October 22, 2021
 by Gabriele Maycher, CEO, GEM Dental Experts Inc. BSc, PID, dip DH, RDH.

Still confused about the 2018 AAP Periodontal classification? Never fear! The next few monthly columns will review some of the most important updates made to the industry’s global periodontal guidelines to help hygiene teams achieve the highest level of care. Once we have exhausted this topic we will move onto other questions about the process of care. If you have any specific questions, you would like answers to, please let me know.

How do I calculate the site of greatest loss using radiographic bone loss (RBL)? I am not even sure what the literature means by “radiographic bone loss measured in percentage of root length1”?


I agree that this concept can be confusing, so follow these steps.

Step 1: Complete a full mouth series (FMS) to determine the site of greatest loss. Remember the site of greatest loss cannot be attributable to non-periodontitis causes (i.e., vertical defects due extractions, traumatic causes, cervical restorations, and so on).

Step 2: Determine the stage using RBL. Use the ruler icon in your program software to measure the distance from the cementoenamel junction (CEJ) to the apex of the root at the site of greatest loss. (If you don’t know where the ruler icon is, ask your doctor. He uses it all the time for endo.) Keep in mind a 2mm distance on average from the CEJ to the alveolar bone crest is considered normal bone-level height,2 so subtract 2mm from the CEJ to apex measurement to determine where the bone levels should be in health. Next measure the current bone level at the site (bone height to apex of the root). Subtract the current bone level from healthy bone level number, and this will give you the bone level loss. Once you have the bone level loss number, divide it by the healthy bone level number and multiply it by 100. This will give you a percentage of bone loss for staging severity when using RBL as a determinant (i.e., Stage I <15%, Stage II 15%–33%, Stage III and IV >33%). See Table 1: Periodontitis Staging.

Example: CEJ to Apex = 27mm – 2mm = 25mm healthy bone height. Current bone level at site of greatest loss = 12mm. 25mm (healthy bone) – 12mm (current bone level) = 13mm bone loss. 13mm (bone loss) ÷ 25mm (healthy bone level) = .52 x 100 = 52% bone loss (Stage III or IV).

Step 3: Determine grade using indirect evidence of progression. Take the percentage of bone loss number and divide it by the patient’s age (i.e., Grade A <0.25, Grade B 0.25 to 1.0, Grade C >1.0). See Table 2: Periodontitis Grading.

Example: 52% bone loss ÷ 40 years of age = 1.3. Grade C.

Staging severity using RBL calculation:

  • CEJ to the Apex of the Root – 2mm = Healthy Bone Level
  • Current Bone Height to the Apex of the Root = Current Bone level
  • Healthy Bone Level – Current Bone Level = Bone Loss Level
  • (Bone Loss Level ÷ Healthy Bone Level) x 100 = % Bone Loss

Grading using Indirect Progression Calculation:

  • % Bone Loss ÷ age = Indirect Evidence of progression


      1. Tonetti MS, Greenwell H, Kornman KS. Staging and grading of periodontitis: Framework and proposal of a new classification and case definition. J Periodontol. 2018;89(Suppl 1):S159– S172. https://doi.org/10.1002/JPER.18-0006.
      2. Chapple ILC, Mealey BL, et al. Periodontal health and gingival diseases and conditions on an intact and a reduced periodontium: Consensus report of workgroup 1 of the 2017 World Workshop on the Classification of Periodontal and Peri-Implant Diseases and Conditions. J Periodontol. 2018;89(Suppl 1):S74–S84. https://doi.org/10.1002/JPER.17-0719.

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