The effect of strain rate on fracture toughness of human cortical bone: a finite element study

Evaluating the mechanical response of bone under high loading rates is crucial to understanding fractures in traumatic accidents or falls. In the current study, a computational approach based on cohesive finite element modeling was employed to evaluate the effect of strain rate on fracture toughness of human cortical bone. Two-dimensional compact tension specimen models were simulated to evaluate the change in initiation and propagation fracture toughness with increasing strain rate (range: 0.08–18 s⁻¹). In addition, the effect of porosity in combination with strain rate was assessed using three-dimensional models of micro-computed tomography-based compact tension specimens. The simulation results showed that bone’s resistance against the propagation of a crack decreased sharply with increase in strain rates up to 1 s⁻¹ and attained an almost constant value for strain rates larger than 1 s⁻¹. On the other hand, initiation fracture toughness exhibited a more gradual decrease throughout the strain rates. There was a significant positive correlation between the experimentally measured number of microcracks and the fracture toughness found in the simulations. Furthermore, the simulation results showed that the amount of porosity did not affect the way initiation fracture toughness decreased with increasing strain rates, whereas it exacerbated the same strain rate effect when propagation fracture toughness was considered. These results suggest that strain rates associated with falls lead to a dramatic reduction in bone’s resistance against crack propagation. The compromised fracture resistance of bone at loads exceeding normal activities indicates a sharp reduction and/or absence of toughening mechanisms in bone during high strain conditions associated with traumatic fracture.     

Effect of aging on the transverse toughness of human cortical bone: evaluation by R-curves

The age-related deterioration in the quality (e.g., strength and fracture resistance) and quantity (e.g., bone-mineral density) of human bone, together with increased life expectancy, is responsible for increasing incidence of bone fracture in the elderly. The present study describes ex vivo fracture experiments to quantitatively assess the effect of aging on the fracture toughness properties of human cortical bone specifically in the transverse (breaking) orientation. Because bone exhibits rising crack-growth resistance with crack extension, the aging-related transverse toughness is evaluated in terms of resistance-curve (R-curve) behavior, measured for bone taken from a wide range of age groups (25–74 years). Using this approach, both the ex vivo crack-initiation and crack-growth toughness are determined and are found to deteriorate with age; however, the effect is far smaller than that reported for the longitudinal toughness of cortical bone. Whereas the longitudinal crack-growth toughness has been reported to be reduced by almost an order of magnitude for human cortical bone over this age range, the corresponding age-related decrease in transverse toughness is merely ∼14%. Similar to that reported for X-ray irradiated bone, with aging cracks in the transverse direction are subjected to an increasing incidence of crack deflection, principally along the cement lines, but the deflections are smaller and result in a generally less tortuous crack path.     

基于不同应变判定准则模拟皮质骨断裂的准确性

文题释义：基于等效应变断裂模拟：即在大鼠股骨皮质骨断裂模拟过程中,应用皮质骨有限元模型在外部载荷作用下所产生的等效应变数值,与皮质骨组织的失效应变进行对比,当等效应变数值大于皮质骨组织失效应变时,有限元模型内的单元便发生失效,直至失效单元达到一定数量,模型便发生整体失效,此过程为基于等效应变的断裂模拟。基于主应变断裂模拟：即在大鼠股骨皮质骨断裂模拟过程中,应用皮质骨有限元模型在外部载荷作用下所产生的主应变数值,与皮质骨组织的失效应变进行对比,当主应变数值大于皮质骨组织失效应变时,有限元模型内的单元便发生失效,直至失效单元达到一定数量,模型便发生整体失效,此过程为基于主应变的断裂模拟。背景：由于意外碰撞等外力因素所产生的皮质骨裂纹是引起骨折的重要原因之一,要防止此类骨折发生,首先需弄清不同载荷作用下皮质骨裂纹的产生与扩展机制。由于实验分析对样本具有破坏性,难以同时了解骨结构在断裂前后的内部力学状态,找到一种能够准确模拟皮质骨从裂纹产生、扩展,直至断裂过程的有限元方法就显得尤为重要。当前模拟方法主要应用主应变或等效应变判定模型单元力学状态,继而进行断裂模拟,却鲜有关于这2种应变进行模拟准确性的探究。目的：验证应用主应变与等效应变进行皮质骨断裂模拟的准确程度。方法：结合实验与仿真分析,应用主应变与等效应变进行皮质骨断裂模拟,将仿真与实验结果进行对比,确定应用哪种应变进行模拟更加准确。结果与结论：①应用主应变模拟的皮质骨断裂时间要明显晚于应用等效应变;②通过与实验对比发现,相比主应变,应用等效应变进行仿真所得结果与实验值更为接近;③因此,应用等效应变进行皮质骨断裂模拟相对更加准确。ORCID: 0000-0003-0313-1359(王伟军)中国组织工程研究杂志出版内容重点：人工关节;骨植入物;脊柱;骨折;内固定;数字化骨科;组织工程     

