What Materials Are Best? Exploring the Selection Criteria for Abutments in Oral Implants？

There are five main categories of commonly used abutment materials for implants: Titanium (machined, polished, laser-etched), Surgical-grade stainless steel, Cast gold alloy, Zirconia, and Polyetheretherketone (PEEK). How can clinicians make informed choices for abutment materials in clinical practice? The author primarily considers the following aspects.

Titanium

Titanium is the only material that seamlessly combines toughness, lightweight, excellent biocompatibility, durability, and high strength. Titanium surpasses any known element in corrosion resistance and boasts the highest strength-to-weight ratio. Titanium abutments are crafted from industrial pure titanium or titanium alloys. Industrial pure titanium finds extensive use in medicine due to its corrosion resistance, high strength, and biocompatibility.

The addition of small amounts of oxygen and iron can impact the mechanical properties of industrial pure titanium. By meticulously controlling the addition of these elements, various grades (Grade 1 to Grade 4) of industrial pure titanium can be produced to meet different applications. Industrial pure titanium with the lowest oxygen and iron content is the most easily formable, while gradually increasing oxygen content enhances material strength.

Titanium alloy (Ti-6Al-4V or Ti6Al4V, also known as Ti-6-4). Titanium alloy, also referred to as Grade 5 titanium, contains 6% aluminum, 4% vanadium, up to 0.25% iron (maximum), up to 0.2% oxygen (maximum), with the remaining composition being titanium. Ti-6Al-4V alloy exhibits significantly higher strength compared to industrial pure titanium, providing increased tensile strength and fracture resistance.

Due to its unique physical properties, titanium abutments are the preferred choice for posterior dental implant restorations (Figure 3). These abutments can be pre-fabricated stock abutments or personalized abutments milled through CAD/CAM technology.

Surgical Stainless Steel

Surgical Stainless Steel is a special type of stainless steel used in the medical field, containing alloy elements such as chromium, nickel, and molybdenum. It is easy to clean and sterilize, has high strength, and is corrosion-resistant. Nickel-chromium-molybdenum alloys are sometimes used for implant abutments, but there is a potential for allergic reactions to nickel by the immune system. Surgical-grade stainless steel can be used as a temporary abutment but is not the ideal material for permanent restorative abutments.

Cast Gold Alloy

The composition of cast gold alloy used for implant abutments includes: 60% to 65% gold, 20% to 25% palladium, 19% platinum, and 1% iridium. Implant manufacturers recognize the limitations of early 'stock abutments' and have developed a castable abutment known as the UCLA abutment.

This type of abutment consists of a machined gold alloy base and an attached plastic sleeve; the gold alloy base matches the implant; the plastic sleeve can be cut and modified to create a wax pattern for gold alloy casting (Figure 4). Since the late 1990s, there has been a consensus that gold and porcelain exhibit a poorer soft tissue response compared to alumina (outdated all-ceramic material) and titanium. These views stem from animal studies conducted by Abrahamsson et al. in 1998. As a result of the findings, many clinicians have completely refrained from using gold alloy cast abutments.

Abrahamsson and Welander reiterated in later research (Welander et al., 2008) that titanium and zirconia exhibit superior soft tissue responses compared to gold.

For different abutment materials, attention is given to their ability to form and maintain the 'implant seal'; in this regard, gold alloy has disadvantages compared to titanium and zirconia, and clinicians are advised to avoid its use in clinical practice.

Zirconia

Zirconia is a white crystalline oxide of zirconium (Figure 7). Its primary natural form is the monoclinic crystal structure mineral known as baddeleyite. Advances in biomaterials and ceramic manufacturing technologies have enabled the use of high-strength, biocompatible zirconia in biomedical devices and implant abutments. The partial stabilization of tetragonal zirconia polycrystals (Y-TZP) by yttria, powder injection molding (PIM), and hot isostatic pressing (HIP) techniques signify milestones in the development of zirconia. Other developments, such as the use of alumina-toughened zirconia and ceria-stabilized zirconia, aim to impede the aging process of zirconia, minimizing its impact.

Due to its superior material performance and strength, zirconia is suitable for various applications, whether for aesthetic considerations or high-load requirements (e.g., aesthetic zone cases, fixed restorations for posterior teeth, implant abutments, multi-unit implant restorations). Zirconia exhibits high flexural strength, fracture toughness, and Young's modulus, similar to steel. In addition to its strength, one of the greatest advantages of zirconia is its outstanding tissue integration capability. Numerous studies have confirmed the success of zirconia abutments in maintaining the stability of soft tissues and marginal bone. Results indicate that the type of abutment material influences the quantity and quality of surrounding tissues (zirconia compared to cast gold alloy). Furthermore, zirconia abutments significantly reduce bacterial adhesion and plaque formation, preventing soft tissue inflammation.

It is crucial to note that adjusting and grinding zirconia poses challenges for dentists and technicians. Modifying sintered zirconia components significantly increases the risk of developing microcracks, which may lead to fractures during subsequent masticatory function. Zirconia abutments with a full zirconia connection to the implant, previously used clinically, have largely been phased out. Currently, it is commonly recommended to combine zirconia abutments with a Ti-base connection, especially in aesthetic zone implant restorations (Figure 8).

Polyether ether ketone (PEEK)

PEEK has become the most popular material for temporary abutments. It is a tan or white organic polymer and crystalline thermoplastic with excellent mechanical and corrosion-resistant properties. It has a Young's modulus of 3.6 GPa and tensile strength of 90–100 MPa. PEEK exhibits high resistance to thermal degradation and is capable of withstanding exposure to organic substances and humid environments.

These robust characteristics make PEEK an ideal material for temporary abutments (Figure 9). As early as 1987, animal studies conducted by Williams et al. demonstrated the biocompatibility of PEEK material. In 1995, Hunter et al. used PEEK, titanium, and cobalt-chromium (CoCr) in reconstructive surgery and compared their performance; no significant differences were found in terms of fibroblast or osteoblast cell attachment.

In the field of dentistry, PEEK polymers are employed for restorative abutments and healing abutments, with PEEK abutments being the preferred choice for temporary restorations or healing abutments. While research on PEEK polymers in the dental context is currently limited, its applications hold promise.

In summary, the clinical recommendations for selecting implant abutments are as follows:

1. For posterior teeth in implant restorations, titanium abutments are the preferred choice.

2. In the aesthetic zone, especially for cases with thin gingival biotype or inadequate soft tissue thickness (less than 3mm), zirconia abutments or titanium abutments with titanium nitride coating are the preferred options.

3. For temporary or healing abutments, PEEK abutments are the preferred choice.

4. Exercise caution when considering cast gold alloy for fabricating abutments.

5. Avoid using stainless steel materials for fabricating abutments.