Jamal F. Nayfeh

Custom-designed orthopedic implants evaluated using finite element analysis of patient-specific computed tomography data: femoral-component case study.

BackgroundConventional knee and hip implant systems have been in use for many years with good success. However, the custom design of implant components based on patient-specific anatomy has been attempted to overcome existing shortcomings of current designs. The longevity of cementless implant components is highly dependent on the initial fit between the bone surface and the implant. The bone-implant interface design has historically been limited by the surgical tools and cutting guides available; and the cost of fabricating custom-designed implant components has been prohibitive.MethodsThis paper describes an approach where the custom design is based on a Computed Tomography scan of the patients joint. The proposed design will customize both the articulating surface and the bone-implant interface to address the most common problems found with conventional knee-implant components. Finite Element Analysis is used to evaluate and compare the proposed design of a custom femoral component with a conventional design.ResultsThe proposed design shows a more even stress distribution on the bone-implant interface surface, which will reduce the uneven bone remodeling that can lead to premature loosening.ConclusionThe proposed custom femoral component design has the following advantages compared with a conventional femoral component. (i) Since the articulating surface closely mimics the shape of the distal femur, there is no need for resurfacing of the patella or gait change. (ii) Owing to the resulting stress distribution, bone remodeling is even and the risk of premature loosening might be reduced. (iii) Because the bone-implant interface can accommodate anatomical abnormalities at the distal femur, the need for surgical interventions and fitting of filler components is reduced. (iv) Given that the bone-implant interface is customized, about 40% less bone must be removed. The primary disadvantages are the time and cost required for the design and the possible need for a surgical robot to perform the bone resection. Some of these disadvantages may be eliminated by the use of rapid prototyping technologies, especially the use of Electron Beam Melting technology for quick and economical fabrication of custom implant components.

Higher cumulative revision rate of knee arthroplasties in younger patients with osteoarthritis

This study was designed to test the hypothesis that younger patients treated for osteoarthritis and similar conditions using total knee arthroplasty and unicompartmental knee arthroplasty have a lower implant survival rate when compared with older patients. Previous studies have been done on a small number of patients and only included the younger patients. In many cases patients treated for rheumatoid arthritis have been included in the studies and exceptional survival rates have been reported. The current study compared the cumulative revision rate of the components in 33,251 patients older than 60 years and 2606 patients younger than 60 years treated with total knee arthroplasty or unicompartmental knee arthroplasty for osteoarthritis or similar conditions. Cox regression was used to compare the risk for revision between the two age groups and between gender and the effect of year of operation. The results showed a higher cumulative revision rate for the group of younger patients in all statistical analyses and the risk ratio for revision was significantly lower for the group of older patients. The risk for revision decreased for both groups when considering the year of surgery. This is probably attributable to better implant components and surgical techniques.

Finite element analyses of two antirotational designs of implant fixtures.

The purpose of this study was to compare the effect of cyclic compressive forces on loosening of the abutment retaining screw of dental implant fixtures with two different antirotational designs using the finite element analysis. A three-dimensional model of externally hexed and trichannel dental implant fixtures with their corresponding abutments and retaining screws was developed. Comparison between the two designs was carried out using finite element analysis. The results revealed that the externally hexed design has significantly higher overall stress, contact stress, and deflection compared with the trichannel design. The trichannel antirotational design has the least potential for fracture of the implant/abutment assembly in addition to its capability for preventing rotation of the prosthesis and loosening of the screw.

NONLINEAR DYNAMICS OF POLAR-ORTHOTROPIC CIRCULAR PLATES

The dynamics of nonlinear polar orthotropic circular plates with simply supported boundary condition are investigated. Kirchhoff strain displacement relations for thin plates plus next higher-order nonlinear terms (von Karman type geometric nonlinearity) are considered. Lagrangian density function and Hamiltons principle are utilized to derive Lagranges equations, from which the equations of motion and associated boundary conditions are derived. Analytical solution is obtained by the perturbation techniques and numerical solution by the Runge–Kutta method. Phase diagrams, discrete Fast Fourier Transform (FFT diagrams) and time history responses are presented for studying the forced vibration behavior. The sub-harmonic and primary resonances are studied as well as the effect of adding damping foil layers. The quadratic term in the governing equation plays a softening role on the overall behavior of the plate due to its relatively large coefficient. The increase of damping tends to smooth out the unstable region (i.e. jump phenomenon) in the system.

