How Microindentation Assesses Fracture Risk Beyond Traditional Bone Density Tests
Imagine being told your bones are "normal" based on standard medical tests, only to later suffer a devastating fracture from a simple stumble. This scenario is more common than you might think. For decades, the gold standard for assessing bone health has been bone mineral density (BMD) testing, which measures how much calcium and other minerals are packed into your bones. While low BMD is clearly associated with increased fracture risk, a startling fact remains: the majority of fractures actually occur in people with normal or only slightly low bone density 1 . This glaring limitation has fueled the search for better assessment tools.
Most fractures occur in individuals with normal bone mineral density, highlighting the need for better assessment tools.
of fractures happen with normal BMD
The missing piece of the puzzle lies in something experts call "bone quality"—a complex combination of bone architecture, tissue properties, and the ability to resist tiny cracks from propagating. Think of it this way: BMD tells you how much material is in the bone, but not whether that material is high-quality. Two bones with identical density could have vastly different resistance to fracture based on their microarchitecture and material properties. Until recently, evaluating these aspects required invasive bone biopsies that were impractical for routine care. Now, an innovative technology called Reference Point Microindentation (RPI) offers a promising window into this hidden dimension of bone strength 1 2 .
Reference Point Microindentation represents a paradigm shift in bone assessment. Instead of measuring how dense the bone is, it measures how well the bone material resists mechanical intrusion—a direct indicator of its quality and strength.
The clinical version uses a handheld device that creates microscopic indentations in bone tissue.
Standardized measurement comparing bone indentation to reference material.
If you've ever pressed your thumbnail into different types of wood, you've intuitively understood material strength properties. Soft pine dents easily while hard oak resists denting. RPI applies this same principle with scientific precision to bone tissue.
This ingenious approach allows clinicians to assess bone material properties directly in living patients, providing information that was previously inaccessible without invasive procedures. The entire process takes just minutes and provides immediate data that complements traditional bone density measurements 1 9 .
For any new medical technology to gain acceptance, it must prove its worth through rigorous validation. A crucial experiment conducted on bovine bone provided compelling evidence linking RPI measurements to actual bone fracture resistance 6 .
Forty-eight rectangular bone specimens were prepared from fresh bovine femurs. The samples were divided into two groups—one preserved in their natural state, and the other boiled for three hours to denature the collagen network while leaving the mineral component intact 6 .
Each bone specimen received a small, precise notch to simulate a stress concentrator—similar to how a small crack in your car windshield might spread under pressure 6 .
All specimens underwent two types of mechanical testing: fracture toughness testing (the traditional gold standard) and impact microindentation using the OsteoProbe® to obtain BMSi measurements 6 .
The boiled bones showed significantly reduced fracture resistance, confirming that collagen integrity plays a crucial role in bone toughness. More importantly, the study revealed a remarkably strong correlation (r = 0.86) between BMSi values and traditional fracture toughness measurements 6 .
| Parameter | Fresh Bone | Boiled Bone | Change | Statistical Significance |
|---|---|---|---|---|
| Fracture Toughness (KIC) | Higher | Significantly Reduced | Decreased | p < 0.0001 |
| Bone Material Strength Index (BMSi) | Higher | Significantly Reduced | Decreased | p < 0.0001 |
| Correlation Between BMSi & Fracture Toughness | r = 0.86 | p < 0.001 | ||
This finding was groundbreaking—it provided the first direct evidence that the simple, minimally invasive RPI test could serve as a reliable surrogate for the complex, destructive fracture toughness test 6 . The implications are significant: a tool that can quickly assess bone fracture resistance without damaging the bone could revolutionize how we identify individuals at risk for fractures.
With the technique validated in the laboratory, the next crucial step was establishing normal reference ranges for human populations. A landmark international study published in 2023 provided exactly this, analyzing data from 479 healthy adults across seven research centers in the United States, Europe, and Australia 9 .
The study revealed that BMSi values typically range from 48 to 101 in healthy adults, with average values of 84.4 for men and 79.0 for women. More importantly, it established reference intervals that clinicians can now use to determine where an individual patient falls relative to healthy peers:
| Population | Mean BMSi | Standard Deviation | Reference Interval |
|---|---|---|---|
| Adult Men | 84.4 | ± 6.9 | 71.0 - 97.9 |
| Adult Women | 79.0 | ± 9.1 | 59.8 - 95.2 |
These reference ranges now enable the calculation of T- and Z-scores for BMSi, similar to how bone density results are reported. This standardization is crucial for translating research findings into clinical practice and identifying individuals with unusually low bone material strength who might benefit from targeted interventions 9 .
BMSi generally declines with age, reflecting cumulative changes in bone quality over time.
Researchers observed geographical variations, suggesting both genetic and environmental factors influence bone material properties.
For those interested in the technical aspects of RPI research, here are the key materials and reagents that make this technology possible:
| Item | Function | Application Notes |
|---|---|---|
| OsteoProbe® Device | Handheld impact microindentation instrument | The only FDA-cleared device for clinical bone material strength assessment 1 . |
| PMMA Reference Phantom | Standardized material for calibration | Allows normalization of measurements across devices and sessions 1 . |
| Local Anesthetic | Patient comfort during measurement | Typically lidocaine; allergy to lidocaine is an exclusion criterion for testing 9 . |
| Ringer's Solution | Bone hydration maintenance | Crucial for in vitro studies as bone mechanical properties are sensitive to hydration state 8 . |
| Vickers Microhardness Tester | Traditional hardness validation | Used in laboratory studies to correlate BMSi with conventional hardness measures 8 . |
Reference Point Microindentation represents more than just a new test—it embodies a fundamental shift in how we conceptualize and evaluate bone strength. By looking beyond density to assess the actual quality of bone tissue, RPI offers the potential to identify at-risk individuals who would otherwise be missed by current standard care.
Patients with chronic kidney disease show impaired BMSi, giving clinicians new insights into their skeletal health 1 .
"The added value of impact microindentation for the evaluation and follow-up of elements of bone fragility, particularly in secondary osteoporosis" is increasingly recognized 1 . However, authors also caution that broader adoption will require continued standardization of testing protocols and more prospective studies demonstrating its value in predicting future fractures.
What makes this technology particularly exciting is its potential to transform preventive care. With a quick, minimally invasive test, clinicians could monitor how bone material quality responds to different treatments, track age-related changes more comprehensively, and ultimately make better-informed decisions to protect their patients from the devastating consequences of fragility fractures. While more research is needed, RPI has already begun to open a new window into the microscopic world of bone quality—and in doing so, offers hope for a future where fewer lives are interrupted by unexpected fractures.