Effect of single and multilevel artificial inter-vertebral disc replacement in lumbar spine: A finite element study.

Degenerative disc disease (DDD) in lumbar spine is one of the major musculoskeletal disorders that cause low back pain (LBP). The intervertebral disc structure and dynamics of the lumbar spine are significantly affected by lumbar DDD, leading to a reduced range of motion (ROM), muscle weakness and gradual degradation. Spinal fusion and inter-vertebral disc replacement prostheses are two major surgical methods used for treating lumbar DDD. The aim of this present study is to examine biomechanical impacts of single level (L3-L4 and L4-L5) and multi level (L3-L4-L5) inter-vertebral disc replacement in lumbar spine (L2-L5) and to compare the performance with intact spine. Finite element (FE) analysis has been used to compare the mobility and stress distribution of all the models for four physiological movements, namely flexion, extension, left and right lateral bending under 6, 8 and 10 Nm moments. Spinal fusion implants completely restrict the motion of the implanted segment and increase disc stress at the adjacent levels. In contrast to that, the results single level ADR models showed closer ROM and disc stress to natural model. At the spinal segments adjacent to the implantation, single level ADR shows lower chance of disc degeneration. However, significantly increased ROM was observed in case of double level ADR.

Dosimetry of pulsed magnetic field towards attaining bacteriostatic effect on Enterococcus faecalis: Implications for endodontic therapy.

AIM: To examine in a laboratory setting the efficacy of moderate to high strength magnetic fields, as a potential bacteriostatic stimulus, against Enterococcus faecalis, one of the causative agents for infection during root canal treatments. METHODOLOGY: Four different strengths (1, 2, 3 and 4 T) of the pulsed magnetic field (PMF) were applied in thirty repetitions to bacterial suspension. A pickup coil setup was used to measure the electromotive force induced inside the bacterial suspensions. The optical density (OD) was monitored over time (for 16 h 40 min) during the post-treatment period to assess bacterial growth. Along with the change in OD values, live/dead assay, membrane depolarization study, atomic force microscopy (AFM), scanning electron microscopy (SEM) and reactive oxygen species (ROS) assay on selected samples were studied to evaluate the effect of PMFs. All results were analysed using one-way ANOVA followed by post hoc Tukey test and considered significant at p < .05. Regression analysis (at a confidence of 95%, α = 0.05) was performed on the bacterial growth and membrane depolarization studies to determine progressive changes of the outcomes. RESULTS: The peak value of the induced electromotive force was recorded as 0.25 V, for the 4 T magnetic field pulse with a pulse width of 16 ms. There was a significant arrest of bacterial cell growth after an exposure to PMFs of 1 T, 3 T and 4 T (ANOVA score: F (4, 495) =395.180 at p = .05). The image-based qualitative results of the live/dead assay using fluorescence microscopy techniques indicated that an exposure to higher PMFs (3 T/ 4 T) induced a bacteriostatic effect in a longer post-exposure timescale. Evidence of altered membrane potential within the 2 h of exposure to 4 T PMF was supported by the incidence of elevated ROS. For the ROS assay, a significant difference occurred for 4 T exposed samples (ANOVA score: calculated F (1, 3) =20.2749 at p = .05). SEM and AFM observations corroborated with the outcomes, by portraying significant membrane damage. CONCLUSION: In a laboratory setting, PMFs with higher magnitudes (3 T and 4 T) were capable of inducing bacteriostatic effects on E. faecalis.

Biomechanical Analysis to Probe Role of Bone Condition and Subject Weight in Stiffness Customization of Femoral Stem for Improved Periprosthetic Biomechanical Response.

Stress shielding due to difference in stiffness of bone and implant material is one among the foremost causes of loosening and failure of load-bearing implants. Thus far, femoral geometry has been given priority for the customization of total hip joint replacement (THR) implant design. This study, for the first time, demonstrates the key role of bone condition and subject-weight on the customization of stiffness and design of the femoral stem. In particular, internal hollowness was incorporated to reduce the implant stiffness and such designed structure has been customized based on subject parameters, including bone condition and bodyweight. The primary aim was to tailor these parameters to achieve close to natural strain distribution at periprosthetic bone and to reduce interfacial bone loss over time. The maintenance of interfacial bone density over time has been studied here through analysis of bone remodeling (BR). For normal bodyweight, the highest hollowness exhibited clinically relevant biomechanical response, for all bone conditions. However, for heavier subjects, consideration of bone quality was found to be essential as higher hollowness induced bone failure in weaker bones and implant failure in stronger bones. Moreover, for stronger bone, thinner medial wall was found to reduce bone resorption over time on the proximo-lateral zone of stress shielding, while lateral thinning was found advantageous for weaker bones. The findings of this study are likely to facilitate designing of femoral stems for achieving better physiological outcomes and enhancement of the quality of life of patients undergoing THR surgery.

Periprosthetic biomechanical response towards dental implants, with functional gradation, for single/multiple dental loss.

The differences in shape and stiffness of the dental implants with respect to the natural teeth (especially, dental roots) cause a significant alteration of the periprosthetic biomechanical response, which typically leads to bone resorption and ultimately implant loosening. In order to avoid such clinical complications, the implant stiffness needs to be appropriately adapted. In this study, hollow channels were virtually introduced within the designed implant screws for reduction of the overall stiffness of the prototype. In particular, two opposing radial gradients of increasing hollow channel diameters, i.e., outside to inside (Channel 1) and inside to outside (Channel 2) were considered. Two clinical situations of edentulism were addressed in this finite element-based study, and these include a) loss of the first molar, and b) loss of all the three molars. Consequently, two implantation approaches were simulated for multiple teeth loss - individual implantation and implant supported dental bridge. The effects of implant length, approach and channel distribution on the biomechanical response were evaluated in terms of the von Mises stress within the interfacial periprosthetic bone, under normal masticatory loading. The results of our FE analysis clearly reveal significant variation in periprosthetic bone stress between the different implant designs and approaches. An implant screw length of 11 mm with the Channel 2 configuration was found to provide the best biomechanical response. This study also revealed that the implant supported dental bridge approach, which requires lower bone invasion, results in favorable biomechanical response in case of consecutive multiple dental loss.

