Clinical evaluation of the integrity of the bone-implant interface of percutaneous implants is important to prescribe loading, to identify the risk of failure, and to monitor the long-term health of the implant. The same concept of interface integrity can be applied in dentistry where the interface is the periodontal ligament (PDL) connecting the tooth root to the surrounding alveolar bone. Similarly, ongoing clinical evaluation of tooth stability (or mobility) has important applications in dental trauma, orthodontics, and periodontology.

This clinical need has led to the development of several noninvasive methods for stability measurement of both percutaneous implant systems as well as natural teeth. Many of these currently available systems rely on resonance frequency analysis (RFA) or similar analysis of the fundamental frequency of vibration of the system. With these measurement techniques, the details of the system are not taken into account in the interpretation. As a result, there is no clear understanding of how the output values relate to the actual stiffness characteristics at the interface. To have a better understanding of the physical properties at the interface, a coupled experimental-analytical modeling approach has been proposed for percutaneous implants. The approach uses an acceleration measurement during an impact with the abutment and an analytical model is included in the interpretation to isolate the properties at the interface.

This study builds upon previous approaches to develop a measurement system for noninvasive evaluation of interface stiffness that is applicable across a range of applications including bone anchored hearing aid (BAHA) implants and natural teeth. The system is termed the Advanced System for Implant Stability Testing (ASIST) and the interface stability measure is termed the ASIST Stability Coefficient (ASC). The ASIST is developed through in vitro laboratory testing for two commercial BAHA systems. A comparison is presented between the developed ASIST and the commercially available OsstellTM ISQ system for BAHA stability measurement. The ASIST is then used in vivo with a longitudinal clinical evaluation for BAHA patients during the first year following surgery. The ASIST technique is also developed for natural teeth and evaluated with longitudinal clinical data during orthodontic treatment.

Through in vitro laboratory testing, the ASIST measure was found to be essentially independent of attached components for BAHA implants with variations due to abutment length of approximately 2.9 ASC (less than 10% of the measure). The ASIST was shown to be sensitive enough to detect changes in interface properties between different implant installations. The ASIST showed significant advantages over the commercially available OsstellTM ISQ system for stability measurement of BAHA systems. The OsstellTM ISQ appears to be more sensitive to geometric variations such as abutment length than actual changes in interface properties, while the ASIST was found to be more sensitive to interface changes and essentially independent of abutment length.

Clinical evaluation of BAHA implants showed a wide variety of stability values and healing patterns across patients. The ASIST shows promise to detect differences in implant stability with different surgical techniques or different implant designs. The ASIST was able to provide an indication of longitudinal changes in implant stability for BAHA patients during the first year following surgery.

The ASIST technique was applied to natural teeth to estimate the PDL stiffness. A three degree of freedom analytical model was presented for a single-rooted tooth system. Several geometric approximations were presented and compared. The model prediction of the acceleration was found to provide a good match with the measured acceleration suggesting that the ASIST technique can be applicable to natural teeth. A longitudinal evaluation was presented showing changes in PDL stiffness properties throughout orthodontic alignment and cuspid retraction. Similar patterns of longitudinal changes were observed for the two maxillary cuspid teeth within individual patients as well as between patients. All patients showed a reduction in PDL stiffness during treatment. This work presented a significant contribution to the experimental-analytical modeling approach for natural teeth, and the work presented here shows promise for future research in this area.

The ASIST has the potential to provide a valuable clinical tool enabling clinicians to quickly and noninvasively evaluate the status of percutaneous implant systems or natural teeth across a variety of applications.