13.1 High-Performance Polymer: PEEK

The high-performance polymer, polyether etherketone (PEEK) , was first introduced as a load-bearing biomaterial for spinal fusion surgery over 15 years ago (Invibio Biomaterials Solutions). Since then, medical grade PEEK polymers have matured into an established biomaterial. Until recently its use in dentistry was limited to healing caps and temporary abutments. Perhaps the most interesting property of PEEK for use as a framework material is its Young’s elastic modulus (4 GPa), which allows the PEEK substructure to more closely match the biomechanical characteristics of the jaw’s natural bone (2–12 GPa). PEEK is quite favourable in that it is strong and resistant to repetitive cyclical loading cycles, yet is slightly elastic, lightweight and able to dissipate stress forces placed on it. It is these ‘bone-like’ properties that make PEEK a more biomechanically engineered substructure material.

The Young’s elastic modulus of PEEK (4 GPa) is similar to the acrylics (2 GPa) and is a lot lower than titanium (100 GPa), but still retains sufficient stiffness for rigidity of structure. However, unlike acrylic, PEEK also has sufficient strength (120 MPa flexural strength versus 40 MPa for acrylic) and excellent flexural fatigue resistance to cyclical loads that make it fit for the purpose of long-term restorations. PEEK’s resistance to failure from flexural fatigue may be of interest to potentially address some of the technical complications associated with cantilever design and lab-based tests have been published that show resistance to failure with distal cantilevers of up to 19 mm length [1].

Shock absorption tests have demonstrated a strong correlation between the damping behaviour of implant-supported crowns and their material composition [2]. Studies have shown that the shock-absorbing capacity of polymer materials was higher than that of ceramics and metals. In recent studies that compared titanium, cobalt chrome, zirconia, lithium disilicate, PMMA (acrylic) and PEEK, it was the latter material that demonstrated the biggest differential between forces applied at an occlusal crown surface and forces transferred through to the implant fixture side [3]. This property of a framework material may be of interest for aspects of patient comfort or for particular cases with parafunctional considerations. The weight of a PEEK framework (e.g. 5 g) is also significantly less than one replicated in titanium (17 g).

PEEK as a permanent framework material has become an option in recent times through the availability of CAD/CAM forms. However, clinical data is typically limited at present to case studies [4–6]. Prospective studies are becoming available but there is currently no long-term follow-up [7].

13.2 Laboratory Fabrication Process

PEEK frameworks can be designed according to the following guidelines for full-arch implant-supported prosthetics [8]. In general, when designing any structural polymer part, there should be the avoidance of any notches or sharp angles into the framework. These notches, scoops and grooves can create potential areas of weakness for polymers and propagate cracks. If unavoidable then the framework thickness behind the area should be at least 2 mm and the angle must be greater than 45°.

Anatomical implant substructures—If the prosthetic is screw retained then a bonded metal connector can be used or in instances where the PEEK material is direct to implant then considerations should be made for the interface of PEEK with the particular implant system and screw design. For example, screw head designs that are conical and of smaller diameter will more readily cut into the polymer if in direct contact, while wider flat-headed screws are more compatible. Abutment wall minimum thicknesses should be 1 mm. Current guidance for minimum widths of the framework anterior and posterior base is 8 and 9 mm, respectively. Minimum framework height should be 5 mm and at sites of the implants the material should be at least 1.5 mm on the buccal side and 2 mm on the lingual side.

Implant bars—The bar height should have a minimum of 4 mm. Posterior wall thickness should be a minimum of 6 mm and anterior wall thickness a minimum of 5 mm. Abutment wall thickness minimum should be 1 mm.

A variety of traditional approaches can be used to create the aesthetics on a PEEK framework, for example, veneering with composite materials, use of injection-moulded acrylic and acrylic teeth to create a hybrid wrap-around, or hybrid designs with the use of individual crowns (ceramic, zirconia or lithium disilicate).

Whatever the preference for finishing the prosthetic framework, it is important to follow the supplier’s guidance as although techniques are straightforward there are specific nuances (Figs. 13.9, 13.10, 13.11, 13.12, 13.13, 13.14, 13.15, 13.16, 13.17 and 13.18).

13.2.1 Acrylic Resin Titanium Hybrid

The original frameworks made by the Branemark group had been fabricated from type III gold. Because of the high cost of this material, alloys of silver palladium and gold palladium had been used in recent years. Passivity in these types of frameworks had been notoriously difficult to achieve. Inaccuracies were the result of multiple variables, which included machining tolerances of components, distortion in the impression material, setting expansion of the die stone, expansion and contraction of alloy and wax and distortion of the framework during heat treatment [12, 13].

Common solutions to provide a passive framework had been sectioning and soldering or fabricating the framework in multiple pieces which were then soldered on a master cast. Another approach to achieve a passive screw-retained framework had been use of the adhesive-corrected implant frameworks where individual cylinders were cemented within the framework after it had been cast (KAL technique). This approach had merit in that it eliminated many of the current prosthetic and laboratory inaccuracies associated with traditional techniques [14–17].

With CAD/CAM technology a lot of the variables have been eliminated and frameworks can be produced with high precision providing that the operator has taken care in producing an accurate impression and the laboratory technician has exercised care in pouring it.

Prior to the CAD/CAM process, accurate impressions, jaw relation records and a wax trial set are completed. The framework is designed with the final tooth position in mind (Fig. 13.19).

Since the transition from gold to titanium frameworks the designs have changed quite considerably (Fig. 13.20). Different manufacturers have different designs available but none rival the designs that were possible with traditional casting techniques. This may be due to limitations in scanning and milling but considerable progress is being made in this area.

There are essentially two types of titanium framework that exist.

Wrap-around—This is a minimalist framework where acrylic resin is wrapped around the titanium framework. The tissue surface is acrylic resin and can be readily relined. Little is known of its biomechanical longevity. In the authors’ experience common problems include fracture of the bar. Whether this is due to inadequate dimensions or excessive cantilever is unknown (Fig. 13.20).

I- or L-shaped design—This design maximises the rigidity, has adequate space and retention for acrylic and has adequate dimensions in the cantilever area (Fig. 13.21).

Length of the cantilever is the most common cause of restoration failure. If looking at deflection of the restoration it is proportional to the length of the cantilever and thickness of the cross section. Small increases in length have a dramatic effect of deflection. Most articles recommend a cantilever length which is two times the anteroposterior spread. It is the authors’ opinion that the cantilever should be minimised as much as possible and should only be extended as minimally as possible [19].

With CAD/CAM technology the problem of wear can also be addressed by milling metal occlusal surfaces to maintain the patient’s existing vertical dimension of occlusion (Fig. 13.22).

13.3 Laboratory Fabrication Process

Attention to detail starts with pouring the impression using a low-expansion die stone. Appropriate powder liquid ratios and vacuum mixing for the time indicated by the manufacturer must be adhered to. A verification index must be provided to the clinician to verify accuracy of the master cast. This index is made from impression plaster. A brittle material that is not forgiving is used. The verification index is tried in with one screw. If the plaster index breaks the master cast is deemed inaccurate and a new impression must be made. The use of an acrylic resin material for verification of the master cast may give the clinician a false-positive result and is not recommended. Jaw relation records utilising a two-part rigid occlusal rim must be accomplished after which a wax trial set-up must be evaluated intra-orally.

The wax trial prosthesis must be verified for correct aesthetics, phonetics, occlusal plane, vertical dimension of occlusion and centric relation.