Long‐term Evaluation

For all these attachment systems, adequate outcomes are documented with regard to function, esthetics, and “oral health‐related quality of life” (OHRQoL) [7, 8]. They have been shown to offer high rates of five‐year overall survival ranging from 95 to 100% [5, 9]. While a complete assessment of long‐term prognoses will also, of course, have to consider the risk of complications, a uniform approach to classifying complications of implant–supported removable prostheses does not currently exist. Back in 2001, Payne et al. [10] proposed that a distinction should be made between routine maintenance performed during periodic recall visits on the one hand, and repair or corrective maintenance requirements on the other. Reports in the literature actually do tend to differentiate between technical complications (often used interchangeably with mechanical complications) and routine maintenance requirement.

Complications and Maintenance

Technical complications in the sense that patrix or matrix components need to be activated, replaced, or repositioned and are the most common type of intervention with implant–supported removable prostheses [11–14]. While ball attachments seem particularly susceptible to requiring such interventions notably within the first year of delivery, these inventions do not translate into a significant difference in overall comparisons between freestanding ball or Locator attachment systems [14, 15]. Within splinted systems, a difference has been reported in favor of rigid (milled) bars, where significantly fewer interventions were required for the retentive part [14].

Inconclusive rates are documented for screw (including abutment screw) loosening or fracture. On one hand, all attachment systems appear to require interventions for these events similarly often [13]. On the other, the incidence of this complication with maxillary prostheses has been reported to be especially high for bar attachments [12].

The risk of prosthetic fractures – including failure of a resin tooth or the denture framework – is influenced by how the restoration has been fabricated in the dental laboratory. Current evidence in the literature indicates a higher rate of technical complications associated with the denture itself for maxillary restorations that do not feature a metal reinforcement or coverage of the palate [12, 16]. Another modifier of intervention rates is for how long a denture has been worn. Systematic reviews have unanimously disclosed that complications increase after a five‐year period [12, 13].

Decision Level 1: People‐related Factors

At this level, the clinician and the patient are the main two factors contributing to the decision. Other modifiers include the clinical and technical expertise of the entire team, the patient’s desires and expectations, and how much he or she can afford to pay for the treatment. Selectable attachment systems may also be limited by health‐related issues, most notably regarding the patient’s motor abilities.

Decision Level 2: Tissue‐related Factors

This level, too, combines various aspects to be considered in the decision‐making. Most importantly, these include the existing prosthetic space, number of implants, inter‐implant distance, implant angulation, and implant position. Of particular note in this context, removable prostheses are generally considered to involve greater horizontal and vertical space requirements than fixed prostheses. It is mandatory to make sure in the diagnostic phase that adequate space will be present for both the attachment and its housing, as well as for the denture itself. In the literature, vertical dimension requirements of 7 or 11 mm have been reported for freestanding or splinted (i.e. bar) attachments, respectively [17]. These values should be regarded as the minimum height capable of avoiding undersized components (Figure 19.2).

Decision Level 3: Simulation‐related Factors

Factors related to the quality of simulation need to enter the decision process as well. They are not only concerned about the question if palatal coverage is required, but also about the dimensions of the future prosthesis. Whenever coverage of the palate is not tolerated by a patient, inserting an adequate number of implants will eliminate this need. Four or more implants are required to this end [18]. The use of double crowns for attachments is considered beneficial by allowing for substantially reduced dimensions of the prosthesis. Experience has shown that implant–supported removable prostheses of this type, similar in design to fixed bridgework, meet with high patient acceptance (Figure 19.3).

This author believes that an ideal attachment system, which will reliably meet a given patient’s individual requirements, can only be selected by taking a synoptic view (Figure 19.4).

History of Telescopic Crowns

Telescopic crowns were invented for fixed dental prostheses. Their use was first described by a Philadelphian dentist named R. W. Starr in 1886 [19]. Initially used for bridgework segmentation, they were later harnessed for removable prostheses.

The conventional use for double‐crown telescopes is to retain dentures on residual teeth. A telescopic double crown is made up of a primary (internal) crown fixed to a natural tooth or implant abutment, and a secondary (external) crown, which, being a rigid component of the denture framework, forms an integral part of the prosthesis.

Friction‐type (Cylindrical Crown) Telescopes

These telescopes are based on a primary crown milled to 0−2° of parallelism, which slides into the secondary crown much like a plunger into a cylindrical receptacle, having contact with its wall circumferentially. Retention is thus achieved by friction, i.e. by the frictional resistance developing between the two contacting surfaces. The original design of these friction‐type telescopes, and their further development into parallel attachments and bars, dates back to Karl Häupl und Hermann Böttger [20].

Resilience‐type (Clearance Crown) Telescopes

This non‐friction variant of telescopic double crowns was introduced by Manfred Hofmann in 1966. Here retention is achieved mainly by mucosal support. Given around 0.5 mm of occlusal clearance between the primary and secondary crowns, retention develops not immediately but after mucosal resilience is established [20]. In European countries, these prostheses are known as “cover dentures.” Good results with this system have been obtained mainly in the presence of up to three natural tooth abutments. Resilience‐type telescopes are not used in implant dentistry.

