Bone remodeling and responsiveness to mechanical stimuli in individuals with type 1 diabetes mellitus

Type 1 diabetes mellitus (T1DM) has been linked to increased osteocyte apoptosis, local accumulation of mineralized lacunar spaces, and microdamage suggesting an impairment of the mechanoregulation network in affected individuals. Diabetic neuropathy might exacerbate this dysfunction through direct effects on bone turnover, and indirect effects on balance, muscle strength, and gait. However, the in vivo effects of impaired bone mechanoregulation on bone remodeling in humans remain underexplored. This longitudinal cohort study assessed consenting participants with T1DM and varying degree of distal symmetric sensorimotor polyneuropathy (T1DM, n = 20, median age 46.5 yr, eight female) and controls (CTRL; n = 9, median age 59.0 yr, four female) at baseline and 4-yr follow-up. Nerve conduction in participants with T1DM was tested using DPNCheck and bone remodeling was quantified with longitudinal high-resolution peripheral quantitative-computed tomography (HR-pQCT, 82 μm) at the standard distal sites. Local trabecular bone formation (Tb.F) and resorption (Tb.R) sites were captured by implementing 3D rigid image registration of HR-pQCT images, and the mechanical environment across the bone microarchitecture at these sites was simulated using micro-finite element analysis. We calculated odds ratios to determine the likelihood of bone formation (ORF) and resorption (ORR) with increasing/decreasing strain in percent as markers for mechanoregulation. At the distal radius, Tb.F was 47% lower and Tb.R was 59% lower in T1DM participants compared with CTRL (P < .05). Tb.F correlated positively with nerve conduction amplitude (R = 0.69, P < .05) in participants with T1DM and negatively with glycated hemoglobin (HbA1c) (R = -0.45, P < .05). Additionally, ORF was 34% lower and ORR was 18% lower in T1DM compared with CTRL (P < .05). Our findings represent in vivo evidence suggesting that bone remodeling in individuals with T1DM is in a state of low responsiveness to mechanical stimuli, resulting in impaired bone formation and resorption rates; these correlate to the degree of neuropathy and level of diabetes control.

Transferability of bone phenotyping and fracture risk assessment by μFRAC from first-generation high-resolution peripheral quantitative computed tomography to second-generation scan data

INTRODUCTION

The continued development of high-resolution peripheral quantitative computed tomography (HR-pQCT) has led to a second-generation scanner with higher resolution and longer scan region. However, large multi-center prospective cohorts were collected with first-generation HR-pQCT and have been used to develop bone phenotyping and fracture risk prediction (μFRAC) models. This study establishes whether there is sufficient universality of these first-generation trained models for use with second-generation scan data.

METHODS

HR-pQCT data was collected for a cohort of 60 individuals, who had been scanned on both first- and second-generation scanners on the same day to establish the universality of the HR-pQCT models. These data were each used as input to first-generation trained bone microarchitecture models for bone phenotyping and fracture risk prediction, and their outputs were compared for each study participant. Reproducibility of the models were assessed using same-day repeat scans obtained from first-generation (n = 37) and second-generation (n = 74) scanners.

RESULTS

Across scanner generations, the bone phenotyping model performed with an accuracy of 93.1%. Similarly, the five-year fracture risk assessment by μFRAC was well correlated with a Pearson's (r) correlation coefficient of r > 0.83 for the three variations of μFRAC (varying inclusion of clinical risk factors, finite element analysis, and dual X-ray absorptiometry). The first-generation reproducibility cohort performed with an accuracy for categorical assignment of 100% (bone phenotyping), and a correlation coefficient of 0.99 (μFRAC), the second-generation reproducibility cohort performed with an accuracy of 96.4% (bone phenotyping), and a correlation coefficient of 0.99 (μFRAC).

CONCLUSION

We demonstrated that bone microarchitecture models trained using first-generation scan data generalize well to second-generation scans, performing with a high level of accuracy and reproducibility. Less than 4% of individuals' estimated fracture risk led to a change in treatment threshold, and in general these dissimilar outcomes using second-generation data tended to be more conservative.

Rapid bone microarchitecture decline in older men with high bone turnover-the prospective STRAMBO study

Older men with high bone turnover have faster bone loss. We assessed the link between the baseline levels of bone turnover markers (BTMs) and the prospectively assessed bone microarchitecture decline in men. In 825 men aged 60-87 yr, we measured the serum osteocalcin (OC), bone alkaline phosphatase (BAP), N-terminal propeptide of type I procollagen (PINP), and C-terminal telopeptide of type I collagen (CTX-I), and urinary total deoxypyridinoline (tDPD). Bone microarchitecture and strength (distal radius and distal tibia) were estimated by high-resolution pQCT (XtremeCT, Scanco Medical) at baseline and then after 4 and 8 yr. Thirty-seven men took medications affecting bone metabolism. Statistical models were adjusted for age and BMI. At the distal radius, the decrease in the total bone mineral density (Tt.BMD), cortical BMD (Ct.BMD), cortical thickness (Ct.Thd), and cortical area (Ct.Ar) and failure load was faster in the highest vs the lowest CTX-I quartile (failure load: -0.94 vs -0.31% yr-1, P < .001). Patterns were similar for distal tibia. At the distal tibia, bone decline (Tt.BMD, Ct.Thd, Ct.Ar, Ct.BMD, and failure load) was faster in the highest vs the lowest tDPD quartile. At each skeletal site, the rate of decrease in Tb.BMD differed between the extreme OC quartiles (P < .001). Men in the highest BAP quartile had a faster loss of Tt.BMD, Tb.BMD, reaction force, and failure load vs the lowest quartile. The link between PINP and bone decline was poor. The BTM score is the sum of the nos. of the quartiles for each BTM. Men in the highest quartile of the score had a faster loss of cortical bone and bone strength vs the lowest quartile. Thus, in the older men followed prospectively for 8 yr, the rate of decline in bone microarchitecture and estimated bone strength was 50%-215% greater in men with high bone turnover (highest quartile, CTX-I above the median) compared to the men with low bone turnover (lowest quartile, CTX-I below the median).

