Tooth loss is one of the most common disabilities affecting mankind, and is a consequence of dental diseases, tumors of the orofacial region and trauma. Missing teeth may also occur because of agenesis. The extent of missing teeth can vary, from single teeth in the anterior region of the jaws, single teeth in posterior segments, extended edentulous spaces and edentulism. This may result in functional, esthetic and phonetic impairment for the individual concerned. Missing teeth have an enormous impact on the psychosocial well-being of these individuals, and as such, dentists have an important role in assisting these individuals to improve the quality of their lives.

After completing this ITI Academy Module, you should be able to list the indications for replacing missing teeth, identify specific indications where implants can be used to replace missing teeth, and recognize the concept of prosthodontically-driven indications for the use of dental implants.

The following section discusses the general indications for replacing missing teeth. The first indication is to restore function. Missing teeth need to be replaced if the individual concerned cannot masticate efficiently or comfortably. Lack of masticatory function can impact negatively on diet and nutritional intake. This can have significant health implications for the individual concerned. Loss of masticatory function may also have social implications, with patients avoiding social events because of their inability to chew properly. This clinical example is of a 40-year-old male patient who is missing most of his teeth in the upper-right quadrant. He has a significant dental disability and cannot masticate properly without a dental prosthesis.

The next indication is to replace teeth so that the individual can speak properly. Dental prostheses, whether fixed or removable, are effective in replacing the dental structures that are necessary for proper speech and articulation. The clinical example shows a patient who received a removable dental prosthesis or RDP to replace his four maxillary front teeth to restore the phonetic function. The vibration of the larynx creates sound waves that are then modified in the vocal tract by the position of the jaws, tongue, lips, teeth and soft palate. The upper anterior teeth play a very important role in phonetics. The interaction of the tongue and lips with the anterior palate and maxillary incisor teeth allows the creation of sounds that are characteristic of human speech. Loss of the upper anterior teeth, and to a lesser extent the lower anterior teeth, will create a significant speech impediment if these teeth are not replaced by a dental prosthesis. Therefore, patients receive a significant benefit when dentists are able to provide stable and comfortable dental prostheses to replace missing anterior teeth.

Teeth also play an integral part in dentofacial esthetics. Missing teeth in any region of the oral cavity that are visible during speaking and smiling detract from the symmetry of the dentition and are unattractive. In this young patient with developmentally missing upper lateral incisors, their replacement with implant-supported restorations has successfully restored dental and facial esthetics.

Loss of multiple maxillary anterior teeth also results in resorption of the alveolar ridge, which in turn diminishes the support of the upper lip. This can have a detrimental effect on the way a person looks. The image shows the profile of a patient who is edentate in the upper and lower jaws. Loss of the teeth and resorption of the maxillary anterior ridge has resulted in loss of support to the upper lip. The psychological impact of missing upper anterior teeth is significant and should not be underestimated. People with missing upper anterior teeth have less positive feelings about themselves, and their dental disability strongly affects their daily life and social interactions. Therefore, provision of a dental prosthesis to replace missing teeth in the visible areas of the dentition is an invaluable health benefit that dentists can provide.

Not all teeth, however, need to be replaced. There are many individuals with missing teeth who continue to function quite well provided they have a sufficient number of occluding tooth units. The concept of a shortened dental arch has been recognized for many years now. There is evidence to suggest that nine to ten pairs of occluding teeth, including the anterior teeth, are sufficient to provide satisfactory masticatory function. In this diagram, the molar teeth in both arches are missing. Ten functioning occluding pairs of teeth are present.

In this example, the lower second premolar and first molar have been missing for over 20 years. The lower left second molar has drifted mesially, but there has been minimal tipping of the tooth. Although the opposing first premolar has over-erupted slightly, the overall occlusal situation has remained stable for many years and the patient does not report any problems with chewing. In this particular case, there are no compelling reasons to replace the missing teeth. On the other hand, this example of a missing lower first molar illustrates significant mesial tipping of the adjacent second molar, and over-eruption of the opposing first molar. Replacement of the first molar with a dental prosthesis could have prevented these occlusal changes from occurring.

The decision to recommend replacement of missing teeth is therefore based on the functional, esthetic and phonetic needs of the patient. The clinical assessment must evaluate these different factors, but must also take into consideration the wishes and subjective needs of the patient. What we, as dental professionals, perceive as a strong indication for tooth replacement may not necessarily be a high priority for the individual concerned. This is particularly so if the consequences of tooth loss are not immediately apparent to the patient. In this clinical example, a lower molar tooth has recently been extracted. The patient observes that the ability to masticate is not too badly affected. The patient, however, needs to be advised of the potential consequences of tooth loss, which could include tipping of the adjacent molar, over-eruption of unopposed teeth, and changes to the functional occlusion.

