In vivo evaluation of bone microstructure in humans: Clinically useful?

Bone fragility: conceptual framework, therapeutic implications, and COVID-19-related issues

Bone fragility is the susceptibility to fracture even for common loads because of structural, architectural, or material alterations of bone tissue that result in poor bone strength. In osteoporosis, quantitative and qualitative changes in density, geometry, and micro-architecture modify the internal stress state predisposing to fragility fractures. Bone fragility substantially depends on the structural behavior related to the size and shape of the bone characterized by different responses in the load-deformation curve and on the material behavior that reflects the intrinsic material properties of the bone itself, such as yield and fatigue. From a clinical perspective, the measurement of bone density by DXA remains the gold standard for defining the risk of fragility fracture in all population groups. However, non-quantitative parameters, such as macro-architecture, geometry, tissue material properties, and microcracks accumulation can modify the bone's mechanical strength. This review provides an overview of the role of different contributors to bone fragility and how these factors might be influenced by the use of anti-osteoporotic drugs and by the COVID-19 pandemic.

Micromechanical evaluation of cortical bone using in situ XCT indentation and digital volume correlation

The overall mechanical behaviour of cortical bone is strongly dependant on its microstructure. X-ray computed tomography (XCT) has been widely used to identify the microstructural morphology of cortical tissue (i.e. pore network, Haversian and Volkmann's canals). However, the connection between microstructure and mechanics of cortical bone during plastic deformation is unclear. Hence, the purpose of this study is to provide an in-depth evaluation of the interplay of plastic strain building up in relation to changes in the canal network for cortical bone tissue. In situ step-wise XCT indentation was used to introduce a localised load on the surface of the tissue and digital volume correlation (DVC) was employed to assess the three-dimensional (3D) full-field plastic strain distribution in proximity of the indent. It was observed that regions adjacent to the imprint were under tensile strain, whereas the volume underneath experienced compressive strain. Canal loss and disruption was detected in regions of higher compressive strains exceeding -20000 με and crack formation occurred in specimens where Haversian canals were running parallel to the indentation tip. The results of this study outline the relationship between the micromechanical and structural behaviour of cortical bone during plastic deformation, providing information on cortical tissue fracture pathways.

Osteocyte numbers decrease only in postcranial but not in cranial bones in humans of advanced age

BACKGROUND

Bone ageing is governed by the linked activities of short-lived osteoblasts and osteoclasts in conjunction with long-lived osteocytes present in osseous structure. Besides their maintenance function, osteogenic cells also gain specific positional information, which may potentially trigger ageing-associated cellular deviations in terminally differentiated osteocytes differently in cranial versus postcranial tissues.

METHODS

We therefore investigated bone taken from deceased aged humans explanted at five distinct anatomical positions throughout the body and assessed physical and biological determinants applying radiologic and histologic measures.

RESULTS

We were able to show that significantly more osteocytes reside in aged cortical bone at cranial positions than within axial or limb skeleton. These cellular states and conditions were not found in the corresponding trabecular bone, where osteocyte numbers remain also high at postcranial positions. Parallel comparative analyses of bone microstructure as analyzed by means of computer tomography showed no significant differences.

CONCLUSIONS

Considering differences and commonalities regarding the bone samples, such as loading, mechanisms of ossification or the surrounding stromal cell compartment, our findings indicate that positional information laid down during ontogenetic processes is instructive during the entire life thus potentially also moulding spatial-specific mechanistic distinctions of bone ageing.

Anisotropic elastic properties of human femoral cortical bone and relationships with composition and microstructure in elderly

The strong dependence between cortical bone elasticity at the millimetre-scale (mesoscale) and cortical porosity has been evidenced by previous studies. However, bone is an anisotropic composite material made by mineral, proteins and water assembled in a hierarchical structure. Whether the variations of structural and compositional properties of bone affect the different elastic coefficients at the mesoscale is not clear. Aiming to understand the relationships between bone elastic properties and compositions and microstructure, we applied state-of-the-art experimental modalities to assess these aspects of bone characteristics. All elastic coefficients (stiffness tensor of the transverse isotropic bone material), structure of the vascular pore network, collagen and mineral properties were measured in 52 specimens from the femoral diaphysis of 26 elderly donors. Statistical analyses and micromechanical modeling showed that vascular pore volume fraction and the degree of mineralization of bone are the most important determinants of cortical bone anisotropic mesoscopic elasticity. Though significant correlations were observed between collagen properties and elasticity, their effects in bone mesoscopic elasticity were minor in our data. This work also provides a unique set of data exhibiting a range of variations of compositional and microstructural cortical bone properties in the elderly and gives strong experimental evidence and basis for further development of biomechanical models for human cortical bone. STATEMENT OF SIGNIFICANCE: This study reports the relationships between microstructure, composition and the mesoscale anisotropic elastic properties of human femoral cortical bone in elderly. For the first time, we provide data covering the complete anisotropic elastic tensor, the microstructure of cortical vascular porosity, mineral and collagen characteristics obtained from the same or adjacent samples in each donor. The results revealed that cortical vascular porosity and degree of mineralization of bone are the most important determinants of bone anisotropic stiffness at the mesoscale. The presented data gives strong experimental evidence and basis for further development of biomechanical models for human cortical bone.

