An assessment of outcomes with intramedullary fixation of fractured ribs.

BACKGROUND: Surgical management of fractured ribs with internal fixation is an increasingly accepted therapy. Concurrently, specific rib fixation prostheses are being developed which should improve results and minimise hardware and rib/splint construct failures. The Synthes titanium intramedullary splint lends itself to difficult to access areas such as posterior rib fractures and fractures under the scapula. We analyse a case series of patients in whom this rib fixation prosthesis has been used. METHODS: Fifteen patients received 35 intramedullary splints. Follow up at 3 and 6 months was performed with three dimensional computed tomography scanning to assess for bone alignment, callus formation and healing, residual deformity, hardware failure or cut through. Computerized finite element analysis (FEA) was used to model forces acting on a posterior fracture with and without an intramedullary fixation splint in situ. RESULTS: Complete healing (bony union) was noted in only 3 (9 %) of the fractures fixed with splints by 3 months. Partial healing (cartilaginous union) was noted in 28 of the 33 fractures (85 %), and non healing was noted in only 2 (6 %). In both those two patients, failure at the rib / splint interface was noted after both patients reported sneezing. No hardware failures were noted. By 6 months the fractures which had shown partial healing, had all completely healed. There were no late failures (between 3 and 6 months) of either hardware or rib/splint interfaces. FEA modelling identified sites of increased stress in the rib at the rib / splint interface and in a modelled intramedullary splint where it spans the fracture. CONCLUSIONS: Further analysis of outcomes with intramedullary splints is warranted as well as further development of intramedullary rib fixation solutions.

Fixation of a human rib by an intramedullary telescoping splint anchored by bone cement.

A novel concept for rib fixation is presented that involves the use of a bioresorbable polymer intramedullary telescoping splint. Bone cement is used to anchor each end of the splint inside the medullary canal on each side of the fracture site. In this manner, rib fixation is achieved without fixation device protrusion from the rib, making the splint completely intramedullary. Finite element analysis is used to demonstrate that such a splint/cement composite can preserve rib fixation subjected to cough-intensity force loadings. Computational fluid dynamics and porcine rib experiments were used to study the anchor formation process required to complete the fixation.

Stress analysis of a centrally fractured rib fixated by an intramedullary screw.

The stress on an intramedullary screw rib fixation device holding together a centrally fractured human rib under in vivo force loadings was studied using finite element analysis (FEA). Validation of the FEA modelling using pullout from porcine ribs proved FEA to be suitable for assessing the structural integrity of screw/bone systems such as rib fixated by a screw. In the human rib fixation investigation, it was found that intramedullary bioresorbable Bioretec screws can fixate centrally fractured human ribs under normal breathing conditions. However, under coughing conditions, simulation showed Bioretec fixating screws to bend substantially. High stresses in the screw are mainly the result of flexion induced by the force loading, and are restricted to thin regions on the outside of the screw shaft. Stiffer screws result in less locally intense stress concentrations in bone, indicating that bone failure in the bone/screw contact regions can be averted with improvements in screw stiffness.

Quantitative guidelines for the prediction of ultrasound contrast agent destruction during injection.

Experiments and theory were undertaken on the destruction of ultrasound contrast agent microbubbles on needle injection, with the aim of predicting agent loss during in vivo studies. Agents were expelled through a variety of syringe and needle combinations, subjecting the microbubbles to a range of pressure drops. Imaging of the bubbles identified cases where bubbles were destroyed and the extent of destruction. Fluid-dynamic calculations determined the pressure drop for each syringe and needle combination. It was found that agent destruction occurred at a critical pressure drop that depended only on the type of microbubble. Protein-shelled microbubbles (sonicated bovine serum albumin) were virtually all destroyed above their critical pressure drop of 109 ± 7 kPa Two types of lipid-shelled microbubbles were found to have a pressure drop threshold above which more than 50% of the microbubbles were destroyed. The commercial lipid-shelled agent Definity was found to have a critical pressure drop for destruction of 230 ± 10 kPa; for a previously published lipid-shelled agent, this value was 150 ± 40 kPa. It is recommended that attention to the predictions of a simple formula could preclude unnecessary destruction of microbubble contrast agent during in vivo injections. This approach may also preclude undesirable release of drug or gene payloads in targeted microbubble therapies. Example values of appropriate injection rates for various agents and conditions are given.

Structural integrity of intramedullary rib fixation using a single bioresorbable screw.

