Significant correlation of bone material strength index as measured by the OsteoProbe with Vickers and Rockwell hardness

The OsteoProbe^1,2 is a microindentation device that has been used to quantify bone’s resistance to indentation at the micro scale with a singular scalar value, named the bone material strength index (BMSi). This value is calculated by taking the ratio of the measured penetration distance into a test material (e.g., bone) and the measured penetration distance into a reference material and then multiplying the ratio by 100 (Fig. 1) as follows:

The OsteoProbe has been used in multiple clinical studies of human patients, and a standard testing procedure has been published previously.³ One study found that the BMSi is lower in patients with type II diabetes,^4,5 while another showed that the BMSi decreased when patients were treated with glucocorticoids and that this decrease could be eliminated or reversed with drug therapy.⁶ A third work examined a decrease in the BMSi in patients with fragility fractures independent of bone mineral density.⁷ 

Some controversy has arisen about what is being measured.^8–10 Although correlations exist for some cases,^11,12 a simple universal relationship between the BMSi and bulk material mechanical properties of the bone that holds for all bone diseases has not been found. Thus, we cannot appeal to comparison with bulk material mechanical properties to answer the question of what is being measured. Instead, OsteoProbe measurements should be compared with standard mechanical tests for measuring surface mechanical properties.¹³ In this study, OsteoProbe measurements are compared to Vickers and Rockwell tests, two standard mechanical tests for measuring hardness—a surface mechanical property. Clinical measurement of the BMSi in a patient can range from the low 40s up to the low 100s. The polymers we selected had BMSi values that represent this range, with the softest scoring 44 and the hardest scoring 104. This range of the BMSI corresponds to indentation depths of 350 µm down to 125 µm both in the patients and in the polymers.^4–7 

To demonstrate that the OsteoProbe’s BMSi is a measure of hardness, measurements performed by the OsteoProbe were compared with conventional indentation-based measurements of hardness, namely, measurements collected through Rockwell and Vickers type testing. Using a scanning electron microscope, a microscale image of an indented polymer by the OsteoProbe was taken, which can be seen in Fig. 2. Conventional engineering materials were used for comparison testing, namely, high-performance polymers. 14 different polymers that span the observed range of OsteoProbe measurements in patients were selected and indented five times by each instrument. The 14 polymers include UHMW polyethylene (ultra-high molecular weight polyethylene), HPDE (high density polyethylene), polystyrene, ABS (acrylonitrile butadiene styrene), Noryl PPO, cast nylon, polyester, extruded nylon, Rexolite, Delrin, ULTEM PEI (polyetherimide), peek, Torlon PAI (polyamide-imide), and PMMA [poly(methyl methacrylate)].

Indentation methods chosen for testing were Rockwell, Vickers, and the BMSi using the OsteoProbe. The Rockwell hardness was measured according to the American Society for Testing and Materials (ASTM) D785-08 (reapproved 2015) standard test method using a United Tru-Blue II hardness testing instrument. Test parameters used a loading force of 60 kg and an indenter geometry of 1/2 in. ball diameter.

The Vickers hardness was measured according to the ASTM E384 standard test method using a United Tru-Blue II hardness testing instrument. Test parameters used a loading force of 10 kg and a 136° high-load Vickers diamond indenter. The Vickers indentations were measured with a 200× and 500× microscope. The Vickers hardness number was calculated from these measurements according to ASTM E384. The OsteoProbe BMSi was measured according to a published, standard testing procedure, with 10 indentations for each polymer sample.³ Every indentation applies a preload force of 10 N at which point a trigger system releases a mechanism that applies an additional 30 N force at high speed, making a microscopic indentation. The penetration distance into the material is automatically measured by the OsteoProbe.

Hardness measurements were performed on 14 selected polymers using Rockwell, Vickers, and OsteoProbe methods, with results shown in Figs. 3–5. OsteoProbe measurements of the selected polymers, as given by values of the BMSi, were found to be comparable with the range of BMSi values measured for the bones of patients.^4–7 

Results showed correlative trends among the measurement methods. As can be seen in Figs. 3 and 4, there are statistically significant correlations between BMSi values and Vickers, with r = 0.94 and p < 0.01, and between BMSi values and Rockwell values, with r = 0.94 and p < 0.01. Scatter seen in the results may be expected, given the different mechanical properties of the 14 different polymers. Figure 5 shows comparable scatter even between the two conventional measures of hardness, Rockwell and Vickers.

Using the Table of Critical Values: Pearson Correlation, the calculated correlation between the BMSi and Rockwell was found significant at p < 0.01, and the correlation between the BMSi and Vickers was significant at p < 0.01. All these tests evaluated the hardness of the materials within a few hundred micrometers of the surface.¹⁴ All three test methods were comparably and significantly correlated: between the BMSi and Rockwell, r = 0.93 at p < 0.01, between the BMSi and Vickers, r = 0.94 at p < 0.01, and between Rockwell and Vickers, r = 0.87 at p < 0.01. The BMSi measurements were slightly more correlated with Rockwell and Vickers hardness measurements than the two standard testing methods were to each other (Tables I and II).

The simple answer to the question “What does the OsteoProbe measure?” is that it measures a normalized indentation distance, the BMSi, which is a measure of the hardness on the surface of the bone. It is similar to other instrumented hardness tests in which an indentation is made according to a standard procedure and the resulting indentation depth is measured: the smaller the indentation depth, the larger the hardness.

Indentation has classically been utilized as a method of material characterization, with the measured results often reported in terms of hardness.¹⁵ The classic Vickers and Rockwell hardness methods use pyramid, cone, or spherical indenter shapes to create permanent, localized deformation on the surface of a part under a given load.¹⁵ Measurement of the deformed geometry gives the hardness. Softer materials are more deformed. Hard materials are less deformed. The OsteoProbe uses a conical indenter shape to create permanent, localized deformation that is monitored by the instrument itself. Specifically, strain gauges in the OsteoProbe monitor the distance that the conical indenter goes into the material under test. Thus, the OsteoProbe can be considered an indentation device, and its results can be compared to conventional devices such as Rockwell and Vickers. The comparison reported in this article revealed strong correlations: correlation coefficients above 0.9, r > 0.9, between BMSi and Rockwell and Vickers hardness instruments.

Why is it important to measure the hardness on the surface of the bone? The answer to this question is that the hardness on the surface of the bone, as quantified by the BMSi, has been shown clinically to be lower in patients with bone degenerating diseases such as type II diabetes^4,5 and bone degenerating treatments such as glucocorticoid treatment.⁶ Patients with fragility fractures have a decreased BMSi.⁷ 

The BMSi, as measured with the OsteoProbe, is significantly correlated with both Rockwell and Vickers hardness on 14 engineering polymers: between the BMSi and Rockwell, r = 0.93 at p < 0.01, between the BMSi and Vickers, r = 0.94 at p < 0.01, and between Rockwell and Vickers, r = 0.87 at p < 0.01. Results support that the OsteoProbe indentation measurement, the BMSi, is a valid measurement of hardness. The value of this measurement of hardness is that it can be done in a clinic on patients without surgically exposing the bone. Decreased hardness, as quantified by the BMSi, has been clinically demonstrated to be correlated with various causes of bone degeneration.^4–7 

The data that support the findings of this study are available from the corresponding author upon reasonable request.

This work was supported, in part, by the NSF, Grant No. NSF1635503, and by the Bill and Melinda Gates Foundation. Guidance of the NSF program director Bruce Kramer is acknowledged.