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Abstract

Patients with a severely resorbed edentulous mandible often suffer from problems with the lower denture. These problems include: insufficient retention of the lower denture, intolerance to loading by the mucosa, pain, difficulties with eating and speech, loss of soft-tissue support, and altered facial appearance. These problems are a challenge for the prosthodontist and surgeon. Dental implants have been shown to provide a reliable basis for fixed and removable prostheses. This has resulted in a drastic change in the treatment concepts for management of the severely resorbed edentulous mandible. Reconstructive, pre-prosthetic surgery has changed from surgery aimed to provide a sufficient osseous and mucosal support for a conventional denture into surgery aimed to provide a sufficient bone volume enabling implants to be placed at the most optimal positions from a prosthetic point of view. The aim of this paper is to review critically the literature on procedures related to the severely resorbed edentulous mandible and dental implant treatment. The study includes the transmandibular implant, (short) endosseous implants, and reconstructive procedures such as distraction osteogenesis, augmentation of the mandibular ridge with autogenous bone, and bone substitutes followed by the placement of implants. The number of patients participating in a study, the follow-up period, the design of the study, the degree of mandibular resorption, and the survival rate of the dental implants all are considered evaluation parameters. Although numerous studies have described the outcome results of dental implants in the edentulous mandible, there have been few prospective studies designed as randomized clinical trials that compare different treatment modalities to restore the severely resorbed mandible. Therefore, it is not yet possible to select an evidence-based treatment modality. Future research has to be focused on long-term, detailed follow-up clinical trials before scientificaly based decisions in treating these patients can be made. This will contribute to a higher level of care in this field.

Introduction

Severe atrophy of the inferior alveolar process and underlying basal bone often results in problems with a lower denture. These problems include insufficient retention of the lower denture, intolerance to loading by the mucosa, pain, difficulties with eating and speech, loss of soft-tissue support, and altered facial appearance. These problems are a challenge for the prosthodontist and surgeon.

In the decades preceding the broad clinical use of dental implants, numerous surgical techniques were developed to improve the starting point for successful prosthetic rehabilitation of the edentulous patient (Jennings, 1989). The main techniques were sulcoplasties (Hillerup, 1979, 1994; Davis and Davis, 1995) and grafting procedures (Härle, 1975; Curtis and Ware, 1977; De Koomen et al., 1979; Lekkas and Wes, 1981; Peterson, 1983; Kent and Jarcho, 1995). Although these techniques provided an enlarged denture-bearing area, thereby contributing to improvement of the retention and stability of the lower denture, most of these techniques only temporarily improved the retention and stability of the lower denture. In addition, a considerable rate of morbidity had to be dealt with (Stoelinga et al., 1986).

Since dental implants have been shown to provide a reliable basis for fixed and removable prostheses, reconstructive pre-prosthetic surgery has changed from surgery aimed to provide a sufficient osseous and mucosal support for a conventional denture into surgery aimed to provide a sufficient bone volume to enable implants to be placed at the most optimal positions from a prosthetic point of view. This treatment is generally accepted for the moderate to severely resorbed edentulous mandible. However, the use of implants in the extremely resorbed mandible, and the selection of a reconstructive surgical procedure to facilitate reliable placement of implants in such a resorbed mandible are still subjects of discussion in the literature. Therefore, the aim of this paper was to review critically the literature on procedures related to implant treatment of the edentulous mandible, with special emphasis on the extremely resorbed edentulous mandible. In the present discussion, an extremely resorbed edentulous mandible is defined as a mandibular height in the symphyseal area of 12 mm or less as measured on a standardized lateral cephalogram.

Definition of Dental Implants

Dental implants, as discussed in this review, are prosthetic devices of alloplastic material implanted into the oral tissues beneath the mucosal and/or periosteal layer, and on/or within the bone, to provide retention and support for a fixed or a removable prosthesis. Although dental implants may be classified by their silhouette or geometric form (i.e., fin, screw, cylinder, basket, root-form), in this review dental implants will be discussed according to their anchorage component (the dental implant body) as this relates to the bone that provides support and stability (Van Blarcom, 1999). The three basic types that will be discussed in this review are eposteal dental implants, transosteal dental implants, and endosteal dental implants.

Search of the Literature

This paper provides a comprehensive review of human studies published in international peer-reviewed literature up to May, 2003, regarding procedures related to implant treatment of the severely resorbed edentulous mandible. A MEDLINE search was completed along with a manual search to locate relevant literature. Publications presented in abstract form were ignored. The following MESH terms for the search in MEDLINE were used: dental implants, edentulous mandible, augmentation, distraction, atrophied, and transmandibular. The number of patients participating in a study, follow-up period, the design of the study, degree of mandibular resorption, and the survival rate of the implants were all considered evaluation parameters.