BACKGROUND: Operative management of flail chest injury is receiving increasing interest. However, we have noticed in our own practice the difficulty in achieving reliable results with posterior rib fracture fixation. In this article, we analyze and model the physiologic forces acting on posterior rib fractures and assess the suitability of an intramedullary screw fixation technique in this site. METHODS: Computerized finite element analysis (FEA) was used to model a typical sixth rib and analyze the physiologic forces that act on the rib in vivo. A fracture in the posterior aspect of the rib was incorporated into the model, and an intramedullary screw fixation concept was assessed, using both a bioabsorbable polymer screw and a stainless steel screw.The records of 120 consecutive patients with flail chest were reviewed, and 26 patients were identified as having multiple posterior rib fractures with displacement. These patients formed a clinical correlation group by which to assess the FEA model. RESULTS: FEA modeling of the posterior rib fracture showed likely posterior displacement in response to physiologic forces. Review of the 26 patients with flail chest and displaced posterior fractures confirmed the direction of displacement. Modeling of an intramedullary screw fixation showed significant stresses in the bone/screw contact areas (stainless steel solution) and the prosthesis itself (bioabsorbable polymer solution) CONCLUSION: This FEA model demonstrates that physiologic forces cause posterior displacement at posterior rib fracture sites. Fixation solutions to counteract these forces need to overcome significant stresses at both the bone/prosthesis contact regions and within the prosthetic material itself. LEVEL OF EVIDENCE: Epidemiologic/therapeutic study, level V.

Development of optimized vascular fractal tree models using level set distance function.

Using the concepts of fractal scaling and constrained constructive optimization (CCO), a branching tree model, which has physiologically meaningful geometric properties, can be constructed. A vascular branching tree model created in this way, although statistically correct in representing the vascular physiology, still does not possess a physiological correct arrangement of the major arteries. A distance-function based technique for "staged growth" of vascular models has been developed in this work to address this issue. Time-dependent constraints based on a signed-distance level set function have been added, so that the tree models will first be grown near the designated surface(s) and, then, gradually allowed to penetrate into the enclosed volume. The proposed technique has been applied to construct a model of the human cerebral vasculature, which is characterized by the above-mentioned distribution of the arteries.

Mode of failure of rib fixation with absorbable plates: a clinical and numerical modeling study.

BACKGROUND: Rib fractures as a result of trauma are a relatively common injury. There is an increasing interest in the operative stabilization of these injuries. Absorbable fixation plates are an option for improved rib fracture treatment. The aim of this study was to review our plating failures and to create a numerical model of muscle forces on fractured ribs to identify the mechanism by which these rib fixations have failed. METHODS: Thirteen patients who had 58 ribs fixed with absorbable prostheses were reviewed. Finite element modeling was used to simulate the fixation of a lateral rib fracture using an absorbable plate and screw system. Internal pressure, intercostal forces, and appropriate displacement and rotational constraints were enforced at the rib ends. RESULTS: Ten rib fixation failures were noted in the clinical series. The modeling results showed that stresses on the plate differ during inspiration and expiration. Failure to use the two central screws resulted in higher stresses on the plating system. During inspiration simulations, the screws on both rib parts are active in keeping the rib and plate surfaces unseparated. However, during expiration, there is a greater stress on the screws on the posterior part of the broken rib, and separation of the plate from the rib seems to be more likely to occur at this site. CONCLUSIONS: This study indicates that the likely mode of failure of this absorbable plating system occurs on the posterior part of the rib, which correlates with the clinical failures seen.

Experimental measurement and mathematical modeling of pulsatile forces on a symmetric, bifurcated endoluminal stent graft model.

The objective of this study was to measure the pulsatile forces acting on a symmetric, bifurcated endoluminal stent graft to validate a computational fluid dynamics (CFD) and analytic model so that they can be used for various graft dimensions. We used a load cell to measure the force owing to the movement of an acrylic model of a bifurcated stent graft under pulsatile flow. This was then simulated with a CFD and analytic model. The main features of the experimental pulsatile force data and the CFD results were consistent. The results showed that the total force was proportional to the inlet pressure cycle. The force rose from 3.32 N at 130 mm Hg systolic to 17.5 N at 250 mm Hg systolic pressure. For the more variable regions of the flow, the experimentally measured forces lagged the computational and analytic results. The CFD and analytic models provide approximate descriptions for the forces acting on a bifurcated stent graft subjected to pulsatile flow. Such models should be of assistance to designers of endoluminal stent grafts.

