1 Introduction

Root planing/root surface decontamination is the primary treatment modality for removing the residual calculus deposits and portion of cementum. Periodontal disease develops due to the adhering plaque on the root surface [1], and its mineralization results in the formation of dental calculus. Therefore, the basic principle behind a successful periodontal therapy is identifying the cause and its complete elimination [2, 3]. Therefore, the best possible means to achieve a completely healthy root surface has been the goal [4, 5].

During root planing, the instruments used on the surface of the roots leave them more irregular and rougher, making them vulnerable to more plaque & bacterial adhesion and hampers adhesion of periodontal ligament fibroblasts. Several in-vivo studies have shown that the treatment using ultrasonic instruments removes less root structure when compared to the hand instruments [6, 7]. Still, the surface of the root becomes rough [8, 9]. Therefore, using hand instruments and ultrasonic instrumentation has been recommended in treating root surfaces affected by the disease [10,11,12].

Sharpening of dulling curettes has been necessary to maintain efficient instrumentation throughout the root planing procedure. However, a dull curette is inefficient and can produce uneven edges that show a negative image on the instrumented surface [13]. Studies show variable stroke numbers of five [14], ten [15], and twelve [16] before sharpening the instruments. The effective removal of the cementum and peripheral dentin is completed in the first 20 strokes; as the number of strokes increases to 40 strokes, the pressure applied increases [17]. It is also noted, very few clinical therapists appear to sharpen their curettes every 5–20 strokes.

Studies have been exploring sharpness as a factor in deciding any instrument's capability to be used on the root surface. For example, standard curettes are provided with the extra width of the cutting edge that would be necessary as a backup for repeated sharpening. However, instruments that are used by repeated sharpening can have an irregular cutting edge or wire edge that could make it inefficient and result in trauma to the surrounding tissue.

Manufacturers and clinicians have been trying to research the effectiveness of instruments on calculus removal from the root surface. Any curette with a long life without repeated sharpening that leaves the root surface smooth and decreases trauma to the surrounding tissues has been a gold standard. Metal alloys such as stainless steel, tungsten carbide, high-speed steel, cryogenically treated steel, and various others have been shown to influence the life and retention of a sharp cutting edge. These curettes have been said to have an edge retention and would retain the edge for a long duration of time. Though few of these materials have been shown to erode, they [18, 19] occupy the market and are claimed by the manufacturers to have good cutting. Hence the present study was undertaken to assess the effectiveness of root planing of the curettes with edge retention technology. The present study compared the root surface roughness and the cutting-edge retention after mechanical root planing using five periodontal curettes. The null hypothesis tested was no statistically significant difference between the root surface roughness between five different kinds of curettes.

2 Materials and Methods

2.1 Specimen Preparation

Thirty maxillary or mandibular central/ lateral incisors from human subjects undergoing extraction due to periodontal disease were selected following the approval from the institutional ethical committee (IEC 294/2018). The tooth samples were anonymized, collected, cold sterilized, and prepared for the procedure. Initially, they were decoronated, and the roots were cut axially into two parts using a rotating diamond disc bur and a micro motor unit. The obtained parts were rinsed in sterile distilled water, and then they were air-dried. The specimen preparation was done according to the protocol described by Arora et al. [20]. The sixty root fragments that were obtained were glued on a prefabricated acrylic block. These specimens were wet polished and finished using sandpaper with consecutive sizes of 1000 grit (Struers). This polishing procedure ensured a comparable surface roughness and served as a baseline before instrumentation to avoid any deviations in profilometric analysis. The baseline values were between 40 to 60 µm, and the specimens were distributed such that the variation was not statistically significant.

2.2 Treatment Groups

The samples were randomly assigned for root planing to one of the five test groups (n = 12 per instrument type). Each group was treated by a different root instrumentation tool. The instruments used in the study were group A: stainless steel curette, group (SS) B: titanium coated curette (SC), group C: titanium curette (Ti), group D: Everedge (EE) curettes, and group E: XP curettes (Fig. 1). Dentin removal was determined after the first 40 strokes and cumulatively for 500, 1000, and 2500 strokes. The strokes were decided based on the previous research to detect the efficiency of the instrument [21].

