Jérôme Chambert

Geometrical analysis of the V–Y advancement flap applied to a keystone flap

BACKGROUND The V-Y advancement flap and, more recently, the keystone flap are commonly used to cover skin defects. Both flaps allow for primary closure after advancement by substituting the initial defect for a narrower defect distributed over a greater length. The first objective of this study was to develop a geometrical analysis of the V-Y advancement flap. The second objective was to explain the benefit of using the keystone flap compared to a single V-Y advancement flap. MATERIAL AND METHOD A geometrical analysis is proposed using a two-dimensional analysis in which the flaps are assumed to have a rigid-body behaviour. First, in the case of the V-Y advancement flap, a trigonometric relationship is defined between the distance of closure before and after advancement, thus implying the value of the flaps apex angle. Second, by considering the keystone flap as the association of three V-Y advancement flaps, the trigonometric relationship is applied to the keystone flap. RESULTS In the case of the V-Y advancement flap, the optimal apex angles are between 20° and 60°. At less than 20°, the length of the flap increases in an exaggerated manner. At greater than 60°, the distance of closure, particularly at the apex of the flap where a corner stitch is performed, is greater than the distance of closure of the initial defect. In the case of the keystone flap, the width of the final defect around the flap is clearly smaller and more regular compared to the final defect around a single V-Y advancement flap. CONCLUSION The geometrical analysis of the V-Y advancement flap in our description illustrates the major benefit of the keystone flap over a single V-Y advancement flap.

Intra-and inter-individual variability in the mechanical properties of the human skin from in vivo measurements on 20 volunteers

The mechanical properties and behavior of the human skin in vivo are of medical importance, particularly to surgeons who have to consider the skin extension capabilities in the preparation of surgical acts. Variable data can be found in literature that result from diverse kinds of tests (in vivo, ex vivo, and postmortem) performed with different instruments.

Experimental and numerical analysis of a V-Y advancement flap on human skin ex vivo

D. Remache*, J. Pauchot, J. Chambert, L. Capek and E. Jacquet Department of Applied Mechanics, University of Franche-Comte, Femto-st Institute – UMR 6174 CNRS, 24 rue de l’ Epitaphe, 25000 Besançon, France; Orthopaedic, Traumatology, Plastic Reconstructive and Hand Surgery Unit, EA 4268I4S IFR133 INSERM, University Hospital of Besançon, Place Saint-Jacques, F-25030 Besançon, France; Department of Mechanical Engineering, Technical University of Liberec, Studenska 2, 46117 Liberec 1, Czech Republic

Numerical analysis of keloid scar in the presternal area

Keloid scars are benign skin tumours that affect only humanity, especially among the Asian and African populations. Keloids consist in the excessive accumulation of dermal collagen that extends beyond the boundaries of the original wound (Kelly 2009). They are located on specific sites, tend to enlarge over time and pose aesthetic and psychological problems for patients. The physiopathology of keloids is unclear, and there is still no therapeutic consensus. Nevertheless, it appears that keloids result from a combination of genetic, cellular and mechanical factors (Salles et al. 2009). The purpose of this study is to predict the likely directions of extension of a presternal keloid as a function of mechanical stress field. Abdel-Fattah (1976) has assumed that a presternal keloid of initially oblong shape evolves into a claw-like pattern (or butterfly shape) over time. Based on geometrical and mechanical data from the literature, a finite element simulation of the skin in vivo in the vicinity of a presternal keloid of oblong shape is performed with Ansys software by assuming a linear elastic behaviour. The numerical results are then post-processed by Matlab software to predict the likely directions of extension of the keloid. Finally, the numerical predictions are compared with experimental results of Abdel-Fattah (1976). In particular, the growth direction of the keloid obtained from the clinical case is compared with the one computed by the maximum shear stress in the neighbourhood of the keloid.

A New Modelling of Blanking for Thin Sheet in Copper Alloys with Dynamic Recrystallization

Precision blanking process is widely used by electronic and micromechanical industries to produce small and thin components in large quantities. To take account of the influence of strain rate and temperature on precision blanking of thin sheet in copper alloys, we have proposed a thermo‐elasto‐visco‐plastic modelling. In addition, dynamic recrystallization takes place in Cual copper alloy during the blanking process of thin sheet. A new modelling of dynamic recrystallization based on the thermodynamics of irreversible processes is presented. Blanking simulations of Cual copper sheet are carried out in order to analyze the softening effect induced by dynamic recrystallization.

Numerical Design Of Experiments to Analyse the Contact Conditions in Microforming

In microforming, the so‐called size effects can be observed in the material flow behaviour as well as in the frictional behaviour. In order to study the frictional behaviour a preliminary numerical characterization of the surface tribology has been carried out. A numerical design of experiments (DOE) is based on cylinder upsetting tests to define the influence of surface geometric properties on the resultant force. The simulations have been performed with the finite element software LS‐Dyna by using an axisymmetric model. The mechanical behaviour of the cylinder specimen was described by an elastic‐plastic material law, whereas the upsetting plates were assumed to be rigid. The workpiece is considered to be a copper alloy (CuZn10). The average roughness Ra and the average mean spacing Sm have been chosen to describe surface roughness properties. The tool and workpiece surfaces have been modelled using a sinusoidal profile. The five input parameters of the DOE are the amplitude and the period of the two si...

A New Modelling of Dynamic Recrystallization — Application to Blanking Process of Thin Sheet in Copper Alloy

Blanking process is widely used by electronic and micromechanical industries to produce small and thin components in large quantities. To take into consideration the influence of strain rate and temperature on precision blanking of thin sheet in copper alloy, a thermo‐elasto‐visco‐plastic modelling has been developed in [1]. Furthermore the blanking of thin sheet in Cual copper presents dynamic recrystallization. A new modelling of dynamic recrystallization based on the thermodynamics of irreversible processes is proposed. Blanking simulations of Cual copper sheet are performed to analyze the softening effect induced by dynamic recrystallization.

Micromechanical nonlocal damage modeling

Publisher Summary This chapter presents a micromechanical nonlocal damage model to predict the damage evolution in metal forming processes, especially in the cold forging of small components. The modeling considered in this chapter is based on a Gurson model modified by Bennani, where the nonlocal damage has been defined in the same manner as Pijaudier-Cabot and Bazant. A nonlocal damage modeling based on Gursons model has been implemented in the large strain finite element framework of POLYFORM code for quasi-static loading conditions. The first results show that nonlocal damage could reduce the problem of mesh dependency. Future works consist in testing this nonlocal formulation on classical tensile tests with axisymmetric notched specimens and then to apply it on metal forming processes—such as blanking or cold forging.