Interrelationship of trabecular mechanical and microstructural properties in sheep trabecular bone

Mechanical strength of ceramic scaffolds reinforced with biopolymers is comparable to that of human bone

Eight groups of calcium-phosphate scaffolds for bone implantation were prepared of which seven were reinforced with biopolymers, poly lactic acid (PLA) or hyaluronic acid in different concentrations in order to increase the mechanical strength, without significantly impairing the microarchitecture. Controls were un-reinforced calcium-phosphate scaffolds. Microarchitectural properties were quantified using micro-CT scanning. Mechanical properties were evaluated by destructive compression testing. Results showed that adding 10 or 15% PLA to the scaffold significantly increased the mechanical strength. The increase in mechanical strength was seen as a result of increased scaffold thickness and changes to plate-like structure. However, the porosity was significantly lowered as a consequence of adding 15% PLA, whereas adding 10% PLA had no significant effect on porosity. Hyaluronic acid had no significant effect on mechanical strength. The novel composite scaffold is comparable to that of human bone which may be suitable for transplantation in specific weight-bearing situations, such as long bone repair.

Comparative anatomical measurements of osseous structures in the ovine and human knee

The ovine stifle has been increasingly used as a large animal model for the human knee. Still, comparative anatomical measurements of the knee in sheep and humans are missing. Thus, the purpose of this study was to describe and measure the osseous anatomy of the ovine stifle in comparison to the human knee. Twenty-four stifles of skeletal-mature merino-sheep and 24 human cadaver knees were obtained and distances between selected anatomical structures of the distal femur, the proximal tibia, and the patella were measured digitally and documented. Based on these, intercondylar ratio, tibial aspect ratio, patella aspect ratio and the cortical index were calculated. Regarding epicondylar width, lateral condylar width, medial condylar width and the tibial dimensions, the ovine stifle can be considered as a human knee scaled down by one third. However, sheep have a smaller trochlear width and a narrower femoral intercondylar notch than humans resulting in lower relative values for intercondylar width and intercondylar height. The distal femur's cortical index is the same in both species. In contrast, sheep have a massive bone stock below their tibial plateau and a proximal tibial shaft with remarkably thick cortical bone. The ovine stifle can be regarded as a useful model for the human knee. However, future studies should consider the differences in the femoral intercondylar notch width, the patellofemoral joint's biomechanics and the proximal tibia's cortical bone stock.

Methodological considerations for analyzing trabecular architecture: an example from the primate hand

Micro-computed tomographic analyses of trabecular bone architecture have been used to clarify the link between positional behavior and skeletal anatomy in primates. However, there are methodological decisions associated with quantifying and comparing trabecular anatomy across taxa that vary greatly in body size and morphology that can affect characterizations of trabecular architecture, such as choice of the volume of interest (VOI) size and location. The potential effects of these decisions may be amplified in small, irregular-shaped bones of the hands and feet that have more complex external morphology and more heterogeneous trabecular structure compared to, for example, the spherical epiphysis of the femoral head. In this study we investigate the effects of changes in VOI size and location on standard trabecular parameters in two bones of the hand, the capitate and third metacarpal, in a diverse sample of nonhuman primates that vary greatly in morphology, body mass and positional behavior. Results demonstrate that changes in VOI location and, to a lesser extent, changes in VOI size had a dramatic affect on many trabecular parameters, especially trabecular connectivity and structure (rods vs. plates), degree of anisotropy, and the primary orientation of the trabeculae. Although previous research has shown that some trabecular parameters are susceptible to slight variations in methodology (e.g. VOI location, scan resolution), this study provides a quantification of these effects in hand bones of a diverse sample of primates. An a priori understanding of the inherent biases created by the choice of VOI size and particularly location is critical to robust trabecular analysis and functional interpretation, especially in small bones with complex arthroses.

Does osteoporosis increase complication risk in surgical fracture treatment? A protocol combining new endpoints for two prospective multicentre open cohort studies

Background

With an ever-increasing elderly population, orthopaedic surgeons are faced with treating a high number of fragility fractures. Biomechanical tests have demonstrated the potential role of osteoporosis in the increased risk of fracture fixation complications, yet this has not been sufficiently proven in clinical practice. Based on this knowledge, two clinical studies were designed to investigate the influence of local bone quality on the occurrence of complications in elderly patients with distal radius and proximal humerus fractures treated by open reduction and internal fixation.

Methods/Design

The studies were planned using a prospective multicentre open cohort design and included patients between 50 and 90 years of age. Distal radius and proximal humerus fractures were treated with locking compression 2.4 mm and proximal humerus internal locking plates, respectively. Follow-up examinations were planned for 6 weeks, 3 and 12 months as well as a telephone interview at 6 months. The primary outcome focuses on the occurrence of at least one local bone quality related complication. Local bone quality is determined by measuring bone mineral density and bone mineral content at the contralateral radius. Primary complications are categorised according to predefined factors directly related to the bone/fracture or the implant/surgical technique. Secondary outcomes include the documentation of soft tissue/wound or general/systemic complications, clinical assessment of range of motion, and patient-rated evaluations of upper limb function and quality of life using both objective and subjective measures.

Discussion

The prospective multicentre open cohort studies will determine the value of local bone quality as measured by bone mineral density and content, and compare the quality of local bone of patients who experience a complication (cases) following surgery with that of patients who do not (controls). These measurements are novel and objective alternatives to what is currently used.

