- In 1970, we were at the tail end of silver point obturation. While they could be done well when the canals were properly shaped and properly fitted with a snug 02 tapered silver point, too often smaller silver points were placed that did not bind apically and were not well filled with cement laterally. The result was leakage and corrosion of the silver point producing a black sediment along the length of the canal. The leakage was further exacerbated by the fact that the cement was usually particulate in nature (as opposed to a polymer) such as zinc oxide and eugenol or calcium hydroxide based that readily dissolve in the presence of saliva. The obvious attraction of silver points were the dense appearance they produce on x-ray. We equated this dense appearance to a job well done.
- To do away with the corrosion of silver points, gutta percha became more popular because it could be cold adapted to the canal walls via lateral and vertical condensation leaving a thinner layer of cement that could potentially breakdown in the presence of saliva reducing the amount of leakage that might occur. While leakage could still occur, corrosion is not a by-product without the presence of silver points.
- With gutta percha the mainstay of obturation, Dr. Schilder pioneered the use of gutta percha in a thermoplastic form developing techniques that adapted the gutta percha intimately along the length of the canals reducing still further the thickness of the cement interface. At the same time, Dr. Schilder promoted greater tapered shaping, producing the continuous taper that is necessary for the thermoplastic obturation of canals. To facilitate greater tapered preparation, peeso and gates glidden reamers were introduced to prepare the more coronal aspects of the canal to a wider dimension in both the mesio-distal and bucco-lingual planes.
- With 02 tapered stainless steel files the predominant instruments used in canal shaping, wider canal preparations took greater time and the larger tipped 02 tapered stainless steel files tended to distort curved canal preparations to the outer wall as they instrumented apically to larger diameters. They also had a tendency to block the apical portion of the canals with debris resulting in a loss of length.
- The introduction of greater tapered rotary NiTi allowed the more rapid shaping of funneled conical preparations that facilitate the thermoplastic obuturation of canals. Being significantly more flexible, particularly in the apical portion of the instrument, their use reduced the degree of distortion associated with the use of K-files. Engine-driven rather than manual, greater tapered shaping reduced hand fatigue and made for a better fit between the ultimate shape of the canal and the corresponding gutta percha point.
- As the use of greater tapered rotary NiTi instrumentation expanded, it became obvious that instrument separation was a concern that in turn led to new heat treated NiTi alloys, single usage and more reliance on a well established glide path prior to their usage.
- Around 2007, research started documenting weaker roots as a result of greater tapered preparations, along with the production of dentinal micro-cracks that were further shown to coalesce and propagate during active forms of obturation, post placement and normal masticatory forces. As a result of this research and the clinical impression of larger numbers of vertical fractures occurring years after endodontic treatment, we are now seeing a reduction in the tapers of the canals as well as the degree of apical preparation. In an attempt to reduce instrument breakage and the production of dentinal micro-cracks, asymmetric reciprocation was introduced. Yet, recent research still demonstrates the continued production of dentinal micro-cracks and the separation of instruments.
- Today the fashion is continuing to change. Most recently, noting the defects and weakened roots that result from greater tapered rotary NiTi instrumentation, an endodontic movement is growing that minimizes the amount of tooth structure removed during access, followed by minimal canal tapers dramatically reducing the amount of tooth structure removed in the mesio-distal plane. The downside of this positive development is the increased amount of tissue remaining in the buco-lingual plane. With lesser tapered preparations thermoplastic obuturation becomes more problematic and is being increasingly replaced by single point room temperature fills. Under these circumstances, the cement must now take up the burden of filling the spaces that are sure to be present in the buccal and lingual extensions of oval canals. If we don’t want to repeat the leakage that occurred with silver points 45 years ago, we need to use epoxy resin polymer cement that are highly resistant to hydrolytic degradation while bonding intimately both physically and chemically to both the dentin and the central gutta percha cone. However, despite the advantages of a cement possessing the properties of epoxy resin, it will mean little if the tissue in the buccal and lingual extensions are not fully removed. While the popular concept today is minimal access and minimal preparation, this approach is not addressing this short coming of consistent full debridement.
- To optimize tissue removal with minimal sacrifice of dentin, we need to use instruments that are efficient in removing tissue in the bucco-lingual extensions without excessively widening the canals in the mesio-distal plane. To perform these tasks, we are far better off, employing non–rotating 30-45º reciprocation instrumentation oscillating at 3000-4000 cycles per minute. With rotation completely eliminated, the instruments will remain intact. The dentist now has the confidence to employ these instruments with vigor against the buccal and lingual extensions. In effect, the canals can now be prepared to a larger version of their original anatomy, something that should have always been the case, but was thwarted by fashion and commercialization. Minimal preparations are in a sense a step forward, but it can lead to two steps backwards if the tissue in the buccal and lingual extensions are not addressed in an effective manner. Instruments designed to cut aggressively in the bucco-lingual plane accomplish two tasks. One, they can enlarge the canal space beyond their own dimensions and the thinner instruments in the sequence delineate the glide path for the thicker somewhat stiffer instruments to faithfully follow. Appreciating these capabilities leads to insights that preserve dentin while improving the tissue removed from all the pulpal recesses. We’re getting there, but have yet to reach the promised land.