A RESONANCE FREQUENCY TECHNIQUE TO MONITOR THE INTEGRITY OF DENTAL IMPLANTS Peter Cawley, Brian Pavlakovic and David Alleyne Department of Mechanical Engineering Imperial College Exhibition Road London SW7 2BX England Neil Meredith Department of Oral and Dental Science Bristol Dental Hospital and School Lower Maudlin Street, Bristol BS 1 2LY England INTRODUCTION Titanium implants are being used increasingly to provide support for prostheses replacing missing teeth in edentulous and partially dentate patients. There is good histological evidence to show that in satisfactory implants bone forms in intimate contact with the implant surface during the healing process following fixture placement [1]. Currently, fixtures are left unloaded for a period of3-6 months following placement in order to allow this healing process to occur. It would be very valuable to be able to monitor the healing process non-invasively in order to determine more accurately when it is safe to load the implant. Similarly, during service, it would be beneficial to be able to detect the onset of problems such as an increase in the mobility of the implant due to infection, or to a decrease in the height of the bone surrounding the implant. Radiography is the most accurate non-invasive method currently available to assess the quality of the implant/tissue interface [2,3]. However, it is difficult to obtain results which are reproducible to the degree of accuracy required. Attempts have been made to use the characteristics of an impact between a striker and the implant abutment to monitor the implantlbone interface [4]. The instrument used comprises a handpiece containing a slug which is accelerated towards a tooth by an electromagnet; the contact duration of the slug on the tooth is measured using an accelerometer. It may readily be shown [5] that this contact duration is related to the stiffuess of the body being struck. However, there is little evidence to suggest that this method is sufficiently sensitive and reproducible to monitor the changes in bone formation during healing at the implant/tissue interface. An alternative technique [6,7] is to screw a beam into the implanted fixture and to measure the first flexural resonance frequency of the resulting system. This resonance frequency will be affected by both the exposed length of fixture and the stiffuess of the interface between the implant and the bone. If the exposed length of the fixture increases due to, for example, bone loss, the effective length of the beam is increased and the resonance frequency is reduced. The resonance frequency is also reduced if the stiffuess of Review o/Progress in Quantitative Nondestructive Evaluation. Vol. /8 Edited by Thompson and Chimenti, Kluwer Academic/Plenum Publishers, 1999 1601