2.1

Ethical standards

The Ethics Committee of the local institution reviewed and authorized the trial (reference number 599) in the 6th of June 2022. The trial is registered ClinicalTrials.gov record ID number NCT05912907. The anticipated outcomes, potential complications, and the necessary intervention in such a condition were explained to the participants and their caregivers. Each child’s parent was asked to sign an informed consent after being carefully explained. All procedures were performed in the paediatric dentistry clinic, Minia University Dental Hospital, Faculty of Dentistry, Minia University.

2.2

Study design, sample size and randomization process

A two-parallel-arm randomised controlled trial with an allocation ratio of 1:1 was designed to recruit children. Based on the Power Analysis and Sample Size Software (PASS, version 23.0.2. NCSS, LLC. Kaysville, Utah, USA), the sample size was calculated using a general estimating equation (GEE) model for binary repeated measures over equal time intervals. An effect size of 0.15, obtained from the difference in clinical success rates between MTA [ , ] and KNO ₃ /PCA that was estimated by the findings of a pilot study that included ten teeth with a clinical success rate of 80%. A total sample size of 104 participants (52 participants per group) after considering an attrition rate of 10% was enough to declare the difference between the two groups at an alpha level of significance of 5% and a power of 80%.

The participants were randomly allocated to either the intervention (KNO ₃ /PCA) group or control (MTA) group using permuted block randomization, considering a block size of four. The randomization process was the responsibility of an independent researcher, who was masked with the trial objectives and steps. The following computer-generated software: https://www.sealedenvelope.com , was used to gain the randomization sequence. Double-folded papers contained the treatment codes enclosed within 104 identical, opaque, and adequately sealed envelopes. Every four envelops (two coded for the intervention group and two coded for the control group) were shuffled and placed in four similar containers. The trial was single blinded (participants were masked to the capping material). After the cavity was accessed and the pulp was exposed, an independent nurse opened the envelope and the capping water was placed.

2.3

Inclusion and exclusion criteria

Children older than 6 years with no history of systemic, allergic, genetic, or behavioural problems were included. Included teeth were carious, immature mandibular, or maxillary first permanent molars with the following criteria: 1) deep occlusal carious cavities; 2) vital pulps that confirmed clinically in terms of normal responses to thermal and electric pulp testing and no history of spontaneous pain, tenderness to percussion, or gingival swelling and/or fistulous tract emergence; and 3) negative findings in the periapical radiographs in terms of the intact lamina dura and the absence of furcal or periapical radiolucency, internal root resorption, or external root resorption. Teeth with iatrogenic or traumatic pulp exposures or non-restorable crowns, no pulp exposure after caries eradication, necrotic pulps, or a large exposed pulp diameter (>2.5 mm), according to Parinyaprom et al. [ ], were excluded.

2.4

Clinical procedures

Prior to beginning the clinical procedures, the EPT (Parkell D640 Digitest II Pulp Vitality Tester) at two successive intervals (2 min apart) and a thermal cold stimulus with a cotton pellet damped with Green Endo-Ice refrigerant; Coltene/Whalkedent Inc., OH, USA) for 5 s were performed. After application of topical anaesthesia (20% benzocaine I-Gel Topical Anesthetic Gel), an aspirating syringe for inferior alveolar nerve block, using articaine hydrochloride 4% and epinephrine 1:100 000 (Septocaine® 1.7 mL, SEPTODONT Ltd. Paris, France) was used to anesthetize the intended tooth. The tooth was isolated with a rubber dam and then accessed with a high-speed carbide round bur with copious water.

Caries was removed from the cavity walls first using a sharp manual excavator and then from the pulpal floor. Once the pulp was exposed, the exposure site was flushed with normal saline to remove debris, and the exposure size was assessed using a periodontal probe. Bleeding was controlled using a cotton pellet socked in 2.5% sodium hypochlorite (NaOCl) (Hyposol; Prevest DenPro Limited, Jammu, India) and left in the cavity for 5 min [ ]. Based on the European Society of Endodontology position statement [ ], if blood haemostasis couldn’t be achieved within 5 min, the tooth was excluded. Once the bleeding from the exposure site had been controlled.