Trial registration numbers

Clinical Trials.gov NCT01144208 and NCT01143675

A novel method of estimating structure model index from gray-level images

According to the standard approach, estimation of the structure model index (SMI) of a trabecular bone sample has to be preceded by segmentation of the bone image and then triangulation of the trabecular surface. However, when analyzing clinical data, image segmentation should be avoided whenever possible, due to difficulties in controlling binarization artifacts. The aim of the present study is to develop a method to estimate SMI directly from gray-level images, without prior segmentation. It is shown that the standard definition of SMI can be formulated in terms of integrals of the gray-level intensity and the magnitude of the gray-level intensity gradient, computed for the analyzed image and the image eroded by an infinitesimal ball structuring element. Because in a real application the size of an eroding element is always finite, a procedure is proposed to reduce the finite size errors. The performance of the proposed method is tested for structures with known SMI. Next, based on a set of μCT images of trabecular bone from the distal radius, the proposed and the standard methods are compared. It is shown that the proposed novel approach is statistically equivalent to the standard one, if applied to high-resolution μCT data. The influence of clinically relevant factors like limited resolution and noise on the estimation of SMI is tested. It is shown that the gray-level approach is more robust against image degradation factors than the standard one.

Trabecular bone structure in the mandibular condyles of gouging and nongouging platyrrhine primates

The relationship between mandibular form and biomechanical function is a topic of significant interest to morphologists and paleontologists alike. Several previous studies have examined the morphology of the mandible in gouging and nongouging primates as a means of understanding the anatomical correlates of this feeding behavior. The goal of the current study was to quantify the trabecular bone structure of the mandibular condyle of gouging and nongouging primates to assess the functional morphology of the jaw in these animals. High-resolution computed tomography scan data were collected from the mandibles of five adult common marmosets (Callithrix jacchus), saddle-back tamarins (Saguinus fuscicollis), and squirrel monkeys (Saimiri sciureus), respectively, and various three-dimensional morphometric parameters were measured from the condylar trabecular bone. No significant differences were found among the taxa for most trabecular bone structural features. Importantly, no mechanically significant parameters, such as bone volume fraction and degree of anisotropy, were found to vary significantly between gouging and nongouging primates. The lack of significant differences in mechanically relevant structural parameters among these three platyrrhine taxa may suggest that gouging as a habitual dietary behavior does not involve significantly higher loads on the mandibular condyle than other masticatory behaviors. Alternatively, the similarities in trabecular architecture across these three taxa may indicate that trabecular bone is relatively unimportant mechanically in the condyle of these primates and therefore is functionally uninformative.

Trabecular bone structure in the humeral and femoral heads of anthropoid primates

The functional significance of three-dimensional trabecular bone architecture in the primate postcranial skeleton has received significant interest over the last decade. Some previous work has produced promising results, finding significant relationships between femoral head trabecular bone structure and hypothesized locomotor loading in leaping and nonleaping strepsirrhines. Conversely, most studies of anthropoid femoral head bone structure have found broad similarity across taxonomic and locomotor groups. The goal of this study is to expand on past analyses of anthropoid trabecular bone structure by assessing the effects of differential limb usage on the trabecular bone architecture of the forelimb and hindlimb across taxa characterized by diverse locomotor behaviors, including brachiation, quadrupedalism, and climbing. High-resolution x-ray computed tomography scans were collected from the proximal humerus and proximal femur of 55 individuals from five anthropoid primate species, including Symphalangus syndactylus, Papio sp., Presbytis rubicunda, Alouatta caraya, and Pan troglodytes. Trabecular bone structural features including bone volume fraction, anisotropy, trabecular thickness, and trabecular number were quantified in large volumes positioned in the center of the humeral or femoral head. Femoral head trabecular bone volume is consistently and significantly higher than trabecular bone volume in the humerus in all taxa independent of locomotor behavior. Humeral trabecular bone is more isotropic than femoral trabecular bone in all species sampled, possibly reflecting the emphasis on a mobile shoulder joint and manipulative forelimb. The results indicate broad similarity in trabecular bone structure in these bones across anthropoids.

Correlation of mechanical properties within the equine third metacarpal with trabecular bending and multi-density micro-computed tomography data

Computed tomography (CT) data can be employed with respect to determining mechanical properties and has been used to predict parameters such as elastic modulus, yield strength, and ultimate strength of intact bone. Micro-computed tomography (muCT) possesses the resolution capable of detecting apparent bone density in extremely local regions and can characterize the trabecular structure. It has been asserted that this micro-structure is susceptible to micro-buckling and bending, which has a controversial role in predicting the global mechanical properties of bone. The current study measured the mechanical properties of relatively high apparent density bone from the equine distal third metacarpal. The mechanical properties were correlated with trabecular morphology parameters and apparent densities of localized regions obtained with muCT. These data were used to test two hypotheses: (1) accounting for trabecular bending using trabecular morphology parameters would provide better global mechanical property predictions than using only apparent density, and (2) regions of low apparent density dominate the overall mechanical behavior and provide greater correlation to the measured mechanical properties than regions of high apparent density. The data indicated that accounting for trabecular bending with morphological parameters resulted in stronger correlations to mechanical properties than correlations that relied only on apparent density (r2= 0.91 versus r2= 0.81). Low apparent density regions were more strongly correlated with mechanical properties than high apparent density regions (r2= 0.85 versus r2= 0.77), demonstrating the importance of selecting appropriate regions when attempting to predict mechanical properties from CT data.

