Tooth loss can occur for a variety of reasons - congenital absence, trauma, dental disease (e.g. caries or periodontal disease), as well as mechanical failure.
Congenital absence : It is not uncommon for a tooth or teeth to be congenitally absent. Most commonly, the primary (baby) tooth is present but there is no successor (permanent tooth) to replace it. Frequently, this will be apparent when the baby tooth exfoliates, or falls out (usually during adolescence). Often however, the baby tooth will remain in place and will function until it fails due to the loss of root support or other dental disease. At this point, it will need to be removed.
Before placing an implant in the site of a congenitally absent tooth, it is important that your doctor verify that there is not a tooth bud (a cyst-like structure) in the jawbone in that area. The most commonly missing teeth are maxillary (upper jaw) lateral incisors and premolars.
Trauma can cause loss of teeth in a variety of ways. Teeth can be "knocked out" from trauma, such as a child falling off her bicycle and onto her face. Frequently however, trauma can affect the teeth in ways that do not manifest until months or years later. Root fracture may not be apparent until some time later when infection develops. Sometimes, after teeth have been traumatized, they can be treated and appear to be doing well until many years later when root resorption becomes apparent. This occurs when the body turns against itself and causes cells to eat away at the root surface, often allowing bone to grow into the defect that has been created.
Trauma of a more pernicious order can also affect the dentition. Significant defects of the jaw bone, in addition to teeth, can occur as a result of trauma. This may be following surgery to remove a tumor from the mouth and/or jaws, or secondary to external trauma such as an automobile accident, other forms of blunt trauma or ballistic wounds. These types of trauma can often be compounded by significant loss of jaw bone volume, or even continuity, and could require other forms of surgery to reconstruct the jaw anatomy as well as provide for prosthetic tooth replacement.
The most common reason for tooth loss is gum (periodontal) disease. This is essentially a localized infection in the gums and supporting structures of the teeth leading to loss of bone. This can progress to the point that teeth fall out on their own or are deemed beyond repair or are too compromised to be useful and must be extracted. Dental caries (decay) can also progress to the extent that teeth are beyond the ability to be restored in function. Decay can also lead to significant infection in the bone around the ends of the root(s) leading to necessary tooth extraction to prevent further infectious complications.
Lastly, teeth may crack or fracture in such a way that they cannot be maintained and must be removed. This can happen as the result of clenching and grinding habits (bruxing), or for mechanical reasons related to the lack of sufficient support from other teeth which causes extreme stress to the teeth that remain in function.