What is C-Factor?
C-Factor, or Configuration Factor, is defined as the ratio of bonded to unbonded surfaces. Higher C-Factors lead to higher stresses.
How is C-Factor Reduced?
Make use of thin layers of composite
A thin layer of composite (e.g. a layer of composite <1 mm in depth) should be placed over the Immediate Dentin Sealing (IDS) and Resin Coating (RC) layers. This thin layer of composite has a lower C-Factor as the ratio of bonded to unbonded surfaces is reduced.
Reduce structurally compromised cusps and include these in the onlay design
By including structurally compromised cusps in the onlay design, this can also help to reduce C-Factor forces as the shape of the preparation is changed.
As per the paper by Fennis et al. in 2004, holding (functional) cusps that measure <2 mm in thickness, and tension (non-functional) cusps <3 mm should be onlayed. The cusp thickness should be measured from the base of the cusp.
Consider the resulting shape of the preparation
A “plate” preparation with 1 side will have a low C-Factor. A “bowl”-shaped preparation with 2-3 sides will have a medium C-Factor. Finally, a “cup” preparation with up to 5 sides will have a high C-Factor. There are many variations within these shapes.
The type of layering technique used
The 2004 paper by Nikolaenko et al. shows that the placement of composite in thin horizontal increments results in greater bond strengths than vertical or oblique increments.
By ensuring that different substrates are not connected too early while the dentin bond is maturing, the hybrid layer is able to form in a stress-free environment. This relates to the concepts of Hierarchy of Bondability (HOB) and Decoupling With Time (DWT).
Research by Bicalho et al. in 2014 confirmed that placing composite in increments, compared to bulk-filling, increases bond strengths between 100-300% depending on the thickness of the increments. The resulting residual stress produced during polymerization is transferred to the residual tooth structure and results in less stress being placed on the maturing dentin bond.
What is the role of fiber in controlling C-Factor?
By placing a fiber insert, such as Ribbond, on the pulpal and/or axial walls, stress on the developing dentin bond and the maturing hybrid layer is reduced, as the Ribbond fibers have been shown to deform under polymerization stress.
A 2 mm ball of composite (or a “Delibit” as per the “Wallpapering Technique”) is compressed onto the floor of the cavity, resulting in a layer of 1 mm. The fiber e.g. Ribbond is wetted appropriately and inserted in the 1 mm layer then cured.
EverX, which is a Short Fiber-Reinforced Composite (SFRC), can also be used to direct the flow of composite towards the developing hybrid layer.
All of the above techniques contribute to creating a Stress Reduced Direct Composite Restoration. More information can be found in Simone Deliperi’s 2017 paper: Stress-reduced Direct Composites for the Restoration of Structurally Compromised Teeth: Fiber Design According to the “Wallpapering” Technique.
What is Deep Margin Elevation (DME)?
Deep Margin Elevation (DME) refers to the technique by which the restorative margin in a deep box preparation is raised to the level, or above the level, of the gingival margin. Gold standard isolation is necessary and complex matricing techniques are often required. DME allows isolation to be more easily achieved for indirect restoration cementation, among many other benefits. DME can be used as an alternative to crown lengthening surgery.
Bressner et al. showed in the 2019 paper that the survival rate of DME over 12 years was 95.9%.
Stress-Reduced Placement of DME Layers
The second thin layer of composite is commonly placed using AP-X composite. A layering technique is used, with either thin horizontal increments or a peripheral layer which is deliberately not connected to the axial wall over the pulp, in order to allow time for the dentin bond over the pulp to mature.
After allowing time for the hybrid layer to mature, the peripheral wall is then connected to the axial wall.
At this point, the Biobase (BB) is formed. The Biobase is made up of the following layers:
- Immediate Dentin Sealing (IDS)
- Resin Coating (RC)
- Thin layer of Fiber-Reinforced Composite
- Deep Margin Elevation (DME)
The placement techniques above allow Decoupling With Time (DWT).