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Simpleware Case Study: Developing a FE Model for Pectus Excavatum Evaluation using Toyota’s THUMS
Overview
Virtual testing of the human body from imaging data is important for mitigating against the ethical and practical problems of physical experiments. In this study, was adapted by JSOL and Nagano Children's Hospital from automotive safety applications to the medical challenge of modeling pectus excavatum, a type of deformity of the thorax that is also known as funnel chest.
草榴社区 Simpleware software was used in this project to create a realistic patient’s thorax structure from CT data suitable for morphing to THUMS' detailed FE model, and provided the basis for simulating the growth of costal cartilage after operations.
Highlights
- Toyota's highly detailed THUMS FE body modeling software adapted for use with medical image data and pathological studies
- Simpleware software used to generate models from CT data
- Morphing of image-based model and THUMS resulted in a detailed simulation resource
- Models created with THUMS enable future simulations of clinical options
Thanks to
- Masahiko Noguchi, Department of Plastic & Reconstructive Surgery, Nagano Children's Hospital
Development of THUMS
THUMS was first released in 2000 as the outcome of a cooperative research project between the Toyota Motor Corporation and Toyota Central R&D Labs, Inc., resulting in the world’s first virtual human body model for reproducing and analyzing whole-body injuries during vehicle collision. The resulting THUMS virtual human body software program was adapted to different body types and circumstances, incorporating the skeleton, brain, internal organs, and muscles.
Development of an occupant model in THUMS.
In addition, THUMS can accurately reproduce the shape and strength of the human body, while also offering opportunities for repeat analysis of different collision patterns. In the automotive industry, THUMS is therefore able to significantly reduce development time and ethical risks when carrying out collision experiments. Toyota made the THUMS software available free-of-charge in January 2021, with the goal of expanding its usage for car safety research. However, JSOL and Nagano Children’s Hospital have now demonstrated how the model can be applied to medical research.
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.
Research looked at deforming thorax shape with reference to actual patient bone shape.
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.
Extraction of the different thoracic shapes from CT images using Simpleware software.
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.
Simulation of thorax growth using LS-DYNA.
Changes were measured in costal cartilage and rib growth, based on CT images of ten patients without any thoracic deformities aged 10 to 15 years, and the resulting growth rate incorporated into the model as a thoracic growth factor. LS-DYNA was used to simulate the result of thorax growth for the different cases in the study.
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.
Results obtained for bone growth.
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.
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