Finger and thumb replantation: From biomechanics to practical surgical applications.

Finger and thumb amputations, which are always dramatic injuries with major functional and psychological repercussions, remain a surgical challenge. This review on digit replantation develops the underlying biomechanical and surgical aspects as well as practical indications. The different stages from trauma to postoperative monitoring are described. We describe the steps to follow from theory to practice in order to optimize the surgical acts that must as effective possible in terms of management and decision-making efficiency. Indications recognized as standards such as thumb amputation, multi-digit amputations and distal amputations are detailed, as well as the more controversial ring finger replantations. The challenge of successful finger and thumb replantation lies in searching for the best functional and cosmetic outcome and not performing irrelevant microsurgical manipulations.

Contribution of flow MRI in the therapeutic management of middle face high flow arteriovenous malformation: A case report.

BACKROUND: The radiosurgical management of high flow arteriovenous malformations (HFAVM) in the "destructive" stage requires a precise hemodynamic and anatomical assessment. PATIENT AND METHODS/CASE REPORT: We report the case of a 32 years-old patient with a large ulcerated face HFAVM, on which Doppler ultrasound was impossible to perform. We show that, by combining 3D PCA and 2D CINE PC-MRI sequences, magnetic resonance imaging is capable to provide a complete morphometric and velocimetric mapping of the nidus and feeding arteries of the HFAVM. CONCLUSION: Although Doppler ultrasound is the reference examination in the HFAVM, Flow MRI without contrast agent provides an advantageous alternative to assess vascular pathologies and choose the therapeutic strategy.

Polymerization kinetics of a mixture of Lipiodol and Glubran 2 cyanoacrylate glue upon contact with a proteinaceous solution.

The Glubran 2 cyanoacrylate glue is a liquid embolic agent used to block blood vessels endovascularly. Typically mixed with an iodized oil (Lipiodol) for visualization under X-ray, it polymerizes when in contact with blood and tissues owing to the presence of ions and proteins. The objective of the study is to determine the influence of plasma proteins in the polymerization reaction. A triggering solution containing bovine serum albumin (BSA) and the main blood ions is used as a model of plasma. The polymerization kinetics of Glubran 2-Lipiodol mixtures is measured upon aspiration in a capillary tube and contact with the proteinaceous solution. Having varied the glue and protein concentrations, we show that glue-Lipiodol mixtures with concentrations larger or equal to 25% polymerize when put in contact with an ionic solution containing at least 4% of BSA. The reaction is decomposed into two phases: a fast zwitterionic polymerization induced by the BSA molecules followed by a slower polymerization phase. The reaction speed and extent of the solidification region mostly depend on the glue concentration. The time for the glue solution to polymerize over a 1mm thickness varies from 5s for pure glue to about 1min for a 50% glue concentration, and 10min for a 25% glue mixture. It is the first time that the kinetics of the two polymerization reactions is quantified for Glubran 2, which will provide the information needed by interventional radiologists to optimize the planning of endovascular glue injection.

Polymerization kinetics of n-butyl cyanoacrylate glues used for vascular embolization.

Vascular embolization is a minimally invasive treatment used for the management of vascular malformations and tumors. It is carried out under X-ray by navigating a microcatheter into the targeted blood vessel, through which embolic agents are delivered to occlude the vessels. Cyanoacrylate liquid glues have been widely used for vascular embolization owing to their low viscosity, rapid polymerization/solidification rate, good penetration ability and low tissue toxicity. The objective of this study is to quantitatively investigate the physical properties of two n-butyl cyanoacrylate (nBCA) glues (Glubran 2 and Histoacryl) mixed with an iodized oil (Lipiodol) at various concentrations. We show that an homogeneous solution results from the mixing of the glue and Lipiodol, and that the viscosity, density and interfacial tension of the mixture increase with the proportion in Lipiodol. We have designed a new experimental setup to systemically characterize the polymerization kinetics of a glue mixture upon contact with an ionic solution. We observe that the whole polymerization process includes two phases: an interfacial polymerization that takes place at the interface as soon as the two liquids are in contact with a characteristic time scale of the order of the minute; a volumetric polymerization during which a reaction front propagates within the mixture bulk with a characteristic time scale of the order of tens of minutes. The polymerization rate, front propagation speed and volume reduction increase with the glue concentrations. It is the first time that such comprehensive results are obtained on liquid embolic agents.

Characterizing the membrane properties of capsules flowing in a square-section microfluidic channel: effects of the membrane constitutive law.

