Clinical hip fracture is accompanied by compression induced failure in the superior cortex of the femoral neck

Predicting fracture classification and prognosis with hounsfield units and femoral cortical index: A simple and cost-effective approach

BACKGROUND

The relationship between bone density and fracture has been widely studied and recognized, and the role of cortical bone in proximal femoral fractures has also been increasingly studied. However, both the determination of bone mineral density (BMD) and the determination of cortical mass are expensive and cumbersome. The purpose of this study is to investigate whether two readily available indicators, Hounsfield Units (HUs) and femoral cortical index (FCI), can be used to predict hip fracture classification and prognosis.

METHODS

A retrospective study was conducted on 110 patients with hip fragility fractures. Cortical index was calculated on fractured and contralateral femur FCI, with HUs calculated on the proximal femur. The correlation of the FCI and HU with diabetes, hypertension, and related indicators, such as albumin, creatinine, and urea nitrogen levels, were also analyzed in the study.

RESULTS

Both the Evans classification of intertrochanteric fractures and the Garden and Pauwels classifications of femoral neck fractures showed that as the severity of the fracture increased, the HUs and FCI decreased. Age and albumin level also had a negative correlation with HUs and FCI. There was also a significant correlation between HUs and FCI.

CONCLUSIONS

The HUs and FCI, which can be easily and quickly obtained, can be used to predict the classification and prognosis of hip fractures.

Exploration of the synergistic role of cortical thickness asymmetry ("Trabecular Eccentricity" concept) in reducing fracture risk in the human femoral neck and a control bone (Artiodactyl Calcaneus)

The mechanobiology of the human femoral neck is a focus of research for many reasons including studies that aim to curb age-related bone loss that contributes to a near-exponential rate of hip fractures. Many believe that the femoral neck is often loaded in rather simple bending, which causes net tension stress in the upper (superior) femoral neck and net compression stress in its inferior aspect ("T/C paradigm"). This T/C loading regime lacks in vivo proof. The "C/C paradigm" is a plausible alternative simplified load history that is characterized by a gradient of net compression across the entire femoral neck; action of the gluteus medius and external rotators of the hip are important in this context. It is unclear which paradigm is at play in natural loading due to lack of in vivo bone strain data and deficiencies in understanding mechanisms and manifestations of bone adaptation in tension vs. compression. For these reasons, studies of the femoral neck would benefit from being compared to a 'control bone' that has been proven, by strain data, to be habitually loaded in bending. The artiodactyl (sheep and deer) calcaneus model has been shown to be a very suitable control in this context. However, the application of this control in understanding the load history of the femoral neck has only been attempted in two prior studies, which did not examine the interplay between cortical and trabecular bone, or potential load-sharing influences of tendons and ligaments. Our first goal is to compare fracture risk factors of the femoral neck in both paradigms. Our second goal is to compare and contrast the deer calcaneus to the human femoral neck in terms of fracture risk factors in the T/C paradigm (the C/C paradigm is not applicable in the artiodactyl calcaneus due to its highly constrained loading). Our third goal explores interplay between dorsal/compression and plantar/tension regions of the deer calcaneus and the load-sharing roles of a nearby ligament and tendon, with insights for translation to the femoral neck. These goals were achieved by employing the analytical model of Fox and Keaveny (J. Theoretical Biology 2001, 2003) that estimates fracture risk factors of the femoral neck. This model focuses on biomechanical advantages of the asymmetric distribution of cortical bone in the direction of habitual loading. The cortical thickness asymmetry of the femoral neck (thin superior cortex, thick inferior cortex) reflects the superior-inferior placement of trabecular bone (i.e., "trabecular eccentricity," TE). TE helps the femoral neck adapt to typical stresses and strains through load-sharing between superior and inferior cortices. Our goals were evaluated in the context of TE. Results showed the C/C paradigm has lower risk factors for the superior cortex and for the overall femoral neck, which is clinically relevant. TE analyses of the deer calcaneus revealed important synergism in load-sharing between the plantar/tension cortex and adjacent ligament/tendon, which challenges conventional understanding of how this control bone achieves functional adaptation. Comparisons with the control bone also exposed important deficiencies in current understanding of human femoral neck loading and its potential histocompositional adaptations.