In contrast, this patient, who has been missing many posterior teeth for most of his life, can function well and has no esthetic or other dental concerns. His dental needs are satisfied even with the reduced number of teeth in his dentition. Although we may feel otherwise, his subjective perception is that he does not require any dental prostheses. However, this needs to be balanced against the possibility of tooth movement following extraction. Most undesirable tooth movement takes place within the first six months of tooth extraction. It is not always an easy task to predict whether teeth opposing or adjacent to a missing-tooth gap will migrate or over- erupt. A careful analysis of the occlusion, often with the aid of study casts, may indicate if there are stable occlusal contacts which may prevent migration of the teeth.

General indications, Key Learning Points: Missing teeth need to be replaced for functional, esthetic and phonetic reasons. However, not all missing teeth need to be replaced. In addition to a thorough clinical evaluation, the patient's wishes and perceived needs should be taken into consideration when recommending treatment.

In the next section, we focus on specific indications for implant-supported reconstructions. There are many treatment options for the replacement of missing teeth. There are specific indications for which dental implants - with their unique property of being anchored in the jawbone - may be suitable to support or retain fixed and removable dental prostheses. For fixed dental prostheses or FDPs, implants may be considered the treatment option of choice when teeth adjacent to a gap in the dental arch are unrestored. Conventional tooth-supported fixed dental prostheses require preparation of the adjacent teeth to serve as abutments. This results in considerable loss of tooth structure, and irreversible alteration to the natural teeth. A dental implant serves as a freestanding prosthesis, independent of the adjacent natural teeth. Dental implants therefore represent a conservative treatment option by avoiding preparation of the neighboring teeth.

Implants may also be considered in situations where the adjacent teeth are already restored, and when there are clinical indications and the patient desires that these teeth are left uninvolved. In this example of a missing upper left lateral incisor, the existing crowns are functional, esthetic and in good condition. A dental implant has been placed to support a single-unit, fixed dental prosthesis which has avoided the need to include the adjacent teeth. A dental implant would also satisfy the patient's desire to have free-standing dental units for ease of maintenance and homecare.

A different clinical scenario arises when the teeth adjacent to a missing-tooth gap are considered to be compromised. In this case, an implant has been used to replace the upper right central incisor. The upper left central incisor has previously been restored and a three-unit, tooth-supported, fixed dental prosthesis could have been made. However, the upper left central incisor was assessed as being structurally too weak to be used as an abutment for a tooth-supported, fixed dental prosthesis. The implant option therefore avoids involving compromised teeth which may still be functional and is satisfactory in terms of replacing the missing tooth.

In extended edentulous spaces, tooth-supported, fixed dental prostheses may have long pontic spans. Biomechanically, this increases the stresses on the abutment teeth. Long pontic spans are also at risk of flexion, which can cause cementation failure of the retainers and the risk of recurrent caries. In this diagram, three missing lower teeth are replaced with a five-unit tooth-supported FDP. This results in a long pontic span and the potential for unfavorable forces on the abutment teeth. With an implant solution, two implants can be used to support a three-unit FDP. The length of the pontic has been reduced and the natural teeth are uninvolved.

Implants can also be used in extended edentulous gaps to replace individual teeth. There is often a strong desire on the part of patients to have teeth that function as independent units, provide ease of maintenance and access for oral hygiene procedures. This example illustrates the use of two implants to replace two premolar teeth. This option can only be considered if there is sufficient mesiodistal space to allow multiple adjacent implants to be placed with sufficient distance between the implants, and between the implants and the adjacent teeth.

Dental implants may also be indicated if the teeth adjacent to the gap are unsuitable as abutments for a fixed dental prosthesis. The adjacent teeth may be endodontically, periodontally or restoratively compromised. Or they may be unfavorably positioned as in this example, where the alignment of the lateral incisors makes them unsuitable for use as abutments for a four-unit FDP.

In free-end edentulous areas of Kennedy Class I and II, dental implants can provide fixed tooth replacements and avoid the need for removable partial dental prostheses. In this example, two implants have been used to replace the lower right first and second molars. The implants have been restored as individual units.

Implants can not only provide fixed replacement of missing teeth, they can also be used in conjunction with removable dental prostheses or RDPs, where they can provide both retention and support of full overdentures. In these examples from different patients, four implants have been used in the maxilla and two implants in the anterior mandible to retain full removable dental prostheses. The attachments provide retention for the prostheses, with support provided by the oral mucosa of the alveolar ridges and hard palate.

These are examples of implants used in conjunction with removable partial dentures. In the image on the left, bars have been attached to two implants on each side of the upper jaw. The bars provide both retention and support to the removable dental prosthesis. The image on the right shows a retentive abutment attached to an implant in the first premolar region. The abutment provides an additional point of retention for the removable dental prosthesis.

In partially edentulous jaws, implants can be used to provide a combination of fixed and removable prostheses. This patient has a three-unit, tooth-supported FDP supported by the upper left central and canine. In the first quadrant, a three unit implant-supported FDP is supported by implants in the upper right central and canine sites. Attachments incorporated into each of the prostheses provide retention for a removable dental prosthesis.

