Gamal Baroud

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.

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.

Material properties of the human calcaneal fat pad in compression: experiment and theory.

The structural behaviour of the human heel pad has been studied extensively due to its ability to absorb shock, protect against excessive local stress, and reduce plantar pressures. However, the material properties of the tissue have not been adequately measured. These must be known in order to perform a finite element analysis of the effect of factors such as foot geometry and shoe/surface construction on heel pad function. Therefore, the purposes of this study were to (a). measure the viscoelastic behaviour of the fat pad in compression, and (b). to determine an appropriate constitutive equation to model the tissue. A series of unconfined compression tests were performed on 8mm diameter cylinders of fat pad tissue, consisting of quasi-static, 175, 350 mm/s and stress-relaxation tests to approximately 50% deformation. The tissue exhibited nonlinear, viscoelastic behaviour. No significant difference was found in the quasi-static behaviour between samples from different locations and orientations in the heel. The stress-relaxation tests were used to determine the time constant (tau(1)=0.5s), the 175 mm/s test to determine the relaxation coefficient (g(1)=28), and the 350 mm/s compression test to determine the material constants (C(100)=C(010)=0.01, C(200)=C(020)=0.1 Pa) of a single-phase, hyperelastic, linear viscoelastic strain energy function (r(2)=0.98).

High-viscosity cement significantly enhances uniformity of cement filling in vertebroplasty: an experimental model and study on cement leakage.

Study Design. Experimental study using a laboratory leakage model. Objective. To examine the working hypothesis that high-viscosity cements will spread uniformly, thus significantly reducing the risk of leakage. Summary of Background Data. In vertebroplasty, forces that govern the flow of bone cement in the trabecular bone skeleton are an essential determinant of the uniformity of cement filling. Extraosseous cement leakage has been reported to be a major complication of this procedure. Leakage occurs due to the presence of a path of least resistance caused by irregularities in the trabecular bone or shell structure. Ideally, cement uniformly infiltrates the trabecular bone skeleton and does not favor specific paths. Cement viscosity is believed to affect the infiltration forces and flow during the procedure. Clinically, altering the time between cement mixing and delivery modifies the viscosity of bone cement. Methods. An experimental model of the leakage phenomenon of vertebroplasty was developed. A path, simulating a blood vessel, was created in the model to perturb the forces underlying cement flow and to favor leakage. Cement of varying viscosities was injected in the model, and, thereafter, the filling pattern, cement mass that has leaked, time at which leakage occurred, and injection pressure were measured. Results. A strong relationship was found between the uniformity of the filling pattern and the elapsed time from cement mixing and viscosity, respectively. Specifically, 3 distinct cement leakage patterns were observed: immediate leakage was observed when cement was injected 5–7 minutes following mixing. The cement was of a low viscosity and more than 50% of the total cement injected leaked. Moderate leakage was observed when injection occurred 7–10 minutes following mixing. Less than 10% of the cement leaked, and the viscosity was at a transient state between the low viscosity of immediate leakage and a higher viscosity, doughy cement. Cement leakage ceased completely when cement was delivered after 10 minutes. The viscosity of the cement in this case was high, and the cement was of a dough-like consistency. Conclusions. High-viscosity cement seems to stabilize cement flow. However, the forces required for the delivery of high-viscosity cement may approach or exceed the human physical limit of injection forces. Although the working time of the cement is about 17 minutes, it may not be manually injectable with a standard syringe and cannula after 10 minutes, at which time cement leakage ceased completely.

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

Abstract 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 Darcys law. The mean permeability of SI and AP cores was 4.45±1.72×10−8 and 3.44±1.26×10−8xa0m2, 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.

Clinical measurements of cement injection pressure during vertebroplasty.

Study Design. Clinical study of injection pressure during vertebroplasty. Objectives. To investigate the range of injection pressures during conventional vertebroplasty interventions and to study the influence of syringe design and cement polymerization time on injection pressure. Summary of Background Data. Vertebroplasty is an efficient procedure for the treatment of painful vertebral fractures. However, cement leakage is a potentially serious complication. Although injection pressure has been suggested as a factor for extravasation risk, to date, there are only anecdotal reports of pressure data for cement augmentation in the clinic. Methods. Using a syringe holder instrumented with load and displacement transducers, injection pressure and volume were recorded in vivo during conventional cement augmentation. Wide (3 mm opening) and normal (1.8 mm opening) syringes were alternated such that each type was evaluated for early (300–500 seconds postmixing) and late (>500 seconds postmixing) cement polymerization time. The influence of syringe type and polymerization time on injection pressure was evaluated using a multifactorial analysis of variance followed by Scheffé post hoc comparison. Results. The maximum peak injection pressure measured was 3215 kPa. The average pressure peaks for normal and wide syringes were 1693 ± 653 kPa and 1727 ± 597 kPa, respectively. No statistically significant differences were found between injection pressures with wide and normal syringes. Higher injection pressures were observed later in the polymerization process. Conclusions. High injection pressures approaching 20 atmospheres are reached during conventional vertebroplasty. Widening the syringe tip diameter did not significantly change injection pressures, whereas elapsed time did. Further research is needed to improve injection equipment and materials for vertebroplasty.

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.

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.

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 4xa0cc/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.

Cement Filling Control and Bone Marrow Removal in Vertebral Body Augmentation by Unipedicular Aspiration Technique An Experimental Study Using Leakage Model

Study Design. Experimental design using a laboratory leakage model. Objective. To examine that a new aspiration technique, with a double conduit cannula design, improves the uniformity of cement filling, thus significantly reducing the risk of extraosseous leakage. Summary of Background Data. In vertebral augmentation, understanding the forces governing the intravertebral cement flow is essential for controlling the cement formation. A path of least resistance posed by the irregularities in the bone matrix or vertebral shell increases the risk of leak. We have previously shown that using viscous cement reduces the leakage risk. However, this may damage the already weak bone due to the high forces required for the cement to enter the bone cavities. Methods. An experimental leakage model for vertebral augmentation was used—with a path, simulating a blood vessel, to provoke leakage. A novel cannula with 2 concentric conduits was used. The inner conduit is used for cement delivery and the outer conduit for aspiration. A mixed level with 2 factors (2 × 22) experiment design was used to examine the ability of aspiration to direct the cement flow in both low and high viscous cement regimes. Results. Aspiration significantly enhanced the filling uniformity and reduced the risk of leakage. The reduction in leak with the suction simultaneous to the injection for low viscosity cement, elapsed time 4 minutes, was 1.5 cc (α = 0.05). In the suction experiments, the reduction in leakage as compared with the reference condition for the 8 minutes elapsed time was 0.5 cc, (α = 0.05). Conclusion. The aspiration technique combined with a new cannula design improved the uniformity of filling. The aspiration technique helps in removal of the displaced bone marrow or tumor tissue. The aspiration applied with the new cannula requires only a single incision. Thus, it does not result in an increased invasiveness.