Success vs. Survival

In evaluation studies, the mere presence of dental implants in the oral cavity can be defined as ‘survival’ of the implant. When certain other criteria—such as radiographic aspects, clinical mobility, and functional aspects—are taken into consideration, the extent of ‘success’ can be defined. These ‘success’ criteria are not uniform, and several criteria have been proposed (Albrektsson et al., 1986; van Steenberghe, 1997). Because the majority of evaluation studies concerning the use of dental implants in the severely resorbed edentulous mandible do not use the same ‘success’ criteria, and other studies merely report ‘survival’ percentages, comparison of studies is hindered. For that reason, study ‘survival’ percentages will be given in the present discussion, unless stated otherwise.

Eposteal Dental Implants

Eposteal implants are dental implants that receive their primary bone support by leaning on the residual bone of the mandible. The subperiosteal dental implant, also known as the subperiosteal frame, is the eposteal implant system most used in this category (Van Blarcom, 1999). Other, rarely used, systems are the intramucosal inserts and ramus frames (Kerley et al., 1981; Worthington and Rubenstein, 1998). These earlier forms of implant rehabilitation could be successful, but long-term studies are lacking. For that reason it can be assumed that this therapy is either not successful in the long-term, is used by few clinicians, or is surpassed by new materials and new techniques.

The subperiosteal frame was introduced by Dahl in 1943. The technique was refined by Goldberg (Goldberg and Gershkoff, 1949), and later on by Linkow (Linkow et al., 1998). Patients had to undergo two surgical interventions. During the first operation, the surgeon uncovered the bony edentulous alveolar process and the surrounding basal mandibular bone by raising a mucoperiosteal flap. Subsequently, an impression was made of the denture-bearing area. A custom-made frame, made of a cobalt-chromium alloy, was placed subperiosteally during the second operation. Fixed or removable prostheses could be connected to several transmucosal posts.

The concept of a subperiosteal frame was innovative. Non-biological materials were inserted into human tissues with open communication with the oral environment, thereby creating transmucosal posts. Fixed or removable prostheses could be anchored to these posts. Although retrospective studies reported ten-year survival rates of between 60% and 75% (Young et al., 1983), various structural problems were clinically experienced, including epithelial ingrowth, dehiscence of the implant, infection, and paresthesia of the mental nerve (Garefis, 1978; Bodine et al., 1996). These major drawbacks resulted in removal of the implant in more than 60% of the patients examined during a 20-year follow-up study (Yanase et al., 1994), although cases have been reported with long-term (over 25 years) success (Kurtzman and Schwartz, 1995; Morrow et al., 2000).

Specific information about the clinical performance of this system in relation to the level of resorption of the inferior alveolar process or comparison with other implant systems is, to the best of our knowledge, not available in the literature. Today, the technique of applying a subperiosteal frame has been practically abandoned, because successful rehabilitation in a high proportion of the cases over a long-term period is apparently inferior to that achieved with other systems, such as transosseous and endosseous systems (Adell et al., 1981). Moreover, the morbidity of the transosseous and endosseous systems is less than that of the subperiosteal frame.

Transosteal Dental Implants

Transosteal or transosseous dental implants are implants composed of a metal plate and transosteal pins or posts. The metal plate is held with retentive pins or screws fixed to the inferior border of the mandible. This metal plate supports the transosteal pins/posts that penetrate the full thickness of the mandible and project into the mouth in the inter-foraminal area (Van Blarcom, 1999). The transosseous dental implants used in humans are the ‘staple bone implant’ system and the ‘transmandibular implant’ system (TMI).

Staple bone implant system

The staple bone implant system was developed as an alternative to subperiosteal frames, because of the major complications that were encountered in the clinical application of subperiosteal frames (Small, 1975, 1980). The main objectives in designing the staple bone implant system were to reduce forces on the implant and to make thin transmucosal perforations. To prevent overloading of this implant system, a tissue-borne overdenture has to be made with stress-breaking attachments to stabilize the denture. The staple bone implant consists of a baseplate with two or four (parallel) transosseous pins and from two to five retentive pins (or screws) to stabilize the baseplate to the inferior border. The implant is made of a titanium alloy to allow for osseointegration (Small et al., 1995).

