Analysis of an elastic ring tourniquet (ERT) using the theory of a Cosserat curve and finite elements

A-TT™ and S-MART™ are novel elastic ring tourniquets (ERTs) used for the control of hemodynamic shock and for bloodless surgical procedures on limbs. The ERT device is essentially a silicone ring surrounded by a stockinette sterile gauze. Since the contact pressure applied by the ERT is determined by its geometric and mechanical properties and cannot be adjusted, it is important to develop a mechanical model that can be used to ensure that the contact pressure remains low enough not to cause local tissue damage. Although the calculation of the contact pressure applied by a silicone ring seems to be a simple classical problem, it is shown that the accurate prediction of this contact pressure requires analysis of a number of complicating features, which are related to nonlinear geometric and material influences. This paper presents a hierarchy of mechanical models which include: a simple strength of materials model; models based on the theory of a Cosserat curve and a generalized string (which includes deformation of the cross-section); exact axisymmetric plane strain nonlinear elasticity; and axisymmetric nonlinear finite element analysis. It is shown that the Cosserat model provides reasonable predictions of the response of the silicone ring. Since numerical simulations with the Cosserat model are much faster than those with the finite element model, the Cosserat analysis is more suitable for optimization of the thickness of the stockinette, which can significantly reduce both the maximum and average contact pressures applied by the ERT on the skin.