Insights into reference point indentation involving human cortical bone: Sensitivity to tissue anisotropy and mechanical behavior

Abstract

Reference point indentation (RPI) is a microindentation technique involving 20 cycles of loading in “force-control” that can directly assess a patient’s bone tissue properties. Even though preliminary clinical studies indicate a capability for fracture discrimination, little is known about what mechanical behavior the various RPI properties characterize and how these properties relate to traditional mechanical properties of bone. To address this, the present study investigated the sensitivity of RPI properties to anatomical location and tissue organization as well as examined to what extent RPI measurements explain the intrinsic mechanical properties of human cortical bone. Multiple indents with a target force of 10 N were done in 2 orthogonal directions (longitudinal and transverse) per quadrant (anterior, medial, posterior, and lateral) of the femoral mid-shaft acquired from 26 donors (25–101 years old). Additional RPI measurements were acquired for 3 orthogonal directions (medial only). Independent of age, most RPI properties did not vary among these locations, but they did exhibit transverse isotropy such that resistance to indentation is greater in the longitudinal (axial) direction than in the transverse direction (radial or circumferential). Next, beam specimens (~ 2 mm × 5 mm × 40 mm) were extracted from the medial cortex of femoral mid-shafts, acquired from 34 donors (21–99 years old). After monotonically loading the specimens in three-point bending to failure, RPI properties were acquired from an adjacent region outside the span. Indent direction was orthogonal to the bending axis. A significant inverse relationship was found between resistance to indentation and the apparent-level mechanical properties. Indentation distance increase (IDI) and a linear combination of IDI and the loading slope, averaged over cycles 3 through 20, provided the best explanation of the variance in ultimate stress (r2=0.25, p=0.003) and toughness (r2=0.35, p=0.004), respectively. With a transverse isotropic behavior akin to tissue hardness and modulus as determined by micro- and nano-indentation and a significant association with toughness, RPI properties are likely influenced by both elastic and plastic behavior of bone tissue.

Tissue-Level Mechanical Properties of Bone Contributing to Fracture Risk

“…Indentation depth assessed from cRPI on cadaveric femoral bone from 34 donors (65±25 [21–99] yo) was inversely correlated with both peak bending strength and toughness derived from three-point bend (3pt) testing [22]. Similar relationships were observed after conducting cRPI and 3pt testing on femurs from non-diabetic and diabetic rats sacrificed at 32 weeks of age (n=18) as well as on ribs from mature female beagles with and with BP-treatment (n=19) [72].…”

“…With the BioDent, the periosteal bone is cyclically loaded at 2 Hz between 0 N and a target force, typically 10 N, for 10 to 20 cycles with a load profile similar to nanoindentation (load-to-maximum force, dwell, unload-to 0 N, with each part of the cycle having the same duration of 167 ms). A number of tissue resistance properties can be derived from the resulting force vs. displacement curves (see [22] for details). Whereas the BioDent has a stand to hold the indentation module over the patient’s bone, the OsteoProbe is a hand-held device that generates a one-time, impact load (from 10 N, the triggering force, to ~ 40 N in 0.25 ms) such that the recorded depth of the tip into the bone (150 μm – 260 μm) is indexed to the harmonic mean of multiple indentation depths into a plastic standard (PMMA).…”

In vivo reference point indentation measurement variability in skeletally mature inbred mice

“…5 These studies show the real potential for this tool in a clinical setting, even while we are only beginning to understand how RPI outcomes compare with properties measured using traditional mechanical testing modalities. [6][7][8][9] The present study is targeted at a more fundamental level than the clinical and preclinical assessments of RPI described above. Rodents are the most common animals used in skeletal research, owing in large part to the biological tools (for example, genetic manipulation) available for these taxa.…”

Spatial distribution and remodeling of elastic modulus of bone in micro-regime as prediction of early stage osteoporosis

“…Recent study has also demonstrated that microindentation on the bone surface, cause micro-fractures and the analysis of which may be able to correlate with the overall mechanical property of bone tissue. In this method a reference point indentation instrument was used which loads the bone surface at a particular load and frequency and measures the surface mechanical properties (Diez-Perez et al, 2010; Gallant et al, 2013; Granke et al, 2014). Hence possibly use of reference point indentation methods can serve as a new tool for clinical diagnosis of early stages of osteoporotic conditions.…”