Q.(a) Write about the mechanism and healing of bone fracture. OR

A fractured bone heals in stages — hematoma formation, soft (fibrocartilaginous) callus, hard (bony) callus, and finally bone remodelling — restoring the bone's original shape and strength, answering part (a) of this either/or question.

This question offers a choice between (a) the mechanism and healing of a bone fracture, and (b) why MRI is good at imaging soft tissue. Since (a) can be answered fully on its own, it is addressed here.

A bone fracture (a break/crack in a bone, from a fall, blow, or excessive stress) heals through the following stages:

1. Hematoma formation: immediately after the fracture, blood vessels at the break rupture, and blood pools to form a clot (hematoma) around and within the fracture site. This clot brings in inflammatory cells and provides a scaffold for the initial repair.
2. Fibrocartilaginous (soft) callus formation: over the following days, fibroblasts and chondroblasts migrate into the hematoma and start laying down collagen fibres and cartilage, forming a soft, somewhat flexible callus that bridges the broken ends and provides initial stability, though it is not yet true bone.
3. Bony (hard) callus formation: osteoblasts then invade the soft callus and begin depositing new bone matrix (through a process resembling ossification), gradually converting the soft cartilaginous callus into a hard, bony callus that firmly unites the fractured ends. …

A broken bone heals through an orderly four-stage sequence -- clotting, soft callus, hard callus, and remodelling -- that together restore both continuity and strength to the bone.

1. Haematoma (blood clot) formation (first few hours-days)

• The fracture tears blood vessels in the bone and periosteum, causing bleeding at the site.
• The blood clots to form a haematoma, which seals off the broken ends and cuts off blood supply to the immediately adjacent bone cells (causing localised cell death).
• Inflammatory cells (neutrophils, macrophages) and fibroblasts migrate in, and new capillaries begin to sprout into the clot -- this is the inflammatory phase.

2. Fibrocartilaginous (soft) callus formation (about 1-3 weeks)

• Fibroblasts produce collagen fibres and chondroblasts produce cartilage within the haematoma, forming a soft, spongy mass called the procallus that bridges the broken ends.
• This soft callus is not rigid but is strong enough to loosely splint the fracture.

3. Bony (hard) callus formation (about 4-6 weeks)

• Osteoblasts migrate into the soft callus and progressively replace the fibrocartilage with spongy (woven/immature) bone through endochondral ossification.
• This converts the soft callus into a hard, calcified callus firmly uniting the fracture.

4. Bone remodelling (months to over a year) …