Gurunathan Saravana Kumar

An experimental and theoretical investigation of surface roughness of poly-jet printed parts

The present study has attempted to investigate and model surface roughness on parts printed using a poly-jet additive manufacturing system. Initially the study investigated the effect of layer thickness, local surface orientation and finish type on surface roughness in poly-jet printed parts. The study shows that the surface orientation and finish type are the major factors affecting surface roughness of poly-jet parts. Then a detailed experimental study was conducted by varying surface orientation in very close intervals to obtain the surface roughness distribution. The study reveals that surface roughness distribution for poly-jet parts is different from that obtained for parts made by other additive manufacturing processes. A detailed experimental and theoretical analysis of droplet geometry, as formed by the jetting process, and its effect on the edge profile of the polymerised layer is presented. A surface roughness prediction model is proposed based on these studies and validated using profilometric measurements.

Personalised bone tissue engineering scaffold with controlled architecture using fractal tool paths in layered manufacturing

The scaffolds for bone tissue engineering should consider the functional requirements such as the external shape of the replacement, porosity for vessel and nutrient conduit, and stiffness in order to avoid stress shielding and to stimulate growth of the new tissue. Layered manufacturing (LM) has shown great promise in fabricating such porous bone scaffold. The present work proposes a biomimetic design and LM of patient- and site-specific controlled porosity scaffolds for optimised mechanical properties for repair and regeneration of bone. Correlation models between porosity and modulus for bone, and known biomaterials processable by LM are used to estimate the site-specific porosity requirements in the scaffold model. A novel method for generating a tool path using space-filling fractal curves eliminates representation difficulties associated with LM of porous objects. A representative study of a hydroxyapatite scaffold for a cortical bone defect site in human femur is presented to illustrate the methodology.

Fractal raster tool paths for layered manufacturing of porous objects

The present work describes an approach for layered manufacturing (LM) of porous objects using an appropriate modelling scheme, a pre-processing algorithm for slicing and a raster tool path generation based on the porosity information. Initially an overall framework of modelling and data transfer that includes controlled porosity information apart from the external geometry of porous objects and its transfer for LM is presented. A novel raster path generation methodology using space-filling fractal curves for LM of porous models is presented later. Specifically, the geometry and space-filling characteristics of fractal curves are studied for application to raster tool path generation in LM. Finally, boundary-constrained raster patterns are generated based on the surface geometry. The resulting data can be translated into a machine language file that can be imported by an LM system. Case studies are presented to illustrate the efficacy of this approach.

Reconstruction of subject-specific human femoral bone model with cortical porosity data using macro-CT

Tissue engineering scaffolds, particularly those for femoral bones, have a very wide application owing to the nature and frequency of damage to the femur in aged people. The present work envisages a modelling method for design and manufacturing of femoral bone scaffolds with subject-specific external geometry as well as internal porous architecture. Specifically a 3D reconstruction methodology is proposed incorporating cortical bone porosity properties by using macro-CT scan data. To accomplish this, statistical models for correlations and stochastic distributions are developed to predict the cortical porosity and pore size distribution from the CT numbers. The models are developed from histological observation and analysis of CT images of a limited number of cadaver femurs. The statistical validity of these models has been tested and an example is illustrated to describe the complete model reconstruction protocol.

Optimization of custom cementless stem using finite element analysis and elastic modulus distribution for reducing stress-shielding effect:

This work proposes a methodology involving stiffness optimization for subject-specific cementless hip implant design based on finite element analysis for reducing stress-shielding effect. To assess the change in the stress–strain state of the femur and the resulting stress-shielding effect due to insertion of the implant, a finite element analysis of the resected femur with implant assembly is carried out for a clinically relevant loading condition. Selecting the von Mises stress as the criterion for discriminating regions for elastic modulus difference, a stiffness minimization method was employed by varying the elastic modulus distribution in custom implant stem. The stiffness minimization problem is formulated as material distribution problem without explicitly penalizing partial volume elements. This formulation enables designs that could be fabricated using additive manufacturing to make porous implant with varying levels of porosity. Stress-shielding effect, measured as difference between the von Mises stress in the intact and implanted femur, decreased as the elastic modulus distribution is optimized.

Effect of various factors on pull out strength of pedicle screw in normal and osteoporotic cancellous bone models

Lower back pain is treated by bone fusion between adjacent vertebrae typically using instrumentation to immobilize and stabilize the spine till bony fusion takes place. Instrumentation is effected surgically using pedicle screws. However breakage and loosening of pedicle screws from the vertebral body are two main clinical concerns particularly for osteoporotic patients. The objective of the present study is to evaluate the effect of bone density, insertion depth and insertion angle of pedicle screw on the pull out strength and insertion torque using rigid polyurethane foam constructs. Insertion torque was measured during the insertion of the screw and pull-out tests were performed on instrumented pedicle screws and foam construct. These rigid foams represented the range of extremely osteoporotic to normal bone densities. The insertion angle represented the range of angle of pedicle and the insertion depth represented the depth to which these pedicle screws are inserted in the vertebra. The results showed that the pull out strength and insertion torque increased with increase in density and insertion depth (p<;0.05) whereas pull out strength decreased with increase in insertion angle. There was significant interaction effect of the various factors on the pull out strength and insertion torque. The findings from the current study suggest that holding power of the pedicle screws can be increased by increasing the purchase depth and reducing the angle of insertion. A statistical model was developed to predict the holding power of the screw which can assist surgeon in pre surgical planning.

