CATEGORY AUSTRALASIAN DENTIST 45 with dental implants. These indicated a baseline measurement for an implant at any time following placement and also revealed the changes in resonance frequency analysis with time attributable to clinical osseointegration. Since then many studies have been carried out demonstrating increases in resonance frequency and hence stability with osseointegration. Equally importantly are the differences observed between the maxilla and the mandible where increases in resonance frequency are typically greater in the maxilla than the mandible. At first this may seem strange but when we consider the maxillary bone has a more open and vital trabecula network compared to the mandible the amounts with more new bone formation demonstrated histologically in the maxilla is significantly greater than that in the mandible. The primary bone to implant contact of an implant in the mandible is typically greater than that of a similarly implant placed in the maxilla and this makes perfect sense. Following the early studies, a €1.8 million European Commission grant to establish the technical viability of using Resonance Frequency Analysis under realistic operating conditions was obtained. This included working with partners in the UK, Spain, Italy and Sweden. The outcome conclusively demonstrated the practical use of the method and it was at this point that Imperial College in London formed a university spinout company, Integration diagnostics. The electronic technology designed and used to measure RFA in patients was highly sophisticated and without the help of colleagues like David Alleyne and Tim Orr at Imperial College this would not have been possible. Tim Orr is an extremely talented electronic design engineer and world renowned for his designs of analogue synthesisers used by bands such as Tangerine Dream. Early transducer designs were right angled titanium cantilever beams which could be attached to an implant with an abutment type screw. Bonded to the sides of the beams were piezoceramic crystals. The piezo electric effect occurs when an electric current is applied across a piezo crystal the crystal deforms and vice versa; if mechanical bending is applied to a crystal a voltage is generated. The RFA transducer had a transmitter crystal transmitting a sine wave of varying frequency in the range 5 to 15 kilohertz and a receiver crystal detecting said resonance frequency. Although cumbersome and rather difficult to use this design did establish the technical and clinical viability of the method and could detect to measure the difference in resonance frequency of different bone qualities and also differences between successful and failed implants. Even at this early stage it was envisioned that in the future there would be the potential for traffic light system indicating the suitability of an implant for immediate loading. Little was it known it would take some 20 years before this could be achieved. Along the journey all the potential nondestructive test methods were investigated. Reverse torque testing was first described by Sullivan as a method to assess the integration of an implant by applying a reverse torque of 32 Newton centimetres three to four weeks after placement. If the implant rotated it had failed and should be removed and replaced. Unfortunately, the application of this potentially destructive test method has persisted for many years. It should be obvious from studies of Histology and clinical implant performance that not all implants integrate at the same rate unless shearing the interface of an implant with the torque of 32 Newton centimetres may not necessarily be an indication that the implant had caused a failure. The Periotest has also been a test method that has had its proponents in dental implantology. The Periotest was designed by Siemens to measure tooth mobility. The instrument used a small electromagnet which accelerated a slug to tap a tooth. An accelerometer in the metal slug measured the contact duration with the tooth and this was cross referenced with a database of a large number of mobile teeth. In this it was quite successful and there was some correlation evident with Miller’s classification of tooth mobility. However, in the case of dental implants the stiffness characteristics between a successful, integrated and potentially failing implant is very different to that of a mobile tooth and as such the measurement range on the instrument was severely limited to just a few PTV units. Irrespective of the lack of sensitivity of the device and its variability related to the contact position with which the instrument touched the implant the Periotest has had a number of advocates. The third method and probably the most widely used and discussed and yet the most enigmatic is the measurement of insertion torque. Strid a paper published in 1984 identifying some of the parameters which related to the insertion torque of an implant. These include the size and diameter of the implant, the difference in diameter between the final osteotomy and the diameter of the implant, friction, cutting geometry of the implant, bone density, thread cutting force. All these variables interact and play a role in determining a torque time or torque distance graph plotted during the insertion of an implant. Most dental implant drill equipment manufacturers provide a drill motor capable of displaying a graph however the interpretation of this graph still remains something of an enigma. Insertion torque can only give a very basic interpretation of implant stability and it should be borne in mind that this relates to a rotational force and therefore is not directly representative of an implant stability under clinical loading which is most likely to be applied by a bending force. A number of investigators have attempted to correlate and understand the relationship between insertion torque and resonance frequency analysis however, there is little if any reason why there should be such a correlation for the reasons previously described. A number of the early clinical studies using resonance frequency analysis gave valuable insights into the relationship between the clinical and biological behaviour of implants placed in bone. At implant placement it is possible to make a resonance frequency measurement of the primary implant stability and a number of studies demonstrated that this can be distributed typically in the range of 40 to 90 ISQ. ISQ (implant stability quotient) was a clinical scale designed directly from resonance frequency measurements to create a more useful and understandable range for clinicians to interpret implant stability. Implants exhibiting a primary stability of 40 to 60 ISQ can be considered to have a very low stability. 60 to 80 is high and as such it is possible to make an informed opinion about potential loading protocols for such implants perhaps with delayed and submerged healing for those implants having lower ISQ values. It must be remembered the suitability of an implant for immediate loading is attributable to a number of parameters, not least the occlusion, the tooth involved, para function or any bruxing habits present, the type and nature of implant and the surrounding bone. A high ISQ value in excess of 70 may support the indication for immediate loading of a single implant or even perhaps as part of full arch bridge. Perhaps the most fascinating element of the measurement of implant stability in using resonance frequency analysis is measuring the change between primary INNOVATIONS Figure 5
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