Observing public specialist part 1

I had a day off during the university break and decided to half a day observing in the public specialist clinic. I observed the final impressions of  35 and 37 implants for fabrication of a fixed bridge. This was part of a larger rehabilitation where the other teeth had already been restored with crowns. There was an existing model with implant analogues in it but the new impression was to capture the implant position with the tooth supported crowns in place. I jotted down some notes in regards to this procedure:

General notes: 

He was extremely neat and particular and knew what to do at each stage of the procedure. his tone was calm and he explained things slowly and thoroughly to his patient and DA. Everything was measured precisely with no haphazardness or waste or sloppiness e.g picking up adhesive on a swab was done precisely with half the swab delicately dabbed in the liquid rather than dunked in as I do. He was precise with the amount of blue wax he put in the embrasures, with the amount of pattern resin he picked up on the microbrush, all done with a light touch. I thought I was a picky dentist but watching him made my clinical demeanour look like an inexperienced student. It is not just important to work the way he does to appear in control, but to keep a level head and conserve your energy throughout long appointments and long days. If you are doing things rushed, this leads to mistakes and burns through more energy.

Once the screwdriver is in the channel and the screw is engaged, rest the handle on the inner surface of the thumb and keep this stationary to support the screwdriver. Then use your forefinger to turn the driver. to firmly tighten the screw, switch to the finger tips of the thumb and index finger. If there is lack of space i.e at the back of the  mouth use tweezers to hold screwdriver down then fingers to turn it. Use shorter screwdriver to get further back.

Gauze at back of mouth to catch components.  especially porcelain is slippery.

 Provisional bridge:

The current temporary bridge was constructed in a previous appointment out of Artglass indirect composite resin. He said these could last a good 12-18 months if there is a good connector size.

The lab composites are less bad compared to chairside composite resins in terms of making a temporary bridge as they shrink less and are done in layers. Once the model is verified  clinically, it is fine to make temporary bridges in the lab out of composite  as you know the stress in the material will not be too great and the bridge should seat. Any shrinkage should be clinically irrelevant but if the shrinkage is so much that the temporary cylinders don't fit back on then this is not good but it will be obvious if there is a mismatch.

Pink teflon tape is thicker than blue so he uses this to block the screw channel cavity as less is needed to close the access. Make a very long sausage of teflon tape and pack it on top of the screw with an amalgam packer until the access is completely full. Then pull the sausage out slightly and have your DA cut the end off with sharp scissors. Pack this tail back in and you'll have just enough space to place restorative material. He used cavit after this as he would be reaccessing the in a day or two as he was  planning to do a model verification appointment. He put cavit in with a flat plastic, used a wet cotton pellet to press it in gently then a dry cotton pellet to pack it properly and clean the excess.

Implant screw:

Gold screw preload stretches more than titanium screw

Most prosthetic screws at the abutment level are 15 Ncm to torque but always check the instructions as different brands will have different specific torque requirements. Be aware of the difference between lab screws and prosthetic screws. Final screws should always be used straight out of the packet. Beware of reusing titanium screws e.g when replacing a prosthesis as they can only be reused once after tightening. The titanium is quite stiff so if you tighten the screw, then remove, then retighten multiple times they risk fracture as they don't have much ability to stretch. Gold screws stretch more so can keep being reused multiple times. Titanium can't be reused more than once but you can always just change to a new titanium screw. This occurs commonly when a patient comes in with a loose screw and crown. Purely tightening the screw risks screw fracture in a short period. It is safer to change to a new titanium screw. Reusing the prosthetic screws during the provisional and impression stage is ok as they are in place for a short term and not highly torqued.  

Prosthetic screws are soaked in savacol when they are out of the mouth. Flush the screw channel of the implant with savacol just before reinserting the temporary bridge. I am wondering if it is ok to have lots of fluid in the implants and will there be any issues with hydraulic pressure?

Lab screws are made of brass. Never use these in the mouth as when you torque above 10 NCM they will always fracture.

Impression jig:

He had an implant verification jig made prior to the appointment. This was made on the previous models. It involves an impression coping being placed in each implant analogue then joined with pattern resin in a flat platform (Figure 1). This serves to rigidly splint each impression coping together to account for the shrinkage, distortion and flex in the impression material. This was made quite precisely with the copings joined as there was an existing model to make it on but they can be made chairside with pattern resin, or generic shaped ones made in the lab that are then trimmed to fit. Alternatively, metal bars can be cut and bent chairside and luted to the impression copings with pattern resin or composite resin. This avoids the issue of inter arch contraction of material. I have also seen floss threaded around the impression copings in a figure 8 shape then flowable composite resin run along the floss and cured. My gut feeling is that the shrinkage on this would be significant but it is an alternative, simple technique.

Figure 1: An example of an implant verification jig.

The concept of the jig is to provide a rigid connector that has minimal shrinkage to provide an undistorted spatial relationship of the implant fixtures. To minimse shrinkage with the pattern resin jigs, the jigs are premade joined together, screwed into the implant fixtures. Then the jig is sectioned and rejoined with chairside pattern resin. Sectioning the jig relieves the assembly of any stress that is contained in the material and rejoining it by curing a small amount of resin minimises the volumetric shrinkage.

Clinical steps (verification jig):

1. Mark the anterior buccal aspect of the jig with a permanent marker. to aid in locating intraorally. 
2. Place gauze at the back of the mouth for airway protection. 
3. Remove the cover screws, insert the jig and finger tighten the screws. This will put tension on the jig acrylic and at the impression coping, implant interface.
4. Section with the jig in the middle with a Long tungsten carbide fissure bur in a high speed handpiece with water spray. This will relieve the tension in the system. The thinner the slice, the less future shrinkage you will get as the volume of acrylic is less. In the past he used to use thin laboratory discs to section the jig but the trade off is there is no guarantee that the material will make it all the way through and join the pieces together.
5. Retighten the screws as he stress in the material may have stopped full seating. Tighten until very firmly finger tight.
   
6.  Rejoin the jig in pattern resin. He prefers GC pattern resin as it shrinks less than Durelay (7-8% vs 15%). 85% of the shrinkage is done in the first 25 minutes so it is wise to premake the bulk of the jig and let the shrinkage occur in the lab rather than in the mouth. The DA holds 2 dishes of pattern resin 1 powder and 1 liquid and he salt and peppers with a microbrush. The tip of the brush is half dipped in the liquid then lightly dipped in the powder. If it is too dry, it is then lightly dipped in the centre of the liquid to pick up more liquid.pickup up too much powder or liquid causes the mass of pattern resin to be too large and makes it more difficult to handle and it may fall off the microbrush.