Background of Reference Point Indentation.

After some fundamental research on bio minerals including bone, we knew enough to invent and build a Reference Point Indentation (RPI) instrument (formerly known as the Bone Diagnostic Instrument, BDI, and the Tissue Diagnostic Instrument, TDI). The RPI was being developed with the long-term goal of providing a way for researchers and clinicians to measure material properties of human bone in vivo. Such measurements could contribute to the overall assessment of bone fragility in the future. The portability, ease of use, and minimal training make this instrument suitable to measure bone material properties in a clinical setting.

Here is a little history of the early days of Reference Point Indentation. These 9 instruments were built in my garage in a few months back in 2005.

Over the next couple of years we build more iterations in my lab. This is iteration #19, which served as a prototype for the first commercial Reference Point Indentation Device. The current version is being sold by Active Life Scientific Inc.

Figure 1 The prototype for what became the first commercial Reference Point Indentation instrument.
Once we had this instrument, we approached Dr. Adolfo Diez Perez, who had suggested the need for such an instrument. Here is a Video of some of the early tests.

Video: Testing of RPI in vivo, from the Hospital del Mar in Barcelona
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The Reference Point Indentation instrument measures microscopic materials properties of the bones of living patients. Our hypothesis is, and our preliminary results are in agreement, that a measure of microscopic fracture resistance with the Reference Point Indentation instrument in an individual’s bone correlates with their resistance to macroscopic bone fractures, or, stated another way, their overall bone fracture risk. Indentation testing is already well-established as a powerful tool in characterizing mechanical properties of materials.

Fundamental research has revealed that bone fractures begin when the organic matrix of the bone, or “glue” holding mineralized collagen fibrils together, fails causing crack propagation. Preliminary results with the Reference Point Indentation instrument have shown that an individual’s susceptibility to this fundamental failure mode can be measured by indentation tests in which bone is forced, on a microscopic scale, into the same types of failure – separation of mineralized collagen fibrils – that is the root event of bone fractures. Preliminary results with the Reference Point Indentation instrument have also shown that the necessary measurement can be performed on bone that is still covered with soft tissue.

Figure 2 Basic operation of the Reference Point Indentation instrument.

The heart of the Reference Point Indentation instrument is a probe assembly that can be inserted through the skin of a living patient to measure the fracture properties of bone by creating microscopic fractures in an indentation on the surface of the bone (Figure 2). At this stage of development, the indentation distance increase (IDI) appears to best distinguish bone that is more easily fractured from bone that is less easily fractured. The IDI is a measure of the difference in indentation after repetitive cycling to a fixed force. These indented fractured areas are very small: of order of one-thousandth of a cubic millimeter, but give important information about how easy or difficult it is to fracture the bone of an individual.

Figure 3 Some of the first results on patients by Dr. Adolfo Diez Perez. The normalized Indentation Distance Increase (IDI) of a 17 year old female and a 79 year old female.

Since these early days there have been over 120 peer-reviewed publications with Reference Point Indentation.

We built these first instruments to be very versatile – to be sure that whatever was wrong with the bone, we could find it. As it turned out, of all the many, many parameters we could measure, total indentation distance was about the best in distinguishing people with fractures from people without. So we went back to basics and built a much simpler instrument, the Osteoprobe.

Here is a page from my home lab notebook with the key breakthrough for the Osteoprobe.

It went through about seven iterations before becoming commercialized by Active Life Scientific

A user interface for the Osteoprobe developed in our lab by Dan Bridges with the help of psychologist Hal Kopeikin, who helped us design the user interface to motivate and support the development of good operator technique. The people at Active Life Scientific Inc. have refined the user interface far beyond this by now.

For more detailed information please refer to our publications and to the website of the company, Active Life Scientific Inc. , that formed to commercialize the devices.