[Mechanical properties of materials used to reinforce aortic anastomoses in thoracic aortic surgery]

One of the causes of anastomotic bleeding during prosthetic repair of the thoracic aorta is aortic wall cut-through by monofilament threads due to insufficient mechanical strength of the vascular wall. The tightness of the anastomosis depends on such components as blood coagulation system state, presence of aseptic inflammation of the vascular wall, surgical technique and properties of the materials used to form anastomoses (felt, pericardium).

The aim of this study was to estimate the deformation and strength properties of materials used for reconstruction of the thoracic aorta, investigating such parameters as the peak stress, peak strain, and initial elastic modulus, as well as evaluating alterations appearing under load on various types of anastomoses with the normal and dilated aorta.

The mechanical properties of the materials were examined in the uniaxial stress-strain testing mode on the Instron 1122 tensile testing machine. To study the properties of the aorta, two groups were formed: group 1 including anastomoses with normal aorta (n=38) and group 2 comprising anastomoses with dilated aorta (n=30). The comparison between the groups was made according to the following parameters: peak stress ( max), peak strain ( max), and initial elastic modulus (Einit). The same parameters were obtained and analyzed for the group of the human pericardium (n=30) and medical felt (n=10).

It was shown that the peak stress ( max) of the normal aorta both in the longitudinal and transverse direction exceeds more than two-fold that of the dilated aorta. At the same time, the patterns of deformation of the normal and altered aorta are similar, which is illustrated by close values of the initial elastic modulus (Einit). The pericardium possesses similar to the aorta strength characteristics and has comparable with the aorta value of the initial elastic modulus. Deformation of medical felt occurs quite differently, for the initial elastic modulus of felt is ten-fold higher than that of biological objects. Such a difference in deformation-strength properties of felt and the aorta may lead to cut-through of the middle layer of the anastomosis (aorta) and intimal dissection. Assessing the aortic wall strength in experiments for cut-through by monofilament threads demonstrated that disruption of integrity of reinforced three-layer anastomoses is due to intimal dissection (under loads not leading to cut-through of the rest layers of the anastomosis). Being a homogenous in structure serous membrane, the pericardium is more resistant to cut-through than the aorta. Unlike the pericardium, aortic layers have different histological structure and hence different resistance to cut-through.

When forming sandwich-type (tree-layer) anastomoses of the thoracic aorta with the use of felt and monofilament threads, the integrity of the aortic wall is disrupted due to intimal dissection under loads not leading to cut-through of the rest anastomotic layers. The pericardium possesses similar to the aorta deformity-strength properties and more likely will contribute to optimal distribution of loads on the anastomosis line compared with medical felt. A decrease in strength properties of the dilated aorta in the area of anastomosis is not critical and the aorta may therefore continue fulfilling its function after reinforcement of its wall with auxiliary materials (pericardium, felt).