#19 What I Learned Today

Introduction to Operative Dentistry

Our session in the sim lab gave us the opportunity to get the feel for condensing amalgam. We started off by going over the characteristics and composition of amalgam (please refer a week prior to our dental materials lecture). Amalgam has been used in dentistry for over 150 years, and is primarily used for posterior restorations, crown build-ups (when left with little tooth structure), and small cervical lesions on buccal surfaces. Amalgam is cheap and easy to use. However, it does have some disadvantages, such as poor esthetics, brittle (tends to shear easily), initially weak before it fully sets, doesn't bond directly to tooth (needs mechanical retention), and may exhibit creep (permanent deformation from constant bending).

Amalgam is composed by mixing an amalgam alloy (typically silver, tin, and copper) with liquid mercury; a process referred to as trituration. The alloy comes in powdered form, and comes in a number of specific powder types, including:

• lathe cut: angular and irregularly shaped chunks
• spherical: microspheres of different sizes
• admixed: both lathe cuts and spheres
• modified spherical: elongated round particles

Spherical amalgams typically require less mercury than lathe cut amalgams to form a workable mass, and they tend to flow better. As well, by overfilling the prep, the mercury accumulates in top layers due to condensing, which is then carved away using a Ward's carver. This helps eliminate as much mercury as possible.

Older amalgam typically used low-copper alloys (5% or less). When mixed, the mercury and metal alloying would form various compounds, with the two main ones being silver mercury (called gamma-1) and tin-mercury (gamma 2). The gamma-2 proved to be a weak, easily corrodible phase which showed high levels of creep.

Current amalgam uses high-copper alloys (13-30%), which results in the elimination of the gamma-2 phase, through tin reacting with copper instead of mercury. This allows for much superior chemical and mechanical properties.

In lab today, we condensed four types of amalgam into cavity preps in wooden blocks. The types experimented with were:

1. Tytin regular set (100% spherical)
2. Tytin FC fast set (modified spherical)
3. Contour regular set (admixed; 70% spherical, 30% lathe)
4. Dispersalloy regular set (admixed; 30% spherical, 70% lathe)

The powdered alloy and liquid mercury came in pre-packaged containers, which were mechanically triturated at different set cycles per minute (CPM) and seconds, based on manufacturer's request. The amalgam was then transfered from carrier to bowl, and added to the blocks using condenser instruments. One needs to work fast and make sure all line angles get filled properly using a smaller condensing instrument, before switching over to the larger one.