<unk> s form for a variety of interacting reasons. Multiple factors, including factors affecting a blood vessel wall and the blood through the vessel, contribute. The pressure of blood within the expanding<unk>  may also injure the blood vessels supplying the artery itself, further weakening the vessel wall. Without treatment, these<unk> s will ultimately progress and rupture. Infection. A mycotic<unk>  is an<unk>  that results from an infectious process that involves the arterial wall. A person with a mycotic<unk>  has a bacterial infection in the wall of an artery, resulting in the formation of an<unk> . One of the causes of mycotic<unk> s is infective endocarditis. The most common locations include arteries in the abdomen, thigh, neck, and arm. A mycotic<unk>  can result in sepsis, or life-threatening bleeding if the<unk>  ruptures. Less than 3% of abdominal aortic<unk> s are mycotic<unk> s. Syphilis. The third stage of syphilis also manifests as<unk>  of the aorta, which is due to loss of the vasa vasorum in the tunica adventitia. Copper deficiency. A minority of<unk> s are caused by copper deficiency, which results in a decreased activity of the lysyl oxidase enzyme, affecting elastin, a key component in vessel walls. Copper deficiency results in vessel wall thinning, and thus has been noted as a cause of death in copper-deficient humans, chickens, and turkeys. Mechanics<unk> al blood vessels are prone to rupture under normal blood pressure and flow due to the special mechanical properties that make them weaker. To better understand this phenomenon, we can first look at healthy arterial vessels which exhibit a J-shaped stress-strain curve with high strength and high toughness (for a biomaterial in vivo). Unlike crystalline materials whose linear elastic region follows Hooke's Law under uniaxial loading, many biomaterials exhibit a J-shaped stress-strain curve which is non-linear and concave up. The blood vessel can be under large strain, or the amount of stretch the blood vessel can undergo, for a range of low applied stress before fracture, as shown by the lower part of the curve. The area under the curve up to a given strain is much lower than that for the equivalent Hookean curve, which is correlated to toughness. Toughness is defined as the amount of energy per unit volume material can absorb before rupturing. Because the amount of energy released is proportional to the amount of crack propagation, the blood vessel wall can withstand pressure and is “tough.” Thus, healthy blood vessels with the mechanical properties of the J-shaped stress-strain curve have greater stability against<unk> s than materials with linear elasticity. Blood vessels with<unk> s, on the other hand, are under the influence of an S-shaped stress-strain curve. As a visual aid,<unk> s can be understood as a long, cylindrical balloon. Because it's a tight balloon under pressure, it can pop at any time stress beyond a certain force threshold is applied. In the same vein, an unhealthy blood vessel has elastic instabilities that lead to rupture. Initially, for a given radius and pressure, stiffness of the material increases linearly. At a certain point, the stiffness of the arterial wall starts to decrease with increasing load. At higher strain values, the area under the curve increases, thus increasing the impact on the material that would promote crack propagation. The differences in the mechanical properties of the<unk> al blood vessels and the healthy blood vessels stem from the compositional differences of the vessels. Compared to normal aortas,<unk> al aortas have a much higher volume fraction of collagen and ground substance (54.8% vs. 95.6%) and a much lower volume fraction of elastin (22.7% vs. 2.4%) and smooth muscles (22.6% vs. 2.2%), which contribute to higher initial stiffness. It was also found that the ultimate tensile strength, or the strength to withstand rupture, of<unk> al vessel wall is 50% lower than that of normal aortas. The wall strength of ruptured<unk> al aortic wall was also found to be 54.2 N/cm2, which is much lower than that of a repaired aorta wall, 82.3 N/cm2. Due to the change in composition of the arterial wall,<unk> s overall have much lower strength to resist rupture. Predicting the risk of rupture is difficult due to the regional anisotropy the hardened blood vessels exhibit, meaning that the stress and strength values vary depending on the region and the direction of the vessel they are measured along.
Aneurysm
Baker's cyst
Coma
Angioma
Hematoma
Vascular tumor
Gliosis
Pseudoaneurysm