Limitations of dual x-ray absorptiometry in children with chronic kidney disease

Dual x-ray absorptiometry (DXA) is the most widely used densitometric method for diagnosing osteoporosis in adults. It has also been widely adopted as a diagnostic tool in the pediatric population. The most significant limitation of DXA is its reliance on areal rather than volumetric bone mineral density (BMD), which results in an artificial underestimation of bone density in short people. Poor longitudinal growth, however, is an eminent problem in children with chronic kidney disease (CKD). There is also no evidence in children that areal BMD is predictive of future fracture risk, which is the traditional rationale for measuring BMD in children with CKD. Therefore, the Kidney Disease Outcomes Quality Initiative guidelines and the current position of the International Society for Clinical Densitometry (ISCD) on pediatric patients, both of which are presented in this issue of Pediatric Nephrology, do not recommend the use of DXA in children with CKD. To date, there is no consensus on the best method to assess the degree of renal osteodystrophy in this patient population, and further collaborative efforts to correlate densitometric findings with clinical outcomes are warranted.

Evaluation of the mechanical properties of rat bone under simulated microgravity using nanoindentation

Exposure to microgravity causes a decrease in bone mass and altered bone geometry due to the lack of weight-bearing forces on the skeleton. The mechanical properties of bone are due not only to the structure and geometry, but also to the tissue properties of the bone material itself. To study the effects of microgravity on bone tissue, the mechanical properties of tail suspension rat femurs were investigated. Twelve Sprague-Dawley rats were randomly divided into two groups, tail suspension (TS) and control (CON). On days 0 and 14, the bone mineral density (BMD) of the femurs was determined by Dual Energy X-ray Absorptiometry. After 14 days, three-point bending was used to test the mechanical properties of the whole femur and nanoindentation was used to measure the mechanical properties of the bone materials. The BMD of femurs in TS was significantly lower than that in CON. In the three-point bending testing, the breaking load, stiffness and energy absorption all decreased significantly in the TS group. In the nanoindentation tests, there was no significant difference between TS and CON in elastic modulus (E), while hardness (H) was significantly decreased and E/H significantly increased in TS. Weightlessness affects the intrinsic mechanical properties of bone at the bone material level. It is necessary to investigate further the effect of microgravity on the collagen bone matrix. Nanoindentation is a relatively new technique that is useful for investigating the above changes induced by microgravity and for assessing the efficacy of intervention.

Noninvasive prediction of vertebral body compressive strength using nonlinear finite element method and an image based technique

Noninvasive prediction of vertebral body strength under compressive loading condition is a valuable tool for the assessment of clinical fractures. This paper presents an effective specimen-specific approach for noninvasive prediction of human vertebral strength using a nonlinear finite element (FE) model and an image based parameter based on the quantitative computed tomography (QCT). Nine thoracolumbar vertebrae excised from three cadavers with an average age of 42 years old were used as the samples. The samples were scanned using the QCT. Then, a segmentation technique was performed on each QCT sectional image. The segmented images were then converted into three-dimensional FE models for linear and nonlinear analyses. A new material model was implemented in our nonlinear model being more compatible with real mechanical behavior of trabecular bone. A new image based MOS (Mechanic of Solids) parameter named minimum sectional strength ((sigma(u)A)(min)) was used for the ultimate compressive strength prediction. Subsequently, the samples were destructively tested under uniaxial compression and their experimental ultimate compressive strengths were obtained. Results indicated that our new implemented FE model can predict ultimate compressive strength of human vertebra with a correlation coefficient (R(2)=0.94) better than usual linear and nonlinear FE models (R(2)=0.83 and 0.85 respectively). The image based parameter introduced in this study ((sigma(u)A)(min)) was also correlated well with the experimental results (R(2)=0.86). Although nonlinear FE method with new implemented material model predicts compressive strength better than the (sigma(u)A)(min), this parameter is clinically more feasible due to its simplicity and lower computational costs. This can make future applications of the (sigma(u)A)(min) more justified for human vertebral body compressive strength prediction.

Trabecular bone mechanical properties in patients with fragility fractures

Fragility fractures are generally associated with substantial loss in trabecular bone mass and alterations in structural anisotropy. Despite the high correlations between measures of trabecular mass and mechanical properties, significant overlap in density measures exists between individuals with osteoporosis and those who do not fracture. The purpose of this paper is to provide an analysis of trabecular properties associated with fragility fractures. While accurate measures of bone mass and 3-D orientation have been demonstrated to explain 80% to 90% of the variance in mechanical behavior, clinical and experimental experience suggests the unexplained proportion of variance may be a key determinant in separating high- and low-risk patients. Using a hierarchical perspective, we demonstrate the potential contributions of structural and tissue morphology, material properties, and chemical composition to the apparent mechanical properties of trabecular bone. The results suggest that the propensity for an individual to remodel or adapt to habitual damaging or nondamaging loads may distinguish them in terms of risk for failure.

A structural approach to skeletal fragility in chronic kidney disease

Renal osteodystrophy is a multifactorial disorder of bone metabolism in chronic kidney disease (CKD). As CKD progresses, ensuing abnormalities in mineral metabolism result in distortions in trabecular microarchitecture, thinning of the cortical shell, and increased cortical porosity. Recent studies have shown significantly increased hip fracture rates in CKD stages 3 and 4, in dialysis patients, and in transplant recipients. The majority of studies of bone loss in CKD relied on dual-energy x-ray absorptiometry (DXA) measures of bone mineral density. However, DXA summarizes the total bone mass within the projected bone area, concealing distinct structural alterations in trabecular and cortical bone. Recent data have confirmed that peripheral quantitative computed tomography (pQCT) measures of cortical density and thickness provide substantially better fracture discrimination in dialysis patients, compared with hip or spine DXA. This review summarizes the growing evidence for bone fragility in CKD stages 3 through 5, considers the effects of CKD on trabecular and cortical bone structure as it relates to fracture risk, and details the potential advantages and disadvantages of DXA and alternative measures of bone density, geometry, and microarchitecture, including pQCT, high-resolution pQCT, and micro-magnetic resonance imaging for fracture risk assessment in CKD.

