Softcallus

Softcallus is an early stage of fracture healing in which a fibrocartilaginous tissue bridges the fracture gap. It develops after the initial inflammatory and hematoma phases as periosteal and endosteal cells, along with inflammatory and mesenchymal cells, invade the fracture site and differentiate into chondrocytes and fibroblasts that produce extracellular matrix.

The tissue of the softcallus is rich in type II collagen and proteoglycans and becomes vascularized, forming a flexible bridge that stabilizes the fracture before mineralization begins. Compared with bone, this tissue is relatively weak, but it provides greater stability than the hematoma alone.

In terms of timing, softcallus typically forms within the first few weeks after injury and progresses over several weeks. It is eventually replaced by hard callus through endochondral ossification, after which remodeling restores normal bone structure over months.

Imaging and histology reflect its composition. Radiographs may show a radiolucent bridging callus at the fracture site, while MRI or CT can better delineate the cartilaginous tissue. Histologically, softcallus contains fibrocartilage with chondrocytes embedded in a proteoglycan-rich matrix, with ongoing neovascularization and invasion by osteogenic cells.

Clinical significance lies in its role as a temporary stabilizing framework during healing. Adequate softcallus formation is essential for proper fracture union; impaired formation or excessive motion can contribute to delayed healing or nonunion, whereas factors such as age, smoking, diabetes, and poor nutrition can adversely affect the process.