The Class 2 restoration is the most-performed procedure in general dental practice in the U.S.1, accounting for significant practice revenue.

In recent years, there has been a significant shift away from silver amalgam as a restorative material and a move towards direct resin composite, which offers advantages such as:

  • Natural aesthetics, for a more pleasing appearance.
  • Mercury-free material, solving the issues of waste management.
  • Adhesive quality, therefore restoring the structural integrity of the tooth and sealing exposed dentinal tubules.

However, placement of dental composite in a Class 2 box is technically demanding for the clinician, and, as a result of common problems and challenges, many Class 2 restorations are doomed to failure before the patient even leaves the office.

A typical Class 2 restoration
Figure 1: A typical Class 2 restoration.
A typical Class 2 restoration
Figure 2: A typical Class 2 restoration.

In this three-part series, we'll explore common failure modalities and solutions to overcome these, starting with this article, where we'll begin by examining the most common problems, with a focus on one area in particular.

The common issues encountered post-operatively after placement of Class 2 direct composite restorations include sensitivity and white lines at the margins (there are several reasons for this, one of which is poor quality preparation and conditioning of the enamel substrate; this is explored in a recent Spear Digest article), poor quality or non-existent contact points, overhangs at the gingival margin or axial wall, and voids at the interface of the composite resin and margin.

Another factor is failure to control polymerisation shrinkage stress in a high C-Factor cavity, which is worth examining at greater length.

Outcomes of Polymerisation Shrinkage Stress in Class 2 Restorations

Polymerisation of dental composite resin is always accompanied by marked volumetric shrinkage of the order of 2 to 6 percent2. This is due to the conversion of monomer molecules into a polymer network, which results in a bulk contraction3.

The composite resin shrinkage strain is translated to stress within the Class 2 restoration since the composite resin is confined within the cavity geometry. When polymerization stress is generated, there are multiple outcomes, most commonly:

  1. Deformation of the tooth structure leading to enamel fracture, cracked cusps, and cuspal movement4.
  2. Failure of the interface between the composite resin and the tooth (adhesive failure). Often the resin is pulled away from the least bonded surface (resin separation concept) and a marginal gap is formed. This margin is typically the gingival box floor since the enamel in this location is often extremely thin or lacking. Clinically, the outcome is white lines and sensitivity in the short-term; oftentimes this will progress to secondary caries in the mid-to-long term.
Marginal gap formation
Figure 3: White line indicating gap formation.

The impact of polymerisation shrinkage stress depends on the size of the cavity: larger Class 2 restorations have lower stress levels in the restoration and larger stresses within the tooth structure; therefore, the tooth tends to fracture. In contrast, in smaller restorations the tooth is more resistant to polymerization shrinkage stress and the stress concentrates within the restoration, resulting in marginal breakdown.

Factors Influencing Polymerisation Shrinkage Stress

So why does polymerisation shrinkage stress occur?

There are four main factors that contribute to the process:

1. Filler Content

Composite resin is a polymer matrix with filler particles. Shrinkage is directly related to the proportion of polymer matrix within the resin: the more monomer converts to polymer, the higher the degree of contraction. Conversely, the space occupied by the filler remains unchanged during polymerisation. Therefore, the higher the filler content the less stress is generated5.

2. Degree of Conversion

Small increases in the degree of conversion result in substantial stress increases. However, this should be balanced by the fact that higher conversion improves mechanical properties6.

3. Elastic Modulus

The polymerisation stress is correlated with the rate of gain of Elastic Modulus (stiffness) of the composite resin. The stiffest material generates the most stress. Further, Elastic Modulus increases as the polymerisation reaction proceeds7. This is expressed in Hooke's Law: Polymerisation Stress=Volumetric Shrinkage X Elastic Modulus. In other words, the higher the elastic modulus and volumetric shrinkage of the composite, the higher the stress generated.

4. C-Factor Ratio

The concept of C-Factor (or Cavity Configuration Factor) was originally described by Feilzer, De Gee and Davidson in 19878

A high C-Factor ratio gives a high propensity toward the generation of higher stresses within the restoration. For example, a direct resin veneer has only one bonded surface (the facial) and one free surface, giving it a low C-Factor. In this restoration, polymerisation contraction is restricted in only one direction, therefore allowing the composite to flow freely in the early rigid stage, reducing stress and allowing development of high bond strengths. In large masses, therefore, management of polymerisation stress is a simple matter and resin masses can usually be placed and polymerized without concern.

In contrast, the Class 2 proximal box has four bonded surfaces (the gingival floor, the facial axial wall, the lingual axial wall and the pulpal wall) and only two unbonded surfaces. Here the contraction stress is hindered by the cavity walls in three directions and there is less potential for stress release by flow, which results in higher polymerisation stress during polymerisation. This requires more complex management strategies, which we will address in the following articles about Class 2 restorations.

Jason Smithson, BDS (Lond), DipRestDentRCS (Eng), is a member of Spear Resident Faculty.


  1. “American Dental Association Procedure Recap Report.” Chicago, IL: American Dental Association; 2006.
  2. Bowen, Rafael L., Kimiya Nemoto, and James E. Rapson. "Adhesive bonding of various materials to hard tooth tissues: forces developing in composite materials during hardening." The Journal of the American Dental Association 106.4 (1983): 475-477.
  3. Patel, M. P., M. Braden, and K. W. M. Davy. "Polymerization shrinkage of methacrylate esters." Biomaterials 8.1 (1987): 53-56.
  4. Meredith, Neil, and Derrick J. Setchell. "In vitro measurement of cuspal strain and displacement in composite restored teeth." Journal of dentistry 25.3-4 (1997): 331-337.
  5. Condon, John R., and Jack L. Ferracane. "Assessing the effect of composite formulation on polymerization stress." The Journal of the American Dental Association 131.4 (2000): 497-503.
  6. Braga, Roberto Ruggiero, and J. L. Ferracane. "Contraction stress related to degree of conversion and reaction kinetics." Journal of Dental Research 81.2 (2002): 114-118.
  7. Braem M, Davidson CL, Van Hearle G, Van Doren V, Lambrechts P. The relationship between test methodology and behaviour of composites. J Dent Res 1987; 66:1036-9.
  8. Feilzer AJ, De Gee AJ and Davidson CL. Setting stress in composite resin in relation to configuration of the restoration. J Dent Res 1987 Nov:66(11): 1636-9