Dinesh Gundapaneni

Finite element analysis of stress and wear characterization in total ankle replacements.

Total Ankle Arthroplasty is performed in order to reduce the pain and loss of ambulation in patients with various forms of arthritis and trauma. Although replacement devices fail by a number of mechanisms, wear in the polyethylene liner constitutes one of the dominating failure modes. This leads to instability and loosening of the implant. Mechanisms that contribute to wear in the liners are high contact and subsurface stresses that break down the material over time. Therefore, it is important to understand the gait that generates these stresses. Methods to characterize and decrease wear in Ohio Total Ankle Replacements (TARs) have been performed in this research. This research utilizes finite element analysis of Wright State University (WSU) patented TAR models. From the Finite element analysis (FEA) results, mathematical models of contact conditions and wear mechanics were developed. The maximum wear rate values obtained in the study (at 25.598MPa, 3.74mm(3)/year) and maximum surface Mises stress obtained with new optimization model (11.52MPa) seem to be comparable with the maximum wear values obtained in other similar studies. These models were used to determine the best methods for wear characterization and reduction. Furthermore, optimization models were developed based on geometry of the implants. These equations optimize geometry, thus congruency and anatomical simulations for total ankle implants.

Wear characteristics of WSU total ankle replacement devices under shear and torsion loads

BACKGROUND There are several factors that contribute to the failure of total ankle replacement (TAR). Aseptic loosening is one of the primary mechanisms of failure in TAR. Since a cross-linked ultrahigh molecular weight polyethylene (UHMWPE) is used as liner material, there is a need to quantify and develop methods to estimate the wear rates of the liners. High contact stresses develop during the gait generates wear debris resulting in osteolysis and early loosening of the prostheses. METHODS In this paper wear characteristics of Wright State University (WSU) TARs were determined by applying shear and torsion loads. Viscoelastic properties were used to model the liner component. Finite element analysis was conducted to determine the wear rate by deriving Von Mises and contact stresses generated in the liner and wear rate equation was used to predict the wear rate. RESULTS Titanium alloy has shown less resistance towards shear forces when compared with other metal alloys. Under torsion, rotation angle plays a significant role in affecting the peak stress values. The maximum average contact stress was 14.46 MPa under torsion load which contributes to a wear rate of 0.67 (mm(3)/year) for one of the mobile bearing models. The maximum average contact stress and wear rate obtained from the analytical study were 10.55 MPa and 0.33 (mm(3)/year), respectively for mobile bearing models. When compared with mobile bearing model, fixed bearing model has shown higher stresses at different degrees of rotation. CONCLUSION Both shear and torsion loads cause significantly lower contact stresses and wear when compared to the axial load. Further studies are necessary to accurately determine the wear behavior of fixed bearing TAR models.

Effect of cement fill ratio in loosening of hip implants

Femoral loosening is one of the most prevalent causes of revision orthopedic surgeries. Cement mantle thickness has been directly correlated with femoral loosening. If the mantle is too thick, there is an increased risk of radiolucent lines and inconsistent densities. Also, the more bone that is reamed out during the procedure can lead to instability, especially if the quality of the bone is compromised due to osteoporosis. Too thin of a mantle can lead to a higher probability for cement fracture, loosening the prosthetic even further. This study has shown that there is an ideal thickness range between two to five that should be kept. From radiographic images one can measure the thickness of the cement mantle showing the loosening characteristics.

Thermal isotherms in PMMA and cell necrosis during total hip arthroplasty.

Background Polymethylmethacrylate (PMMA), also known as bone cement, is a commonly used adhesive material to fix implants in Total Hip Arthroplasty (THA). During implantation, bone cement undergoes a polymerization reaction which is an exothermic reaction and results in the release of heat to the surrounding bone tissue, which ultimately leads to thermal necrosis. Necrosis in the bony tissue results in early loosening of the implant, which causes pain and reduces the life of the implant. Purpose The main objective of the present study was to understand the thermal isotherms in PMMA and to determine the optimal cement mantle thickness to prevent cell necrosis during THA. Methods In this study, the environment in the bony tissue during implantation was simulated by constructing 3D solid models to observe the temperature distribution in the bony tissue at different cement mantle thicknesses (1 mm, 3 mm and 5 mm), by applying the temperature conditions that exist during the surgery. Stems made with Co-Cr-Mo, 316L stainless steel and Ti6Al4V were used, which acted as heat sinks, and a thermal damage equation was used to measure the bone damage. FEA was conducted based on temperature conditions and thermal isotherms at different cement mantle thicknesses were obtained. Results Thermal isotherms derived with respect to distance in the bony tissue from the center of the cement mantle, and cell necrosis was determined at different mantle thicknesses. Based on the deduced results, cement mantle thickness of 1-5 mm does not cause thermal damage in the bony tissue. Conclusion Considering the long term stability of the implant, cement mantle thickness range from 3 mm-5 mm was found to be optimal in THA to prevent cell necrosis.

Simulation of ankle joint kinematics in sagittal plane using passive imaging data – a pilot study

Abstract Purpose: The purpose of this pilot study was to determine the radius of curvature of the tibia and talus, and to deduce ankle joint kinematics in the sagittal plane using passive imaging d...

Applicability of PEEK and its composites in total ankle replacement devices and wear rate predictions

PEEK and carbon fiber reinforced PEEK (CFR-PEEK) materials have emerged as leading high-performance materials for replacing metal components in orthopedic devices. In this study these materials were critically evaluated as main load bearing components in total ankle replacement (TAR) devices. Wear characterization was determined by conducting finite element analysis on second generation Wright State University (WSU) TAR devices. Maximum von Mises stress value of 25.06 MPa was obtained for the optimized model with CFR-PEEK/UHMWPE as tibial/talar and bearing components, which is comparable with stress value of 26.85 MPa obtained for Ti-6Al-4V/UHMWPE material model. A similar trend was observed for contact stress values as well, where an average contact stress value of 6.61 MPa was obtained for CFR-PEEK/UHMWPE model which corresponds to a wear rate of 0.03 (mm3 yr−1). Based on the obtained stress and wear results, and in the absence of PEEK debris causing osteolysis data, CFR-PEEK was recommended as a potential alternative to Titanium alloy in TAR devices. Further wear simulator testing is necessary to accurately determine the wear behavior of TAR models.