Fracture toughening mechanism of cortical bone: an experimental and numerical approach

In this investigation, the crack propagation mechanisms contributing to the toughness of cortical bone were studied using a combination of experimental and numerical approaches. Compact tension (CT) specimens were prepared from bovine cortical bones to achieve crack propagation in the longitudinal and transverse directions. Stable crack extension experiments were conducted to distinguish the crack growth resistance curves, and virtual multidimensional internal bond (VMIB) modeling was adopted to simulate the fracture responses. Results from experiments indicated that cortical bone exhibited rising resistance curves (R-curves) for crack extension parallel and perpendicular to the bone axis; the transverse fracture toughness was significantly larger, indicating that the fracture properties of cortical bone are substantially anisotropic. Microscopic observations showed that the toughening mechanisms in the longitudinal and transverse directions were different. When the crack grew in the transverse direction, the crack deflected significantly, and crack bifurcations were found at the crack wake, while, in the longitudinal direction, the crack was straight and uncracked ligaments were observed. Numerical simulations also revealed that the fracture resistance in the transverse direction was greater than that in the longitudinal direction.     

Respective roles of organic and mineral components of human cortical bone matrix in micromechanical behavior: an instrumented indentation study

Bone is a multiscale composite material made of both a type I collagen matrix and a poorly crystalline apatite mineral phase. Due to remodeling activity, cortical bone is made of Bone Structural Units (BSUs) called osteons. Since osteon represents a fundamental level of structural hierarchy, it is important to investigate the relationship between mechanical behavior and tissue composition at this scale for a better understanding of the mechanisms of bone fragility. The aim of this study is to analyze the links between ultrastructural properties and the mechanical behavior of bone tissue at the scale of osteon.Iliac bone biopsies were taken from untreated postmenopausal osteoporotic women, embedded, sectioned and microradiographed to assess the degree of mineralization of bone (DMB). On each section, BSUs of known DMB were indented with relatively high load (∼500 mN) to determine local elastic modulus (E), contact hardness (H_c) and true hardness (H) of several bone lamellae. Crystallinity and collagen maturity were measured by Fourier Transform InfraRed Microspectroscopy (FTIRM) on the same BSUs. Inter-relationships between mechanical properties and ultrastructural components were analyzed using multiple regression analysis.This study showed that elastic deformation was only explained by DMB whereas plastic deformation was more correlated with collagen maturity. Contact hardness, reflecting both elastic and plastic behaviors, was correlated with both DMB and collagen maturity. No relationship was found between crystallinity and mechanical properties at the osteon level.     

