Microstructural and compositional contributions towards the mechanical behavior of aging human bone measured by cyclic and impact reference point indentation

Abstract

The assessment of fracture risk often relies primarily on measuring bone mineral density, thereby accounting for only a single pathology: the loss of bone mass. However, bone’s ability to resist fracture is a result of its biphasic composition and hierarchical structure that imbue it with high strength and toughness. Reference Point Indentation (RPI) testing is designed to directly probe bone mechanical behavior at the microscale in situ, although it remains unclear which aspects of bone composition and structure influence the results at this scale. Therefore, our goal in this study was to investigate factors that contribute to bone mechanical behavior measured by cyclic reference point indentation, impact reference point indentation, and three-point bending. Twenty-eight female cadavers (age 57–97) were subjected to cyclic and impact RPI in parallel at the unmodified tibia mid-diaphysis. After RPI, the middiaphyseal tibiae were removed, scanned using micro-CT to obtain cortical porosity (Ct.Po.) and tissue mineral density (TMD), then tested using three-point bending, and lastly assayed for the accumulation of advanced glycation end-products (AGEs). Both the indentation distance increase from cyclic RPI (IDI) and bone material strength index from impact RPI (BMSi) were significantly correlated with TMD (r = −0.390, p = 0.006; r = 0.430, p = 0.002; respectively). Accumulation of AGEs was significantly correlated with IDI (r = 0.281, p = 0.046), creep indentation distance (CID, r = 0.396, p = 0.004), and BMSi (r = −0.613, p < 0.001). There were no significant relationships between tissue TMD or AGEs accumulation with the quasi-static material properties. Toughness decreased with increasing tissue Ct. Po. (r = −0.621, p < 0.001). Other three-point bending measures also correlated with tissue Ct. Po. including the bending modulus (r = −0.50, p < 0.001) and ultimate stress (r = −0.56, p < 0.001). The effects of Ct.Po. on indentation were less pronounced with IDI (r = 0.290, p = 0.043) and BMSi (r = −0.299, p = 0.037) correlated modestly with tissue Ct. Po. These results suggest that RPI may be sensitive to bone quality changes relating to collagen.

Tissue-Level Mechanical Properties of Bone Contributing to Fracture Risk

“…Also, the assessment of indentation resistance is not necessarily independent of porosity as with traditional tissue indentation tests because pores cannot be purposely avoided. For the most part, cortical bone loss with aging or menopause occurs primarily through endocortical resorption [23], but indentation depth at the periosteal surface of cadaveric tibia mid-shafts (left and right combined) was recently correlated with regional cortical porosity as determined by micro-computed tomography with a voxel size of 30 μm (r=0.290, p=0.043 for indentation distance increase vs. porosity and r=−0.299, p=0.037 for BMSi vs. porosity) [24]. …”

Clinical Measurements of Bone Tissue Mechanical Behavior Using Reference Point Indentation

“…After the completion of all testing on a specimen (typically 5-10 indents 32 ), the user indents an actual PMMA block for calibration and the final BMSi values are calculated. Since the BioDent is a cyclic microindentation device and OsteoProbe utilizes impact microindentation, the two devices are inherently different and may measure different behaviors of bone 33 . It is worth noting that a higher IDI generally reflects poor resistance to indentation, while a higher BMSi suggests relatively higher resistance against indentation.…”

“…In particular, individuals with Type 2 Diabetes (T2D) mellitus has been shown to exhibit compromised BMSi despite normal or higher-than-normal BMD and favorable microarchitecture across multiple studies 42-44 . Though the mechanisms driving reduced BMSi in diabetics are unclear, the changes BMSi has been associated with the accumulation of advanced glycation end products (AGEs) in human bone 33 and may be the putative mechanism for the fragility observed in T2D. BMSi has also been reported in other clinical morbidities, including chronic kidney disease and transplantation 45 , acromegaly 46 , Paget’s disease 47 , Camurati-Engelmann disease, Type 1 Gaucher disease 48 , and obesity 49 , with BMSi observed to be lower in the respective pathological cohorts.…”

“…However, cadaver studies provide the opportunity to compare the BioDent and Osteoprobe with other mechanical testing modalities 33,54,55 . Karim and co-authors suggest BioDent measures creep or fatigue crack growth from cyclic loading, whereas OsteoProbe is sensitive to energy dissipated from rapid loading 54 .…”

“…For example, Uppuganti and colleagues 55 found cyclic RPI to be sensitive to tissue mineral density (r = −0.89), and OsteoProbe to be sensitive to cortical porosity (r = −0.90; Table 2). Abraham et al 33 found that BMSi was more strongly correlated with advanced glycation end product (AGEs) content (r = −0.613) than IDI (r = 0.281). In the same study, IDI and BMSi correlated with tissue mineral density roughly to the same degree (r = −0.390; r = 0.430, respectively).…”