Luiza Muraru

Individualised, micro CT-based finite element modelling as a tool for biomechanical analysis related to tissue engineering of bone

Load-bearing tissues, like bone, can be replaced by engineered tissues or tissue constructs. For the success of this treatment, a profound understanding is needed of the mechanical properties of both the native bone tissue and the construct. Also, the interaction between mechanical loading and bone regeneration and adaptation should be well understood. This paper demonstrates that microfocus computer tomography (microCT) based finite element modelling (FEM) can have an important contribution to the field of functional bone engineering as a biomechanical analysis tool to quantify the stress and strain state in native bone tissue and in tissue constructs. Its value is illustrated by two cases: (1) in vivo microCT-based FEM for the analysis of peri-implant bone adaptation and (2) design of biomechanically optimised bone scaffolds. The first case involves a combined animal experimental and numerical study, in which the peri-implant bone adaptive response is monitored by means of in vivo microCT scanning. In the second case microCT-based finite element models were created of native trabecular bone and bone scaffolds and a mechanical analysis of both structures was performed. Procedures to optimise the mechanical properties of bone scaffolds, in relation to those of native trabecular bone are discussed.

Influence of Implant Connection Type on the Biomechanical Environment of Immediately Placed Implants – CT-Based Nonlinear, Three-Dimensional Finite Element Analysis

PURPOSE The purpose of the present study was to evaluate the biomechanical environment of immediately placed implants, before and after osseointegration, by comparing three different implant-abutment connection types. MATERIALS AND METHODS A computer tomography-based finite element model of an upper central incisor extraction socket was constructed containing implants with either external hex, internal hex, or Morse-taper connection. Frictional contact elements were used in the bone, implant, abutment, and abutment screw interfaces in the immediately placed simulations. In osseointegrated simulations, the repair of bone alveolar defect and a glued bone-to-implant interface were assumed. By analysis of variance, the influence was assessed of connection type, clinical situation, and loading magnitude on the peak equivalent strain in the bone, peak von Mises stress in the abutment screw, bone-to-implant relative displacement, and abutment gap. RESULTS The loading magnitudes had a significant contribution, regardless of the assessed variable. However, the critical clinical situation of an immediately placed implant itself was the main factor affecting the peak equivalent strain in the bone and bone-to-implant displacement. The largest influence of the connection type in this protocol was seen on the peak equivalent stress in the abutment screw. On the other hand, a higher influence of the various connection types on bone stress/strain could be noted in osseointegrated simulations. CONCLUSIONS The implant-abutment connection design did not significantly influence the biomechanical environment of immediately placed implants. Avoiding implant overloading and ensuring a sufficient initial intraosseous stability are the most relevant parameters for the promotion of a safe biomechanical environment in this protocol.

Gait assessment during the initial fitting of customized selective laser sintering ankle foot orthoses in subjects with drop foot

Background: Recently, additive fabrication has been proposed as a feasible engineering method for manufacturing of customized ankle foot orthoses (AFOs). Consequently, studies on safety, comfort and effectiveness are now carried out to assess the performance of such devices. Objective: Evaluate the clinical performance of customized (selective laser sintering) SLS-AFOs on eight subjects with unilateral drop foot gait and compare to clinically accepted (polypropylene) PP-AFOs. Study Design: Active control trial. Methods: For each subject two customized AFOs were fabricated: one SLS-AFO manufactured following an additive fabrication framework and one thermoplastic PP-AFO manufactured according to the traditional handcraft method. Clinical performance of both AFOs was evaluated during gait analysis. Results: A significant beneficial effect of both custom-moulded PP-AFO and customized SLS-AFO in terms of spatial temporal gait parameters and ankle kinematic parameters compared to barefoot gait of adults with drop foot gait are observed. No statistically significant difference between the effect of PP-AFO and of SLS-AFO was found in terms of spatial temporal gait parameters and ankle kinematic parameters. Conclusion: AFOs manufactured through the SLS technique show performances that are at least equivalent to the handcrafted PP-AFOs commonly prescribed in current clinical practice. Clinical relevance Manufacturing personalized AFOs with selective laser sintering (SLS) in an automated production process results in decreased production time and guarantees the consistency of shape and functional characteristics over different production time points compared to the traditional manufacturing process. Moreover, it reduces the dependency of the appliance on the experience and craftsmanship of the orthopaedic technician.

