Periodontal Indices: CPI, PSR, and Reversible vs. Irreversible Measures

Core Review

Community Periodontal Index (CPI)

Developed by the World Health Organization (WHO) and previously known as the Community Periodontal Index of Treatment Needs (CPITN), the CPI is a rapid, population-level screening tool for periodontal status. It was designed for epidemiological surveys where comprehensive full-mouth periodontal charting is not feasible.

Examination method: The mouth is divided into sextants (sixths):

• Sextant 1: Maxillary right posterior (teeth #1–#5)
• Sextant 2: Maxillary anterior (teeth #6–#11)
• Sextant 3: Maxillary left posterior (teeth #12–#16)
• Sextant 4: Mandibular left posterior (teeth #17–#21)
• Sextant 5: Mandibular anterior (teeth #22–#27)
• Sextant 6: Mandibular right posterior (teeth #28–#32)

A sextant must have at least two functional teeth (not indicated for extraction) to be scored. A sextant with fewer than two functional teeth is excluded.

Index teeth: Within each sextant, specific index teeth are probed. For adults aged 20+, all teeth in the sextant are examined and the worst finding is recorded. In younger populations (under 20), only index teeth are used: #3, #8, #14, #19, #24, #30 (the same Ramfjord teeth used in OHI-S — the NBDHE may test this overlap).

CPI Probe: A specially designed lightweight probe with a 0.5 mm ball tip, a colored band from 3.5–5.5 mm, and markings at 3.5 mm and 5.5 mm. The pocket depth is assessed as "within the colored band" or "beyond the colored band."

CPI Codes (0–4):

Note: For Code 3, the colored band of the CPI probe (3.5–5.5 mm) is partially visible. For Code 4, the entire band disappears into the pocket. This is a highly testable distinction.

Periodontal Screening and Recording (PSR)

PSR is the American adaptation of the CPI, endorsed by the American Academy of Periodontology (AAP) and the American Dental Association (ADA) for use in general practice as a quick screening tool. PSR uses the same sextant approach and the same 0–4 coding system with some modifications:

PSR Codes:

The asterisk () is appended to the code if any of these additional findings are present. A sextant coded 3 means a 4–5 mm pocket PLUS furcation involvement.

Treatment implications from PSR:

• Code 0 or 1: Preventive care, oral hygiene instruction
• Code 2: Scaling and oral hygiene instruction; remove plaque-retentive factors
• Code 3 or higher: Comprehensive full-mouth periodontal charting indicated; comprehensive periodontal evaluation needed

Reversible vs. Irreversible Indices

This is one of the most conceptually important distinctions on the NBDHE:

Reversible indices measure conditions that can resolve — the score can return to zero with successful treatment:

• Gingival Index (GI): Measures gingival inflammation. Successfully treated gingivitis returns GI to 0. The tissue heals.
• Plaque Index (PI, Silness & Löe): Measures plaque accumulation. Brushing returns PI to 0.
• OHI-S DI-S (debris component): Measures current oral debris. Toothbrushing reduces the score.
• Bleeding indices (GBI, SBI): Measure current bleeding on probing. As inflammation resolves, bleeding stops and score returns to 0.
• CPI Code 1 (bleeding): Bleeding on probing resolves when gingivitis is treated. Code 3 and 4 (pockets) may or may not be reversible depending on whether true attachment loss has occurred (pseudopockets from edema can resolve; true attachment loss cannot).

Irreversible indices measure cumulative past disease experience — the score NEVER decreases:

• DMFT/DMFS: Once a tooth is Decayed, Missing, or Filled, it stays that way. A filling replaces decay but doesn't erase the experience — D converts to F, same count.
• Attachment loss (CAL): Clinical attachment loss measures destruction of the periodontal attachment apparatus. Lost attachment does not regenerate (with rare exceptions from regenerative procedures). CAL is the "DMFT of periodontics."
• CPI Code 3 or 4 due to true attachment loss: If the probing depth reflects supracrestal tissue destruction rather than pseudopocketing from inflammation, it will not fully resolve.
• Tooth loss: Once a tooth is extracted, the M component is permanent.

The OHI-S CI-S (calculus component) occupies a middle ground: calculus is neither immediately reversible like plaque nor permanently irreversible like a filling. It requires professional removal (scaling) to resolve, but once removed, the score can return to 0.

Why This Distinction Matters

On the NBDHE, you may be asked to evaluate the effectiveness of an intervention. If the intervention was a community oral hygiene education program, the appropriate outcome measure would be a reversible index (OHI-S, Plaque Index, or GI) because you expect improvement. Using DMFT to evaluate an oral hygiene program would be inappropriate — the DMFT scores of the population cannot decrease regardless of how effective the program is.

Conversely, if evaluating a school sealant program, you would eventually want to measure DMFS to see if fewer new carious surfaces develop — but this requires a long-term study, as DMFS can only stay the same (if prevention works) or increase (if it fails).

Clinical/Board Application

Board-style question: "A dental public health program aims to reduce gingivitis in a community through school-based oral hygiene instruction. Six months after the program, the hygienist should expect improvement in which index?" Answer: Gingival Index (GI) or Plaque Index (PI). These measure reversible conditions. DMFT would not show improvement.

Common Traps

• Trap: Thinking CPI/PSR measures attachment loss. It measures probing depth, which can reflect either pseudopocketing (reversible, from edema) or true attachment loss (irreversible). CPI alone cannot distinguish between them.
• Trap: Confusing index reversibility with the disease itself. Gingivitis is reversible, periodontitis (attachment loss) is not — but CPI/PSR codes for pockets (3, 4) may or may not represent reversible pseudopocketing.
• Trap: Evaluating a prevention program with an irreversible index and expecting the score to decrease. The appropriate measure is a reduction in the INCIDENCE (rate of new disease), not a reduction in the prevalence (existing cumulative score).