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Split increment horizontal layering: A simplified placement technique for direct posterior resin restorations Khamis Hassan, BDS, MSD, PhD, FADI | Salwa Khier, BDS, MSD, MSc, PhD, FADM
A new simplified technique for restoring moderate-to-large Class I occlusal cavities has been proposed. This technique utilizes split increment horizontal placement of composite resin, combined with shade or chroma stratification. It is believed that this technique will minimize the detrimental effects of polymerization shrinkage on the adhesive interface and cavity walls. Received: February 18, 2005
In recent years, the use of composite resin for restoring posterior teeth has increased dramatically. It is well-known that all composite resins shrink during polymerization; this shrinkage presents several challenges during placement and photocuring.1 Photocuring generates polymerization shrinkage stresses, which may initiate adhesive failure at the compositetooth interface, resulting in gap formation between composite resin and cavity walls. Oral fluids containing bacteria may fill this gap and cause microleakage and secondary caries.2,3 Cohesive failure also may occur in the form of microcracks in composite resin; in addition, coronal deformation may result from these shrinkage stresses transferring to the tooth, resulting in postoperative sensitivity and propagation of existing enamel microcracks.3,4 The magnitude of these stresses depends on several factors, including the resin’s modulus of elasticity, the rate of polymerization, and the restorative techniques utilized, as well as cavity configuration (also known as the C-factor). The C-factor is the ratio between bonded and unbonded surfaces; as the ratio increases, the polymerization stress increases.5 Class I occlusal cavity preparations have the highest C-factor, as they have only one free, unbonded surface that is able to deform as polymerization shrinkage occurs, causing the highest and most unfavorable stresses at the cavity walls and margins.5 Dentists and researchers are aware of the adverse effects of the shrinkage stresses. Several efforts have been made to decrease these stresses; these efforts were di-
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Accepted: April 11, 2005
rected toward improving composite resin formulation, photocuring methods, and restorative placement techniques. Incremental placement techniques are widely recognized as a major factor in the reduction of shrinkage stresses.3,6-9 These techniques include the horizontal occlusogingival layering technique, the wedgeshaped oblique layering technique, and the successive cusp buildup technique.10-16 The horizontal placement technique utilizes composite resin layers, each less than 2.0 mm thick. This technique has been reported to increase the C-factor and consequently increases the shrinkage stresses between the opposing cavity walls.10-12 The oblique technique is accomplished by placing a series of wedge-shaped composite increments. Each increment is photocured twice, first through the cavity walls and then from the occlusal surface, to direct the vectors of polymerization toward the adhesive surface. This technique reduces the C-factor and prevents the distortion of cavity walls.9-13 The successive cusp buildup technique involves applying the first composite increment to a single dentin surface without contacting the opposing cavity walls. At that point, each cusp is built up separately by placing a series of wedge-shaped composite increments, using an oblique layering to minimize the C-factor.14-16 The stratified layering technique was proposed and oriented to the development of functional and anatomic restoration utilizing the “esthetic” composite resin restorative materials that include shades of dentin and enamel as well as var-
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ious translucencies and intensive colors.16-18 This technique is designed to reproduce the various degrees of chroma present within a tooth. It involves placing dentin shades of composite resin with a higher chroma in the middle of the preparation and placing a lower chroma resin close to the cusp walls. The stratified layering technique is accomplished by placing initial dentin layers of composite shades or chromas that are two or three degrees higher than the selected basic shade or chroma. Subtle variations in dentin color can be achieved by changing the thickness of each chroma layer in specific areas of the restoration. The enamel layer is placed following the contours established by the dentin layers; it varies in thickness depending on the desired effect. The enamel layer can be modified by placing various shades of opalescent or intensive enamels on different areas of the restoration. Further effects can be produced using resin intensive colors or stains.16,17 A clinical restorative protocol introduced in 2001 combined the incremental successive cusp buildup technique and the stratified horizontal layering technique. This protocol involved wedgeshaped oblique increments with shade or chroma stratification.18 Although this technique produces excellent esthetic posterior composite restorations, a long learning curve is necessary, as an internal systematic buildup of each cusp must be performed separately while the different color variations of natural posterior dentition also must be considered. The objective of this article was to propose a new simplified restorative technique for moderate-to-large Class I occlusal cavities that would utilize split increment horizontal placement of composite resin, combined with shade or chroma stratification.
