A pseudo-elastic effective material property representation of the costal cartilage for use in finite element models of the whole human body

Influences of human thorax variability on population rib fracture risk prediction using human body models

Rib fractures remain a common injury for vehicle occupants in crashes. The risk of a human sustaining rib fractures from thorax loading is highly variable, potentially due to a variability in individual factors such as material properties and geometry of the ribs and ribcage. Human body models (HBMs) with a detailed ribcage can be used as occupant substitutes to aid in the prediction of rib injury risk at the tissue level in crash analysis. To improve this capability, model parametrization can be used to represent human variability in simulation studies. The aim of this study was to identify the variations in the physical properties of the human thorax that have the most influence on rib fracture risk for the population of vehicle occupants. A total of 15 different geometrical and material factors, sourced from published literature, were varied in a parametrized SAFER HBM. Parametric sensitivity analyses were conducted for two crash configurations, frontal and near-side impacts. The results show that variability in rib cortical bone thickness, rib cortical bone material properties, and rib cross-sectional width had the greatest influence on the risk for an occupant to sustain two or more fractured ribs in both impacts. Therefore, it is recommended that these three parameters be included in rib fracture risk analysis with HBMs for the population of vehicle occupants.

Stiffness reduction of the rib cage to perform a minimally invasive pectus excavatum repair: biomechanical evaluation

OBJECTIVES

To study the influence of sternal transection and costal chondrotomies on the stiffness and stresses in the rib cage of adult patients undergoing Nuss pectus excavatum procedure.

METHODS

Four pectus excavatum models with different Haller indexes were created by parameterizing a 3D model of a rib cage obtained based on a computed tomography scan of a patient with no pectus deformity. Using the finite element method, insertion of intrathoracic bars into all models was simulated in 3 conditions, namely, non-intervened, transverse sternal section and costal chondrotomies. Stiffness, stress distribution and maximum stresses for each case were obtained and compared.

RESULTS

Transverse sternotomy provided a reduction of 44% to 54% in the stiffness of the rib cage, depending on the Haller index analysed, while chondrotomies promoted a stiffness reduction of 70%. Stress distribution in the rib cage followed similar pattern for all the tested Haller index, but the maximum stress decreased by 36% when performing a transverse sternotomy, whereas when performing costal chondrotomies, it decreased by 47%.

CONCLUSIONS

Computational results report that transverse sternotomy reduces appreciably the stiffness of the rib cage, while costal chondrotomies promote even a higher stiffness reduction. Thus, these surgical procedures could improve the clinical outcomes of adult patients undergoing a pectus excavatum repair.

Anisotropic and age-dependent elastic material behavior of the human costal cartilage

Compared to articular cartilage, the biomechanical properties of costal cartilage have not yet been extensively explored. The research presented addresses this problem by studying for the first time the anisotropic elastic behavior of human costal cartilage. Samples were taken from 12 male and female cadavers and unconfined compression and indentation tests were performed in mediolateral and dorsoventral direction to determine Young’s Moduli E_C for compression and E_i5%, E_i10% and E_imax at 5%, 10% and maximum strain for indentation. Furthermore, the crack direction of the unconfined compression samples was determined and histological samples of the cartilage tissue were examined with the picrosirius-polarization staining method. The tests revealed mean Young’s Moduli of E_C = 32.9 ± 17.9 MPa (N = 10), E_i5% = 11.1 ± 5.6 MPa (N = 12), E_i10% = 13.3 ± 6.3 MPa (N = 12) and E_imax = 14.6 ± 6.6 MPa (N = 12). We found that the Young’s Moduli in the indentation test are clearly anisotropic with significant higher results in the mediolateral direction (all P = 0.002). In addition, a dependence of the crack direction of the compressed specimens on the load orientation was observed. Those findings were supported by the orientation of the structure of the collagen fibers determined in the histological examination. Also, a significant age-related elastic behavior of human costal cartilage could be shown with the unconfined compression test (P = 0.009) and the indentation test (P = 0.004), but no sex effect could be detected. Those results are helpful in the field of autologous grafts for rhinoplastic surgery and for the refinement of material parameters in Finite Element models e.g., for accident analyses with traumatic impact on the thorax.

