A bony callus, also called a fibrocartilage callus, comprises a critical early step in the body's attempt to heal a bone fracture. It is marked by the formation of bony and cartilaginous materials into a bridge-like structure around the break.
The process starts when a provisional callous forms around the site of the fracture, usually between 1 to 2 weeks after the original break occurs. In the weeks to follow, each side of the break begins to form what is called a collar, a composite of osteoblasts (cells forming the new bone) and the periosteum (where the new bone is formed). Subsequently, through rapid proliferation, the two collars grow towards one another, closing the gap of the fracture.
The resulting bony callus is usually the first sign of healing that a physician views on an X-ray, typically within 2 to 3 weeks following the injury. The cartilage produced in the process gives way during this period to stronger bony extensions that form at the fracture ends, joining them once more. Within subsequent months or years, the callus is eventually reabsorbed by the body after remodeling the bone.
Bony callus formation is generally quicker in children than with adults, and cortical (compact) bone heals quicker than cancellous (trabecular or spongy) bone.