Abdominal Aneurysms

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This was a joint project between Medical Engineering and the Vascular Laboratory, Hull Royal Infirmary (Hull and East Yorkshire Hospitals NHS Trust). The project is supported by the EPSRC and private funding.

Background

The decision to repair an asymptomatic abdominal aortic aneurysm (AAA) is currently based on aneurysm diameter, with patients advised to consider a repair if the diameter of the aneurysm is 5.5 cm or greater or if they are symptomatic. However, although aneurysm size is an important predictor of rupture, not all large aneurysms rupture, while over 20% of small aneurysms (<5.5 cm) may rupture. Currently, no reliable criterion exists to predict risk of rupture on an individual patient basis, and a decision to operate based on aneurysm diameter alone may subject a significant proportion of patients to unnecessary surgery with significant mortality and morbidity. Patients with a stable aneurysm are more likely to die of other causes. Hence, the arbitrary setting of a single threshold diameter for all patients may be inappropriate.

Preliminary study

In the study at Hull, finite element analysis was used to examine the stresses in individual patient aneurysms to see whether stress is a better indicator of rupture risk than diameter.

The aneurysms of 27 patients were analysed, of which 12 had ruptured and 15 had not. Aneurysm geometry was obtained directly from the patient’s CT scan data and used to create patient-specific finite element models, which were loaded with individual patient’s recorded blood pressure. The output was a contour map of the wall stress throughout each patient’s aneurysm.

The results showed that there was no significant difference between the mean diameters of the two groups, but the peak wall stress was significantly higher in the ruptured aneurysms than the non-ruptured aneurysms. (In statistical terms, an independent sample t-test gave a value of p=0.2 for the diameters and p=0.004 for the wall stresses). Furthermore, the site of peak wall stress correlated with the site of rupture in most cases where the location was easily identifiable in the CT or was recorded by the surgeon (see for example the figure below).

CT scan showing the site of rupture		Finite element model showing the stress distribution and predicted site of rupture

Conclusions

This was only a small study, and a number of issues in the methodology need further investigation. However, the results are promising and show that peak wall stress in aneurysms can be calculated from routinely performed CT scans. Early indications are that these stresses may be a better predictor of risk of rupture than aneurysm diameter on an individual patient basis.