The challenge of establishing preclinical models for segmental bone defect research

A considerable number of international research groups as well as commercial entities work on the development of new bone grafting materials, carriers, growth factors and specifically tissue-engineered constructs for bone regeneration. They are strongly interested in evaluating their concepts in highly reproducible large segmental defects in preclinical and large animal models. To allow comparison between different studies and their outcomes, it is essential that animal models, fixation devices, surgical procedures and methods of taking measurements are well standardized to produce reliable data pools and act as a base for further directions to orthopaedic and tissue engineering developments, specifically translation into the clinic. In this leading opinion paper, we aim to review and critically discuss the different large animal bone defect models reported in the literature. We conclude that most publications provide only rudimentary information on how to establish relevant preclinical segmental bone defects in large animals. Hence, we express our opinion on methodologies to establish preclinical critically sized, segmental bone defect models used in past research with reference to surgical techniques, fixation methods and postoperative management focusing on tibial fracture and segmental defect models.

Stability of connected mini-implants and miniplates for skeletal anchorage in orthodontics

The aim of this study was to examine the primary stability of connected mini-implants and miniplates. Three different skeletal anchorage systems were investigated: (1) two 1.5 mm diameter cylindrical mini-implants connected with a 0.021 x 0.025 inch stainless steel (SS) wire, (2) two 1.6 mm diameter tapered mini-implants connected with a 0.021 x 0.025 inch SS wire, and (3) two 2.0 mm diameter cylindrical mini-implants connected by a titanium locking miniplate. Fifteen standardized bovine bone specimens were prepared, five specimens for each experimental group. The connected mini-implants were fixed on the bone specimens. The systems underwent uniaxial pull-out tests at the midpoint of the connecting wire or miniplate using a mechanical testing machine. One-way analysis of variance was used to determine the difference of the pull-out test results between the groups. Both the titanium miniplate and SS wire connection systems showed severe deformation at the screw head, which broke before the mini-implants failed. The 2.0 mm miniplate system showed the highest pull-out force (529 N) compared with the other two wire connection systems (P < 0.001). The 2.0 mm system was also stiffer than the 1.6 and 1.5 mm systems (P < 0.001). The yield force of the 2.0 mm miniplate (153 N) was significantly higher than the 1.5 mm (88 N) and 1.6 mm (76 N) systems (P < 0.001). This in vitro study demonstrated that the connection of two mini-implants with a miniplate resulted in higher pull-out force, stiffness, and yield force to resist pulling force and deformation. Such a set-up could thus provide a stable system for orthodontic skeletal anchorage.

Trabecular microstructure at the human scaphoid nonunion

PURPOSE

Scaphoid fractures have the highest prevalence of nonunion in the human body, but despite this, little is known about the microstructure of the bone. Given that microstructural changes in the bone at the site of nonunion define the nonunion, it was our purpose to characterize the trabecular microarchitecture on both sides of the nonunion and compare these to the normal structure.

METHODS

Fifteen human scaphoid nonunions (14 men, 1 woman; average age, 29 years; interval between injury and examination ranged from 7 to 156 months) were excised during surgical reconstruction and assessed by high-resolution micro-computed tomography.

RESULTS

The bone from the nonunions was clearly different from that of normal scaphoids (obtained from nonreplantable hand amputations or postmortem). Bone on both sides of the nonunions was characterized by higher bone mineral density and an increased number of trabeculae, which were thinner and more tightly packed together compared with the normal scaphoid bones. Further evaluation showed that the bone on the proximal side of the nonunion had higher bone mineral density, higher bone volume fraction, decreased bone surface area, increased trabecular thickness, and increased trabecular number when compared with the distal side.

CONCLUSIONS

These data demonstrate the differing microstructure of the bone at the scaphoid nonunion, particularly on the proximal side, and indicate that the biology also differs from normal. The data also show that scaphoid nonunions produce new bone at the site, which may have positive implications toward hardware fixation and healing stimulation.

Resolution dependence of the non-metric trabecular structure indices

Non-metric indices of topological features of trabecular bone structure, such as structure model index (SMI), connectivity density (Conn.D), and degree of anisotropy (DA), provide unique information relevant to bone quality. With recent technological advancement, in vivo assessment of these indices may be possible from images acquired using high-resolution imaging techniques such as high-resolution peripheral quantitative computed tomography (HR-pQCT). However, more detailed investigation of the dependence of non-metric indices on spatial resolution is needed to determine their applicability. The purpose of this study was to determine whether these three non-metric indices are affected by the spatial resolution of CT images. First, the SMI, Conn.D, and DA were calculated for trabecular bone specimens with varying plate-like and rod-like structures from resampled muCT images across a range of spatial resolutions and compared to the reference values. To account for differences in size across different species and anatomical sites, the results are reported in normalized resolution units. Next, the impact of resolution on the non-metric indices for cores of human distal tibia trabecular bone from clinical HR-pQCT images was evaluated to determine the applicability of the non-metric indices to in vivo imaging. We found that the non-metric indices of trabecular bone structure were affected by spatial resolution of CT images. Particularly, the SMI deviated from the high-resolution muCT reference value depending on the structure type, whether plate-like or rod-like. Both Conn.D and DA were underestimated in the images obtained at an in vivo resolution. It is not trivial to determine absolute threshold for validity of these non-metric indices without considering a specific study design (e.g. relative resolution, the size of the treatment effect to detect, and specimen type). The results of this study provide an upper bound for the accuracy of the non-metric indices under limited resolution scenarios.

