Reconstruction of patient specific abdominal aortic aneurysm models for 3D printing for use on a physiological test bed in the assessment of AAA and endovascular aneurysm repair

Abdominal aortic aneurysms (AAA) are a permanent and irreversible localised dilation of the vessel. The prevalence of AAA’s is rising with tens of thousands of new cases diagnosed each year giving rise to a high number of fatalities per year. Clinical practice currently employs the maximum diameter criterion to judge the need for surgical intervention, which is AAA’s larger than 5.0-5.5 cm in diameter.

The majority of AAA’s are now treated with Endovascular Aneurysm Repair (EVAR). Long term complications and late rupture risks remain a primary concern of this therapeutic approach. Stent graft migration is one of the foremost long term complication post EVAR and is an issue for all types of stent grafts and is one of the principal causes of aneurysm rupture and transabdominal intervention in patients. The risk rate of migration can increase over time, mandating long term surveillance and this is one of the core motivations for further investigations. With long term studies reporting increasing rates of secondary intervention, the development of in-vitro replicas of the in- vivo state have become essential in helping to study the environment to understand key influencing factors and to aid in predicting long term behaviour. A test rig was developed to study and characterize performance and behaviour post endovascular aneurysm repair with consistent inclusion of complications in an effort to regulate the testing approach based on a review of the range of experimental systems described in the literature for measuring and monitoring different behaviours of AAA’s. This experimental assembly is capable of providing physiologically realistic flow and pressure conditions; realized by the inclusion of a number of major components including a flow pump, pressure catheter and an Arduino controller capable of producing a cyclical pulsatile waveform.

Due to the constraints of the manufacturing processes involved in silicone AAA modelling, alternative methods of producing in-vitro models were explored. Multi axial testing was performed on silicone and a range of materials available for commercial 3D printing to examine the viability of manufacturing AAA models that emulate diseased-tissue-like behaviour. The results demonstrate particular printed materials behaving in a similar manner to AAA tissue in terms of mechanical factors such as failure stress and failure strain, with other characteristics performing in similar ranges as diseased AAA tissue.

To study the long term complications of EVAR, AAA patient scans were identified and provided by the cardiovascular team at the University Hospital, Limerick to reflect a range of real life cases. Three patient specific scans were reconstructed using MIMICS® software and subsequently manufactured on the Stratasys Objet Connex 500 Printing System. These printed models comprise of individual layers (lumen, thrombus, calcium) with in-vivo like material properties.Two main body stent graft devices, both “unhooked “ and “hooked” were modified to allow for the measurement of dislodgement and force gauge pull out forces, under physiological in-vivo conditions. The force gauge pullout forces are higher in each patient case than in the idealised model.

The results and conclusions presented contribute to the knowledge of AAA biomechanics and demonstrate the potential use of 3D printed materials as AAA tissue surrogate. The experimental assembly has the potential to act as a preoperative assessment support and post-operative behaviour performance estimation and ultimately aid in the clinical management of the disease.