Dynamics of pulsatile flow in fractal models of vascular branching networks.

Efficient regulation of blood flow is critically important to the normal function of many organs, especially the brain. To investigate the circulation of blood in complex, multi-branching vascular networks, a computer model consisting of a virtual fractal model of the vasculature and a mathematical model describing the transport of blood has been developed. Although limited by some constraints, in particular, the use of simplistic, uniformly distributed model for cerebral vasculature and the omission of anastomosis, the proposed computer model was found to provide insights into blood circulation in the cerebral vascular branching network plus the physiological and pathological factors which may affect its functionality. The numerical study conducted on a model of the middle cerebral artery region signified the important effects of vessel compliance, blood viscosity variation as a function of the blood hematocrit, and flow velocity profile on the distributions of flow and pressure in the vascular network.

Type II endoleaks: when is intervention indicated and what is the index of suspicion for types I or III?

One of the principal reasons for failure of endovascular aneurysm repair (EVAR) is the occurrence of endoleaks, which regardless of size or type can transmit systemic pressure to the aneurysm sac. There is little debate that type I endoleaks (poor proximal or distal sealing) are associated with continued risk of aneurysm rupture and require treatment. Similarly, with type III endoleak, there is agreement that the defect in the device needs to be addressed; however, what to do with type II endoleaks and their effect on long-term outcome are not so clear. Aneurysm sac change is a primary parameter for determining the presence of an endoleak and assessing its impact. While diameter measurement has been the most commonly used method for determining sac changes, volume measurement has now been proven superior for monitoring structural changes in the 3-dimensional sac. Determining the source of an endoleak and the direction of flow are necessary for proper classification; however, while computed tomographic angiography has high sensitivity and specificity for detecting endoleaks, it is limited in its ability to show the direction of flow. Contrast-enhanced duplex ultrasound, on the other hand, is better able to quantify flow and characterize endoleaks. Flow is evidence of pressure, and increasing intrasac pressure increases wall tension, thus inducing progressive aneurysm expansion until rupture. Hence, determining intrasac pressure is becoming a vital component of endoleak assessment. All endoleaks can create systemic pressure inside the aneurysm sac, and there are a variety of intrasac pressure transducers being evaluated to assess this effect. A clinical pathway for patients with suspected type II endoleaks is based on a combination of imaging and pressure measurements. Imaging alone requires at least two interval examinations to determine the trend, while pressure measurements give immediate reassurance or an indication to intervene. Although still under development, pressure measurement is destined for general use and will provide a scientific basis for the management of type II endoleaks.

Modeling of antegrade and retrograde flow into a branch artery of the aorta: implications for endovascular stent-grafting and extra-anatomical visceral bypass.

PURPOSE: To compare antegrade and retrograde flow characteristics in a branch of a conduit under typical pulsatile pressure and flows, seeking an answer to the question: "Does it matter whether inflow to a branch vessel is antegrade or retrograde?" METHODS: A model was built to simulate an abdominal aorta with a branch designed to approximate a typical renal artery. Experiments were conducted to measure the flow rates from 40- and 200-mm-long inflow conduit tubes simulating a branch with antegrade and retrograde inflow configurations. For the base case with a flush origin of the branch, the pressure difference between the main conduit and branch vessel was adjusted so that the average branch flow rate was 1.22 L/min, representing average renal artery flow. A pump produced a pulsatile 5-L/min flow of a glycerol/water solution through a tube to mimic blood flow through the aorta at a mean inlet pressure of 97 mmHg, with systolic and diastolic pressures of 121 and 78 mmHg, respectively. Computational fluid dynamics (CFD) simulations were performed for the flush, antegrade inflow, and retrograde inflow cases. The CFD-predicted flow rates at the branch vessel outlet for all 3 geometries were compared with the experiments. RESULTS: From the experiments, the mean time-average branch vessel outflow rate through a 40-mm conduit for the antegrade case was 1.22+/-0.01 L/min, which was the same as the retrograde case (1.21+/-0.01 L/min; within the experimental error). However, the branch vessel outflow flow rate through a 200-mm conduit for the retrograde case was 0.07 L/min lower than the antegrade. The results from the CFD model were in good agreement with the experiments. CONCLUSION: The experiments and CFD results suggest that there is negligible difference in the outflow rates to a branch vessel in antegrade and retrograde directions for 40-mm-long conduits. However, for a 200-mm conduit, the flow to a branch vessel through the retrograde path is lower than for the antegrade direction, which has implications for the insertion of branches to stent-grafts and extra-anatomical surgical bypass for visceral revascularization.

