At 3-months, at medium depth, heterogeneity was great (I2=90%, Fig.2a). After sensitivity analysis, four studies were found highly heterogeneous to each other and were unsuitable for meta-analysis. After a bias analysis, the heterogeneity source was thought to be: (1) small number of studies; (2) the fact that tissue healing took time and early probing disrupted attachment gains. At 3-months, PPD and CAL reductions were unstable, causing large heterogeneity.
At medium depth, 6-months, Kargas et al. was found to have significant heterogeneity. After excluded, heterogeneity decreased to 0% (Tau² = 0.00; Chi² = 0.02, df = 2 (P = 0.99); I² = 0%). The results showed statistically significant differences between manual and ultrasound groups. PPD reduction after manual subgingival scaling was greater than ultrasonic subgingival scaling (MD 0.19,95%CI [0.11, 0.27], P<0.00001). Compared with the other three studies, only non-smokers were included in this study, which might be the reason for heterogeneity (Fig. 4a).
When initial PPD was deep at 6 months, D'Ercole was a major origin of heterogeneity. After exclusion, heterogeneity decreased to (Tau² = 0.03; Chi² = 3.86, df = 2 (P = 0.15); I² = 48%) (Fig. 4a).
When initial PPD was medium, at 3-months follow-up, the heterogeneity of CAL was high (Tau² = 0.01; Chi² = 10.92, df = 3 (P = 0.01); I² = 73%). According to a sensitivity analysis, four studies were highly heterogeneous with each other, making them unsuitable for meta-analysis (Fig. 4b). The heterogeneity source was also thought to be: (1) too few studies; (2) tissue healing took time and early intervening probing may damage attachment gain. When the follow-up period was only 3 months, CAL were unstable which caused great heterogeneity.
Heterogeneity was also large in the following three groups: 1) deep pocket, at 3-months follow-up; 2) medium pocket, at 6-months follow-up; 3) deep pocket, at 6-months follow-up. After excluding Ioannou 2009, heterogeneity decreased from: 87%, 52%, 71% to 24%, 0%, 0%. This study was the only one in which 50% of the patients were smokers, while other papers were unclear about the ratio of smokers or had a small number of smokers, which might be the reason for heterogeneity. After exclusion, at deep pocket depth of a 6-months follow-up, after manual subgingival scaling, CAL reduction was more than ultrasonic subgingival scaling and were statistically different. (MD 0.58, 95%CI [0.27, 0.89], P=0.0002) (Fig. 4b, Fig. 4c).
In the same study, heterogeneity of PPD and CAL was much greater at 3-months than at 6-months follow-up. The influence was quite apparent at medium PPD, 3-months. At deep initial depths, either 3-months or 6-months, heterogeneity was acceptable. The reason could be that the tissue took time to heal. In deep pocket, tissue contacted and attached to the bone better, resulting in shorter healing time and more stable condition within 3 months. At medium pocket depths, tissue did not contact bone as readily as at deeper pockets, so healing time was longer. Probing too early in healing process may damage tissue and influence attachment gain, and led to unstable results.
Quality of evidence
After evaluating the quality of data with the GRADE system, the following results were obtained: the data of PPD at 3-months and 6-months follow-up of shallow pocket, and CAL at 3-months follow-up of shallow pocket was of very low quality; the data of PPD at 3-months follow-up of medium pocket, CAL at 3-months follow-up of medium and deep pocket and at 6-months follow-up of shallow pocket was of low quality; the data of PPD at 3-months follow-up of medium pocket, at 6-months follow-up of medium and deep pocket and CAL at 6-months follow-up of medium and deep pocket was of moderate quality. Details were given in the Appendix 7. The data of shallow pocket mostly was of very low quality because of too small sample size and publication bias, so that we could not draw a reliable conclusion and more studies are required. The data of 3-months follow-up was of low or very low quality, which might due to the fact that tissue healing took time and early probing disrupted attachment gains. We thought 3-months follow-up was too short and we cannot draw reliable conclusions according to the data of it. The data of PPD and CAL at 6-months follow-up for the medium and deep pocket groups was of moderate quality. Based on the above, we thought the conclusion should be drawn according to the data of 6-months follow-up of medium and deep pocket groups.
According to the above analysis, different indicators showed statistical significance between ultrasonic and manual subgingival scaling, which indicated the different effectiveness in clinic after the treatment of ultrasonic and manual instruments.
In addition, in shallow pocket, CAL increased after both ultrasonic and manual subgingival scaling, which might be resulted from junctional epithelium attachment damage. We have also found it clinically. Therefore, manual subgingival scaling is not recommended when PPD is less than 4mm. In clinical practice, when PPD is less than 4mm and there is symptom such as bleeding on probing or subgingival dental calculus, ultrasonic subgingival working tip can be used for deep cleaning.
In terms of GR, at medium or deep PPD, there was no differences between manual and ultrasonic subgingival scaling, whatever the roots were single or multiple.[15, 18]
BOP results of one study showed, at 6-months follow-up, more BOP reduction at deep depths after manual scaling than ultrasound.
Residual calculus provided different results. Two studies[20, 21] indicated that, regardless of depth, there were no statistical differences in calculus clearance rates between ultrasound and manual treatment. Schwarz indicated when PPD was deep, for a single-root tooth, ultrasonic dental calculus removal was more effective than manual subgingival scaling.
Above all, ultrasonic subgingival scaling is an efficient non-surgical treatment, yet manual subgingival scaling is also essential and cannot be replaced by ultrasonic method.
According to the newly-released 2018 periodontitis classifications, there are something about it:
- Different economic and health care developments between developed and developing countries make different influences on periodontitis.[22, 23] Primary CAL in developing countries was three times of developed countries. Only one study involved a developing country (Turkey). Whether the conclusions reached in this paper apply to developing countries is unknown.
- Smoking is confirmed as affecting progress of periodontitis and was considered in the new classification. Most of the included studies chose patients according to the 1999 classification and did not consider the impact of smoking, which may lead to heterogeneity.
- Significance to clinical practice
(1) Combining our above analysis and quality of evidence, we believed that only 6-months follow-up results could be used to reach following conclusions:
- When initial PPD was shallow, no conclusions were drawn due to the limited number of studies.
- When initial PPD was medium, PPD reductions proved that manual subgingival scaling was superior. CAL and GR results showed no statistical differences. More studies are needed before any conclusion can be drawn.
- When initial PPD was deep, manual subgingival scaling was superior in terms of PPD, CAL and BOP results, while GR results showed no statistical differences. This conclusion also needs more study because of the limited number of studies.
(2) In terms of residual dental calculus, there was no conclusion could be drawn.
- Significance to research
(1) Inclusion and exclusion criteria could refer to the new classification to reduce bias;
(2) Studies should consider other indicators such as BOP, PI, and GI, bacterial changes, when comparing in different PPDs;
(3) More studies are needed in developing countries;
(4) Single and multiple root teeth should be measured separately;
(5) Further studies should enlarge sample sizes to improve credibility;
(6) Inclusion or exclusion criteria for smokers should be standardized.
(7) Studies with a follow-up period of six months or longer are suggested to determine reliable results.