K.R. Williams

Surface Analysis of Titanium Implants

Medical grade titanium samples were examined using X-ray photoelectron spectroscopy before and after immersion in various proteins. Additionally, an implant removed from a patient following clinical failure was examined using scanning ion and electron microscopy. The surface of the as-received samples was found to be mainly TiO2, with contaminants of H2O/OH-, calcium and nitrogen which remained after autoclaving. The immersed proteins adhered to the titanium surface, possibly via a Ca-O link. The failed clinical sample was found to be partially fibrously encapsulated with evidence of calcification. Small amounts of TiOOH were detected at the fibrous periphery, supporting the theory of Tengvall and co-workers that in vivo titanium implants are covered in a gel of this material.

Finite element analysis of stress distribution at the tooth-denture base interface of acrylic resin teeth debonding from the denture base

Acrylic resin teeth present a problem when they detach unexpectedly from the denture base resin. Detachment is caused by stress concentrations at the tooth/denture base resin interface. In this study, the finite element method was used to examine the stress distribution at this interface when a single static force that resembled incisal bite force was applied. The results indicated that irrespective of the type of acrylic resin teeth used, maximum tensile stresses were found at the palatal aspect of the interface. It is suggested that boxing the tooth in the acrylic resin will help redistribute stress concentrations favorably.

Finite element stress analysis of restored teeth

Abstract A bucco-lingual descretization of a sectioned molar tooth was developed from which a plane stain finite element analysis was utilized to determine the principal, shear and effective stresses at each of nine Guass points within each element. The model was uniformly loaded along the inclined occlusal surfaces to a level of 100 Newtons. The maximum effective and principal stresses were always at the fissure between the cusps at a value of 5.4 MPa in the enamel of sound whole tooth and 5.35 MPa in the amalgam of a pinned restoration. It was generally found that the insertion of a liner attenuates the stress dependent on cavity size. Wider cavities tend to reduce the maximum effective stresses. Stresses at the liner amalgam interface are lower than those at the tooth fissure, suggesting that fracture or deformation is unlikely to initiate from the amalgam base and propogate to the occlusal surface. Insertion of stainless steel retaining pins returns the maximum effective stress at the fissure to the higher levels found for a class I restoration.

A Dynamic and Harmonic Damped Finite Element Analysis Model of Stapedotomy

This study was undertaken in an attempt to better understand the mechanics of sound transmission at the footplate following stapedotomy. The insertion of a Teflon (polytetrafluoroethylene) stapes prosthesis introduces new constraints within the reconstructed ossicular chain which have an effect on the normal vibration patterns of the tympanic membrane. In a finite element model of the ear, constraints have been reproduced as a series of spring constants in the incus/prosthesis/footplate interfaces incorporating damping to simulate the impedance of the inner ear. At zero damping, the frequency response at the pseudo stapes footplate exhibit several maxima and minima between 800 Hz and 2.5 Hz. At higher damping values, these maxima and minima become smoothened out with two or three naturals occurring over the same frequency range. Severe ankylosis of a diseased footplate is reproduced by over-damped conditions. The umbo, incus and stapes footplate vibrate in phase with similar frequencies at light damping levels. The movement of the prosthesis at the pseudo-footplate can be large in the out of plane axis of the ossicular chain, unless sufficient support is provided at the reconstructed footplate. Clinically, this would suggest the vein graft interposed between the piston and stapedotomy hole should endow resistance and elasticity to the system.

Problems in Alloplastic Middle Ear Reconstruction

Alloplastic materials have been used in middle ear reconstruction since 1952. Biologically, biomaterials and specifically ceramics can be defined as being bioinert, bioactive or biodegradable; these terms being descriptive of the biochemical response provoked by the material in the host tissue. Various ceramics, carbons and polymers and their reactions have been studied by the author and his colleagues over the past 5 years utilising animal implantation and fibroblast culture studies aligned with experience from clinical usage. The qualities of the ideal alloplastic implant namely: biocompatibility, stabilisation and incorporation, the consistency of the implant and bioactive bonding and the biofunctionality or mechanics, are applied to the materials currently available. No single alloplastic material fulfills all criteria, the problems encountered in relation to the biomaterials are reported. A better understanding of the microstresses, fatigue and microfractures is necessary, before the ideal alloplastic biomaterial becomes available.