DEFENSE HOLE DESIGN FOR A SHEAR DOMINANT LOADED PLATE

Baseline data is produced for designing an optimum Defense Hole System (DHS) for a large plate with a circular hole in shear dominant-load range. Stress concentration associated with circular holes for tensile/shear ratio ranging from 0% to 25% is reduced by 13.5% to 16.67%, respectively. This reduction is achieved by introducing auxiliary elliptical holes (i.e., DHS) along the principal stress directions. Each pair lies along the same principal direction has the same geometry and placement on either side of the main hole. These holes are placed in the low stress regions. With such reduction in the maximum stress level, the improvement in fatigue life of a structural part can be very significant. Both redesign optimization and parametric optimization techniques are utilized to reach the optimum solutions and to generate the baseline data. Finite Element Analysis (FEA) is used to evaluate the stresses and to optimize the size and location of the DHS. The optimum cases are validated using the RGB-photoelasticity technique. Three main goals are achieved by introducing such holes: maximum stress reduction, working as crack arrest in case a crack propagates, and material reduction.

THERMALLY INDUCED DISPLACEMENT IN SIMPLY-SUPPORTED LAMINATES

Thermally-induced transverse displacement of unidirectional ply, cross-ply and anti-symmetric angle-ply composite plate is investigated. The laminates are assumed to be simply supported along the four edges. The dynamic flexural response due to sudden surface heating is examined, with emphasis on the effects of plate thickness and stacking sequence on the maximum plate deflection of a graphite-epoxy composite. The total displacement is obtained by superposition of the first mode quasi-static and dynamic solutions. The quasi-static displacement is derived using a Levy-type solution while the dynamic displacement is formulated by utilizing the Classical Lamination Theory (CLT), Galerkins Method and the Laplace Transform. The results show that thermally-induced displacements in unidirectional and cross-ply laminates for the same thickness are far less than those in angle-ply laminates.

Utilization of a dependency-tracking language to reduce computational time during multidisciplinary design optimization

Reduction in computational time was desired and achieved in optimizing a multidisciplinary missile design system. The time reduction was realized using an object-oriented, dependency-tracking, demand-driven language called the adaptive modeling language (AML). The features of this language allowing for reduction in computational time are referred to as dependency-tracking and demand-driven computations. The dependency-tracking feature keeps track of the relationship amongst properties and objects within the hierarchy. This feature ensures that only necessary computations be carried out and it also ensures that computations that have previously been performed not be carried out again so long as the input to these computations have not changed. The timesaving features of this language make it an attractive choice when performing optimizations. A computational reduction in time of between 33 and 44% was achieved in the case when the language was used in conjunction with design of experiment and response surface models. The missile design system, interactive missile design and the optimization interface are coded in AML. The efficiency of the language was studied in conjunction with design of experiment, response surface analysis, and gradient-based optimization. The advancement of the missile design software by integrating optimization functionality is also discussed.

Numerical and Perturbation Analysis of Dynamic Post-buckling of Rectangular Laminated Plates

A new phenomenon for laminated composite plates undergoing dynamic post-buckling is presented. The buckled composite plate subjected to in-plane dynamic load not only gains more non-linearity by the quadratic term, which arises from static buckling, but also changes its dynamic behavior from pure parametric vibration to combined parametric and forcing traverse vibrations. The nonlinear behavior of a simply supported laminated composite plate undergoing dynamic post-buckling is investigated. The range of bifurcation parameter (forcing frequency) for chaotic motion is obtained, and the characteristics of the dynamic behavior of the laminated composite plate in post-buckling are unveiled. Hamiltons Principle, Galerkins Method, and Lagranges Equation are utilized to obtain the equations of motion with higher-order shear deformation. The differential equation has quadratic and cubic non-linearities as well as parametric and harmonic excitation terms. The method of multiple scales is used to determine the equations that describe the second order modulation of the amplitude and phase. Floquet theory is used to analyze the stability of periodic responses.

Using Stereolithography Models in Planning Spinal Surgery

CT data of two patients was used to create computer generated 3-dimensional models of portions of their spines. One of the patients suffered from scoliosis and the other patient from tortecolis. The virtual models were then used to create stereolithography models in order to assist the planning of surgery of the two patients. The models increased the surgeons understanding of the deformations of the patients spines by providing a visualization tool. Also, the surgeon was able to use the models for patient education.Copyright

OBJECT ORIENTED MULTIDISCIPLINARY DESIGN OPTIMIZATION

Multidisciplinary Design Optimization (MDO) will be applied as a decision support tool to a large-scale industrial system. The multidisciplinary system will be modeled in an object-oriented language, Adaptive Modeling Language (AML). As it is specifically designed to model concurrent engineering systems, it will be used to capture the knowledge of the overall system including the geometry and the optimization process. This paper will discuss the advantages of modeling large-scale systems in an object-oriented language.