Comparative Analysis of Effect of Density, Insertion Angle and Reinsertion on Pull-Out Strength of Single and Two Pedicle Screw Constructs Using Synthetic Bone Model.

Study Design Biomechanical study. Purpose To determine the effect of density, insertion angle and reinsertion on pull-out strength of pedicle screw in single and two screw-rod configurations. Overview of Literature Pedicle screw pull-out studies have involved single screw construct, whereas two screws and rod constructs are always used in spine fusions. Extrapolation of results using the single screw construct may lead to using expensive implants or increasing the fusion levels specifically in osteoporotic bones. Methods Single screw and two screw pull-out strength tests were carried out according to American Society for Testing and Materials F 543-07 on foam models to test the effect of density, insertion angle and reinsertion using poly axial pedicle screws. Results Bone density was the most significant factor deciding the pull-out strength in both single and two screw constructs. The difference in pull-out strength between single screw and two screw configurations in extremely osteoporotic bone model (80 kg/m3) was 78%, whereas in the normal bone model it was 48%. Axial pull-out value was highest for the single screw configuration; in the two screw configuration the highest pull-out strength was at 10°–15°. There was an 18% reduction in pull-out strength due to reinsertion in single screw configuration. The reinsertion effect was insignificant in the two screw configuration. Conclusions A significant difference in response of various factors on holding power of pedicle screw between single and two-screw configurations is evident. The percentage increase in pull-out strength between single and two screw constructs is higher for osteoporotic bone when compared to normal bone. Reinsertion has no significant effect on pull-out strength in the two screw rod configuration.

Patient specific parametric geometric modelling and finite element analysis of cementless hip prosthesis

This work proposes a framework for subject-specific cementless hip implant design and analysis. A set of pre-specified femoral features that can be used for custom implant design have been identified and extracted from the femur geometry reconstructed from computed tomography images. A parametric implant design is proposed based on the extracted femur features. Virtual assembly of the instantiated stem with femur model is done to check for the form fit. A finite element analysis of the resected femur with implant assembly is done for a known experimental loading condition to assess change in the stress-strain state of the femur and the stress shielding effect due to insertion of the implant. A commercially used modular implant design is chosen for the comparison studies in terms of form fit as well as stress shielding effect. The study shows that the proposed custom implant fairs well in both these aspects.

Optimisation of double wishbone suspension system using multi-objective genetic algorithm

This paper presents an application of multi-objective optimisation for the design of an important component of automobiles, namely the suspension system. In particular, we focus on the double wishbone suspension, which is one of the most popular suspensions in use today and is commonly found on midrange to high-end cars. The design of such mechanical systems is fairly complicated due to the large number of design variables involved, complicated kinematic model, and most importantly, multiplicity of design objectives, which show conflict quite often. The above characteristic of the design problem make it ideally suited for a study in optimisation using non-classical techniques for multi-objective optimisation. In this paper, we use NSGA-II [5] for searching an optimal solution to the design problem. We focus on two important performance parameters, namely camber and toe, and propose objective functions which try to minimise the variation of these as the wheel travels in jounce and rebound. The pareto-optimal front between these two objectives are obtained using multiple formulations and their results are compared.

Pull out strength calculator for pedicle screws using a surrogate ensemble approach.

BACKGROUND AND OBJECTIVE Pedicle screw instrumentation is widely used in the treatment of spinal disorders and deformities. Currently, the surgeon decides the holding power of instrumentation based on the perioperative feeling which is subjective in nature. The objective of the paper is to develop a surrogate model which will predict the pullout strength of pedicle screw based on density, insertion angle, insertion depth and reinsertion. METHODS A Taguchis orthogonal array was used to design an experiment to find the factors effecting pullout strength of pedicle screw. The pullout studies were carried using polyaxial pedicle screw on rigid polyurethane foam block according to American society for testing of materials (ASTM F543). Analysis of variance (ANOVA) and Tukeys honestly significant difference multiple comparison tests were done to find factor effect. Based on the experimental results, surrogate models based on Krigging, polynomial response surface and radial basis function were developed for predicting the pullout strength for different combination of factors. An ensemble of these surrogates based on weighted average surrogate model was also evaluated for prediction. RESULTS Density, insertion depth, insertion angle and reinsertion have a significant effect (p <0.05) on pullout strength of pedicle screw. Weighted average surrogate performed the best in predicting the pull out strength amongst the surrogate models considered in this study and acted as insurance against bad prediction. CONCLUSIONS A predictive model for pullout strength of pedicle screw was developed using experimental values and surrogate models. This can be used in pre-surgical planning and decision support system for spine surgeon.