股骨近端不同平面皮质骨厚度值、X射线灰度值与女性髋部脆性骨折的相关性

文题释义：脆性骨折：是指在无创伤或较微创伤情况下引起的骨折,一般是指在平地或人体站立高度下活动过程中发生的骨折,X射线下常可见骨骼有骨质疏松征象,也称为骨质疏松性骨折。髋部骨折：是指发生在髋部及周围的骨折,包括股骨颈骨折、股骨转子间骨折、转子间骨折及髋臼骨折等,是骨质疏松性骨折最常发生的骨折部位,患者常因高龄且伴有基础疾病而1年内死亡率可达50%,5年存活率为20%。背景：随着人口老龄化,脆性骨折疾病受到临床越来越多的关注,骨密度值的单独使用已经不能满足临床对脆性骨折的风险评估。目前骨皮质厚度作为评估脆性骨折发生风险的新方向和新方法之一,受到越来越多学者的研究和讨论。目的：探讨在X射线DR摄影下,股骨近端不同平面骨皮质厚度值、X射线灰度值与50岁以上女性髋部脆性骨折的相关性,以此探寻股骨近端不同平面中,用于预测女性髋部脆性骨折发生风险最合适的测量平面。方法：依照纳入标准收集2018年7月至2019年6月在广西医科大学第九附属医院放射科行X射线DR骨盆正位片检查的50岁以上女性患者100例的相关临床资料。所有患者对试验方案均知情同意,且得到医院伦理委员会批准。股骨近端骨皮质厚度值的测量平面设计为股骨颈中段、小转子上1 cm以内和小转子下1 cm以内,共3个测量平面。X射线灰度值测量以大转子和小转子的中点连线作为矩形对角线取矩形区域测量。结果与结论：①50-64岁年龄组记为A组,65岁及以上年龄组记为B组,每组各50例;② B组患者的小转子上1 cm以内和小转子下1 cm以内平面的骨皮质厚度值、X射线灰度值比A组低,骨折率比A组高,差异均有显著性意义(P < 0.05);而股骨颈中段平面的骨皮质厚度值2组间比较差异无显著性意义(P > 0.05);③骨皮质厚度值的测量平面比较,骨皮质厚度测量值在整体上由高到低依次为小转子下1 cm以内、小转子上1 cm以内和股骨颈中段平面,差异有显著性意义(P < 0.05);④观察对象小转子下1 cm以内及小转子上1 cm以内骨皮质厚度值、X射线灰度值、年龄,与脆性骨折相关系数依次为-0.303,-0.205,-0.272及0.346(P < 0.05),但股骨颈中段平面皮质骨厚度与脆性骨折无明显相关性(P > 0.05);⑤A组中骨折和非骨折患者小转子上1 cm以内和小转子下1 cm以内测量平面的骨皮质厚度值比较差异有显著性意义(P < 0.05);B组中骨折和非骨折患者各指标比较差异均无显著性意义(P > 0.05);在50岁以上的所有患者中,骨折和非骨折患者各测量平面的骨皮质厚度值和X射线灰度值的比较差异均有显著性意义(P < 0.05);⑥说明骨皮质厚度值变薄,X射线灰度值变小,则髋部脆性骨折可能性变高。利用股骨近端骨皮质厚度评估50岁以上女性髋部脆性骨折风险时,推荐在股骨小转子下1 cm以内平面进行测量。ORCID: 0000-0003-1961-7468(覃海阔)中国组织工程研究杂志出版内容重点：人工关节;骨植入物;脊柱;骨折;内固定;数字化骨科;组织工程     

自体块状皮质骨修复种植区颌骨缺损

背景:自体骨有骨传导、骨诱导及骨生成的特性,同时具有良好的生物相容性,移植后无排斥反应。但关于自体骨块移植后的骨块是完全被吸收替代?还是能保留细胞活性长期存留?仍存在争议。目的:观察皮质骨移植后改建再生过程中的组织学变化。方法:6只健康比格犬麻醉后拔除两侧上颌前磨牙,并去除颊侧宽约10 mm、长约15 mm、厚度约2 mm的骨板,建立上颌骨缺损模型,在双侧下颌骨体颊侧切取相应大小的块状皮质骨并修整边缘。在上颌骨缺损区移植皮质骨块,一侧为单纯块状皮质骨移植,另一侧在块状皮质骨移植同期植入种植体。分别于骨移植后3,6个月取材,进行大体观测和组织学观察,分析移植骨块的吸收率、移植骨中的细胞存活率。实验方案经大连医科大学动物实验伦理委员会批准。结果与结论:①大体观察可见移植骨块体积逐渐缩小,边缘圆钝,与基骨结合稳固;②6个月时全部种植体脱落;硬组织磨片可见移植骨块与基骨间存在新生骨连接;6个月时移植骨骨陷窝孔隙率明显低于3个月,移植骨吸收率显著高于3个月(均P<0.05);③结果说明,块状皮质骨移植后,能够与受植区基骨发生骨结合,其内部的骨细胞部分保持活力,随着愈合期延长,新生骨细胞的比例增加,移植骨块的体积逐渐变小。皮质骨移植同期种植后移植物吸收明显,种植体骨结合不良。     

Fracture resistance of human cortical bone across multiple length-scales at physiological strain rates

