Correlation between vertical misfits and stresses transmitted to implants from metal frameworks

Abstract

An inappropriate prosthetic fit could cause stress over the interface implant/bone. The objective of this study was to compare stresses transmitted to implants from frameworks cast using different materials and to investigate a possible correlation between vertical misfits and these stresses. Fifteen one-piece cast frameworks simulating bars for fixed prosthesis in a model with five implants were fabricated and arranged into three different groups according to the material used for casting: CP Ti (commercially pure titanium), Co-Cr (cobalt-chromium) or Ni-Cr-Ti (nickel-chromium-titanium) alloys. Each framework was installed over the metal model with all screws tightened to a 10 N cm torque and then, vertical misfits were measured using an optical microscope. The stresses transmitted to implants were measured using quantitative photoelastic analysis in values of maximum shear stress (τ), when each framework was tightened to the photoelastic model to a 10 N cm standardized torque. Stress data were statistically analyzed using one-way ANOVA and Tukey's test and correlation tests were performed using Pearson's rank correlation (α = 0.05). Mean and standard deviation values of vertical misfit are presented for CP Ti (22.40 ± 9.05 μm), Co-Cr (66.41 ± 35.47 μm) and Ni-Cr-Ti (32.20 ± 24.47 μm). Stresses generated by Co-Cr alloy (τ = 7.70 ± 2.16 kPa) were significantly higher than those generated by CP Ti (τ = 5.86 ± 1.55 kPa, p = 0.018) and Ni-Cr-Ti alloy (τ = 5.74 ± 3.05 kPa, p = 0.011), which were similar (p = 0.982). Correlations between vertical misfits and stresses around the implants were not significant as for any evaluated materials.

Technical complications with implant-supported dental prostheses

“…This is in disagreement with findings reported in the literature that state that discrepancies in the fit between a prosthesis and abutment can be overcome by the use of a bonding agent, thereby conferring greater stability to the set of components (17). It should be stressed that the abovementioned studies are laboratory studies (13,18) and bonding in such cases is performed under ideal, pre-established conditions. A number of factors can affect the bonding of prostheses on both teeth and implants, such as the preparation of the abutment, the taper of the abutment, surface area, surface roughness, type of cement and experience of the dentist (19).…”

“…Forming sections and soldering structures can minimize discrepancies, but may not assure passive adaptation (10). Although there is no consensus regarding the degree of clinically-acceptable misfit (13), a fit which is not capable of causing any complication in the long term is defined as passive. It has been suggested that misfits smaller than 150µm are acceptable (8).…”

Adaptação de infraestruturas metálicas de próteses fixas sobre implantes instalados na maxila por meio da cirurgia virtual guiada

“…A passividade ou adaptação passiva é caracterizada pela existência de contato circunferencial simultâneo de toda a superfície de assentamento da prótese com todos os pilares de suporte, com apenas um dos parafusos distais apertados (Torres et al, 2011).…”

“…Está claro na literatura que desajustes podem provocar complicações biomecânicas, como fratura de componentes do sistema, afrouxamento dos parafusos, reabsorções ósseas, alterações dos tecidos moles e até mesmo perda da osseointegração (Brånemark, 1983;Goll, 1991;Wee et al, 1999;Torres et al, 2011).…”

“…Esses achados reforçam os relatos da literatura (Goll, 1991;Tan, 1995;Torres et al, 2011) Esses desajustes das peças protéticas, assim como os desvios lineares e angulares dos implantes na cirurgia virtual guiada, podem estar relacionados às diferentes etapas envolvidas no processo de fabricação, incluindo desde a aquisição e manipulação das imagens da TC até o acabamento e polimento da prótese final (Tan, 1995;Castilio et al, 2006).…”