Food impaction is a very difficult problem after full crown restorations and is commonly found in the interstitial space of the terminal teeth. However, the cause of such embedding is not clear, making its prevention and treatment difficult. Previous clinical studies have suggested that food impaction with intact adjacent surfaces can be treated by adjusting the contact points of the cementation surfaces. Whether or not the occlusion after crown restoration is also related to the distribution of occlusal contact points is yet to be further investigated. In this paper, we compared the incidence of food impaction in the group of final teeth with that in the group of non-final teeth by following up cases of second molar full crown restorations, and analyzed the relationship between the occurrence of food impaction and occlusal contact points after final teeth full crown restorations, so as to provide a reference for the prevention and treatment of food impaction after crown restorations. 1. Materials and methods 1.1 Medical records and grouping Cases of second molars requiring single tooth full crown restoration due to large dental defects were selected from 2010 to 2011 in our restorative department. The affected teeth had no obvious loosening and no obvious periodontal pockets; the root canal treatment was perfect and there was no percussion pain or gingival abnormalities; the teeth on the affected side were intact and the occlusion was basically normal; there were no serious chronic diseases, good compliance and they were permanent residents of the city. The medical records were divided into two groups: the study group had the tooth as the terminal tooth, i.e., no third molar; the control group had the tooth as the non-terminal tooth, i.e., there was a third molar behind the tooth. 1.2 Methods Case manipulation, review and result statistics were performed by three different physicians. The case operators had no prior knowledge of the factors affecting this study. All cases were fabricated according to full or pile crown requirements. After the crown was fully seated, there was some resistance to the adjacent surface passing through with the forming piece, and there was no early contact between the median and lateral coaptation, and the glass ionomer cement was cemented after the completion of the coaptation. After clearing the cement, the looseness of the neighboring surface contact was recorded using the plug gauge method, and the contact relationship between the occlusal surfaces of the full crown was recorded with high-precision occlusal paper. The occlusal surface contact relationship and the tightness of the adjacent surface contact and food embedment were reviewed 3 months after surgery; for those with complaints of food embedment, fibrous foods such as beef jerky or enoki mushrooms were requested to be eaten before the examination, and food embedment was considered only when fibrous foods were found locally. In this study, food embedding between the first and second molars in the full-crown restoration area was used as the study index, and food embedding in other areas was not included in the scope of this study. 1.3 Full-crown occlusal surface contact relationship typing: In this paper, the occlusal surface contact relationship is divided into 3 types: Type I refers to the full-crown cusp or the distal mesial slope of the crest has obvious contact points; Type II refers to the cusp or the distal mesial slope of the crest has no obvious contact points, but the cusp or the proximal mesial slope of the crest has obvious contact points; Type III refers to the cusp contact points are not obvious. 1.4 Classification of contact tightness of adjacent surfaces: according to the resistance of floss through the contact area, it was divided into 3 categories: tight type means difficult to pass; moderate type means passing with certain resistance; no contact type means passing without resistance. 1.5 Statistical methods: SPSS10.0 statistical software was applied to perform one-way ANOVA on clinical data, and p<0.05 was considered statistically significant difference. 2, Results Relationship between contact of adjacent surfaces after full crown adhesion, occurrence of food impaction at 3 months after surgery, and relationship between food impaction and occlusal contact in the experimental group. Food embedding is a very difficult problem encountered after full-crown restorations and is commonly found in the interstitial space of the terminal teeth [1]. However, the cause of such embedding is not clear, making its prevention and treatment difficult. Previous clinical studies have suggested that food impaction in the intact adjacent surface type can be treated by adjusting the contact point of the cementum. Whether or not the occlusion after crown restoration is also related to the distribution of occlusal contact points remains to be further investigated. In this paper, we compared the incidence of food impaction in the group of final teeth with that in the group of non-final teeth by following up cases of second molar full crown restorations, and analyzed the relationship between the occurrence of food impaction and occlusal contact points after final teeth full crown restorations, so as to provide a reference for the prevention and treatment of food impaction after crown restorations. 