While most fracture-mechanics investigations on bone have been performed at low strain rates, physiological fractures invariably occur at higher loading rates. Here, at strain rates from 10⁻⁵ to 10⁻¹ s⁻¹, we investigate deformation and fracture in bone at small length-scales using in situ small-angle x-ray scattering (SAXS) to study deformation in the mineralized collagen fibrils and at the microstructural level via fracture-mechanics experiments to study toughening mechanisms generating toughness through crack-tip shielding. Our results show diminished bone toughness at increasing strain rates as cracks penetrate through the osteons at higher strain rates instead of deflecting at the cement lines, which is a prime toughening mechanism in bone at low strain rates. The absence of crack deflection mechanisms at higher strain rates is consistent with lower intrinsic bone matrix toughness. In the SAXS experiments, higher fibrillar strains at higher strain rates suggest less inelastic deformation and thus support a lower intrinsic toughness. The increased incidence of fracture induced by high strain rates can be associated with a loss in toughness in the matrix caused by a strain rate induced stiffening of the fibril ductility, i.e., a “locking-up” of the viscous sliding and sacrificial bonding mechanisms, which are the origin of inelastic deformation (and toughness) in bone at small length-scales.     

Muscle area estimation from cortical bone

This article investigates the relationship between the cortical bone of the radius and the muscle area of the forearm. The aim of this study was to develop a method for muscle area estimation from cortical bone area at 65% of radius length where the muscle area at the forearm is largest. Muscle area and cortical area were measured directly in vivo by peripheral Quantitative Computed Tomography (pQCT). We found significant correlations between muscle area and cortical area (r = 0.881) in the forearm that are in line with previous studies. We have set up a regression model by testing relevant parameters such as age, sex, forearm length, and stature that were all highly correlated to muscle area. The influence of age and sex on the proportion of muscle area to cortical area is strong and potentially related to the effects of testosterone and estrogen on the muscle‐bone‐unit. Muscle area estimation from cortical bone is possible with a Percent Standard Error of Estimate (%SEE) ranging from 12.03% to 14.83%, depending on the parameters available and the age and sex of the individual. Muscle area estimation from cortical bone can provide new information for the study of skeletal and/or fossil human remains. Anat Rec, 296:1695–1707, 2013. © 2013 Wiley Periodicals, Inc.     

Elastic anisotropy and off-axis ultrasonic velocity distribution in human cortical bone

Elastic structure in cortical bone is usually simplified as orthotropic or transversely isotropic, which allows estimates of three-dimensional technical constants from ultrasonic and density measurements. These elastic property estimates can then be used to study phenotypic changes in cortical bone structure and function, and to create finite element models of skeletal structures for studies of organismal variation and functional adaptation. This study examines assumptions of orthotropic or transversely isotropic material structure in cortical bone through the investigation of off-axis ultrasonic velocities in the cortical plane in 10 samples each from a human femur, mandible and cranium. Longitudinal ultrasonic velocities were measured twice through each bone sample by rotating the perimeter of each sample in 1 ° angular intervals between two ultrasonic transducers. The data were fit to sine curves f(x)=(A × sin(x + B) + C) and the goodness of fit was examined. All the data from the femur fit closely with the ideal sine curve model, and all three coefficients were similar among specimens, indicating similar elastic properties, anisotropies and orientations of the axes of maximum stiffness. Off-axis ultrasonic velocities in the mandible largely fit the sine curve model, although there were regional variations in the coefficients. Off-axis ultrasonic velocities from the cranial vault conformed to the sine curve model in some regions but not in others, which shows an irregular and complex pattern. We hypothesize that these variations in ultrasonic velocities reflect variations in the underlying bulk microstructure of the cortical bone, especially in the three-dimensional patterns of osteonal orientation and structure. Elastic property estimates made with ultrasonic techniques are likely valid in the femur and mandible; errors in estimates from cranial bone need to be evaluated regionally. Approximate orthotropic structure in bulk cortical bone specimens should be assessed if ultrasound is used to estimate three-dimensional elastic properties.     

Estimation of the elastic modulus of child cortical bone specimens via microindentation

Purpose: Non-pathological child cortical bone (NPCCB) studies can provide clinicians with vital information and insights. However, assessing the anisotropic elastic properties of NPCCB remains a challenge for the biomechanical engineering community. For the first time, this paper provides elastic moduli values for NPCCB specimens in two perpendicular directions (longitudinal and transverse) and for two different structural components of bone tissue (osteon and interstitial lamellae).

