Translation of Clinical Rupture Risk Factors for the Biomechanics based AAA Simulations

• Main Author: Winther, Viktor
• Format: Others
• Language: English
• Published: KTH, Skolan för teknik och hälsa (STH) 2017
• Subjects: Medicin teknik; Medical Engineering; Medicinteknik
• Online Access: http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-214690

The abdominal aorta is the largest blood vessel in the abdomen and the main supplier of blood to the lower body. An abdominal aortic aneurysm (AAA) is an unnatural enlargement of the abdominal aorta, which is a serious condition with a high risk of mortality. If the aneurysm exceeds a certain diameter or growth rate, surgical interventions are justified. Use of a diameter-based criterion has been proven to be inaccurate though since some smaller aneurysms can rupture whilst some larger aneurysms remain quiescent. A biomechanical rupture risk assessment (BRRA) that utilizes the finite element method can be used to evaluate the risk of aneurysm rupture. The BRRA calculates the stresses in the aneurysm based upon CT scans and patients blood pressure. Comparing the stresses with the strength of wall in the aneurysm makes it possible to evaluate the risk of rupture. If the stress exceeds the strength, the aneurysm will rupture. To calculate the strength of the vessel wall, a strength equation is used. The strength equation consists of risk factors such as family history, gender, intra luminal pressure and aneurysm diameter. To individualize the assessment further it would be possible to identify and use other risk factors. Rupture risk factors were searched for through two spate literature searches. To identify the risk factors the search utilized keywords such as “rupture risk factors” and “abdominal aortic aneurysm” together with “peak wall stress” or “wall stress”. The search also used a state of the art article from previous research, which contained a list of risk factors that could be searched for. For a factor to be used in this study they had to be global risk factors. Instead of increasing the risk of rupture in a localised point in the aneurysm, a global factor affects the aneurysm uniformly throughout its entirety. The search focused on statistical trials that evaluate the factors impact on wall stress or wall strength. An AAA wall strength equation was constructed based on the rupture risk factors that were identified. This equation was translated into the Finite element analysis program (FEAP) to evaluate its behaviour. A statistical analysis was performed in Matlab using data from the program A4CLINICS developed by VASCOPS gMBh. Using 41 patients along with known patient characteristics and CT scans Biomechanical rupture risk assessment (BRRA) was conducted using the new strength equation. The assessment resulted in a new peak wall rupture index (PWRI). The resulting data was separated into two groups based upon their volume growth rate, one fast growing and one slow growing group. This separation was done for both the VASCOPS strength equation and the new one. Pearson correlation testing was used to test the correlation between both strength equations and volume growth or diameter growth. To evaluate the sensitivity of the strength equation, receiver operating characteristics (ROC) curves were also used. The PWRI in fast and slow groups were not different (p-values of 0.1257 for VASCOPS and 0.0679 for the new equation). The Pearson correlation coefficients showed a higher correlation between new PWRI and volume growth compared to diameter growth. The new PWRI had a higher sensitivity for predicting the volume growth compared to the diameter growth. Initial volume and diameter had the highest sensitivity of all predictors. The new PWRI could be used to predict volume growth. Volume growth is a potential predictor of aneurysm rupture, which indicates the new PWRI can be used in the BRRA. But to achieve results with statistical significance, new simulations using larger population must be performed and the strength equation must be revised.