Effect of grain size and microporosity on the in vivo behaviour of ß-tricalcium phosphate scaffolds.

Defining the most adequate architecture of a bone substitute scaffold is a topic that has received much attention over the last 40 years. However, contradictory results exist on the effect of grain size and microporosity. Therefore, the aim of this study was to determine the effect of these two factors on the in vivo behaviour of ß-tricalcium phosphate (ß-TCP) scaffolds. For that purpose, ß-TCP scaffolds were produced with roughly the same macropore size (≈ 150 µm), and porosity (≈ 80 %), but two levels of microporosity (low: 10 % / high: ≈ 25 %) and grain size (small: 1.3 µm /large: ≈ 3.3 µm). The sample architecture was characterised extensively using materialography, Hg porosimetry, micro-computed tomography (µCT), and nitrogen adsorption. The scaffolds were implanted for 2, 4 and 8 weeks in a cylindrical 5-wall cancellous bone defect in sheep. The histological, histomorphometrical and µCT analysis of the samples revealed that all four scaffold types were almost completely resorbed within 8 weeks and replaced by new bone. Despite the three-fold difference in microporosity and grain size, very few biological differences were observed. The only significant effect at p < 0.01 was a slightly faster resorption rate and soft tissue formation between 4 and 8 weeks of implantation when microporosity was increased. Past and present results suggest that the biological response of this particular defect is not very sensitive towards physico-chemical differences of resorbable bone graft substitutes. As bone formed not only in the macropores but also in the micropores, a closer study at the microscopic and localised effects is necessary.

Phase and size separations occurring during the injection of model pastes composed of ß-tricalcium phosphate powder, glass beads and aqueous solutions.

Glass beads a few hundred micrometers in size were added to aqueous ß-tricalcium phosphate pastes to simulate the effect of porogens and drug-loaded microspheres on the injectability of calcium phosphate cements and putties. The composition of the pastes was monitored during the injection process to assess the effect of glass bead content, glass bead size and paste composition on the paste injectability. The results revealed that the injection process led to both liquid and glass bead segregations: the liquid flowed faster than the glass beads, which themselves flowed faster than the ß-tricalcium phosphate microparticles. In fact, even the particle size distribution of the glass beads was modified during injection. These results reveal that a good design of multiphasic injectable pastes is essential to prevent phase separation.

Effect of subvoxel processes on non-destructive characterization of ß-tricalcium phosphate bone graft substitutes.

The geometric features of bone graft substitutes, such as the pore and pore interconnection sizes, are of paramount importance for their biological performance. Such features are generally characterized by micro-computed tomography (µCT). Unfortunately, the resolution of µCT is often too limited. The aim of this study was to look at the effect of µCT resolution on the geometric characterization of four different bone graft substitutes. An attempt was also made to improve the characterization of these materials by applying a subvoxelization algorithm. The results revealed that both approaches increased the accuracy of the geometric characterization. They also showed that the interconnection size in particular was affected. Comparing the results obtained from the scanned and numerical subvoxelization datasets revealed a minor difference of less than 2.5% for the porosity values. The difference for the pore sizes was up to 10%. Considerable differences of up to 35-50% were found for the interconnection sizes. The present study demonstrates how complex geometric characterization is and how important it is for biomaterial researchers to be aware of the impact of µCT resolution on the pore and pore interconnection sizes.

Commentary: Deciphering the link between architecture and biological response of a bone graft substitute.

Hundreds of studies have been devoted to the search for the ideal architecture for bone scaffold. Despite these efforts, results are often contradictory, and rules derived from these studies are accordingly vague. In fact, there is enough evidence to postulate that ideal scaffold architecture does not exist. The aim of this document is to explain this statement and review new approaches to decipher the existing but complex link between scaffold architecture and in vivo response.

Aqueous impregnation of porous beta-tricalcium phosphate scaffolds.

