Primary Bone Healing: Radiographic Definition
The statement is TRUE: radiographic success of primary bone healing is defined by the disappearance of the fracture line without periosteal callus formation. 1
Understanding Primary vs. Secondary Bone Healing
Primary (direct) bone healing occurs through direct bone repair without endochondral ossification and without cartilaginous or periosteal callus formation. 1 This healing pattern represents a fundamentally different biological process than the more common secondary healing:
Key Distinguishing Features
Primary healing requires:
- Rigid fracture fixation with absolute stability 1
- Minimal to no gap formation at the fracture site 1
- Direct Haversian canal remodeling across the fracture line 1
- Reestablishment of blood vessels without intermediate callus 1
Secondary healing demonstrates:
- Visible periosteal callus formation on radiographs 2
- Three overlapping stages: inflammation, soft/hard callus formation, and remodeling 1
- Endochondral ossification with cartilaginous intermediate 1
Clinical Context and Implications
Primary healing does not typically occur in the natural fracture healing process. 1 It is an iatrogenic pattern induced by surgical intervention with compression plating or other rigid internal fixation methods that eliminate motion at the fracture site.
Radiographic Appearance Over Time
In primary healing, the fracture line gradually disappears without the formation of visible periosteal callus that characterizes secondary healing. 1 This occurs because:
- Bone remodeling occurs directly through cutting cones crossing the fracture gap 1
- Periosteal osteoprogenitor cells are not activated to form external callus 3, 4
- The healing process is entirely intracortical and endosteal 5
Contrast with Secondary Healing Radiographs
In secondary healing, radiographic success includes visible periosteal callus formation. 2 The ACR Appropriateness Criteria consistently describe that when cortical bone healing occurs through secondary mechanisms, "endosteal/periosteal callus may be visible with or without a fracture line through the cortex." 2
Important Clinical Caveats
Absence of periosteal callus does not always indicate primary healing or successful union. 5 Research demonstrates that:
- Some fractures heal rapidly through endosteal mechanisms without periosteal callus 5
- Delayed union can occur with cessation of periosteal response before bridging is complete 5
- Complete absence of both periosteal and endosteal responses indicates nonunion 5
When to Be Concerned
If a fracture managed non-operatively shows no periosteal callus formation by 10 weeks, this presages delayed union. 5 However, this same radiographic appearance in a rigidly fixed fracture may represent normal primary healing progression.
The clinical context—specifically whether rigid internal fixation was used—is essential for interpreting the absence of periosteal callus correctly. 1
Practical Algorithm for Interpretation
For surgically fixed fractures with rigid stabilization:
- Absence of periosteal callus = expected finding in primary healing 1
- Progressive disappearance of fracture line = successful healing 1
- Persistent visible fracture line beyond expected timeframe = potential delayed union 5
For conservatively managed fractures: