The characterization of broadband ultrasound attenuation and fractal analysis by biomechanical properties

Effects of loading rate, age, and morphology on the material properties of human rib trabecular bone

The material and morphometric properties of trabecular bone have been studied extensively in bones bearing significant weight, such as the appendicular long bones and spine. Less attention has been devoted to the ribs, where quantification of material properties is vital to understanding thoracic injury. The objective of this study was to quantify the compressive material properties of human rib trabecular bone and assess the effects of loading rate, age, and morphology on the material properties. Material properties were quantified via uniaxial compression tests performed on trabecular bone samples at two loading rates: 0.005 s^-1 and 0.5 s^-1. Morphometric parameters of each sample were quantified before testing using micro-computed tomography. Rib trabecular bone material properties were lower on average compared to trabecular bone from other anatomical locations. Morphometric parameters indicated an anisotropic structure with low connectivity and a sparser density of trabeculae in the rib compared to other locations. No significant differences in material properties were observed between the tested loading rates. Material properties were only significantly correlated with age at the 0.005 s^-1 loading rate, and no morphometric parameter was significantly correlated with age. Trabecular separation and thickness were most strongly correlated with the material properties, indicating the sparser trabecular matrix likely contributed to the lower material property values compared to other sites. The novel trabecular bone material properties reported in this study can be used to improve the thoracic response and injury prediction of computational models.

Prediction of mechanical properties of human rib cortical bone using fractal dimension

A large number of post mortem human subjects was used to investigate the relation between the micro-structure of rib cortical bone and the mechanical properties using Fractal Dimension. Uniaxial tensile tests were performed on coupons of rib cortical bone. Tensile strength, yield stress, Young's Modulus, maximum strain, and work to fracture were determined for each coupon. Fractal dimension was computed using CT images and Digital Image Correlation procedures. A highly significant effect of fractal dimension in the mechanical properties was found. In addition, the variation in mechanical properties was found to be adequately represented by Generalized Extreme Value type distributions.

Standardizing compression testing for measuring the stiffness of human bone

Objectives

The ability to determine human bone stiffness is of clinical relevance in many fields, including bone quality assessment and orthopaedic prosthesis design. Stiffness can be measured using compression testing, an experimental technique commonly used to test bone specimens in vitro. This systematic review aims to determine how best to perform compression testing of human bone.

Methods

A keyword search of all English language articles up until December 2017 of compression testing of bone was undertaken in Medline, Embase, PubMed, and Scopus databases. Studies using bulk tissue, animal tissue, whole bone, or testing techniques other than compression testing were excluded.

Results

A total of 4712 abstracts were retrieved, with 177 papers included in the analysis; 20 studies directly analyzed the compression testing technique to improve the accuracy of testing. Several influencing factors should be considered when testing bone samples in compression. These include the method of data analysis, specimen storage, specimen preparation, testing configuration, and loading protocol.

Conclusion

Compression testing is a widely used technique for measuring the stiffness of bone but there is a great deal of inter-study variation in experimental techniques across the literature. Based on best evidence from the literature, suggestions for bone compression testing are made in this review, although further studies are needed to establish standardized bone testing techniques in order to increase the comparability and reliability of bone stiffness studies.

Cite this article: S. Zhao, M. Arnold, S. Ma, R. L. Abel, J. P. Cobb, U. Hansen, O. Boughton. Standardizing compression testing for measuring the stiffness of human bone. Bone Joint Res 2018;7:524–538. DOI: 10.1302/2046-3758.78.BJR-2018-0025.R1.

Abnormal bone quality in adolescent idiopathic scoliosis: a case-control study on 635 subjects and 269 normal controls with bone densitometry and quantitative ultrasound

STUDY DESIGN

A case-control study comparing bone quality in Adolescent Idiopathic Scoliosis (AIS) with normal controls.

OBJECTIVE

To evaluate bone quality with quantitative ultrasound (QUS) in AIS and normal controls so as to detect any derangement in bone quality among AIS subjects.

SUMMARY OF BACKGROUND DATA

AIS is characterized by complex spinal deformities. Despite its high prevalence and clinical impact in adolescents, etiology of AIS remains unknown but one possible mechanism is related to derangement of bony mechanical stability, as quantified by bone mineral density (BMD) and bone quality. AIS is known for its association with osteopenia, but little is known about the bone quality in AIS. With technological advancement, QUS can provide objective measurement of bone quality. In this study, we sought to compare bone quality in AIS with normal controls using QUS in addition to the conventional BMD measurement.

METHODS

Six hundred thirty-five AIS girls and 269 age-matched normal girls were investigated. Broadband ultrasound attenuation (BUA), velocity of sound (VOS), and stiffness index (SI) were measured over the nondominant calcaneus using QUS. The results were correlated with anthropometric measurement, radiologic assessment, and BMD of both hips.

RESULTS

The z-score of BMD at the femoral neck of AIS subjects (-0.47 ± 0.97) was significantly lower than that of normal controls (-0.12 ± 1.01, P < 0.001). Crude comparison showed that BUA, VOS, and SI of AIS group were 3.8% (P < 0.01), 0.5% (P = 0.042), and 6.9% (P < 0.01) lower than controls, respectively. After controlling confounding from maturity, body weight, body height, and BMD with multiple linear regression analysis for both mild (Cobb's angle ≤ 25°) and severe (Cobb's angle > 25°) curves, BUA and SI were found to be statistically significantly lower in AIS as compared with controls (P < 0.05).

CONCLUSION

In addition to higher prevalence of osteopenia, AIS patients were also found to have deranged bone quality. These might contribute to the etiopathogenesis of spinal deformities in AIS.

Biomechanical considerations of animal models used in tissue engineering of bone

Tissue engineering combines the aspects of cell biology, engineering, material science, and surgery to generate new functional tissue, and provides an important approach to the repair of segmental defects and in restoring biomechanical function. The development of tissue-engineering strategies into clinical therapeutic protocols requires extensive, preclinical experimentation in appropriate animal models. The ultimate success of any treatment strategy must be established in these animal models before clinical application. It is clear that the demands of the biological and mechanical environment in the clinical repair of critical size defects with tissue-engineered materials is significantly different from those existing in experimental animals. The major considerations facing any tissue-engineering testing logic include the choice of the defect, the animal, the age of the animal, the anatomic site, the size of the lesion, and most importantly, the micro-mechanical environment. With respect to biomechanical considerations when selecting animals for tissue- engineering of bone, it is evident that no common criteria have been reported. While in smaller animals due to size constraint only structural properties of whole bones can be measured, in larger animals and humans both material properties and structural properties are of interest. Based on reported results, comparison between the tissue-engineered bone across species may be of importance in establishing better model selection criteria. It has already been found that the deformation of long bones is fairly constant across species, and that stress levels during gait are dependent on the weight of the animal and the material properties of the bone tissue. Future research should therefore be geared towards developing better biomechanical testing systems and then finding the right animal model for the existing equipment.

Comparison between quantitative calcaneal and tibial ultrasound in a Dutch Caucasian pediatric and adolescent population

In the field of bone densitometry, attention has recently been focused on the pediatric population. Quantitative ultrasound (QUS) as bone assessment technique has many advantages for children in comparison with bone assessment techniques that use ionizing radiation. In this pilot study, we investigated the use of calcaneal and tibial QUS systems in a healthy Caucasian pediatric population. We studied 120 healthy Caucasian Dutch children between ages 7 and 19 yr: 53 boys (mean age of 12.5 yr, range 4.5-18) and 67 girls (mean age of 13.5 yr; range 7.1-19). We recruited children from a large population who previously had participated in a bone assessment study performed at our hospital. Two operators performed calcaneal QUS of the right calcaneus and tibial QUS of the right tibia. The correlation between calcaneal and tibial ultrasound was modest but significant (r = 0.29; p < 0.01). Using the calcaneal device, we found in girls a weak positive correlation between skeletal age and speed of sound (SOS) (r = 0.38), broadband ultrasound attenuation (r = 0.57), and quantitative ultrasound index (r = 0.46), all with a value of p < 0.01. For boys all parameters failed to reach significance. Using the tibial device, we found a good correlation between skeletal age and SOS in girls (r = 0.76) and modest correlation in boys (r = 0.50), both with a value of p < 0.01. This is one of the first studies to present a comparison between two ultrasound techniques in children. At present we feel that, in light of the poor correlation with skeletal age, calcaneal ultrasound has yet to prove its efficacy in children. Tibial ultrasound seems to be a good bone assessment technique in children.

Bone properties as estimated by mineral density, ultrasound attenuation, and velocity

Quantitative ultrasound (QUS) analysis of bone has been suggested to have a level of performance equal to dual-energy X-ray absorptiometry (DXA) for the assessment of fracture risk. In this study, QUS and DXA measurements were conducted on bovine trabecular bone in vitro using commercially available clinical instruments. The samples were then mechanically tested to obtain Young's modulus and ultimate strength. In addition, QUS and DXA parameters of the human calcaneus (n = 34) were measured in vivo. The measurements revealed a significant effect of bovine bone size on broadband ultrasound attenuation (BUA) and speed of sound (SOS) in vitro. By normalizing the DXA and QUS results with bone thickness we could systematically improve their ability to predict bone strength. However, in bovine trabecular bone, BUA showed no significant linear correlation with either bone mineral density (BMD), Young's modulus, or ultimate strength. This finding may be typical of only high-density and low-porosity bovine bone. We significantly improved prediction of ultimate strength by combining density and ultrasound velocity results as compared with assessments of volumetric BMDvol (p < 0.05) or SOS (p < 0.001) alone. However, the improvement was not significant if BMDvol, instead of wet density, was used. Altogether, 88% of the variation in the ultimate strength of bovine bone could be explained by combined density and ultrasound velocity. In vivo, SOS showed a weak negative correlation with heel width (r = -1.350). The in vivo measurements also showed a close correlation for BUA with BMD in the human calcaneus. This suggests that BUA is more suitable for quantitative analysis of low-density trabecular bone.

