Finite element and photoelastic modelling of an abdominal aortic aneurysm: a comparative study

A PHOTOELASTIC MODEL OF THE PATELLOFEMORAL JOINT TO STUDY PATELLAR HEIGHT

Background: Patellar height has been related to anterior knee pain and Osgood–Schlatter disease. To study the influence of patellar height on knee biomechanics, a bidimensional photoelastic prototype of the patellofemoral joint was developed. Methods: Nine tests were performed at different knee flexion angles and patellar heights. A free body diagram was constructed for each test. The following parameters were calculated: lengths (patella, patella thickness and ligamentum patellae); patellar height index; angles (knee flexion, patellar flexion, quadriceps and ligamentum patellae force and reaction force at the ankle); moment arms (extensor, quadriceps and ligamentum patellae in relation to both the tibiofemoral and patellofemoral contact points), effective moment arm and forces around the extensor mechanism relative to the applied force. Results: Femoropatellar and femorotibial reaction forces were greater with increased knee flexion and a fixed patellar height. With fixed knee flexion and different patellar heights, these forces were greater if the patella was elevated. A decrease in the angle between the tibial axis and ligamentum patellae was observed when patellar height increased. Patellar flexion angle increased when patellar height increased. This was accompanied by an increase in the angle of action of the quadriceps force. Extensor moment arm decreased with increased patellar height when knee flexion and the tibiofemoral contact point were fixed. Conclusion: A new application of photoelasticity is presented. The preliminary findings obtained confirm the influence of patellar height on patellar biomechanics, and specifically on forces around the extensor mechanism of the knee.

Biomechanical Restoration Potential of Pentagalloyl Glucose after Arterial Extracellular Matrix Degeneration

The objective of this study was to quantify pentagalloyl glucose (PGG) mediated biomechanical restoration of degenerated extracellular matrix (ECM). Planar biaxial tensile testing was performed for native (N), enzyme-treated (collagenase and elastase) (E), and PGG (P) treated porcine abdominal aorta specimens (n = 6 per group). An Ogden material model was fitted to the stress-strain data and finite element computational analyses of simulated native aorta and aneurysmal abdominal aorta were performed. The maximum tensile stress of the N group was higher than that in both E and P groups for both circumferential (43.78 ± 14.18 kPa vs. 10.03 ± 2.68 kPa vs. 13.85 ± 3.02 kPa; p = 0.0226) and longitudinal directions (33.89 ± 8.98 kPa vs. 9.04 ± 2.68 kPa vs. 14.69 ± 5.88 kPa; p = 0.0441). Tensile moduli in the circumferential direction was found to be in descending order as N > P > E (195.6 ± 58.72 kPa > 81.8 ± 22.76 kPa > 46.51 ± 15.04 kPa; p = 0.0314), whereas no significant differences were found in the longitudinal direction (p = 0.1607). PGG binds to the hydrophobic core of arterial tissues and the crosslinking of ECM fibers is one of the possible explanations for the recovery of biomechanical properties observed in this study. PGG is a beneficial polyphenol that can be potentially translated to clinical practice for preventing rupture of the aneurysmal arterial wall.

Stent graft performance in the treatment of abdominal aortic aneurysms: the influence of compliance and geometry

The long-term success of the endovascular procedure for the treatment of Abdominal Aortic Aneurysms (AAAs ) depends on the secure fixation of the proximal end and the geometry of the stent-graft (SG) device. Variations in SG types can affect proximal fixation and SG hemodynamics. Such hemodynamic variations can have a catastrophic effect on the vascular system and may result from a SG/arterial wall compliance mismatch and the sudden decrease in cross-sectional area at the bifurcation, which may result in decreased distal perfusion, increased pressure wave reflection and increased stress at the interface between the stented and non-stented portion of the vessel. To examine this compliance mismatch, a commercial SG device was tested experimentally under a physiological pressure condition in a silicone AAA model based on computed tomography scans. There was a considerable reduction in compliance of 54% and an increase in the pulse wave velocity of 21%, with a significant amount of the forward pressure wave being reflected. To examine the SG geometrical effects, a commercial bifurcated geometry was compared computationally and experimentally with a geometrical taper in the form of a blended section, which provided a smooth transition from the proximal end to both iliac legs. The sudden contraction of commercial SG at the bifurcation region causes flow separation within the iliac legs, which is known to cause SG occlusion and increased proximal pressure. The blended section along the bifurcation region promotes a greater uniformity of the fluid flow field within the distal legs, especially, during the deceleration phase with reduced boundary layer reversal. In order to reduce the foregoing losses, abrupt changes of cross-section should be avoided. Geometrical tapers could lead to improved clinical outcomes for AAA SGs.