Biomechanical characterization of human lung bullae using uniaxial tensile tests and CT-derived geometry-specific finite element analysis.

Introduction: Lung bullae are air-filled spaces with thin walls located within the lungs, and they are closely associated with spontaneous pneumothorax. However, quantitative mechanical data on human bullae and subject-specific stress analyses remain sparse. Methods: We performed uniaxial tensile tests on surgically resected human tissues, including 39 bulla specimens and 34 parenchyma specimens, and fitted the nonlinear stress-strain curves using hyperelastic models. Rupture stress was calculated for specimens that ruptured within the gauge region, comprising 15 bulla specimens and 14 parenchyma specimens. We subsequently constructed 12 CT-derived geometry-specific finite element models of 12 bullae from 10 patients using preoperative CT images, with segmentation performed in Mimics and geometric smoothing performed in Geomagic. High-pressure loading of 110 cmH Results: A second-order Yeoh model best captured the marked strain-stiffening behavior of both tissues. The mean ultimate tensile strength was 2.318 MPa for bulla specimens and 0.390 MPa for parenchyma specimens. In the CT-derived geometry-specific finite element models, the 95th-percentile equivalent von Mises stress ranged from 50.3 to 515.8 kPa, with a median of 106.8 kPa. Elevated stresses were consistently localized to the bulla vertex or apex, defined as the most remote convex peak with the least parenchymal tethering, rather than along the equator. Under the simulated worst-case loading, no case reached the mean experimental ultimate tensile strength of bulla tissue, although one case exceeded the minimum measured bulla ultimate tensile strength of 462 kPa. Discussion: Human bulla tissue exhibits greater tensile strength than adjacent lung parenchyma but remains vulnerable to elevated local stresses generated by curvature and reduced parenchymal tethering at the apex. CT-derived geometry-specific finite element analysis using hyperelastic constitutive models provides a biomechanical explanation for the clinically observed apical predilection of bulla rupture.