André Castro

Long-term creep behavior of the intervertebral disk: comparison between bioreactor data and numerical results

The loaded disk culture system is an intervertebral disk (IVD)-oriented bioreactor developed by the VU Medical Center (VUmc, Amsterdam, The Netherlands), which has the capacity of maintaining up to 12 IVDs in culture, for approximately 3 weeks after extraction. Using this system, eight goat IVDs were provided with the essential nutrients and submitted to compression tests without losing their biomechanical and physiological properties, for 22 days. Based on previous reports (Paul et al., 2012, 2013; Detiger et al., 2013), four of these IVDs were kept in physiological condition (control) and the other four were previously injected with chondroitinase ABC (CABC), in order to promote degenerative disk disease (DDD). The loading profile intercalated 16 h of activity loading with 8 h of loading recovery to express the standard circadian variations. The displacement behavior of these eight IVDs along the first 2 days of the experiment was numerically reproduced, using an IVD osmo-poro-hyper-viscoelastic and fiber-reinforced finite element (FE) model. The simulations were run on a custom FE solver (Castro et al., 2014). The analysis of the experimental results allowed concluding that the effect of the CABC injection was only significant in two of the four IVDs. The four control IVDs showed no signs of degeneration, as expected. In what concerns to the numerical simulations, the IVD FE model was able to reproduce the generic behavior of the two groups of goat IVDs (control and injected). However, some discrepancies were still noticed on the comparison between the injected IVDs and the numerical simulations, namely on the recovery periods. This may be justified by the complexity of the pathways for DDD, associated with the multiplicity of physiological responses to each direct or indirect stimulus. Nevertheless, one could conclude that ligaments, muscles, and IVD covering membranes could be added to the FE model, in order to improve its accuracy and properly describe the recovery periods.

Biomechanical behaviour of cancellous bone on patellofemoral arthroplasty with Journey prosthesis: a finite element study.

Isolated patellofemoral (PF) arthritis of the knee is a common cause of anterior knee pain and disability. Patellofemoral arthroplasty (PFA) is a bone conserving solution for patients with PF degeneration. Failure mechanisms of PFA include growing tibiofemoral arthritis and loosening of components. The implant loosening can be associated with bone resorption or fatigue-failure of bone by overload. This research work aims at determining the structural effects of the implantation of PF prosthesis Journey PFJ (Smith & Nephew, Inc., Memphis, TN, USA) on femoral cancellous bone. For this purpose, the finite element method is considered to perform computational simulations for different conditions, such as well-fixed and loosening scenarios. From the global results obtained, in the well-fixed scenario, a decrease in strain on cancellous bone was noticed, which can be related to bone resorption. In the loosening scenario, when the cement layer becomes inefficient, a significant increase in cancellous bone strain was observed, which can be associated with bone fatigue-failure.These strain changes suggest a weakness of the femur after PFA.

Validation of an Open Source Finite Element Biphasic Poroelastic Model. Application to the Intervertebral Disc Biomechanics.

A great amount of experimental and numerical studies have studied the Intervertebral Disc (IVD) and proven that it presents hyper-visco-poro-elastic behavior. However, the accessibility and flexibility of the software used in numerical studies is often hampered by the proprietary nature of the FEM-packages used. A novel biphasic poroelastic formulation was implemented on a home-developed open source FE solver, in order to approach to the biomechanical behavior of the IVD in the Human Spine. Firstly, a simple consolidation test with three different u/p-c elements was compared with Terzaghi’s model. Secondly, three IVD FE models were built with the same three types of elements and tested through Heuer’s experimental protocol. Terzaghi’s test was important to validate the innovative biphasic poroelastic formulation. Such verdict was confirmed by the application of this formulation to the IVD FE model. Therefore, the in-development open source FE solver shall become an appropriate to the description of IVD.

Development of a new femoral component for patellofemoral prosthesis

The purpose for this work was to analyze the structural effects of the implantation of contemporary patellofemoral prosthesis on femur cancellous bone. In fact, studies on the state of strain/stress on patellofemoral arthroplasty are still very limited. The influence of the geometry and rigidity of the bone-implant interface is discussed, applying the Finite Element Method. One of the major reasons for the failure of currently available knee implants is the stress shielding effect. In loading situations, the bone is less requested, so the rate of bone remodeling decreases. The results showed that elastic strain is lower on bone areas close to the studied models of patellofemoral prosthesis, on the period after the implantation. Later on, bone rupture by fatigue may also occur, due to high strain, particularly on the models with fixation pins for the prosthesis. Furthermore, contact analyses showed that stability is best promoted without fixation pins.

Computational Modelling of Collagen Hydrogel

Collagen is one of the most used biomaterials in tissue engineering applications. It is abundant in biological tissues, namely, constituting approximately 30% of all musculoskeletal tissues, through the presence in the extracellular matrix. Along the years, several works tried to characterize this biomaterial, as isolated fibres or as part of hydrogels, but the fragility of this material has been deferring its full characterization. This chapter presents an experimental and numerical characterization of a highly hydrated collagen hydrogel, with 0.20% of collagen concentration. The experimental data obtained through rheology experiments with three hydrogel samples was complemented with numerical simulation through finite element techniques. This framework was complemented with literature data, resulting on a set of biomechanical parameters that can describe the hydrogel behaviour, namely, a shear modulus of 0.023 kPa and a bulk modulus of 0.769 kPa, corresponding to a Poisson’s ratio of 0.485. The ultimate aim for this work is to contribute for the determination of the load transfer mechanisms through a collagen medium, i.e. to understand the contribution of collagen on the mechanical stimuli that affect cell behaviour on scaffold cell seeding.