By Barry L. Musikant, DMD
When we talk about three-dimensional obturation various thermoplastic techniques come to mind. Studies have shown that thermoplasticized gutta percha obturation results in a larger volume of gutta percha in the canal compared to cold lateral and vertical condensation and obviously single point obturation. The higher volume of gutta percha has been taken as proof of a superior seal when, in fact, the seal is dependent upon the intimate relationship of the cement to both the gutta percha and the canal walls. Consequently, the inference of a higher volume of gutta percha being proof of a superior seal is secondary to the fact that the cement is forced into a more intimate relationship with the canal walls.
Thermoplastic obturation gives the gutta percha the ability to adapt to the canal walls. In this case, the thermoplastic property is a plus. Upon placement, the thermoplasticized gutta percha cools to body temperature, a drop from 201º C to 37º C. In the process, the gutta percha shrinks anywhere from 3-5%. This occurs after the small amount of cement originally placed has been displaced coronally (Fig 1: https://youtu.be/z5TVzNUrl8k) . Gap formation is unavoidable. To the extent thermoplasticized gutta percha shrinks, three-dimensional oburation is compromised. This is true even if multiple lateral canals are initially filled, an event that occurred when the gutta percha was its most flowable and before shrinkage started.
Thermoplastic obturation takes several different forms. Carrier-based thermoplastic systems have evolved away from metal and plastic carriers to the point where they are now constructed of two different phases of gutta percha, with the outer layer thermoplasticized far more than the inner core which acts as the carrier. All thermoplastic techniques require the minimum placement of cement because the coronal pathway for the release of excess cement is occluded first eliminating the escape valve when room temperature systems are employed. Another thermoplastic approach is the use of a heated spreader that first requires the placement of a conventional gutta percha point to the constriction followed by heated vertical condensation within 5 mm of the apex and conventional condensation within 2 mm of the apex. This form of thermoplastic obturation adapts the gutta percha to the walls of the canal within the last 2-3 mm of the canal. Interestingly, the gutta percha that was placed at room temperature is not affected by the heat introduced by the spreader and remains more or less in the same position as when it was originally placed. Strictly speaking, the most apical placement of the gutta percha has not been thermoplasticized and is essentially a single point fill. The advantage is no shrinkage.
After adaptation of the point to the canal wall via the heated spreader, a void extending from the most apical 2 mm of gutta percha to the orifice is present. This space is then filled using one of several available glue guns that deposit thermoplasticized gutta percha through a thin silver needle. Being thermoplasticized, the entire deposited body of gutta percha will shrink as it cools to body temperature again compromising the concept of a true three-dimensional fill. The same problem occurs when the squirt technique is applied, a form of obturation that skips a heated spreader and goes directly to the extrusion of molten gutta percha from the constriction to the orifice. Again, shrinkage is unavoidable.
While thermoplastic obturation results in shrinkage, it minimizes the excessive stresses that cold lateral and vertical condensation can be transferred to the root. The application of heat can also be excessive resulting in thermal shock to the dentin as well as potentially damaging the surrounding periodontal ligament. In fact, one should realize when using a thermoplastic spreader that the duration of the application of heat is critical. Manufacturers suggest no more than a 2 second application of 201º heat. A duration longer than that increases the temperature to the ligament beyond 10º C that has been documented to damage the ligament. This type of thermoplastic obturation imposes a narrow window for success and subsequently a large window for unintentional iatrogenic damage.
We can avoid the problems of gap formation and excessive application of heat by using room temperature methods of obturation, namely lateral and vertical condensation. These are traditional methods of obturation that have been used by dentists for many decades. The goal is similar to that of thermoplastic obturation, to increase the intimacy of the gutta percha to the canal walls thereby driving the cement into a more intimate relationship with the canal walls. There are two major shortcomings to this technique. One, lateral and vertical condensation has the potential to create and propagate micro-cracks in the dentin that may already be present leading to a premature vertical root fracture. Two, if enough pressure is applied to gutta percha it will distort to the shape being imposed. However, gutta percha is a rubber-like material and will rebound when the pressure is removed. In the actual process of lateral and vertical condensation, the molded gutta percha will conform to the canal walls displacing excess cement coronally , only to rebound to its original shape when the pressure is removed and subsequently leaving a void where the cement was displaced.