Materials and Methods: Microindentation is one of the reference methods used to measure bone stiffness. Here, 8 adult femurs (mean age 82 ± 8.9 years), 3 child femurs (mean age 13.3 ± 2.1 years), and 16 child fibulae (mean age 10.2 ± 3.9 years) were used to assess the elastic moduli of adult and child bones by microindentation.

Results: For adult specimens, the mean moduli measured in this study are 18.1 (2.6) GPa for osteons, 21.3 (2.3) GPa for interstitial lamellae, and 13.8 (1.7) GPa in the transverse direction.

For child femur specimens, the mean modulus is 14.1 (0.8) GPa for osteons, lower than that for interstitial lamellae: 15.5 (1.5) GPa. The mean modulus is 11.8 (0.7) GPa in the transverse direction. Child fibula specimens show a higher elastic modulus for interstitial lamellae 15.8 (1.5) than for osteons 13.5 (1.6), with 10.2 (1) GPa in the transverse direction.

Conclusion: For the first time, NPCCB elastic modulus values are provided in longitudinal and transverse directions at the microscale level.     

Histomorphometric and osteocytic characteristics of cortical bone in male subtrochanteric femoral shaft

The histomorphometric properties of the subtrochanteric femoral region have rarely been investigated. The aim of this study was to investigate the age‐associated variations and regional differences of histomorphometric and osteocytic properties in the cortical bone of the subtrochanteric femoral shaft, and the association between osteocytic and histological cortical bone parameters. Undecalcified histological sections of the subtrochanteric femoral shaft were obtained from cadavers (n = 20, aged 18–82 years, males). They were cut and stained using modified Masson‐Goldner stain. Histomorphometric parameters of cortical bone were analysed with ×50 and ×100 magnification after identifying cortical bone boundaries using our previously validated method. Within cortical bone areas, only complete osteons with typical concentric lamellae and cement line were selected and measured. Osteocytic parameters of cortical bone were analyzed under phase contrast microscopy and epifluorescence within microscopic fields (0.55 mm² for each). The cortical widths of the medial and lateral quadrants were significantly higher than other quadrants (P < 0.01). Osteonal area per cortical bone area was lower and cortical porosities were higher in the posterior quadrant than in the other quadrants (P < 0.05). Osteocyte lacunar number per cortical bone area was found higher in the young subjects (≤ 50 years) than in the older ones (> 50 years) both before and after adjustments for body height and weight (P < 0.05). Moreover, significant but low correlations were found between the cortical bone and osteocytic parameters (0.20 ≤ R² ≤ 0.35, P < 0.05). It can be concluded that in healthy males, the cortical histomorphometric parameters differ between the anatomical regions of the subtrochanteric femoral shaft, and are correlated with the osteocytic parameters from the same site. These findings may be of use when discussing mechanisms that predispose patients to decreasing bone strength.     

可降解聚合物在骨折内固定和骨缺损修复中的应用

可降解聚合物具有可降解、能被人体吸收、生物相容性好等优点 ,在骨折内固定和骨缺损修复方面有广阔的应用前景。近年的研究表明 :其作为骨折内固定物 ,已得到成功的应用 ;作为骨组织填充物 ,需与 BMP复合 ,以提高其骨诱导活性 ;作为组织工程用的支架材料 ,需进一步提高其机械强度。总之 ,作为可降解聚合物 ,在生物相容性、降解时间的可控性、机械性能和制作工艺等方面有待进一步完善。     

Direct measurement of tibial cortical bone surface area

Cortical bone surface area estimation is generally performed by analysis of medical images but its accuracy has not yet been verified by direct bone measurements. This study was performed to evaluate the reproducibility of direct ex vivo estimations of cross sectional tibial cortical bone surface area. Ten tibiae were evaluated by two observers using direct 3D‐digitization at 38 and 66% of total tibial length using a ‘critical’ and a ‘general’ criterion for differentiation between cortical and cancellous bone. The results of the analysis using the more severe ‘critical’ differentiation criterion yielded high intra and interobserver agreement. Mean intraobserver reliability was demonstrated by R = 0.98 (Spearman rank correlation between 0.98 and 0.97; P < 0.01) and inter observer reliability by R = 0.91 (Spearman rank correlation between 0.95 and 0.88; P < 0.01). When using strict criteria to differentiate between cortical and cancellous bone, intra and interobserver reliability of the presented method of direct cortical bone surface area estimation is very high. The results indicate that cross‐sectional cortical bone surface estimation by direct 3D‐digitization can serve as a gold standard for validation of other estimations methods. Clin. Anat. 23:720–725, 2010. © 2010 Wiley‐Liss, Inc.     