A microfluidic method is presented to measure the elastic membrane properties of a population of microcapsules with diameter of order 60 µm. The technique consists of flowing a suspension of capsules enclosed by a polymerized ovalbumin membrane through a square-section microfluidic channel with cross dimension comparable with the capsule mean diameter. The deformed profile and the velocity of a given capsule are recorded. A full mechanical model of the motion and deformation of an initially spherical capsule flowing inside a square-section channel is designed for different flow strengths, confinement ratios, and membrane constitutive laws. The experimental deformed profiles are analyzed with the numerical model. This allows us to find the ratio between the viscous and elastic forces and thus the shear elastic modulus of the membrane. We show that the ovalbumin membrane tends to have a strain-softening behavior under the conditions studied here.

Tension of red blood cell membrane in simple shear flow.

When a red blood cell (RBC) is subjected to an external flow, it is deformed by the hydrodynamic forces acting on its membrane. The resulting elastic tensions in the membrane play a key role in mechanotransduction and govern its rupture in the case of hemolysis. In this study, we analyze the motion and deformation of an RBC in a simple shear flow and the resulting elastic tensions on the membrane. The large deformation of the red blood cell is modelled by coupling a finite element method to solve the membrane mechanics and a boundary element method to solve the flows of the internal and external liquids. Depending on the capillary number Ca, ratio of the viscous to elastic forces, we observe three kinds of RBC motion: tumbling at low Ca, swinging at larger Ca, and breathing at the transitions. In the swinging regime, the region of the high principal tensions periodically oscillates, whereas that of the high isotropic tensions is almost unchanged. Due to the strain-hardening property of the membrane, the deformation is limited but the membrane tension increases monotonically with the capillary number. We have quantitatively compared our numerical results with former experimental results. It indicates that a membrane isotropic tension O(10{-6} N/m) is high enough for molecular release from RBCs and that the typical maximum membrane principal tension for haemolysis would be O(10{-4} N/m). These findings are useful to clarify not only the membrane rupture but also the mechanotransduction of RBCs.

Comparison between spring network models and continuum constitutive laws: application to the large deformation of a capsule in shear flow.

A capsule is a liquid drop enclosed by a solid, deformable membrane. To analyze the deformation of a capsule accurately, both the fluid mechanics of the internal and external fluids and the solid mechanics of the membrane must be solved precisely. Recently, many researchers have used discrete spring network models to express the membrane mechanics of capsules and biological cells. However, it is unclear whether such modeling is sufficiently accurate to solve for capsule deformation. This study examines the correlations between the mechanical properties of the discrete spring network model and continuum constitutive laws. We first compare uniaxial and isotropic deformations of a two-dimensional (2D) sheet, both analytically and numerically. The 2D sheet is discretized with four kinds of mesh to analyze the effect of the spring network configuration. We derive the relationships between the spring constant and continuum properties, such as the Young modulus, Poisson ratio, area dilation modulus, and shear modulus. It is found that the mechanical properties of spring networks are strongly dependent on the mesh configuration. We then calculate the deformation of a capsule under inflation and in a simple shear flow in the Stokes flow regime, using various membrane models. To achieve high accuracy in the flow calculation, a boundary-element method is used. Comparing the results between the different membrane models, we find that it is hard to express the area incompressibility observed in biological membranes using a simple spring network model.

Comparison between measurements of elasticity and free amino group content of ovalbumin microcapsule membranes: discrimination of the cross-linking degree.

An inverse method is used to characterize the membrane mechanical behavior of liquid filled microcapsules. Cross-linked ovalbumin microcapsules are flowed and deformed into a cylindrical microchannel of comparable size. The deformed shape is compared to predictions obtained numerically when modeling a capsule under the same flow conditions. The unknown shear modulus value corresponds to the best fit. The degree of reticulation is estimated in parallel by determining the free amino groups remaining on the microcapsules after the cross-linking reaction. We characterize microcapsule populations fabricated at different reaction pH (5-8) and times (5-30 min) to study different cross-linking degrees. The capsule shear modulus and the amino groups are nearly constant with the reaction pH for the capsules fabricated after 5 min of reticulation. The shear modulus increases with the reaction time, while the NH(2) content decreases with it. A global increase in shear modulus with pH is also observed, together with an unexpected increase in NH(2) content. The study shows that the inverse method is capable of discriminating between various cross-linking degrees of microcapsules. Moreover, for this type of microcapsules, the mechanical method appears more reliable than the chemical one to obtain an estimation of their cross-linking degree.