Investigation of the integration process of dental implants by means of a numerical analysis

OBJECTIVES The aim of this work was to present a preliminary numerical analysis of the integration process of dental implants using a finite element simulation of the dynamic response following impulse excitation. Assessment of the osseointegration process has been previously examined using a numerical approach by calculating the natural frequency of a cantilever attached to the implant. The methodology adopted in this work allows a direct measurement of the implant response following impulse loading and avoids the addition of a bulky cantilever set-up. METHODS The geometric configuration was obtained by averaging the coordinate data from tomographic scans of 14 mandibles. The materials properties were approximated from experimental analysis performed on trabecular and cortical bone tissue. A load was applied to the top of the implant in one direction resulting in an initial displacement. The implant was then freed and allowed to vibrate over approximately 10 cycles. Three fixity conditions were assumed by changing the properties of the surrounding bone ranging from full integration to a poorly integrated implant typical of the situation during bone healing following surgery. The results of the three fixity conditions were compared by calculating the fundamental displacement amplitudes and frequencies of the vibrating impact. RESULTS The calculated results indicated that the implant vibrated at a predominant frequency when partially integrated with a displacement principally in the direction of the applied impulse. However, when the implant was fully integrated a more complex vibration pattern ensued, suggesting the superposition of two or more fundamentals. SIGNIFICANCE Attention has been paid to the formulation of the numerical model for validation purposes as well as a reliable reference for the optimum interpretation of the experimental data. In this way it was possible to establish a simulation procedure to investigate the response of the tissues surrounding the implant and their properties at different stages of healing. It should be pointed out that the numerical procedures represented a valid preliminary approach to the problem and were capable of indicating a guide to the optimum design of the experimental apparatus for measurement of displacement and frequency in vivo.

Influence of titanium oxide and titanium peroxy gel on the breakdown of hyaluronan by reactive oxygen species

The molecular events occurring at the interface between titanium and connective tissue were investigated in order to help explain the unique biocompatible properties of titanium implants and their successful osseointegration into bone tissue. In this study the influence of commercially pure titanium and titanium peroxy gels on the breakdown of the connective tissue component and serum derived factor, hyaluronan, by reactive oxygen species (ROS), produced during the insertion of an implant in vivo, was examined. Hyaluronan breakdown was monitored in vitro in the presence of a hydroxyl radical flux, generated in the presence and absence of titanium powder and discs. Parallel studies examined the breakdown of hyaluronan by hydroxyl radicals in the presence of a titanium peroxy gel, prepared by incubation of the titanium powder or discs in concentrated hydrogen peroxide. The hyaluronan degradation products were separated according to their hydrodynamic size by gel exclusion chromatography. Similarly, experiments were also performed examining the degradation of 2-deoxy-D-ribose by a hydroxyl radical flux in order to demonstrate the detrimental potential of the hydroxyl radicals and to provide a measure of the effectiveness of titanium and titanium peroxy gels as scavengers of ROS. Titanium reduced the harmful effects of the hydroxyl radicals on the breakdown of hyaluronan, presumably acting as a scavenger for the reactive species, possibly by absorbing them into its surface oxide layer, which spontaneously forms on the surface. In contrast, the formation of a titanium peroxy gel from the titanium powder or on the surface of titanium discs enhanced breakdown of both the hyaluronan chains and 2-deoxy-D-ribose. The implications of these findings with regards to the biocompatible nature of the titanium and the ability of these implants to successfully osseointegrate are discussed.

Cyclical deformation behaviour of denture soft lining materials

OBJECTIVES The purpose of the study was to examine and describe the deformation behaviour of five denture soft lining materials under dynamic cyclic loading over a range of applied forces using apparatus designed to simulate masticatory performance. METHODS Five lining materials were tested over 5-min periods using an Instron tensile testing machine that was modified to measure load and strain semi-continuously during cyclic loading. RESULTS Strain energy and strain energy densities and moduli values for each material were calculated. The strain energy absorbed at separate times during the cycling period demonstrated the elastic behaviour of the materials over the 5-min simulated chewing cycle. CONCLUSIONS All materials demonstrated elastic but not perfectly linearly elastic behaviour over the chosen loading and cycling frequency. Molloplast B absorbed most energy at the lower testing loads and Novus absorbed most energy at the higher loads.

An experimental technique for determining middle ear impedance

A two-microphone technique was used to determine the middle ear impedance of a live subject. The procedure involved the application of standing wave tube theory and the assumption that the ear canal behaves like an homogeneous cylinder with plane acoustic wave propagation up to a certain frequency--2 kHz for the current analysis. During experimentation the subject lay on a bench with his head braced against a wooden fixture. Acoustic pressures were recorded from the ear canal by the use of a spectrum analyser and probe microphones with flexible tips. Resultant impedance curves show middle ear natural frequencies at 831 Hz and 1,970 Hz with high levels of damping. The reactive impedance curves show the influence of stiffness and ossicular mass on middle ear sound transmission. An advantage of the approach is that using features of the recorded data it is possible to calculate the effective probe tip to eardrum distance required for the calculation of the middle ear impedance. The two-microphone technique appears to be a promising tool for assessing healthy and diseased middle ear function.

An optical and electron microscopy study of materials implanted in the rat middle ear: I. Carbon

Glassy carbon implants in the middle ear of the rat model appear well tolerated over periods of 6 months. The implants are rapidly covered with a fibrous capsule resulting in some bonding to the surrounding tissue. At a later stage (12 wk), further development of this bond takes place through growth of bone onto the implant surface from adjoining bone lamellae. No adverse tissue reactions were found in the histological sections produced.