Evaluation of salivary pH with self-ligation, elastomeric ligation, and stainless steel ligation in fixed orthodontic treatment: A pilot study

INTRODUCTION

Treatment for malocclusion with fixed orthodontic appliances typically lasts between 18 and 36 months.^[1,2] The material used for archwire ligation affects plaque accumulation around brackets, leading to colonization by Lactobacilli and Streptococcus mutans.^[3-8] The three common ligation methods, self-ligation (SL), elastomeric ligation, and stainless steel ligation, have distinct surface topographies and clinical behaviors; stainless steel ligatures have been associated with less plaque retention than elastomeric modules (EM).^[7] Elastomeric ligatures, in particular, have been linked to higher levels of acidogenic bacteria such as S. mutans and Lactobacilli.^[9] Salivary pH is a useful indicator of caries risk, since values below the critical threshold of 5.5 increase the likelihood of enamel demineralization.^[10]

Previous studies report conflicting results regarding salivary pH changes during orthodontic treatment. Some describe a reduction in pH, particularly with elastomeric ligatures, whereas others show an increase or no significant change.^[11-14] Most existing studies have compared only two ligation methods at a time, limiting direct comparison among self-ligating, elastomeric, and stainless-steel (SS) systems under identical clinical conditions. Moreover, randomized trials evaluating salivary pH across all three methods within a single protocol are scarce. This gap restricts clear clinical guidance for ligature selection in patients at increased caries risk. The present study addresses this by comparing salivary pH changes among all three ligation techniques in a single randomized controlled trial.

MATERIAL AND METHODS

Interventions and allocation

A consolidated standards of reporting trials (CONSORT) flow diagram summarizing screening, exclusions, randomization and analysis is shown in [Figure 1]. Recruitment occurred between January 2025 and March 2025 at (Government Dental College and Hospital, Nagpur). Analyses were conducted on an intention-to-treat basis: All randomized participants (n = 33) were analyzed in their assigned groups regardless of protocol deviations; there were no losses to follow-up (CONSORT flow diagram, [Figure 1]).

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Figure 1: CONSORT flow diagram.

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Participants were randomized 1:1:1 to receive SS ligatures, EM, or self-ligating brackets (SL) at treatment initiation. Randomization used computer-generated block sequences (block size = 3) prepared by an independent statistician; allocation concealment employed sequentially numbered, opaque, sealed envelopes opened at appliance placement. Treating clinicians applied the assigned ligation per standard protocols; oral hygiene instruction and fluoride toothpaste (standard domestic formulation) were provided to all participants.

RESULTS

Baseline demographic and clinical characteristics were comparable across the three groups [Table 1]. Unadjusted group means [Table 2; Figures 2 and 3] show SL and SS remained near-neutral and rose slightly over time (SL: 6.96 → 7.05 → 7.07; SS: 7.01 → 7.12 → 7.17), whereas EM fell markedly at 4 weeks (6.93 → 5.96) with partial rebound at 8 weeks (6.17). Repeated-measures ANOVA on the observed values indicated significant time effects for EM (F = 26.996, p < 0.001), SS (F = 20.893, p < 0.001), and SL (F = 10.263, p = 0.005) [Table 2].

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Figure 2: Intragroup comparison of mean unstimulated salivary pH at baseline, 4 weeks and 8 weeks (line chart). SL: Self-ligation, EM: Elastomeric modules, SS: Stainless-steel.

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Figure 3: Intergroup comparison of mean unstimulated salivary pH at baseline, 4 weeks and 8 weeks (grouped bar chart). SL: Self-ligation, EM: Elastomeric modules, SS: Stainless-steel.

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At 4 weeks, the ANCOVA showed a highly significant group effect on salivary pH (F[2,29] = 35.41, p = 1.65 × 10^-8; partial η² = 0.709). Estimated marginal (adjusted) means at the sample baseline pH were: EM 6.02 (SE 0.10; 95% CI 5.82–6.23), SL 7.05 (SE 0.10; 95% CI 6.85–7.25), and SS 7.05 (SE 0.10; 95% CI 6.85–7.25). Pairwise adjusted contrasts (ANCOVA) at 4 weeks: EM versus SL MD = −1.02 (95% CI −1.31 to −0.74), p < 0.001, Cohen’s d ≈ −3.16; EM versus SS MD = −1.02 (95% CI −1.31 to −0.74), p < 0.001, Cohen’s d ≈ −3.17; SL versus SS MD = −0.003 (95% CI −0.29 to 0.28), p = 0.983 [Table 3-5].

At 8 weeks, ANCOVA again showed a significant group effect (F[2,29] = 24.83, p = 5.21 × 10^-7; partial η² = 0.631). Adjusted means were: EM 6.23 (SE 0.10; 95% CI 6.03–6.43), SL 7.07 (SE 0.10; 95% CI 6.88–7.27), and SS 7.09 (SE 0.10; 95% CI 6.89–7.29). Pairwise adjusted contrasts at 8 weeks: EM versus SL MD = −0.84 (95% CI −1.12 to −0.56), p < 0.001, Cohen’s d ≈ −2.62; EM versus SS MD = −0.86 (95% CI −1.15 to −0.57), p < 0.001, Cohen’s d ≈ −2.68; SL versus SS MD = −0.02 (95% CI −0.30 to 0.26), p = 0.878.

