The present study was carried out to assess the correlation of VD with the FGG shrinkage. In this study, the KGW and AGW increased significantly after surgery compared to baseline (P <0.001). The amount of new KG and AG averaged 5.74 mm and 5.76 mm respectively. This amount of increase was in accordance with the results of McGuire’s study.(33) In the McGuire’s study, the results showed that the average increase of KGW was 3.65 mm in the control group (FGG). This amount was less than in our study, because of the different baseline widths of FGG in the two studies (4mm v 9 mm). Silva et al. reported 5.4 mm increase in KGW which was in accordance with our study (34).
The change in PD was not significant after surgery (P =0.10). This result was similar to the other related studies. (35, 36) The shrinkage of FGG was one of the variables which assessed in this study. The shrinkage of FGG in horizontal and vertical dimensions was statistically significant at months 1,3, and 6 after surgery (P <0.001).
HGD shrinkage was 0.82 mm (8.05%) and 1.26 mm (12.3%) at 1 and 6 months after surgery. The greatest amount of shrinkage of HGD was occurred in the first month after surgery.
In Hatipoglu’s study, an average of 10.2% horizontal shrinkage was reported (37). In another study, Guncu et al. evaluated the effect of tissue adhesives on dimensional shrinkage of FGG. In this study, 14.25% horizontal shrinkage was shown in the control group (using suture for graft stabilization) at 3 months after surgery (38). The different amount of horizontal shrinkage of FGG in these three studies can be because of the different dimensions and thickness of FGG at the baseline. In Hatipoglu’s and Guncu’s studies, the baseline dimensions of FGGs were not equal and standard, and the thickness of FGG varied between 1 to 2 mm. In the present study, the primary dimensions of FGG were about 10×9 mm and the thickness of FGG was about 1.5 mm because of the use of mucotome.
In a study conducted by Silva et al., the mean horizontal graft shrinkage was 22% in non-smokers and 25% in smokers after three months, which was higher than our results (10.1%) (34). The primary horizontal dimension of prepared FGG in Silva’s study was about 14 mm and in our study was about 10 mm. This difference in amount of shrinkage may be partly related to different horizontal dimension of FGG. It can be concluded that if the primary horizontal dimension of FGG is greater, the horizontal shrinkage of FGG will be more. This is only a hypothesis and more studies will be needed to approve it.
In our study, the mean vertical graft shrinkage was 3 mm (33.74%) after six months. Mörmann et al. showed an average vertical graft shrinkage of 42.3% after 12 months postoperatively (39). In the study of Wei et al., the mean vertical graft shrinkage in the control group (FGG) was 16% six months after surgery (40). Hatipoglu et al. reported that the mean vertical graft shrinkage after 6 months was 24.8%.(12)The difference in baseline vertical dimension of FGGs, the thickness of FGGs and the type of suturing techniques may be the causes of these different results.
The most horizontal and vertical graft shrinkage were occurred during the first month after surgery (8.05%, 25.19% respectively) and the shrinkage of the vertical dimension was more than the horizontal dimension. This finding was in accordance with the other studies.(12, 34, 41) In the present study, the mean GA decreased by 42.2% during the follow-up period, which was statistically significant (P <0.001). Hatipoglu et al. obtained similar result and showed that graft shrinkage was 35.3% after six months.(12) The shrinkage of FGGs is a well-known clinical event that happens during graft healing in the first postoperative year and the width of new keratinized gingiva remains stable thereafter (41).
The most important variable which was evaluated in this study was VD. The results of the present study showed that the VD had increased 2.05 mm after 6 months and this change was statistically significant (P <0.01). In an old study which was carried out by Egli et al., the mean increase of VD was 2.3 mm after 12 months (30). In Egli’s study, the VD was measured from the incisal edge to the floor of the vestibular fold, minus the distance from the incisal edge to the gingival margin. This measurement may be encountered with some errors during realizing the floor of the vestibular fold. But in the present study, all VD measurements were done on the prepared casts to increase the precision.
The results of repeated measures with covariance structure, showed that after adjusting the changes of KG, AG, HGD, VGD, and GA by VD, the effect size of VGD was reduced from 0.993 to 0.303. Based on this finding, the VD which is determined by the level of muscle attachment is an effective factor on shrinkage of VGD. Decrease of vertical dimension of FGG during this study may be in accordance to tendency of dissected muscular fibers to insert into their original sites. In cases with less VD, the muscular fibers are located more coronal and reattachment to them prevent apical stabilization of graft during healing phase. Adjustment of these variables by VD showed that the changes in HGD were not statistically significant. Considering the place of attachment and function of muscles in the mandible, it was expected that horizontal dimension of FGG was not influenced by VD. There was a significant correlation between VGD shrinkage (r=-0.614, p=0.005) and GA shrinkage (r=-0.476, p=0.039) with VD by performing Pearson correlation coefficient. This correlation was moderate and negative. This means that there is a reverse correlation between the shrinkage of VGD and VD. The correlation between VGD shrinkage and VD was greater than the correlation of the GA shrinkage and VD, because of the impact of FGG horizontal dimension on calculating GA. The correlation of HGD shrinkage with VD was not significant (r=0.065, p=0.792). Based on the obtained results by doing a simple linear regression, the percentage of VGD shrinkage decreased by 2.55 percent for each mm increase of VD. Therefore, greater VD result in less shrinkage of VGD and vice versa.
In recent literature review, we did not find a similar study to compare the results. To assess the shrinkage of FGG, some factors such as preparation of recipient bed (partial thickness or denuded)(20), dimensions of FGG, thickness of FGG(42), performing of periosteal fenestration or not, the flap apical to the recipient bed, phenotype of periodontium(43), suturing techniques, type and size of suture(17, 44), preoperative VD and muscular forces may interfere in healing of graft and affect the final results(28). Therefore, doing a study with this wide range of confounders is very difficult, but considering these factors is necessary to design the future studies.