The mandibular staple bone implant has been evaluated in several retrospective studies that have reported survival rates of between 86% and 100% (Small and Misiek, 1986; Small, 1993; Meijer et al., 1998). The most common complications are gingival hyperplasia, crestal bone loss, and infections around the transmucosal part of the transosseous pins. Serious, but rarely observed, complications are fracture or mobility of a transosseous pin, and fracture of the mandible. The transosseous pins, rigidly connected to the baseplate, are always parallel to each other. This makes prosthetics in general easier, but limits this procedure’s application in compromised situations, because individual angulation of the pins is not possible. In contrast, individual placement of transosteal posts is possible in the TMI system, making placement of the TMI in compromised situations possible. Another disadvantage of the staple bone implant system is that it is difficult to remove, because of the bony integration of the mushroom-shaped retentive pins. Although no specific study has been performed concerning the survival of the mandibular staple bone implant in the extremely resorbed mandible, Meijer et al.(1998) reported a tendency of higher failure rates in patients with a mandibular height less than 12 mm. A similar tendency to increased failure rate in a severely resorbed mandible has been reported for the transmandibular implant system (Versteegh et al., 1995).

Transmandibular implant system

The transmandibular implant system (TMI) was especially developed for the extremely atrophied mandible (Bosker, 1986). Although in both the original and subsequent reports on TMI research the term ‘extremely atrophied mandible’ was never defined, the majority of the patients included in these studies had an anterior mandibular bone height of less than 12 mm (Bosker and Van Dijk, 1989; Maxson et al., 1989; Bosker et al., 1991b). The TMI consists of a baseplate, five cortical screws, and four transosseous posts. In contrast to most other implant systems made of a titanium alloy, all TMI components are made of Implator^® (Cendres et Métaux, Bien-Brille, Switzerland), a gold alloy containing 70% gold, 5% platinum, 12.8% silver, and 12.2% copper. It is claimed that this bioinert material osseointegrates into human bone, although histologic studies are, to our knowledge, limited to animal studies (Arvier et al., 1989).

Like the staple bone implant system, the TMI is inserted by an extra-oral approach while the patient is under general anesthesia. The baseplate is fixed to the inferior border of the mandible with the cortical screws. The transosseous posts, connected to the baseplate, perforate the mandible and the oral mucosa and are post-operatively connected to each other with a bar equipped with two distal cantilevers (Powers et al., 1994). Three months after placement, an implant-supported overdenture is usually constructed.

The TMI has been frequently evaluated (Bosker and Van Dijk, 1989; Maxson et al., 1989; Bosker et al., 1991b; Versteegh et al., 1995; Kwakman et al., 1996; Meijer et al., 2001; Verhoeven et al., 2001; Paton et al., 2002). High survival rates (95%-100%) have been reported (Bosker and Van Dijk, 1989; Maxson et al., 1989; Powers et al., 1989; Bosker et al., 1991b), but other studies report lower (56%-75%) survival rates (Versteegh et al., 1995; Kwakman et al., 1996; Meijer et al., 2001; Verhoeven et al., 2001; Paton et al., 2002). These differences can be ascribed to the different definitions used for complications and failures in the various studiesand to different protocols for placement of the implant and construction of the overdenture, and could be related to the experiences of the surgical and prosthetic teams (Van Pelt, 1997; Powers, 2001; Paton et al., 2002). An overview of studies that have evaluated the transmandibular implant system is given in Table 1. Complications include infections, loss of osseointegration of the transosseous posts, fracture of posts, hyperplasia of the oral mucosa, peri-implant bone loss, disturbances of the mental nerve, and prosthetic complications such as bar and clip corrections.

Several studies have concluded that the TMI is especially suitable for the extremely resorbed mandible, although these studies do not provide information regarding mandibular height (Bosker and Van Dijk, 1989; Betts et al., 1995). In a comparative retrospective study, however, an opposite conclusion was reached, namely, that the failure rate was higher in extremely resorbed mandibles (bone height less than 12 mm) (Versteegh et al., 1995). A similar tendency was observed for the staple bone implant (Meijer et al., 1998).

To date, two prospective studies have compared the TMI with endosseous implants (Geertman et al., 1996; Stellingsma et al., 2004a,b). With regard to the severely resorbed mandible, the one-year results showed no significant differences between the two systems (Geertman et al., 1996), but thereafter, significantly more complications were reported with the TMI system. These complications included loss of osseointegration, infection, and non-fitting superstructures. After six years, a survival rate of 97% was reported for the endosseous implants vs. a survival rate of 72% for the TMI group (Meijer et al., 2001). Also, with regard to the extremely resorbed mandible, short endosseous implants perform significantly better than the transmandibular implant (Stellingsma et al., 2004a,b).

