Laboratory feature
big change Small angle,
Kevin Lochhead, specialist in prosthodontics and clinical director at Edinburgh Dental Specialists, explains how a small change in angle is producing a big change in implant dentistry
F
or many years the standard approach to restoring implants has been with cemented restora-
tions, which are effectively porcelain-bonded crowns or bridgework glued to angled posts on the implants. Fabricating this type of
restoration allowed for the limitation in knowledge, mate- rials and technical skills within the profession at the time. The current trend in implant dentistry, however, is moving away from cement-retained restorations towards one-piece screw-retained restorations. Screw-retained restorations avoid the issues associated with excess cement, are easier to handle and allow for restora- tion retrieval when necessary. This changing trend in the
market has become possible largely due to the increased knowledge on how the bone implant interface behaves under static loading (and the misfit which will always occur) and, crucially, the advancement of computer numerical controlled (CNC) milling units which are able to produce solid one-piece frameworks in tita- nium, zirconium and cobalt chrome that fit perfectly, without any of the challenges imposed by a cast restoration. (Note: Milled cobalt chrome should not be confused with the cobalt chrome used to cast partial dentures. Milled
chrome is a medical grade solid structure with none of the potentially allergenic additives that are necessary for casting.) One of the continuing
challenges in delivering a screw-retained restoration, however, has been that the implant has to be placed correctly (angled to the cingulum) to allow for the screw channel to emerge where it is not seen. This can be a more challenging surgical procedure, which some implant designs (and drilling systems) make it more diffi- cult to achieve (every clinician who places implants will have experienced placing the direc- tion indicator to show correct orientation only for the implant to drift into a more labially angulation during placement.) The implant companies have
put some thought into this and the result is angled screw channels coupled with newly designed screws and screw drivers. As the image of the new ‘Angled Screw Channel Abutment’ and ‘Omnigrip’ tooling from Nobel Biocare shows, the design works by the edge of the driver engaging the screw head and still having enough grip to torque correctly. It is now possible to change
the angulation of the screw access by up to 25 degrees without any compromise in aesthetics (which would occur with angled abutments) or strength (which occurs with
screw-retained crowns using cemented link abutments). As the restoration is fixed to the implant head, there is no need for additional components. With this solution, it is
possible to take an implant that previously had to be restored with a two-part cemented solution (and the inherent challenges of establishing the correct marginal depth with provisional restorations) to a one-piece screw-retained solu- tion – in two relatively simple restorative appointments, saving time and money for you and the patient. Restoration retrieval is a huge
bonus. In a cemented situation if a patient is unhappy with the shade of a crown, removing it may be challenging or impos- sible. The screw retained process is much simpler. Porcelain margins can be taken deeper for better aesthetics, without concern for excess cement. No one milling company has a solution for all implant systems and platforms.
Combining the innovations from all companies means that, for most main implant systems, it is possible to use angled screw channels for small bridges up to full arch, one-piece, implant bridges in all the common materials. Most recently, it has become
possible to provide an angled screw channel in milled chrome for single implants. Milled chrome behaves like cast gold, allowing ceramists to make beautiful strong restorations, using ceramics they are used to, at a significant cost saving from conventional casting. To provide this solution to
patients, you only need the relevant screw driver, which is supplied by the laboratory with your first case. Providing a fixture head impression is all that is needed. It is, of course, always advisable to contact the laboratory beforehand to check what is possible for your implant system.
Scottish Dental magazine 51
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