The proposed technique This technique splits each composite
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Fig. 1. A diagram of a large Class I occlusal composite resin restoration placed using the proposed technique.
Fig. 2. The first uncured horizontal increment of dentin shade composite (A3) covers the cavity floor.
Fig. 3. The two diagonal cuts split the first uncured increment into four triangular-shaped portions. This step is followed by photocuring from the buccal, lingual, and occlusal directions.
Fig. 4. Complete filling of one diagonal cut with dentin shade composite (A3) is followed by photocuring from the occlusal direction.
Fig. 5. Left: One-half of the other diagonal cut filled with dentin shade composite (A3) and photocured. Right: The other half of the cut is filled and photocured.
Fig. 6. The second and third increments (dentin shade composites A2 and A1) are used to cover the first cured increment. The top fifth of the third increment in the middle area of the diagonal cuts is left unfilled for internal characterization.
horizontal increment into four portions before curing. During photocuring, each portion of the split increment contacts only two cavity surfaces that do not oppose each other. It is assumed that this technique will minimize the detrimental effects of polymerization shrinkage on the adhesive interface and cavity walls. A diagram of the composite resin restoration that was placed using the proposed technique is presented in Figure 1. The following is a step-by-step description of a composite resin restoration placed in a large Class I occlusal cavity preparation using the proposed technique. None of the horizontal increments placed were more than 1.5 mm thick. The selected shade was A1. To replace dentin in a stratified manner, three composite dentin shades (A1, A2, and A3) were used. The dentin shade composite with the highest chroma (A3) was used as the first increment. It was placed horizontally to cover the cavity floor and was left without curing initially (Fig. 2). Using a small-bladed instrument, two diagonal cuts were made in the first un-
cured increment, splitting it into four triangular-shaped portions so that each portion contacted only one cavity wall and part of the floor. Each cut was 1.5 mm wide and extended through the thickness of the entire increment. The split increment was photocured for 40 seconds from each direction (buccal, lingual, and occlusal) (Fig. 3). The two diagonal cuts were filled completely with the same dentin shade composite used earlier. One diagonal cut was filled and photocured for 20 seconds from the occlusal direction (Fig. 4). At that point, the second diagonal cut was filled and photocured, one half at a time (Fig. 5). The second horizontal increment of dentin shade composite (A2), which is one chroma less than the previous one, was placed to cover the first photocured increment. As with the first increment, the same steps of increment splitting and photocuring were performed, followed by separate composite filling and photocuring of the each of the two diagonal cuts. The third horizontal increment of
dentin shade composite (A1) was placed to cover the two previous increments. This increment (one chroma less than the second increment) matched the selected shade and was extended to the dentinoenamel junction (DEJ). This increment received treatment similar to that performed for the previous two increments, except that the top fifth of the middle area of the diagonal cuts was not filled with composite. This area was used for internal characterization with pit and fissure stain (Fig. 6). Following the internal characterization, the enamel shade composite increment that matched the selected shade (A1) was placed horizontally to replace the lost enamel and extended from the DEJ to contact the preparation cavosurface margin. This increment was shaped to establish the occlusal morphology; at that point, two diagonal cuts were made and the increment was photocured for 40 seconds from the buccal, lingual, and occlusal directions (Fig. 7). The four ends of the diagonal cuts next to the enamel wall and cavosurface
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Fig. 7. Enamel shade composite (A1) increment is placed after internal characterization replaces the lost enamel and establishes the occlusal morphology. This increment was provided with two diagonal cuts and photocured.
Fig. 8. The four ends of the diagonal cuts next to the enamel wall and cavosurface margin are filled with enamel shade composite (A1) and photocured.
Fig. 9. The rest of the diagonal cuts are filled with the final increment of translucent shade composite (T), covering the A1 enamel composite without touching the cavosurface margin. This increment was photocured without splitting.
Fig. 10. The first horizontal split increment of dentin shade (A3) of Point-4 composite resin, prior to photocuring.
Fig. 11. One diagonal cut is filled completely with A3 dentin composite and photocured.