When the prescribed prosthodontic protocol was strictly adhered to, controlled bone growth in the mandible has been reported to occur (Powers et al., 1994; Bosker and Powers, 1995). The explanation given for this phenomenon is that the bending forces in the area distal to the lateral implants that develop during functional loading of the implant might serve as a mechanical stimulus for bone (re)modeling processes. There are reports of bone growth in the areas adjacent to the implant of up to 9 mm (Bosker et al., 1991a). This so-called ‘rejuvenation’ has been claimed to occur in large groups of patients (Bosker et al., 1991a; Betts et al., 1993; Powers et al., 1994). However, the evaluation instrument that was used, i.e., panoramic radiographs, is questionable; standardization of the recording technique was not done, and this makes comparison of subsequent radiographs hazardous due to distortion and magnification errors (Batenburg et al., 1997). Moreover, statistical analysis of the data is lacking, and consequently the interpretation of the results is subject to considerable bias. Bone growth has also been reported in other studies, although not to as great an extent as in earlier studies (Kwakman et al., 1997; Verhoeven et al., 2001). Again, this differing result may be due to the different protocols used (Powers, 2001).

In addition to providing retention and stability to the lower denture and therefore rehabilitating masticatory function, it is possible (Bosker and Wardle, 1999) to reconstruct the function and appearance of the lower face following the insertion of the transmandibular implant (Powers and Bosker, 1996). By using an extra-oral approach, the surgeon relocated the position of several facial muscles and, additionally, removed redundant skin and fat. The results were evaluated in 146 patients: The reported satisfaction was claimed to be high, although this subjective increase in satisfaction was not objectively assessed by means of, e.g., a validated questionnaire, and no pre-treatment data were recorded. Thus, there is a high risk of an interpretation bias.

Endosteal Dental Implants

Prior to the evolution of transosseous implant systems, which were exclusively used in the edentulous mandible, there was the development of endosteal implant systems capable of replacing one or more teeth in the partial or complete edentulous mandible or maxilla. An endosteal or endosseous dental implant is a dental implant placed into the alveolar and/or basal bone of the mandible or maxilla and transecting only one cortical plate. With regard to the edentulous mandible, the apical part of the endosseous implant occasionally extends into the caudal cortical plate. An implant placed this way is still considered ‘endosseous’. In contrast to transosseous systems, where the retentive components form a unit, these endosseous implants can be regarded as solitary components, so that adjustments or replacement of individual implants is possible. This is a major advantage compared with the transosseous systems where, in cases of complications, a complete revision of the entire system is needed.

The endosseous dental implant is composed of an anchorage component, termed the ‘endosseous dental implant body’, which ideally is within the bone, and a retentive component, termed the ‘endosseous dental implant abutment’. Descriptions of the dental implant body that use silhouette or geometric forms—such as cylinder, conical, screw, or blade—may be used as adjectives to enhance the understanding of the geometry of endosseous dental implants (Van Blarcom, 1999).

The Swedish research group led by Brånemark and the Swiss research group led by Schroeder were the first to study the direct contact between bone and titanium endosseous implants (Brånemark et al., 1969; Schroeder et al., 1976). This direct bone-to-implant contact, a phenomenon called ‘osseointegration’ (Brånemark et al., 1977), led to the development of various endosseous implant systems that could be used in a clinical setting. It became clear that both the geometric design and the surface conditions of the implant, and a meticulous surgical technique combined with an optimal condition of the implant site were prerequisites for successful osseointegration. Geometric designs were, among other reasons, developed to make possible selection with respect to location and/or application (Buser et al., 1994; Mericske-Stern et al., 2000). This way, one can choose an implant design to reach the optimal conditions for both functional and esthetic rehabilitation of the patient. From studies that have focused on surface properties of endosseous implants, it can be concluded that these properties not only play an important role in qualitative and quantitative aspects of the bone-implant interface, but are also decisive in the time needed to reach a certain level of osseointegration (Buser et al., 1998; Cochran et al., 2002).

The clinical use for endosseous implants in prosthetic dentistry is obvious. The first clinical results published by Swedish research groups (Brånemark et al., 1977) showed favorable survival rates. After confirmation of these results by Zarb and his co-workers, the use of titanium endosseous implants was widely accepted (Zarb and Symington, 1983). Today, endosseous titanium implants are utilized in both partially and completely edentulous patients.

Application of endosseous implants in the edentulous mandible has changed the treatment concepts enormously; with the use of these implants, it is possible to provide retention for fixed and removable prostheses. This kind of treatment improves oral function and has considerable patient satisfaction (Boerrigter et al., 1995; Bakke et al., 2002).

Fixed bridges and removable overdentures on endosseous implants

The concept of installing five or six endosseous implants in the interforaminal region, followed by the construction of a fixed bridge, was developed by the Brånemark group and has been evaluated in several studies (Adell et al., 1981; Albrektsson et al., 1986; Naert et al., 1992; Quirynen et al., 1992; Lindquist et al., 1996). Provided that a strict protocol is followed, it is a reliable treatment option, and the survival rates of the endosseous implants are high (between 90 and 98%).