In summary, the baseline-adjusted analyses are consistent with the unadjusted findings: Elastomeric ligatures (EM) produced a large, statistically significant acidifying effect at 4 and 8 weeks relative to SL and SS, while SL and SS did not differ meaningfully.

DISCUSSION

Oral hygiene during orthodontic treatment is critical, as inadequate hygiene contributes to the development of white spot lesions.^[15] With the introduction of self-ligating systems, a new mode of archwire ligation has been adopted, and its influence on oral hygiene has been widely studied.^[16-18] Freitas et al. reported that fixed appliances alter oral microbial ecology, increasing acid-producing bacteria such as S. mutans and Lactobacillus, which can contribute to a reduction in salivary pH during treatment.^[19] While increased acidogenic potential around elastomeric ligatures has been reported previously, this randomized three-arm trial provides a controlled comparison quantifying both the magnitude and temporal pattern of salivary pH shifts with uniform sampling conditions. Our contribution is therefore incremental but clinically informative for early preventive planning.

The effect of archwire ligation on oral biomarkers, including plaque, gingival and bleeding indices, and microbial colonization, has been variably reported.^[5-8] Al-Haifi et al.^[11] found that elastomeric (EM) ligatures produced a significant decrease in salivary pH to levels considered unfavorable for enamel, while Bergamo et al.^[20] reported differing contamination patterns between conventional and self-ligating brackets. Other studies show mixed outcomes: some report pH reductions after fixed-appliance therapy,^[21,22] whereas others find no significant change.^[23-25] In our trial, EM behaved as the clear outlier, producing the largest acidifying effect, while SS and self-ligating (SL) groups remained neutral to slightly alkaline. The SS/EM findings align with Al-Haifi et al.,^[11] and - to the best of our knowledge - this is the first randomized trial to include SL in a direct pH comparison with EM and SS, limiting direct prior comparisons for SL.

Türkkahraman et al.,^[5] in a split-mouth study, observed slightly higher colonization of S. mutans and Lactobacilli with EM than with SS ligatures, although the difference was not statistically significant. In contrast, De Souza et al.,^[6] using PCR analysis, reported significantly higher microbial plaque indices around elastomeric ligatures compared with SS ligatures after 6 months of treatment.

Sukontapatipark et al.^[4] used scanning electron microscopy to evaluate microbial colonization on elastomeric and SS ligatures and found no difference in microbial morphotypes between ligation types. This likely reflects the dominant influence of excess composite and marginal gaps, which create the primary ecological niche for plaque formation, rather than the ligation material itself. EM, as organic and porous materials, favor early biofilm accumulation compared with smoother metallic or self-ligating surfaces, which likely explains the increased acidogenesis observed with EM.^[26] The temporal pattern observed is biologically plausible. The sharp pH drop in the EM group at 4 weeks likely reflects rapid early plaque accumulation on the porous elastomeric surface, promoting acidogenic biofilm activity. The partial recovery at 8 weeks may result from stabilization of microbial colonization and improved patient compliance with oral-hygiene instructions. In contrast, the smoother inorganic surfaces of SS and SL accumulate plaque more slowly, maintaining relatively stable pH values. This pattern indicates that the highest acidogenic challenge with elastomeric ligation occurs in the early treatment phase, emphasizing the need for intensified preventive measures during the 1^st month.^[12,13]

A plausible cause for the mild alkalinization in SL and SS is early low-grade gingival inflammation after appliance placement. Plaque retention and gingival exudate can boost proteolytic/ureolytic bacterial activity, producing ammonia and raising local pH; this effect has been described previously.^[27,28] Direct measurement of plaque/GI indices, urease/ammonia, and targeted microbiology would test this hypothesis.

Future studies should include longer follow-up to determine whether these pH changes persist or normalize during treatment. The addition of plaque and gingival indices, salivary flow, and buffering capacity, and species-specific microbiology (e.g., S. mutans, Lactobacilli, ureolytic bacteria) would clarify whether ligation-related pH shifts translate into clinically relevant demineralization. Larger multicentre randomized trials would improve external validity and permit analysis of high-risk subgroups.

CONCLUSION

• Within the limitations of this trial, elastomeric ligatures caused a sustained reduction in salivary pH during the first 8 weeks of treatment, whereas self-ligating and SS ligation maintained neutral to slightly alkaline pH values.

• The intergroup difference appeared after 4 weeks and persisted thereafter, indicating that the ligation method influences the salivary environment, with EM posing a higher potential risk for enamel demineralization.

• Self-ligating or SS ligation may provide a more favorable salivary pH profile, although ligature selection should be individualized to patient risk and clinical needs.