1. Materials and methods 1.1 Medical records and grouping Cases of second molars requiring single tooth full crown restoration due to large dental defects were selected from 2010 to 2011 in our restorative department. The affected teeth had no obvious loosening and no obvious periodontal pockets; the root canal treatment was perfect and there was no percussion pain or gingival abnormalities; the teeth on the affected side were intact and the occlusion was basically normal; there were no serious chronic diseases, good compliance and they were permanent residents of the city. The medical records were divided into two groups: the study group had the tooth as the terminal tooth, i.e., no third molar; the control group had the tooth as the non-terminal tooth, i.e., there was a third molar behind the tooth. 1.2 Methods Case manipulation, review and result statistics were performed by three different physicians. The case operators had no prior knowledge of the factors affecting this study. All cases were fabricated according to full or pile crown requirements. After the crown was fully seated, there was some resistance to the adjacent surface passing through with the forming piece, and there was no early contact between the median and lateral coaptation, and the glass ionomer cement was cemented after the completion of the coaptation. After clearing the cement, the looseness of the neighboring surface contact was recorded using the plug gauge method, and the contact relationship between the occlusal surfaces of the full crown was recorded with high-precision occlusal paper. The occlusal surface contact relationship and the tightness of the adjacent surface contact and food embedment were reviewed 3 months after surgery; for those with complaints of food embedment, fibrous foods such as beef jerky or enoki mushrooms were requested to be eaten before the examination, and food embedment was considered only when fibrous foods were found locally. In this study, food embedding between the first and second molars in the full-crown restoration area was used as the study index, and food embedding in other areas was not included in the scope of this study. 1.3 Full-crown occlusal surface contact relationship typing: In this paper, the occlusal surface contact relationship is divided into 3 types: Type I refers to the full-crown cusp or the distal mesial slope of the crest has obvious contact points; Type II refers to the cusp or the distal mesial slope of the crest has no obvious contact points, but the cusp or the proximal mesial slope of the crest has obvious contact points; Type III refers to the cusp contact points are not obvious. 1.4 Classification of contact tightness of adjacent surfaces: according to the resistance of floss through the contact area, it was divided into 3 categories: tight type means difficult to pass; moderate type means passing with certain resistance; no contact type means passing without resistance. 1.5 Statistical methods: SPSS10.0 statistical software was applied to perform one-way ANOVA on clinical data, and p<0.05 was considered statistically significant difference. 2.Results The relationship between the contact of adjacent surfaces after full crown adhesion, the occurrence of food impaction at 3 months after surgery, and the relationship between food impaction and occlusal contact in the experimental group. 3, Discussion Food impaction after crown restoration has been one of the difficult problems plaguing the restorative clinic [. Although restorative clinicians generally feel that the incidence of food impaction after crown restoration in the terminal position is higher than in other sites, there has been a lack of corresponding research studies to objectively confirm and explain this phenomenon. In this study, the incidence of food impaction after full crown restoration was 20.6%, which was higher than that of 6.9% for non-final teeth, and the p-value was statistically close to 0.05, suggesting that food impaction should be given high priority when crown restoration is performed on final teeth. However, what is the reason for the higher incidence of food ingrowth after crown restoration in the terminal teeth than in the non-terminal teeth? It is generally accepted that the root cause of food ingrowth is the gap in the contact area of the adjacent surfaces. If the adjacent contact between the two teeth is maintained during occlusion, food impaction is unlikely to occur. In all the cases in this study, there was a close or moderate adjacent surface contact relationship before and after cementation, and the occurrence of food impaction must be due to a dynamic gap between the crown and the anterior teeth during occlusion. The generation of the gap can be attributed to the uncoordinated movement of the adjacent teeth, and there are two scenarios: one is a large anterior tilt of the teeth in front of the crown and insufficient anterior tilt of the crown; the other is a posterior tilt of the