For me, the most irrational aspect of cold vertical and lateral condensation is the application of force via a tapered spreader that can easily exceed the tensile strength of dentin. According to some articles, as long as the metal spreader does not wedge into the canal to the point where it is touching dentin this is of no concern. From a practical point of view, it is difficult to determine whether this particular criteria is being met and even if it is, is the generation of excessive stress truly avoided? This is a perfect example of where a route that completely avoids these possibilities is the safest.
With thermoplastic obturation in its several forms and cold lateral and vertical condensation producing voids and the potential for excess stresses to the roots, my choice for trauma-free obturation is a single point technique. It is a passive technique that will not generate excessive stresses. The question asked by those using alternate techniques is whether or not it is an effective means for canal obturation. Like all obturation systems the effectiveness of the single point technique is based on the cement. However, the single point technique requires that the cement be an effective seal in both thick and thin layers and that it be present in sufficient quantity that it can fill all the gaps between the master gutta percha cone and the walls of the canal. For this to happen, we need an instrument that can flood the canal with cement prior to the placement of the prefitted gutta percha point. The bi-directional spiral (Fig 2) is the applicator incorporating a design where the coronal flutes drive the cement apically while the last three apical flutes drive the cement coronally. When the bi-directional spiral is loaded with cement two flows occur, one going corono-apically and the other going from the apex coronally. These two flows of cement collide three mm from the tip of the instrument and are driven laterally (https://youtu.be/Hq3240SjJ9o) . With the bi-directional spiral being used in an up and down motion within the canal never closer than 3-4 mm from the apex, the cement is dispersed vertically as well as laterally along the entire length of the canal flooding all but the last 2-3 mm of the canal. The master prefitted gutta percha point is then liberally coated with cement (https://youtu.be/FRzWIpIKmbY)and placed into position in the canal driving excess cement coronally and creating a three dimensional seal along length.
A word must be said about the cement being used since the long term seal is a function of its properties. We prefer the use of an epoxy resin (EZ-Fill cement). It is highly flowable. When the prefitted gutta percha point is placed it requires very little pressure to drive the cement into an intimate relationship with the canal walls. Furthermore, being a room temperature system, it will warm not cool to body temperature expanding slightly in the process improving what is already an intimate seal. EZ-Fill is a polymer that does not contract upon polymerization maintaining dimensional stability. Being a polymer, it is highly resistant to hydrolytic degradation, something that all particulate cements (ZOE, calcium hydroxide and glass ionomer) are prone to. It bonds both chemically and physically to dentin and gutta percha to maintain the seal. It is highly radiopaque clearly seen on x-ray, but is able to be digested by the macrophage periapically if by chance some is extruded beyond the apex. Perhaps, most importantly, it has 70 plus years of research behind it demonstrating its effectiveness and kindness to the periapical tissues.
We started this discussion with the assumption that it is thermoplastic obturation that is synonymous with three-dimensional obturaton. In fact, this is so if the only parameter to measure such fills is the quantity of gutta percha in the canal. As we can see this is an irrelevant parameter that does not reflect upon the main goal of obturation, namely to seal the canal. If we measure the quality of a seal, we now understand that while it has always been the function of the cement to create that seal, the seal is not compromised by the varying thicknesses of the cement. This allows us to use cold passive single point techniques that create a three-dimensional seal with the least stress transferred to the root possible.
One last question that someone might ask is what to do when the canals are highly oval with a lot of buccal and lingual space after the master point has been placed? Once can go two ways. The cement already flooding the canal is occupying that space effectively and needs no further gutta percha points. Alternatively, one can use a spreader to create a lateral space for the placement of an auxillary point with the proviso that the pressure applied to the spreader does not exceed the weight of one’s hand. We have no desire to mold the gutta percha that would induce the rebound effect or expose the roots to any further stresses. Rather, we apply just enough pressure to create a space where a subsequent well-coated point is then placed.
By using a passive obturation technique we scrupulously avoid the introduction of stresses, maintain the accurate placement of the gutta percha point and avoid the introduction of voids that would compromise the seal. The fact that the employment of this technique is simpler and far less expensive is a wonderful marketing feature, but does not surpass the fact that it is safer and kinder to the tooth.