CATEGORY 44 AUSTRALASIAN DENTIST The 25th of September 2024 saw in Gothenburg, Sweden a celebration of the 25th anniversary of the introduction of Osstell as a diagnostic technique for measuring implant stability (Figure 1) (Figure 2). This article is designed to entertain and inform clinicians and academics alike about the origins of this novel test method, its academic evolution, commercial development and how it has become the premier and worldwide recognised gold standard for measuring dental implant stability. This may also serve in some small way to interest potential future inventors (Figure 3). As I believe may be often the case good ideas can often originate over a cup of coffee and this was the case with resonance frequency analysis later to become Osstell. Whilst undertaking a research fellowship at Imperial College in London for my first PhD a casual but fortunate meeting occurred one day with a colleague Peter Cawley. Peter is a mechanical engineer specialising in non-destructive test methods and has been running a very active department at the college for many years. In true and fair recognition Peter has received the FRS. The Fellowship of The Royal Society is the highest accolade that can be awarded to a scientist and in this case an engineer in the United Kingdom. Peter and I started to have discussions about what was new in engineering and also in dentistry. Peter was highly skilled in developing diagnostic test methods for assessing the reliability and potential failure mechanisms have rather larger structures than dental implants; aircraft, railway engines and oil pipeline (Figure 4). Nonetheless when I explained in 1992 that dental implants were one of the latest things in dentistry but there were still some still uncertainty about the potential causes and identification of early failures. The opportunity to apply non-destructive test methods was quite attractive as an alternative to radiographic examination which took some months to reveal bony changes or indeed tapping an implant with a mirror handle as advocated by clinicians at the time and which probably told us more about the mirror handle than it did about the implant.. There were two major physical, and biomechanical problems with dental implants. They are surrounded by a non-elastic, non-homogeneous and non-isotropic material called bone and implants are rather small, smaller than a turbine blade making them more difficult to excite. Non-destructive test methods commonly impart energy into a structure and then measure the reflected signal. This energy can take the form of an impact, sonic or commonly ultrasonic frequencies. An analogy is the sonar used by submarines to detect the sea bottom and its irregularities by measuring the reflection of a transmitted sound. We explored all the possible methods for non-destructive testing to assess the nature of the implant bone interface. In practise, dental implants were too small to give useful and accurate data using an impact method which could not impart sufficient energy. The most successful, repeatable and accurate measurements were achieved using sonic excitation, actually in the audible range of some 7 to 15,000 Hertz. Although the extremely low signal levels would mean the patients were unaware of the signal. The reflected signal on the test bench detected a resonance frequency from a simple cantilever beam transducer not unlike a musical tuning fork attached to an implant. The parallel to a tuning fork is close and the length of the fork imparts the frequency related to its length and the stiffness with which it is held (Figure 5.). There is clearly a parallel with dental implants where clinicians would like to know in detail the stiffness of the implant bone interface and the level of the marginal bone. The original invention made for resonance frequency analysis measurements for dental implants was conceived and patented in 1992. In subsequent years laboratory and animal studies were undertaken to understand if such measurements had the potential for clinical application. In 1993 there was close collaboration with Professor Thomas Albrektsson, Lars Senneby and the Bränemark Clinic in Gothenburg. It was here where studies revealed that clinical measurements of resonance frequency analysis could be accurately and repeatably made on patients INNOVATIONS Osstell and implant stability – A quarter of a century By Professor Neil Meredith, BDS.,MSc.,FDS RCS.,PhD,PhD Professor Neil Meredith Figure 1 Figure 3 Figure 2 Figure 4
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