Three-dimensional mapping of cortical porosity and thickness along the superolateral femoral neck in older women

Although several studies have analyzed inter-individual differences in the femoral neck cortical microstructure, intra-individual variations have not been comprehensively evaluated. By using microCT, we mapped cortical pore volume fraction (Ct.Po) and thickness (Ct.Th) along the superolateral femoral neck in 14 older women (age: 77.1 ± 9.8 years) to identify subregions and segments with high porosity and/or low thickness—potential “critical” spots where a fracture could start. We showed that Ct.Po and Ct.Th significantly differed between basicervical, midcervical, and subcapital subregions of the femoral neck (p < 0.001), where the subcapital subregion showed the lowest mean Ct.Th and the highest mean Ct.Po. These cortical parameters also varied substantially with age and with the location of the analyzed microsegments along the individual’s neck (p < 0.001), showing multiple microsegments with high porosity and/or low thickness. Although the highest ratio of these microsegments was found in the subcapital subregion, they were also present at other examined subregions, which may provide an anatomical basis for explaining the fracture initiation at various sites of the superolateral neck. Given that fractures likely start at structurally and mechanically weaker spots, intra-individual variability in Ct.Po and Ct.Th should be considered and the average values for the entire femoral neck should be interpreted with caution.

Spatial distribution of hip cortical thickness in postmenopausal women with different osteoporotic fractures

Few studies h ave discussed the association between cortical bone outside the fracture site and the fracture itself. Focusing on hip cortical thickness, this study revealed distinct distributions of the parameters for hip (trochanteric or femoral neck), vertebral, and peripheral osteoporotic fractures and suggested that the spatial distribution of hip cortical thickness was fracture-specific.

PURPOSE

Cortical bone is critical for bone strength. Hip cortical thickness is reported to be closely associated with the incidence of hip fractures, but its relationship with nonhip fractures is rarely studied. As the hip is a major site for fracture risk assessment, it would be of great benefit to investigate the association between hip cortical thickness and different osteoporotic fractures.

METHODS

One hundred age-matched postmenopausal women were equally assigned to 4 osteoporotic fracture groups (trochanteric, femoral neck, vertebral, and peripheral fractures) and a nonfracture group. Each subject had a clinical quantitative computed tomography scan of the bilateral hips and the lumbar spine. A cortical bone mapping algorithm was adopted to calculate hip cortical thickness. Hip and lumbar trabecular density and the hip cortical thickness distribution were compared among the groups.

RESULTS

All the fracture groups presented lower lumbar trabecular density. Compared with nonfracture controls, patients with hip or vertebral fractures but not peripheral fractures showed decreased cortical thickness and trabecular density of the hip. Fracture-specific distributions of cortical thickness were revealed, including zonal defects on the neck-intertrochanter junction, greater trochanter, and the periphery of the lesser trochanter for trochanteric fractures, a focal defect on the anterosuperior neck for femoral neck fractures, a moderate and average distribution for vertebral fractures, and focally thicker cortices on the anterosuperior greater trochanter and the periphery of the lesser trochanter for peripheral fractures.

CONCLUSION

The spatial distribution of hip cortical thickness was different for each type of osteoporotic fracture, and patients with centrally located fractures demonstrated more severe cortical deterioration. This finding needs to be validated in a larger sample.

Global and local characterization explains the different mechanisms of failure of the human ribs

Knowledge of the mechanics and mechanistic reasons inducing rib fracture is fundamental for forensic investigations and for the design of implants and cardiopulmonary resuscitation devices. A mechanical rationale to explain the different rib mechanisms of failure is still a challenge. The aim of this work was to experimentally characterize human ribs to test the hypothesis that a correlation exists between the ribs properties and the mechanism of failure. 89 ribs were tested in antero-posterior compression. The full-field strain distribution was measured through Digital Image Correlation. The fracture load ranged 7-132 N. Two main different mechanisms of failure were observed: brittle and buckling. The strain analysis showed that the direction of principal strains was either aligned with the ribs, or oblique, around 45°, with a rather uniform direction in the most strained area. The maximum principal strains were in the range between 1000 and 30000 microstrain and the minimum principal strain between -30000 and -800 microstrain. The ribs undergoing brittle fracture had significantly thicker cortical bone than those undergoing buckling. Also, larger tensile strains were observed in the specimens with brittle fracture than in the buckling ones. These findings support the focus of cortical thickness modelling which could help in sharpening computational models for the aforesaid purposes.