Fracture risk based on HR-pQCT measures does not vary with age in older adults - the bone microarchitecture international consortium (BoMIC) prospective cohort study

Fracture risk increases with lower areal BMD (aBMD); however, aBMD-related estimate of risk may decrease with age. This may depend on technical limitations of 2-dimensional (2D) DXA which are reduced with 3D high-resolution peripheral quantitative computed tomography (HR-pQCT). Our aim was to examine whether the predictive utility of HR-pQCT measures with fracture varies with age. We analyzed associations of HR-pQCT measures at the distal radius and distal tibia with two outcomes: incident fractures and major osteoporotic fractures. We censored follow-up time at first fracture, death, last contact or 8 years after baseline. We estimated hazard ratios (HR) and 95%CI for the association between bone traits and fracture incidence across age quintiles. Among 6835 men and women (ages 40-96) with at least one valid baseline HR-pQCT scan who were followed prospectively for a median of 48.3 months, 681 sustained fractures. After adjustment for confounders, bone parameters at both the radius and tibia were associated with higher fracture risk. The estimated HRs for fracture did not vary significantly across age quintiles for any HR-pQCT parameter measured at either the radius or tibia. In this large cohort, the homogeneity of the associations between the HR-pQCT measures and fracture risk across age groups persisted for all fractures and for major osteoporotic fractures. The patterns were similar regardless of the HR-pQCT measure, the type of fracture, or the statistical models. The stability of the associations between HR-pQCT measures and fracture over a broad age range shows that bone deficits or low volumetric density remain major determinants of fracture risk regardless of age group. The lower risk for fractures across measures of aBMD in older adults in other studies may be related to factors which interfere with DXA but not with HR-pQCT measures.

Characterizing Bone Phenotypes Related to Skeletal Fragility Using Advanced Medical Imaging

PURPOSE OF REVIEW

Summarize the recent literature that investigates how advanced medical imaging has contributed to our understanding of skeletal phenotypes and fracture risk across the lifespan.

RECENT FINDINGS

Characterization of bone phenotypes on the macro-scale using advanced imaging has shown that while wide bones are generally stronger than narrow bones, they may be more susceptible to age-related declines in bone strength. On the micro-scale, HR-pQCT has been used to identify bone microarchitecture phenotypes that improve stratification of fracture risk based on phenotype-specific risk factors. Adolescence is a key phase for bone development, with distinct sex-specific growth patterns and significant within-sex bone property variability. However, longitudinal studies are needed to evaluate how early skeletal growth impacts adult bone phenotypes and fracture risk. Metabolic and rare bone diseases amplify fracture risk, but the interplay between bone phenotypes and disease remains unclear. Although bone phenotyping is a promising approach to improve fracture risk assessment, the clinical availability of advanced imaging is still limited. Consequently, alternative strategies for assessing and managing fracture risk include vertebral fracture assessment from clinically available medical imaging modalities/techniques or from fracture risk assessment tools based on clinical risk factors. Bone fragility is not solely determined by its density but by a combination of bone geometry, distribution of bone mass, microarchitecture, and the intrinsic material properties of bone tissue. As such, different individuals can exhibit distinct bone phenotypes, which may predispose them to be more vulnerable or resilient to certain perturbations that influence bone strength.

A multi-stack registration technique to improve measurement accuracy and precision across longitudinal HR-pQCT scans

BACKGROUND

Recent applications of high-resolution peripheral quantitative computed tomography (HR-pQCT) have demonstrated that changes in local bone remodelling can be quantified in vivo using longitudinal three-dimensional image registration. However, certain emerging applications, such as fracture healing and joint analysis, require larger multi-stack scan regions that can result in stack shift image artifacts. These artifacts can be detrimental to the accurate alignment of the bone structure across multiple timepoints. The purpose of this study was to establish a multi-stack registration protocol for evaluating longitudinal HR-pQCT images and to assess the accuracy and precision error in comparison with measures obtained using previously established three-dimensional longitudinal registration.

METHODS

Three same day multi-stack HR-pQCT scans of the radius (2 stacks in length) and tibia (3 stacks in length) were obtained from 39 healthy individuals who participated in a previous reproducibility study. A fully automated multi-stack registration algorithm was developed to re-align stacks within a scan by leveraging slight offsets between longitudinal scans. Stack shift severity before and after registration was quantified using a newly proposed stack-shift severity score. The false discovery rate for bone remodelling events and precision error of bone morphology and micro-finite element analysis parameters were compared between longitudinally registered scans with and without the addition of multi-stack registration.

RESULTS

Most scans (82 %) improved in stack alignment or maintained the lowest stack shift severity score when multi-stack registration was implemented. The false discovery rate of bone remodelling events significantly decreased after multi-stack registration, resulting in median false detection of bone formation and resorption fractions between 3.2 to 7.5 % at the radius and 3.4 to 5.3 % at the tibia. Further, precision error was significantly reduced or remained unchanged in all standard bone morphology and micro-finite element analysis parameters, except for total and trabecular cross-sectional areas.

