Can Plaque Indices Effectively Indicate the True Amount of Plaque?

Introduction: Clinical plaque indices are well-established methods for quantifying the effects of plaque control, for example, with different toothbrush types. However, effect sizes observed in such studies depend on the index used. Therefore, this study aimed to evaluate how Turesky-modified Quigley-Hein Plaque Index (T-QHPI), Rustogi-modified Navy Plaque Index (RMNPI), and RMNPI modified by Bretz (RMNPI-Bretz) scores correspond to actual plaque amounts.

Methods: Thirty participants, 24.0 ± 4.1 years old, were included. Following 72 h without oral hygiene and after subsequent habitual toothbrushing, disclosed plaque (Mira-2-Ton®) was captured using intraoral scans (Carestream 3800). Scoring grids corresponding to the indices were projected onto standardised images on the oral and vestibular sides of the Ramfjord teeth (16, 21, 24, 36, 41, and 44) and scored. Afterwards, actual plaque coverage was quantified planimetrically (P%).

Results: All methods yield significant plaque reduction after brushing (P%: 42.1 ± 14.0%, T-QHPI: 32.7 ± 9.5%, RMNPI: 19.7 ± 9.5%, RMNPI-Bretz: 30.2 ± 9.9; p ≤ 0.001 each). However, Bland-Altman analysis revealed distinct systematic and proportional biases in relation to P%. As P% was not linearly related to the index values, equations were derived, enabling the conversion into actual plaque coverage. Those were applied to T-QHPI and RMNPI data from the literature to allow a deeper understanding of effect sizes.

Conclusions: Plaque indices only partially reflect actual plaque coverage. Compared to RMNPI, T-QHPI and RMNPI-Bretz gave closer agreement with the planimetric measurements. These findings highlight the limitations of traditional plaque indices in accurately representing plaque reduction, while offering a new approach to enhance the interpretability of oral hygiene studies.