Determination of bone's mechanical matrix properties by nanoindentation

Osteoporosis is a devastating disease that is characterized not only by a reduction in bone quantity but also by deterioration in bone quality. The quality of bone tissue is greatly influenced by its mechanical properties and, therefore, investigations into the etiology and enhanced detection of osteoporosis, or the efficacy of interventions, may require the assessment of bone's mechanical properties at the level of the tissue. Nanoindentation is a relatively new technique that is capable of evaluating bone's quasi-static and dynamic mechanical properties on extremely small volumes of tissue. These data can be used directly to describe the pre-yield properties of the matrix, but can also be combined with imaging techniques and mechanical models to extrapolate the mechanical properties from the level of the tissue to that of the organ.

A structural approach to the assessment of fracture risk in children and adolescents with chronic kidney disease

Children with chronic kidney disease (CKD) have multiple risk factors for impaired accretion of trabecular and cortical bone. CKD during childhood poses an immediate fracture risk and compromises adult bone mass, resulting in significantly greater skeletal fragility throughout life. High-turnover disease initially results in thickened trabeculae, with greater bone volume. As disease progresses, resorption cavities dissect trabeculae, connectivity degrades, and bone volume decreases. Increased bone turnover also results in increased cortical porosity and decreased cortical thickness. Dual-energy X-ray absorptiometry (DXA)-based measures of bone mineral density (BMD) are derived from the total bone mass within the projected bone area (g/cm(2)), concealing distinct disease effects in trabecular and cortical bone. In contrast, peripheral quantitative computed tomography (pQCT) estimates volumetric BMD (vBMD, g/cm(3)), distinguishes between cortical and trabecular bone, and provides accurate estimates of cortical dimensions. Recent data have confirmed that pQCT measures of cortical vBMD and thickness provide substantially greater fracture discrimination in adult dialysis patients compared with hip or spine DXA. The following review considers the structural effects of renal osteodystrophy as it relates to fracture risk and the potential advantages and disadvantages of DXA and alternative measures of bone density, geometry, and microarchitecture, such as pQCT, micro-CT (microCT), and micro magnetic resonance imaging (microMRI) for fracture risk assessment.

Structural parameters and mechanical strength of cancellous bone in the femoral head in osteoarthritis do not depend on age

For normal bone, aging has been associated with a decrease of both density and failure strength, and with the development of pathologies such as osteoporosis. Conversely, it has been reported that another common disease, osteoarthritis, may alter these age-related changes in cancellous bone, suggesting that it may have a protective role against osteoporosis and the correspondent fracture risk. It was reported that in the principal compressive region of the femoral head in osteoarthritis the bone density does not depend on age. However, it is not clear if this independence on age of the cancellous bone density corresponds also to a reduced dependence on age of the strength to failure. The present work examined cancellous bone from the principal compressive region of the femoral head of 37 patients having severe osteoarthritis. The aim was (1) to investigate the dependence on age of both the structural parameters and the ultimate stress and (2) to investigate the relationships between the ultimate stress and the structural parameters. Using X-ray microcomputed tomography, three-dimensional structural parameters, such as bone volume fraction, direct trabecular thickness and structure model index were calculated. Then the specimens were compressed to failure to determine the ultimate stress. It was found that none of the investigated structural parameters did depend on age, and also the ultimate stress did not depend on age (p>0.05 for all regressions on age). In addition, the ultimate stress was significantly correlated with the structural parameters, primary with the minimum bone volume fraction and the average bone volume fraction (R(2)=0.95 and R(2)=0.84, respectively). These findings show that severe osteoarthritis or a related factor may change the age dependences of both the structural parameters and the mechanical properties usually reported for normal cancellous bone. These results suggest for this pathology to have a protective role against the age-related decrease in density, the age-related deterioration of the microarchitecture and the age-related decrease of the failure strength for the cancellous bone in the principal compressive region of the human femoral head.

Evaluation of trabecular mechanical and microstructural properties in human calcaneal bone of advanced age using mechanical testing, microCT, and DXA

Early detection of fracture risk is important for initiating treatment and improving outcomes from both physiologic and pathologic causes of bone loss. While bone mineral density (a quantity measure) has traditionally been used for this purpose, alternative structural imaging parameters (quality measures) are proposed to better predict bone's true mechanical properties. To further elucidate this, trabecular bone from cadaveric human calcanei were used to evaluate the interrelationship of mechanical and structural parameters using mechanical testing, dual energy X-ray absorptiometry (DXA) scanning, and micro computed tomography (microCT) imaging. Directional specific structural properties were assessed in three-dimensional (3-D) and correlated to mechanical testing and DXA. The results demonstrated that microCT-derived indices of bone quality (i.e., volume fraction and structural model index) are better than DXA-derived bone mineral density for the prediction of the mechanical parameters of bone (i.e., elastic modulus, yield stress, and ultimate stress). Diagnostically, this implies that future work on the early prediction of fracture risk should focus as much on bone quality as on quantity. Furthermore, the results of this study show that a loss of bone primarily affects the connectedness and overall number of trabeculae. Ultimate stress, however, is better correlated with trabecular number than thickness. As such, primary prevention of osteoporosis may be more important than later countermeasures for bone loss.

Staged-injection procedure to prevent cement leakage during vertebroplasty: an in vitro study

STUDY DESIGN

Fibrin sealant (FS) combined with or without growth factor was used to improve the micro-architectural and biomechanical properties of vertebral body in osteoporotic ovine spine.