Cone‐type (Conical Crown) Telescopes

In 1968, Karl‐Heinz Körber worked on refining the friction‐type telescopes by developing a primary crown no longer cylindrical in shape, but featuring a taper of 4°–8°, which reduced frictional resistance to telescope closure [20]. Depending on the convergence angle of the primary crown, either a retentive or a supportive function is accomplished. Smaller angles translate into more retention, the strength of which is also influenced by different surface characteristics and materials. A high‐gold alloy has proven to be the material of choice for the components of this system (Figure 19.5).

Gold‐on‐zirconia Double Crowns

This system also uses a conical primary crown with a defined cone angle of 2° but with a high‐strength oxide ceramic as the primary crown material and a secondary crown made of electroformed gold. This principle was developed and introduced by Paul Weigl at the turn of the millennium, has entered the daily clinical practice with the introduction of advanced CAD/CAM technology, and is today widely accepted. This system, also known as the “Frankfurt principle,” does not rely on friction between the patrix and matrix components for retention (which therefore remains unaffected by intraoral service). Rather, the retention is accomplished tribologically, with the surfaces interacting in relative motion via a gap of 5 μm that is created between the primary and secondary crowns. In contact with saliva, this feature serves as a capillary gap allowing cohesive forces to develop that will ensure continuous retention of the prosthesis. The design and material selected for the primary crowns are also beneficial, as zirconia minimizes the “demasking” effect and given a predefined vertical height of at least 7 mm, acts as a rest seat allowing for a delicate removable prosthesis similar to what would normally be the design of a fixed prosthesis. In addition, primary crowns of zirconia offer more stable retention than crowns made of gold alloy (Figure 19.6) [5,21–23].

To summarize, a variety of double‐crown telescopic systems are available, all of them having their own strengths and weaknesses. The characteristics of the most widely used systems are listed in Table 19.1.

Indications and Limitations

A challenge routinely posed by severely reduced dentitions is for the clinical team and the patient to decide whether the proposed rehabilitation should incorporate any compromised residual teeth or should be supported and retained exclusively by implants. While there is never going to be a universally applicable solution to this dilemma, the concept of telescopic retention by double crowns, notably in accordance with the “Frankfurt principle,” does offer several advantages in prostheses combining both existing teeth and strategically inserted implants.

Combinations of tooth and implant abutments are known to offer mutual stabilization. While the tactile sensitivity of natural abutments, given intact periodontal receptors, will counteract any excessive masticatory forces, the much higher stability of the implants will protect the compromised abutment teeth from excessive loading [25]. The goal is to achieve total integration of abutments, i.e. of all residual natural teeth with all implants placed. As an additional consideration, the prosthetic usefulness of each potential abutment tooth should be evaluated in advance based on periodontal, endodontic, and reconstructive criteria [26, 27]. Its suitability for a hybrid prosthesis of this type can be derived from the result of this evaluation outlined in Table 19.2.

Another important advantage of these restorations is that they can potentially survive any loss of one or several tooth abutments. In a scenario like this, the prosthesis will usually continue to serve its purpose. Later, an additional implant can be inserted to re‐establish stability at the site of the lost abutment tooth.

A distinctive feature of the Frankfurt treatment protocol is to fabricate and provide these patients with a “travel denture” (described in detail later in this chapter). As a result, the patient can resort to an acceptable copy of the definitive denture whenever the need arises, so that any maintenance and/or repair jobs can be conducted in the laboratory as required without creating an unpleasant waiting period for the patient.

Compared to fixed restorations, it is easier to maintain high levels of plaque control with a prosthesis that can be removed by the patient at all times. Compared to dentures retained by splinted bars, it is also much easier for the patient to maintain self‐performed hygiene of the freestanding implant–supported crowns serving as primary telescopes. One potential limitation to keep in mind, however, is the long‐term prognosis of the patient’s general health, including his or her motor abilities (Figure 19.7 and 19.8).

Correct implant positioning is essential to the long‐term success of the prosthesis and, therefore, should be driven by the design of the future prosthesis, to avoid any malpositioning that might cause deviations from the uniform orientation that is needed for all abutments to properly insert into the secondary crowns. These requirements may necessitate a more extensive surgical approach, which, in turn, might exceed the patient’s willingness to accept risks or be contraindicated in the presence of health problems. While few compromises are acceptable with regard to the position, length, and angulation of implants for fixed prostheses, there is nothing wrong with selecting between several options of implant positioning for removable prostheses to strike an appropriate balance between the extensiveness, cost, and benefit of treatment. For instance, major surgical procedures like bone augmentation can often be avoided or their invasiveness reduced without repercussions on the function or esthetics attainable by prosthetic treatment. An effort should be made, however, to achieve an abutment distribution as quadrangular as possible for optimal support of the prosthesis. Pertinent follow‐up examinations have demonstrated a statistically significant difference between the success rates offered by quadrangular versus both triangular and linear transverse distributions, in the absence of a difference between these latter two support distributions (Figure 19.9) [5].