CONCLUSION

Multi-stack registration is an effective strategy for accurately aligning multi-stack HR-pQCT scans without modification of the image acquisition protocol. The algorithm presented here is a viable approach for performing accurate morphological analysis on multi-stack HR-pQCT scans, particularly for advanced application investigating local bone remodelling in vivo.

A Fracture Risk Assessment Tool for High Resolution Peripheral Quantitative Computed Tomography

Most fracture risk assessment tools use clinical risk factors combined with bone mineral density (BMD) to improve assessment of osteoporosis; however, stratifying fracture risk remains challenging. This study developed a fracture risk assessment tool that uses information about volumetric bone density and three-dimensional structure, obtained using high-resolution peripheral quantitative compute tomography (HR-pQCT), to provide an alternative approach for patient-specific assessment of fracture risk. Using an international prospective cohort of older adults (n = 6802) we developed a tool to predict osteoporotic fracture risk, called μFRAC. The model was constructed using random survival forests, and input predictors included HR-pQCT parameters summarizing BMD and microarchitecture alongside clinical risk factors (sex, age, height, weight, and prior adulthood fracture) and femoral neck areal BMD (FN aBMD). The performance of μFRAC was compared to the Fracture Risk Assessment Tool (FRAX) and a reference model built using FN aBMD and clinical covariates. μFRAC was predictive of osteoporotic fracture (c-index = 0.673, p < 0.001), modestly outperforming FRAX and FN aBMD models (c-index = 0.617 and 0.636, respectively). Removal of FN aBMD and all clinical risk factors, except age, from μFRAC did not significantly impact its performance when estimating 5-year and 10-year fracture risk. The performance of μFRAC improved when only major osteoporotic fractures were considered (c-index = 0.733, p < 0.001). We developed a personalized fracture risk assessment tool based on HR-pQCT that may provide an alternative approach to current clinical methods by leveraging direct measures of bone density and structure. © 2023 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Precision of bone mechanoregulation assessment in humans using longitudinal high-resolution peripheral quantitative computed tomography in vivo

Local mechanical stimuli in the bone microenvironment are essential for the homeostasis and adaptation of the skeleton, with evidence suggesting that disruption of the mechanically-driven bone remodelling process may lead to bone loss. Longitudinal clinical studies have shown the combined use of high-resolution peripheral quantitative computed tomography (HR-pQCT) and micro-finite element analysis can be used to measure load-driven bone remodelling in vivo; however, quantitative markers of bone mechanoregulation and the precision of these analyses methods have not been validated in human subjects. Therefore, this study utilised participants from two cohorts. A same-day cohort (n = 33) was used to develop a filtering strategy to minimise false detections of bone remodelling sites caused by noise and motion artefacts present in HR-pQCT scans. A longitudinal cohort (n = 19) was used to develop bone imaging markers of trabecular bone mechanoregulation and characterise the precision for detecting longitudinal changes in subjects. Specifically, we described local load-driven formation and resorption sites independently using patient-specific odds ratios (OR) and 99 % confidence intervals. Conditional probability curves were computed to link the mechanical environment to the remodelling events detected on the bone surface. To quantify overall mechanoregulation, we calculated a correct classification rate measuring the fraction of remodelling events correctly identified by the mechanical signal. Precision was calculated as root-mean-squared averages of the coefficient of variation (RMS-SD) of repeated measurements using scan-rescan pairs at baseline combined with a one-year follow-up scan. We found no significant mean difference (p < 0.01) between scan-rescan conditional probabilities. RMS-SD was 10.5 % for resorption odds, 6.3 % for formation odds, and 1.3 % for correct classification rates. Bone was most likely to be formed in high-strain and resorbed in low-strain regions for all participants, indicating a consistent, regulated response to mechanical stimuli. For each percent increase in strain, the likelihood of bone resorption decreased by 2.0 ± 0.2 %, and the likelihood of bone formation increased by 1.9 ± 0.2 %, totalling 38.3 ± 1.1 % of strain-driven remodelling events across the entire trabecular compartment. This work provides novel robust bone mechanoregulation markers and their precision for designing future clinical studies.

Rapid cortical bone loss at the distal radius is associated with higher risk of fracture in older men - the STRAMBO study

Rapid loss of areal bone mineral density (aBMD) is associated with higher fracture risk after adjustment for confounders including initial aBMD. However, the link between bone microarchitecture decline and fracture is not clear. We studied the association between bone microarchitecture deterioration assessed prospectively over 4 years and the subsequent fracture risk in older men. Bone microarchitecture at the distal radius and tibia was assessed by high resolution peripheral QCT (HR-pQCT; XtremeCT, Scanco Medical) (baseline, 4 years) in 732 men aged 60-87 years. During the 8-year follow-up, 109 men had fragility fractures. Areal BMD was assessed by DXA. After adjustment for age, weight, prior falls and fractures, distal radius aBMD (baseline, slope), and baseline distal radius total volumetric BMD (Tt.BMD), the faster decrease in distal radius Tt.BMD was associated with higher fracture risk (HR=1.53 /SD, 95%CI: 1.20-1.95, p<0.005). Rapid cortical bone loss was associated with higher fracture risk (cortical thickness: HR=1.48; 1.15-1.90, p<0.01; cortical BMD: HR=1.38; 1.11-1.72, p<0.01). The rate of trabecular bone loss at the distal radius and the rate of bone microarchitecture decline at the distal tibia were not associated with fracture risk. After adjustment for aBMD and distal radius HR-pQCT measures assessed after 4 yrs, changes in Tt.BMD were associated with higher fracture risk (e.g., Tt.BMD: HR=1.37; 1.11-1.69, p<0.005). Compared with the reference model (age, weight, prior fractures and falls, baseline and slope of aBMD, baseline HR-pQCT value), further addition of the slope of the HR-pQCT measure did not improve the fracture prediction. Thus, rapid cortical bone loss at the distal radius is associated with higher fracture risk in the multivariable models including baseline values of the HR-pQCT measure. However, repeated HR-pQCT measurements did not improve the assessment of the fracture risk in older men (compared with the reference model defined above).