Acoustic and ultrasonic tissue characterization--assessment of osteoporosis

Osteoporosis, often termed the 'silent epidemic', has been defined as 'a decrease in bone mass and architectural deterioration of bone tissue, leading to enhanced bone fragility and consequent increase in fracture risk'. In the United Kingdom alone, the annual health costs are in excess of 750 million Pounds, with 60,000 patients suffering a hip fracture each year. A quarter of these will die within 12 months of their fracture, half of the remainder will never regain independent living. The established procedure for assessing the risk of osteoporotic fracture is via bone mineral density (BMD) assessment using dual-energy X-ray absorptiometry (DXA). However, DXA is an expensive technique and is not widely available. Within the past 15 years, ultrasound assessment of bone has rapidly advanced in scientific understanding, technical development and clinical utility. Measurements of cancellous bone (particularly at the calcaneus) are generally performed in preference to those of cortical bone (tibial cortex). There are currently 15 commercial systems available and over 3500 systems are in use world-wide. The low cost and portability offered by ultrasound systems should enable an integrated community-based screening programme to be established in the near future. Ultrasound measurements of bone are generally obtained using transmission rather than pulse-echo techniques owing to its highly attenuating nature. Ultrasound velocity and attenuation measurements are utilized. For velocity, there are well-defined fundamental relationships describing the dependence upon the elasticity and density of bone.

Fractal analysis of radiographs: assessment of trabecular bone structure and prediction of elastic modulus and strength

The purpose of this study was to determine whether fractal dimension of radiographs provide measures of trabecular bone structure which correlate with bone mineral density (BMD) and bone biomechanics, and whether these relationships depend on the technique used to calculate the fractal dimension. Eighty seven cubic specimen of human trabecular bone were obtained from the vertebrae and femur. The cubes were radiographed along all three orientations--superior-inferior (SI), medial-lateral (ML), and anterior-posterior (AP), digitized, corrected for background variations, and fractal based techniques were applied to quantify trabecular structure. Three different techniques namely, semivariance, surface area, and power spectral methods were used. The specimens were tested in compression along three orientations and the Young's modulus (YM) was determined. Compressive strength was measured along the SI direction. Quantitative computed tomography was used to measure trabecular BMD. High-resolution magnetic-resonance images were used to obtain three-dimensional measures of trabecular architecture such as the apparent bone volume fraction, trabecular thickness, spacing, and number. The measures of trabecular structure computed in the different directions showed significant differences (p<0.05). The correlation between BMD, YM, strength, and the fractal dimension were direction and technique dependent. The trends of variation of the fractal dimension with BMD and biomechanical properties also depended on the technique and the range of resolutions over which the data was analyzed. The fractal dimension showed varying trends with bone mineral density changes, and these trends also depended on the range of frequencies over which the fractal dimension was measured. For example, using the power spectral method the fractal dimension increased with BMD when computed over a lower range of spatial frequencies and decreased for higher ranges. However, for the surface area technique the fractal dimension increased with increasing BMD. Fractal measures showed better correlation with trabecular spacing and number, compared to trabecular thickness. In a multivariate regression model inclusion of some of the fractal measures in addition to BMD improved the prediction of strength and elastic modulus. Thus, fractal based texture analysis of radiographs are technique dependent, but may be used to quantify trabecular structure and have a potentially valuable impact in the study of osteoporosis.

Fractal dimension predicts broadband ultrasound attenuation in stereolithography models of cancellous bone

There has been considerable debate on the relative dependence of broadband ultrasound attenuation (nBUA, dB MHz(-1) cm(-1)) upon the density and structure of cancellous bone. A nonlinear relationship between nBUA and porosity has recently been demonstrated using stereolithography models, indicating a high structural dependence for nBUA. We report here on the measurement of trabecular perimeter and fractal dimension on the two-dimensional images used to create the stereolithography models. Adjusted coefficients of determination (R2) with nBUA were 94.4% (p < 0.0001) and 98.4% (p < 0.0001) for trabecular perimeter and fractal dimension respectively. The feature of fractal dimension representing both the porosity and connectivity of a given structure is most exciting. Further work is required to determine the relationship between broadband ultrasound attenuation and fractal dimension in complex three-dimensional cancellous bone structures.