OBJECTIVE

To analyze the treatment effects of bovine bone morphogenetic protein (bBMP) combined with FS on osteopenic ovine vertebral architecture, bone mineral density, and biomechanics in vivo.

SUMMARY OF BACKGROUND DATA

Vertebroplasty and kyphoplasty were used to treat spinal osteoporosis. They can increase strength of vertebrae physically. However, each has specific disadvantages. bBMP could rapidly increasing bone formation and suppressing bone resorption, but little is known about its effect on ovariectomized-induced osteoporosis.

METHODS

Six sheep underwent ovariectomy and were placed on a low-calcium diet. Twelve months later, according to Ladin square design, L4-L6 vertebrae in all sheep were randomly assigned to 3 treatment groups: A (30 mg bBMP/1.5 mL FS), B (30 mg bBMP) and C (1.5 mL FS). All materials were injected into the assigned vertebra transpedicularly. Animals were killed 3 months after injection, and bone mineral density (BMD), biomechanics, and histomorphometry were assessed. Analysis of variance was used to determine effects of bBMP/FS (alpha = 0.05).

RESULTS

The BMD in Group 1 was 17.1% and 14.7% higher than that in Group 2 and Group 3, respectively. The micro-CT reconstruction analysis showed that the density and connectivity of trabecular bone in bBMP/FS treated vertebrae were higher than those in control groups. The mechanical properties (yield stress, ultimate stress, energy absorption, bone modulus) of the vertebrae were also significantly higher. In this study, local bBMP/FS treatment showed a positive trend in improving BMD, histomorphometric parameters, and mechanical strength of osteoporotic vertebra. Slow release of bBMP using FS appeared to be an effective method of protein delivery.

CONCLUSION

The use of bBMP/FS in the treatment of vertebral osteoporosis in an attempt to enhance bone strength merits further study.

Acoustic properties of trabecular bone--relationships to tissue composition

In osteoporosis, changes in tissue composition and structure reduce bone strength and expose it to fractures. The current primary diagnostic technique, i.e., dual energy X-ray absorptiometry, measures areal bone mineral density (BMD) but provides no direct information on trabecular structure or organic composition. Although still poorly characterized, ultrasound techniques may bring about information on bone composition and structure. In this study, relationships of 2.25-MHz ultrasound speed, attenuation, reflection and backscattering with composition of human trabecular bone (n=26) were characterized experimentally, as well as by using numerical analyses. We also determined composition of the trabecular sample (fat and water content, bone volume fraction) and that of the calcified matrix (mineral, proteoglycan and collagen content of trabeculae). In experimental analyses, bone volume fraction and mineral content of the calcified matrix were the only determinants of BMD. Further, bone volume fraction served as the strongest determinant of ultrasound parameters (r=0.51-0.87). In numerical simulations, density and mechanical properties of the calcified matrix systematically affected ultrasound speed, attenuation, reflection and backscattering. However, partial correlation coefficients revealed only low associations(|r|<or=0.4) between the composition of calcified matrix and ultrasound parameters in experimental measurements. To conclude, the content and structure of calcified matrix, rather than its composition, affect more significantly acoustic properties of healthy trabecular bone.

High-frequency oscillatory motions enhance the simulated mechanical properties of non-weight bearing trabecular bone

Extremely low-level oscillatory accelerations, applied without constraint, can increase bone formation. Here, we tested the hypothesis that high-frequency oscillations, applied in the absence of functional weight bearing, can be sensed by trabecular bone to produce a structure that is more efficient in sustaining applied loads. The left leg of anesthetized adult female mice (n=18) was subjected to high-frequency oscillations at 45 Hz, 0.6g for 20 min/day, 5 days/week for 3 weeks, while the contralateral leg served as an internal control. To remove the potential interference of the habitual strain environment with the imposed physical signal, the hindlimbs of these mice were chronically unloaded. In vivo microCT scans of the proximal metaphyseal region of the tibia were transformed into finite element meshes to evaluate trabecular and cortical mechanical properties. Simulated longitudinal compression tests showed that the short applications of high-frequency oscillations were sensed primarily by trabecular bone. At the end of the experimental period, apparent trabecular stiffness of the oscillated bones was 38% (p<0.001) greater than that of non-weight bearing controls. Simulated uniaxial loads applied to trabecular bone induced 21%, 52%, and 131% greater (p<0.05) median, peak compressive, and peak tensile longitudinal stresses in control than in stimulated bones. Non-weight bearing control bones were also characterized by greater transverse normal and shear stresses (77% and 54%, respectively, p<0.001) as well as 35% greater (p=0.03) longitudinal shear stresses. Compared to normal age-matched controls (n=18), oscillations were able to attenuate, but not fully prevent, the decline in trabecular mechanical properties associated with the removal of weight bearing. These data indicate not only that bone cells can sense low-level, high-frequency oscillatory accelerations, but also that they can orchestrate a structural response that produces a stiffer trabecular structure that may be less prone to fracture.

Effects of microarchitecture on bone strength

Bone strength and stiffness depend strongly on bone mass, but they also depend on the microarchitecture and tissue quality of both cancellous and cortical bone. All these aspects differ between individuals and between anatomic sites. This review discusses ways to characterize the three-dimensional cancellous architecture as well as changes in architecture and bone composition caused by bone remodeling. The methods used range from detailed descriptions of sizes and distances in cancellous bone to coarser texture analysis methods using clinical data. As the resolution of clinical images increases, it may become possible to use knowledge of the relationship between bone microarchitecture and strength to predict fracture risk clinically.