C-reactive protein predicts endocortical expansion but not fracture in older men: the prospective STRAMBO study

In older men, higher high-sensitivity C-reactive protein (hsCRP) concentrations were associated with faster prospectively assessed endocortical expansion (distal radius, distal tibia) and slightly higher cortical bone loss at distal tibia, but not with the fracture risk. High hsCRP level has a limited impact on bone decline in older men.

PURPOSE

Data on the link of the high-sensitivity C-reactive protein (hsCRP) with bone loss and fracture risk are discordant. We studied the association of the hsCRP with the prospectively assessed decrease in areal bone mineral density (aBMD), bone microarchitecture decline, and fracture risk in older men.

METHODS

At baseline, hsCRP was measured in 823 men aged 60-88. Areal BMD and bone microarchitecture (distal radius, distal tibia) were assessed by dual-energy X-ray absorptiometry and high-resolution peripheral QCT, respectively, at baseline and after 4 and 8 years. Data on incident fractures were collected for 8 years.

RESULTS

Higher hsCRP concentration was associated with faster increase in aBMD at the whole body and lumbar spine, but not other sites. Higher hsCRP levels were associated with faster decrease in cortical area and more rapid increase in trabecular area at the distal radius (0.048 mm²/year/SD, p < 0.05) and distal tibia (0.123 mm²/year/SD, p < 0.001). At the distal tibia, high hsCRP level was associated with greater decrease in total and cortical volumetric BMD (vBMD) and in failure load. The hsCRP levels were not associated with the fracture risk, even after accounting for competing risk of death.

CONCLUSION

Higher hsCRP levels were associated with greater endocortical expansion at the distal radius and tibia. Higher hsCRP was associated with slightly faster decrease in total and cortical vBMD and failure load at distal tibia, but not with the fracture risk. Thus, high hsCRP levels are associated with faster cortical bone loss, but not with fracture risk in older men.

Meta-analysis of Diabetes Mellitus-Associated Differences in Bone Structure Assessed by High-Resolution Peripheral Quantitative Computed Tomography

PURPOSE OF REVIEW

Diabetes mellitus is defined by elevated blood glucose levels caused by changes in glucose metabolism and, according to its pathogenesis, is classified into type 1 (T1DM) and type 2 (T2DM) diabetes mellitus. Diabetes mellitus is associated with multiple degenerative processes, including structural alterations of the bone and increased fracture risk. High-resolution peripheral computed tomography (HR-pQCT) is a clinically applicable, volumetric imaging technique that unveils bone microarchitecture in vivo. Numerous studies have used HR-pQCT to assess volumetric bone mineral density and microarchitecture in patients with diabetes, including characteristics of trabecular (e.g. number, thickness and separation) and cortical bone (e.g. thickness and porosity). However, study results are heterogeneous given different imaging regions and diverse patient cohorts.

RECENT FINDINGS

This meta-analysis assessed T1DM- and T2DM-associated characteristics of bone microarchitecture measured in human populations in vivo reported in PubMed- and Embase-listed publications from inception (2005) to November 2021. The final dataset contained twelve studies with 516 participants with T2DM and 3067 controls and four studies with 227 participants with T1DM and 405 controls. While T1DM was associated with adverse trabecular characteristics, T2DM was primarily associated with adverse cortical characteristics. These adverse effects were more severe at the radius than the load-bearing tibia, indicating increased mechanical loading may compensate for deleterious bone microarchitecture changes and supporting mechanoregulation of bone fragility in diabetes mellitus. Our meta-analysis revealed distinct predilection sites of bone structure aberrations in T1DM and T2DM, which provide a foundation for the development of animal models of skeletal fragility in diabetes and may explain the uncertainty of predicting bone fragility in diabetic patients using current clinical algorithms.

Hip Fractures in Older Adults Are Associated With the Low Density Bone Phenotype and Heterogeneous Deterioration of Bone Microarchitecture

