Understanding Implant-Abutment Connections: Internal Hex vs. Conical Connection Under Load

Current implantology demands for competent implant-abutment connections for predictable, high-quality results in two-piece implant systems. Without this essential engineering feature, the long-term success of our dental implants is unfortunately doomed to failure. Implant abutments not only transmit functional loads from the prosthesis to the implant body and surrounding alveolar bone, but also influences microleakage, screw stability, and crestal bone preservation. 

Among the multiple connection designs developed over the past decades, internal hex and conical (Morse taper) systems dominate modern clinical practices. Yet, understanding how these connection geometries behave under functional loading help clinicians make evidence-based decisions for each case. 

The Role of Implant-Abutment Connection Design

Why Use a Two-Piece Internal Connection System

Two-piece implant systems offer clinicals greater prosthetic flexibility and surgical control. This system separates the implant body from the abutment, allowing clinicians to achieve subcrestal placement for better esthetics and easier angulation correction during prosthetic restorations. Internal connections enhance this design by housing the joint within the implant body, protecting the structure from lateral forces and minimizing micromotion. 

Compared with external hex systems, where the connection sits above the implant platform, internal connection provides improved mechanical stability, reduces risk of screw loosening, and superior sealing against bacterial leakage.

Connection Design’s Importance

The implant-abutment interface serves as both a mechanical joint and a biological seal. In short terms, in influences every aspect of the implant restoration: 

• Mechanically, it determines how occlusal forces are transmitted through the implant structure to the peri-implant bone. 
• Biologically, it defines how well the connection resists bacterial infiltration and micromotion. As a result, even minimal instability at this junction can lead to mechanical complications of inflammatory complications that can jeopardize osseointegration. 
• Aesthetically, it plays a vital role providing a steady and functional biological gingival contour that restores the emergence profile, giving the restoration and natural appearance.

Load Transmission in Implant Systems 

During mastication, the occlusal load passes from the crown to the abutment, then into the implant body, and finally to the surrounding tissues. The geometry and fit of the connection influence how these forces are distributed. Thus, designs that minimize bending and microgap formation tend to maintain crestal bone levels more effectively. Some fundamental engineering parameters include: 

• Preload: the tension in the abutment screw.
• Micromotion: the subtle, relative displacement between the implant surface and the surrounding tissues under load.
• Microgap: the space that exists between the implant surface and the abutment interface in a two-piece dental implant system.

Internal Hex Connection

Structural Design and Characteristics

The internal hex connection is a flat-to-flat joint that features a hexagonal recess within the implant platform. The abutment engages this hex, providing anti-rotational stability and accurate prosthetic alignment. This design remains as one of the most widely used connections due to its simplicity and compatibility across multiple restorative systems.

Behavior of Hex Connection Under Functional Load

The flat interface of the internal hex transmits occlusal forces primarily through the abutment screw, whether under vertical or lateral forces. It performs as a stable and reliable joint, reducing lateral micromovement due to its internal positioning, especially when compared to older external designs. As a result, it helps to stabilize the prosthetic component under vertical and oblique loads. Also, the hexagonal geometry provides anti-rotational stability, ensuring accurate seating and maintaining prosthetic orientation over time, and with the appropriate torque application, the screw proload creates elastic tension that keeps the joint firmly compressed, allowing it to resist loosening during cyclic mastication. 

On the other hand, some studies have shown that internal hex connections exhibit higher stress concentration at the screw joint, increasing the risk of screw loosening and proload loss over time. However, when properly torqued and maintained, they demonstrate reliable mechanical endurance, depending greatly on the appropriate preload. 

Clinical Implications

Clinically, internal hex systems offer significant prosthetic versatility as they facilitate easier indexing, retrievability, and wide component availability. All these advantages make them highly practical for full-arch or multi-unit restoration where component interchangeability matters. 

As a drawback, they require consistent torque calibration, proper screw design, and maintenance checks to mitigate the risks of microleakage and screw loosening over time.  

Conical (Morse Taper) Connection

Structural Design and Characteristics

The conical connection, often called a morse taper, employs a friction-fit engagement where the abutment and implant form a tapered interface, typically between 6° and 12°. This design provides an intimate mechanical contact, producing a cold-weld-like stability with a high resistance to micromotion while enhancing scaling at the interface.

Behavior of Conical Connections Under Functional Load

Conical connections, such as the GDT CON NP and CON RP, perform exceptionally well under functional loading. Their tapered geometry disperses occlusal force more evenly throughout the implant body and crestal bone. Some recent analyses show lower stress peaks and better resistance to bending moments when compared to other designs.

Additionally, the intimate fit minimized the microgap space, reducing the likelihood of screw loosening and bacterial infiltrations. The frictional retention between components also stabilizes torque over time, maintaining preload better than flat-to-flat interfaces.

Clinical Implications 

Conical connections demonstrate superior long-term marginal bone preservation and biological sealing. Multiple systematic reviews have reported significantly lower marginal bone loss around conical connection implants compared with external or internal hex types. 

However, disassembling a well-seated conical abutment can represent a challenge to the clinician. In some cases, tapping instruments are required to disengage the taper, which demands caution to prevent deformations and other structural issues. Despite this, the mechanical and biological benefits of this internal connection make it an excellent choice for esthetic zones and single-tooth restorations where stability and tissue integrity are priorities. 

Comparative Analysis Under Load

Micromotions and Fatigue Resistance

Under cyclic loading, conical connections generally display lower micromotions and higher fatigue resistance than internal hex systems. This stability is the result of the tapered interface that distributes forces along a broader contact area, reducing reliance on the abutment screw alone. Internal hex systems can show similar results, especially when supported by adequate torque and precise machining tolerances, but their mechanical load path remains screw-dominated. 