Fig. 1
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Tips of the instruments used for root planing procedure in the study

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2.3 Description of the Instruments

All the curettes used were identical to the Universal curettes or the design of the universal curette (Columbia curette 2R—2L). The titanium-coated curette was prepared by coating the stainless-steel curette with a 1-micron thickness of titanium nitride using the cathode arc evaporation PVD technique [22] and the plain titanium curettes were prepared using the titanium alloy (Medical grade—Ti 6Al 4 V). The EE curettes were commercially available with edge retention technology achieved by cryogenic treatment (EverEdge™ curette by hufriedy). The commercially available XP curettes were prepared by a patented nano-optimized technology making the instrument durable and hard. The material used for the curette has a Rockwell C hardness of 89 as claimed by the manufacturer (XP curette by American eagle®).

The samples were subjected to 40, 500, and 1000 stokes by manual instrumentation. A single operator performing the strokes was calibrated to ensure that the applied force was exerted within the defined range (3–4 N) [23]. The performance was determined using the force application platform (a modified micro weighing scale), and it showed improper performance using the curettes for 2500 strokes and resulted in operator fatigue. Hence 2500 strokes were carried out using a robotic arm that was calibrated to perform this maneuver. Calibration of the instrument was done using the working strokes that ran from apical to coronal, parallel to the axis of the tooth, and the length was fixed at 3 mm.

2.4 Instrumentation Procedure

After the first wear phase of 40 strokes using a new curette, the second sequence (500–40) strokes were performed with the same curette applying the same pressure on new standardized tooth root samples. Then, a 1000 and 2500 final wear sequence was performed using the fresh root sample.

Sharpening of the curettes was not performed at any time during the instrumentation cycle. Instead, chemical, and thermal stress induced during sterilization processes were simulated after the first two wear procedures. Before the sterilization procedure, the curettes were immersed in an instrumentation disinfection solution for 30 min, then carefully rinsed with water and dried using a Stericlave® drying device at a standard program for ten minutes. The instrument was finally subjected to sterilization comprised of a pre-vacuum period of 8 min, an actual sterilization period of 18 min at 134 °C, and a drying period of ten minutes. After this sterilization procedure, total mechanical wear of 1000 strokes and 2500 strokes were performed.

The final 2500 strokes were carried out using a calibrated industrial robot manipulator IRB2600 by ABB Robotics (Fig. 2a). It is a 6-axis manipulator fitted with a 2-jaw gripper. The gripper opening and closing are operated with pneumatic control. The manipulator's axis is positioned by AC servo motors controlled by programming the IRC5 controller using RAPID programming.

Fig. 2
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a Calibrated industrial robot manipulator IRB2600 by ABB Robotics. b Preprogrammed Jaw gripper holding the curette during the procedure. c During the instrumentation procedure showing the removal of cementum surface from the root embedded on an acrylic at 2500 strokes

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2.5 Programming and the Setup of the Robotic Arm

For the root surface instrumentation, the robot was programmed to simulate the root instrumentation strokes. First, the curette is attached to the 2-jaw gripper of the robotic arm (Fig. 2b), and the root dentin specimen is embedded in an acrylic model that is fixed to a firm platform (Fig. 2c). The programming was done as follows: (1) The stroke length was calibrated to be 3 mm with every stroke, (2) the cutting edge of the instrument was reengaged 0.005 mm deeper, (3) The total number of strokes and the endpoint was entered at 1000 or 2500 strokes. Once the programming was completed, the program was executed to make 1000/2500 strokes with a velocity of 50 mm/s. An examiner was constantly present at the site to inspect the movement of the arm during the complete cycle.

2.6 Scanning Electron Microscopic (SEM) and Energy-Dispersive X-ray Spectroscopy (EDS) Analysis

After completing the procedure, the root samples were mounted on an SEM mounting platform, gold-sputtered, and made conductive (JFC 1600, Auto fine coater, Tokyo, Japan). The specimen was then analyzed under the SEM (JSM-6380LA, Analytical scanning electron microscope, JEOL, Germany). The root surface morphology before and after instrumentation at 100, 250, 500, and 1000 X Magnification and the EDS analysis of the curettes.

2.7 Profilometric Analysis

Profilometric analysis was done using the Surtronic 3 + profilometer (Taylor/Hobson Pneumo). The profilometer was checked for sensitivity before the beginning of the measurement. The sensitivity was limited using a standard specimen with the stylus placed at the right angles to the grooves. The cut-off length (Lc) was kept at 0.8 mm; the specimen's Ra parameter was measured and compared with the Ra of the reference specimen. The measurement was repeated till it was with 2% of the actual. The Ra and Rz value of the specimen was calculated with a cut-off length of the profilometer fixed to 0.8 mm, the tracing length was 1.25 mm, the traverse length was 1.45 mm, and the speed was 0.1 mm/sec. The Ra and Rz values were taken as an average of five readings.