Femoral neck areal bone mineral density (FN aBMD) is a key determinant of fracture risk in older adults; however, the majority of individuals who have a hip fracture are not considered osteoporotic according to their FN aBMD. This study uses novel tools to investigate the characteristics of bone microarchitecture that underpin bone fragility. Recent hip fracture patients (n = 108, 77% female) were compared with sex- and age-matched controls (n = 216) using high-resolution peripheral quantitative computed tomography (HR-pQCT) imaging of the distal radius and tibia. Standard morphological analysis of bone microarchitecture, micro-finite element analysis, and recently developed techniques to identify void spaces in bone microarchitecture were performed to evaluate differences between hip fracture patients and controls. In addition, a new approach for phenotyping bone microarchitecture was implemented to evaluate whether hip fractures in males and females occur more often in certain bone phenotypes. Overall, hip fracture patients had notable deterioration of bone microarchitecture and reduced bone mineral density compared with controls, especially at weight-bearing sites (tibia and femoral neck). Hip fracture patients were more likely to have void spaces present at either site and had void spaces that were two to four times larger on average when compared with non-fractured controls (p < 0.01). Finally, bone phenotyping revealed that hip fractures were significantly associated with the low density phenotype (p < 0.01), with the majority of patients classified in this phenotype (69%). However, female and male hip fracture populations were distributed differently across the bone phenotype continuum. These findings highlight how HR-pQCT can provide insight into the underlying mechanisms of bone fragility by using information about bone phenotypes and identification of microarchitectural defects (void spaces). The added information suggests that HR-pQCT can have a beneficial role in assessing the severity of structural deterioration in bone that is associated with osteoporotic hip fractures. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Association of Vitamin D and Parathyroid Hormone Status With the Aging-Related Decline of Bone Microarchitecture in Older Men: The Prospective Structure of Aging Men's Bones (STRAMBO) Study

Poor vitamin D status and high parathyroid hormone (PTH) level are associated with impaired bone microarchitecture, but these data are mainly cross-sectional. We studied the association of the baseline PTH and 25-hydroxycholecalciferol (25OHD) levels with the prospectively assessed deterioration of bone microarchitecture and in estimated bone strength in older men. Distal radius and tibia bone microarchitecture was assessed by high-resolution peripheral quantitative computed tomography (HR-pQCT) at baseline, then after 4 and 8 years in 826 men aged 60-87 years. At distal radius, total bone mineral density (Tt.BMD), cortical thickness (Ct.Th^d ), cortical area (Ct.Ar), cortical BMD (Ct.BMD), and trabecular BMD (Tb.BMD) decreased, whereas trabecular area (Tb.Ar) increased more rapidly in men with 25OHD ≤20 ng/mL versus the reference group (>30 ng/mL). Men with 25OHD ≤10 ng/mL had faster decrease in reaction force and failure load than men with 25OHD >30 ng/mL. At the distal tibia, Tt.BMD, Ct.Th^d , Ct.Ar, Ct.BMD, failure load, and reaction force decreased, whereas Tb.Ar increased more rapidly in men with 25OHD between 10 and 20 ng/mL versus the reference group. The results were similar when 12 ng/mL was used as a threshold of severe vitamin D deficiency. At distal radius, men with PTH levels above the median (>44 pg/mL) had more rapid decrease in Tt.BMD, Ct.Ar, Ct.BMD, Ct.Th^d , reaction force, and failure load, and more rapid increase in Tb.Ar versus the lowest quartile (≤34 pg/mL). At the distal tibia, men in the highest PTH quartile had faster decrease in Tt.BMD, Ct.Th^d , Ct.Ar, Ct.BMD, reaction force, and failure load and faster increase in Tb.Ar versus the lowest quartile. The results were similar in men with glomerular filtration rate >60 mL/min. The results were similar in men who took no vitamin D or calcium supplements for 8 years. In summary, vitamin D deficiency and secondary hyperparathyroidism are associated with more rapid prospectively assessed cortical and trabecular bone decline in older men. © 2022 American Society for Bone and Mineral Research (ASBMR).

Bone Microarchitecture Phenotypes Identified in Older Adults Are Associated With Different Levels of Osteoporotic Fracture Risk

Prevalence of osteoporosis is more than 50% in older adults, yet current clinical methods for diagnosis that rely on areal bone mineral density (aBMD) fail to detect most individuals who have a fragility fracture. Bone fragility can manifest in different forms, and a "one-size-fits-all" approach to diagnosis and management of osteoporosis may not be suitable. High-resolution peripheral quantitative computed tomography (HR-pQCT) provides additive information by capturing information about volumetric density and microarchitecture, but interpretation is challenging because of the complex interactions between the numerous properties measured. In this study, we propose that there are common combinations of bone properties, referred to as phenotypes, that are predisposed to different levels of fracture risk. Using HR-pQCT data from a multinational cohort (n = 5873, 71% female) between 40 and 96 years of age, we employed fuzzy c-means clustering, an unsupervised machine-learning method, to identify phenotypes of bone microarchitecture. Three clusters were identified, and using partial correlation analysis of HR-pQCT parameters, we characterized the clusters as low density, low volume, and healthy bone phenotypes. Most males were associated with the healthy bone phenotype, whereas females were more often associated with the low volume or low density bone phenotypes. Each phenotype had a significantly different cumulative hazard of major osteoporotic fracture (MOF) and of any incident osteoporotic fracture (p < 0.05). After adjustment for covariates (cohort, sex, and age), the low density followed by the low volume phenotype had the highest association with MOF (hazard ratio = 2.96 and 2.35, respectively), and significant associations were maintained when additionally adjusted for femoral neck aBMD (hazard ratio = 1.69 and 1.90, respectively). Further, within each phenotype, different imaging biomarkers of fracture were identified. These findings suggest that osteoporotic fracture risk is associated with bone phenotypes that capture key features of bone deterioration that are not distinguishable by aBMD. © 2021 American Society for Bone and Mineral Research (ASBMR).

Bone Microarchitecture Decline and Risk of Fall and Fracture in Men With Poor Physical Performance-The STRAMBO Study

CONTEXT

High fracture risk in individuals with low muscle strength is attributed to high risk of falls.

OBJECTIVE

This work aims to study the association of muscle mass and physical performance with bone microarchitecture decline and risk of fall and nonvertebral fracture in men.