Quantitative ultrasound (QUS) axial transmission method reflects anisotropy in micro-arrangement of apatite crystallites in human long bones: A study with 3-MHz-frequency ultrasound

Anisotropic arrangement of apatite crystallites, i.e., preferential orientation of the apatite c-axis, is known to be an important bone quality parameter that governs the mechanical properties. However, noninvasive evaluation of apatite orientation has not been achieved to date. The present paper reports the potential of quantitative ultrasound (QUS) for noninvasive evaluation of the degree of apatite orientation in human bone for the first time. A novel QUS instrument for implementation of the axial transmission (AT) method is developed, so as to achieve precise measurement of the speed of sound (SOS) in the cortex (cSOS) of human long bone. The advantages of our QUS instrument are the following: (i) it is equipped with a cortical bone surface-morphology detection system to correct the ultrasound transmission distance, which should be necessary for AT measurement of long bone covered by soft tissue of non-uniform thickness; and (ii) ultrasound with a relatively high frequency of 3 MHz is employed, enabling thickness-independent cSOS measurement even for the thin cortex by preventing guide wave generation. The reliability of the proposed AT measurement system is confirmed through comparison with the well-established direct transmission (DT) method. The cSOS in human long bone is found to exhibit considerable direction-dependent anisotropy; the axial cSOS (3870 ± 66 m/s) is the highest, followed by the tangential (3411 ± 94 m/s) and radial (3320 ± 85 m/s) cSOSs. The degree of apatite orientation exhibits the same order, despite the unchanged bone mineral density. Multiple regression analysis reveals that the cSOS of human long bone strongly reflects the apatite orientation. The cSOS determined by the AT method is positively correlated with that determined by the DT method and sensitively reflects the apatite orientation variation, indicating the validity of the AT instrument developed in this study. Our instrument will be beneficial for noninvasive evaluation of the material integrity of the human long-bone cortex, as determined by apatite c-axis orientation along the axial direction.

Previous Damage Accumulation Can Influence Femoral Fracture Strength: A Finite Element Study

To manage osteoporotic hip fracture risk, it is necessary to understand failure mechanisms of bone at both the material and organ level. The structural response of bone is dependent on load history. Repeated loading causes progressive microstructural cracking, resulting in reduced apparent-level stiffness and, if damage is significant, reductions to peak load bearing capability. However, the effect of previous damage accumulation has not been well explored at the organ level. It was hypothesized that femoral fracture load and fracture pattern may be sensitive to damage accumulation from previous loading events. Six cadaveric specimens were used to develop patient specific finite element (FE) models from quantitative tomographic (qCT) scans. Material properties were assigned from qCT intensity at each element location, and damage evolution was predicted using a previously validated quasi-brittle FE model. Three scenarios were investigated: stumble followed by another stumble (S-S), fall followed by another fall (F-F), and stumble followed by a fall (S-F). Fracture load and pattern were compared to FE predictions for a single stumble (S) or single fall (F) loading event. Most specimens were resilient to accumulated damage, showing little (<5%) change in fracture load from the multiple-load scenarios (S-S, F-F, and S-F) compared to an equivalent single load scenario (S or F). Only one specimen demonstrated moderate (5-15%) reductions in strength from all three multiple-load scenarios. However, two specimens experienced moderate (20-30%) increase in fracture load in some load cases. In these cases, initial damage caused the load to be more evenly distributed upon subsequent loading events. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:2197-2203, 2019.

