From two to three weeks onwards a process begins whereby the fragile cartilage material of the Soft Callus is transformed completely into Woven bone. This process typically continues for between six and twelve weeks, depending on the location and type of fracture (generally six weeks for the upper limb and twelve weeks for the lower limb). Hard Callus formation is a complex process that is guided by the release of mineral compounds such as Calcium and Phosphate into the Cartilage tissue, which subsequently transforms into a bridge of Hard Callus over the fracture site.
Once the Hard Callus has formed at the former fracture site, then fracture Union is said to have occurred. Fracture Union can be seen on x-ray at around six weeks in upper limb fractures and twelve weeks in lower limb fractures. One of the factors that encourages Hard Callus formation in lower limb fractures is gentle weight bearing exercise. This is why Orthopaedic Consultant encourage the use of Removable Plastic Cast Walkers, as they allow controlled weight bearing and stimulate Hard Callus formation, because bone responds to repeated gentle loading by laying down more bone tissue.