METHODS

A prospective, 8-year follow-up of a cohort was conducted among the general population. A total of 821 volunteer men aged 60 and older participated. Hip areal bone mineral density (aBMD) and appendicular lean mass (ALM) were assessed at baseline by dual x-ray absorptiometry. Lower-limb relative ALM (RALM-LL) is ALM-LL/(leg length)2. The physical performance score reflects the ability to perform chair stands and static and dynamic balance. Bone microarchitecture was assessed by high-resolution peripheral quantitative computed tomography (HR-pQCT) at baseline and after 4 and 8 years. Statistical analyses were adjusted for shared risk factors. Outcome measurements included the rate of change in the HR-pQCT indices, incident falls, and fractures.

RESULTS

Cortical bone loss and estimated bone strength decline were faster in men with low vs normal RALM-LL (failure load: -0.74 ± 0.09 vs -0.43 ± 0.10%/year; P < .005). Differences were similar between men with poor and those with normal physical performance (failure load: -1.12 ± 0.09 vs -0.40 ± 0.05%/year; P < .001). Differences were similar between men having poor performance and low RALM-LL and men having normal RALM-LL and performance (failure load: -1.40 ± 0.17 vs -0.47 ± 0.03%/year; P < .001). Men with poor physical performance had a higher risk of fall (hazard ratio [HR] = 3.52; 95% CI, 1.57-7.90, P < .05) and fracture (HR = 2.68; 95% CI, 1.08-6.66, P < .05).

CONCLUSION

Rapid decline of bone microarchitecture and estimated strength in men with poor physical performance and low RALM-LL may contribute to higher fracture risk.

Validation of Bone Density and Microarchitecture Measurements of the Load-Bearing Femur in the Human Knee Obtained Using In Vivo HR-pQCT Protocol

High resolution peripheral quantitative computed tomography (HR-pQCT) was designed to study bone mineral density (BMD) and microarchitecture in peripheral sites at the distal radius and tibia. With the introduction of the second generation HR-pQCT scanner (XtremeCT II, Scanco Medical) that has a larger, longer gantry it is now possible to study the human knee in vivo using HR-pQCT. Previous validation of HR-pQCT measurements at the distal radius and tibia against micro-CT is not representative of the knee because the increased cross-sectional area, greater amount of soft tissue surrounding the scan region, and different imaging protocol result in potentially increased beam hardening effects and photon scatter and different signal-to-noise ratio. The objective of this study is to determine the accuracy of density and microarchitecture measurements in the human knee measured by HR-pQCT using an in vivo protocol. Twelve fresh-frozen cadaver knees were imaged using in vivo HR-pQCT (60.7 µm) protocol. Subsequentially, distal femurs were extracted and imaged using a higher resolution (30.3 µm) ex vivo protocol, replicating micro-CT imaging. Scans were registered so that agreement of density and bone microarchitecture measurements could be determined using linear regression and Bland-Altman plots. All density and microarchitecture outcomes were highly correlated between the 2 protocols (R² > 0.89) albeit with statistically significant differences between absolute measures based on paired t tests. All parameters showed accuracy between 4.5% and 8.7%, and errors were highly systematic, particularly for trabecular BMD and trabecular thickness (R² > 0.93). We found that BMD and microarchitecture measurements in the distal femur obtained using an in vivo HR-pQCT knee protocol contained systematic errors, and accurately represented measurements obtained using a micro-CT equivalent imaging protocol. This work establishes the validity and limitations of using HR-pQCT to study the BMD and microarchitecture of human knees in future clinical studies.

Proximal Tibia Bone Stiffness and Strength in HR-pQCT- and QCT-Based Finite Element Models

Injury to the ACL significantly increases the risk of developing post-traumatic osteoarthritis. Following injury, considerable focus is placed on visualizing soft tissue changes using MRI, but there is less emphasis on the alterations to the underlying bone. It has recently been shown using high-resolution peripheral quantitative computed tomography (HR-pQCT) that significant reductions in bone quality occur in the knee post ACL-injury. Despite the ability of HR-pQCT to show these changes, the availability of scanners and computational time requirements required to assess bone stiffness and strength with HR-pQCT limit its widespread clinical use. As such, the objective of this study was to determine if clinical quantitative CT (QCT) finite element models (QCT-FEMs) can accurately replicate HR-pQCT FEM proximal tibial stiffness and strength. From FEMs of 30 participants who underwent both QCT and HR-pQCT bilateral imaging, QCT-FEMs were strongly correlated with HR-pQCT FEM stiffness (R² = 0.79). When QCT-FEM bone strength was estimated using the reaction force at 1% apparent strain, strong correlations were observed (R² = 0.81), with no bias between HR-pQCT FEMs and non-linear QCT-FEMs. These results indicate that QCT-FEMs can accurately replicate HR-pQCT FEM stiffness and strength in the proximal tibia. Although these models are not able to replicate the trabecular structure or tissue-level strains, they require significantly reduced computational time and use widely available clinical-CT images as input, which make them an attractive choice to monitor bone density, stiffness and strength alterations, such as those that occur post ACL-injury.