Subject-specific ex vivo simulations for hip fracture risk assessment in sideways falls

The risk of hip fracture of a patient due to a fall can be described from a mechanical perspective as the capacity of the femur to withstand the force that it experiences in the event of a fall. So far, impact forces acting on the lateral aspect of the pelvic region and femur strength have been investigated separately. This study used inertia-driven cadaveric impact experiments that mimic falls to the side from standing in order to evaluate the subject-specific force applied to the hip during impact and the fracture outcome in the same experimental model. Eleven fresh-frozen pelvis-femur constructs (6 female, 5 male, age = 77 years (SD = 13 years), BMI = 22.8 kg/m² (SD = 7.8 kg/m²), total hip aBMD = 0.734 g/cm² (SD = 0.149 g/cm²)), were embedded into soft tissue surrogate material that matched subject-specific mass and body shape. The specimens were attached to metallic lower-limb constructions with subject-specific masses and subjected to an inverted pendulum motion. Impact forces were recorded with a 6-axis force plate at 10,000 Hz and three dimensional deflections in the pelvic region were tracked with two high-speed cameras at 5000 Hz. Of the 11 specimens, 5 femur fractures and 3 pelvis fractures were observed. Three specimens did not fracture. aBMD alone did not reliably separate femur fractures from non-fractures. However, a mechanical risk ratio, which was calculated as the impact force divided by aBMD, classified specimens reliably into femur fractures and non-fractures. Single degree of freedom models, based on specimen kinetics, were able to predict subject-specific peak impact forces (RMSE = 2.55% for non-fractures). This study provides direct evidence relating subject-specific impact forces and subject-specific strength estimates and improves the assessment of the mechanical risk of hip fracture for a specific femur/pelvis combination in a sideways fall.

Inter-site Variability of the Human Osteocyte Lacunar Network: Implications for Bone Quality

PURPOSE OF REVIEW

This article provides a review on the variability of the osteocyte lacunar network in the human skeleton. It highlights characteristics of the osteocyte lacunar network in relation to different skeletal sites and fracture susceptibility.

RECENT FINDINGS

Application of 2D analyses (quantitative backscattered electron microscopy, histology, confocal laser scanning microscopy) and 3D reconstructions (microcomputed tomography and synchrotron radiation microcomputed tomography) provides extended high-resolution information on osteocyte lacunar properties in individuals of various age (fetal, children's growth, elderly), sex, and disease states with increased fracture risk. Recent findings on the distribution of osteocytes in the human skeleton are reviewed. Quantitative data highlighting the variability of the osteocyte lacunar network is presented with special emphasis on site specificity and maintenance of bone health. The causes and consequences of heterogeneous distribution of osteocyte lacunae both within specific regions of interest and on the skeletal level are reviewed and linked to differential bone quality factors and fracture susceptibility.

Hypermineralization in the femoral neck of the elderly

Hip fragility depends on the decline in bone mass as well as changes in bone microstructure and the properties of bone mineral and organic matrix. Although it is well-established that low bone mass or osteoporosis is a key factor in hip fracture risk, it is striking to observe that 92% of 24 patients who have sustained an intracapsular hip fracture showed hypermineralization at the superior-anterior quadrant, a critical region associated with increased hip fracture risk. In-depth material studies on a total of 12 human cadaver femurs revealed increased degree of mineralization in the hypermineralized tissue: calcium weight percentage as measured by quantitative backscattered electron imaging increased by approximately 15% compared with lamellar bone; mineral-to-matrix ratio obtained by Raman microspectroscopy imaging also increased. Immunohistochemistry revealed localized type II collagen in the hypermineralized region, implying its cartilaginous nature. At the ultrastructural level, X-ray scattering revealed significantly smaller (on average 2.3 nm thick and 15.6 nm long) and less ordered bone minerals in the hypermineralized tissue. Finally, the hypermineralized tissue was more brittle than lamellar bone under hydrated state - cracks propagated easily in the hypermineralized region but stopped at the lamellar boundary. This study demonstrates that hypermineralization of femoral neck cortical bone is a source of bone fragility which is worth considering in future fracture risk assessment when the origin of hip fracture is unclear based on current evaluation standards. STATEMENT OF SIGNIFICANCE: Hypermineralization of femoral cortical bone in older adults might occur in many more hip fracture cases than presently known. Yet, this tissue remains largely unknown to the orthopedic community possibly due to coarse resolution of clinical imaging. The current study showed the hypermineralized tissue had reduced fracture resistance which could be attributed to the material changes in mineral content, organic matrix, and mineral platelets properties. It thus could be a source for fracture initiation. Consequently, we believe hypermineralization of femoral neck cortical bone should be considered in hip fragility assessment, especially when low bone mass cannot be identified as a primary contributor to hip fracture.