The ability of a porous bone graft substitute to be impregnated with an aqueous solution is of great importance for tissue engineering and in vivo applications. This study presents an impregnation test setup and assesses the effect of various synthesis parameters such as sintering temperature, composition, macroporosity and macropore size on the impregnation properties of porous beta-tricalcium phosphate scaffolds dipped in water. Among those parameters, the macropore size had by far the largest effect; generally, the bigger the macropore size, the lower the saturation level. The results also showed that impregnation was less complete when the samples were fully dipped in water than when they were only partially dipped, owing to the requirement for the system to create air bubbles under water.

Geometric analysis of porous bone substitutes using micro-computed tomography and fuzzy distance transform.

There is increased interest in resorbable bone substitutes for skeletal reconstruction. Important geometric design measures of bone substitute include pore size, interconnection size, porosity, permeability and surface area of the substitute. In this study, four substitute groups with variable geometric features but constant porosity were scanned using micro-computed tomography (microCT) and their geometric measures were determined using an advanced image-processing algorithm based on fuzzy distance transform and new pore size definition. The substitutes were produced using the calcium phosphate emulsion method. The geometric analysis revealed that the reproducibility of the emulsion method was high, within 5%. The average porosity of the four groups was 52.3 + or - 1.5. The pore diameter of the four bone substitute groups was measured to be 170 + or - 1.7, 217 + or - 5.2, 416 + or - 19, and 972 + or - 11 microm. Despite this significant change in pore size, the interconnection size only increased slightly with an increase of pore size. The specific surface decreased with increasing pore size. The permeability increased with the pore size and was inversely proportional to the specific surface. The combination of microCT and the fuzzy image-processing tool enables accurate geometric analysis, even if pore size and image resolution are in the same range, such as in the case of the smallest pore size. Moreover, it is an exciting tool to understand the structure of the substitute with the hope of designing better bone substitutes.

Mechanisms underlying the limited injectability of hydraulic calcium phosphate paste. Part II: particle separation study.

Calcium phosphate cements (CPCs) are of great interest for bone augmentation procedures. In these a hydraulic calcium phosphate paste is injected through a small bore needle into the bone. The injectability of these pastes is relatively poor, resulting into partial injection only. In earlier studies we have shown that phase separation brings the injection process to a halt. Phase separation is characterized by a faster flow of the liquid than of the solid during paste extrusion. So far it is unclear whether or not particle separation contributes to the poor injectability of such hydraulic pastes. It is hypothesized that fine particles behave like a liquid and thus separate under the injection pressure, leaving larger particles behind. A factorial experimental design was used to examine this hypothesis. The particle size distribution (PSD) of the extrudate was measured over the course of each injection experiment using laser diffraction. The solid content of the paste was further inspected using scanning electron microscopy. A total of 48 experiments covering four factors at two levels each were performed. One factor was the ultrasound exposure duration, to ensure the dispersion quality of the particles during the PSD measurements. Another factor was the location of the samples over the course of the injection, so as to compare the extrudate with the PSDs remaining in the syringe. The liquid:powder ratio (LPR) in the injected paste was another factor investigated. Specifically, two different pastes with 40% and 50% LPR were examined. The dispersal medium was a fourth factor investigated, to ensure adequate dispersion of the particles during the PSD measurements. Analysis of variance showed that sample location did not significantly affect PSD. No apparent PSD change for the extruded paste and the paste remaining in the syringe could be detected by scanning electron microscopy. In conclusion, the present study did not show any evidence suggesting that particle separation occurred over the course of injection and thus that phase separation remains the main phenomenon leading to the poor injectability of CPCs.

Pore-scale analysis of Newtonian flow in the explicit geometry of vertebral trabecular bones using lattice Boltzmann simulation.

