JOURNAL OF ENDODONTICS [ VOL 2, NO 12, DECEMBER 1976
R e s t o r a t i o n of e n d o d o n t i c a l l y treated posterior teeth Brian G. T i d m a r s h , BDS, F R A C D S , O t a q o , N e w Z e a l a n d
T h e b i o m e c h a n i c a l b e h a v i o r of teeth u n d e r l o a d i n d i c a t e s the n e e d for a p p r o p r i a t e m e t h o d s of restorinq endodontically treated p o s t e r i o r t e e t h to e n s u r e their r e t e n t i o n a s f u n c t i o n i n q units of the d e n t i t i o n .
Refinement of endodontic techniques, particularly those related to the preparation and filling of narrow curved root canals, enables the dentist to treat many posterior teeth successfully. However, f r o m the patient's point of view, success depends not only on the adequacy of the root canal filling but also on restoring the tooth to function with a strong, well-retained, biologically acceptable restoration with good margins, contacts, contours, occlusion, and appearance. Failure to do so frequently results in the breakdown of the remaining coronal tooth structure and ultimate loss of the tooth. Biomeehanical Considerations In recent years some insight has been gained into the biomechanical behavior of teeth under load. A n intact tooth may be considered as a hollow, laminated structure that deforms under load; it may shorten, its sides may bulge, and its cusps m a y 374
be wedged apart by opposing cusps. Although under physiologic loads complete elastic recovery takes place, permanent deformity m a y follow very high or sustained loads. 1 In addition, the intact tooth appears to behave as a prestressed laminate. 2 It is characteristic of such a structure that it can withstand greater loads in the prestressed rather than the unstressed state. It is difficult to see how this prestressed state comes about in the tooth, but one hypothesis suggests that as the crown develops, the outward movement of the ameloblasts and the inward movement of the odontoblasts sets up the stressed condition, which is then frozen or stabilized by mineralization of the matrix. The significance of this phenomenon is that any cavity preparation, however small, destroys the prestressed state and releases the stresses. Experimental work has shown that after cutting a cavity in a tooth the cusps move together very slightly, almost as if the central tooth structure had been holding the cusps apart. The crazing or cracks that appear in surrounding enamel when a cavity is cut is probably further evidence of the release of stresses accompanying the loss of the prestressed condition. The clinical significance of this is that a tooth in which a cavity has been prepared deforms to a much greater extent under applied loads.
There is a direct relationship between the amount of central tooth structure that is lost in cavity preparation and the deformations that occur under load. 1 Access cavities for endodontics remove a very substantial part of the coronal dentin, and relatively low loads can cause a significant deformity or even fracture. Thus, two reasons can now be identified why pulpless teeth are weak: first, the loss of the stressed laminate state and second, the greatly reduced amount of tooth structure on which loads will fall. A third reason may exist. It has been shown that the calcified tissues of pulpless teeth contain 9% less water than those of vital teeth. 3 In addition, as teeth age and more peritubular dentin is laid down within the odontoblast tubules, less space exists for organic material and tissue fluid, both of which lend flexibility to living tissue. F o r example, a dry, dead branch of a tree certainly breaks more easily than its living counterpart. However, the role that water plays in the biomechanical behavior of teeth is, at present, unclear.
