The Effectiveness of Calcium Phosphates in the Treatment of Dentinal Hypersensitivity: A Systematic Review

Dentin hypersensitivity (DH) pain is a persistent clinical problem, which is a common condition known to affect patients’ quality of life (QoL), but no treatment has ever been agreed upon. Calcium phosphates, available in different forms, have properties that allow sealing the dentinal tubules, which may relieve dentin hypersensitivity. The aim of this systematic review is to evaluate the ability of different formulations of calcium phosphate to reduce dentin hypersensitivity pain level in clinical studies. The inclusion criterion was as follows: clinical randomized controlled studies using calcium phosphates in treating dentin hypersensitivity. In December 2022, three electronic databases (Pubmed, Cochrane and Embase) were searched. The search strategy was performed according to Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. The bias assessment risks results were carried out using the Cochrane Collaboration tool. A total of 20 articles were included and analyzed in this systematic review. The results show that calcium phosphates have properties that reduce DH-associated pain. Data compilation showed a statistically significant difference in DH pain level between T0 and 4 weeks. This VAS level reduction is estimated at about −2.5 compared to the initial level. The biomimetic and non-toxic characteristics of these materials make them a major asset in treating dentin hypersensitivity.


Introduction
Dentin hypersensitivity (DH) is an oral complaint frequently reported in clinical dental practice. It is characterized by a short, sharp pain arising from exposed dentin in response to thermal, evaporative, tactile, osmotic, or chemical stimuli that cannot be ascribed to any other form of dental defect or pathology [1,2]. A review outlined a prevalence of DH ranging from 1 to 34% after clinical examination; the highest level has been reported to be on the cervical surface of the canine as well as first premolar permanent teeth and also in patients with periodontal alterations [3]. In their daily life, patients with dentin hypersensitivity complain of discomfort and pain while consuming hot or cold foods and beverages (coffee and ice cream) while toothbrushing or sometimes even while breathing. These symptoms and problems may be highly relevant, leading to restrictions on everyday activities and be a determinant of the individual's oral-health-related quality of life (OHRQoL) [4].
Several theories have been proposed in order to explain the biological mechanism of dentin hypersensitivity [3, 5,6]. To date, the most widely accepted theory of DH is the hydrodynamic theory of Brännström [7][8][9]. This theory is based on a rapid movement of the Hydroxyapatite is already used as a DH desensitizer [27]. Other molecules similar to HA, such as synthetic nano-Hydroxyapatite (n-HA) or soluble molecules able to self-set to a hard mass under HA form, such as Tetracalcium phosphate (TTCP) and dicalcium phosphate dihydrate (DCPD) [28], have already been tested in clinical conditions in the treatment of DH. These studies show encouraging results. However, no systematic reviews evaluated the effects of these molecules on the DH pain level.
This systemic review aims to evaluate the effect of various calcium phosphate molecules such as hydroxyapatite, nano-hydroxyapatite, TTCP, DCPD, dicalcium phosphate anhydrous (DCPA), or/and amorphous calcium phosphate (ACP) on the reduction in DH pain level.

Materials and Methods
The study protocol was registered in the International Prospective Register of Systematic Reviews (PROSPERO) under ID = CRD42022336712. The present systematic review was conducted per the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA 2020) 2020 guidelines [29]. Population, Intervention, Comparison, Outcomes, and Study design components of this systematic review are as follows: Participants (P) were adult patients suffering from dentin hypersensitivity due to non-carious cervical lesions and not associated with post-bleaching hypersensitivity and periodontal therapy. Interventions (I) were in-office or in-home treatments of dentin hypersensitivity with products containing calcium phosphate. For Comparison (C), the comparison with other molecules is not applicable, but we looked at the variation in the level of pain felt by the patients before and after treatment with calcium phosphates. Outcome (O) was the reduction in pain associated with dentin hypersensitivity after treatment with calcium phosphate molecules. The study design (S) selected was a randomized controlled trial (RCT). Case reports, in vitro studies, in situ studies, systematic reviews, meta-analysis, letters to editors, and non-randomized trials, as well as studies on tooth decay or studies with no good molecule tested, were excluded. The research question was as follows: Are calcium phosphate able to reduce the DH pain?

