Modal analysis for implant stability assessment: Sensitivity of this methodology for different implant designs.

OBJECTIVE: To investigate the influence of implant design on the change in the natural frequency of bone-implant system during osseointegration by means of a modal 3D finite element analysis. METHODS: Six implants were considered. Solid models were obtained by means of reverse engineering techniques. The mandibular bone geometry was built-up from a CT scan dataset through image segmentation. Each implant was virtually implanted in the mandibular bone. Two different models have been considered, differing in the free length of the mandibular branch ('long branch' and 'short branch') in order to simulate the variability of boundary conditions when performing vibrometric analyses. Modal analyses were carried out for each model, and the first three resonance frequencies were assessed with the respective vibration modes. RESULTS: With reference to the 'long branch' model, the first three modes of vibration are whole bone vibration with minimum displacement of the implant relative to bone, with the exception of the initial condition (1% bone maturation) where the implant is not osseointegrated. By contrast, implant displacements become relevant in the 'short branch' model, unless osseointegration level is beyond 20%. The difference between resonance frequency at whole bone maturation and resonance frequency at 1% bone maturation remained lower than 6.5% for all modes, with the exception of the third mode of vibration in the 'D' implant where this difference reached 9.7%. With reference to the 'short branch', considering the first mode of vibration, 61-68% of the frequency increase was achieved at 10% osseointegration; 72-79% was achieved at 20%; 89-93% was achieved at 50% osseointegration. The pattern of the natural frequency versus the osseointegration level is similar among different modes of vibration. SIGNIFICANCE: Resonance frequencies and their trends towards osseointegration level may differ between implant designs, and in different boundary conditions that are related to implant position inside the mandible; tapered implants are the most sensitive to bone maturation levels, small implants have very little sensitivity. Resonance frequencies are less sensitive to bone maturation level beyond 50%.

Influence of thread shape and inclination on the biomechanical behaviour of plateau implant systems.

OBJECTIVE: To assess the influence of implant thread shape and inclination on the mechanical behaviour of bone-implant systems. The study assesses which factors influence the initial and full osseointegration stages. METHODS: Point clouds of the original implant were created using a non-contact reverse engineering technique. A 3D tessellated surface was created using Geomagic Studio® software. From cross-section curves, generated by intersecting the tessellated model and cutting-planes, a 3D parametric CAD model was created using SolidWorks® 2017. By the permutation of three thread shapes (rectangular, 30° trapezoidal, 45° trapezoidal) and three thread inclinations (0°, 3° or 6°), nine geometric configurations were obtained. Two different osseointegration stages were analysed: the initial osseointegration and a full osseointegration. In total, 18 different FE models were analysed and two load conditions were applied to each model. The mechanical behaviour of the models was analysed by Finite Element (FE) Analysis using ANSYS® v. 17.0. Static linear analyses were also carried out. RESULTS: ANOVA was used to assess the influence of each factor. Models with a rectangular thread and 6° inclination provided the best results and reduced displacement in the initial osseointegration stages up to 4.58%. This configuration also reduced equivalent VM stress peaks up to 54%. The same effect was confirmed for the full osseointegration stage, where 6° inclination reduced stress peaks by up to 62%. SIGNIFICANCE: The FE analysis confirmed the beneficial effect of thread inclination, reducing the displacement in immediate post-operative conditions and equivalent VM stress peaks. Thread shape does not significantly influence the mechanical behaviour of bone-implant systems but contributes to reducing stress peaks in the trabecular bone in both the initial and full osseointegration stages.

Multibody modelling of ligamentous and bony stabilizers in the human elbow.

The elbow ligamentous and bony structures play essential roles in the joint stability. Nevertheless, the contribution of different structures to joint stability is not yet clear and a comprehensive experimental investigation into the ligament and osseous constraints changes in relation to joint motions would be uphill and somehow unattainable, due to the impossibility of obtaining all the possible configurations on the same specimen. Therefore, a predictive tool of the joint behavior after the loss of retentive structures would be helpful in designing reconstructive surgeries and in pre-operative planning. In this work, a multibody model consisting of bones and non-linear ligamentous structures is presented and validated through comparison with experimental data. An accurate geometrical model was equipped with non-linear ligaments bundles between optimized origin and insertion points. The joint function was simulated according to maneuvers accomplished in published experimental studies which explored the posteromedial rotatory instability (PMRI) in coronoid and posterior medial collateral ligament (PB) deficient elbows. Moreover, a complete design of experiments (DOE) was explored, investigating the influence of the elbow flexion degree, of the coronoid process and of the medial collateral ligaments (MCL) structures (anterior and posterior bundles) in the elbow joint opening. The implemented computational model accurately predicted the joint behavior with intact and deficient stabilizing structures at each flexion degree, and highlighted the statistically significant influence of the MCL structures (P<0.05) on the elbow stability. The predictive ability of this multibody elbow joint model let foresee that future investigations under different loading scenarios and injured or surgically reconstructed states could be effectively simulated, helping the ligaments reconstruction optimization in terms of bone tunnel localizations and grafts pre-loading. LEVEL OF EVIDENCE: V.