It is not always possible or advisable to install five or six endosseous implants in the edentulous mandible. Therefore, the treatment concept of a removable overdenture anchored to two to four endosseous implants was introduced. The superstructure connecting the implants with the overdenture can be divided into ball attachments, clip-bar attachments, magnet attachments, and a milled bar with precision attachments (Davis and Davis, 1995). Although the differences in functional aspects are minimal, patients prefer implant-supported prostheses (Tang et al., 1997; Van Kampen et al., 2002).

The treatment concept of the mandibular overdenture retained by endosseous implants has been evaluated in several studies (Wismeijer et al., 1995; Batenburg et al., 1998a,b; Sadowsky, 2001). Studies that have focused on clinical behavior and radiological aspects confirm that this kind of treatment is very predictable in showing high survival rates (> 90%) of the implants, along with healthy peri-implant tissues, on condition that a high level of oral hygiene is maintained (Geertman, 1995; Boerrigter et al., 1997; Roynesdal et al., 1998). With respect to patient satisfaction and psychosocial functioning, it is clear that patients regard implant-supported mandibular dentures as very beneficial (Kent, 1992; Bouma et al., 1997; Locker, 1998; Raghoebar et al., 2000b; Stellingsma et al., 2003). These results are comparable with those for implant-supported bridges (Adell et al., 1990; Johns et al., 1992; Hemmings et al., 1994). The choice between a fixed bridge and a removable overdenture in the edentulous mandible is dependent on several factors. Not only are anatomic factors such as the inter-foraminal space and inter-maxillary relations important, but oral hygiene and speech-related factors play a role as well. Finally, patient-related factors such as costs and the preference for a fixed or removable prosthesis must also be considered.

Short endosseous implants

The placement of short endosseous implants is another option to treat the extremely resorbed mandible. In the case of severe ridge atrophy and short implants (< 12 mm), the ratio between implant length and the distance to the occlusal plane is compromised, resulting in unfavorable biomechanics. Since the latter could jeopardize long-term osseointegration, this mode of treatment is not widely used (Brånemark et al., 1985; Worthington, 1992). There are some reports, however, concerning the use of short endosseous implants in the extremely resorbed mandible. The scale of these studies is limited, and comparison with other modalities is limited to the studies by Geertman et al.(1996) and Stellingsma et al. (2003a,b). Nevertheless, it is an attractive treatment option because of the relatively simple surgical procedure and limited morbidity (Triplett et al., 1991; Keller, 1995; Geertman et al., 1996; Bruggenkate et al., 1998; Friberg et al., 2000; Stellingsma et al., 2000, 2004a, b; Deporter et al., 2002). Survival rates vary from 88 to 100%. Recently, Deporter et al.(2002) reported excellent ten-year outcomes from the use of short endosseous implants to support mandibular overdentures: a ten-year implant survival of 92.7% and an average annual bone loss limited to 0.03 mm since the first year of implant placement. Although more randomized clinical trials are needed, survival rates thus far are comparable with those of implants in less severely resorbed edentulous mandibles (Batenburg et al., 1998b). An overview of the literature concerning the use of short endosseous implants in the edentulous mandible is given in Table 2. In a randomized clinical trial that compared three treatment modalities (transmandibular implant, augmentation of the mandible with an autologous bone graft followed by placement of four endosseous implants, and the placement of four short endosseous implants) for the extremely resorbed edentulous mandible, it was concluded that treatment with short endosseous implants is the treatment of choice due to the minimal complications, the high survival rate, the stable bone-implant interface, and the fact that patients can be treated in an outpatient clinic setting (Stellingsma et al, 2004a,b). The major complication of treatment with short endosseous implants is the risk of a (partial) fracture of the mandible (Mason et al., 1990; Triplett et al., 1991). Even though the incidence of this complication was shown to be rare (Raghoebar et al., 2000c), treatment is often difficult if a fracture does occur (Tolman and Keller, 1991).

Grafting Procedures

In the case of severe atrophy of the edentulous mandible, it is possible to augment the mandible prior to the placement of endosseous implants. Various techniques and materials have been developed to increase mandibular height. Onlay techniques as well as interposition of the graft in the inter-foraminal area are used. Autogenous materials, such as bone and cartilage, and allogenic materials, such as hydroxyapatite or bone substitutes, as well as combinations of these materials, are used for ridge augmentation (Stoelinga et al., 1986; Vanassche et al., 1988; Tolman, 1995). Depending on the clinical conditions, endosseous implants can be inserted at the same treatment session or after the graft has been incorporated for 3-4 months.