Suitability of natural teeth for use as abutments
Treatment goal	Tooth should be preserved	Tooth should be pretreated for a safe prognosis	Tooth should be extracted
PERIODONTAL criteria	Safe	Uncertain	Hopeless
Attachment loss	Probing depth: ≤3 mm	Probing depth: ≥6 mm	Attachment loss: up to apex
Residual pockets after completion of active treatment for periodontitis	Attachment loss: ≤25% Furcation involvement: grade ≤1	Attachment loss: ≈ 50% Bone loss: 50–70% Furcation involvement: grade I, II or III	Probing depth: >9 mm Mobility: grade III Bone loss: ≥70% Furcation involvement: grade III
ENDODONTIC criteria	Safe	Uncertain	Hopeless
No clinical signs of inflammation	Vital, or root canal adequately treated	Nonvital without root canal treatment
No radiolucencies	No clinical signs of inflammation No or diminishing radiolucency	No clinical signs of inflammation Persistent radiolucency Teeth requiring revision Apicoectomy required	Clinical and radiographic problems, treatment no longer useful Longitudinal root fracture Symptomatically obliterated teeth Periodontal‐endodontic lesion Cysts Root resorption
RECONSTRUCTIVE criteria	Safe	Uncertain	Hopeless
Residual hard tissue	Circular ferrule: ≥1.5–2 mm	Reduced retention and resistance form	Residual hard tissue inadequate (less than 1.5 mm of circular ferrule)
Convergence angle	Abutment height (retentive surfaces): ≥3 mm	Abutment height: <3 mm	Crown lengthening and extrusion not possible
Ferrule	Resistance form (abutment height/diameter: ≥0,4)	Convergence angle: >25° in a strategic position
Strategic value	Convergence angle: 15–20° Biologic width: 2–3 mm Adequate crown‐root ratio	Requires post‐and‐core restoration Requires surgical crown lengthening Circular cervical defects

Also, removable restorations supported by implants allow for better flexibility in optimizing the prosthetic design for aesthetics and function. The denture‐mucosa interface can be positioned more freely, and the tooth axes more readily corrected. Occlusal relationships can be transformed or adjusted in the vertical and horizontal planes with modest effort. Limitations of these prostheses, notably when natural tooth abutments are to be incorporated, arise from the total space requirements for a primary crown, secondary crown, tertiary structure, and veneer layer. Also, preparing natural teeth with abrasive instruments for use as abutments is somewhat invasive. Esthetic limitations may arise from a need for over‐contoured prosthetic restorations notably in the anterior segment of the maxilla or mandible. Giving due consideration to all these limitations in the planning stage is absolutely required.

The treatment and fabrication process are more complex for hybrid prostheses than for other restorations. A team approach to planning is required, followed by close coordination of the successive clinical and laboratory steps. Meticulous care should be devoted to “passivation,” which is required to compensate for the physiological mobility of natural tooth abutments as compared to the relative rigidity of osseointegrated implants. The process of laboratory fabrication, too, is technique‐sensitive and should be performed by a considerably experienced and skilled technician.

Clinical and Technical Process

The rest of this chapter will be dedicated to case reports illustrating the chairside and laboratory workflows for prostheses retained by gold‐on‐zirconia double crowns.

Primary and Secondary Crowns

Using CAD/CAM technology, the primary crowns are milled from zirconia, an oxide‐ceramic material known for its high strength. Based on an esthetic try‐in (mock‐up) of a digitized denture setup, a backward‐planned design process is carried out first for ideal abutment shapes and then for the primary crowns. Once milled, these primary crowns are then processed using diamond finishing instruments without polishing.

Workflow: figures 1–6.

A milling device is used to create the contact surfaces of the conical crowns. Care should be taken to ensure smooth surfaces, as any tiny “dent” will be replicated in the process of electroforming the matrix component, which then, in the process of being dislodged from the conical crown, is bound to expand by the amount of this dent, thus compromising the high precision of fit. Any polishing will indent these smooth surfaces in ways not always visible to the naked eye. Therefore, following processing in the milling unit, the conical crowns should never be polished.

Workflow: figures 7 and 8

The secondary crowns, or matrix components, are created directly on the surface of the conical primary crowns using a fully automated process of electroforming by deposition of fine‐gold particles. Thus, rather than duplicating the primary crown for indirect forming of the matrix component, the internal surface of the fine‐gold coping is directly defined by the external surface of the conical crown. To render the surface conductive for electroforming, the conical crown is covered with a very thin layer of silver varnish by air‐spraying, which also creates a separating layer allowing the electroformed coping, once finished, to be dislodged from the conical crown. The layer thickness (and especially evenness) of conductive silver varnish is critical for an accurate fit of the electroformed matrix component. Given a correct procedure, a 5 μm fit is created between the primary and secondary crown.

Workflow: figures 9−12: Fabrication of the secondary crown by deposition of fine‐gold particles.