The geometric and transport properties of trabecular bone are of particular interest for medical engineers active in orthopaedic applications and more specifically in hard tissue implantations. This article resorts to computational methods to provide some understanding of the geometric and transport properties of vertebral trabecular bone. A fuzzy distance transform algorithm was used for geometric analysis on the pore scale, and a lattice Boltzmann method (LBM) for the simulation of flow on the same scale. The transport properties of bone including the pressure drop, elongation, and shear component of dissipated energy, and the tortuosity of the bone geometry were extracted from the results of the LBM flow simulations. Whenever suitable, dimensionless numbers were used for the analysis of the data. The average pore size and distribution of the bone were found to be 746 microm and between 75 and 2940 microm, respectively. The permeability of the flow in the cavities of the specific bone sample was found to be 5.05 x 10(-8) m2 for the superior-inferior direction which was by a factor of 1.5-1.7 higher than the permeability in the other two anatomical directions (anterior-posterior). These findings are consistent with experimental results found 3 years prior independently. Tortuosity values approached 1.05 for the superior-inferior direction, and 1.13 and 1.11 for the other two perpendicular directions. The low tortuosities result mainly from the large bone porosity of 0.92. The flow on the pore scale seems to be shear dominated but 30 per cent of the energy dissipation was because of elongational effects. The converging and diverging geometry of the bone explains the significant elongation and deformation of the fluid elements. The transition from creeping flow (the Darcy regime), which is of interest to vertebral augmentation and this study, to the laminar region with significant inertia effects took place at a Reynolds number of about 1-10, as usual for porous media. Finally, the authors wish to advise the readers on the significant computational requirements to be allocated to such a virtual test bench.

Theoretical and experimental approach to test the cohesion of calcium phosphate pastes.

Recent studies have revealed that the ability of a calcium phosphate cement paste to harden in a physiological environment without desintegrating into small particles might be a key property to ensure a safe and reliable clinical use of calcium phosphate cements. However, this property called cohesion is not well understood and has not been studied extensively. The goal of the present study was to better understand which factors affect the cohesion of a calcium phosphate paste using the combination of a theoretical and experimental approach. In the theoretical approach, factors expected to influence the paste cohesion such as Van der Waals forces, electrostatic and steric interactions, as well as osmotic effects were listed and discussed. In the experimental approach, a new method to measure the cohesion of a non-setting calcium phosphate paste was presented and used to assess the effects of various factors on this property. The new method allowed a continuous measurement of cohesion and gave reproducible results. The experimental results confirmed the theoretical predictions: an increase of the liquid-to-powder ratio of the paste and of the powder particle size, as well as the addition of citrate ions and in limited cases dissolved xanthan polymer chains reduced the paste cohesion.

A new cannula to ease cement injection during vertebroplasty.

One of the main limitations of vertebroplasty is the excessive pressure required to inject a sufficient amount of cement into a vertebral body. Based on previous work that shows that approximately 95% of the injection pressure is required to deliver the cement through the cannula, we proposed a new cannula design with a larger internal diameter in the proximal section. The objective of this study is to determine whether the new cannula geometry significantly reduces the delivery pressure and eases cement injection during vertebroplasty. Two different methods were employed to examine the delivery pressure in a conventional and two redesigned cannulae: (1) analytical model: Hagen-Poisseuille's flow through a tube was used to predict the pressure drop in the cannulae; (2) experiment: first a Newtonian silicone oil and then an acrylic bone cement was injected through the cannulae at a constant rate of 4 cc/min, and the delivery pressure was recorded. Both the experimental and analytical findings confirmed that the redesigned cannula reduces the delivery pressure significantly. Specifically, when the internal diameter of the proximal section was increased by a factor of two, which is clinically feasible, the delivery pressure dropped by about 63%. The redesigned cannula appears to have the potential to improve vertebroplasty. The key benefits are that (1) it eases cement injection, (2) it can be easily integrated into the existing procedure, and (3) it is cost-effective.

Rheological characterization of concentrated aqueous beta-tricalcium phosphate suspensions: the effect of liquid-to-powder ratio, milling time, and additives.

The field of injectable calcium phosphate suspensions and cements is experiencing vigorous research activity. This is stimulated by their importance for the cement augmentation procedure (vertebroplasty), which is an emerging procedure to treat osteoporotic fragility fractures. The rheological properties such as the yield stress and viscosity play an important role in the process of cement delivery and infiltration into the cancellous bone cavities. However, the number of studies relating to their rheological properties is very limited. The objective of this first study was to examine the effects of the following three variables on the rheological properties of a non-setting beta-tricalcium phosphate suspension: liquid-to-powder ratio, milling of powder particles, and additives. The broad finding is that all the variables affect the rheological properties remarkably. The more specific salient finding is the large variation in viscosity and in the yield stress. The viscosity spanned three orders of magnitude and the yield stress spanned five orders of magnitude. It appears that the rheological properties can be altered at will. However, one has to exercise extreme caution because these changes are not without cost to other important properties such as the cohesiveness and mechanical properties of the cement. Another important finding is that a linear correlation between the yield stress and the viscosity was found. Measurement of one of these variables might be enough to determine the other.