Specific Indications, Key Learning Points: Implants can be used to support fixed dental prostheses when adjacent teeth are unrestored, or if the adjacent teeth are restored but there are specific clinical reasons not to involve the teeth in the prosthesis, when adjacent teeth are compromised or unsuitable as abutment teeth, in long edentulous gaps, in free-end edentulous areas, to provide individual freestanding replacements, and to support full fixed dental prostheses.

With removable dental prostheses, implants can be used to retain upper and lower full overdentures, to retain and/or support removable partial dentures, and in fixed and removable dental prostheses combinations.

The final part of this module introduces the concept of "prosthodontically-driven" implant placement. To be able to provide our patients with implant-based prostheses that are functional, comfortable and esthetic, implants need to be placed in the optimal position for the chosen prosthesis design. Therefore, the final restoration and tooth position should be used to plan the implant position. For fixed dental prostheses, the implant must be placed in the correct mesiodistal and vertical position. The implant must also be placed in the correct orofacial position. The green area in this diagram, also referred to as the safety zone, is the optimal region to position the facial part of the shoulder of the implant. There are no indications for implant placement that are not based on this prosthodontically-driven concept. In other words, all implant indications must be based on a prosthodontic plan, whether for a fixed or removable dental prosthesis. It is not acceptable for an implant to be placed into the available bone if this results in a malpositioned implant. If the position of the bone does not allow an implant to be correctly placed, then the bone needs to be augmented or other corrections made before the implant is placed.

When using fixed implant reconstructions, the implants should be located in the position of the missing teeth. In this clinical example, the implant has been tipped too far towards the facial aspect. This has resulted in recession of the peri-implant mucosa. The cone-beam CT scan shows that the implant has been placed into the available bone. However, this did not result in correct orofacial implant positioning. This case should have been treated with a bone graft to increase the horizontal dimension of the ridge, followed by implant placement into the correct orofacial axis.

This case illustrates a poor treatment outcome with FDPs through lack of a prosthodontic treatment plan. Three adjacent implants have been placed, none of which has been placed in the correct three-dimensional position. The proximity of the implants has resulted in loss of bone and soft tissue between the implants. A prosthodontically driven treatment plan would have dictated the use of a diagnostic set-up and a surgical template to position the implants correctly. Adjacent implants in the anterior maxilla should also have been avoided.

In this case, an implant has been placed into the available bone in a lower second premolar site without a prosthodontic treatment plan. The malposition of the implant has rendered it unusable as support for a fixed dental prosthesis. The implant had to be removed.

The same care in treatment planning needs to be undertaken with removable dental prostheses. The final prosthesis design should determine the position of the implants. In this case, an implant-supported and retained removable dental prosthesis was planned. The bars and attachments on the implants have a certain height which needs to be factored into the surgical plan. Based on a diagnostic set-up, the implants have been placed where the prosthesis is thickest in order to accommodate the bars and attachments, the denture base and the prosthetic teeth. The two anterior implants have been placed far enough apart to allow for a bar of adequate length to provide support to the prosthesis in the anterior region.

Prosthodontically Driven Implant Indications, Key Learning Points: All implant indications must be based on a prosthodontic plan, whether for a fixed or removable reconstruction. For fixed reconstructions, the final tooth position and restoration determine where the implants should be placed. For removable reconstructions, the prosthesis design determines where the implant should be placed. If the position and volume of the available bone do not allow implants to be placed in the correct prosthodontic position, then the site should first be grafted to optimize the bone conditions prior to implant placement.

Module "Role of Implants in Dentistry", module summary: To summarize this module, the general indications for replacing missing teeth are based on the functional, esthetic and phonetic needs of the patient. Not all missing teeth need to be replaced. The wishes and perceived needs of the patient should be considered as part of the clinical examination.

There are specific indications where implants can be used to replace missing teeth. Implants can be used to support fixed dental prostheses in a wide variety of clinical indications, as outlined here: When adjacent teeth are unrestored, or if the adjacent teeth are restored but there are specific clinical reasons not to involve the tooth in the prosthesis, when adjacent teeth are compromised or unsuitable as abutment teeth, in long edentulous gaps, in free-end edentulous areas, to provide individual freestanding replacements, and to support full fixed dental prostheses.

In addition, implants can also be used in conjunction with removable reconstructions in fully edentulous and partially edentulous clinical situations, to retain upper and lower full overdentures, to retain and/or support removable partial dentures, and in fixed and removable dental prostheses combinations.

Implants should only be placed according to a prosthodontic treatment plan. For fixed implant reconstructions, the final restoration determines the optimum position of the implant. For removable dental prostheses, implants should be placed in accordance with the type and design of the prosthesis. If implants cannot be placed in the correct prosthodontic position, the ridge should first be augmented to optimize the bone conditions.