The surface roughness micromorphology data concerning the measurements of roughness (Ra) and (Rz) and the Scanning electron microscopy (SEM) analysis were all obtained from the center of the specimen. The roughness measurements Ra and Rz values were tabulated for analysis using the Two-way ANOVA using Jamovi software (The jamovi project. Jamovi. 1.2 ed2020, Team RC. R: A Language and environment for statistical computing. Version 3.6 ed2019). For the individual posthoc comparisons Tukey test was carried out after the assessment of the homogeneity of variance. The statistical significance was set at 95%, and p < 0.05 was considered statistically significant.

3 Results

3.1 Profilometric Analysis

After the initial 40 strokes (baseline), the lowest surface roughness and roughness depth change was recorded for the titanium coated instrument (Ra; 2.33 μm, Rz; 18.643). At the second test interval of 500 strokes, the XP curettes presented with the lowest mean surface roughness and roughness depth (Ra; 1.39 μm, Rz; 12.319 μm). Before the third instrumentation cycle, the instrument was sterilized. The mean surface roughness and the roughness depth at the endpoint of 1000 strokes showed XP curettes having a mean surface roughness and a roughness depth less than the EE curettes (Ra; 1.02 μm, Rz; 9.61 μm). The final cycle of 2500 strokes was carried out using the automated robotic arm and showed the lowest mean surface roughness after instrumentation using the XP curettes (Ra; 0.71 μm, Rz; 7.19 μm) (Figs. 3, 4).

Fig. 3
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Bar graph showing the descriptives of mean roughness value (Ra)

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Fig. 4
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Bar graph showing the descriptives of the maximum height of the profile (Rz)

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A statistically significant difference was seen using 2-way ANOVA analysis within the instrument groups, and the strokes applied. Hence the null hypothesis was rejected (Table 1). The post hoc analysis showed a statistically significant difference in the Ra values between the XP curettes and other types of curettes (p < 0.05) (Tables 2). The post hoc analysis between each stroke showed the roughness value reduced as the strokes increased p < 0.05 (Table 3). Similar findings were shown, and the difference was statistically significant when the Rz values were observed (p < 0.05). Post hoc analysis also showed the XP curettes to be better than other curettes, and it was statistically significant (p < 0.05) (Tables 4, 5, 6).

Table 1 Analysis of mean surface roughness (Ra) using two-way ANOVA showing statistically significant difference between the instruments and the strokes (p < 0.005)
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Table 2 Post hoc analysis of Ra values between each instruments showing statistically significant difference between the XP instruments and other instruments (p < 0.05)
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Table 3 Post hoc analysis of Ra between each strokes showing statistically significant difference between the 2500 strokes and all other strokes (p < 0.05)
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Table 4 Analysis of average surface depth (Rz) using Two-Way ANOVA showing statistically significant difference between the instruments and the strokes (p < 0.005)
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Table 5 Post hoc analysis of Rz values between each instruments showing statistically significant difference between the XP instrument and the strokes (p < 0.005)
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Table 6 Post hoc analysis of Rz between each strokes showing statistically significant difference between the SS instruments and the strokes (p < 0.005)
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3.2 Scanning Electron Microscopic Analysis (SEM)

3.2.1 Changes Seen on the Instruments

New instruments were observed under the scanning electron microscope before the treatment was started. The images showed the cutting edge of the stainless-steel curettes to have irregular margins and uneven surfaces. The lateral surface and the face of these blades showed numerous striations attributed to the alterations seen following sharpening. In addition, multiple cutting edges were seen to form the face and the lateral surface (Fig. 5). On the other hand, the titanium-coated curettes had a smoother lateral surface than the face of the blade. There were multiple striations on the face of the blade, and the cutting edge was very irregular, with striations running along the cutting edge. After 1000 and 2500 strokes, the cutting edge of the curette showed loss of substance and exhibited physical signs of wear. There was a minimal noticeable change after 40 500 strokes but after 1000 and 2500 strokes, the cutting edge showed a noticeable edge loss, and there were signs of blunting (Fig. 6).

Fig. 5
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Scanning electron microscope (SEM) images of a stainless-steel curette (SS) showing: a The sharpness of the cutting edge of the blade and b the root surface morphology after 40, 500, 1000 and 2500 root planing strokes

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Fig. 6
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Scanning electron microscope (SEM) images of a titanium-coated stainless-steel curette (SC) showing: a The sharpness of the cutting edge of the blade and b the root surface morphology after 40, 500, 1000 and 2500 root planing strokes

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The titanium curettes had a smoother lateral surface, and there were multiple striations on the face of the blade. In addition, the cutting edge was very irregular, with striations running along this edge. There was a minimal noticeable change after 40 strokes but after 500, 1000 and 2500 strokes, the cutting edge showed noticeable loss with evidence of blunting (Fig. 7).