A finite element rheological model for polymethylmethacrylate flow: analysis of the cement delivery in vertebroplasty.

Polymethylmethacrylate (PMMA) is increasingly used in orthopaedics. Finite element (FE) modelling can play an important role in understanding the PMMA flow behaviour. However, FE models have not been used so far because conventional FE packages do not allow for the rheopectic and pseudoplastic behaviour of PMMA to be taken into consideration and because it requires multiple expertise to incorporate these behaviours into an FE package. The objectives of the present paper are to: (a) propose a rheological model that describes PMMA flow behaviour; (b) implement this model into ANSYS using FORTRAN; and (c) validate the implementation by comparing it with analytical solutions. After the validation showed good agreement, an FE model of PMMA delivery through an eight-gauge cannula was developed to examine the extra-vertebral flow conditions of vertebroplasty. The FE analysis showed a logarithmic increase of the injection pressure, where it almost doubled from 1.2 to 2.3 MPa over two minutes. This unanticipated non-linear increase is due to the highly non-uniform viscosity profile in the cannula. It can be concluded that: (a) the rheological model implemented in ANSYS can be used to analyse practical flow problems related to PMMA and (b) time and shear-rate effects of PMMA are crucial to estimate its flow behaviour accurately.

Injection biomechanics of bone cements used in vertebroplasty.

The incidence of osteoporotic bone fractures is growing exponentially as the western population ages and as life expectancy increases. Vertebroplasty, where acrylic or calcium phosphate cement is injected into the weakened vertebrae to augment them, is an emerging procedure for treating spinal fragility fractures. However, cement injection is currently limited because there are no clear standards for a safe, reproducible and predictable procedure. The purpose of this paper is to examine the role that bone cements play in the underlying bio-mechanisms that affect the outcomes of cement injection. Our most important finding after combining clinical, laboratory and theoretical research is that the process of cement injection poses conflicting demands on bone cements. The cements are required to be more viscous and less viscous at the same time. The challenge therefore is to develop biomaterials, techniques and/or devices that can overcome or manage the conflicting demands on cement viscosity.

Experimental and theoretical investigation of directional permeability of human vertebral cancellous bone for cement infiltration.

The use of acrylic polymers in infiltrating the porous bone structure is an emerging procedure for the augmentation of osteoporotic vertebrae. Although this procedure is employed frequently, it is performed based on empirical knowledge, and therefore, does not take into consideration the porosity-dependent permeability of human vertebral cancellous bone. The purpose of this study was to: (a). experimentally and theoretically investigate interdependence of the vertebral cancellous bone permeability and porosity, and (b). examine if the bone permeability of spinal cancellous bone can be predicted using bone mineral density measurements. If these relations can be established, they can be useful in optimizing the injection conditions for predicable cement infiltration. To determine the porosity-dependent and directional permeability, 34 bone cores-20 samples in the superior-inferior (SI) direction and 14 in the anterior-posterior (AP) direction-were cut from 20 lumbar vertebrae and infiltrated with silicone oil with a viscosity matching that of PMMA. The permeability of the cores was determined based on Darcy's law. The mean permeability of SI and AP cores was 4.45+/-1.72 x 10(-8) and 3.44+/-1.26 x 10(-8)m(2), respectively. An interesting finding of this study was that the permeability of the AP cores was approximately 78% of that of SI cores, though the porosity of the SI and AP cores taken from the same vertebra was approximately equal. In addition, we provided a theoretical model for the porosity-dependent permeability that accurately described non-linear dependency of the bone permeability and porosity in both directions. Although the relation of the bone permeability and porosity was established, bone mineral density was a weak predictor of the bone permeability. The experimental and theoretical results of this study can be used to understand polymer flow in cement infiltration procedures.

