In fracture healing by secondary (indirect) intention, the mechanical stimulus that drives conversion of the cartilaginous soft callus to hard (bony) callus is:

• A Cyclic hydrostatic compression favours endochondral ossification while interfragmentary motion inhibits direct bone formation ✓
• B Complete rigid immobility, which stimulates intramembranous ossification throughout the callus
• C Continuous tensile strain that activates osteoblast differentiation in the fibrocartilage
• D Intramedullary vascular invasion that is the primary driver independent of mechanical environment

Explanation

According to the mechano-biological principles of fracture healing (Perren's strain theory), regions of high interfragmentary strain (deformity) form fibrous tissue; intermediate strain produces fibrocartilage; low strain allows bone formation. In soft callus, regions with appropriate cyclic compression undergo endochondral ossification as cartilage is progressively replaced by woven bone. As fracture stiffness increases, strain decreases, promoting progressive bone bridge maturation. This is why controlled micro-motion (e.g., with flexible fixation) promotes abundant callus, while excessive motion leads to non-union.

Reference: Maheshwari Essential Orthopaedics, 6th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.