Reflection: Teaching session class II composites

The other day I had a session as a clinical educator in the university simulation clinic. I was surprised to find when I turned up that I was the only dentist there and the person who normally led the class was on leave. Eventually it fell to me to "lead" the session which just involved announcing what the exercise was for the day and what steps were involved. On reflect it wasn't a spectacular effort but I will definitely be able to do better next time. The topic was class II composites and I had some thoughts about some common things that I explained during the session and some common errors that I noted. Hopefully when I am in this position in the future I would have a bit more of an idea of what I am doing.

We were using the V3 matrix system and covered the basics of how to use this system. Some notes on each component: Band, wedge, ring.

There are different sizes of matrix band, the coloured plastic markers in the container will tell you which size the band is. When choosing the band size, look at the cavity and estimate the occlusogingival dimension. The gingival aspect of the band should be below the cavity floor and the occlusal aspect has to be just above the future marginal ridge. The band has to be above the marginal ridge so the curve of the matrix band can be used to place composite against which will make the shape of the marginal ridge and occlusal embrasure.When rubber dam is used, a band that is too wide will bounce back if you try to push it down and can deform. it will also sit up too high and will make it harder to judge where the marginal ridge should be. In general 5.5mm for molars 4.5 for premolars and 3.5mm primary molars. Subgingival extensions may necessitate a 6.5 or a 7.5mm band. Variations on normal can be possible with short clinical crowns and recession.

The plastic wedge in the V3 system doesn't separate teeth but just seals the band against the tooth. You want the wedge that fits into the tooth with friction but if the wedge is too large it may not fit in without deforming. The good thing about the wedges is that it has a V shaped cross section so if the wedge is slightly too large it will fold slightly and can be forced in. However, if you have it much too large the cross section makes it weak and it will bend. This is compared to wooden wedges which have a role in separating teeth. So if you push wooden wedges in with force, the teeth will move apart and squeeze in even if it is slightly too large. 

If there is lots of friction when pushing the wedge in it commonly pushes the band in the direction you're pushing the wedge in. Use your finger to hold the band down to avoid it being displaced. If rubber dam is being used, the interdental dam can elastically pull the wedge out so use your other finger to stretch the dam out then push the wedge in so the recoil of the dam pulls the wedge in instead.

Check the seal of the band before proceeding with ring placement. Each step needs to be done well before proceeding as with any matrix system, it is either done right or it isn't and if it isn't then the placement of restorative material will be substandard. When you are checking your matrix band, be self critical. Check 3 things:

1. The little holder tab of the band is in buccolingual centre of the cavity, this matters less with v3 but with other systems there is a dip in the occlusal aspect of the band i.e bean shape bands and the marginal ridge shape will be off if the band is placed off centre. 

2. Check there is contact between the band and the adjacent tooth. If there is an open contact between the matrix band and the adjacent tooth, there will definitely be an open contact with the restoration. If there is no contact with the adjacent tooth, it may be due to the ring pulling the band away. To solve this you can attempt to burnish it back out out or re-place the ring. Take the ring off and see if the band is able to sit passively against the adjacent tooth. If not then it may be the wedge that is displacing the band by impinging into the cavity space. If this is the problem, you can trim the wedge with a bur or try a different wedge. I often go through a few different sizes and types of wedges to find the one that fits. 

3. Check the seal of the band at the base of the cavity. If there is a poor seal, there will be overhangs in the worst place to clean up. If there is a black line (gap) or coloured line (rubber dam or wedge) at the base then it may be the band is too short to seat far enough down or the wedge is too small to seal the band against the tooth.  There may be a gap part way across the base of the cavity where you've cut a dent in your prep or there is a concavity in the tooth surface. If this occurs, it is very difficult to seal the band against the tooth so as your first increment you can seal the base of the box with flowable resin and cure to prevent overhangs when you come to put resin in under pressure.

The ring is used to adapt the band against the tooth and provides separation of the teeth that is normally provided by wooden wedges. The ring squeezes in, contacts the adjacent teeth on an inclined plane on the buccal and lingual. The net force is in the direction that separates the teeth. This works only when there is enough buccal and lingual tooth structure to support it the ring. Some ring systems contact the tooth in different positions and can even be used against the wedge to separate the teeth. If there is insufficient tooth structure to support a ring you can consider using an alternative system band system e.g circumferential band or using the sectional matrix with a wooden wedge with no ring. 

Understanding the why is as important as understanding the how. For example:

-The reason why we use a sectional band is because it has an anatomic contour. The reason why is can has an anatomic contour is it doesn't rely on circumferential tension to stay on the tooth which would otherwise straighten the band out. 

-The reason why the band is thinner is it takes up less space and therefore makes it easier to achieve a contact. The downside of this is that it is more flimsy and if you try and force it in it will deform. As a result, the reason why we break the contact in a simulation session is mainly for convenience and "to have the margins in an accessible location" to allow finishing, polishing and hygiene. Clinically, if the caries is removed, most of the time the contact will be broken as almost by definition the caries appears under the contact point. Very conservative preparations may not break the contact on the buccal and lingual and it is up to the call of the operator to decide whether or not to cut healthy tooth structure for restorative convenience.

-The reason why we layer composite is to minimise the effect of polymerisation shrinkage on the material properties of the restoration. Shrinkage occurs towards centre of curing composite mass. During the gel stage of setting, stress is dissapated but in the post gel stage there will be pulling on walls. If shrinkage stress overcomes the bond strength of the composite resin to the bond layer, the composite will pull away from the walls and result in marginal leakage, sensitivity staining, also to allow light to penetrate. darker more opaque composites need thinner layers. Therefore what we want to avoid is a situation where there are opposing walls in a single layer as the opposing walls will pull against each other and increase the shrinkage stress. The resin will therefore pull more towards the best bonded surface usually the largest surface area of one with the most enamel.

When handling composite there is a technique to make it easier. The constant difficulty I faced as a student was trying to pick up and put down the composite. To make this easier, a clean instrument is essential. materials smeated onto the metal surface will allow the composite to stick so use a tissue or gauze to clean any unset material off or scrape any set material off. An alcohol wipe can be used to clean any residues off the instrument surface. When picking up the composite , use a very light tough. The harder you press the instrument into the mass of composite, the more it will adapt onto the instrument and stick. Instead, use the instrument to cut off the size of increment you want and lightly nudge the increment until it comes away from the mixing pad. Keep nudging it until the increment sticks to the instrument. The amount of pressure you need to pick it up is similar to the amount you need to put it down. Take it slowly over to the tooth surface and nudge it against the tooth to stick it to the tooth.

When the rubber dam septum is bunched up in the interproximal area, it will take up room where the wedge should sit and stop the wedge from entering. This will force the wedge coronally above the base of the cavity and the wedge will get become jammed between the teeth and deform the wedge. The wedge should be sitting in the space as apically as possible.