Influence of oscillatory mixing on the injectability of three acrylic and two calcium-phosphate bone cements for vertebroplasty.

Injecting acrylic and, increasingly, calcium-phosphate cements into the porous bone structure is an emerging procedure, referred to as vertebroplasty, for the augmentation of osteoporotic vertebrae. Despite the benefits of vertebroplasty, it has limitations. The limitations of interest in this study are the injectability of bone cements and their mixing variability (i.e., low reproducibility of resulting viscosity). The objective of this study is to investigate the effect of oscillatory versus manual mixing on cement viscosity and mixing variability. Five cements are tested: (a) Vertebroplastic, (b) DP-Pour, (c) Antibiotic Simplex, (d) chronOS Inject, and (e) Biopex. Compared to manual mixing, oscillatory mixing significantly decreased the mean viscosity and the mixing variability, which was inferred from the coefficient of variation. For example, under oscillatory mixing, the viscosity and the variability for Vertebroplastic decreased to one-third of the corresponding values for manual mixing. Similar results were obtained for the other cements. The decrease in viscosity is attributed to the pseudo-plastic behavior of bone cements. The decrease in the variability of cement viscosity was attributed to greater dispersive mixing of the cement components under oscillatory mixing. The decrease in viscosity eases the injection by reducing the pressure required. The decrease in the variability of cement viscosity increases reproducibility of the cement injection. Oscillatory mixing appears to have the potential to contribute to improving vertebroplasty.

Influence of mixing method on the cement temperature-mixing time history and doughing time of three acrylic cements for vertebroplasty.

Acrylic cements are increasingly being used to augment osteoporotic vertebrae in a procedure called vertebroplasty. Two significant factors that may complicate the use of acrylic cements are: (a) short handling time, which may result in insufficient filling of the vertebra; and (b) exothermic setting (curing) behavior, which may result in thermal damage of the surrounding tissue. It has been previously reported that mixing the cement components under oscillation, as compared to manual mixing, increases the handling time. More specifically, it seems that oscillatory mixing slows down the cement polymerization process and, consequently, widens the time window during which cement is injectable. However, the effect of oscillatory mixing on the exothermic setting behavior of cement undergoing polymerization has not been examined. In this study, the exothermic setting behavior of three commercially available acrylic cements--Antibiotic Simplex, DP-Pour&trade, and Vertebroplastic--were examined for both manual and oscillatory mixing methods. For each combination of cement and mixing method, the parameters that were measured were the exothermic setting curve (and hence the cement setting temperature and setting time) and the cement doughing time. It was found that oscillatory mixing had no significant effect on any of these parameters. Based on the results of this study, it can be concluded that, for the tested cements, the setting process is a reaction-controlled process rather than a diffusion-controlled one. Clinically, this implies that oscillatory mixing may be used to increase the working period for acrylic cements without increasing the risk of thermal damage to surrounding tissue.

Material changes in osteoporotic human cancellous bone following infiltration with acrylic bone cement for a vertebral cement augmentation.

Bone cement infiltration can be effective at mechanically augmenting osteoporotic vertebrae. While most published literature describes the gain in mechanical strength of augmented vertebrae, we report the first measurements of viscoelastic material changes of cancellous bone due to cement infiltration. We infiltrated cancellous core specimen harvested from osteoporotic cadaveric spines with acrylic bone cement. Bone specimen before and after cement infiltration were subjected to identical quasi-static and relaxation loading in confined and free compression. Testing data were fitted to a linear viscoelastic model of compressible material and the model parameters for cement, native cancellous bone, and cancellous bone infiltrated (composite) with cement were identified. The fitting demonstrated that the linear viscoelastic model presented in this paper accurately describes the mechanical behaviour of cement and bone, before and after infiltration. Although the composite specimen did not completely adopt the properties of bulk bone cement, the stiffening of cancellous bone due to cement infiltration is considerable. The composite was, for example, 8.5 times stiffer than native bone. The local stiffening of cancellous bone in patients may alter the load transfer of the augmented motion segment and may be the cause of subsequent fractures in the vertebrae adjacent to the ones infiltrated with cement. The material model and parameters in this paper, together with an adequate finite-element model, can be helpful to investigate the load shift, the mechanism for subsequent fractures, and filling patterns for ideal cement infiltration.

