Unless you have eliminated direct restorations from your repertoire, you will need a product that will provide a temporary wall against which a material can polymerize. This is the basic function of a matrix. While your choice used to be confined to which size band you were going to place into the Tofflemire retainer, the current offerings come in a variety of forms, including self-retained, metal or plastic, regular or pre-contoured, sectional or circumferential.
Typically easier to use and thinner than plastic ones. Due to their malleability, metal bands can usually be sealed at the gingival margin more completely to prevent overhangs and are less likely to lose their contour if aggressively wedged. They can also be burnished against the adjacent tooth to help overcome less than ideal tooth contours.
The main advantage of the clear plastic matrices is the ability to cure the gingival increment of composite from a position other than the occlusal. Since some researchers believe composite will "shrink toward the light" source (although recent studies refute this theory), curing from a position other than occlusal has been thought to reverse the direction of this shrinkage and thereby reduce microleakage at the more susceptible gingival margin. (It is more susceptible to microleakage due to being in cementum or in poor quality enamel.)
However, the studies showing improved marginal integrity with clear bands (and light-reflecting wedges) are in vitro, have variable results, and do not take into account the contemporary adhesives which are much more effective in resisting polymerization shrinkage. In addition, there are now a number of materials available that have minimal shrinkage properties. Consequently, we still favor pre-contoured metal matrices for their ease of use and predictability.
Conventional matrix retainers are not always needed or desirable since overtightening of a matrix band may cause compression of the cusps. When the band is loosened, the compression pressure may be released, allowing the cusps to rebound with resulting microleakage. It may also cause the cusps to be bonded together under stress and may be responsible for postoperative sensitivity as well as cuspal fractures. In addition, normal Tofflemire-type retainers may tear or deform the less rigid bands in this category.
On the other hand, retaining rings, which are used in conjunction with sectional matrices, are very important in providing stability for the band as well as separating force to ensure tight contacts.
Achieving a positive contact with all the bands can usually be enhanced by prewedging the restoration and by holding the band tightly against the adjacent tooth with an instrument during curing. However, prewedging can also have deleterious effects on the papilla. Prewedging is usually not necessary when using a sectional band with a retaining ring except when the tightness of the pre-treatment contact makes placing the band very difficult.
For the most part, these bands are only necessary when there is no proximal contact, since one of their main purposes is making the restoration of that contact predictable. In other words, if your preparation is very conservative and does not break contact, a more conventional Tofflemire-like matrix is quite satisfactory, less expensive, and easier to place. Sectional bands are difficult to insert if the contact is not broken.
But assuming the Class II lesion or previous defective restoration is wide enough to require opening the contact, these bands will be quite helpful. Your first task is to select the size band that best fits the tooth you are restoring. The small bands are especially helpful for premolars, the standard size fits most molars, and the large bands come in handy when the gingival wall extends onto the root surface.
An individual matrix is used for each proximal box, meaning two are necessary for an MOD restoration. Once you have the matrix in position (usually with the concave edge toward gingiva, notched edge or dimple at occlusal), wedge passively. This wedge is merely placed to eliminate overhangs, not to separate the teeth for a more positive contact. Therefore, place the wedge with just enough pressure to keep the band against the gingival margin. It is sometimes helpful to cut the wedge short so it does not protrude from the gingival embrasure. Cutting the wedge short prepares the area for placement of the retaining ring. However, new tine designs have recesses for wedges, making the sectioning task less necessary.
Using a dental dam or dedicated forceps, engage the bow of the retaining ring and exert opening pressure, which will separate the tines. Once the tines are positioned over the proximal line angles, the expansion pressure from the dental dam forceps is released, allowing the ring to grab the matrix and hold it against the tooth. This placement of the tines not only minimizes or eliminates excess composite at the line angles, but also exerts pressure on the band against the adjacent tooth to produce a positive and tight contact.
The small rings would be used typically for premolars while the rings with long tines would be used primarily for molars. However, select the ring based on the occlusogingival dimension of the restoration, regardless of what type of tooth is being restored. Once the band and ring are placed, aggressively burnish the contact area against the adjacent tooth surface with the convex back surface of a spoon excavator or any other instrument of your choosing.
Even though there are subtle differences between the brands, they all work similarly. Since the retaining ring can be adjusted, use adjusting pliers to open or close the tines to better fit the tooth if necessary. However, our experience shows adjusting the rings is rarely necessary. Assuming the ring and band are stable, burnish it to further refine its shape and to assure a tight contact. This whole process may sound confusing and laborious, but once learned, progresses quickly and produces a superb contact. However, due to the contour of the bands, composite placement is sometimes more difficult at the gingival floor. Extra care must be taken to prevent voids.
Since they are precut and contoured, it is sometimes difficult to get a sectional matrix through a tight contact, especially if the contact has not been opened during the preparation. However, if you anticipate this problem, the retaining ring can sometimes be placed at the beginning of the procedure, assuming it doesn't interfere with the cavity preparation. This is one of the main reasons for the elongated bow design by one manufacturer. In this way, the ring will apply pressure at the contact for the time it takes to prepare the cavity and may be able to provide enough separation to allow the matrix to be placed without a problem.
If you still can't get the matrix through the contact, a conventional band, being thinner and longer, can be see-sawed through it. Once it is in proper position, it can be cut to a convenient length. A retaining ring can also be used with a section of a conventional Tofflemire-type band.
NOTE: Numerous examples of the placement techniques for different matrices are presented in The Techniques, Volume 1.