Simpleware Case Study: Developing a FE Model for Pectus Excavatum Evaluation using Toyota’s THUMS

Application to Pectum Excavatum Research

Pectus excavatum is a structural deformity of the anterior thoracic in which the sternum and rib cage are abnormally shaped. With this condition, the chest can appear caved-in or sunken, and is present at birth or after puberty. Surgical intervention, most commonly using the Nuss method, is a minimally invasive procedure that inserts a curved metal bar under the depressed breastbone. The bar or bars then elevate the breastbone to a more conventional position, and are subsequently removed after several years.

In this study, JSOL and Nagano Children’s Hospital created a pectus excavatum model by morphing the chest of a THUMS model of a 10-year-old in relation to patient CT images, reproducing patient bone shape and pathological conditions. They then worked on simulating post-operative development by imitating the growth of costal cartilage after surgery, using the phenomenon of thermal expansion to help reproduce this physiological change.

Pectus Excavatum Model Creation Workflow

The thorax shape and features after surgery were obtained from patients using CT scanning, in order to morph the skeletal shape of the THUMS model based on these extracted features. Conditions were also added to simulate bone growth using thermal expansion. Two forms of thorax shape were targeted for this study, specifically overcorrection and flat types formed after Nuss surgeries.

Simpleware software was able to extract the thoracic shape from the CT images and apply image processing tools to accurately model the different anatomies. Comparison was made between a "normal" thorax and patient data by morphing a THUMS model of a ten-year-old to match over-corrective and flat features from the image data, such as the angle of the costal cartilage.

Simulating Thermal Expansion

For thermal expansion, it was assumed that the costovertebral joints and costal cartilage of each model are growth parts, and thermal expansion conditions were thus assigned to them for simulation. In addition, the researchers limited expansion to the longitudinal direction of the ribs to prevent expansion from thickening the bone. To control bone growth, beam elements were created for length measurement on the surface of the ribs and costal cartilage, and a setting employed to stop the temperature rise of thermal expansion at a predetermined length.

Results

The simulation results demonstrated differences in thorax growth, including how, compared with normal thorax growth, a flat thoracic wall operation model displays abnormal protrusion of a hypochondrium region as part of growth. Intercostal opening that did not occur in actual growth was also identified during the simulation but was not considered as the focus of the study was on growth near costal cartilage and costovertebral joints. In practice, there are many different factors that potentially contribute to thoracic shapes, such as ossification of costal cartilage and intercostal muscle behavior, which require further study.

Conclusions

By using the shape features extracted from patient CT data in Simpleware software, the project was able to reproduce them on the established THUMS model through morphing. 

This model set now provides the basis for further verification of how growth patterns align with clinical practice, with the goal of utilizing new simulations to help predict long-term outcomes for treatment of pectus excavatum. More generally, the Toyota THUMS model offers a valuable resource for adapting an established automotive resource to solving challenges in pre-surgical planning.