The advantage of a one-stage procedure is that the graft and the implant can be placed at the same time, thereby eliminating a second operation. An important disadvantage is that the positioning and angulation of the implants are more complicated, thereby making this one-stage procedure undesirable from a prosthetic point of view (Bell et al., 2002).

Another drawback of the one-step reconstruction with onlay bone grafts and endosseous implants (Keller and Tolman, 1992; Vermeeren et al., 1996; Verhoeven et al., 1997) is the unpredictable resorption of the grafted bone around the implants (Vermeeren et al., 1996). Resorption of the graft is less extensive than in the onlay technique, when one interposes a bone graft in the inter-foraminal area, in combination with the placement of endosseous implants in a one-stage (Lew et al., 1991; Keller and Tolman, 1992) or a two-stage procedure (Satow et al., 1997; Stellingsma et al., 1998; Bell et al., 2002). In particular, when the two-stage procedure is used, the bone-implant interface can, for the most part, be preserved (Gratz et al., 1994; Stellingsma et al., 1998). Although most studies report the use of an intra-oral grafting approach for the edentulous mandible, a submental extra-oral approach to prevent oral contamination of the graft has also been used (Lew et al., 1991; Bell et al., 2002).

With both onlay or interposed bone grafts, hydroxyapatite can be used as a filler, alongside autologous bone, to achieve the desired volume and contour for the augmented mandible (Haers et al., 1991; McGrath et al., 1996). Complications that are experienced with the use of hydroxyapatite are migration, displacement, and dehiscence of hydroxyapatite particles that can cause mucosal erosions (Kent et al., 1986). Stabilization of endosseous implants with hydroxyapatite as a primary retentive material does not seem adequate; it is just not a substitute for viable bone (Kent and Jarcho, 1995). However, to restore the extremely resorbed mandible, hydroxyapatite can be used posteriorly in the lateral parts, with dental implants placed in the inter-foraminal area (Lew et al., 1991).

The most significant complications that occur following grafting procedures in the mandible are sensory disturbances of the mental nerve, wound dehiscence, infections of the grafted area, and, with autogenous bone grafts, donor area morbidity (Tolman, 1995). An overview of the literature concerning augmentation of the edentulous mandible in combination with endosseous implants and an implant-retained mandibular overdenture is given in Table 3. As discussed previously, the placement of short endosseous implants gives results at least as predictable as those achieved with the insertion of longer endosseous implants after a grafting procedure or the use of a transmandibular implant system (Stellingsma et al., 2004a,b). Future randomized clinical trials are needed to prove the hypothesis that short endosseous implants are the treatment of choice to rehabilitate patients with an extremely resorbed edentulous mandible.

Distraction Osteogenesis

Besides grafting techniques, distraction osteogenesis can be performed to improve the starting point for the placement of implants in the inter-foraminal area of the severely resorbed edentulous mandible (Chin and Toth, 1996; Hidding et al., 1999). Distraction osteogenesis is a technique of gradual bone-lengthening, allowing natural healing mechanisms to generate new bone. When applied to the reconstruction of a severely resorbed edentulous mandible, an osteotomy in the inter-foraminal area of the mandible is made, after which the distraction device is placed. Five to seven days after surgery, active distraction is started at a rate of 0.5 to 1 mm per day. Between four and eight weeks after the last day of active distraction, mineralization of the newly formed bone matrix in the distraction area has progressed sufficiently to allow for the placement of endosseous implants with sufficient primary stability. During the next three months, the implants are left unloaded to allow for further mineralization and remodeling of the distracted area (Raghoebar et al., 2000a, 2002). In comparison with grafting procedures, the advantages of distraction osteogenesis are the absence of donor site morbidity, the presence of vital bone in the distraction area, and the gain of soft tissues. Possible complications of the distraction technique for the edentulous (severely resorbed) mandible are fracture of the mandible, infection, and necrosis of the superior fragment, but such complications are rarely reported in the literature.

The results of distraction osteogenesis have been evaluated in several studies (Urbani et al., 1999; McAllister, 2001; Raghoebar et al., 2002). The short-term clinical, radiographic, and histomorphologic results are very promising. There is some evidence for the assumption that, in the near future, distraction osteogenesis can develop into a reliable tool for augmentation of the anterior segment of a severely resorbed edentulous mandible. However, because long-term results of this technique are still unavailable, some caution still needs to be exercised in the recommendation of this mode of treatment in general practice. Comparison of the distraction method with other techniques—like the placement of a transmandibular implant, augmentation of the edentulous mandible in combination with implants, or the placement of short endosseous implants—will facilitate an assessment of the efficacy of distraction osteogenesis, in combination with endosseous implants in the treatment of the severely resorbed mandible. Such studies have not yet been published.

