Partial vs Full Coverage Restorations Part 2
In our last post we talked about how we fall into the trap of having to ‘choose’ whether to prepare and deliver a partial or full coverage restoration. Considering how dental materials developed to be retained by adhesion, we shifted our scope to comparing restorations on whether it is a conventional restoration retained by mechanical retentive features cut into the tooth; or adhesive restoration retained by chemical bonding via resin cement
Charting and Maintaining Tooth Longevity
Our two jobs as prosthodontists are rehabilitating natural teeth or replacing them. Replacing previously restored teeth is a straightforward decision in treatment planning, but the more difficult one is deciding when a tooth is suitable to be rehabilitated or if the patient’s quality of life would be better if it were to be replaced. That’s where prognostication plays the role.
When looking at prognostic factors for a natural tooth, remaining tooth structure (even after preparation) is placed front and centre. McDonald’s Restorability Index relies solely on remaining tooth structure as the criteria or suitability to rehabilitate a tooth and Patel’s Practicality Index puts tooth structure as one of the prognostic criteria alongside periodontal and endodontic health.
What neither prognostic index looks at is the type of restoration on the tooth. There are indices for restorations, but prognostic indices that focus on the longevity of natural teeth will not consider either the type of restoration or its material.
What we focus on here is the preservation of tooth structure. There is a reason it’s the first of Shillingburg’s principles of tooth preparation. Maintaining tooth structure will provide a less complicated treatment pathway, help fabricate more biologically oriented preparations and provide treatment options other than extraction as the restoration’s service life comes to an end.
How Much Tooth Structure Gets Lost at Preparation?
In 2002 the Journal of Prosthetic Dentistry published a paper by Edlehoff and Sorensen where typodont teeth underwent preps for different restorations and bridge retainers. The mean amount of tooth structure removal for preparation design increased in the following order:
When understanding this more streamlined decision making, we need to shift the lens from ‘crown strong, onlay weak’ or ‘crown invasive, composite conservative,’ or the absolute worst: ‘vertiprep best.’
- Adhesive attachment box (all ceramic) (5.5%)
- Adhesive attachment wing with two grooves (metal) (10%)
- Mesio-Occlusal or Disto-Occlusal inlay excluding transverse ridge or central groove (20%)
- Mesio-Occlusal inlay including transverse ridge or central groove I3 (27.2%)
- Half-crown abutment (all ceramic (35.5%)
- MOD Onlay (39%) (that one surprised me the most)
- Partial Crown abutment (46.7%)
- Complete crown (all ceramic) with chamfer margin (67.5%)
- Complete crown (all ceramic) with rounded shoulder margin (72.3%)
- Complete crown (ceramometal) with buccal shoulder and lingual chamfer(75.6%)
What raises eyebrows with a little deep reading is that the most invasive preparation in the list is actually the conservative design for ceramometal or even prorcelain fused to zirconia crowns. Therefore, when you cut a shoulder margin in visible areas for veneering and limit the invisible areas to chamfers for the substructure material only to preserve tooth structure or to maintain ferrule, you’re still sacrificing three quarters of the coronal tooth structure by weight for your restoration.
Another point to not is that the teeth used in the study were assumed to be straightforward, upright cases (undergrad benchwork cases in other words). It doesn’t take into account tilted teeth or even ‘instant ortho’ applications that are undergone in some practices and schools
In conclusion, full coverage restorations should be avoided when not indicated for replacing old crowns or for severely destructed teeth. The current technology regarding fabricating and retaining partial coverage
restorations makes them a preferable option in terms of preserving tooth structure and maintaining prognosis of the natural tooth.
In the coming part we’ll go deeper into the concept of minimally invasive dentistry and how the contemporary partial coverage restoration can serve that purpose.
See you soon!
