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A constitutive model is developed to characterize a general class of polymer and polymer-like materials that displays hyperelastic orthotropic mechanical behavior. The strain energy function is derived from the entropy change associated with the deformation of constituent macromolecules and the strain energy change associated with the deformation of a representative orthotropic unit cell. The ability of this model to predict nonlinear, orthotropic elastic behavior is examined by comparing the theory to experimental results in the literature. Simulations of more complicated boundary value problems are performed using the finite element method.

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Purpose Contemporary total knee arthroplasty femoral component designs offer various degrees of fit amongst the global population. The purpose of this study was to assess component fit of contemporary femoral component design families against multiple ethnicities.MethodsUsing a multi-ethnic dataset including Caucasian, Indian, and Korean subjects, this study investigated component fit in six contemporary femoral component design families (A: Persona™, B: NexGen®, C: Sigma®, D: GENESIS™ II, E: Triathlon®, F: Vanguard®). Component overhang/underhang was measured between the resected distal femur and its corresponding component size and compared across design families and ethnicities. The severity of overhang/underhang and propensity of downsizing due to clinically significant overhang were quantified for the overall dataset and each ethnicity.ResultsIn all the overhang cases, Designs A and B had significantly lower component overhang than the other designs (p < 0.02). In all the underhang cases, Designs C and E had significantly greater underhang than the other designs (p < 0.01). Component design influenced the occurrence (% bones) of component downsizing due to clinically significant overhang (>3 mm), with the highest incidence observed in Designs D (20.5 %) and F (17.7 %), and the lowest incidence observed in Designs A (0 %) and B (0.4 %). Variation in component fit was significantly impacted by designs (p < 0.01) but not ethnicities (n.s.).ConclusionsThe inclusion of multiple ML/AP shape offerings and the increased number of available sizes in Design A, as compared to other contemporary femoral component design families studied, result in improved femoral component fit across various ethnicities.

Although traditional constitutive models for rubbery elastic materials are incompressible, many materials that demonstrate nonlinear elastic behavior are somewhat compressible. Clearly important in hydrostatic deformations, compressibility can also significantly affect the response of elastomers in applications for which several boundaries are rigidly fixed, such as bushings, or triaxial states of stress are realized. Compressibility is also important for convergence of finite element simulations in which a rubbery elastic constitutive law is in use. Volume changes that reflect compressibility have been observed historically in both uniaxial tension and hydrostatic compression tests; however, there appear to be no data obtained from both types of tests on the same material by which to validate a compressible hyperelastic law. In this paper, we propose a new compressible hyperelastic constitutive law for elastomers and other rubbery materials in which entropy and internal energy changes contribute to the volume change. Using data from the literature, we show that this law is capable of reproducing both the pressure—volume response of elastomers in hydrostatic compression, as well as the stress—stretch and volume change—stretch data of elastomers in uniaxial tension.

PurposeThe aim of this study was to comprehensively evaluate contemporary tibial component designs against global tibial anatomy. We hypothesized that anatomically designed tibial components offer increased morphological fit to the resected proximal tibia with increased alignment accuracy compared to symmetric and asymmetric designs.MethodsUsing a multi-ethnic bone dataset, six contemporary tibial component designs were investigated, including anatomic, asymmetric, and symmetric design types. Investigations included (1) measurement of component conformity to the resected tibia using a comprehensive set of size and shape metrics; (2) assessment of component coverage on the resected tibia while ensuring clinically acceptable levels of rotation and overhang; and (3) evaluation of the incidence and severity of component downsizing due to adherence to rotational alignment and overhang requirements, and the associated compromise in tibial coverage. Differences in coverage were statistically compared across designs and ethnicities, as well as between placements with or without enforcement of proper rotational alignment.ResultsCompared to non-anatomic designs investigated, the anatomic design exhibited better conformity to resected tibial morphology in size and shape, higher tibial coverage (92 % compared to 85–87 %), more cortical support (posteromedial region), lower incidence of downsizing (3 % compared to 39–60 %), and less compromise of tibial coverage (0.5 % compared to 4–6 %) when enforcing proper rotational alignment.ConclusionsThe anatomic design demonstrated meaningful increase in tibial coverage with accurate rotational alignment compared to symmetric and asymmetric designs, suggesting its potential for less intra-operative compromises and improved performance.Level of evidenceIII.