Analysis of total knee replacements in a South Indian Institute

Background: Total knee replacement (TKR) is the most widely practised elective surgical procedure for advanced osteoarthritic patients globally. There is paucity of data on TKR’s in south Indian population. This study was aimed at analysing the peri-operative data of primary TKR’s to ease in customising the imported implants for our population.Methods: Data on 261 TKR’s performed in our institute over 40 months was collected. Categorical variables like age, gender, size of components used, co-morbidities and type of implants used were analysed.Results: Out of 220 patients with 261 knees, 141 were females (173 knees) and 79 were males (88 knees). The age group of the cohort was 60±11.2 years (Mean±SD). Diabetes and Hypertension (n=152) were the commonest co morbidities. The Cruciate Retaining (CR) type of prosthesis was used to the maximum extent (n=218). Size 3 femoral and tibial component was the commonly used in males and size 2 in females. 143(54.8%) knees had tibio-femoral component mismatch.  Conclusions: This study gives a reasonable insight into various aspects of the total knee replacements in a South Indian Institute. This data would be useful for manufacturing suitable implants customised for our population in the context of imported implants being in vogue. It also helps the operating surgeon to appropriately order for the desirable implants well in advance lest he should struggle intra operatively for want of better clinical outcome

Pore-resolved investigation of turbulent open channel flow over a randomly packed permeable sediment bed

Pore-resolved direct numerical simulations (DNS) are performed to investigate the interactions between streamflow turbulence and groundwater flow through a randomly packed porous sediment bed for three permeability Reynolds numbers, $Re_K$, of 2.56, 5.17, and 8.94, representative of natural stream or river systems. Time-space averaging is used to quantify the Reynolds stress, form-induced stress, mean flow and shear penetration depths, and mixing length at the sediment-water interface (SWI). The mean flow and shear penetration depths increase with $Re_K$ and are found to be nonlinear functions of non-dimensional permeability. The peaks and significant values of the Reynolds stresses, form-induced stresses, and pressure variations are shown to occur in the top layer of the bed, which is also confirmed by conducting simulations of just the top layer as roughness elements over an impermeable wall. The probability distribution functions (PDFs) of normalized local bed stress are found to collapse for all Reynolds numbers and their root mean-squared fluctuations are assumed to follow logarithmic correlations. The fluctuations in local bed stress and resultant drag and lift forces on sediment grains are mainly a result of the top layer, their PDFs are symmetric with heavy tails, and can be well represented by a non-Gaussian model fit. The bed stress statistics and the pressure data at the SWI can potentially be used in providing better boundary conditions in modeling of incipient motion and reach-scale transport in the hyporheic zone.Comment: 34 pages, 15 figures, 9 tables. arXiv admin note: text overlap with arXiv:2204.1387

A model for the degradation of polyimides due to oxidation

Polyimides, due to their superior mechanical behavior at high temperatures, are used in a variety of applications that include aerospace, automobile and electronic packaging industries, as matrices for composites, as adhesives etc. In this paper, we extend our previous model in [S. Karra, K. R. Rajagopal, Modeling the non-linear viscoelastic response of high temperature polyimides, Mechanics of Materials, In press, doi:10.1016/j.mechmat.2010.09.006], to include oxidative degradation of these high temperature polyimides. Appropriate forms for the Helmholtz potential and the rate of dissipation are chosen to describe the degradation. The results for a specific boundary value problem, using our model compares well with the experimental creep data for PMR-15 resin that is aged in air.Comment: 13 pages, 2 figures, submitted to Mechanics of Time-dependent Material

A thermodynamic framework to develop rate-type models for fluids without instantaneous elasticity

In this paper, we apply the thermodynamic framework recently put into place by Rajagopal and co-workers, to develop rate-type models for viscoelastic fluids which do not possess instantaneous elasticity. To illustrate the capabilities of such models we make a specific choice for the specific Helmholtz potential and the rate of dissipation and consider the creep and stress relaxation response associated with the model. Given specific forms for the Helmholtz potential and the rate of dissipation, the rate of dissipation is maximized with the constraint that the difference between the stress power and the rate of change of Helmholtz potential is equal to the rate of dissipation and any other constraint that may be applicable such as incompressibility. We show that the model that is developed exhibits fluid-like characteristics and is incapable of instantaneous elastic response. It also includes Maxwell-like and Kelvin-Voigt-like viscoelastic materials (when certain material moduli take special values).Comment: 18 pages, 5 figure