Clinical Considerations Successful restoration of weak, pulpless teeth implies the prevention of large-magnitude deformations of
JOURNAL OF ENDODONTICS ] VOL 2, NO 12, DECEMBER 1976
the remaining tooth structure. Amalgam is probably only barely adequate as a restorative material when there has been minimal loss of tooth structure. For example, a mandibular premolar that has been endodontically treated through a small occlusal access cavity but is otherwise intact probably can be restored adequately with amalgam. However, once the mesial and distal surfaces have been lost, and this is usually the case with posterior teeth requiring endodontic treatment, only a major restoration will serve. In particular, it must be ensured that the cusps are overlaid to prevent their deformity and subsequent fracture. Although amalgam may be used to overlay such cusps, gold is generally better because its superior strength and lack of flow under load makes cuspal protection more certain. Apparently, the important factor in the ability of an overlay to protect the cusp is the thickness of the gold and not the particular design of the overlay. 2 Photoelastic stress studies on plastic tooth replicas would seem to indicate the superiority of the reverse bevel, but this is yet to be verified in the clinical situation. However, the inlay with cuspal overlay has some drawbacks. It is difficult to achieve good retention from the small amount of remaining dentin, and enamel must be removed to obtain mutual divergence of the numerous walls of the cavity preparation. This factor more than any other makes some form of veneer crown a better choice for many endodontieally treated teeth. Extracoronally retained restorations that encompass the remaining weakened tooth are generally the most suitable, but it is very important that the remaining central core of the tooth is itself made sufficiently strong and well retained. Merely filling the central defect with cement or amalgam is of little help because fracture and
loss of both the core and the enveloping crown can result. Two basic methods are available for creating strong, well-retained cores for veneer crowns, The first is by using plastic filling materials such as amalgam or composite resin retained by one or more posts in the root canals, with perhaps additional pins in surrounding dentin. Obviously, the need to use a canal for retention should be realized at an early stage and a sectional root filling technique used to facilitate the placement of the retention posts. Screw posts =:: are available in a variety of lengths and diameters and are quite suitable. However, it should be realized that the tapered shape of the post and the very efficient wrench used for screwing it in can easily split a weak root. F o r this reason, it is a safer practice to select a post that has a snug, sliding fit in the canal, cement it with zinc phosphate cement, and engage the screw threads with no more than a single turn. Photoelastic analysis of the distribution of stresses around various types of endodontic posts has indicated that tapped and threaded posts offer superior retention, particularly where the root length is short. However, they are liable to generate high levels of stress, particularly if the countersink is fully engagedA Threaded retention pinst also can exert stresses on dentin. These stresses can be minimized by cementing the pins into oversize pin channels, using zinc phosphate or cyanoacrylate cements. Rodda and Hood~ have cast doubt as to the retention offered by the latter. The second basic method is to make a cast post and core from a wax pattern or to cast a core onto a nickel-chrome-cobalt post. Whichever method is used, provision should be made for adequate length and prevention of rotation. Generally, a post length at least as long as the crown
is necessary. In addition, the core should rest on a flat bed, otherwise axial loading may cause the root to split. Internal angles, particularly those at the junction of the canal and root face, should be rounded to avoid the generation of high levels of stress at these critical points. Treatment then is completed by constructing the veneer crown. Summm~/ Successful treatment of the rootfilled posterior tooth depends on an awareness of its weak nature and the design and construction of a strong, well-retained restoration. The weakened tooth structure must be protected by overlaying the cusps, or it must be replaced by a well-retained core before construction of a veneer crown. Failure to observe these fundamental points usually leads to the ultimate loss of the tooth. * A. B. Dentatus, Stockholm, Sweden. t Thread Mate System, Whaledent International, New York. Dr. Tidmarsh is senior lecturer in the department of conservative dentistry, University of Otago Dental School, Dunedin, New Zealand. Requests for reprints should be directed to: Dr. B. G. Tidmarsh, University of Otago Dental School, PO Box 647, Dunedin, New Zealand. References
1. Grimaldi, J. Measurement of the lateral deformation of the tooth crown under axial compressive cuspal loading, thesis, University of Otago, Dunedin, New Zealand, 1971. 2. Malcolm, P. Cast restoration and cusp flexibility, thesis, University of Otago, Dunedin, New Zealand, 1973. 3. Helfer, A.R.; Melnick, S.; and Schilder, H. Determination of the moisture content of vital and pulpless teeth. Oral Surg 34:661 Oct 1972. 4. Standlee, J.P.; Caputo, A.A.; Collard, E.W.; and Pollack, M.H. Analysis of stress distribution by endodontic posts. Oral Surg 33:952 June 1972. 5. Rodda, J.C., and Hood, J.A. Pins retained with a cyanoacrylate adhesive. NZ Dent J 71:198 Oct 1975.