Optical Coherence Tomography for Non-Invasive ex vivo Investigations in Dental Medicine - a Joint Group Experience (Review)

This review emphasizes the current knowledge related to optical coherence tomography (OCT) as a non-invasive diagnostic tool to perform ex vivo and showing great potential for in vivo structural imaging of features in the oral cavity. OCT technology can generate high-resolution cross-sectional and en-face images of the internal architecture of the investigated sample (2-3 mm in depth). To this goal, en-face time domain OCT (TD-OCT) and spectral domain OCT (SD-OCT) were employed. Topics included in this review refer to OCT non-destructive evaluations of: dental abfraction and attrition, material defects and micro-leakages at the tooth-filling interface, temporal-mandibular joint disc, quality of bracket bonding on dental hard tissue, prosthetic restorations and micro-leakages at prosthetic interfaces, root canals, presence or absence of apical micro-leakages, and osteo-integration of dental implants and of bone grafting materials. OCT revealed internal features of the material investigated with greater sensitivity than current diagnostic methods. We put our research in context with others' results but the review reflects primarily our joint group experience and it presents images collected with our OCT systems only. The studies demonstrate the viability of OCT as a useful tool in dental medicine practice, as well as in research. Being completely non-invasive, OCT can be extended to soft tissue. Both TD and SD implementations prove the unique capabilities of OCT. For handheld scanning devices it is expected that the swept source principle (as one of the SD possibilities) will prevail, due to its high speed that allows for the reduction of distorting effects caused by the involuntary movements of the hand and of the patient. For high transversal resolution investigations, especially in more research oriented studies, it is expected that en-face TD-OCT will continue to coexist with SD-OCT methods, offering additionally a low cost quick provision of en-face view and compatibility with dynamic focus. Dynamic focus, that is the simultaneous adjustment of focus and coherence gate in depth is incompatible with SD-OCT methods and require repetitions of acquisitions under different focus in order to improve the transversal resolution, or more complex heads with division of the optical path in the object arm along different focus adjustments. In this respect, en-face TD-OCT provides a lower cost alternative to high transversal resolution of static samples. We have shown that complementary studies are possible embracing OCT with more traditional methods, such as confocal microscopy and microCT. Combination of principles is expected to evolve due to their limitations when considered separately.