Immediate Full-Arch Restoration on 4/6 Implants. Part 3

In the previous part, we concluded with the selection of implant placement sites for full-arch rehabilitations supported by 4 or 6 implants. Let’s briefly recap that this design often requires angulated implant placement. In the maxilla, the implants can be tilted anteriorly, as shown in the following illustration.

Series: Immediate Full-Arch Restoration on 4/6 Implants
→ Part 1. Planning, immediate implantation and conditions for long-term success
→ Part 2. Selection and positioning of implants
→ Part 3. Abutment selection: the concept of multi-unit abutments (you are here)

The same applies to the maxillary tuberosities, especially if the patient is prone to bruxism and/or requires a full complement of teeth. The illustration below shows a version with six implants, the two distal ones being anchored in the maxillary tuberosities (tuber maxillae), which allows the maxillary sinuses to be bypassed.

In the mandible, only straight implant placement or anterior tilting is possible.

Abutment selection: the concept of multi-unit abutments

Let us recall the difference between abutment-level and implant-level restorations.

Let’s start with implant-level restorations. Everything is extremely simple here: the abutment is secured to the implant, and the prosthesis is rigidly fixed to the abutment. With this approach, the implant/abutment connection bears the maximum mechanical load (indicated by the red arrows in the illustration below). The illustration shows two implant/abutment connection options: on the left, without platform switching, and on the right, with platform switching. However, both options involve implant-level restorations; the difference in platform switching is important for the formation of the biological width (gingival attachment), not for the mechanical properties of the restoration.

It’s also worth briefly mentioning the crestal and subcrestal placement of implants. There are clear indications for subcrestal placement. If the soft tissue thickness above the alveolar crest is 2 mm or less, subcrestal placement is recommended, meaning the implant platform is inserted 1-1.5 mm below the crestal bone level. A steep and smooth emergence angle and double platform switching are essential. The image below shows a correctly performed subcrestal placement of the implant.

IMPORTANT! Subcrestal implant placement without platform switching is highly discouraged. Instead of benefits, complications may result.

Let’s return to the concept of implant-level screw retention. This is most often used for single-tooth restorations, where the crown is fabricated on a Ti-base and a single screw secures the entire structure to the implant. Direct single-tooth screw fixation can also be used for two-unit and, very rarely, three-unit restorations; see the images below. These provisional restorations are shown with implant-level screw retention.

The examples were intentionally chosen to illustrate rather complex cases with significant axial divergence between the implants. They highlight one of the main drawbacks of multi-unit implant-level screw retention: the difficulty of ensuring a passive fit and tightening the screws without gaps or strain in restorations other than single units.

Abutment-level screw retention is used for multi-unit restorations and avoids these issues. It utilizes two screws and a multi-unit abutment. One screw securely attaches the abutment to the implant, while the second secures the restoration to the abutment. For straight multi-unit abutments, the retaining screw is integrated into the abutment itself, while angled abutments are secured with a separate screw (see the illustration below).

In this case, improper seating and excessive stress at the implant/abutment level are eliminated. Furthermore, trauma to the peri-implant soft tissue is reduced, as multi-unit abutments are placed once, immediately after soft tissue contouring. They sometimes act as healing abutments themselves and are not removed for try-ins and adjustments of the restoration. The general concept of screw-retained abutment-level restorations is shown in the illustration below.

The load distribution also shifts from the implant/abutment level to the abutment/restoration level. The bone around the implant is not overloaded and does not undergo excessive resorption. Even if a screw breaks or loosens, the problem can be corrected in minutes. This is why this retention method is the standard for full-arch restorations supported by 4/6 implants.

Advantages of abutment-level restorations using multi-unit abutments

• Redistribution of occlusal loads from the implant platform to the abutment platform.
• Achieving a passive fit of the prosthesis is easier—the abutment cone is short and tapers occlusally, unlike the internally-facing implant interfaces. Therefore, even with a 15-20° axial divergence between the implants, no seating issues due to poor passive fit will arise. The image below of the restoration shows that the implants were not placed in a prosthetically driven position, but the restoration successfully compensates for the axial divergence. Furthermore, angled multi-unit abutments were not required.

• Screw retention of prostheses eliminates contact of cement with soft tissues, and the restoration can be easily removed at any time for repair, replacement, or hygienic maintenance of the soft tissues under the prosthesis, which is difficult or almost impossible to do with cement-retained prostheses.
• To compensate for significant axial divergences between implants, there are several options for angled abutments (see the illustration below)—this allows for the optimal positioning of the screw access channels on the prosthesis.

• The “One Abutment, One Time” concept, as we mentioned earlier, means that the less often the soft tissue attachment needs to be disturbed, the better. The gingival margin is more stable and dense, which results in reduced marginal bone loss and a lower risk of gingival recession.

