13

The Posterior Regions: Free-End Situations

In the posterior regions, too, the decision-making process for augmentation procedures follows the treatment-planning concept in chapter 11. In the posterior regions, bone can thus be reliably built up horizontally and vertically. The topic of this chapter is predominantly the differential indication for augmentation alternatives. In the posterior regions, the regenerative approach must be weighed against prosthetic tissue replacement. In the posterior mandible, for example, a vertical deficit can usually be compensated by short implants and long crowns without major esthetic disadvantages. In the posterior maxilla, for example, there are efforts to avoid sinus elevation surgery.

13.1 Clinical Considerations in Free-End Situations

The basic approach of this chapter is to compensate for bone defects in partially edentulous patients with free-end situations in order to create conditions that mimic natural bone for dental implants and dentures. This is also the best prophylaxis for the atrophied adjacent teeth, which are otherwise prone to periodontal overload, gingival recession, and root caries (Fig 13-1). In general, the adequate restoration of a free-end situation is, to a certain extent, a conservative treatment, because the restoration of the occlusal support zones protects the temporomandibular joint and either prevents or is a prerequisite for the functional treatment of temporomandibular joint problems by occlusal splints. The perioprosthetic stabilization of the residual dentition by implant restoration of free-end situations should also be mentioned. If an anterior residual dentition has to bear the same chewing force as the previous dentition with 14 occlusal pairs, periodontal tooth migration and tooth loosening due to overloading are possible. Restoration of the supporting zones by means of bone augmentation and dental implants can prevent this tendency and thus preserve a compromised dentition. Finally, the esthetic and functional (eg, angular cheilitis) effect of an increase in bite height via reconstruction of the supporting zones by means of augmentation and dental implants should be mentioned, because in the case of free-end situations there has usually been a reduction in dimension of occlusion beforehand due to abrasion and tooth migration. In the ideal situation, ie, with bone augmentation and dental implants, the patient is restored to the youthful condition of a full dentition. Such a regenerative approach is, of course, associated with expense and surgical interventions. A large part of this chapter deals with the coordination of the treatment aims and the extent to which compromises can be made to this ideal.

Fig 13-1 Vertical augmentation, to a certain extent as a conservative service. Remodeling adjacent to a vertical defect leads to gingival recession and root caries on the distal tooth. Augmentation removes the stimulus for bone resorption at the distal tooth. This prevents root caries and attachment loss.

Fig 13-2 Illustration of why augmentations in the posterior mandible usually heal well. The scar tissue attached to the ascending mandibular ramus and the periodontium of the most distal tooth tends to migrate cranially during shrinkage and thus cover the augmentation.

13.2 Comparison of Soft Tissue Healing in Free-End Situations in the Maxilla and Mandible

In the posterior mandible, conditions are more favorable for soft tissue healing than in the maxilla. A scar always has a tendency to shrink, and a flap has a tendency to retract. In the maxilla as a whole, the retraction tendency works in the direction of exposure of the augmentations because the soft tissue behind the tuberosity is attached behind the tuberosity relatively far cranially, and the convex arch shape, which protrudes in relation to the narrow apical base, tends to create tension in soft tissue flaps. More favorable conditions exist in the posterior mandible. Here, the soft tissue is attached to the ascending ramus and the cranially attached pterygomandibular raphe. Therefore, together with an elevated anterior residual dentition, scarring pulls the soft tissue over the concave jaw shape of the posterior mandible and tends to cover the augmentation with flap tension (Fig 13-2). The posterior oral vestibule is also not as deep in the mandible as in the maxilla. More favorable conditions for bone graft healing are present, lending credence to the reports of guided bone regeneration (GBR) exceeding the 3 to 4 mm of bone formation that would otherwise occur.¹ In the posterior jaws, flap retraction can be countered by tunneling techniques so that no midcrestal incision is made at all, but instead a subperiosteal posterior preparation is made via a vertical relief incision in the canine region.²

13.3 General Information on Short Versus Regular Implants

The length of natural tooth roots, between 10 and 14 mm, was for a long time the model for the length of dental implants. This overlooked the fact that tooth roots in the periodontal ligament are movably attached in contrast to implants. Lower transmission lengths of the masticatory force to the bone are sufficient for the osseointegrated fixed bond. It looks strange for an expirienced clinician, but according to prospective studies, long single crowns for mandibular molars can be anchored on implants of 4-mm length with only 3-mm osseointegration length.3 Even the maxilla, with its reduced cortical structure compared with the mandible, could be successfully restored with long single crowns on 4-mm implants according to prospective data.⁴ It should be noted that prospective data on ultra-short 4-mm implants have so far covered only short time periods of up to 2 years.⁵ Prospective data on short implants between 6 and 8 mm in length with up to 8 years of follow-up are available.⁶ As a result of the learning process mentioned above, implants from 8 mm in length are now considered normal length, and implants shorter than 8 mm are considered short implants.

Splinting for prosthetic compensation of short endosteal implant lengths in the maxilla⁷ is also a prosthetic concept adopted from experience with periodontally anchored teeth. Whether this principle also applies to dental implants remains to be seen, as the survival rates of nonsplinted single crowns on short implants in the mandible are very good.⁸

Load-bearing capacity and type of abutment connection for short and reduced-diameter implants

If occlusal rehabilitation is considered to be a sustainable overall medical treatment, and if it is also indicated in some studies that the bone level on osseointegrated implants may remain stable for life, then the mechanics of the implant-abutment connection should also be adapted to this potentially lifelong function.

