escribe the differences between primary and secondary bone healing .

The two mechanisms of bone healing are primary bone healing and secondary bone healing. (THIS IS JUST LIKE SKIN: you can sew it up or it can scab.)

Primary bone healing involves a direct attempt by the cortex to re-establish itself after interruption without the formation of a fracture callus.

Just like in skin, primary healing only works when the edges are touching exactly. And since touching edges are not common, primary healing is the less commonly seen type of healing.

In fact, this method is employed only after rigid surgical fixation, or in case with a partial crack in the bone, a so-callled “unicortical” fracture, where the remaining bone holds everything rigid.

The basic science, in brief: Primary bone healing is lead by the formation of a so-called cutting cone (consisting of osteoclasts at the front of the cone to remove bone and trailing osteoblasts to lay down new bone) across the gaps to form a secondary osteon.

Secondary bone healing involves the classical stages of injury, hemorrhage inflammation, primary soft callus formation, callus mineralization, and callus remodeling. This method of bone healing closely resembles endochondral ossification (which involves a cartilage template being replaced by bone).

This suggests that indirect bone healing results in re-formation of bone with essentially the same mechanical properties as the original bone, if not better.

Key point: healing in a sense recapitulates growth. So fracture healing can lead to completely new bone, not scar. (At the other extreme: cartilage: it heals poorly—it not only forms just scar, the scar is poor mechanical quality. Scientists among you: fix this!)

SCHEMATIC: SECONDARY BONE HEALING

1. Bone breaks
2. Hematoma (blood clot) forms at once.
3. From this hematoma, a primary callus forms This is composed of granulation tissue (fibroblasts and new blood vessels).
4. The cells in this soft callus make cartilage
5. The cartilage is then mineralized producing “woven” or “lamellar” ie disorganized, bone.
5. Last, the woven bone remodels into normal bone (oriented in direction of load)