Edentulism
Edentulism is the condition of being toothless to at least some degree; it is the result of tooth loss.  Loss of some teeth results in partial edentulism, while loss of all teeth results in complete edentulism.
Even organisms that never possessed teeth can be referred to as being edentulous, such as members of the former zoological classification order of Edentata, which included anteaters, sloths and armadillos, all of which possess no anterior teeth and either no or poorly-developed posterior teeth.

# Importance of teeth and consequences of edentulism
For people, the relevance and functionality of teeth can be easily taken for granted, but a closer examination of their considerable significance will demonstrate how they are actually very important.  Among other things, teeth serve to:
- support the lips and cheeks, providing for a fuller, more aesthetically pleasing appearance
- maintain an individual's vertical dimension of occlusion
- along with the tongue and lips, allow for the proper pronunciation of various sounds
- preserve and maintain the height of the alveolar ridge
- cut, grind, and otherwise chew food

## Facial support and aesthetics
When an individual's mouth is at rest, the teeth in the opposing jaws are nearly touching; there is what is referred to as a freeway space of roughly 2-3 mm.  However, this distance is partially maintained as a result of the teeth limiting any further closure past the point of maximum intercuspation.  When there are no teeth present in the mouth, the natural vertical dimension of occlusion is lost and the mouth as a tendency to overclose.  This causes the cheeks to exhibit a "sunken-in" appearance and wrinkle lines to form at the commisures.  Additionally, the anterior teeth, when present, serve to properly support the lips and provide for certain aesthetic features, such as an acute nasiolabial angle.
Loss of muscle tone and skin elasticity due to old age, when most individuals begin to experience edentulism, tend to further exacerbate this condition.
The tongue, which consists of a very dynamic group of muscles, tends to fill the space it is allowed, and in the absence of teeth, will broaden out.  This makes it initially difficult to fabricate both complete dentures and removable partial dentures for patients exhibiting complete and partial edentulism, respectively; however, once the space is "taken back" by the prosthetic teeth, the tongue will return to a narrower body.

## Vertical dimension of occlusion
As stated, the position of maximal closure in the presence of teeth is referred to as maximum intercuspation, and the vertical jaw relationship in this position is referred to as the vertical dimension of occlusion.  With the loss of teeth, there is a decrease in this vertical dimension, as the mouth is allowed to overclose when there are no teeth present to block further upward movement of the mandible towards the maxilla.  This may contribute, as explained above, to a sunken-in appearance of the cheeks, because there is now "too much" cheek than is needed to extend from the maxilla to the mandible when in an overclosed position.  If this situation is left untreated for a many years, the muscles and tendons of the mandible and the TMJ may manifest with alterred tone and elasticity.

## Pronunciation
The teeth play a major role in speech.  Some letter sounds require the lips and/or tongue to make contact with teeth for proper pronunciation of the sound, and lack of teeth will obviously affect the way in which an edentulous individual can pronounce these sounds.
For example, the fricative consonant sounds of the English language s, z, x, d, n, l, j, t, th, ch and sh are achieved with tongue-to-tooth contact, and the fricative f and v are achieved through lip-to-tooth contact.  These sounds are very difficult to properly enunciate for the edentulous individual.

## Preservation of alveolar ridge height
The alveolar ridges are columns of bone that surround and anchor the teeth and run the entire length, mesiodistally, of both the maxillary and mandibular dental arches.  The alveolar bone is unique in that it exists for the sake of the teeth that it retains; when the teeth are absent, the bone slowly resorbs.  The maxilla resorbs in a superioposterior direction, and the mandible resorbs in an inferioanterior direction, thus eventually converting an individual's occlusal scheme from a Class I to a Class III.
In addition to this resorption of bone in the vertical and anterioposterior dimensions, the alveolus also resorbs faciolingually, thus diminishing the width of the ridge.  What initially began as a sort of tall, broad, bell curve-shaped ridge (in the faciolingual dimension) eventually becomes a short, narrow, stumpy sort of what doesn't even appear to be a ridge.  Resorption is exacerbated by pressure on the bone; thus, long-term complete denture wearers will experience more drastic ridges to their ridges that non-denture wearers.  Those individuals who do wear dentures can decrease the amount of bone loss by retaining some tooth roots in the form of overdenture abutments or have implants placed.  Note that the depiction above shows a very excessive change and that this many take many years of denture wear to achieve.
Ridge resorption may also alter the form of the ridges to less predictable shapes, such as bulbous ridges with undercuts or even sharp, thin, knife-edged ridges, depending of which of many possible factors influenced the resorption.

## Masticatory efficiency
Physiologically, teeth provide for greater chewing ability.  They allow us to masticate food thoroughly, increasing the surface area necessary to allow for the enzymes present in the saliva, as well as in the stomach and intestines, to digest our food.  Chewing also allows food to be prepared into small boli that are more readily swallowed than haphazard chunks of considerable size.  For those who are even partially endentulous, it may become extremely difficult to chew food efficiently enough to swallow comfortably, although this is entirely dependent upon which teeth are lost.  When an individual loses enough posterior teeth to make it difficult to chew, he or she may need to cut their food into very small pieces and learn how to make use of their anterior teeth to chew.  If enough posterior teeth are missing, this will not only affect their chewing abilities, but also their occlusion; posterior teeth, in a mutually protected occlusion, help to protect the anterior teeth and the vertical dimension of occlusion and, when missing, the anterior teeth begin to bear a greater amount of force for which they are structurally prepared.  Thus, loss of posterior teeth will cause the anterior teeth to splay.  This can be prevented by obtaining dental prostheses, such as removable partial dentures, bridges or implant-supported crowns.  In addition to reestablishing a protected occlusion, these prostheses can greatly improve one's chewing abilities.
As a consequence of a lack of certain nutrition due to altered eating habits, various health problems can occur, from the mild to the extreme.  Lack of certain vitamins (A, E and C) and low levels of riboflavin and thyamin can produce a variety of conditions, ranging from constipation, weight loss, arthritis and rheumatism. There are more serious conditions such as heart disease and Parkinson's disease and even to the extreme, certain types of Cancer.
Numerous studies linking edentulism with instances of disease and medical conditions have been reported. In a cross-sectional study, Hamasha and others found significant differences between edentulous and dentate individuals with respect to rates of atherosclerotic vascular disease, heart failure, ischemic heart disease and joint disease.

# Etiology
The etiology, or cause of edentulism, can be multifaceted.  While the extraction of non-restorable or non-strategic teeth by a dentist does contribute to edentulism, the predominant cause of tooth loss in developed countries is periodontal disease.  While the teeth may remain completely decay-free, the bone surrounding and providing support to the teeth may resorb and disappear, giving rise to tooth mobility and eventual tooth loss.  In the photo at right, tooth #21 (the lower left first premolar, to the right of #22, the lower left canine) exhibits 50% bone loss, presenting with a distal horizontal defect and a mesial vertical defect.  Tooth #22 exhibits roughly 30% bone loss.