How to locate subgingival calculus

Subgingival calculus as the name implies is out of sight and therefore can be difficult to locate and clean. In this case, out of sight doesn't mean out of mind as it is our responsibility to locate these deposits and clean the tooth surface during the patient's regular periodontal maintenance visit. Locating these deposits in the first place is the difficulty and this can become much more difficult after debridement has begun due to bleeding of gums. Below are some points on how to identify subgingival calculus.

• Supragingival calculus tends to be yellower, chalkier and rougher than the surrounding tooth structure. Once you know the proper shape of the tooth and root surface, any deviation is easy to identify as calculus. Drying the tooth will help differentiate the two surfaces. Subgingival calculus is often proximally related to supragingival calculus as the same poor oral hygiene practices that have led to one lead to the other. Areas where subgingival calculus is present without supragingival calculus may be where supragingival calculus has become submerged under the gum due to gingival swelling, the supragingival calculus has been cleaned off by the patient or dentist without clearing the subgingival deposits or when the patient has an aciding oral environment which has dissolved the supragingival calculus but not the subgingival deposits as they are protected by the gingival crevicular fluid. 
• Associated with subgingival calculus: Recession will exposure previous subgingival calculus as a rim of green-black buildup around the tooth. This can indicate further subgingival deposits in a similar location. What I see in my practice most apparent on periodontally involved extracted teeth is a ring of obvious calculus just below the CEJ and much less subgingical calculus associated below this. This is because initially when the periodontal disease process was starting slowly there was enough time to form supra and subgingival calculus jsut above the level of attachment (usually just below the CEJ). As the disease progresses and attachment is lost. Plaque accumulates below this rim of calculus and the rate of loss of attachment increases as the exposed root surface area increases. If recession is slower than attachment loss which is almost always the case then the rate of attachment loss will increase at an increasing rate. Therefore the plaque under the initial rim of calculus will not have enough time to form into subgingival calculus.
• Radiographs: Bite wings, periapicals and opgs can identify the extent of calculus formation. As thin deposits are often burnt out by the radiographic exposure, this can give a false negative. Deposits that are thick enough and mineralised enough to show up on radiographs implies that they have been present for an extended period of time. They are often adjacent to obvious bone loss areas on the radiograph. Due to the direction of the xray beams passing through the dental tissue, only interproximal calculus deposits will appear on the radiograph. These are often associated with buccal and lingual deposits that are more simple to identify visually.  Where there are significant interproximal calculus deposits, it may be useful to perform a debridement before radiographs are taken to use the radiographs to screen for missed areas. Either this or retake a set of radiographs after a debridement for the same purpose.
• Dark colour: Subgingival calculus is darker than their supragingival equivalents due to chronic exposure to gingival crevicular fluid, blood and blood breakdown products. In thin biotypes, this dark colour can show up as pigmentation on the gingival margin as the tissue will be slightly translucent. 
• Due to the inflammatory reaction of the body to subgingival calculus, adjacent gingival areas will often be inflamed. Red, rolled gingival margins or inflamed interdental papillae should clue you in to search for subgingival calculus at that site.
• Periodontal probing: The use of a periodontal probe can actually physically feel subgingival calculus deposits. Light downward pressure and definite pressure against the tooth surface is necessary to avoid false negatives. Smaller deposits or burnished calculus can be difficult to locate with this method.
• Dry the tooth: A wet tooth will cause scattering of the light due to the optical properties of the water. Drying the tooth will allow you to see the porosities present in supra and subgingival calculus to more easily differentiate it from the surrounding tooth structure. Blowing air at the right angle (quite parallel to the tooth) will cause retraction of the gingiva allowing you to look down the gingival crevice with magnification and illumination. I find this the most reliable way to check for calculus as it concurrently allows you to clear GCF and blood away from the site. More severe periodontal conditions will have better tissue retraction due to looser tissue and this will allow a wider field of view.
• Bleeding on probing: This is a positive sign of inflammation. It is associated with bacterial deposits some of which may be calculus.
• Raise a flap: During surgical debridement, raising a flap will allow you direct visual access to the calculus deposits 
• Periodontal review: During review, refractory sites with worsening or no improvement of probing depths may be due to missed calculus. Check these sites carefully with the above methods.
• Check common sites:  lingual surfaces of all lower teeth and buccal surfaces of upper molars due to their proximity to major salivary gland ducts. I also notice quite frequently significant calculus buildup around the upper anterior teeth on the palatal and buccal in mouthbreathers as they grow plaque quite well due to the drying action of mouth breathing. Checking inteproximal areas is important as most of the population doesn't floss ass effectively as they brush so calculus will tend to build up faster in the proximal areas. due to the papillary fill of the interdental space in virgin teeth any calculus present interproxmially will be by default subgingival calculus.

Having multiple methods to detect subgingival calculus is important to ensure that your periodontal cleans are effective and thorough. These methods must be used with a watchful eye and methodical nature. Following a routine during debridement and review will ensure that you do not miss sites. Ensure that you finish the work in one spot before continuing on as this will avoid jumping back and forth around the mouth which is quite tiring and will result in more widespread bleeding that will further obscure your target area. Use of local anaesthetic and local hemostatic precautions can assist during your procedures.