Thread, Taper, and Reverse Taper: How Implant Shape Determines Its Application. Part 1

In this article, we’ll explore how to choose dental implants based on bone density and other factors. We will also examine why such a wide variety of implants exists and whether a universal solution is truly possible for all bone types, jaw locations, etc.

Why There Can’t Be a Perfect Implant for Every Occasion

The reason is simple: an implant that is perfect for one clinical situation may be completely unsuitable for another.

Series: Thread, Taper, and Reverse Taper: How Implant Shape Determines Its Application
→ Part 1. Choosing an implant by bone density (you are here)
→ Part 2. Clinical application of different implant shapes

Some overconfident surgeons claim they could place a nail and it would osseointegrate. More often than not, these are inexperienced specialists. While it’s true that the properties of titanium and bone tissue allow any biocompatible object to osseointegrate, this does not guarantee it will function effectively as a long-term support for a prosthesis.

Specifically, more and more patients now desire immediate restoration rather than waiting 4-6 months, as required by traditional deferred loading protocols. In immediate loading protocols, primary stability is critical. Immediately after placement, an implant’s stability depends on the frictional force with which it engages the bone. Bone structure and density vary throughout the jaw, even in the same patient. The mandible is typically denser than the maxilla, for example. Achieving consistent primary stability in both dense and soft bone requires different implant designs and surgical approaches.

It’s no wonder there is such a wide variety of implant shapes and sizes. In this article, we’ll explore some of the options, as illustrated below.

Here is a specific case. The top photo shows the patient’s initial clinical situation. On one side of the mandible (quadrant 4), there is an old plate-form implant and a bridge with partial root support. The other side (quadrant 3) presents a well-preserved alveolar ridge suitable for straightforward implantation. The complexity of the work and the choice of implants for these two areas are fundamentally different.

Implantation in quadrant 3 was very simple; the implants were installed using a flapless technique, and the entire procedure took no more than 15-20 minutes. Naturally, the cost of placing implants under such favorable conditions is much lower compared to the work required in quadrant IV for the same patient. There, the plate-form implant had to be removed, and short, large-diameter implants with wide threads were placed. The procedure for quadrant IV took over an hour.

Many clinics base their pricing on the cost of the implants themselves. Their dialogue with the patient often follows the principle that certain implants are better and therefore more expensive, increasing the overall cost of the surgery. In reality, all FDA-cleared implants have a survival rate above 96%. XGATE meets these standards with CE marking, ISO 13485, and FDA-cleared products on parts of its portfolio. It would be more honest and transparent to base the cost of treatment on the complexity of the procedure and the surgeon’s expertise. However, we do not impose our opinions on anyone.

Typical Clinical Situations and Challenges – Choosing the Right Implant

1. Achieving primary stability in different bone types (hard/soft/heterogeneous).
2. Addressing vertical bone deficiency.
3. Immediate implantation in a narrow extraction socket.
4. Immediate implantation in a wide extraction socket.
5. Treating a narrow alveolar ridge.

Let’s look at these challenges and situations in more detail.

Primary Stability and Bone Density

For immediate implantation with immediate loading, implants must be exceptionally stable. While there are several methods for verifying stability, the simplest and most effective is measuring the insertion torque. If the torque is less than 35 Ncm, it is best to avoid immediate loading. Conversely, insertion torque exceeding 60 Ncm can also be problematic. High torque, especially in a very dense bone, can lead to compression necrosis. While in some cases high torque may not cause issues, it is best to stay within reasonable limits, typically up to 70 Ncm.

Let’s start by classifying bone types according to the classical system.

1. Type D1: The densest bone. The cortical layer is very thick (3 mm or more). The cancellous layer is often poorly defined, and the implant may be more than 50% encased in cortical bone.
2. Type D2: The optimal type for implantation. The cortical layer is 1.5 to 3 mm thick, and the cancellous bone is also quite dense.
3. Type D3: A thin cortical layer (less than 1.5 mm) and fairly soft cancellous bone with large trabeculae.
4. Type D4: The cortical layer is extremely thin or virtually absent (average 0.5 mm). The cancellous bone is very soft, demineralized, and barely visible on X-rays.

