The Development of Population-Wide Descriptions of Human Rib and Rib Cage Geometry

Abstract

Elderly individuals, obese individuals, and females all have greater risk of rib fractures and other associated thoracic injuries than younger mid-sized male adults in motor vehicle crashes. Differences in body morphology between these vulnerable populations and the subjects represented by physical or computational human body models is a potential source for this risk disparity, and efforts are required to quantify these differences in order to protect a wider population.

We present a novel parametric shape model of the human rib centroidal path using logarithmic spirals. It provides a more accurate and efficient fit than previous models of overall rib geometry, and it utilizes direct geometric properties such rib size, aspect ratio, and "skewness" in its parameterization.

The model was fitted to 21,124 ribs from 1000 adult CT scans, and regression analyses produced a flexible rib shape model to build ribs typical for any population of a given age, height, weight, and sex. Significant differences in rib shape were quantified across populations, and a new aging effect was uncovered whereby rib span and rib aspect ratio are seen to increase with age, producing characteristically shallower and flatter overall rib shapes in elderly populations. This effect was more strongly and directly associated with age than previously documented age-related changes in rib angulation. Simulated mechanical loading of ribs showed that the specific changes in shape found with age also had implications on their ability to resist deformation. Stiffness to body-anterior loading was seen to increase with age by up to 30% across a 70-year age difference.

Finally, we place ribs into their appropriate thoracic context by building a similar parametric model of the surrounding skeleton. A modular approach is used that ensures accuracy in key geometric measures, and results show the accumulated effects on overall chest shape that come from individual variations in the ribs, spine, sternum, and their relative positions.

This study can be used to help build population-specific computational models of the thoracic rib cage. Furthermore, results provide quantitative population corridors for rib shape parameters which can be used to improve the assessment and treatment of rib skeletal deformity and disease.