Theoretical and experimental model to describe the injection of a polymethylmethacrylate cement into a porous structure.

A theoretical approach was used to determine the distribution of a poly(methylmethacrylate) cement after its injection into a porous structure. The predictions of the model were then compared to experimental results obtained by injecting a polymethylmethacrylate cement into an open-porous ceramic filter. The goal was to define a model that could predict what factors affect the risk of cement extravasation and hence how the risk of cement extravasation can be minimized. The calculations were based on two important rheological laws: the law of Hagen-Poiseuille and the law of Darcy. The law of Hagen-Poiseuille describes the flow of a fluid in a cylindrical tube. The law of Darcy describes the flow of a fluid through a porous media. The model predicted that the extravasation risk was decreased when the cement viscosity, the bone pore size, the bone permeability and the bone porosity were increased, and when the diameter of the extravasation path and the viscosity of the marrow were decreased. Experimentally, the effect of the marrow viscosity and extravasation path could be evidenced. Therefore, the model was believed to be an adequate approximation of the experimental behavior. In conclusion, the experimental results demonstrated that the model was adequate and that the best practical way to decrease the risk of extravasation is to increase the cement viscosity.

Load shift of the intervertebral disc after a vertebroplasty: a finite-element study.

Infiltrating osteoporotic cancellous bone with bone cement (vertebroplasty) is a novel surgical procedure to stabilize and prevent osteoporotic vertebral fractures. Short-term clinical and biomechanical results are encouraging; however, so far no reports on long-term results have been published. Our clinical observations suggest that vertebroplasty may induce subsequent fractures in the vertebrae adjacent to the ones augmented. At this point, there is only a limited understanding of what causes these fractures. We have previously hypothesized that adjacent fractures may result from a shift in stiffness and load following rigid augmentation. The purpose of this study is to determine the load shift in a lumbar motion segment following vertebroplasty. A finite-element (FE) model of a lumbar motion segment (L4-L5) was used to quantify and compare the pre- and post-augmentation stiffness and loading (load shift) of the intervertebral (IV) disc adjacent to the augmented vertebra in response to quasi-static compression. The results showed that the rigid cement augmentation underneath the endplates acted as an upright pillar that severely reduced the inward bulge of the endplates of the augmented vertebra. The bulge of the augmented endplate was reduced to 7% of its value before the augmentation, resulting in a stiffening of the IV joint by approximately 17%, and of the whole motion segment by approximately 11%. The IV pressure accordingly increased by approximately 19%, and the inward bulge of the endplate adjacent to the one augmented (L4 inferior) increased considerably, by approximately 17%. This increase of up to 17% in the inward bulge of the endplate adjacent to the one augmented may be the cause of the adjacent fractures.

How to determine the permeability for cement infiltration of osteoporotic cancellous bone.

Cement augmentation is an emerging surgical procedure in which bone cement is used to infiltrate and reinforce osteoporotic vertebrae. Although this infiltration procedure has been widely applied, it is performed empirically and little is known about the flow characteristics of cement during the injection process. We present a theoretical and experimental approach to investigate the intertrabecular bone permeability during the infiltration procedure. The cement permeability was considered to be dependent on time, bone porosity, and cement viscosity in our analysis. In order to determine the time-dependent permeability, ten cancellous bone cores were harvested from osteoporotic vertebrae, infiltrated with acrylic cement at a constant flow rate, and the pressure drop across the cores during the infiltration was measured. The viscosity dependence of the permeability was determined based on published experimental data. The theoretical model for the permeability as a function of bone porosity and time was then fit to the testing data. Our findings suggest that the intertrabecular bone permeability depends strongly on time. For instance, the initial permeability (60.89 mm(4)/N(*)s) reduced to approximately 63% of its original value within 18 seconds. This study is the first to analyze cement flow through osteoporotic bone. The theoretical and experimental models provided in this paper are generic. Thus, they can be used to systematically study and optimize the infiltration process for clinical practice.