A new approach for quantifying localized bone loss by measuring void spaces

The application of high resolution peripheral quantitative computed tomography (HR-pQCT) for the study of bone health has provided valuable insight into the role bone microarchitecture has in determining bone strength and fracture risk. However, conventional density and morphological parameters struggle to distinguish whether localized bone loss is present, visible as heterogeneous deterioration in the trabecular network. This is because current HR-pQCT parameters quantify a global average of properties in the cortical or trabecular compartment. This study proposes a new metric we term "void space" that segments volumes of localized deterioration in the trabecular bone network from HR-pQCT scans and quantifies void space as the void space to total volume ratio (VS/TV, %). A simple and fully automated protocol for segmenting and quantifying void space in HR-pQCT scans is presented, along with the assessment of accuracy, precision, and cross-calibration between generations of HR-pQCT systems. Finally, prevalence of void space and the association with standard HR-pQCT parameters is demonstrated using a large population-based cohort (n = 1236). Void space detection was found to be highly reproducible (accuracy >95%, least significant change <1.76% VS/TV) and correlation between scanner generations was strong (R² = 0.87), although the first generation system struggled to identify small voids. Assessment of void space prevalence in the population-based cohort revealed that void spaces were more common in females than males, prevalence increased with age, and void spaces were typically systemic (occurring at both scan sites rather than only one). A comparison of group-wise differences between participants with and without void space demonstrated that individuals with void spaces had significantly worse trabecular properties for both sexes and at both scan sites. Specifically, the median trabecular bone mineral density, bone volume fraction, and trabecular number were below the 25th percentile of the population, while trabecular separation and inhomogeneity were above the 75th percentile of the population in participants with void spaces. Cortical bone characteristics did not differ between participants with and without void spaces. When the void space region was excluded from morphological analysis so that only the remaining "functional bone" was considered, trabecular properties of participants with void spaces were greatly improved, especially for those who were the greatest outliers. Void space is an intuitive morphological parameter that captures localized deterioration in the trabecular bone network, and has the potential to provide valuable insight into the assessment of bone fragility.

Improvements in radiographic and clinical assessment of distal radius fracture healing by FE-estimated bone stiffness

Bone strength determined from finite element (FE) modelling provides an estimate of fracture healing progression following a distal radius fracture (DRF), but how these measures relate to patient-reported outcomes and functional outcomes remains unknown. We hypothesized that changes in bone stiffness and bone mineral density measured using high-resolution peripheral quantitative computed tomography (HR-pQCT) are associated with clinically available measures of functional and patient-reported outcomes. We also aimed to identify which clinical outcome measures best predict fracture stiffness and could therefore be used to inform cast removal.

Participants (n = 30) with stable distal radius fractures were followed for two week intervals from the time of fracture until two months post-fracture, then at three months and six months post-fracture. At each follow-up, participants underwent clinical, radiographic, and functional assessments, as well as had their fractured wrist scanned using HR-pQCT. Recovery of bone stiffness during fracture healing was determined from micro-FE (μFE) models generated from HR-pQCT image data.

During the DRF healing process, significant longitudinal changes were found in μFE-estimated stiffness, patient-reported outcomes, grip strength, range of motion (ROM), tenderness, number of cortices healed based on radiographs, and fracture line visibility (p < 0.05); however, no significant change was detected in HR-pQCT based total bone mineral density. Patient-reported outcomes, such as the Patient-Rated Wrist Evaluation (PRWE) and the Quick Disability of the Arm, Shoulder and Hand (QuickDASH) questionnaire, correlated strongly with μFE-estimated stiffness (0.61 ≥ r_m ≥ 0.66). Based on μFE-estimated stiffness, PRWE and QuickDASH are the best predictors of stiffness recovery (p < 0.05) and may be used to guide duration of cast immobilization in the clinical setting.

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Recovery of fracture stiffness may inform time required for cast immobilization.

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Patient reported outcomes predict rate of fracture stiffness recovery.

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Radiographic outcomes correlate weakly with fracture stiffness.

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Patient reported outcomes may inform duration of cast immobilization.

Sex- and Site-Specific Reference Data for Bone Microarchitecture in Adults Measured Using Second-Generation HR-pQCT

There are currently no population-based reference data sets available for volumetric bone mineral density and microarchitecture parameters measured using the second-generation high-resolution peripheral quantitative computed tomography (HR-pQCT), yet the technology is rapidly becoming a standard for studies of bone microarchitecture. Although cross-calibrated data sets from the first-generation HR-pQCT have been reported, they are not suitable for second-generation bone microarchitecture properties because of fundamental differences between scanner generations. This study provides site- and sex-specific centile curves across the adult life span for second-generation HR-pQCT properties. A total of 1236 adult participants (768 female and 468 male) from the Calgary area between the ages of 18 and 90 years were scanned at the distal tibia and radius using the second-generation HR-pQCT. Bone densities, microarchitectural properties, and failure load estimated using finite element analysis were determined using standard in vivo protocol. Site- and sex-specific centile curves were generated using the generalized additive models for location, scale, and shape (GAMLSS) method. These data provide reference curves appropriate for predominantly white male and female adults, which can be used as a tool to assess patient- or cohort-specific bone health. © 2020 American Society for Bone and Mineral Research.

Guidelines for the assessment of bone density and microarchitecture in vivo using high-resolution peripheral quantitative computed tomography

INTRODUCTION

The application of high-resolution peripheral quantitative computed tomography (HR-pQCT) to assess bone microarchitecture has grown rapidly since its introduction in 2005. As the use of HR-pQCT for clinical research continues to grow, there is an urgent need to form a consensus on imaging and analysis methodologies so that studies can be appropriately compared. In addition, with the recent introduction of the second-generation HrpQCT, which differs from the first-generation HR-pQCT in scan region, resolution, and morphological measurement techniques, there is a need for guidelines on appropriate reporting of results and considerations as the field adopts newer systems.

