Fixation of a frontozygomatic fracture with a shape-memory staple

Determination of the Effect of TiN Coating on Self-Fitting Properties of Dental Implants Made of NiTi Alloy

Design and material research continues to increase dental implants' success rates, which is a widely applied treatment type. The size and morphology of the implant-bone interface are essential for implant stability. Our study produced a dental implant with two artificial tooth roots from NiTi alloy to increase the implant-bone contact surface. The properties of NiTi alloy, such as transformation temperature and composition, were determined by material characterization tests. Using NiTi alloy's shape memory effect, these artificial roots at body temperature were programmed with appropriate heat treatments for the self-fitting feature. Dental-implant-like models are coated with TiN to prevent Ni ion release. The corrosion tests were performed in Ringer's solution to determine the effect of TiN coating on Ni ion release. The nickel ion emission values showed that the TiN coating inhibited the release. In addition, it was determined that the TiN coating increased the shape memory transformation time of the NiTi alloy. In in vitro tests of NiTi and TiN-coated NiTi implants, it was observed that they completed self-fitting by deforming the trabecular bone, but the placement in the cortical bone was not complete. During the use of a shape memory implant, it should complete its transformation without contacting the cortical bone and should not cause a stress concentration.

Analysis of One-Dimensional Ivshin-Pence Shape Memory Alloy Constitutive Model for Sensitivity and Uncertainty

Shape memory alloys (SMAs) are classified as smart materials due to their capacity to display shape memory effect and pseudoelasticity with changing temperature and loading conditions. The thermomechanical behavior of SMAs has been simulated by several constitutive models that adopted microscopic thermodynamic or macroscopic phenomenological approaches. The Ivshin–Pence model is one of the most popular SMA macroscopic phenomenological constitutive models. The construction of the model requires involvement of parameters that possess inherent uncertainty. Under varying operating temperatures and loading conditions, the uncertainty in these parameters propagates and, therefore, affects the predictive power of the model. The propagation of uncertainty while using this model in real-life applications can result in performance discrepancies or failure at extreme conditions. In this study, we employed a probabilistic approach to perform the sensitivity and uncertainty analysis of the Ivshin–Pence model. Sobol and extended Fourier Amplitude Sensitivity Testing (eFAST) methods were used to perform the sensitivity analysis for simulated isothermal loading/unloading at various operating temperatures. It is evident that the model’s prediction of the SMA stress–strain curves varies due to the change in operating temperature and loading condition. The average and stress-dependent sensitivity indices present the most influential parameters at several temperatures.

Electroassisted Functionalization of Nitinol Surface, a Powerful Strategy for Polymer Coating through Controlled Radical Surface Initiation

Coating Nitinol (NiTi) surfaces with a polymer layer has become very appealing in the past few years owing to its increased attraction in the biomedical field. Although its intrinsic properties helped ensure its popularity, its extensive implementation is still hampered by its nickel inclusion, making it sensitive to pitting corrosion and therefore leading to the release of carcinogenic Ni²⁺ ions. Among all recent ways to modify NiTi surfaces, elaboration of self-assembled monolayers is of great interest as their high order confers a reinforcement of the metal surface corrosion resistance and brings new functionalities to the metal for postmodification processes. In this work, we compare the electroassisted and thermally assisted self-assembling of 11-(2-bromoisobutyrate)-undecyl-1-phosphonic acid (BUPA) to the classical immersion process on NiTi surfaces initially submitted to a hydrothermal treatment. Among all tested conditions, the electroassisted grafting of BUPA at room temperature appears to be the most promising alternative, as it allows grafting in very short times (5-10 min), thus preventing its degradation. The thus-formed layer has been proven to be sufficient to enable the surface-initiated atom transfer radical polymerization (SI-ATRP) of 2-(dimethylamino)ethyl methacrylate.

Biomechanical evaluation of shape-memory alloy staples for internal fixation-an in vitro study

Background

The field of orthopaedics is a constantly evolving discipline. Despite the historical success of plates, pins and screws in fracture reduction and stabilisation, there is a continuing search for more efficient and improved methods of fracture fixation. The aim of this study was to evaluate shape-memory staples and to compare them to a currently used implant for internal fracture fixation. Multi-plane bending stability and interfragmentary compression were assessed across a simulated osteotomy using single and double-staple fixation and compared to a bridging plate.

Methods

Transverse osteotomies were made in polyurethane blocks (20 × 20 × 120 mm) and repairs were performed with one (n = 6), or two (n = 6) 20 mm nitinol staples, or an eight-hole 2.7 mm quarter-tubular plate (n = 6). A pressure film was placed between fragments to determine contact area and compressive forces before and after loading. Loading consisted of multi-planar four-point bending with an actuator displacement of 3 mm. Gapping between segments was recorded to determine loads corresponding to a 2 mm gap and residual post-load gap.

Results

Staple fixations showed statistically significant higher mean compressive loads and contact areas across the osteotomy compared to plate fixations. Double-staple constructs were superior to single-staple constructs for both parameters (p < 0.001). Double-staple constructs were significantly stiffer and endured significantly larger loads before 2 mm gap formation compared to other constructs in the dorsoventral plane (p < 0.001). However, both staple constructs were significantly less stiff and tolerated considerably lower loads before 2 mm gap formation when compared to plate constructs in the ventrodorsal and right-to-left lateral loading planes. Loading of staple constructs showed significantly reduced permanent gap formation in all planes except ventrodorsally when compared to plate constructs.

Conclusions

Although staple fixations were not as stable as plate fixations in particular loading planes, double-staple constructs demonstrated the most consistent bending stiffness in all planes. Placing two perpendicular staples is suggested instead of single-staples whenever possible, with at least one staple applied on the compression side of the anticipated loading to improve construct stability.

Biomedical Applications of Shape Memory Alloys

Shape memory alloys, and in particular NiTi alloys, are characterized by two unique behaviors, thermally or mechanically activated: the shape memory effect and pseudo-elastic effect. These behaviors, due to the peculiar crystallographic structure of the alloys, assure the recovery of the original shape even after large deformations and the maintenance of a constant applied force in correspondence of significant displacements. These properties, joined with good corrosion and bending resistance, biological and magnetic resonance compatibility, explain the large diffusion, in the last 20 years, of SMA in the production of biomedical devices, in particular for mini-invasive techniques. In this paper a detailed review of the main applications of NiTi alloys in dental, orthopedics, vascular, neurological, and surgical fields is presented. In particular for each device the main characteristics and the advantages of using SMA are discussed. Moreover, the paper underlines the opportunities and the room for new ideas able to enlarge the range of SMA applications. However, it is fundamental to remember that the complexity of the material and application requires a strict collaboration between clinicians, engineers, physicists and chemists for defining accurately the problem, finding the best solution in terms of device design and accordingly optimizing the NiTi alloy properties.