Search Strategy
Three databases (PubMed/Medline, Cochrane Library, and EMBASE) were searched using relevant keywords to identify articles published until December 2022, with no language restriction, as shown in Table 1. Additionally, bibliographies of all selected articles, specialized journals, and other related publications, including reviews and metaanalyses, were also searched to identify further relevant articles. The records obtained from this extensive literature search were transferred to an EndNote ® library, and duplicates were removed.

Screening and Study Selection
The research and selection process articles were carried out independently by two authors (M.M. and S.M.). First, the retrieved articles were imported into a bibliographic reference management software program (EndNote), where duplicates were removed. Then, the records' titles and abstracts obtained were screened, based on determined eligibility criteria. Finally, the full texts of the remaining studies were assessed by the same authors. Discrepancies were resolved, and consensus was built by engaging a third author (E.R.). Only randomized controlled trials that assessed the dentinal desensitized effect of calcium phosphate were included.

Data Extraction
When available, the data of included studies were extracted by both reviewers (M.M and S.M) and verified and confirmed by two other authors (O.N.B. and E.R). An Excel file was previously established to provide support for collecting demographic data (name of first author, year and country of publication, number of participants, and mean age), study methodology (study design, number and characteristics of the participating groups, number of follow-up visits, method of measuring dentin hypersensitivity, composition, concentration and use of calcium phosphate as a desensitizing agent for dentine hypersensitivity), and main results. All these extracted data were listed in Table 2.  TTCP/DCPA showed a significant difference between T0 and T1 and a significantly lower CAS score at T4 than T2 and T3.   n-HA showed significant difference for CAS and cold test between T0 and T3 days.

Quality Assessment
The same two authors (M.M. and S.M.) assessed the risk of bias in the included studies using Cochrane's Collaboration tool for assessing the risk of bias in randomized controlled trials [50]. Disagreements were resolved via discussion, and a third researcher (E.R.) was approached when necessary. This evaluation concerned the generation of the randomization sequence (selection bias), concealment of the allocation (reporting bias), blinding of the investigator and the participant (confusion bias), blind evaluation of the results (performance bias), management of missing data (attrition bias), selection of the reporter, and other types of bias. From these criteria, the bias risk level was determined to be low, unclear, or high.

Synthesis of Results
A qualitative and quantitative synthesis of the results of the included studies, structured around different outcomes, was performed. The data from these different studies were extracted, and the results are summarized in Table 3. For studies in which the authors reported results as medians and interquartile ranges, the values were converted to means and SDs using the formula (q1 + median + q3)/3, where q1 indicates the 25th percentile and q3 the 75th percentile. An approximation of the standard deviation was obtained by applying this formula (q3 − q1)/1.35. Analysis groups between baseline and 4 weeks of follow-up were formed according to the method of assessment of dental hypersensitivity to determine whether calcium phosphates are effective in reducing pain associated with dental hypersensitivity associated pain. Data from these different groups were pooled to determine the mean pain reduction value. When studies used the same type of intervention and comparison groups with the same outcome measure, the results were pooled with mean differences for continuous outcomes.

Study Selection
The initial search of all sources yielded 10,019 records. Of these, 2435 duplicated studies were removed using the reference manager EndNote ® . A total of 7515 articles were excluded after reading titles and/or abstracts, 12 records were excluded since reports were not retrieved, 57 records from database registers and 18 identified through other methods were read and analyzed in their full-text, and 55 records were excluded for reasons such as not good drugs tested (n = 34), in vitro studies (n = 11), or study on tooth decay (n = 10), as shown in Figure 1