Conclusions

Dental implantology has evolved from an experimental to a mature evidence-based discipline. It is currently a valuable treatment modality in the prosthetic treatment of edentulous patients. Numerous techniques have been developed for the use of dental implants for retention and stabilization of fixed or removable mandibular dentures. Today, the options for the restoration of the extremely resorbed mandible with implants can be categorized as follows:

the use of (short) endosseous implants in combination with either a fixed or removable prosthesis;

augmentation of the mandible by means of distraction techniques or grafting procedures, followed by the placement of endosseous implants in combination with either a fixed or removable prosthesis; and

the installation of a transosseous implant system in combination with a removable prosthesis.

Although numerous studies have been published about the outcomes of dental implants in the edentulous mandible, there are still certain questions that have to be answered. For example, comparative studies, designed as prospective clinical trials to evaluate various treatment modalities for the extremely resorbed mandible with dental implants, are still scarce. In fact, they are limited to the studies by Geertman et al.(1996) and Stellingsma et al. (2004a,b). Future research concerning implant treatment of the extremely resorbed edentulous mandible should be focused not only on long-term, detailed follow-up clinical trials in which clinical and radiographic aspects are analyzed, but also on the evaluation of the restoration of function and other patient-based parameters. Only by taking all of the factors into account can one arrive at evidence-based decisions in treating these patients, thereby contributing to a higher level of care in this field.

TABLE 1
Overview of the Literature Concerning the Evaluation of the Transmandibular Implant System

Author # of Patients Follow-up Period (yrs) Retro/Prospective Study Mandibular Height (mm) Type of (evaluative) Radiograph Survival Rate (%)

^a Not specifically stated.

^b In these studies, replacement of an individual implant post due to, e.g., fracture or loss of osseointegration is considered to be a reversible complication that does not reduce the survival rate, and was therefore not scored as a failure. In the other studies, all reporting lower survival rates, such a complication was scored as a failure.

Bosker and van Dijk, 1989 368 0.5-12 retrospective 4-20 panoramic 97^b

Maxson et al., 1989 190 0.25-5 retrospective 4-18 panoramic 95^b

Powers et al., 1989 13 2 prospective —^a panoramic 100^b

Bosker et al., 1991b 1356 0.5-13 retrospective 4-24 panoramic 96^b

Versteegh et al., 1995 37 2.3-6.5 retrospective 7.5-16.0 panoramic 74.8

Meijer et al., 2001 30 6 prospective 8-15 panoramic 72.0

Verhoeven et al., 2001 70 3-13 retrospective ^< 14 panoramic 84.8-100^b

Paton et al., 2002 58 5-15 retrospective 5-16 panoramic 56

Stellingsma et al., 2003b 20 2 prospective 6-12 oblique lateral 93.8

TABLE 2
Overview of the Literature Concerning the Use of Short Endosseous Implants in the Edentulous Mandible

Author # of Patients/# of Implants Follow-up Period (yrs) Retro/Prospective Study Mandibular Height (mm) Type of (evaluative) Radiograph Survival Rate (%) Type of Prosthesis

^a Not specifically stated.

Triplett et al., 1991 28/130 1-5 retrospective < 10 —^a 94 fixed, removable

Keller, 1995 52/260 1-10 prospective < 10 panoramic, lateral 93 fixed, removable

Kwakman et al., 1996 29/ 58 5 prospective 8-15 panoramic 100 removable

Bruggenkate et al., 1998 126/253 1-7 retrospective —^a panoramic, periapical 94 fixed, removable

Stellingsma et al., 2000 17/ 68 5 retrospective < 12 oblique lateral 88 removable

Friberg et al., 2000 49/260 1-14 prospective —^a panoramic, Scanora^® 92.3 fixed/removable

Stellingsma et al., 2003b 20/ 80 2 prospective 6-12 oblique lateral 100 removable

TABLE 3
Overview of the Literature Concerning Augmentation of the Edentulous Mandible in Combination with Endosseous Implants and an Implant-retained Mandibular Overdenture

Author # of Patients/# of Implants Follow-up Period (yrs) Retro/Prospective Study Mandibular Height (mm) Immediate/Delayed Implant Placement Type of (evaluative) Radiograph Survival Rate (%) Type of Graft

^a Onlay graft via extra-oral submental approach.

^b HA = Hydroxyapatite.

