Operative Dentistry MCQ Answer 01

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Answer is c. 4, 2, 1

During polymerization the organic matrix of a composite resin can undergo volumetric shrinkage on the order of 2% ~ 7%. This contraction crates stresses at the hybrid layer upwards 18 MPa. These forces can severely strain the bond between composite and tooth structure. This leads to Marginal gap formation, microleakage, stress cracking of composite, or enamel fractures along the margins.

The degree of stress development can be controlled to an extent by the cavity design, expressed in C-Factors, uses of bases, the size, shape, and position of increments, light or chemical curing. Most important and easily controlled in respects to stress relief can be accomplished by maintaining the C-Factor as low as possible.

The calculation of C-Factor is the number of bonded surfaces verses non-bonded surfaces. The higher number of tooth structure walls that will be bonded to yields a higher C-Factor. Conversely the fewer number of tooth structure walls the lower the C- Factor. Thus a standard cavity preps represent the following: Class 1 preps have 5 bonded to 1 non-bonded for a C-Factor of 5. Two surface Class 2 preps have 4 bonded to 2 non-bonded for a C-Factor of 2. Class 3 preps have 4 bonded to 2 non-bonded for a C-Factor of 2. Class 4 preps have 1 to 4, C-Factor 1. Class 5 varies according to depth and can range from 1 to 5. Of course multi-surface Class 2 (MOD, MODB, ETC), 3 (MFL, DFL), and 4 (MIFL, MIDFL) have lower C-Factors than the standard preps in that there are more non-bonded surfaces than bonded.

To compensate for this contraction the following should be factored in to the procedure:

All enamel margins, incremental fill, highly filled material (less matrix = less shrinkage, don’t bridge cusps, clear matrix.

Carvalho, R.M. et al. A Review of Polymerization Contraction : The Influence of Stress Development verses Stress Relief. Oper Dent 1996 (21): 17-24 {source of photo}