Remarkably, modern implant designs and placement techniques now make it possible to achieve excellent primary stability in all bone types. Until recently, achieving stability in the soft bone of the maxilla was a significant challenge for dentists.

How to Achieve Good Primary Stability in Bone of Different Densities

The solution lies in understanding material properties, using an analogy with construction fasteners for different materials.

1. Type D1 Bone: A smooth nail is best for oak boards. Driving a screw into oak is difficult and often impossible; a long screw will likely get stuck and break under continued force. Similarly, for D1 bone, implants with a very shallow thread profile and a slightly tapered body are used. In essence, the design is as close as possible to a simple nail.

   

    

2. Type D2 Bone: Pine boards are effectively joined with standard construction screws, which create a stronger and more stable connection than nails.

   

    

3. Type D3 Bone: This bone is similar to the particleboard used in inexpensive furniture. It requires special furniture fasteners with a larger diameter and more aggressive threads. A slightly undersized pilot hole is drilled first, a technique very similar to implant placement protocols in D3 bone.

   

    

4. Type D4 Bone: This is analogous to drywall or dense Styrofoam. Even in these materials, a secure hold is achievable with fasteners that have wide, blade-like threads and a sufficient diameter. Corresponding implant designs also exist.

   

The illustration below highlights the parallels between construction fasteners and implant designs for different bone densities.

On the far left is a tapered-cylindrical implant with a shallow thread profile. This shape is ideal for fixation in dense D1 bone, and its construction equivalent is a nail. This simple form, lacking sharp protrusions and deep threads, avoids over-compressing or traumatizing dense bone. This is crucial because D1 bone has poor vascularity, and excessive compression can lead to bone necrosis rather than remodeling.

The second implant from the left is quite different. The neck is flared and features microthreads. This design provides secure fixation in the 2 mm of cortical bone typical of the D2 type. The threads on the implant body are similar to the first implant.

The third implant also has a flared neck with microthreads, but the main body threads have wide blades designed for fixation in the cancellous bone layer. In D3 bone, the cortical layer is thin and cannot bear the primary mechanical load, so wide threads are needed to engage the cancellous bone.

The fourth type of implant lacks a specialized collar for cortical anchorage, as D4 bone has virtually no cortical layer. The threads are wide, the pitch (distance between threads) is increased, and the implant body has a distinctly tapered shape.

To simplify the classification, dental implants can be divided into two broad categories:

1. Cortical Implants: These feature microthreads on the neck and a shallow body thread profile (no more than 0.3 mm) for fixation in hard D1-D2 bone. Primary stability is achieved through friction and slight compression of the bone by the implant body and neck. Insertion torque should not exceed the elastic limit of bone (ideally no more than 50 Ncm) to avoid the risk of necrosis.
2. Cancellous Implants: These have aggressive, deep threads (0.5 to 1.5 mm) for fixation in soft D3-D4 cancellous bone. Primary stability here is also achieved through friction, but it relies on the significantly increased surface area of the threads. Due to the loose structure of D3-D4 bone, there is little risk of over-compression, so the insertion torque can safely exceed 50 Ncm, though it is still advisable not to exceed 70 Ncm.

The remaining implant designs are variations or hybrids of these two types. The clinician’s task is to select the appropriate implant or combination of implants for each specific clinical case.

It is worth noting that in many clinical situations, universal implants can be used effectively, even for immediate loading. An example is the XGate Dental implant system, which combines several clever technical solutions.

Unlike conventional cortical implants where the neck is flared, the neck of the XGate implant features a reverse taper. This innovative design reduces compression in the cortical bone. As the implant is inserted, its widest part, located below the neck, gently expands the osteotomy. When the implant is fully seated, the narrower neck relieves this pressure. The natural elasticity of the bone then allows the cortex to close in and gently grip the implant neck. This simple yet brilliant design provides secure engagement without the risk of over-compression. XGate Dental implants are ideal for both flapless surgery in D1-D2 bone and for immediate placement protocols, thanks to their self-tapping apex.