Description of Included Studies
The characteristics of the 20 included articles are presented in Table 2. The number of subjects included varied from 8 to 208. The age range of patients ranged from 18 to 80 years. The follow-up range varied from immediately to 6 months. Most of the studies performed a 4-week follow-up phase [30,[32][33][34][35][36][37][38][39][40][41][42][47][48][49].   A statistically significant decrease in the level of pain associated with dentin hypersensitivity was observed between 4 weeks of follow-up and baseline according to VAS score of air blast, tactile sensitivity, and cold water tests (p < 0.05).
All studies showed significant reductions in VAS or Schiff of dentin. In order to determine the efficacy of calcium phosphate in the reduction in DH pain level, we synthetized data in Table 3 accordingly with the realized test. Data compilation showed a statistically significant difference in DH pain level between T0 and 4 weeks. This reduction is estimated at about −2.5 compared to the initial level of pain.
Six of seven studies showed a significant decrease in VAS or Schiff at 4 weeks for tactile stimulation [33,34,36,38,45,47]. The decrease was not significant in one study [41]. The total calculated mean difference score of the seven studies was −2.53 ± 0.07 (−2.66 to −2.39) p < 0.05. All data are compiled in Table 3.
Three studies showed a significant decrease in VAS at 4 weeks for the cold water test [36,39,41]. The total calculated mean difference score of all studies between the baseline T0 and 4 weeks was −2.56 ± 0.16 (−2.88 to −2.23) p < 0.05. Desensitized agents were n-HA. All data are compiled in Table 3.

Analysis of the Risks of Bias
The results of the risk of bias assessment are presented in Figure 2. This analysis was carried out using the Cochrane Collaboration tool [50]. This assessment involved randomized clinical trials and was carried out on all the studies included in this systematic review. The assessment revealed that seven studies were considered to have a low risk of bias [32,[34][35][36]41,44,48]. Five studies were considered to be at high risk of bias for the following reasons: in the study of Eyuboglu et al. [33], because the randomization was performed after the initial pain assessment; in the study of Gopinath et al. [41], because the randomization method and the description of the sample size were not clearly exposed; and in the study of Jena et al. [42], because the absence of description of the sample size and of duration and location of the study also constituted a risk of bias.

