- Early phase – space occupation without guidance
The defect is packed with non-vital, non-resorbing granules. - Fibrous encapsulation instead of bone
he host responds by laying down fibrovascular connective tissue around each particle (a quiescent foreign-body–type response). These fibrous sleeves mechanically and biologically isolate particles. - Distance osteogenesis stalls at the margins
Osteogenesis proceeds only from viable bone edges. When advancing front meets the fibrous capsules/impermeable particles, it cannot bridge across to the next island; there’s no continuous osteoconductive path. The defect stabilizes as particles + fibrous tissue, not bone. - Long-term persistence
The front moves into the osteoid, rapidly raising mineral content to ~60–70% of mature levels over days–weeks. Histologically, the osteoid’s pale eosinophilia gives way to darker, more basophilic mineralized matrix. - Maturation (secondary mineralization)
Because particles persist, the central defect never converts to vital bone; functionally it behaves like a fibrous non-union within a grafted site.
Fibrous encapsulation is the body’s “plan B” when the conditions for bone aren’t met.
Biology/signaling problems
- Poor vascular ingress / hypoxia → capillaries don’t penetrate; osteoprogenitors don’t arrive → fibroblasts dominate
- Insufficient osteogenic signals (low BMP/TGF-β activity, scant marrow access)
- Pro-inflammatory macrophage polarization (M1) → foreign-body–type response and collagen capsule instead of remodeling (vs. pro-healing M2)
- Persistent low-grade inflammation produces fibrosis