Movement and dislocation of modular stent-grafts due to pulsatile flow and the pressure difference between the stent-graft and the aneurysm sac.

PURPOSE: To investigate the stability and movement of modular aortic stent-grafts subjected to oscillating forces from pulsatile blood flow, with particular reference to the thoracic aorta. METHODS: Analytical mathematical modeling was used to understand the forces on modular grafts. In a benchtop experiment, a transparent acrylic box was filled with water to mimic an aneurysm. Two stent-grafts were placed inside the box in a nested, arched configuration where one component was partly inside the other. A pump produced a pulsatile approximately 5-L/min flow of water through the stent-grafts at a mean inlet pressure of approximately 100 mmHg (approximately 13,330 Pa), with systolic and diastolic pressures of approximately 130 and approximately 80 mmHg, respectively (pulse pressure 50 mmHg). The movement of the 2 modular stent-grafts was observed. RESULTS: The curved stent-graft system oscillated transversely when there was zero mean pressure difference between the stent-graft and the aneurysm. As the mean pressure difference was increased, this transverse graft movement was damped and then disappeared. A relatively large pressure difference caused the stent-graft to inflate and become sturdier. In terms of stability, the analytical mathematical model for a 30-mm-diameter Zenith modular stent-graft curved through 90 degrees (with the ends of the graft fixed in place) showed that the modular components will separate at a pressure difference of 0 mmHg for 1 stent segment overlap (20 mm) and at an average 59 mmHg pressure difference for 2 stent overlaps, but the device would not separate at a pressure difference of 90 mmHg for 3 stent overlaps. CONCLUSION: Transverse cyclic movement of the curved stent-graft system with pulsation indicates a pressurized sac. When the pressure difference is large and there is a blood-tight seal between the aneurysm and the stent-graft, then the transverse movement of the stent-graft is minimal, but the risk for modular separation is highest. Curved thoracic endografts are subject to forces that may cause migration or separation, the latter being more likely if the seal between the graft and the sac is blood tight, if the blood pressure is high, and if the diameter of the graft is small and the sac large. Operators should plan for maximum overlap of modular components when treating large or long thoracic aneurysms.

Experimental force measurements on a bifurcated endoluminal stent graft model: comparison with theory.

The goal of this study was to experimentally validate a steady-state mathematical model, which can be used to compute the forces acting on a bifurcated endoluminal stent graft. To accomplish this task, an acrylic model of a bifurcated graft was used for the force measurements. The graft model was connected to the inlet piping with a flexible rubber membrane that allowed the graft model to move. This allowed us to measure the force owing to the movement of the graft model with a calibrated load cell. Steady-state blood flow was assumed, and the working fluid was water. The experimental data were found to be consistent with the results from a previously published mathematical model: the graft force is strongly dependent on the proximal or inlet pressure and the inlet area. The force tends to be weakly dependent on flow rate. More research work will be required to determine whether the steady-state force model examined in this article provides a realistic determination of the forces on an endoluminal stent graft that is subject to pulsatile blood flow.

Suprarenal fixation: effect on blood flow of an endoluminal stent wire across an arterial orifice.

PURPOSE: To investigate what effect, if any, the presence of a stent wire in front of a renal artery has on the volume flow rate of blood through the renal artery. METHODS: Experimental, numerical, and analytical modeling methods were used to test 4 separate stent wire configurations: a stent wire across the center of an artery orifice, an off-center wire placed at one-quarter the arterial diameter, a V-shaped wire with its vertex at the center, and 2 stent wires at one-third-diameter spacing. RESULTS: For all the configurations studied, the presence of stent wires has a minimal effect on the blood flow rate into an artery of >/=3-mm diameter, with most flow rates decreasing by around 1%. This is true provided that there is no buildup of material on the wire. When material buildup was "encouraged" to occur, then decreases in flow rate of up to 40% were observed. The numerical and analytical methods indicated that the flow rates would, in most cases, decrease by around 3% to 10%. CONCLUSIONS: A bare stent wire in front of a >3-mm-diameter artery decreases the flow rate minimally, providing there is no material on the wire. Although the numerical and analytical methods indicated a greater effect on flow, the approximations required for these 2 methods to obtain meaningful solutions suggest that the experimental results are the most accurate. Nonetheless, the analytical equations provided a useful approximation for determining the effect on blood flow due to the presence of a stent wire.