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How does callus help repair a broken bone?

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Internal structure of a human long bone — bone A long bone of the human body, showing a magnified cross section of the interior.

In osteology, a callus acts as a vital bridge of bony and cartilaginous material that connects the segments of a fractured bone during the healing process.1

The repair occurs in stages:

Provisional Callus: Within one to two weeks of an injury, a provisional callus envelops the fracture site. Osteoblasts—specialized cells located in the periosteum—proliferate rapidly to form collars around the ends of the break. These collars grow toward each other to physically unite the bone fragments.1
Definitive Callus: As healing progresses, the initial cartilage within the callus is resorbed and replaced by bone tissue. Two to three weeks post-injury, strong bony extensions successfully join the fractured ends, allowing the bone to regain its organized structure.1
Remodeling: Over a period of months to years, the callus is gradually resorbed as the bone undergoes bone remodeling to restore its original form.1