crown. Since the anterior tilt of the crown tooth needs to fight against the resistance of the periodontal elastic modulus common to the first molar itself, the second premolar, the first premolar, and the cuspid, in the non-periodontal patients with basically normal occlusion in this study, the anterior tilt movement of the first molar will be very small, and it is difficult to have a significant gap even if the crown tooth does not tilt anteriorly, and the embedding is not easy to occur, so it is considered that the embedding occurs mainly due to the second scenario, that is, the crown tooth Therefore, it is considered that the occurrence of embedding is mainly due to the second scenario, that is, the crown tooth is tilted distomedially. When the crown tooth is the terminal tooth, there is no other resistance to the distomedial tilt; however, when there is a third molar behind the crown tooth, the posterior tilt of the crown tooth has to resist the upper and lower occlusal relationship of the third molar, making the posterior tilt much more difficult; this explains why the incidence of food impaction after crown restoration of the terminal tooth is much higher than that of the non-terminal tooth in this study. The natural dentition has a tendency to shift proximally in the occlusal process, while the implant does not have a corresponding proximity movement, thus creating a gap between the implant and the anterior teeth . In the case of full crown restorations, the periodontal movement and the tendency of the forces to move do not change, but the configuration and the contact relationship of the surfaces change in a complex way. The force of the distal mesial tilt of the crowned tooth comes mainly from the joint force on the proximal mesial bevel of the cusp or crest of the symphyseal plane. The forces during occlusion are mainly transmitted through the contact points of the cementum. During crown restoration alignment, attention is usually paid to the occlusal high point, and little attention is paid to the distribution of occlusal contact points on the symphyseal surfaces. Currently, there is no corresponding classification of occlusal surface contact types of crowns. In this study, the type of occlusal contact was divided into 3 types, and the type of occlusal contact in the group of crown restorations with food impaction was mostly type II, i.e., the contact point of the proximal mesial bevel on the crown was obvious but the contact point of the distal mesial bevel was not obvious, and the type II crown was subjected to the force of displacement of the cusps of the opposing teeth to the distal mesial direction during the occlusal contact. The probability of food impaction in the total type II facet contact reached 57.1%, suggesting that type II facet contact should be prevented in the clinical restoration of the final teeth, thus greatly reducing the occurrence of food impaction; while food impaction rarely occurred in type I, i.e., those with uniform facet contact or obvious distocentral oblique contact; for type III, which has no obvious facet contact, 1 out of 2 cases occurred 3 months after surgery. Food impaction is caused by the vertical movement of the teeth to re-establish unfavorable occlusal contact. The significant correlation between the type of contact point and the occurrence of food impaction suggests the importance of occlusal adjustment to prevent this type of food impaction. There can be other causes of food impaction after crown restoration, such as poor recovery of the contact area, periodontal disease, chipped porcelain, and tooth loss, which can be easily taken into account and avoided, and these confounding factors were excluded in this study by case selection and operative requirements. The study was double-blinded so that the crown restorer was not aware of the experimental observation factors in advance, avoiding the influence of subjective factors on the results. In cases where food impaction occurs after a crown restoration in the terminal dentition, the forces that make it move distally and medially can be completely eliminated by adjusting the contact points on the proximo-medial bevel of the cusps. In the case of the dynamic gap type, the blockage can be resolved with one adjustment; in the case of the static gap type, several iterations are required and in conjunction with other methods. Food impaction after crown restoration has been one of the difficult problems plaguing the restorative clinic. Although restorative clinicians generally feel that the incidence of food impaction is higher after crown restoration in the terminal position compared to other sites, there has been a lack of research studies to objectively confirm and explain this phenomenon. In this study, the incidence of food impaction after full crown restoration was 20.6%, which was higher than that of 6.9% for non-final teeth, and the p-value was statistically close to 0.05, suggesting that food impaction should be given high priority when crown restoration is performed on final teeth. However, what is the reason for the higher incidence of food ingrowth after crown restoration in the terminal teeth