Comparison of periprosthetic bone remodeling after implantation of anatomic and tapered cementless femoral stems in total hip arthroplasty: A prospective cohort study protocol

INTRODUCTION

Current total hip arthroplasty (THA) implant usage trends favor cementless fixation, and plenty studies have demonstrated that numbers of cementless femoral stems are associated with excellent long-term survivorship and functional outcomes. Various types of cementless femoral stems have been developed and utilized in multiple applications, including straight, tapered, anatomic, customized, short, and even neck stems. All of these designs aimed to achieve maximal primary stability and promote osseointegration. Nevertheless, stress-shielding and periprosthetic bone loss continue to occur and remain critical issues in promoting long-term survivorship of THA. Considering anatomic and tapered stems are the most popular cementless designs today, this prospective cohort study aimed to investigate the effect of stem design on stress-shielding and periprosthetic bone remodeling after implantation of an anatomic stem with proximal fixation (Ribbed Hip system; Waldemar Link, Hamburg, Germany) and the direct comparison to a fully coated tapered stem (LCU Hip system; Waldemar Link).

MATERIALS AND METHODS

This prospective cohort study will comprise patients who receive primary unilateral THA with the Ribbed anatomic hydroxyapatite (HA)-coated stem or LCU tapered fully HA-coated stem. The changes in periprosthetic bone mineral density after insertion of Ribbed and LCU stem prostheses will be assessed by means of dual-energy X-ray absorptiometry in the periprosthetic region of interest according to Gruen and colleagues. Standard anteroposterior and lateral plain radiography will be performed for qualitative assessment of the periprosthetic bone remodeling. The following items will be analyzed or measured on follow-up radiographs to compare with the initial appearance on the radiographs taken immediately postoperatively: cortical thickness in each Gruen zone, fitness of the distal stem within the isthmus, femoral stem alignment, radiolucent line, reactive line, periosteal bone reactions, and subsidence. Biologic fixation and stability of the cementless implant will be evaluated using Engh grading scale, and heterotopic ossification will be graded according to Brooker classification. Furthermore, Harris hip score and Western Ontario and McMaster Universities Osteoarthritis Index Score will also be assessed for postoperative functional evaluation. These radiologic and clinical assessments will be taken postoperatively, at 6 months, 1, 2, 3, 4, and 5 years after surgery.

ETHICS AND DISSEMINATION

This study was approved by The First Affiliated Hospital of Chongqing Medical University Ethics Committee. The study results will be disseminated at national and international conferences and published in peer-reviewed journals.

STUDY REGISTRATION

Chinese Clinical Trial Registry (http://www.chictr.org.cn): ChiCTR1800017841.

The QUALYOR (QUalité Osseuse LYon Orléans) study: a new cohort for non invasive evaluation of bone quality in postmenopausal osteoporosis. Rationale and study design

The diagnostic performance of densitometry is inadequate. New techniques of non-invasive evaluation of bone quality may improve fracture risk prediction. Testing the value of these techniques is the goal of the QUALYOR cohort.

INTRODUCTION

The bone mineral density (BMD) of postmenopausal women who sustain osteoporotic fracture is generally above the World Health Organization definition for osteoporosis. Therefore, new approaches to improve the detection of women at high risk for fracture are warranted.

METHODS

We have designed and recruited a new cohort to assess the predictive value of several techniques to assess bone quality, including high-resolution peripheral quantitative computerized tomography (HRpQCT), hip QCT, calcaneus texture analysis, and biochemical markers. We have enrolled 1575 postmenopausal women, aged at least 50, with an areal BMD femoral neck or lumbar spine T-score between - 1.0 and - 3.0. Clinical risk factors for fracture have been collected along with serum and blood samples.

RESULTS

We describe the design of the QUALYOR study. Among these 1575 women, 80% were aged at least 60. The mean femoral neck T-score was - 1.6 and the mean lumbar spine T-score was -1.2. This cohort is currently being followed up.

CONCLUSIONS

QUALYOR will provide important information on the relationship between bone quality variables and fracture risk in women with moderately decreased BMD.

In vivo Visualisation and Quantification of Bone Resorption and Bone Formation from Time-Lapse Imaging

PURPOSE OF REVIEW

Mechanoregulation of bone cells was proposed over a century ago, but only now can we visualise and quantify bone resorption and bone formation and its mechanoregulation. In this review, we show how the newest advances in imaging and computational methods paved the way for this breakthrough.

RECENT FINDINGS

Non-invasive in vivo assessment of bone resorption and bone formation was demonstrated by time-lapse micro-computed tomography in animals, and by high-resolution peripheral quantitative computed tomography in humans. Coupled with micro-finite element analysis, the relationships between sites of bone resorption and bone formation and low and high tissue loading, respectively, were shown. Time-lapse in vivo imaging and computational methods enabled visualising and quantifying bone resorption and bone formation as well as its mechanoregulation. Future research includes visualising and quantifying mechanoregulation of bone resorption and bone formation from molecular to organ scales, and translating the findings into medicine using personalised bone health prognosis.