A method for patient-specific evaluation of vertebral cancellous bone strength: in vitro validation

BACKGROUND

In the context of osteoporosis, important determinants of the fracture risk are the apparent strength and stiffness of cancellous bone, as well as its brittleness and energy absorption capacity. Standard medical imaging, however, cannot measure these mechanical properties directly. Consequently, an estimation of the risk for fracture is made by correlating relative density or mineral density at a skeletal site with statistics of fracture occurrence, which provides limited and partial indications on fracture risks. A better method for evaluating the patient-specific mechanical properties of cancellous bone is therefore required.

METHODS

In order to asses the mechanical properties of vertebral cancellous bone, we developed a finite element parametric model of lattice trabecular architecture that, in the future, will be suitable for use with bone imaging modalities. The model inputs are apparent morphological parameters (trabecular thickness and trabecular separation) and the bone mineral density. We conducted uniaxial compression tests on 36 canine vertebral cancellous bone specimens (C7 and L1) to validate model predictions of strength and stiffness in vitro.

FINDINGS

Predictions of strength and stiffness matched the experimental results within relative absolute errors of 17.7% and 12.8%, respectively (average of differences between model-predicted and measured values, divided by the average of measured values). We also employed the model for evaluation of strength and stiffness of human L1 and L5 vertebrae and found mean strength of 1.67 MPa (confidence interval 0.42 MPa) and mean elastic modulus of 190 MPa (confidence interval 50 MPa), which are well within the range of previously reported apparent strength and stiffness properties.

INTERPRETATION

The present model can be used to improve medical imaging-based evaluation of the spine in osteoporotic individuals by providing more specific information on the individual bone's susceptibility to fracture once clinical bone scans will be able to provide more reliable measures of trabecular thickness and separation.

Vertebral osteoporosis and trabecular bone quality

Vertebral fractures due to osteoporosis commonly occur under non-traumatic loading conditions. This problem affects more than 1 in 3 women and 1 in 10 men over a lifetime. Measurement of bone mineral density (BMD) has traditionally been used as a method for diagnosis of vertebral osteoporosis. However, this method does not fully account for the influence of changes in the trabecular bone quality, such as micro-architecture, tissue properties and levels of microdamage, on the strength of the vertebra. Studies have shown that deterioration of the vertebral trabecular architecture results in a more anisotropic structure which has a greater susceptibility to fracture. Transverse trabeculae are preferentially thinned and perforated while the remaining vertical trabeculae maintain their thickness. Such a structure is likely to be more susceptible to buckling under normal compression loads and has a decreased ability to withstand unusual or off-axis loads. Changes in tissue material mechanical properties and levels of microdamage due to osteoporosis may also compromise the fracture resistance of vertebral trabecular bone. New diagnostic techniques are required which will account for the influence of these changes in bone quality. This paper reviews the influence of the trabecular architecture, tissue properties and microdamage on fracture risk for vertebral osteoporosis. The morphological characteristics of normal and osteoporotic architectures are compared and their potential influence on the strength of the vertebra is examined. The limitations of current diagnostic methods for osteoporosis are identified and areas for future research are outlined.

Interrelationships between electrical properties and microstructure of human trabecular bone

Microstructural changes, such as reduction of trabecular thickness and number, are characteristic signs of osteoporosis leading to diminished bone strength. Electrical and dielectric parameters might provide diagnostically valuable information on trabecular bone microstructure not extractable from bone mineral density measurements. In this study, structural properties of human trabecular bone samples (n=26) harvested from the distal femur and proximal tibia were investigated using the computed microtomography (microCT) technique. Quantitative parameters, e.g. structural model index (SMI) or trabecular bone volume fraction (BV/TV), were calculated. In addition, the samples were examined electrically over a wide frequency range (50 Hz-5 MHz) using a two-electrode impedance spectroscopy set-up. Relative permittivity, loss factor, conductivity, phase angle, specific impedance and dissipation factor were determined. Significant linear correlations were obtained between the dissipation factor and BV/TV or SMI (|r| 0.70, p<0.01, n=26). Principal component analyses, conducted on electrical and structural parameters, revealed that the high frequency principal component of the dissipation factor was significantly related to SMI (r=0.72, p<0.01, n=26). The linear combination of high and low frequency relative permittivity predicted 73% of the variation in BV/TV. To conclude, electrical and dielectric parameters of trabecular bone, especially relative permittivity and dissipation factor, were significantly and specifically related to a trabecular microstructure as characterized with microCT. The data gathered in this study constitute a useful basis for theoretical and experimental work towards the development of impedance spectroscopy techniques for detection of bone quality in vitro or in special cases of open surgery.

Parathyroid hormone 1-34 enhances titanium implant anchorage in low-density trabecular bone: a correlative micro-computed tomographic and biomechanical analysis