It’s also worth noting that some manufacturers specify a multi-unit abutment diameter based on the implant diameter. However, most implant platforms use a standard system with a single diameter, which is sufficient for 90% of cases. The only difference is the transgingival height. For example, XGATE Dental uses color-coded transgingival height indicators.

Of course, screw retention on multi-unit abutments is not without its drawbacks:

• Inferior marginal seal compared to cement retention. Biological fluids with a high microflora content accumulate in microgaps, which not only causes an unpleasant odor but also increases the risk of localized inflammation. Therefore, screw-retained prostheses should ideally be removed approximately every six months for soft tissue hygiene and cleaning of the prosthesis.
• It’s not always possible to position the screw access holes optimally. The anatomy of the alveolar ridge in the anterior regions often prevents implant placement without the screw access holes emerging through the aesthetic zone.
• The screw access channel itself weakens the restoration slightly, but if we are utilizing zirconia or milled bar structures, this disadvantage is mitigated.

Important considerations when choosing an implant system for screw-retained restorations

1. The implant/abutment interface is best with an internal conical connection. This provides greater stability and sealing, and also facilitates double platform switching.

But this isn’t essential—platform switching can also be implemented with an internal hex connection. This interface is very common and compatible across many manufacturers.

2. Compatibility of the superstructure with other brands. A real-life example involved a patient with an implant-supported bridge placed several years ago. Since then, several more teeth were lost. The prosthesis needed to be replaced and several more implants placed. It is highly advantageous that the new abutments are compatible with both the old and new implants. Consequently, there are no problems in fabricating the new prosthetic structure.
3. Compatibility with CAD/CAM technologies. This primarily refers to the availability of a CAD library with 3D geometries of all components and the ease of accessing them.
4. Availability of additional prosthetic components – impression copings, implant analogs, scan bodies, healing caps from the MUA level, etc.

   

    

   

5. Manufacturing accuracy of all components. This parameter is often underestimated when choosing components. It is impossible to notice visually, and even during insertion, one cannot feel that the gap between components is 150 µm instead of 50 µm. In reality, a large gap allows for micromovement, a bacterial pumping effect, and a high risk of abutment fracture. Therefore, choose manufacturers that declare strict manufacturing tolerances for their implant system components.
6. Material. We won’t discuss surface topography now. All FDA-cleared implant systems have a surface quality sufficient for reliable osseointegration. However, the type of titanium alloy plays a crucial role in full-arch prosthetic protocols supported by 4/6 implants. For example, XGATE Dental products are made of Grade 23 Ti-6Al-4V ELI (6% aluminum, 4% vanadium). This is a medical-grade, extra-low interstitial (ELI) alloy in which the content of interstitial elements (primarily oxygen, but also nitrogen, carbon, and iron) is strictly limited. Grade 23 contains almost half as much oxygen (0.13%) as Grade 5 (0.20%). Grade 23 reduces the risk of fracture in small-diameter screws used to retain dental prostheses due to its significantly higher cyclic fatigue strength, ductility, and fracture toughness.

Why Grade 23 protects screws from breakage

Dental prosthetic screws have a critically small diameter (often M1.4–M2.0) and are subject to enormous cyclic loads during mastication. Under these conditions, Grade 23 outperforms Grade 5 in three respects:

• High ductility (elongation at break). Reduced oxygen makes the metal less brittle. Grade 23 has a 10-15% higher elongation than Grade 5. With excessive tightening or micromovement of the prosthesis, a Grade 23 screw will slightly deform plastically, whereas Grade 5 would suffer immediate brittle fracture.
• Excellent fracture toughness. Grade 23’s fracture toughness is 75–90 MPa√m, while Grade 5’s is only 55–75 MPa√m. Small screws have stress concentrators—the threads. Microcracks inevitably form at the transition from the head to the body of the screw. Grade 23 inhibits the propagation of these microcracks much more effectively, preventing them from causing the screw to fail prematurely.
• Fatigue resistance in a wet environment. In the oral cavity, metal is susceptible to corrosion fatigue. Grade 23 has lower micro-notch sensitivity. During chewing (millions of micro-impacts), a Grade 23 screw lasts much longer without showing signs of fatigue failure.

Comparison of characteristics for dentistry

These same properties also apply to other parts of the superstructure and even to the walls of small-diameter implants in the interface area.

In the upcoming publications in our series, we will continue to delve into the nuances of full-arch restorations and will sequentially analyze the key clinical stages:

• Provisionalization and its protocols. The concept of intraoral welding.
• Assessment of the patient’s systemic condition.
• Rehabilitation protocols in actual clinical cases.

Reliable biomechanics and predictable results begin with the details—from the correct angulation to the physical properties of the titanium alloy. Explore the full range of MUA V-type and D-type solutions, as well as additional prosthetic components, on our website: XGATE Catalog.

Previous: ← Part 2. Selection and positioning of implants