than in the non-terminal teeth? It is generally accepted that the root cause of food ingrowth is the gap between the adjacent contact areas[. If the adjacent contact between the two teeth is maintained during occlusion, food impaction is not likely to occur. In all cases in this study, there was a close or moderate adjacent surface contact relationship before and after mucoadhesion, and the occurrence of food impaction must be due to a dynamic gap between the crown and the anterior teeth during the occlusion process. The generation of the gap can be attributed to the uncoordinated movement of the adjacent teeth, and there are two scenarios: one is a large anterior tilt of the teeth in front of the crown and insufficient anterior tilt of the crown; the other is a posterior tilt of the crown. Since the anterior tilt of the crown tooth needs to fight against the resistance of the periodontal elastic modulus common to the first molar itself, the second premolar, the first premolar, and the cuspid, in the non-periodontal patients with basically normal occlusion in this study, the anterior tilt movement of the first molar will be very small, and it is difficult to have a significant gap even if the crown tooth does not tilt anteriorly, and the embedding is not easy to occur, so it is considered that the embedding occurs mainly due to the second scenario, that is, the crown tooth Therefore, it is considered that the occurrence of embedding is mainly due to the second scenario, that is, the crown tooth is tilted distomedially. When the crown tooth is the terminal tooth, there is no other resistance to the distomedial tilt; however, when there is a third molar behind the crown tooth, the posterior tilt of the crown tooth has to resist the upper and lower occlusal relationship of the third molar, making the posterior tilt much more difficult; this explains why the incidence of food impaction after crown restoration of the terminal tooth is much higher than that of the non-terminal tooth in this study. The natural dentition has a tendency to shift proximally in the occlusal process, while the implant does not have a corresponding proximity movement, thus creating a gap between the implant and the anterior teeth . In the case of full crown restorations, the periodontal movement and the tendency of the forces to move do not change, but the configuration and the contact relationship of the surfaces change in a complex way. The force of the distal mesial tilt of the crowned tooth comes mainly from the joint force on the proximal mesial bevel of the cusp or crest of the symphyseal plane. The forces during occlusion are mainly transmitted through the contact points of the cementum. During crown restoration alignment, attention is usually paid to the occlusal high point, and little attention is paid to the distribution of occlusal contact points on the symphyseal surfaces. Currently, there is no corresponding classification of occlusal surface contact types of crowns. In this study, the type of occlusal contact was divided into 3 types, and the type of occlusal contact in the group of crown restorations with food impaction was mostly type II, i.e., the contact point of the proximal mesial bevel on the crown was obvious but the contact point of the distal mesial bevel was not obvious, and the type II crown was subjected to the force of displacement of the cusps of the opposing teeth to the distal mesial direction during the occlusal contact. The probability of food impaction in the total type II facet contact reached 57.1%, suggesting that type II facet contact should be prevented in the clinical restoration of the final teeth, thus greatly reducing the occurrence of food impaction; while food impaction rarely occurred in type I, i.e., those with uniform facet contact or obvious distocentral oblique contact; for type III, which has no obvious facet contact, 1 out of 2 cases occurred 3 months after surgery. Food impaction is caused by the vertical movement of the teeth to re-establish unfavorable occlusal contact. The significant correlation between the type of contact point and the occurrence of food impaction suggests the importance of occlusal adjustment to prevent this type of food impaction. There can be other causes of food impaction after crown restoration, such as poor recovery of the contact area, periodontal disease, chipped porcelain, and tooth loss, which can be easily taken into account and avoided, and these confounding factors were excluded in this study by case selection and operative requirements. The study was double-blinded so that the crown restorer was not aware of the experimental observation factors in advance, avoiding the influence of subjective factors on the results. In cases where food impaction occurs after a crown restoration in the terminal dentition, the forces that make it move distally and medially can be completely eliminated by adjusting the contact points on the proximo-medial bevel of the cusps. In the case of the dynamic gap type, one adjustment can resolve the embedding; in the case of the static gap type, several iterations are required and in conjunction with other methods.