A numerical study towards shape memory alloys application in orthotic management of pediatric knee lateral deviations.

Exerting a constant load would likely improve orthosis effectiveness in treating knee lateral deviations during childhood and early adolescence. Shape memory alloys are potential candidates for such applications due to their so called pseudoelastic effect. The present study aims to quantitatively define the applicable mechanical loads, in order to reduce treatment duration while avoiding tissular damage and patient discomfort. This is essential for performing a more efficient design of correction devices. We use a patient-specific finite elements model of a pediatric knee to determine safe loading levels. The achievable correction rates are estimated using a stochastic three-dimensional growth model. Results are compared against those obtained for a mechanical stimulus decreasing in proportion to the achieved correction, emulating the behavior of conventional orthoses. A constant flexor moment of 1.1 Nm is estimated to change femorotibial angle at a rate of (7.4 ± 4.6) deg/year (mean ± std). This rate is similar to the achieved by more invasive growth modulation methods, and represents an improvement in the order of 25% in the necessary time for reducing deformities of (10 ± 5) deg by half, as compared with conventional orthoses.

A tool for solving bone growth related problems using finite elements adaptive meshes.

Long bones geometry changes in response to longitudinal growth in the epiphyseal plates and hydroxyapatite apposition in the periosteum. Due to its relevance for growth modulation and orthotics performance, researchers have extensively modeled these phenomena, using the finite elements method for it almost since the introduction of modern computers. This is a rather complex task that, besides the inherent difficulty of solving the models equations, requires considering a moving boundary. Here, the development of a new computational tool for its resolution is described. A generalized formulation of these problems is established based on the most common approaches taken in the literature and a novel finite elements algorithm is proposed for its resolution. The later allows a significant reduction of the spatial discretization requirements, the computational cost and the numerical errors associated with more classical approaches. The potentiality of the method is demonstrated by its application to three cases of practical interest, namely, hemiepiphysiodesis treatment, growth in the distal femur and bone remodeling around hip prosthesis. Eight relevant cases of study and an open source implementation of the proposed algorithm are also provided as supplementary material.

Mechanobiological based long bone growth model for the design of limb deformities correction devices.

A mechanobiological model of bone growth aimed for the design of medical devices for the treatment of limb deformities during childhood and adolescence was developed. Dimensional analysis was introduced as a tool for the systematic evaluation of the influence attributed to different factors that might modify the bone growth process. Simplifications were proposed, allowing the reduction of bone growth relevant parameters to four non-dimensional numbers, representing the chondrocyte sensitivity to stress, the epiphyseal plate geometry, the bone rigidity and the time. Benchmark situations considered for model validation were bone growth under normal conditions and an epiphyseal stapling treatment. A finite elements approach was used to analyze bone growth in the distal portion of the femur. Results are shown to be consistent with corresponding clinical data published in the literature, which indicates the potential of the here proposed method for the design of specific devices and treatments.

Fretting damage of Ni-rich ultrafine grained NiTi superelastic wires.

The effects of fretting on Ni-rich ultra-fine grained NiTi superelastic wires have been characterized. Fretting tests have been performed using wire on wire in 90° cross-cylinder configuration until 105 cycles in air at 25 °C. Constant displacement amplitude of 50 µm and normal loads of 10, 20 and 50 N were considered. For a normal load of 10 N, the tribosystem performed in Gross Slip Regime and the predominance of wear damage was observed. Mixed Fretting Regime was instead observed for normal loads of 20 N and 50 N. In these cases, the predominant damage mechanism was crack formation with the cracks oriented normal to the displacement direction. Occurrence of martensitic transformation in the contact region was inferred from the particular shape of the fretting loops. Due to their possible impact on biocompatibility, the debris detached from the tribosystem during the different experiments were collected and characterized by TEM. They consisted in agglomerations of nano-crystalline TiO2 (rutile) and NiO oxide particles sized between 10 and 20 nm.