2.2 Pathophysiology of Bone Fragility & Fracture Healing
Key Takeaways
Bone strength depends on loading conditions, structure and material properties as well as mineral density.
A trabecular column model can illustrate geometry effects when its assumptions are stated. DXA cannot quantify the modeled load at failure.
A fracture through a focal lesion requires a diagnostic assessment even when the patient also has low BMD.
Direct and secondary fracture healing differ in the role of callus. Trial results about healing apply to the fractures and treatment settings studied.
Relate loading to structural capacity
A fracture occurs when the applied load exceeds the bone's capacity in that loading condition. The risk depends on the size and direction of the load, bone dimensions, cortical and trabecular structure and the tissue's properties. DXA estimates mineral mass per projected area. QCT estimates volumetric density in a calibrated region and can assess compartments with appropriate acquisition and analysis. Neither measure directly tests the load at which a particular bone will fail. A study-specific correlation between density and strength cannot be treated as a fixed percentage for every site or patient.
Cortical thickness, porosity and the distribution of bone relative to the loading axis affect mechanical behavior. Trabecular plates and rods can contribute differently to directional stiffness, as shown in studies of human specimens. Loss of connections can change load paths. Reduced connectivity and thinner trabeculae are distinct changes, and routine DXA cannot identify either one. At the tissue scale, type I collagen cross-links and the mineral matrix help determine stiffness and deformation. Experiments on trabecular specimens have linked non-enzymatic glycation with altered mechanical behavior. In patients with type 2 diabetes, fracture risk can be higher at a given BMD, but a normal-range DXA result cannot establish glycation as the cause of an individual fracture.
Worked buckling model
For an ideal straight, slender column under axial compression, use the Euler relation with effective length L, elastic modulus E and second moment of area I.
If the effective length doubles while material, section properties and end conditions stay the same, the modeled critical buckling load falls to one-quarter of the original value. For example, 8 N becomes 2 N. This helps explain why cross-bracing can matter. A real trabecular network includes nonuniform plates, rods, joints and several loading modes. Losing a connection does not establish a particular effective length change or a 75% strength loss in the patient.
Classify the event before assigning a cause
| Mechanism or finding | What it supports |
|---|---|
| Fracture after a fall from standing height or less | Consider skeletal fragility. Review the site, trauma and imaging, and assess other contributors. |
| Fracture after substantial trauma | Record the mechanism and evaluate the injury. Cohort data in older adults link high-trauma fractures with lower BMD and future fractures, so the history can still matter in risk assessment. |
| Fracture through a focal lytic or otherwise abnormal lesion | Consider a pathologic fracture from local disease. Further diagnostic evaluation is needed regardless of DXA classification. |
| Repetitive loading | Distinguish fatigue injury from unusual repetitive stress on otherwise normal bone and insufficiency injury under ordinary loads on weakened bone. Use the clinical and imaging context. |
Recognize healing patterns and limits of drug evidence
With very stable fixation and close fracture surfaces, direct repair can involve osteonal remodeling across the site with little or no external callus. Under relative stability, secondary healing generally involves hematoma and inflammation, formation of a soft callus, replacement by mineralized woven bone, and remodeling toward lamellar bone. Cartilage replacement and direct bone formation can both contribute to a healing callus. The phases overlap. The visibility of a callus depends partly on the healing mode and timing. Blood supply, fracture characteristics, stability and patient factors influence timing.
In a prespecified FREEDOM analysis, denosumab did not show evidence of delayed healing of the studied nonvertebral fractures. A HORIZON analysis found no association between timing of zoledronic acid and delayed hip-fracture healing in that trial. These are study-specific clinical findings. They do not predict the microscopic course of a given fracture or replace decisions about atypical fractures. A distal radius trial of teriparatide did not meet its prespecified 40-mcg versus placebo comparison for faster healing. A shorter time in the 20-mcg group came from a post hoc comparison and warrants caution. The U.S. Forteo label does not include acceleration of fracture union as an indication.
Trabecular mechanics experiment · Trabecular glycation and mechanics experiment · Type 2 diabetes cohort · High-trauma fracture cohort · WHO fragility fractures · ACR vertebral fracture guidance · Fracture healing biology · FREEDOM healing analysis · HORIZON healing analysis · Teriparatide healing trial · Forteo label
An ideal column model gives a critical buckling load of 12 N. The effective length doubles while the material and section stay the same. What does the model predict?
A load of 24 N.
A load of 3 N, under the stated model assumptions.
A load of 6 N because the load is inversely proportional to length.
A universal 75% decrease in human vertebral failure strength.
A patient with low BMD fractures through a focal lytic lesion. What conclusion is appropriate?
Treat the lesion as a routine osteoporotic fracture based on the T-score alone.
Exclude any contribution from a local process because the patient fell.
Evaluate for a focal pathologic cause and assess the patient’s broader skeletal risk separately.
Calculate the likelihood of a lytic lesion from the hip BMD.
A splinted fracture is healing under relative stability. Which sequence is consistent with secondary healing?
Hematoma and inflammation, soft callus, mineralized woven bone, then remodeling.
Immediate mature osteon bridging with no role for a callus.
Lamellar remodeling before the inflammatory response.
A fixed delay of union whenever a patient takes an antiresorptive.
Sections you finish are checked off in the contents.