Collagen fiber orientation pattern, osteon morphology and distribution, and presence of laminar histology do not distinguish torsion from bending in bat and pigeon wing bones

Bone can adapt to its habitual load history at various levels of its hierarchical structural and material organization. However, it is unclear how strongly a bone's structural characteristics (e.g. cross-sectional shape) are linked to microstructural characteristics (e.g. distributions of osteons and their vascular canals) or ultrastructural characteristics [e.g. patterns of predominant collagen fiber orientation (CFO)]. We compared the cross-sectional geometry, microstructure and ultrastructure of pigeon (Columba livia domestica) humeri, and third metacarpals (B3M) and humeri of a large bat (Pteropus poliocephalus). The pigeon humerus is habitually torsionally loaded, and has unremodeled ('primary') bone with vessels (secondary osteons are absent) and high 'laminarity' because a large majority of these vessels course circularly with respect to the bone's external surface. In vivo data show that the bat humerus is also habitually torsionally loaded; this contrasts with habitual single-plane bending of the B3M, where in vivo data show that it oscillates back and forth in the same direction. In contrast to pigeon humeri where laminar bone is present, the primary tissue of these bat bones is largely avascular, but secondary osteons are present and are usually in the deeper cortex. Nevertheless, the load history of humeri of both species is prevalent/predominant torsion, producing diffusely distributed shear stresses throughout the cross-section. We tested the hypothesis that despite microstructural/osteonal differences in these pigeon and bat bones, they will have similar characteristics at the ultrastructural level that adapt each bone for its load history. We postulate that predominant CFO is this characteristic. However, even though data reported in prior studies of bones of non-flying mammals suggest that CFO would show regional variations in accordance with the habitual 'tension regions' and 'compression regions' in the direction of unidirectional habitual bending, we hypothesized that alternating directions of bending within the same plane would obviate these regional/site-specific adaptations in the B3M. Similarly, but for other reasons, we did not expect regional variations in CFO in the habitually torsionally loaded bat and pigeon humeri because uniformly oblique-to-transverse CFO is the adaptation expected for the diffusely distributed shear stresses produced by torsion/multidirectional loads. We analyzed transverse sections from mid-diaphyses of adult bones for CFO, secondary osteon characteristics (size, shape and population density), cortical thickness in quadrants of the cortex, and additional measures of cross-sectional geometry, including the degree of circular shape that can help distinguish habitual torsion from bending. Results showed the expected lack of regional CFO differences in quasi-circular shaped, and torsionally loaded, pigeon and bat humeri. As expected, the B3M also lacked CFO variations between the opposing cortices along the plane of bending, and the quasi-elliptical cross-sectional shape and regional microstructural/osteonal variations expected for bending were not found. These findings in the B3M show that uniformity in CFO does not always reflect habitual torsional loads. Osteon morphology and distribution, and presence of laminar histology also do not distinguish torsion from bending in these bat and pigeon wing bones.

Assessment of hip fracture risk by cross-sectional strain-energy derived from image-based beam model

BACKGROUND

Clinicians have been looking for a simple and effective biomechanical tool for the assessment of hip fracture risk. Dual-energy X-ray absorptiometry (DXA) is currently the primary bone imaging modality in clinic, and the engineering beam is the simplest model for a mechanical analysis. Therefore, we developed a DXA-based beam model for the above purpose.

METHODS

A beam model of the proximal femur was constructed from the subject's hip DXA image and denoted DXA-beam. Femur stiffness was calculated at cross-sections of interest using areal bone-mineral-density profile. Impact force induced in a sideways fall was applied as a critical loading. Fracture risk index at a cross-section was defined as the ratio of strain-energy induced by the impact force to the allowable strain-energy. A clinic cohort was used to study the discriminability of DXA-beam, which was measured by the area under the curve and odds ratio, both with 95% confidential interval.