This creates a dual-fixation effect: the wider portion of the implant is anchored in the cancellous bone, while the narrower neck is stabilized by the cortical plate without the risk of excessive pressure and subsequent necrosis. This solution is ideal for bone types D2-D3 and also performs well in D1 bone. Since D2, D3, and the transitional forms between them are the most commonly encountered bone types, this design offers broad applicability.

The variable thread profile of XGate Dental implants also deserves attention.

Sharp, aggressive threads at the apex act like a tap, allowing the implant to cut its own path and penetrate the bone with ease. This not only simplifies the surgeon’s work but is also crucial for achieving high primary stability. For instance, in an immediate placement protocol where only the implant apex engages native bone beyond the socket, these aggressive threads provide a sufficient grip for immediate loading.

Another advantageous feature of the XGate Dental implant system is its well-designed restorative components. For example, the combination of the reverse taper neck and the V-type multi-unit abutment system creates a significant platform switching effect.

Platform switching is a proven method for preserving crestal bone around the implant, as confirmed by numerous studies. The pronounced horizontal offset created by the XGate system provides ample space for the development of a thick, robust soft tissue seal around the abutment, which is key to the long-term health and esthetic success of the restoration.

So, can the same type of implant be used in different bone types? The answer is yes, but with some important caveats. If a standard cortical implant is placed in soft bone, immediate and even early loading protocols should be avoided, and the osseointegration time will need to be extended. Here are some rough estimates for a standard cortical implant:

• D1 Bone (Standard): Secondary stability after ~2 months
• D2 Bone: Secondary stability after ~3 months
• D3 Bone: Secondary stability after ~4 months
• D4 Bone: Secondary stability no earlier than 6 months

Conversely, placing a cancellous implant with aggressive, deep threads into very dense D1 bone using a standard protocol is challenging, if not impossible. The osteotomy would need to be significantly widened, meaning the implant would only be held by the outer edges of its threads, increasing the risk of it binding or stripping the bone. Furthermore, the poor vascularity of dense bone means that osseointegration may be delayed.

However, using a universal implant like the XGate Dental system can yield good or at least acceptable results in most cases, with reasonable healing times. The main exception would be extremely soft, friable bone (D4), where achieving adequate primary stability is the primary challenge. In such cases, a specialized implant with a more pronounced taper and wider threads may still be the better choice.

The illustration below shows a treatment plan using implants specialized for different bone types.

A combination of cancellous implants for the softer posterior maxilla and cortical implants for the denser mandible can achieve excellent primary stability across the board. This allows for the placement of temporary restorations and promotes osseointegration within a predictable timeframe. By using highly specialized cortical and cancellous implants in their ideal environments, the timeline for achieving secondary stability can be optimized:

• Cortical Implant in D1 Bone: Secondary stability after ~2 months
• Cortical Implant in D2 Bone: Secondary stability after ~3 months
• Cancellous Implant in D3 Bone: Secondary stability after ~2 months
• Cancellous Implant in D4 Bone: Secondary stability after ~3 months

We do not advocate for specific brands but aim to provide clinicians with a better understanding of the benefits of each implant design. The brand and price of an implant are not sufficient criteria for selection; it is far more important to choose an implant based on the specific clinical situation.

The concept of combining different implant designs within the same patient has been validated by over a decade of clinical use, and the long-term results are available for review. The images below show stable bone levels with minimal resorption around the implant necks, even after 10-11 years.

Compatibility of Superstructures from Different Manufacturers – Why Is It Important?

Since 2020, implants with a conical interface have continued to gain market share. Most manufacturers use an 11° cone (a 22° total included angle), while some use a 5° interface or other variations. The standardization of connection types and the resulting compatibility of abutments with implants from different manufacturers is a major benefit for both clinicians and patients. It is common for patients to present with existing implants who need to replace an old restoration or add new implants to create a larger prosthesis. If the implant interfaces are compatible, a clinician can easily use new stock or custom abutments to create a seamless and high-quality restoration.

That’s all for now. In the next part, we will look at several clinical examples and discuss the application of implants with different body profiles and thread designs.

Continue reading: Part 2. Clinical application of different implant shapes →