Current literature tells us that it isn’t possible to achieve a truly “passive” framework that splints multiple implants. No matter what techniques or materials are used, there will always be some measurable stress or strain created. It is known that framework misfits can cause screw breakage as well as component fracture but it’s effect on the supporting bone is still something of a mystery. Some researchers feel that the bone can remodel in response to an ill fitting framework but the amount that can be tolerated isn’t quantifiable. Best practice procedures tell us that we should strive for frames that show no clinical or radiographic marginal opening, so it is important to review some simple ways to control clinical and laboratory generated errors.
The most common cause of implant framework misfits is error introduced during the impression phase. Choice of technique and materials can help or hinder the production of an accurate master model. Open tray impression copings avoid error introduced by reinsertion of copings back into the impression in the closed tray technique. Sandblasting and use of tray adhesive on the impression copings further ensures that no movement will occur during removal of the impression from the mouth. In the case of implant level impressions, internal connection platforms preclude pre-impression acrylic splinting of copings because of potential path of withdrawal issues. Abutment level impressions of multiunit abutments offer the clinician the ability to connect copings with one another via acrylic splinting which lessens the possibility of elastic deformation of the impression material upon removal from the mouth. In effect the impression is taken with acrylic resin and the elastic impression material is only utilized to record the soft tissues. Care must be taken in acrylic resin splinting of impression copings to minimize the inevitable shrinkage of these materials. Use of fine grained polymers such as those found in pattern resins exhibit less shrinkage than others. This shrinkage can occur over a number of hours, so its best to make the bulk of the splint ahead of time. Laboratory sectioning of each unit with a thin diamond disc and intra oral reassembly will keep shrinkage errors to a minimum. If direct vision of the marginal interface between the impression coping and the implant platform isn’t possible, radiographic confirmation is recommended. Once the impression is made you can breathe a sigh of relief and the laboratory phase can begin at your discretion.
Before pouring the implant impression it is important to control the expansion of the stone which can introduce large linear errors. Use of a Type 4 dental stone is recommended as it typically exhibits an expansion of approximately .08%. Isolating the analogs with straws prior to pouring will further minimize this expansion. Insertion of the straws into the soft tissue moulage before setting is a convenient way to close the gap at the base of the straws which will prevent the intrusion of stone. Only after a setting time of 24 hours is polyurethane die material injected around each analog to connect it to the cast base.
And so, a thorough knowledge of techniques and materials allows us to choose the best for each purpose and an understanding of the potential errors induced by each gives us the opportunity to minimize errors in the production of the most accurate cast that can reasonably be made. Frameworks from master casts made in this manner allow the clinician and technician to take full advantage of the inherent accuracy of digital milling strategies. Expect to enhance the long term prognosis of your implant reconstructions.