The use of endosseous titanium implants is the standard of care in dentistry and orthopaedic surgery. Nevertheless, implantation in low-density bone has a poor prognosis and experimental studies show delayed implant anchorage following gonadectomy-induced bone loss. Intermittently administered human parathyroid hormone 1-34 [iahPTH(1-34)] is the leading bone anabolic therapy. Hence, this study assessed whether iahPTH(1-34) enhances titanium implant integration in low-density bone. Threaded titanium implants, 0.9 mm in diameter, were inserted horizontally into the proximal tibial metaphysis of 5-month-old rats, 7 weeks postorchiectomy (ORX). Subcutaneous administration of iahPTH(1-34), at 5, 25 and 75 microg/kg/day commenced immediately thereafter and lasted for 8 weeks. Quantitative micro-computed tomography (muCT) at the implantation site was carried out at 15 microm resolution using high energy and long integration time to minimize artifacts resulting from the high implant radiopacity. Osseointegration (OI) was calculated as percent implant surface in contact with bone (%OI) quantified as the ratio of "bone"-to-total voxels in contact with the implant. Additionally, the trabecular bone volume density (BV/TV), trabecular thickness (Tb.Th), trabecular number (Tb.N) and connectivity density (Conn.D) were measured in the peri-implant bone. All microCT parameters were stimulated by iahPTH(1-34) dose-dependently; the percent maximal enhancement was %OI = 143, BV/TV = 257, Tb.Th = 150, Tb.N = 140 and Conn.D = 193. The maximal values of %OI, BV/TV and Tb.Th in iahPTH(1-34)-treated ORX rats exceeded significantly those measured in the implantation site of untreated sham-ORX controls. The same specimens were then subjected to pullout biomechanical testing. The biomechanical parameters were also enhanced by iahPTH(1-34) dose-dependently, exceeding the values recorded in the sham-ORX controls. The percent iahPTH(1-34)-induced maximal enhancement was: ultimate force = 315, stiffness = 270 and toughness = 395. Except for the BV/TV and Tb.Th, there was no significant difference between the effect of the 25 and 75 microg/kg/day doses. There was a highly significant correlation between the morphometric and biomechanical parameters suggesting the use of quantitative CT as predictive of the implant mechanical properties. These findings demonstrate that iahPTH(1-34) effectively stimulates implant anchorage in low-density trabecular bone and thus the feasibility of administering iahPTH(1-34) to improve the clinical prognosis in low-density trabecular bone sites.

A physical phantom for the calibration of three-dimensional X-ray microtomography examination

X-ray microtomography is rapidly gaining importance as a non-destructive investigation technique, especially in the three-dimensional examination of trabecular bone. Appropriate quantitative three-dimensional parameters describing the investigated structure were introduced, such as the model-independent thickness and the structure model index. The first parameter calculates a volume-based thickness of the structure in three dimensions independent of an assumed structure type. The second parameter estimates the characteristic form of which the structure is composed, i.e. whether it is more plate-like, rod-like or even sphere-like. These parameters are now experiencing a great diffusion and are rapidly growing in importance. To measure the accuracy of these three-dimensional parameters, a physical three-dimensional phantom containing different known geometries and thicknesses, resembling those of the examined structures, is needed. Unfortunately, such particular phantoms are not commonly available and neither does a consolidated standard exist. This work describes the realization of a calibration phantom for three-dimensional X-ray microtomography examination and reports an application example using an X-ray microtomography system. The calibration phantom (external size 13 mm diameter, 23 mm height) was based on various aluminium inserts embedded in a cylinder of polymethylmethacrylate. The inserts had known geometries (wires, foils, meshes and spheres) and thicknesses (ranging from 20 microm to 1 mm). The phantom was successfully applied to an X-ray microtomography device, providing imaging of the inserted structures and calculation of three-dimensional parameters such as the model-independent thickness and the structure model index. With the indications given in the present work it is possible to design a similar phantom in a histology laboratory and to adapt it to the requested applications.

Relationship between CT intensity, micro-architecture and mechanical properties of porcine vertebral cancellous bone

BACKGROUND

In vivo assessment of bone density is insufficient for the evaluation of osteoporosis in patients. A more complete diagnostic tool for the determination of bone quality is needed. Micro-computed tomography imaging allows a non-destructive method for evaluating cancellous bone micro-architecture. However, lengthened exposure to ionizing radiation prevents patients to be imaged by such a system. The aim for this study was to elucidate the relationships between image intensity (of Hounsfield units), cancellous bone micro-architecture and mechanical properties.

METHODS

Using pig vertebral cancellous bone, the bone specimens were imaged using clinical and micro-computed tomography scanners and subsequently subjected to uniaxial compression testing.

RESULTS

Results indicate that micro-architecture can be predicted using clinical image intensity. Micro-architectural parameters relevant to osteoporosis study, such as percent bone volume, trabecular bone pattern factor, structure model index, trabecular thickness and trabecular separation have shown significant correlation with R2 values of 0.83, 0.80, 0.70, 0.72, and 0.54, respectively, when correlated to Hounsfield units. In addition, the correlation of mechanical properties (E, sigma yield, and sigma ult) in the superior-inferior direction (the primary loading direction), to micro-architecture parameters has also been good (R2 > 0.5) for all except tissue volume, tissue surface and degree of anisotropy.

INTERPRETATION

This proves that the predictive power of bone strength and stiffness was improved with the combination of bone density and micro-architecture information. This work supports the prediction of micro-architecture using current clinical computed tomography imaging technology.

How to select the elastic modulus for cancellous bone in patient-specific continuum models of the spine

Patient-specific finite element (FE) modelling is a promising technology that is expected to support clinical assessment of the spine in the near future. To allow rapid, robust and economic patient-specific modelling of the whole spine or of large spine segments, it is practicable to consider vertebral cancellous bone in the spine as a continuum material, but the elastic modulus of that continuum material must reflect the quality of the individual vertebral bone. A numerical parametric model of lattice trabecular architecture has been developed for determining the apparent elastic modulus of cancellous bone Ecb in vertebrae. The model inputs were apparent morphological parameters (trabecular thickness TbTh and trabecular separation TbSp) and the bone mineral density (BMD), which can all be measured in vivo, using the spatial resolution of current clinical quantitative computed tomography (QCT) commercial whole-body scanners. The model predicted that Ecb values between 30 and 110 MPa represent normal morphology and BMD of human spinal cancellous bone. The present Ecb to TbTh, TbSp and BMD relationships pave the way for automatic generation of patient-specific continuum FE spine models that consider the individual's osteoporotic or other degenerative condition of cancellous bone.