Session 1 - The Nature of the Dental Unit Waterline Problem 1.1 Introduction.  1.2 The Problem of Biofilm. 1.3 What Makes Dental Unit Waterlines Unique. 1.4 A Historical Perspective.   1.3 What Makes Dental Unit Waterlines Unique Unlike household waterlines, dental unit waterlines provide living conditions, which are particularly suited to biofilm formation. Ideal conditions for biofilm are highly dependent on particular water flow characteristics. The following chart compares and contrasts the different water flow characteristics found between dental unit water and municipal and household water. DIFFERING FLOW CHARACTERISTICS Dental Unit Water - laminar flow - low flow rate - low volume of use - small diameter tubes - high surface/volume ratio - long lengths of tubing - room temperature or warmer - plastic tubing - dental water aerosols   Municipal Water - turbulent flow - high flow rate - high volume - larger diameter pipes - low surface/volume ratio - copper pipes Long lengths of narrow-bore tubing lead to low volumes of slow moving water. These conditions are ideal for substantial water stagnation. These flow characteristics are ideal for microrganisms because they ensure minimal disruption, which encourages further colonization. In many ways, the dental unit is an ideal incubator for microganisms. For example, 1. The dental waterlines are made of plastic. The typical water bacteria are heterotrophic bacteria. This means the bacteria need carbon as a source to metabolize. Our plastic waterlines (opposed to the copper pipes in our house) provide the ideal environment to enhance bacterial growth and replication. 2. Dental unit water is usually stagnant. Dental units are not in operation most of the time - only operating 8 hours per day. Out of every hour of operation, water is actually flowing only about 10 minutes. Even when the unit is in use, the water is usually stagnant. 3. Low flow rate. Even when the treatment water is on and flowing, the rate is very low - often only 5-50 ml per minute. This low flow rate with little turbulence helps stagnation and encourages bacterial replication. 4. Laminar flow. Even with the water flowing, the water flows fastest in the middle of the tube, slowing towards the edge. As the surface is approached, the flow rate decreases. Inside the biofilm spongelike layer, there is probably little if any flow, or stagnant water. 5. Surface to volume ratio. As the diameter of the tubes decreases, the surface area for any given volume increases. This algebraic ratio appears to be a major factor. The dental waterlines are only 1.5 millimeters in diameter (inside measurement, or 1/16 inch). It virtually guarantees that any bacteria in the water have the opportunity to be in contact with the surface, and then attaching themselves to the surface to grow, much like plaque in our mouths. Some dental units even have water heaters to keep the temperature warm for patient comfort. This increased temperature helps select for bacteria that live well at body temperatures. Potential human pathogens are actually encouraged to grow by our process of selection. The biofilm is like a "sponge layer" inside the narrow tubing. There is an abundance of pores within this layer with a tremendous surface area to support bacterial growth. One way to picture this phenomenon is to visualize water flowing over a sponge filled with paint. How much water has to flow over the sponge to remove all the paint? How easy is it to remove the paint even if the sponge is repeatedly squeezed while the water flows? Although flushing with water may temporarily decrease bacterial levels, it is not a predictable method of control. In fact, in dental waterlines, the low flow rate of water actually helps remove toxins from the biofilm and bring in more fresh water, which in turn nourishes the bacteria. The removed toxins come out with the coolant water. Until fairly recently, most dentists did not become aware of the: 1) extent of the biofilm problem and 2) it's implication for their practice of dentistry. In the next section we will take a step back in time to see how these two issues developed over the past 40 years. This completes 1.3.

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