METHODS

A joint working group between the International Osteoporosis Foundation, American Society of Bone and Mineral Research, and European Calcified Tissue Society convened in person and by teleconference over several years to produce the guidelines and recommendations presented in this document.

RESULTS

An overview and discussion is provided for (1) standardized protocol for imaging distal radius and tibia sites using HR-pQCT, with the importance of quality control and operator training discussed; (2) standardized terminology and recommendations on reporting results; (3) factors influencing accuracy and precision error, with considerations for longitudinal and multi-center study designs; and finally (4) comparison between scanner generations and other high-resolution CT systems.

CONCLUSION

This article addresses the need for standardization of HR-pQCT imaging techniques and terminology, provides guidance on interpretation and reporting of results, and discusses unresolved issues in the field.

Optimizing HR-pQCT workflow: a comparison of bias and precision error for quantitative bone analysis

Manual correction of automatically generated contours for high-resolution peripheral quantitative computed tomography can be time consuming and introduces precision error. However, bias related to the automated protocol is unknown. This study provides insight into error bias that is present when using uncorrected contours and inter-operator precision error based on operator training.

INTRODUCTION

High-resolution peripheral quantitative computed tomography workflow includes manually correcting contours generated by the manufacturer's automated protocol. There is interest in minimizing corrections to save time and reduce precision error; however, bias related to the automated protocol is unknown. This study quantifies error bias when contours are uncorrected and identifies the impact of operator training on bias and precision error.

METHODS

Forty-five radii and tibiae scans across a representative range of bone density were analyzed using the automated and manually corrected contours of three operators, with training ranging from beginner to expert, and compared with a "ground truth" to estimate bias. Inter-operator precision was measured across operators.

RESULTS

The tibia had greater error bias than the radius when contours were uncorrected, with compartmental bone mineral densities and cortical microarchitecture having greatest biases, which could have significant implications for interpretation of studies using this skeletal site. Bias and precision error were greatest when contours were corrected by the beginner operator; however, when this operator was removed, bias was no longer present and inter-operator precision was between 0.01 and 3.74% for all parameters except cortical porosity.

CONCLUSION

These findings establish the need for manual correction and provide guidance on operator training needed to maximize workflow efficiency.

The Correction of Systematic Error due to Plaster and Fiberglass Casts on HR-pQCT Bone Parameters Measured In Vivo at the Distal Radius

Due to difficulty assessing healing of distal radius fractures using conventional radiography, there is interest in using high resolution peripheral quantitative computed tomography (HR-pQCT) to track healing at the microarchitectural level. Unfortunately, the plaster-of-Paris and fiberglass casts used to immobilize fractures affect HR-pQCT measurements due to beam hardening, and increased noise. The challenge is compounded because casts have variable thickness, and an individual patient will often have their cast changed 2-3 times during the course of treatment. This study quantifies the effect of casts within a clinically relevant range of thicknesses on measured bone parameters at the distal radius, and establishes conversion equations to correct for systematic error in due to cast presence. Eighteen nonfractured participants were scanned by HR-pQCT in three conditions: no cast, plaster-of-Paris cast, and fiberglass cast. Measured parameters were compared between the baseline scan (no cast) and each cast scan to evaluate if systematic error exists due to cast presence. A linear regression model was used to determine an appropriate conversion for parameters that were found to have systematic error. Plaster-of-Paris casts had a greater range of thicknesses (3.2-9.5 mm) than the fiberglass casts (3.0-5.4 mm), and induced a greater magnitude of systematic error overall. Key parameters of interest were bone mineral density (total, cortical, and trabecular) and trabecular bone volume fraction, all of which were found to have systematic error due to presence of either cast type. Linear correlations between baseline and cast scans for these parameters were excellent (R² > 0.98), and appropriate conversions could be determined within a margin of error less than a ±6% for the plaster-of-Paris cast, and ±4% for the fiberglass cast. We have demonstrated the effects of cast presence on bone microarchitecture measurements, and presented a method to correct for systematic error, in support of future use of HR-pQCT to study fracture healing.

Harmonizing finite element modelling for non-invasive strength estimation by high-resolution peripheral quantitative computed tomography

The finite element (FE) method based on high-resolution peripheral quantitative computed tomography (HR-pQCT) use a variety of tissue constitutive properties and boundary conditions at different laboratories making comparison of mechanical properties difficult. Furthermore, the advent of a second-generation HR-pQCT poses challenges due to improved resolution and a larger region of interest (ROI). This study addresses the need to harmonize results across FE models. The aims are to establish the relationship between FE results as a function of boundary conditions and a range of tissue properties for the first-generation HR-pQCT system, and to determine appropriate model parameters for the second-generation HR-pQCT system. We implemented common boundary conditions and tissue properties on a large cohort (N = 1371), and showed the relationships were highly linear (R² > 0.99) for yield strength and reaction force between FE models. Cadaver radii measured on both generation HR-pQCT with matched ROIs were used to back-calculate a tissue modulus that accounts for the increased resolution (61 µm versus 82 µm), resulting in a modulus of 8748 MPa for second-generation HR-pQCT to produce bone yield strength and reaction force equivalent to using 6829 MPa for first-generation HR-pQCT. Finally, in vivo scans (N = 61) conducted on both generations demonstrated that the larger ROI in the second-generation system results in stronger bone outcome measures, suggesting it is not advisable to convert FE results across HR-pQCT generations without matching ROIs. Together, these findings harmonize FE results by providing a means to compare findings with different boundary conditions and tissue properties, and across scanner generations.