Application of the advanced system for implant stability testing (ASIST) to natural teeth for noninvasive evaluation of the tooth root interface.

In this paper we present the development of the Advanced System for Implant Stability Testing (ASIST) for application to natural teeth. The ASIST uses an impact measurement combined with an analytical model of the system and surrounding support to provide a measure of the interface stiffness. In this study, an analytical model is developed for a single-rooted natural tooth allowing the ASIST to estimate the stiffness characteristics of the periodontal ligament (PDL). The geometry and inertia parameters of the tooth model are presented in two ways: (1) using full CT scans of the individual tooth and (2) using an approximate geometry model with estimates of only the tooth length and diameter. The developed system is evaluated with clinical data for patients undergoing orthodontic treatment. This study shows that ASIST technique can be applied to natural teeth to estimate the stiffness characteristics of the PDL. The developed system can provide a valuable clinical tool for assessment of tooth stability properties and PDL stiffness in a variety of clinical situations such as dental trauma, orthodontics, and periodontology.

Advanced System for Implant Stability Testing (ASIST).

This study presents the Advanced System for Implant Stability Testing (ASIST) which provides a non-invasive, quantitative measure of the stability of percutaneous implants used for craniofacial rehabilitation such as bone anchored hearing aids or dental implants. The ASIST uses an impact technique coupled with an analytical model which allows the measure to be independent of the system components. This paper presents a laboratory evaluation of the ASIST for the Oticon Medical Ponto and the Cochlear Baha Connect bone anchored hearing aid (BAHA) systems. There is minimal effect of abutment length on the ASIST Stability Coefficient (ASC) value, indicating that the method is able to isolate the interface properties from the overall system and the measurement is independent of attached components. Additionally, the ASIST was able to detect differences between different implant installations suggesting that it may be sensitive to changes in interface stiffness.

A dynamic analytical model for impact evaluation of percutaneous implants.

The ongoing need for a clinically effective noninvasive technique for monitoring implant stability has led to a number of testing methods based on the concept of resonant frequency. Resonant frequency measurements are an indirect measure of the bone-implant interface integrity and do not provide any specific measures of the physical properties of the interface itself. In this study, an analytical model has been developed to interpret the measurement results of an impact testing method based on the Periotest handpiece. Model results are compared to a variety of in vitro tests to verify model predictions and to gain an understanding of the parameters influencing the measurements. Model simulations are then used to predict how changes in the supporting stiffness properties, material loss around the neck of the implant, and the presence of an implant flange will affect the measurements. The developed analytical model, in conjunction with the impact measurements, allows direct estimation of the bone properties that support implants. Model simulations show the impact testing technique to be sensitive to bone loss and stiffness changes that would correspond to poorly integrated implants (ones which may be in danger of failing). Similarly, for implants with very stiff support, little useful quantitative data can be obtained about the bone supporting the implant, as the stiffness of the other components of the system dominate the response. However, such implants are generally considered healthy.

Simulation of impact test for determining "health" of percutaneous bone anchored implants.

There is an ongoing requirement for a clinically relevant, noninvasive technique to monitor the integrity of percutaneous implants used for dental restorations, bone-anchored hearing aids, and to retain extra-oral prostheses (ear, eye, nose, etc). Because of the limitations of conventional diagnostic techniques (CT, MRI), mechanical techniques that measure the dynamic response of the implant-abutment system are being developed. This paper documents a finite element analysis that simulates a transient response to mechanical impact testing using contact elements. The detailed model allows for a specific interface between the implant and bone and characterizes potential clinical situations including loss of bone margin height, loss of osseointegration, and development of a soft connective tissue layer at the bone-implant interface. The results also show that the expected difference in interface stiffness between soft connective tissue and osseointegrated bone will cause easily measurable changes in the response of the implant/abutment system. With respect to the loss of bone margin height, changes in the order of 0.2 mm should be detectable, suggesting that this technique is at least as sensitive as radiography. A partial loss of osseointegration, while not being as readily evident as a bone margin loss, would still be detectable for losses as small as 0.5 mm.

Three-dimensional force systems from vertically activated orthodontic loops.

When both vertical alignment and first-order rotation of teeth are to occur simultaneously, a 3-dimensional force system is required. This numerical study evaluated several appliances (rectangular loops and L-loops) used to vertically align teeth. Consideration was given to how these designs might be modified to produce the appropriate force system to allow both movements to occur simultaneously. It was found that the rectangular loop was the most appropriate choice for first-order corrections. For the rectangular loops studied, the in-plane force system was shown to be essentially independent of the out-of-plane effects, which allowed the 2 corrections to be controlled separately.

Simulation of the superelastic response of SMA orthodontic wires.

Shape-memory alloys have properties that make them well suited to a variety of applications. One application for which their unique combination of properties (large elastic range, low modulus of elasticity, ability to deliver nearly constant forces over a wide range of deformations) seems ideally suited is for orthodontic retraction appliances where these properties are very desirable. The mechanical response of shape-memory alloys is modeled by a simple constitutive model that captures the essential superelastic behavior of the shape-memory wires. An initial value approach that iteratively converges to the appropriate boundary conditions is utilized to deliver numerical solutions. Qualitative agreement is shown with previous experimental works. The possible benefits of using such wires in an orthodontic retraction appliance are then investigated.

Three-dimensional effects in retraction appliance design.

The clinical importance of the three-dimensional effects of the force systems supplied by appliance designs used for retraction has long been appreciated. However, quantification of these force systems is not as well known. In this work, a numerical method is used to provide quantitative insight into three-dimensional effects for typical appliance designs. One problem that occurs clinically is the axial rotation of a single rooted tooth as a result of the forces being applied by the retraction device on the tooth's buccal surface. An out-of-plane preactivated bend can be used to counteract this rotation. The proposed numerical method can accurately determine the force systems resulting from this out-of-plane preactivation as well as the in-plane force systems. It is shown that the out-of-plane effects are independent of the in-plane behaviour so that the usual forces and moment to force ratios are maintained.