Computational intelligence based design of implant for varying bone conditions.

The objective is to make the strain deviation before and after implantation adjacent to the femoral implant as close as possible to zero. Genetic algorithm is applied for this optimization of strain deviation, measured in eight separate positions. The concept of composite desirability is introduced in such a way that if the microstrain deviation values for all eight cases are 0, then the composite desirability is 1. Artificial neural network (ANN) models are developed to capture the correlation of the microstrain in femur implants using the data generated through finite element simulation. Then, the ANN model is used as the surrogate model, which in combination with the desirability function serves as the objective function for optimization. The optimum achievable deviation was found to vary with the bone condition. The optimum implant geometry varied for different bone condition, and the findings act as guideline for designing patient-specific implant.

Finite Element Analysis to Probe the Influence of Acetabular Shell Design, Liner Material, and Subject Parameters on Biomechanical Response in Periprosthetic Bone.

The implant stability and biomechanical response of periprosthetic bone in acetabulum around total hip joint replacement (THR) devices depend on a host of parameters, including design of articulating materials, gait cycle and subject parameters. In this study, the impact of shell design (conventional, finned, spiked, and combined design) and liner material on the biomechanical response of periprosthetic bone has been analyzed using finite element (FE) method. Two different liner materials: high density polyethylene-20% hydroxyapatite-20% alumina (HDPE-20%HA-20%Al2O3) and highly cross-linked ultrahigh molecular weight polyethylene (HC-UHMWPE) were used. The subject parameters included bone condition and bodyweight. Physiologically relevant load cases of a gait cycle were considered. The deviation of mechanical condition of the periprosthetic bone due to implantation was least for the finned shell design. No significant deviation was observed at the bone region adjacent to the spikes and the fins. This study recommends the use of the finned design, particularly for weaker bone conditions. For stronger bones, the combined design may also be recommended for higher stability. The use of HC-UHMWPE liner was found to be better for convensional shell design. However, similar biomechanical response was captured in our FE analysis for both the liner materials in case of other shell designs. Overall, the study establishes the biomechanical response of periprosthetic bone in the acetabular with preclinically tested liner materials together with new shell design for different subject conditions.

Optimal selection of dental implant for different bone conditions based on the mechanical response.

PURPOSE: Bone quality varies from one patient to another extensively. Young's modulus may deviate up to 40% of normal bone quality, which results into alteration of bone stiffness immensely. The prime goal of this study is to design the optimum dental implant considering the mechanical response at bone implant interfaces for a patient with specific bone quality. METHOD: 3D models of mandible and natural molar tooth were prepared from CT scan data, while dental implants were modelled using different diameter, length and porosity and FE analysis was carried out. Based on the variation in bone density, five different bone qualities were considered. First, failure analysis of implants, under maximum biting force of 250 N had been performed. Next, the implants that remained were selected for observation of mechanical response at bone implant interfaces under common chewing load of 120 N. RESULT: Maximum Von Mises stress did not surpass the yield strength of the implant material (TiAl4V). However, factor of safety of 1.5 was considered and all but two dental implants survived the design stress or allowable stress. Under 120 N load, distribution of Von Mises stress and strain at the boneimplant interface corresponding to the rest of the implants for five bone conditions were obtained and enlisted. CONCLUSION: Implants exhibiting interface strain within 1500-3000 microstrain range show the best bone remodelling and osseointegration. So, implant models having this range of interface strains were selected corresponding to the particular bone quality. A set of optimum dental implants for each of the bone qualities were predicted.

Microstructure and compression properties of 3D powder printed Ti-6Al-4V scaffolds with designed porosity: Experimental and computational analysis.

The osseointegration of metallic implants depends on an effective balance among designed porosity to facilitate angiogenesis, tissue in-growth and bone-mimicking elastic modulus with good strength properties. While addressing such twin requirements, the present study demonstrates a low temperature additive manufacturing based processing strategy to fabricate Ti-6Al-4V scaffolds with designed porosity using inkjet-based 3D powder printing (3DPP). A novel starch-based aqueous binder was prepared and the physico-chemical parameters such as pH, viscosity, and surface tension were optimized for drop-on-demand (DOD) based thermal inkjet printing. Micro-computed tomography (micro-CT) of sintered scaffolds revealed a 57% total porosity in homogeneously porous scaffold and 45% in the gradient porous scaffold with 99% interconnectivity among the micropores. Under uniaxial compression testing, the strength of homogeneously porous and gradient porous scaffolds were ~47MPa and ~90MPa, respectively. The progressive failure in homogeneously porous scaffold was recorded. In parallel to experimental measurements, finite element (FE) analyses have been performed to study the stress distribution globally and also locally around the designed pores. Consistent with FE analyses, a higher elastic modulus was recorded with gradient porous scaffolds (~3GPa) than the homogenously porous scaffolds (~2GPa). While comparing with the existing literature reports, the present work, for the first time, establishes 'direct powder printing methodology' of Ti-6Al-4V porous scaffolds with biomedically relevant microstructural and mechanical properties. Also, a new FE analysis approach, based on the critical understanding of the porous architecture using micro-CT results, is presented to realistically predict the compression response of porous scaffolds.