Simulated impact response of a 3-D printed skull, with an ellipsoidal excision, using finite element analysis

This paper investigates methods of determining the influence of an ellipsoidal excision (14.2x11.8 mm occipital region) on the structural integrity of a human skull when exposed to impact loading. Experimental and simulation-based analyses were conducted, using 3-D printed replicas and a finite element model; both were derived from a clinical CT scan of the patient (28 YO MC, with no previous health concerns). Previous simulation studies [1] have achieved managed to predict skull fracture locations effectively for nonexcised skulls

Subject-specific functional model of hard and soft tissues; skull and spine

© Cranfield University, 2018There is a strong demand for mechanically and morphologically accurate models of the human musculoskeletal system, particularly of the spine. Such models would have multiple applications, including surgical guides, the analysis of implant fitment and design, as well as individual strength evaluation. Current standards such as the ASTM F1717 (devised for the static and dynamic testing of implants) represent complex spine morphologies using simplified blocks of homogeneous material generally constructed from ultra-high-molecular-weight polyethylene (UHMWPE). These do not attempt to replicate morphological characteristics, and therefore do not reproduce mechanical loading properties, especially when considering the complexity of vertebral bodies and their facets. The work described in this thesis investigated the creation of a compressively accurate and validated model of a lumbar motion segment, specifically the validity of technologies such as computed tomography (CT) scanning, computer-aided scan reconstruction, rapid prototyping, digital image correlation (DIC) and finite element analysis (FEA) modelling. In particular, DIC (an optical measurement method) allowed full-field measurements of the displacements and strains. This was used to determine loading paths and magnitudes during the testing procedure. To complement this approach, FEA modelling identified the location and severity of maximum strains for subsequent comparison to the DIC and mechanical testing data. All FEA models were based on CT scan datasets of the modelled cadaveric material, and were validated against the ex vivo mechanical test measurements. The research followed a number of core stages: 1. First, the applicable technologies were tested and verified, with all channels indicating closely related data. This was achieved by the compressive loading of two types of analogue skulls, allowing the validation of DIC as a data acquisition technique in complex structures. Validation against FEA models demonstrated their potential to provide further insight into the experimental results. The initial testing identified a well-defined pathway for a sample manufacturing and preparation process, making it much easier to produce reliable analogues for subsequent experiments. ii 2. In the second stage, analogue motion segments (AMss) were created using the CT scan datasets obtained from the cadaveric porcine specimens. Motion analysis provided a better understanding of the loading paths again by using DIC as an appropriate data acquisition system. Following the creation of the AMS, different materials were considered for the creation of intervertebral discs (IVDs). The mechanically most biofidelic material was selected. 3. Finally, a sensitivity study was carried out to determine a relationship between the scanning resolution and model accuracy for both the mechanical analogue and the FEA model. The use of 3D printing was found to be an effective, efficient and economical strategy for the creation of accurate biomechanical analogues. Furthermore, DIC was a useful tool when looking at individual component strains and displacements. Finally, when considering a motion segment, the majority of the elastic loading – and thus its behaviour on the whole – was governed by the material properties of the IVD simulant. This research demonstrated a clear path towards the creation of a reliable, biofidelic motion segment, or even a partial lumbar spine analogue, that would comply in dynamic and static loading scenarios as well as conformity in compression. The capability of the techniques and the compliance and accuracy of the resulting models was confirmed by developing both analogue mechanical models and FE simulations. Given their potential advantages, it is only a matter of time before mechanical analogues and their corresponding digital models replace the outdated and inaccurate testing standards in our current medical facilities and research centres

On the shock behaviour and response of Ovis Aries vertebrae

When investigating a biological system during shock loading, it is best practice to isolate different components to fully comprehend each individual part [1,2] before building up the system as a whole. Due to the high acoustic impedance of bone in comparison to other biological tissues [3] the majority of the shock will be transmitted into this medium, and as such can cause large amounts of damage to other parts of the body potentially away from the impact area

Spinal Motion Segments — II: Tuning and Optimisation for Biofidelic Performance

Most commercially available spine analogues are not intended for biomechanical testing, and the few that are suitable for using in conjunction with implants and devices to allow a hands-on practice on operative procedures are very expensive and still none of these offers patient-specific analogues that can be accessed within reasonable time and price range. Man-made spine analogues would also avoid the ethical restrictions surrounding the use of biological specimens and complications arising from their inherent biological variability. Here we sought to improve the biofidelity and accuracy of a patient-specific motion segment analogue that we presented recently. These models were made by acrylonitrile butadiene styrene (ABS) in 3D printing of porcine spine segments (T12–L5) from microCT scan data, and were tested in axial loading at 0.6 mm·min−1 (strain rate range 6×10−4 s −1 – 10×10−4 s−1 ). In this paper we have sought to improve the biofidelity of these analogue models by concentrating in improving the two most critical aspects of the mechanical behaviour: the material used for the intervertebral disc and the influence of the facet joints. The deformations were followed by use of Digital Image Correlation (DIC) and consequently different scanning resolutions and data acquisition techniques were also explored and compared to determine their effect. We found that the selection of an appropriate intervertebral disc simulant (PT Flex 85) achieved a realistic force/displacement response and also that the facet joints play a key role in achieving a biofidelic behaviour for the entire motion segment. We have therefore overall confirmed the feasibility of producing, by rapid and inexpensive 3D-printing methods, high-quality patient-specific spine analogue models suitable for biomechanical testing and practic

Spinal Motion Segments — I: Concept for a Subject-specific Analogue Model

Most commercial spine analogues are not intended for biomechanical testing, and those developed for this purpose are expensive and yet still fail to replicate the mechanical performance of biological specimens. Patient-specific analogues that address these limitations and avoid the ethical restrictions surrounding the use of human cadavers are therefore required. We present a method for the production and characterisation of biofidelic, patient-specific, Spine Motion Segment (SMS = 2 vertebrae and the disk in between) analogues that allow for the biological variability encountered when dealing with real patients. Porcine spine segments (L1–L4) were scanned by computed tomography, and 3D models were printed in acrylonitrile butadiene styrene (ABS). Four biological specimens and four ABS motion segments were tested, three of which were further segmented into two Vertebral Bodies (VBs) with their intervertebral disc (IVD). All segments were loaded axially at 0.6 mm·min−1 (strain-rate range 6×10−4 s−1–10×10−4 s−1). The artificial VBs behaved like biological segments within the elastic region, but the best two-part artificial IVD were ∼15% less stiff than the biological IVDs. High-speed images recorded during compressive loading allowed full-field strains to be produced. During compression of the spine motion segments, IVDs experienced higher strains than VBs as expected. Our method allows the rapid, inexpensive and reliable production of patient-specific 3D-printed analogues, which morphologically resemble the real ones, and whose mechanical behaviour is comparable to real biological spine motion segments and this is their biggest asset