FINDINGS

Fracture risk measured by DXA-beam model at the femoral neck [odds ratio 2.23, 95% confidence interval (1.83, 2.57)], inter-trochanter [2.49, (2.14, 3.25)] and sub-trochanter [2.82, (2.38, 3.51)] were strongly associated with hip fracture. The area under the curve by DXA-beam at the femoral neck [0.74, 95% confidence interval (0.70, 0.76)], inter-trochanter [0.77, (0.75, 0.82)] and sub-trochanter [0.76, (0.74, 0.81)] were higher than that by femoral neck bone mineral density [0.71, (0.65, 0.78)].

INTERPRETATION

The DXA-beam model is a simple and yet effective mechanical model. It had promising performance in discrimination of fracture cases from controls.

Spatial Distribution of Microcracks in Osteoarthritic Femoral Neck: Influence of Osteophytes on Microcrack Formation

Osteophytes have been suggested to influence the bone mechanical properties. The aim of this study was to compare the microcrack density in osteophytes with that in the other parts of the osteoarthritic femoral neck (FN). The presence of microcracks was investigated in the ultra-distal FN and in the osteophytes in samples obtained during hip arthroplasty in 24 postmenopausal women aged 67 ± 10 years. Furthermore, the 3D microarchitecture and the collagen crosslinks contents were assessed by high-resolution peripheral quantitative computed tomography and high-performance liquid chromatography, respectively. Osteophytes were present in the 24 FN, mainly at the level of the inferior quadrant. Microcracks were present in all FN with an average of 2.8 per sample. All observed microcracks were linear. The microcrack density (Cr.N/BV; #/mm²) was significantly higher in cancellous than in cortical bone (p = 0.004), whereas the microcrack length (Cr.Le, µm) was significantly greater in cortical bone (p = 0.04). The collagen crosslinks ratio pyridinoline/deoxypyridinoline was significantly and negatively correlated with Cr.N/BV in the posterior (r' = - 0.68, p = 0.01) and inferior (r' = - 0.53, p = 0.05) quadrants. Microcracks were observed in seven osteophytes in seven patients. When microcracks were present in the osteophyte area, Cr.N/BV was also significantly higher in the whole FN and in the quadrant of the osteophyte compared to the cases without microcrack in the osteophyte (p < 0.03). In conclusion, in FN from hip osteoarthritis microcracks are present in all FNs but in only 23% of the osteophytes. The microcrack formation was greater and their progression was smaller in the cancellous bone than in the cortex. The spatial distribution of microcracks varied according to the proximity of the osteophyte, and suggests that osteophyte may influence microcrack formation related to changes in local bone quality.

The Influence of Cortical Porosity on the Strength of Bone During Growth and Advancing Age

PURPOSE OF REVIEW

Bone densitometry provides a two-dimensional projected areal apparent bone mineral density that fails to capture the heterogeneity of bone's material composition and macro-, micro-, and nano-structures critical to its material and structural strength. Assessment of the structural basis of bone fragility has focused largely on trabecular bone based on the common occurrence of fragility fractures at sites with substantial amounts of trabecular bone. This review focuses on the contribution of cortical bone to bone fragility throughout life.

RECENT FINDINGS

Accurately differentiating cortical and trabecular bone loss has important implications in quantifying bone fragility as these compartments have differing effects on bone strength. Recent advances in imaging methodology have improved distinction of these two compartments by (i) recognition of a cortico-trabecular transitional zone and (ii) quantifying bone microstructure in a region of interest that is a percentage of bone length rather than a fixed point. Additionally, non-invasive three-dimensional imaging methods allow more accurate quantification of changes in the cortical, trabecular, and cortico-trabecular compartments during growth, aging, disease, and treatment. Over 75% of the skeleton is assembled as cortical bone. Of all fragility fractures, ~ 80% are appendicular and involve regions rich in cortical bone and ~ 70% of all age-related appendicular bone loss is cortical and is mainly due to unbalanced intracortical remodeling which increases cortical porosity. The failure to achieve the optimal peak bone microstructure during growth due to disease and the deterioration in cortical and trabecular bone produced by bone loss compromise bone strength. The loss of strength produced by microstructural deterioration is disproportionate to the bone loss producing this deterioration. The reason for this is that the loss of strength increases as a 7th power function of the rise in cortical porosity and a 3rd power function of the fall in trabecular density (Schaffler and Burr in J Biomech. 21(1):13-6, 1988), hence the need to quantify bone microstructure.