异体皮质人工骨复合物治疗骨折不愈合及骨缺损

背景：异体皮质干燥异体骨为多孔状结构,孔均匀且相互贯通,孔径200 µm 左右,在形态结构上与人类无机骨极为相似。 目的：观察应用骨形态发生蛋白-骨髓间充质干细胞-异体皮质人工骨复合物治疗骨折不愈合及骨缺损的临床效果。 方法：选择33例创伤性骨缺损患者,其中男19例,女14例;年龄26-85岁。首先提取患者自体骨髓间充质干细胞,进一步以异体皮质人工骨为支架材料,同时导入骨形态发生蛋白和骨髓间充质干细胞,通过人工合成方式制成骨形态发生蛋白-骨髓间充质干细胞-异体皮质人工骨复合物,植入骨缺损处,每例植入人工骨复合物20-45 g。 结果与结论：随访6个月-2.5年,根据Mankin及Komender等对同种异体骨移植结果的评分标准,满意32例,不满意1例。复查X射线片示32例达骨性愈合,其中新鲜骨折愈合时间2-7个月,陈旧性骨折愈合时间4-9个月。同种异体松质骨6-9周开始与自体骨融合爬行替代。1例感染性骨缺损移植后迟发性感染,去除内固定及同种异体骨,置管冲洗、局部稳定后再次植骨愈合,随访2年1个月骨感染未复发;1例钢板松动致骨折未愈合,再次行内固定、自体骨移植后骨折愈合。表明应用骨形态发生蛋白-骨髓间充质干细胞-异体皮质人工骨复合物治疗骨折不愈合及骨缺损无明显不良反应,修复效果良好。中国组织工程研究杂志出版内容重点：生物材料;骨生物材料; 口腔生物材料; 纳米材料; 缓释材料; 材料相容性;组织工程全文链接：     

Periosteal PTHrP regulates cortical bone modeling during linear growth in mice

The modeling of long bone surfaces during linear growth is a key developmental process, but its regulation is poorly understood. We report here that parathyroid hormone‐related peptide (PTHrP) expressed in the fibrous layer of the periosteum (PO) drives the osteoclastic (OC) resorption that models the metaphyseal–diaphyseal junction (MDJ) in the proximal tibia and fibula during linear growth. PTHrP was conditionally deleted (cKO) in the PO via Scleraxis gene targeting (Scx‐Cre). In the lateral tibia, cKO of PTHrP led to a failure of modeling, such that the normal concave MDJ was replaced by a mound‐like deformity. This was accompanied by a failure to induce receptor activator of NF‐kB ligand (RANKL) and a 75% reduction in OC number (P ≤ 0.001) on the cortical surface. The MDJ also displayed a curious threefold increase in endocortical osteoblast mineral apposition rate (P ≤ 0.001) and a thickened cortex, suggesting some form of coupling of endocortical bone formation to events on the PO surface. Because it fuses distally, the fibula is modeled only proximally and does so at an extraordinary rate, with an anteromedial cortex in CD‐1 mice that was so moth‐eaten that a clear PO surface could not be identified. The cKO fibula displayed a remarkable phenotype, with a misshapen club‐like metaphysis and an enlargement in the 3D size of the entire bone, manifest as a 40–45% increase in the PO circumference at the MDJ (P ≤ 0.001) as well as the mid‐diaphysis (P ≤ 0.001). These tibial and fibular phenotypes were reproduced in a Scx‐Cre‐driven RANKL cKO mouse. We conclude that PTHrP in the fibrous PO mediates the modeling of the MDJ of long bones during linear growth, and that in a highly susceptible system such as the fibula this surface modeling defines the size and shape of the entire bone.     

