Finite Element Modelling of Intramural Haematoma in the Thoracic Aorta – size matters

P Sastry1, R. Aihong Zhao, M Field , D Richens

• ‘[the] plan of urgent surgery for unstable patients and initial medical treatment for stable patients with surgery reserved for those who develop complications seems reasonable......

• [an] imaging variable with prognostic significance, reported by Asian groups, is IMH thickness between 10 and 15 mm

Objective

1. To design a computational model of the thoracic aorta

2. Program the model to behave according to known properties of aortic tissue and blood flow interaction

3. Implant clot of varying size in the media, and observe the impact on stress in each layer (intima, media, adventitia)

Designing the model• The Arbitrary Lagrangian

Eulerian (ALE) algorithm is a well established methodology for modelling tissue-fluid interactions, and was utilised for our study.

• The model consists of a three-layer aortic wall, an ellipsoid blood clot, blood in the lumen, and ambient inlet pressure

schematic of the aorta model

• The thicknesses of the three layers are intima 0.33mm, media 1.32mm and adventitia 0.96 mm

• A blood clot was planted within the media, and given an ellipsoid shape with three axes denoted as a, b and c in x, y and z axis

• A series of clots of different dimensions were studied• Inlet pressure was assigned to be 117 mmHg• Mechanical properties of aorta, clot and flowing blood

were taken from published literature

Evaluating aortic wall stress

• Output measure was ‘von Mises’ stress.... – the resultant stress experienced at a specific locale,

rationalising the effect of all the stresses acting in different directions

• ..and prinical strain– the change in dimension (caused by loading) relative to the

original length.

• These two parameters were observed for each of the three layers of aorta, as well as at different positions relative to the clot.

Model with no clotStress distribution in the intima (top), media (middle) and adventitia (bottom)

Medial Von Mises stress peaks 0.097MPa while its value in adventitia is only 0.016 MPa. At the same moment, intimal stress peaks at 0.64 Mpa

i.e. the intima is subjected to much greater loading than the other two layers

Measurements of strain follow the same pattern

Case 2, Clot 1: 2mmx5mmx5mm, 91mm33

• Although the volume of clot is only 91 mm 3, the clot changes the path of blood flow dramatically

Intima

Media

Adventitia

Case 7, Clot 6: 10mmx16mmx20mm, 6637mm33

• the stress in the intima breaches the yield stress of the intima, so that a dissection flap is created, and blood leaves the haematoma

Intima

Media

Adventitia

Abbreviated summary of results

Case Clot dimensions

(mm)

Volume (mm3) Von Mises stress

(MPa)

Principal strain

a b c

1 0 0 0 0 0.64 0.26

2 2 5 5 91 0.7 0.28

3 4 5 5 199 0.7 0.285

4 4 8.6 10 645 0.7 0.29

5 8 8 8 1064 0.7 0.29

6 10 8 10 1549 0.8 0.32

7 10 16 20 6637 0.9 0.38

• Aortic wall stresses and strains do not change much with clot volumes between 91 mm3 and 646 mm3.

• However, when the three clot radii increase to 10mm, 16mm and, 20mm in the three directions respectively, the stresses and strains dramatically increase, ultimately exerting enough pressure to breach the intima, causing a dissection in the model.

Conclusions

• Our model corroborates clinical findings that IMH diameter 10-15mm may be predictive of dissection/ rupture

• With further fine-tuning, this type of modelling could help guide management of IMH according to the anatomy presented.