Normal Bone Remodeling
Healthy bone constantly rebuilds itself in balance.
- ~120 days: Remodeling cycle (resorption through repair)
- 1:1: Osteoclast:osteoblast ratio (coupled activity)
- ~10%: Annual bone turnover (of adult skeleton)
- ~5 mm: Tibial cortex thickness (mid-shaft average)
Coupled resorption and formation
Osteoclasts dig, osteoblasts refill, in sequence.
Mechanical loading signal
Everyday load nudges bone to strengthen slightly.
Baseline imaging appearance
A healthy bone scan shows only faint, even uptake.
Reactive Bone Edema
Training outpaces repair, and fluid starts pooling.
- 2–3 wk: Time to onset (of ramped-up training)
- I–II: Bone scan grade (mild focal uptake)
- 1: MRI edema grade (mild marrow signal)
- Post-activity: Symptom pattern (ache after loading)
Remodeling falls behind
Resorption briefly outruns new bone formation.
Fluid accumulation begins
Interstitial fluid seeps into stressed marrow space.
Scintigraphy sensitivity
Bone scan detects change before symptoms flare.
Periosteal Reaction
The bone surface itself becomes inflamed and reactive.
- STIR/T2: MRI sequence (best periosteal contrast)
- Periosteum: Edema location (along posteromedial tibia)
- II: Bone scan grade (moderate linear uptake)
- Focal tenderness: Palpation finding (along shaft)
Periosteal signal appears
A bright rim traces the outer bone surface.
Load site correlation
Reaction clusters where bending stress peaks.
Still reversible
Rest at this stage usually restores normal bone.
Stress Reaction
Marrow edema deepens as microdamage quietly accumulates.
- 3: MRI edema grade (moderate-severe marrow signal)
- III: Bone scan grade (intense focal uptake)
- ~78%: Cortical integrity (microcrack accumulation)
- Elevated: Return-to-run risk (without load reduction)
Microdamage outpaces repair
Microcracks accumulate faster than osteoblasts fill them.
Marrow signal intensifies
MRI edema now spans a wider marrow region.
Last stage before fracture
Continued load pushes bone toward a true break.
Stress Fracture Line
A visible cortical line marks true bone failure.
- IV: Bone scan grade (fusiform intense uptake)
- Fracture line: MRI finding (low-signal cortical break)
- ~55%: Cortical integrity (and falling under load)
- 6–12 wk: Typical healing time (with load modification)
Cortical line becomes visible
A dark fracture line crosses the cortex on MRI.
High-risk anterior tibial cortex
Anterior cortex fractures heal slowly, need caution.
Healing reverses the picture
Rest lets uptake and edema gradually recede.