Real time monitoring of screw insertion using acoustic emission can predict screw stripping in human cancellous bone.

BACKGROUND: To develop experience, orthopaedic surgeons train their own proprioception to detect torque during screw insertion. This experience is acquired over time and when implanting conventional/non-locked screws in osteopenic cancellous bone the experienced surgeon still strips between 38 and 45%. Technology needs to be investigated to reduce stripping rates. Acoustic-Emission technology has the ability to detect stress wave energy transmitted through a screw during insertion into synthetic bone. Our hypothesis is Acoustic-Emission waves can be detected through standard orthopaedic screwdrivers while advancing screws through purchase and overtightening in cancellous human bone with different bone mineral densities replicating the clinical state. METHODS: 77 non-locking 4 mm and 6.5 mm diameter cancellous bone screws were inserted through to stripping into the lateral condylar area of 6 pairs of embalmed distal femurs. Specimens had varying degrees of bone mineral density determined by quantitative CT. Acoustic-Emission energy and axial force were detected for each test. RESULTS: The tests showed a significant high correlation between bone mineral density and Acoustic-Emission energy with R = 0.74. A linear regression model with the mean stripping load as the dependent variable and mean Acoustic-Emission energy, bone mineral densities and screw size as the independent variables resulted in r2 = 0.94. INTERPRETATION: This experiment succeeded in testing real time Acoustic-Emission monitoring of screw purchase and overtightening in human bone. Acoustic-Emission energy and axial compressive force have positive high correlation to bone mineral density. The purpose is to develop a known technology and apply it to improve the bone-metal construct strength by reducing human error of screw overtightening.

The acoustic emission from asperity interactions in mixed lubrication.

Gears typically operate in mixed lubrication conditions, where the lubricant film is too thin to prevent opposing surface asperities from interacting with each other. The likelihood/intensity of interactions is indicated by the Λ ratio: the ratio of smooth surface film thickness to surface roughness. Researchers have asserted that asperity interactions are the predominant cause of acoustic emission (AE) in healthy gear contacts. However, direct experiments on gears have yet to yield a clear relationship between the Asperity AE (AAE) and Λ ratio, this is in part due to the complexity of gear tooth contacts. In this paper, a disc rig was used to simulate a simplified gear contact so that the fundamental relationship between AAE and Λ could be investigated more effectively. By varying oil temperature and entrainment speed, a wide spectrum of lubrication conditions was generated. In contrast to other published studies, an independent measurement technique, the contact voltage (CV), was used to verify the amount of interactions, and repeated roughness measurements were used to confirm minimal surface wear. A simple, consistent and precise relationship between AAE amplitude and Λ was identified and defined for changes from full-film to mixed lubrication. Within the mixed lubrication regime, the AAE amplitude increased exponentially as Λ decreased at all speeds tested. It was also observed that an increase in speed always resulted in an increase in AAE amplitude, independently of any changes in Λ. This direct effect of speed was modelled so that the AAE could be predicted for any combination of speed and Λ within the tested envelope. This paper links gear contact tribology and AE with new precision and clearly demonstrates the potential of using AAE as a sensitive monitoring technique for the lubrication condition of gears.

The stress reducing capacity of unfilled resin in a Class V cavity.

This study examined the stress reducing capacity of varying thicknesses of unfilled resin in a Class V cavity. A two dimensional plane strain mesh of a Class V cavity, 3 mm in diameter and 2 mm deep, was produced and the thickness of the unfilled resin layer was varied from 0 to 80 microm. A polymerization shrinkage of 1.5% was applied to the composite resin and the interfacial forces examined. The maximum shear stresses were found to occur along the pulpal floor of the restoration at the unfilled resin-dentine interface. The maximum shear stress values varied from 11.1 to 22.4 MPa and the shear stresses decreased by up to 38% as the thickness of the unfilled resin increased to 80 microm.

The surface properties of silicone elastomers Exposed to seawater.

The performances of some silicone elastomers as compliant coatings which are resistant to marine fouling have been assessed from a sea-water exposure trial covering three fouling seasons. Measurements of contact angles (polar and non-polar liquids, recently-advanced and recently-receded liquid drops and air bubbles) have been used to investigate the surface properties of materials and of coatings resistant to fouling after two years' exposure. The unmodified poly (dimethyIsiloxane) elastomer General Electric (GE) 21 was still resistant to marine settlement after three seasons and the poly(dimethyldiphenylsiloxane) GE655 only became fouled during the third season. No other unmodified material showed resistance to fouling beyond two seasons. The addition of a low-viscosity poly(dimethylsiloxane) oil to GE655 in a sufficient quantity (20 mass %) to cause blooming resulted in a material that remained free of fouling. Time-dependent behaviour by drops of all liquids on freshly prepared samples was observed in recently-advanced contact angles but not by recently-receded contact angles. With polar liquids, hard clear elastomers showed stepwise changes and also gave considerable contact-angle hysteresis effects. Immersion in water over a period of several weeks brought about a slow decrease in the hydrophobicity of all elastomers. GE21, after exposure in seawater for over two years, also showed a decrease as indicated by the contact angle of distilled water drops on its surface. The slow changes in the interfacial properties of silicones with polar liquids are attributed to rearrangements of polymer chains close to the surface, driven by the formation of hydrogen bonds between the solvent and oxygen atoms in the backbone. Penetration of the material by water gradually increases the surface energy and, sooner or later, the material becomes susceptible to fouling. For GE655, this may be delayed by incorporating with the formulation a relatively incompatible low-viscosity silicone oil.