gap through the Haversian canals. Cells from the inner and outer periosteum are incorporated and remodeled in such a way that the load-bearing capacity of the bone is restored after about three weeks. Secondary fracture healing occurs when the gap between the bone parts is too large or the fracture ends are slightly displaced. Secondary healing with callus formation is also necessary if movement between the fracture parts is possible. Secondary fracture healing occurs in five stages. First, force is applied to the bone, which destroys the bone structure and results in the formation of a hematoma (injury phase). In the subsequent inflammatory phase, macrophages, mast cells and granulocytes invade the hematoma. Simultaneously with the breakdown of the hematoma, bone-forming cells are established. After four to six weeks, the inflammation subsides and the granulation phase occurs. Now a soft callus is formed from fibroblasts, collagen and capillaries. New bone tissue is built up in the periosteum area. In the fourth phase (callus hardening), the soft callus hardens and the newly formed tissue mineralizes. After about three to four months, the physiological load-bearing capacity is restored. In the final phase (remodeling phase), the original bone structure with medullary cavity and Haversian canals for nutrient supply is restored. Secondary bone healing can take six months to two years. The length of time depends on various factors such as the type of bone or the age of the affected person.
Diseases
Bone healing does not always proceed physiologically. Disturbances in the healing process can occur due to a lack of supply of acidic and nutrient-rich blood.In addition, a normal anatomical position of the bone parts with close contact to each other is required. The mobility of the two parts should be reduced to the minimum, and permanent compression forces also accelerate fracture healing. Open fractures can delay the healing process or make it impossible if it results in infection of the bone or surrounding tissues. Regular nicotine consumption and diseases that impair blood circulation, such as diabetes or osteoporosis, also have a negative effect on fracture healing. If one or more of these conditions are present, a pathological course may result. Failure of the bony callus to form within the regular period is referred to as delayed fracture healing. If this lasts longer than six months, pseudarthrosis may occur. This is an additional, pathological joint in the bone. The reason for this is usually inadequate immobilization. However, not only a lack of callus formation, but also excessive callus formation can lead to the occurrence of pseudarthrosis. This is caused by excessive compression of the fracture sites, which is also due to inadequate immobilization. If the fracture is located in or near a joint, movement may be restricted in the course of healing and, subsequently, contracture of the affected joint may occur. In very rare cases, callus formation damages nerves and vessels near the bone by compression.
