Histologic, histochemical, and biomechanical properties of fragments isolated from the anterior wall of abdominal aortic aneurysms

Objective: To analyze biomechanical, histologic, and histochemical properties of anterior fragments of abdominal aortic aneurysms (AAA) and to correlate them with the maximum transverse diameter (MTD) and symptoms associated to the aneurysms. Methods: Fragments of the anterior aneurysm wall were obtained from 90 patients submitted to open repair of AAA of degenerative etiology from 2004 to 2009 in the Clinics Hospital of Sao Paulo University Medical School. Two specimens were produced from the fragments: one for histologic analysis for quantification of collagen fibers, elastic fibers, smooth muscle cells, and degree of inflammatory activity and the other for uniaxial tensile test to assess biomechanical failure properties of the material, such as strength, tension, and stress. Cases were classified according to symptoms and to the AAA MTD. Results: Fragments from AAA with MTD >= 5.5 cm showed higher values for biomechanical failure properties than those of AAA with MTD < 5.5 cm (strength, 5.32 +/- 2.07 x 4.1 +/- 2.41 N; tension, 13.83 +/- 5.58 x 10.82 +/- 6.48 N/cm; stress, 103.02 x 77.03 N/cm(2); P < .05). No differences were observed between the groups in relation to failure strain (0.41 +/- 0.12 x 0.37 +/- 0.14; P = .260) and thickness of the fragments (1.58 +/- 0.41 x 1.53 +/- 0.42 mm; P = .662). The average values of fiber compositions of all the fragments were as follows: collagen fibers, 44.34 +/- 0.48% and 61.85 +/- 10.14% (Masson trichrome staining and Picrosirius red staining, respectively); smooth muscle cells, 3.46 +/- 2.23% (immunohistochemistry/alpha-actin); and elastic fibers, less than 1% (traces) (Verhoeff-van Gieson staining). No differences in fiber percentages (collagen, elastic, and smooth muscle) were observed in fragments from AAA with MTD >= 5.5 cm and <5.5 cm, but more intense inflammatory activity was seen in larger AAA (grade 3; 70% x 28.6%; P = .011). Compared with asymptomatic aneurysms, symptomatic aneurysms showed no differences in the biomechanical failure properties (strength, 5.32 +/- 2.36 x 4.65 +/- 2.05 N; P = .155; tension, 14.08 +/- 6.11 x 12.81 +/- 5.77 N/cm; P = .154; stress, 103.02 x 84.76 N/cm(2); P = .144), strain (0.38 +/- 0.12 x 0.41 +/- 0.13; P = .287), thickness of the fragments (1.56 +/- 0.41 x 1.57 +/- 0.41 mm; P = .848), and histologic composition (collagen fibers, 44.67 +/- 11.17 x 44.02 +/- 13.79%; P = .808; smooth muscle fibers, 2.52 x 2.35%; P = .751; elastic fibers, <1%) Conclusions: Fragments of the anterior wall from larger aneurysms were more resistant than those from smaller AAA, with no tissue properties that could explain this phenomenon in the histologic or histochemical analyses utilized.