Fig. 7
figure 7

Scanning electron microscope (SEM) images of a titanium curette (Ti) showing: a The sharpness of the cutting edge of the blade and b the root surface morphology after 40, 500, 1000 and 2500 root planing strokes

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The Everedge (EE) curette had a smooth face of the blade, but the lateral surface was irregular with multiple vertical striations (Fig. 8). However, a single uniform cutting edge traversed that was continuous along the instrument's blade could indicate the sharpness. There was no noticeable change in the cutting edge at 500 and 1000 strokes. The changes at 2500 strokes were also very minimal.

Fig. 8
figure 8

Scanning electron microscope (SEM) images of an Everedge curette (EE) showing: a The sharpness of the cutting edge of the blade and b the root surface morphology after 40, 500, 1000 and 2500 root planing strokes

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The XP curettes had a smooth face as well as a lateral surface (Fig. 9). The cutting edge of the blade was uniform, and it appeared comparatively precise than other curettes. There were no changes on the cutting edge of the instrument's blade until 1000 strokes, and minimal changes occurred at 2500.

Fig. 9
figure 9

Scanning electron microscope (SEM) images a XP curette (XP) showing: a The sharpness of the cutting edge of the blade and b the root surface morphology after 40, 500, 1000 and 2500 root planing strokes

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3.2.2 Changes Seen on the Root Surface

The scanning electronic microscopy (SEM) of the root surface after 40 and 500 strokes, using stainless steel (SS), stainless steel coated (SC), and the titanium curette (Ti), showed tooth surface loss. In addition, the root surface showed irregular lines and a rough surface (Figs. 5, 6, 7).

After instrumentation of 1000 and 2500 strokes using SS, SC, and the Ti curettes, progressive loss of root substance and surface changes correspond to the wear/blunting of the curette blade. After using the titanium curette, the root surface showed irregular wavy lines and deep gouges resembling tree-bark or tyre-patch (Fig. 7) corresponding to the curette blade's surface. The XP and the EE curette root surface images showed shallow gouges at 1000 strokes, but the textures were uniform and smooth at 2500 strokes (Figs. 8, 9). The EDS analysis showed no changes on the surfaces of the curettes other than the titanium and titanium coated curettes. The titanium curette (Ti) showed corrosion on the lateral surfaces, and it was greater than the Titanium coated (SC) curettes (Fig. 10).

Fig. 10
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EDS analysis of the a titanium coated stainless steel curette and b titanium curette showing the corrosion on the lateral surface of the blade of the instrument

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4 Discussions

Subgingival instrumentation is an integral part of non-surgical periodontal therapy and is achieved by ultrasonic and hand instruments. A root surface that is smooth and hard favors healing and periodontal tissue attachment [24]. Hand instruments have been considered the gold standard for this purpose as it renders the root surface smooth and helps in better attachment of the cells [12, 25,26,27]. Recent clinical practice guidelines stated in an EFP workshop also give a unanimous grade A consensus based on the quality of evidence for using either hand instruments or the ultrasonic instruments, used alone or in combination [28].

Several studies have observed irregular morphological changes of the root surface under SEM [13, 29, 30]. It was the reflection of the cutting edge of the curettes and hence was directly related to its cutting-edge deficiency. The surface changes resembled a wavy pattern, tree bark, tire patch, and these patterns appeared when improper instrumentation or a damaged curette. Similar observations were seen in the present study with a wavy pattern of the titanium curette after instrumentation, a negative impression on the root surface that showed the cutting-edge variation. Profilometric findings provide a good tool for assessing this roughness and quantifying the quality of the instrumentation [31, 32].

Moreover, root surface roughness can be measured using the amplitude parameters such as Rt; the maximum height/ total height of the profile measures the height between the deepest valley and the highest peak of the evaluation length. Rz is a frequently used parameter to check whether the profile has protruding peaks that might affect the static or sliding contact function. The Rq—root mean square deviation of the assessed profile provides the same information as the Ra value. Hence in this study the reason for including the Ra value is its global acceptance in evaluating the roughness amplitude on a profile, and it is meaningful for random surface roughness that has changed with the use of surface polishing [33].