When two alternately loaded parts are joined, micromovements and elastic deformations occur. This also applies to the cone, which is considered to be the best pairing of two parts from a mechanical standpoint. But under the constant alternating load during mastication, the inner cone can slip further into the outer cone of the abutment connection over time with just a small amount of material removal and thus gain a small degree of freedom by rotating around the abutment screw. Elastic deformations of the abutment connection are also known from detailed synchroton radiographic studies of implant connections under high load,⁹ especially in the case of reduced-diameter implants.¹⁰ All of these micromovements can expand into macromovements over the years with hundreds of masticatory contacts per day. Then the failure of the component or the breakage of the abutment screw is not far away. Mechanical failure of the abutment connection results in a defective osseointegrated implant that can only be removed at great expense, also in terms of tissue loss.

This suggests that, to protect the patient, clinicians should prefer to use thicker and more stable implants and perform bone augmentation if necessary to accommodate them, as long as this is feasible to do while staying in compliance with the biologic rules of distance from adjacent teeth.

Short implants often have flat connections with an external hexagon, which have worse mechanical properties than the regular tapered internal connections.¹¹ Implants with internal connections are available from different lengths: Camlog from 9 mm, Straumann BL from 8 mm, Conelog (Camlog) from 7 mm, and Astra EV (Dentsply Sirona) from 6 mm. The vertical space requirement resulting from the screw, anti-rotation device, and cone does not allow for shorter versions. One exception is the ultra-short 4-mm Straumann TL implant, which is largely hollow on the inside, but still maintains the normal coupling of TL implants because of the tulip shaped transgingival portion (Fig 13-3). The other exception is Bicon implants, whose connection works by wedging a very steep taper without screwing and without an antirotation element, and which also use the same internal connection type for all implants down to the shortest implants of 5 mm. The abutment connection compromises may speak for avoiding short implants and instead augmenting the bone vertically and using regular implants.

Peri-implantitis and short implants

In prospective studies, bone loss is always about 1 mm less in short implants than in regular implants in comparable situations.^12,13 But in relation to the total osseointegrated length, for short implants even a small bone loss is higher in percentage than in regular impalnts. Also a small bone loss may therefore become critical for an ultrashort dental implant.

Fig 13-3 The minimum implant length is determined by the space needs for the internal tapered connection, anti-rotation device, and screw threading. Most very short implants can only solve this by stepping back to external hex connections. An exception is the 4-mm Straumann TL implant, in which the internal taper connection is accommodated by its tulip shape and which therefore has the same proven coupling as, eg, 14-mm implants.

If inflammatory bone resorption is then added, a short implant is quickly lost. The usual pocket depth in peri-implant lesions that can lead to the establishment of pathogenic pocket flora, is about 6 to 8 mm. At this depth, a short implant has long since been lost; it cannot withstand any degree of peri-implantitis. A regular implant, however, is still functionally stable, if it retains an apical 4 mm of osseointergated length.

Therefore, a rescue attempt by surgical periimplantitis therapy is also worthwhile for regular implants. The success rate of short-term infection elimination with surgical peri-implantitis therapy is almost 100%, and more than two-thirds of all infected implants can be retained long-term after periimplantitis therapy (see chapter 15). This is an argument for vertical augmentation and use of implants of regular length.

13.4 General Information on Narrow- Versus Regular-Diameter Implants

Implants with a diameter less than 3.5 mm are defined as narrow-diameter implants.¹⁴ This is mainly for loading reasons,¹⁵ because in implants with internal connections, the wall thickness will eventually fall below the critical range as they become narrower. An implant with an internal connection and/or platform switching must be drilled hollow to serially accommodate the elements of the connection, anti-rotation device, and screw threading. This problem is further exacerbated with tapered internal connections, where the taper as a wedge exerts an explosive effect on the implant. Implants with a diameter of less than 3 mm are referred to as mini-implants. For reasons of space, many of these no longer have an abutment connection and are used in one piece. An exception is the Straumann 2.9-mm BL implant, which has a conical internal connection.

The problem of diameter reduction is the risk of implant fracture. This occurred in a large sample of over 10,000 mixed implant types at a rate of 0.44% (about 1 every 200 implants), mostly between 2 and 8 years after placement. One millimeter of diameter reduction compared to a standard diameter brought a 96.7% increase in the risk of fractures. Harder titanium alloys were able to compensate for this effect with a 72.9% risk reduction. There was a twenty-fold increase in risk in the presence of bruxism.¹⁶

When it comes to molar replacement, narrow-diameter implants not only reach the limits of load-bearing capacity; due to the large diameter of molar crowns (about 10 mm), such small implants also create a flat emergence profile with a corresponding impediment to oral hygiene (Fig 13-4). Based on this premise, molars should rather be restored with implants of increased diameter (wide platform), which are around 5 mm in diameter. In addition, tissue-level implants, such as the Straumann TL implant, have the added benefit of a connection that gradually widens toward the platform (tulip shape), resulting in favorable loading values and harmonious emergence profiles with wide-platform implants. A 5-mm implant requires at least 1 mm, preferably 2 mm, of bone vestibularly and orally, meaning that a minimum ridge width of 7 to 9 mm is required for safe placement. If this bone is not present in the site, it can be built up by augmentation.

Fig 13-4 In the case of pronounced crown overhangs with an inconsistent emergence profile, paths for interdental brushes (hygiene access, red) should be planned. Sometimes it is more hygienic to place the implant eccentrically in a very wide gap and create hygiene access with a pontic. The most favorable solution is an implant with a diameter adapted to the crown (crown-down concept, far right). (Modified from Wolfart.¹⁷)

13.5 Pros and Cons of Vertical Augmentation in the Posterior Mandible

Prospective data on short implants in sufficient bone initially show equally good survival as for regular implants, in some cases even better. However, after 5 years, the statistics shifted in favor of regular implants; longer periods have not yet been studied.¹² A recent meta-analysis by the author of randomized prospective studies on short implants versus vertical augmentation followed by regular implants in the posterior mandible¹³

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