Validated thoracic vertebrae and costovertebral joints increase biofidelity of a human body model in hub impacts

Objective: A recent emphasis on nontraditional seating and omnidirectional impact directions has motivated the need for deformable representation of the thoracic spine (T-spine) in human body models. The goal of this study was to develop and validate a deformable T-spine for the Global Human Body Models Consortium (GHBMC) M50-O (average male occupant) human model and to demonstrate improved biofidelity.Methods: Eleven functional spinal units (FSUs) were developed with deformable vertebrae (cortical and trabecular), spinal and costovertebral ligaments, and intervertebral discs. Material properties for all parts were obtained from the literature.FSUs were subjected to quasistatic loads per Panjabi et al. (1976) in 6 degrees of freedom. Stiffness values were calculated for each moment (Nm/°) and translational force (N/µm). Updated costovertebral (CV) joints of ribs 2, 6, and 10 were subjected to moments along 3 axes per Duprey et al. (2010). The response was optimized by maximum force and laxity in the ligaments. In both cases, updated models were compared to the baseline approach, which employed rigid bodies and joint-like behavior. The deformable T-spine and CV joints were integrated into the full M50-O model Ver. 5.0β and 2 full-body cases were run: (1) a rear pendulum impact per Forman et al. (2015) at speeds up to 5.5 m/s. and (2) a lateral shoulder impact per Koh (2005) at 4.5 m/s. Quantitative evaluation protocols were used to evaluate the time history response vs. experimental data, with an average correlation and analysis (CORA) score of 0.76.Results: All FSU responses showed reduced stiffness vs. baseline. Tension, extension, torsion, and lateral bending became more compliant than experimental data. Like the experimental results, no trend was observed for joint response by level. CV joints showed good biofidelity. The response at ribs 2, 6, and 10 generally followed the experimental data.Conclusions: Deformable T-spine and CV joint validation has not been previously published and yielded high biofidelity in rear impact and notable improvement in lateral impact at the full body level. Future work will focus on localized T-spine injury criteria made possible by the introduction of this fully deformable representation of the anatomy.

The rabbit costal cartilage reconstructive surgical model

Rib grafts in facial plastic surgery are becoming more frequently used. Small animal models, although not ideal may be used to emulate costal cartilage-based procedures. A surgical characterization of this tissue will assist future research in the selection of appropriate costal segments, based on quantitative and qualitative properties. The objective of this study is to assess the surgical anatomy of the rabbit costal margin and evaluate costal cartilage for use in either in vivo or ex vivo studies and to examine reconstructive procedures. Detailed thoracic dissections of 21 New Zealand white rabbits were performed post-mortem. Costal cartilage of true, false, and floating ribs were harvested. The length, thickness, and width at proximal, medial, and distal locations of the cartilage, with perichondrium intact were measured. Further qualitative observation and digital images of curvature, flexibility, and segmental cross-sectional shape were recorded. The main outcome measure(s) is to characterize, describe, and assess the consistency of dimensions, location, and shape of costal cartilage. In this study, 12 to 13 ribs encase the thoracic cavity. Cartilage from true ribs has an average length, width, and depth of 23.75 ± 0.662, 3.02 ± 0.025, and 2.18 ± 0.018 mm, respectively. The cartilage from false ribs has an average length, width, and depth of 41.97 ± 1.48, 2.00 ± 0.07, 1.19 ± 0.03 mm, and that of floating ribs are 7.66 ± 0.29, 1.98 ± 0.04, and 0.96 ± 0.03 mm. Rib 8 is found to be the longest costal cartilage (49.10 ± 0.64 mm), with the widest and thickest at ribs 1 (3.91 ± 0.08 mm) and 6 (2.41 ± 0.11 mm), respectively. Cross-sectional segments reveal the distal cartilage to maintain an hourglass shape that broadens to become circular and eventually ovoid at the costochondral junction. The New Zealand white rabbit is a practical source of costal cartilage that is of sufficient size and reproducibility to use in surgical research where the long-term effects of operations, therapies, devices, and pharmacologic on cartilage can be studied in vivo.

The effect of calcification on the structural mechanics of the costal cartilage

The costal cartilage often undergoes progressive calcification with age. This study sought to investigate the effects of calcification on the structural mechanics of whole costal cartilage segments. Models were developed for five costal cartilage specimens, including representations of the cartilage, the perichondrium, calcification, and segments of the rib and sternum. The material properties of the cartilage were determined through indentation testing; the properties of the perichondrium were determined through optimisation against structural experiments. The calcified regions were then expanded or shrunk to develop five different sensitivity analysis models for each. Increasing the relative volume of calcification from 0% to 24% of the cartilage volume increased the stiffness of the costal cartilage segments by a factor of 2.3-3.8. These results suggest that calcification may have a substantial effect on the stiffness of the costal cartilage which should be considered when modelling the chest, especially if age is a factor.