What are the Key Characteristics of Mainstream Implant Designs?

Delve into the realm of dental implantology with the question, "What are the Key Characteristics of Mainstream Implant Designs?" Explore the fundamental aspects that define contemporary implant structures, from innovative designs to essential features. Join us in unraveling the intricacies of mainstream implant designs and their impact on modern dental practices.

 

1. Implant-Abutment Connection

1.1 Connection Methods Between Implant and Abutment

After implantation, an abutment must be attached to the implant to complete dental crown restoration. Currently, internal connection, taper connection, and platform switching are mainstream designs for implant-abutment connections, with many practitioners choosing such implants.

While internal connection is generally considered superior, there are situations where external connection is necessary. Internal connection involves a protrusion from the abutment embedded into the interior of the implant. This design requires a certain accommodating space within the implant, and in cases where the implant diameter is small, the presence of this space may compromise the implant's strength. In such instances, external connection, where the protrusion extends upward from the implant into the abutment, becomes a viable choice, ensuring the implant's strength. Therefore, in scenarios where smaller-diameter implants are necessary due to limited bone volume, and reducing the load is not an option, external connection implants can be a suitable choice.

Internationally, alloys made from zirconia and titanium are used to enhance strength, surpassing that of pure titanium. Implants made from such materials, offering sufficient strength and featuring internal connection in small diameters, have been developed. However, these materials and designs have not yet been introduced domestically.

1.2 Neck Bone Resorption

Historically, bone resorption around the neck has been observed within one year of implant placement. Researchers, through years of extensive basic and clinical studies, identify two main factors causing bone resorption around the implant neck: bacteria and micro-movement at the implant-abutment connection.

Bacteria bred in the gap between the abutment and implant can overflow from the interface, causing inflammation in the adjacent area, leading to bone resorption around the implant neck. Micro-movement at the interface can hinder bone formation at the micro-movement site. The concept of platform switching involves shifting the interface inward to move micro-movement away from the bone surface. Coupled with taper connection, it achieves a friction-welding effect (no micro-gaps, prevents bacterial leakage, and eliminates micro-movement), thus avoiding or reducing neck bone resorption caused by bacterial leakage and abutment micro-movement.

However, clinical observations show that one-piece implants can also experience neck bone resorption, indicating that, in addition to the two factors mentioned above, stress concentration around the implant neck is also a significant factor that cannot be ignored.

1.3 Effective Measures to Prevent Neck Bone Resorption

Currently, platform switching and taper connection are found to be the most effective measures to prevent neck bone resorption. The platform-switching concept refers to the shift of the abutment-implant connection toward the center, reducing bone resorption around the implant neck.

Some scholars have found that when connecting larger-diameter implants with smaller-diameter abutments, significant reduction in bone resorption around the implant neck can be achieved. Further studies confirm that this connection method leads to the migration of bacteria and micro-movement away from the bone-implant interface, thus keeping them away from the osseointegration area. Analyzing this principle, platform switching not only refers to the inward shift of the abutment connection but also an upward shift away from the bone surface (for soft tissue-level implants), which can similarly reduce neck bone resorption.

Many experts in China translate "platform switching" as "platform transfer" , but it is suggested that "platform migration"  is a more accurate translation based on the described mechanism. Since bacteria are a significant factor leading to neck resorption, using a taper connection design to ensure no bacteria at the abutment interface is currently a popular implant-abutment connection method. In addition, for ease of subsequent restorative procedures, there should be a sliding process during implant and abutment insertion.

2. Common Designs of Mainstream Implant Systems

2.1 Common Shape Classifications

The purpose of implant design is to convert shear forces into pressure as much as possible and distribute stress to appropriate locations. Implant designs generally fall into root form, column form, and two-way taper forms. Early Straumann, Branemark systems were representatives of columnar designs.

Currently, common root form designs include Anthogyr, Ankylos, Replace, and other implant systems.

Two-way taper design is the latest implant design, with tapering present in both the upper and lower parts of the implant.

2.2 Implant Site Preparation and Design

To achieve ideal initial stability, the implant site diameter should be smaller than the implant diameter. But how much smaller is appropriate?

In Dr. Frost's biomechanical reaction theory proposed in 1987, physiological load zones with 2000 microstrains stimulate bone growth. In overload zones, bone thickness easily increases in young patients, while absorption is more likely in older patients. In pathological load zones, regardless of the patient's condition, bone absorption occurs. In the micro-load zone, disuse absorption occurs.

Therefore, appropriate loading is crucial for bone healing.

3. Implant Surface Treatment

3.1 Early Implants

The surface of early implants was mechanically smooth. After implantation, it primarily relied on new bone growth from the bone walls of the implant site, known as distance osteogenesis. During this phase, any micro-movement of the implant could lead to the formation of fibrous connective tissue on the implant surface, resulting in early implant failure. Therefore, early mechanically smooth implants required minimizing the distance between the implant and adjacent bone. This distance was often determined based on the initial stability of the implant – the better the initial stability, the closer the distance.

3.2 Current Mainstream Implants

Present-day implant surfaces undergo specific treatments to achieve a roughened texture. After implantation, bone-forming cells can directly attach and proliferate on the surface, known as contact osteogenesis. Implants with contact osteogenic characteristics enable rapid deposition of bone cells on the implant surface during the bone healing process, overcoming the critical period of osseointegration. Therefore, modern implant systems can successfully complete osseointegration even with relatively low initial stability.

3.3 Transformation of Bone Formation Patterns Leading to Changes in Implant Surgery

Bone healing is related to the pressure it experiences, and excessive pressure can lead to bone necrosis. However, in clinical settings, it is challenging to determine whether the implant site is under excessive pressure. Therefore, in clinical practice, it is generally preferred to minimize pressure, forming a new surgical concept. Due to improvements in implant design, implants no longer need to be tightly pressed against the bone surface after implantation. The requirement for initial stability has shifted from being more significant to being more moderate. This change is attributed to implants transitioning from mechanically smooth surfaces to rough surfaces, increasing tissue compatibility. This enhanced compatibility results in increased attachment of fibrous proteins, other proteins, and growth factors, as well as increased chemotaxis of bone-forming cells and platelets. Consequently, bone tissue can directly deposit on the implant surface, transforming the bone formation pattern from distance osteogenesis to contact osteogenesis. Therefore, successful healing and osseointegration can occur even when there is a considerable gap between the implant surface and the bone.

4. Implantation Techniques

According to Wolff's law, the formation of bone trabeculae is related to functional pressure – no function means no bone trabeculae. Additionally, Hassler demonstrated in 1980 that when stress exceeds 69N/mm², cell death occurs, but at 24.8N/mm², bone growth accelerates. Therefore, bone loading should be moderate. In clinical practice, the degree of bone compression can only be estimated by the insertion torque during implantation. The greater the insertion torque, the greater the bone compression. Excessive insertion torque results in undue pressure on the bone, which is detrimental to bone metabolism. According to clinical experience, the optimal torque is in the range of 25–50N. If it exceeds 35N, immediate loading is possible. However, it is crucial not to exceed 60N.

As cortical bone has low plasticity and poor blood supply, it has low tolerance for compression. The bone trabeculae surrounding the medullary cavity contain connective tissue rich in blood vessels. After compression, the displacement of bone trabeculae generally does not lead to local circulation disorders. Therefore, when implanting implants, the compression part should be dispersed into the medullary cavity to avoid excessive compression of the cortical bone.