# Modified Gingival Index (MGI) Classification Using Dental Selfies
## 1 Introduction  
### Dental Scientific Background  
The two most common oral diseases among adults are dental caries (tooth decay) and periodontal inflammations (gum diseases) [1]. Caries are caused by bacteria present in dental plaque, which ferment sugars, producing acid which demineralizes tooth enamel, then the bacteria enter the tissues under the enamel [2]. Periodontal diseases [3] are classified as either gingivitis or periodontitis based on severity. Gingivitis is the reversible stage, only the gums are affected, exhibiting symptoms of redness, swelling, and bleeding. With professional treatment, and maintaining a regimen of daily oral hygiene, recovery time from gingivitis is usually about 10-14 days [4]. Gingivitis which goes untreated usually escalates to periodontitis, the irreversible stage of gum disease. Acids, which are the immune system's response to the presence of toxic bacteria, cause a deterioration and finally destruction of the tooth-support tissuesand can lead to teeth loss [5]. There are several pathways of reversing inflammation. The two modes most studied in the literature are (1) mechanical plaque removal, and (2) chemical plaque removal. It has been demonstrated [6] that alcohol-free mouthwash along with regular toothbrushing is capable of reducing gingival inflammation. The significant difference between caries and periodontal disease is that caries often causes pain even in the early stages, while gum infections are often asymptomatic until a quite advanced stage [7]. What the two diseases have in common is their progression and escalation, producing a situation where delayed care can necessitate complex and expensive treatment or lead to loss of teeth [1]. Dentists have developed numerous indices to assess the severity of gingivitis [8; 9], based on one or more of the following criteria: gum coloration (redness), gum contour, bleeding presence, stippling, and crevicular fluid flow [10; 11]. Another method for distinguishing unhealthy oral tissue from healthy tissue is bioimpedance, unhealthy tissue has been found to offer lower resistance to electrical current than healthy ones [12]. Most of the indices developed require both visual and invasive measures (probing of the gums with instruments) to assess gum health status and reach a rating. Exceptional is the Modified Gingival Index (MGI) [8] which is completely noninvasive (i.e., exclusively visual). A survey study demonstrated that the variety of gingivitis indices in common use are all strongly correlated, including in particular MGI [9]. The MGI (Table 1) uses a rating score between 0 and 4, with 0 indicating a tooth with healthy gums and 4 the most severe inflammation with spontaneous bleeding, for a precise definition of each rating see Table 1.  
Numerous epidemiological studies concluded that gingivitis [8] is prevalent in children, adolescents, and adults [11; 13; 14; 15] and more than 80% of the world's population suffers from mild to moderate forms of gingivitis from time to time [1]. Treating gingivitis is relatively simple, mostly at home, based on oral hygiene maintenance methods, including brushing twice daily, mouthwash, interproximal flossing, and dental toothpicks when appropriate [16; 17; 18]. In the clinic, gingivitis is usually treated in a single visit to a dental hygienist to remove plaque and calculus (tartar, or hardened plaque) [19]. If a proper oral hygiene regimen is not maintained, gingivitis is likely to prolong and progress to periodontitis. Despite the fact that routine checkups are essential for monitoring and maintaining oral health, most people do not follow a recommended checkup schedule [20]. The problem is intensified even beyond irregular checkups when people are instructed to practice social distancing and avoid all unnecessary contact, such as during the current COVID-19 pandemic.