Assessment of integration of titanium to bone using acoustic emission transmission
Increasingly, titanium is being used as an implantable material to make best use of its ability to integrate with bone. The replacement of missing teeth is likely the most widespread use of this technology but there is an increasing number of uses in orthopaedics and audiology. One of the challenges with this technique is being able to assess whether the bone-titanium interface is intact or is being subject to breakdown. Currently there is not a reliable and sensitive instrument able to monitor changes in bone-titanium interface. This study sought to develop a reliable and simple-to-use test for monitoring osseointegration using dental implants as a model system with an approach that would allow early detection of compromise to the bone to implant interface. A series of systematic investigations were conducted to examine the reliability of the acoustic emission technique (AE) for measuring changes in the osseointegration of dental implants using an in vitro model system. The model system involved dental implants installed into bovine rib bones with models for; primary stability, partial and full osseointegration, and degraded osseointegrated interfaces. The AE (a high-frequency ultrasonic wave) was produced by a simple source and was injected into the abutment of the implant. The transmitted energy was measured on the surface of the rib bone using a proprietary sensor. Some energy is lost at the implant-bone interface, but also in transmission along and through the bone. The effect of bone micro- and macro-structure on acoustic transmission through and along bone has been measured in the primary stability model and a quantitative relationship developed to allow this patient-specific aspect to be taken into account in a clinical situation. The primary stability model simply involved installing the implant using the normal surgical procedure. For secondary stability, glass ionomer cement was used as a model interfacial material giving partial and full coverage. It has been found that the transmitted energy could distinguish between primary stability and partial and full integration. Finally, to gauge how effective the acoustic emission technique could be in detecting early changes in the marginal bone around osseointegrated implants, simulated circumferential and vertical peri-implant bone defects of various vertical and circumferential extent were tested. It was found that the acoustic energy could effectively detect small changes in marginal bone level around osseointegrated implants. Changes in transmission of the AE signal were able to show both circumferential and narrow vertical bone defects including the most coronal 1 mm of the marginal bone. These findings suggest a role for AE in monitoring the development of osseointegration in the weeks following implant placement and could be coupled with an assessment of bone density on an individual patient basis. The technique could also have a potential application in the early diagnosis of the peri-implantitis in the oral environment or other forms of loss of integration when used elsewhere in the body. These findings are promising, although a number of practical issues need to be resolved before the technique can be validated in the clinical setting. Whereas the dental application is a useful model system, a clinical validation could lead to more general application in cases of monitoring bone-implant integration.