Microgap and Bacterial Leakage

Microgaps as small as 1–10 µm can harbor bacteria, contributing to peri-implant complications, particularly inflammatory conditions like mucositis and peri-implantitis. Conical connections typically achieve a tighter seal than internal hex designs, minimizing microbial leakage even after thermomechanical cycling. Also, a recent study reported that Morse taper connections exhibited less bacterial penetration and stress under load compared with internal hex counterparts.

Long-Term Marginal Bone Stability

Current evidence consistently associates conical connection implants with reduced marginal bone loss, typically attributed to decreased microgap motion and improved biological width maintenance. While flat-interface systems designs, like hex abutments, may experience slightly higher resorption rates (especially in thin biotypes and high-load regions), both systems maintain bone levels within 0.5–1.0 mm over a 5–10 year span. 

Practical Considerations for Clinicians 

Selecting the Right Connection Type

Ultimately, the choice between internal hex and conical systems depends on clinical context. Here you have some practical common cases:

• Single-tooth and esthetic zones: Conical Connections are preferable to obtain a superior sealing, torque stability, and marginal bone preservation.
• Multi-unit and full-arch restorations: These types of restorations benefit from internal hex systems that offer greater prosthetic versatility and simpler retrievability for modifications or repairments. Biomechanics of these types of prostheses also play in favor of flat-to-flat interfaces
• Dense cortical bone or posterior regions: Both systems can succeed, though conical designs better manage occlusal stress.
• High esthetic demand: Conical interfaces maintain soft tissues stability due to minimal microgap and micro-movement. 

Remember that both systems can provide excellent results if case selection, torque protocols, and maintenance schedules are respected. 

Torque Application and Maintenance

Achieving the correct preload is critical to prevent screw loosening. Studies show that improper torque accounts for a majority of mechanical complications in internal hex systems- Thus, clinicians should use calibrated torque wrenches and adhere strictly to manufacturer specifications during their surgical protocols. 

Periodic retightening after 10-15 minutes post-installation allows the abutment screw to settle, compensating for elastic relaxation while keeping long-term stability.

Prosthetic and Laboratory Implications

Internal hex connections provide more flexibility for custom abutments and component interchange, facilitating restorative workflows in multi-unit cases. Conical designs require higher precision in abutment fabrication to maintain the integrity of the taper fit. This is where digital workflow and CAD/CAM milling show their true potential, ensuring consistency in taper angle and surface finish. 

Current Clinical Evidence and Long-Term Outcomes

Most systematic reviews report similar screw loosening rates but showing slightly favorable measures for conical systems when compared to internal hex designs after several years of function. On the other hand, internal connections have shown consistent better results than their external counterparts. Both designs demonstrate comparable survival rates over 95%, showing that clinicians can choose either system depending on patient needs and other factors. 

However, most studies show a higher tendency for internal hex connections to be more prone to mechanical issues like screw loosening and microgap wear, while conical internal designs display potentially more challenging maintenance or retrieval. 

Influence on Peri-Implant Tissue Health

Clinical evidence indicates that minimizing micromotion and bacterial ingress directly benefits peri-implant tissue stability. Conical interfaces limit bacterial ingress due to their tight seal limits, reducing inflammatory potential. However, internal hex systems can achieve excellent outcomes with the appropriate torque, fit, and consistent hygiene and maintenance. 

Future Perspective and Innovation  

As implant designs evolve, new hybrid connections have emerged, combining internal hex index with a conical frictional fit. This dual geometry seeks to merge the anti-rotational stability of the hex design with the sealing advantages of the conical taper. Early mechanical in vitro studies suggest that such hybrids may offer the best balance between retrievability and biomechanical strength. 

Furthermore, digital implantology is improving the precision of abutment seating and torque calibration, reducing human error in mechanical assembly. Moreover, innovation in surface engineering aims to enhance the biological seal and reduce bacterial adhesion.

References

1. Sailer, I., Karasan, D., Todorovic, A., Ligoutsikou, M., & Pjetursson, B. E. (2022). Prosthetic failures in dental implant therapy. Periodontology 2000, 88(1), 130–144. https://doi.org/10.1111/prd.12416
2. Kofron, M. D., Carstens, M., Fu, C., & Wen, H. B. (2019). In vitro assessment of connection strength and stability of internal implant-abutment connections. Clinical Biomechanics, 65, 92–99. https://doi.org/10.1016/j.clinbiomech.2019.03.007
3. Cannata, M., Grandi, T., Samarani, R., Svezia, L., & Grandi, G. (2017). A comparison of two implants with conical vs internal hex connections: 1-year post-loading results from a multicentre, randomised controlled trial. European journal of oral implantology, 10(2), 161–168.
4. Szyszkowski, A., & Kozakiewicz, M. (2019). Effect of Implant-Abutment Connection Type on Bone Around Dental Implants in Long-Term Observation: Internal Cone Versus Internal Hex. Implant dentistry, 28(5), 430–436. https://doi.org/10.1097/ID.0000000000000905
5. Grandi T, Cannata M, Samarani R. A 5-Years Report from a Multicenter Randomized Controlled Trial: Dental Implants with Conical Versus Internal Hex Connections. Clinics in Surgery. 2021.
6. Shen, L., Dong, C., Chen, J., Bai, X., Yang, F., & Wang, L. (2023). The mechanical and clinical influences of prosthetic index structure in Morse taper implant-abutment connection: a scoping review. BMC oral health, 23(1), 775. https://doi.org/10.1186/s12903-023-03545-3