ENAMEL AXIOM #6- SUFFICIENT THICKNESS FOR ADEQUATE LIFESPAN
In this Handbook a clinical lifespan of 25 years is considered reasonable. This corresponds to the point in the life of amalgam restorations when cusp fractures begin to occur, spelling the end of the restoration. If the profession considers amalgam a viable material, then, logically, an equivalent lifespan in resin should be acceptable. It also reflects the point in the clinical lifespan of crowns and onlays when one begins to see perforation of metal occlusal surfaces, porcelain fracture in ceramo-metal crowns, and de-bond due to luting agent fatigue or dissolution in all indirect treatment modes.
In resin, many surveys indicate shorter lifespan is being achieved, due to re-decay, marginal leakage, or other reasons. Thin composite worn through and exposing dentin is another failure mode. By following axiom #6 we will eliminate wear as a cause of failure.
Anticipating that both teeth and resin will wear over time, what thickness of resin must be provided on an occluding or incising surface to prevent exposure of dentin at 25 years?
SCIENTIFIC MEASUREMENT OF TOOTH WEAR
The study below measured wear in vivo. It concluded that in average mouths, enamel on occluding surfaces wears at an average of 30 microns per year. As with most averages, individual patients will lie outside of this range, by a number of standard deviations, to a lesser or greater extent.
- Lambrechts P,Braem M,Vuylsteke-Wauters M, Vanherle G. Quantitative in vivo wear of human enamel J Dent Res 1989;68: 1752-1754
At 30 microns per year, we can conclude that enamel margins should be minimally 25 x 30 =750 microns thick, i.e. 0.75 mm. This will provide the desired 25 year lifespan. If greater wear is anticipated, or a longer lifespan is desired, greater thickness can be instrumented to the limits of the presenting tooth structure. See Occlusal Functional Categorization.
If adequate thickness cannot be instrumented, cusps can be shoed. See Cusp shoes and bevels.
RESIN WEAR: SCIENTIFIC STUDIES
Modern resin wear at the same rate as enamel and have been doing so for over 15 years. They continue to improve, See Omnichroma for a contemporary resin that exhibits a very low wear rate.
Clinician’s Report, April 2004, established that resins of that era were capable of achieving from 60 to 90 microns of wear over a 3-year period of observation.Thus, annual wear of 30 to 20 microns is attainable.
Since that time filler loading and particle formulation have improved, with commensurate improvement of wear rates.
CLINICAL JUDGEMENT OF ENAMEL DEPTH.
The 330 bur is a convenient judge of depth; its dentate portion is 1.8mm. If the enamel margin is just over 40% of its dentate length, you have the necessary depth. However, the 330 bur is not recommended for preparation of thin enamel walls on the occlusal surface because it is occlusally-convergent. This bur shape will undermine enamel, which is the exact opposite to what is required. Rod end exposure requires either parallel or divergent walls towards the occlusal,. It is however a useful reference to judge enamel thickness. See Enamel Axiom#2 and Enamel Axiom #3