In the experimental group, the ratio of KNO ₃ was speculated based on the study conducted by Ahmed et al. [ ]. A consistent putty mix of 5-percent KNO ₃ /PCA was obtained after mixing 95 mg of zinc oxide powder and 5 mg of KNO ₃ (Sigma-Aldrich, Germany) with a liquid of polyacrylic acid (SpofaDental Poland), considering a 1:1 powder/liquid ratio. In the MTA group, the final restoration was placed a day after placing a wet cotton pellet over the MTA capping material to allow its setting, and the cavity was temporized with a glass ionomer (Medifill, Promdica, Germany). The pulp sensibility was tested at each follow-up interval using electric and thermal pulp test. In both groups, a layer of reinforced glass ionomer base (RIVA self-cure, SDI Limited) was added. Finally, a resin composite (Filtek z250 universal restorative, ESPE, St. Paul, MN, USA) filled the cavity ( Fig. 1 ).

Direct pulp capping clinical procedures of a first permanent molar with: (a) occlusal caries; (b) pulp exposure after caries removal; (c) placement of potassium nitrate in polycarboxylate cement (KNO ₃ /PCA) capping material; (d) resin composite final restoration.

Conventional periapical radiographs were taken at baseline immediately after the clinical procedures and one at each follow-up recall. To standardize the radiographic measurements, digital periapical images were obtained using the long cone (paralleling) technique (Vista Scan Mini Easy X-ray System, Germany). A PSP plate size 2 with a film holder was attached to a custom-made index of silicon.

2.5

Study outcomes of definition of success

Clinical and radiographic success (primary outcomes): At follow-up intervals, each tooth was assessed clinically and radiographically. Clinical failure was reported in the presence of one of the following signs and symptoms: pain, sensitivity to percussion, gingival swelling, and/or sinus tract formation. The clinical diagnosis was confirmed by thermal and EPT at each follow-up interval. Radiographic failure was considered in the presence of one of the following features: periapical and/or furcal radiolucency, internal root resorption, or external root resorption.

Radiographic evidence of root maturogenesis (secondary outcomes): Root maturity was assessed for the following measures using the ImageJ software (version 1.50i; National Institutes of Health, Bethesda, MD, USA).

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  Root length (RL) was measured with a straight line extended from the CEJ to the apical foramen in the radiograph ( Fig. 2 a).

  

  Radiographic measurements to assess root maturogenesis: (a) root length (RL) measured with a straight line extended from the CEJ to the apical foramen in the radiograph; (b) apical foramen width (AFW) measured with a line extended between the mesial and distal radicular ends; (c) radiographic root area (RRA) calculated by subtracting the total root area from the root canal space.

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  Apical foramen width (AFW) was measured with a line extended between the mesial and distal radicular ends ( Fig. 2 b).

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  Radiographic root area (RRA) was calculated by subtracting the total root area from the root canal space ( Fig. 2 c).

All radiographic parameters were measured at the baseline and at each follow-up recall.

2.6

Standardization and calibration

All clinical procedures were performed by single operator (S.M.M). At each follow-up recall, clinical assessment and radiographic measures were assessed by two independent pediatric dental professions with 15 years of experience (A.M.A and H.O.S). Cohen’s kappa coefficient (κ) was used to check the inter-examiner agreement. For clinical outcomes, Cohen’s kappa (κ) was 0.98, 0.96, and 0.96 clinically and 0.90, 0.92, and 0.90 radiographically at 6, 12, and 18 months, respectively.

2.7

Statistical data analysis

Statistical Program for Social Sciences for Windows (SPSS), version 22 (IBM© Corporation, NY, USA) was considered to analyse the study findings. Descriptive data, including frequencies (gender, tooth location, exposure size, and clinical and radiographic success rates), were analysed using the Chi-square and Fisher’s exact tests. Continuous data (children’s age and the percentage change in radiographic parameters) was first confirmed for normality and then tested using an independent sample t-test . Likewise, the influence of independent variables was tested on clinical outcomes. The effect of time on the mean values of radiographic parameters (RL, AFW, and RRA) was tested using the general linear model (GLM), which initially confirmed the normality assumption and variance homogeneity. The alpha level of significance was adjusted to 0.05.