Fig. 12. The other diagonal cut is filled with A3 dentin composite and photocured, one half at a time.
dentoform model was used for a demonstration of the proposed technique. Prime&Bond NT adhesive system (Caulk/Dentsply, Milford, DE; 800.532. 2855) was applied to all cavity walls, following the manufacturer’s instructions. Figure 10 shows the first horizontal split increment (A3 dentin shade) of Point-4 composite resin (Kerr Dental, Orange, CA; 800.537.7123) before photocuring. Figure 11 shows one diagonal cut filled completely with the same chroma dentin shade composite, followed by photocuring for 20 seconds from the occlusal direction using the Elipar Highlight curing light unit (3M ESPE, St. Paul, MN; 888.364.3577). Figure 12 shows the other diagonal cut filled with A3 dentin shade composite and photocured, one half at a time. Figure 13 shows the finished restoration.
ratio of 5 when it is photocured.10-12 Concern has been expressed about placing individual increments against opposing walls simultaneously before photocuring, as the resulting polymerization shrinkage stress may cause the cusps to bend toward each other and deform as a result.8,12 This stress may cause postoperative sensitivity and can be detrimental to the tooth and the marginal integrity over time.3,19 For the proposed technique, each horizontal increment was split, prior to curing, into four triangular-shaped portions, with each portion placed against only one cavity wall and part of the floor. This would reduce the C-factor ratio from 5, which is the highest and the most unfavorable, to the second most favorable Cfactor ratio of 0.5. Such small increment portions with a low C-factor ratio would relieve the shrinkage stress by the free composite surface flowing at the diagonal cuts and not at the bonded interfaces, minimizing the adverse effects of polymerization shrinkage stresses. With the present technique, one diagonal cut was filled completely with dentin shade composite and photocured. At this
Fig. 13. The finished composite restoration.
margin were filled with the same enamel shade composite that was used earlier and photocured for 40 seconds from the occlusal direction (Fig. 8). The final increment of translucent shade composite was placed to fill the rest of the diagonal cuts and cover the enamel shade composite increment without touching the cavosurface margin. This increment, which was not split, was photocured for 40 seconds from the occlusal direction (Fig. 9). A moderately large Class I occlusal cavity preparation in a plastic tooth (mandibular right first molar) on a
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Discussion When the conventional horizontal technique is utilized, each composite increment that connects the cavity floor with the four surrounding walls produces the highest and the most unfavorable C-factor
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point, the other diagonal cut was filled and photocured, one half at a time. This sequence would prevent composite resin from connecting two opposing cavity walls simultaneously, minimizing the negative effects of polymerization shrinkage on the cavity walls and adhesive interfaces. The horizontally placed enamel shade increment was shaped to establish the occlusal morphology and provided with two diagonal cuts before photocuring to minimize the development of shrinkage stresses at the enamel and the cavosurface margin. To avoid creating contrasting shade variations in the final restoration next to the cavosurface margin, the four ends of the diagonal cuts next to the enamel wall and cavosurface margin were filled with the same enamel shade composite and not with the translucent incisal shade composite. Such a shade contrast would make the final restoration look artificial and would not match the surrounding tooth structure. Clinicians are well aware that placing a high-quality posterior composite resin restoration is demanding and techniquesensitive. For the proposed technique, the clinicians need to sculpt only the last dentin increment and the enamel increment overlying it. By comparison, the cusp successive buildup technique requires clinicians to place and sculpt several wedge-shaped oblique increments for building up each cusp separately. This would require considerably more chairside time than that needed to place a similar restoration using the proposed technique. The increased time probably would require dentists to increase their fees. It is widely accepted that Class II cavities are restored with posterior composite resins by first building up the proximal wall with composite resin and converting Class II into Class I.20-22 The proposed technique also can be utilized to restore the remaining part of the proximal cavity as well as the occlusal cavity. Marginal integrity and microleakage in vitro experiments are being conducted to evaluate the effects of the proposed technique on the quality of margins in Class I occlusal composite restorations and to compare them with other placement techniques.
Summary The split increment horizontal technique proposed for placing posterior composite resin in moderate-to-large Class I occlusal cavities offers several advantages over currently available techniques. In addition to minimizing the detrimental effects of polymerization shrinkage stresses at the adhesive interface and cavity walls by reducing the C-factor ratio from 5.0 to 0.5, this technique would make placing composite resin in Class I occlusal cavities easier and faster. The proposed technique would benefit lessexperienced general dentists and those who work in busy dental offices at governmental hospitals, as it would encourage them to satisfy the esthetic dental needs of their patients by providing highquality posterior composite restorations.
Author information Dr. Hassan is a professor and Head of the Operative Dentistry Division, Restorative Dental Sciences Department, College of Dentistry, King Saud University, Riyadh, Saudi Arabia, where Dr. Khier is a professor, Dental Biomaterials Division.
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[email protected].
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