This is a radiograph (an Orthopantomograph..OPG) of a 42 year old female patient suffering from Chronic Suppurative Periodontitis.
The disease starts with deposition of plaque on the teeth. The plaque is basically a bacterial film which gets deposited due to improper oral hygiene. With the addition of salivary chemicals, chemicals and salts in the food the plaque starts becoming rigid and forms whitish yellow chalk like deposits on the teeth called calculus. This cannot be removed by brushing and one has to visit the dentist to get rid of this.
Alongside, the gums also become infected with the bacteria in the plaque causing Gingivitis. This may lead to swelling and bleeding of the gums. If uncared for, the bacteria present in the calculus and the ones causing gingivitis entrench themselves deep. Underneath the gums is the alveolar bone which supports the teeth by physically enclosing the roots which is attached to the bone by the specialised periodontal fibres. Once the infection reaches these fibres, the case is termed Periodontitis. If left uncontrolled, the infection spreads to the supporting bone and starts degrading the same. As the bone starts receding, the tooth it supports starts getting mobile and paining and the infection starts emaning pus. It reaches a point where there is hardly any bone support left for the tooth. (See the second and third teeth on the left in the lower jaw. They are devoid of any bone support and are just hanging in there attaced to the gums. It would not take me more than a couple of seconds to pull it out without even using a single instrument!
That is all what I meant by 'Hanging in the air' !
Now for the mysterious 'dog bone' on the x-ray that you all were so curious about. Well, I will not reveal that. Instead would request you to read the first line of this post again and then start making intelligent guesses !!