Regional maps of rib cortical bone thickness and cross‐sectional geometry

Here we present detailed regional bone thickness and cross‐sectional measurements from full adult ribs using high resolution CT scans processed with a cortical bone mapping technique. Sixth ribs from 33 subjects ranging from 24 to 99 years of age were used to produce average cortical bone thickness maps and to provide average ± 1SD corridors for expected cross‐section properties (cross‐sectional areas and inertial moments) as a function of rib length. Results obtained from CT data were validated at specific rib locations using direct measurements from cut sections. Individual thickness measurements from CT had an accuracy (mean error) and precision (SD error) of ?0.013 ± 0.167 mm (R² coefficient of determination of 0.84). CT‐based measurement errors for rib cross‐sectional geometry were ?0.1 ± 13.1% (cortical bone cross‐sectional area) and 4.7 ± 1.8% (total cross‐sectional area). Rib cortical bone thickness maps show the expected regional variation across a typical rib's surface. The local mid‐rib maxima in cortical thickness along the pleural rib aspect ranged from range 0.9 to 2.6 mm across the study population with an average map maximum of 1.4 mm. Along the cutaneous aspect, rib cortical bone thickness ranged from 0.7 to 1.9 mm with an average map thickness of 0.9 mm. Average cross‐sectional properties show a steady reduction in total cortical bone area from 10% along the rib's length through to the sternal end, whereas overall cross‐sectional area remains relatively constant along the majority of the rib's length before rising steeply towards the sternal end. On average, male ribs contained more cortical bone within a given cross‐section than was seen for female ribs. Importantly, however, this difference was driven by male ribs having larger overall cross‐sectional areas, rather than by sex differences in the bone thickness observed at specific local cortex sites. The cortical bone thickness results here can be used directly to improve the accuracy of current human body and rib models. Furthermore, the measurement corridors obtained from adult subjects across a wide age range can be used to validate future measurements from more widely available image sources such as clinical CT where gold standard reference measures (e.g. such as direct measurements obtained from cut sections) are otherwise unobtainable.     

Histovariability in human clavicular cortical bone microstructure and its mechanical implications

The human clavicle (i.e. collarbone) is an unusual long bone due to its signature S‐shaped curve and variability in macrostructure observed between individuals. Because of the complex nature of how the upper limb moves, as well as due to its complex musculoskeletal arrangement, the biomechanics, in particular the mechanical loadings, of the clavicle are not fully understood. Given that bone remodeling can be influenced by bone stress, the histologic organization of Haversian bone offers a hypothesis of responses to force distributions experienced across a bone. Furthermore, circularly polarized light microscopy can be used to determine the orientation of collagen fibers, providing additional information on how bone matrix might organize to adapt to direction of external loads. We examined Haversian density and collagen fiber orientation, along with cross‐sectional geometry, to test whether the clavicle midshaft shows unique adaptation to atypical load‐bearing when compared with the sternal (medial) and acromial (lateral) shaft regions. Because fractures are most common at the midshaft, we predicted that the cortical bone structure would show both disparities in Haversian remodeling and nonrandomly oriented collagen fibers in the midshaft compared with the sternal and acromial regions. Human clavicles (n = 16) were sampled via thin‐sections at the sternal, middle, and acromial ends of the shaft, and paired sample t‐tests were employed to evaluate within‐individual differences in microstructural or geometric properties. We found that Haversian remodeling is slightly but significantly reduced in the middle of the bone. Analysis of collagen fiber orientation indicated nonrandom fiber orientations that are overbuilt for tensile loads or torsion but are poorly optimized for compressive loads throughout the clavicle. Geometric properties of percent bone area, polar second moment of area, and shape (I_max/I_min) confirmed the conclusions drawn by existing research on clavicle macrostructure. Our results highlight that mediolateral shape changes might be accompanied by slight changes in Haversian density, but bone matrix organization is predominantly adapted to resisting tensile strains or torsion throughout and may be a major factor in the risk of fracture when experiencing atypical compression.     

Longitudinal MRI evaluations of human global cortical thickness over minutes to weeks

Magnetic resonance imaging (MRI) was used to evaluate within-subject variability in global mean cortical thickness over test-retest intervals of minutes-weeks in five healthy adults. Within-subject measures of global mean thickness were consistent over these intervals. Test-retest assessments of absolute thickness differences and percent thickness differences indicated variations of, respectively, < or =0.05-0.06 mm and < or =+/-1.9-2.3%. There have been few evaluations of normal within-subject variations in cortical thickness. The present results suggest that within-subject variability in global mean cortical thickness can be low over test-retest intervals of minutes-weeks, and that longitudinal scans can establish useful baseline estimates of variability from which to assess changes due to injury, disease, or other experiences.