Short Communication: Three-dimensional finite element models based on in vivo microfocus computed tomography: Elimination of metal artefacts in a small laboratory animal model by registration with artefact-free reference images

A method is presented to generate metal-artefact-free finite element (FE) models based on in vivo micro-CT images of bone-implant structures in the case of the Guinea Pigs tibiae. A bone-implant composite FE model was constructed based on both pre- (just before implant insertion) and post-operative (4days after implant insertion) sets of micro-CT scans. Definition of bone geometry and attribution of material properties to the volumetric elements was based on pre-operative images while post-operative scans were mainly used for registration. Standard Triangulation Language (STL) representations of implant and bone were generated after segmentation of CT images in MIMICS^(R) (Materialise). Registration was performed in 3-matic^(R) (Materialise). By taking two sets of scans at a 4days interval, undisturbed bone geometry before implant insertion and implant position after insertion were recorded. After adequate validation, FE models constructed with this method can be used to study bone adaptive response to controlled mechanical loading.

Peri-Implant Bone Adaptation Under Dynamic Mechanical Stimulation: The Guinea Pig Model

The present work is part of a larger project to analyse adaptive bone remodelling around implants that receive controlled mechanical stimulation immediately post-operatively. Percutaneous implants in the tibiae of guinea pigs are used as an implant model [1]. For evaluation, microfocus computed tomography (μCT) can be used to complement or partially substitute conventional histology [2]. In the studied model implant system, μCT-based histomorphometry can be used as a substitute for histology in regions at a distance of more than 1000 μm from the titanium implant. Within this limitation, a significant effect of mechanical stimulation can be observed also under in vivo μCT conditions. The optimally osteogenic stimulus in the studied model should cause a strain rate amplitude of 1600 microstrain/s or less in the cortical bone at a distance of 2.3 mm from the implant centre. Future work will include a detailed study of strains in the peri-implant bone with in vivo micro CT-based finite element models.Copyright

The use of a robotic gait simulator for the development of an alignment tool for lower limb prostheses

Background This tool meets the demand of prosthetists to optimise and objectify the dynamic alignment of transtibial prostheses. Nowadays, the prosthetists mainly rely on their own expertise and experience. Current methods to align the prosthesis start with a static alignment of transtibial prosthesis. This method does not take the individual patient comfort into account. Afterwards, adjustments to the alignment are done by trial and error. This is a time-consuming and exhausting activity for both prosthetist and patient.

The effect of a subject-specific AFO on the muscle activation during gait of a test subject suffering from a hemiparetic anterior muscle insufficiency in the lower leg

Background An Ankle Foot Orthosis (AFO) is commonly used in clinical practice to assist gait of patients with different pathologies. The flexibility of the AFO depends on different design characteristics while specific mechanical requirements of the AFO are correlated with patient anatomy and pathology. To this day, the correlation between AFO-design and patient pathology is mainly based on the orthopaedic technician’s experience. The aim of this study is to investigate the influence of the stiffness of an AFO on the muscle-activation pattern of a subject suffering from an anterior muscle insufficiency of the lower leg using a personalized musculoskeletal model. Materials and methods Test subject was a 40-year old male suffering from a hemiparetic anterior muscle insufficiency of the lower leg. A musculoskeletal model of the lower limbs with 23 degrees of freedom and 92 muscles was scaled in OpenSim to match the test subject’s anthropometric data [1]. Muscle-definitions were adapted to simulate the patients’ pathology. A subject specific AFO was constructed using the selective laser sintering technique [2]. The actual stiffness of the AFO was determined using finite element analysis [3] and was 258 Nm/rad. Marker trajectories of an AFO-gait were used to calculate kinematic parameters and muscle-activation during gait using the musculoskeletal model. The AFO was simulated as an angle-dependent torque around the ankle with a neutral angle of 0°. The stiffness of the AFO was varied between 150 and 350 Nm/rad in steps of 25 Nm/rad. Results