Lew et al., 1991 10/43 1-3 prospective 4-8 immediate panoramic 93 onlay^a, bone/HA^b

Keller and Tolman, 1992 7/32 1-4 retrospective < 7 immediate panoramic 94 onlay, bone

Gratz et al., 1994 23/78 < 5 prospective 15-23 immediate lateral 98 interposition, bone

McGrath et al., 1996 18/36 1-3 retrospective 6-12 immediate panoramic 92 onlay, bone/HA

Vermeeren et al., 1996 31/78 5 prospective < 8 immediate panoramic 90 onlay, bone/HA

Verhoeven et al., 1997 13/26 2-4 prospective 7-12 immediate oblique lateral 100 onlay, bone

Satow et al., 1997 32/73 1-7 prospective < 12 delayed lateral, panoramic 95 interposition, bone/HA

Stellingsma et al., 1998 10/40 2-5 retrospective 9-14 delayed panoramic 100 interposition, bone

Bell et al., 2002 14/60 2 retrospective 6-10 delayed panoramic 100 onlay^a, bone

Stellingsma et al., 2003b 20/80 2 prospective 6-12 delayed oblique lateral 88 interposition, bone

Author	# of Patients	Follow-up Period (yrs)	Retro/Prospective Study	Mandibular Height (mm)	Type of (evaluative) Radiograph	Survival Rate (%)
^a Not specifically stated.
^b In these studies, replacement of an individual implant post due to, e.g., fracture or loss of osseointegration is considered to be a reversible complication that does not reduce the survival rate, and was therefore not scored as a failure. In the other studies, all reporting lower survival rates, such a complication was scored as a failure.
Bosker and van Dijk, 1989	368	0.5-12	retrospective	4-20	panoramic	97^b
Maxson et al., 1989	190	0.25-5	retrospective	4-18	panoramic	95^b
Powers et al., 1989	13	2	prospective	—^a	panoramic	100^b
Bosker et al., 1991b	1356	0.5-13	retrospective	4-24	panoramic	96^b
Versteegh et al., 1995	37	2.3-6.5	retrospective	7.5-16.0	panoramic	74.8
Meijer et al., 2001	30	6	prospective	8-15	panoramic	72.0
Verhoeven et al., 2001	70	3-13	retrospective	^< 14	panoramic	84.8-100^b
Paton et al., 2002	58	5-15	retrospective	5-16	panoramic	56
Stellingsma et al., 2003b	20	2	prospective	6-12	oblique lateral	93.8

Author	# of Patients/# of Implants	Follow-up Period (yrs)	Retro/Prospective Study	Mandibular Height (mm)	Type of (evaluative) Radiograph	Survival Rate (%)	Type of Prosthesis
^a Not specifically stated.
Triplett et al., 1991	28/130	1-5	retrospective	< 10	—^a	94	fixed, removable
Keller, 1995	52/260	1-10	prospective	< 10	panoramic, lateral	93	fixed, removable
Kwakman et al., 1996	29/ 58	5	prospective	8-15	panoramic	100	removable
Bruggenkate et al., 1998	126/253	1-7	retrospective	—^a	panoramic, periapical	94	fixed, removable
Stellingsma et al., 2000	17/ 68	5	retrospective	< 12	oblique lateral	88	removable
Friberg et al., 2000	49/260	1-14	prospective	—^a	panoramic, Scanora^®	92.3	fixed/removable
Stellingsma et al., 2003b	20/ 80	2	prospective	6-12	oblique lateral	100	removable

Author	# of Patients/# of Implants	Follow-up Period (yrs)	Retro/Prospective Study	Mandibular Height (mm)	Immediate/Delayed Implant Placement	Type of (evaluative) Radiograph	Survival Rate (%)	Type of Graft
^a Onlay graft via extra-oral submental approach.
^b HA = Hydroxyapatite.
Lew et al., 1991	10/43	1-3	prospective	4-8	immediate	panoramic	93	onlay^a, bone/HA^b
Keller and Tolman, 1992	7/32	1-4	retrospective	< 7	immediate	panoramic	94	onlay, bone
Gratz et al., 1994	23/78	< 5	prospective	15-23	immediate	lateral	98	interposition, bone
McGrath et al., 1996	18/36	1-3	retrospective	6-12	immediate	panoramic	92	onlay, bone/HA
Vermeeren et al., 1996	31/78	5	prospective	< 8	immediate	panoramic	90	onlay, bone/HA
Verhoeven et al., 1997	13/26	2-4	prospective	7-12	immediate	oblique lateral	100	onlay, bone
Satow et al., 1997	32/73	1-7	prospective	< 12	delayed	lateral, panoramic	95	interposition, bone/HA
Stellingsma et al., 1998	10/40	2-5	retrospective	9-14	delayed	panoramic	100	interposition, bone
Bell et al., 2002	14/60	2	retrospective	6-10	delayed	panoramic	100	onlay^a, bone
Stellingsma et al., 2003b	20/80	2	prospective	6-12	delayed	oblique lateral	88	interposition, bone

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