Discussion
Recent systematic reviews with meta-analyses have compared the effectiveness of several desensitizing toothpaste formulations, including some containing nano-hydroxyapatite or potassium combined with hydroxyapatite [10,18,19,[51][52][53]. In our review, we focused specifically on the effectiveness of hydroxyapatite and other calcium phosphate materials able to self-set to a hard mass [24], such as tetracalcium phosphate (TTCP) and dicalcium phosphate dihydrate (DCPD) powders, which are able to produce a supersaturated solution and faster hydroxyapatite precipitation due to their high solubility at neutral PH [28]. In this systematic review, we show the beneficial clinical effects of all different calcium phosphate formulations on dentin hypersensitivity. Whatever the test used (air blast, tactile or cold water), calcium phosphate induced a reduction in a mean of 2.5 pain level on the VAS scale after 4 weeks. Additionally, significant beneficial effects appeared immediately after treatment in eight studies [31,33,34,37,39,42,43,45].
These results may be explained by the ability of calcium phosphates to spontaneously form hydroxyapatite at physiological pH and to adhere to the exposed dentine, forming a layer of calcium phosphate components, which may allow them to seal exposed dentinal tubules and consequently be a good candidate for the treatment of dentin hypersensitivity with a VAS drop immediately after the application.
The beneficial effect, as described in our review, is in accordance with the results of a large systematic review and meta-analysis conducted by Marto et al., regarding numerous molecules in the treatment of DH [19]. In this review, hydroxyapatite and other calcium phosphate molecules showed a significant reduction in DH pain at different points in time.
A previous systematic review and meta-analysis conducted by de Melo Alencar et al. in 2019 [18] underlined the effectiveness of nano-hydroxyapatite in the relief of dentin hypersensitivity compared to n-HA free treatment. Indeed, Alencar et al. [18] showed a significant desensitizing effect against evaporative and tactile stimuli but not against cold stimulation. They hypothesized that cold stimulus, the most disturbing test, could involve not only the hydrodynamic theory but also other factors such as TRPM8 channels in odontoblasts. Concerning the air blast test and the tactile test, their results are in accordance with our systematic review, except for the cold water, for which we show a significant reduction in DH pain level after phosphate calcium treatment. De Melo Alencar et al. also compared n-HA to placebo or other desensitizing agents, particularly arginine. This amino acid when combined with calcium carbonate mimics saliva's ability to occlude and seal open dentinal tubules, which renders that tooth surface resistant to acid and thermal attacks. It has been shown by two previous meta-analyses and one systematic review as promising bioactive agent [13,54,55] in DH. In their review, Alencar et al. [18] showed, in the 4-week follow-up, a better result with nano-hydroxyapatite than those presented by arginine in the treatment of dentin hypersensitivity. In our study, we did not compare calcium phosphate with other products or placebo, since this was not our research question.
However, another systematic review and meta-analysis conducted by Hu et al. [56], comparing numerous dentin desensitizing agents, showed a very low level of evidence of nano-hydroxyapatite and amorphous calcium phosphate toothpaste compared with other desensitizing agents. Indeed, this study included fifty-three clinical studies, but only four with calcium phosphate, two with nano-hydroxyapatite and two with amorphous calcium phosphate, which considerably reduces the effect of this evidence [56].
The review by Cunha-Cruz et al. [57] showed a significant effect of n-HA on DH pain levels in two studies but no effects from amorphous calcium phosphate, but this author took into account only results included in previous systematic reviews and meta-analyses [56].
It is worth noting that the hypersensitivity reduction efficacy of nano-hydroxyapatite was increased du to the adjunction of sodium [58] or ionometric sealant [59] or when combined with laser treatment [60].
Furthermore, the concentration of calcium phosphate used is probably an important factor for effectiveness. Shetty et al. [61], in an in vitro study, reported an enhanced desensitizing action with 100% nano-hydroxyapatite over 8 weeks of treatment compared to 25% nano-hydroxyapatite, and the authors concluded that increased concentrations of the molecule increased its penetration into the tubules and probably improved its desensitizing ability. Our study does not allow us to determine the suitable concentration to use to reach the most efficient effect.
Other biomaterials containing calcium such as calcium sodium phosphosilicate seem to have interest in the treatment of dentin hypersensitivity [30,46,47]. Finally, it is important to note that the hypersensitivity was not completely resolved regardless of the treatment applied. According to our results, a level of pain persists after 4 weeks of treatments as shown in Table 3, which could be explained by the fact that the effectiveness of the treatment tends to decrease or disappear over time [34,49].
Alharith et al. [31], De Oliveira et al. [39], Porciani et al. [46], and Geiger et al. [49] showed positive results in the placebo groups, resulting in a significant effect of treatment but no significant reduction compared to the placebo group. In [31,39,46,49], patients felt significant reductions of up to 60% of dentin hypersensitivity following the application of the placebo treatment. These positive effects of placebo treatments are important to consider, since there may be other factors that could explain the effectiveness of desensitizing therapeutics agents in the reduction in dentin hypersensitivity such as desensitizing agents in control groups. Moreover, these lower levels of sensitivity in placebo groups can also be attributed to the well-known Hawthorne effect, which describes the modification of behavior when individuals are aware that they are being observed, which could influence the patient's responses and ca lead to bias in healthcare studies [62]. The placebo effect may also be involved, since positive motivation and emotional stimuli could activate pain inhibitors in the central nervous system [63].
Our systematic review presents limits that required results to be carefully interpreted. Indeed, included studies are still heterogenous in terms of the age of patient from 18 to 80 years, where we do not know if age is an influencing factor of DH level pain. Additionally, different desensitized agents in different concentrations and different application modalities were used by different authors, and the evaluation of the DH realized with three different tests could be different according to the team's research; all these limits did not allow the generalization of results.

Conclusions
This systematic review shows a reduction in pain perception after calcium phosphate application immediately and 4 weeks after treatment, making this biomaterial a good candidate for the relief of dentin hypersensitivity.