Studies have shown the mean surface roughness between 0.09 and 0.34 at the initial variability observed after the first ten strokes and 0.18 to 0.05 at 1,010 strokes. In the present study, the mean roughness after 40 strokes was between 2.33 to 4.66; after 2500 strokes, the Ra value was between 0.71 to 2.58. The titanium nitride coated [Patented LAFAD Coating] [34] curettes had shown superior capabilities upto 15,000 strokes before the layer was removed and the underlying stainless-steel alloy exposed [35]. Whereas in the present study, the Titanium coated instrument (PVD-coated) performed like the stainless instrument, resulting in a Ra value of 1.933 at 2500 strokes.

The quality of instrumentation depends on the type of alloy used for manufacturing the instrument. Stainless steel instruments have been used previously for a long time, and the studies that have been done on this material have shown a need for repeated sharpening. Comparative studies to examine the cutting-edge retention compared stainless steel with carbon steel to see how it affects the hardness of the cutting edge. The stainless-steel curettes showed significant edge attrition after 45 strokes compared to the high-speed steel, cemented carbide steel, and high carbon steel instruments tested [18]. Studies testing titanium instruments have come across mixed results, with titanium curettes being the best in one [36] and aggressive in another [37]. The cutting/root planing efficiency of titanium nitride-coated curettes and cryogenically treated curette with edge retention technology have been compared. A simulation of the wear efficiency was checked on bovine dentin. Both the instruments were comparable and showed insignificant differences even after 1,010 strokes. The present study noticed notching or change in the cutting edge to an irregular margin in three instruments: stainless steel, titanium alloy, and titanium nitride coated instrument. It has been accepted that subgingival instrumentation requires re-sharpening after every 5 to 40 strokes [9, 13]. Coldiron et al. (1990) observed that even after 35 strokes, the curettes were cutting smoothly and sounding sharp [15]. In the present study, the stainless steel, titanium, and titanium coated curette had a similar surface roughness at 40 strokes, but there was variation afterward between the instruments. Hence it could be interpreted that over 40 strokes, these curettes needed to be sharpened. The sharpness retention also depended on the quality of the alloy and their processing during the instrument production; the retention of sharpness (or root planing capacity) can be present for many more strokes than previously thought. The corrosion of the titanium curettes seen during this study can be due to the exposure of the titanium coated and the titanium curette to the environment following sterilization. Corrosion behavior of these curettes have not been discussed in literature till date. The SS curettes (316L grade) and carbon steel curettes have shown to be resistant to corrosion, but some authors have pointed out that saturated vapors due to heat and moisture can cause corrosion [38, 39].

Two commercially available curettes selected in this study were made of a specially tempered, cryogenically treated stainless steel alloy and a patented nanotech instrument preparation technology. These were the critical retention feature of the cutting edge of the ever-edge and the XP curettes even after 2500 strokes. Furthermore, in an in-vitro study, Er: YAG laser (VersaWave, Delight; Hoya-Con Bio) chisel tip Laser was compared with that of the TIN (American eagle XP curette) and, the TIN was proved to be better [36]. Therefore, similar findings have been presented in this study and have shown that the XP curettes have a better edge retention property than other curettes.

4.1 Limitations of the Study

While the study successfully identified the curette with an excellent cutting-edge retention and provides a good root surface morphology after usage, some limitations need to be considered. First, the minimum number of strokes that could give the final smoothness could not be determined. Second, the titanium coating was done with PVD rather than the LAFAD; hence the resulting variability could be due to the different coatings. Third, the force used during the scaling procedure by the robotic arm could not be assessed during the entire cycle. If the force application is uneven, then the interpretation of the results could have a variation. Therefore, we limited the number of strokes to 2500, which was less than the study done by Gorokhovsky et al. [35] but more than the number of strokes reported by Sisera et al. [21]. Also, in the present study, we could not determine the amount of tooth substance loss after completing the scaling procedure as per the previous studies [21, 31]. Future work could choose the lowest number of strokes to smoothen the root surface or the maximum number of strokes to produce blunting of the XP curettes. Estimated tooth substance loss and a test/training kit to assess the force application that could provide this optimum result could also be undertaken.

5 Conclusions

All four test instruments held their cutting capacity at initial forty strokes under conditions managed in the present study. Further, cutting edge was retained for both the XP and the EE curettes even after 2500 strokes, as seen under the SEM analysis. In addition, the root surface was smooth even after 2500 strokes when XP curettes were used. Therefore, under the experimental conditions maintained in the present study, periodontal curettes with edge retention technology are practical tools, and clinicians could be used for a long time. Hence conventional curettes could be replaced with these curettes during root planing during routine periodontal therapy.