Managing Challenging Bone Conditions: How Implant Thread Design Influences Primary Stability

Achieving high primary stability is the paramount objective in dental implant placement as it serves as the critical prerequisite for successful osseointegration. This mechanical engagement between the implant and the surrounding bone is the base upon which biological healing depends. However, as clinicians, we tend to encounter challenging bone conditions, such as low-density bone, compromised post-extraction ridges or anatomically limited sites, which threaten to undermine this initial stability. 

In these clinically demanding scenarios, the macro-geometric design of the implant, specifically its thread architecture, allows us to transition from a passive feature to an active determinant of success. 

Biomechanical Principles of Thread Design and Bone Engagement 

The primary stability of a dental implant is a function of the frictional forces and mechanical interlocking generated at the bone-implant interface. Thread design is the primary engineering variable that optimizes this interface by managing and distributing occlusal forces into the surrounding bone as a compressive stress indeed of shear stress. 

Defining Key Thread Geometry Parameters 

Thread geometry involves several interdependent parameters, each contributing uniquely to stability: 

• Thread Pitch: It describes the distance between corresponding points on adjacent thread forms. A narrower pitch, meaning more threads per unit length, increases the total surface area for bone contact and distributes functional loads over a greater number of threads, providing a significant advantage in low-density bone. 
• Thread Depth: It encompasses the distance between the crest and root of the thread. Deeper threads engage a larger volume of bone, significantly enhancing anchorage in soft, cancellous bone by reaching into regions of better-quality bone.
• Thread Shape: It refers to the cross-sectional profile of the thread, including V-shaped, square, or buttress designs. Square and power threads offer a larger surface area perpendicular to the occlusal force vector, facilitating a more efficient transmission of compressive forces to the bone. V-threads, as seen in the GDT CFI implant, on the other hand, generate higher shear forces.

Force Distribution Biomechanics

The main goal of thread designs is to convert detrimental shear forces into beneficial compressive forces. Under axial load, a well-designed thread profile presses against the bone in a direction perpendicular to the bone surface, inducing a state of hydrostatic compression within the bone tissue. 

As the maxillary bone is notably stronger in compression than in shear forces, designs that maximize compressive force transfer protect the interfacial bone from microdamage and resorption. 

In low-density bone, with lower modulus of elasticity and yield strength, a deep-threaded, aggressive pitch design is crucial to engage a sufficient volume of bone to achieve this crucial compressive state. 

Clinical Indications for Strategic Thread Design Selection

The selection of an implant system with a specific thread design should be a deliberate decision based on pre-operative bone quality assessment, typically following the Lekholm and Zarb Classification. 

Low-Density (Type III & IV) Bone 

Type IV bone is characterized by a thin cortical plate and a core of low-density cancellous bone, presenting the highest risk of inadequate primary stability. In these conditions, implants featuring a dual-thread design or aggressive, deep threads are indicated. The dual-thread concept often combines a widely spaced deep thread for efficient insertion and macro-anchorage with a closely spaced thread for increased surface area and optimized load distribution. 

A recent study on implants placed in soft bone found that those with progressive thread design achieved significantly higher insertion torques and ISQ values compared to standard designs. 

Immediate Post-Extraction Sites 

The mismatch between a cylindrical implant and a funnel-shaped extraction socket often results in a gap, particularly in the coronal third. Implants with an apically positioned aggressive thread pattern can engage the denser apical and lateral bone, bypassing the empty coronal socket to achieve stability in the basal bone. This apical anchorage strategy is critical for immediate placement protocols. 

Under-Prepared Osteotomies and Compromise Scenarios

When initial stability is less than ideal, or when an osteotomy has been oversized for multiple reasons, a rescue strategy is required. Implants with a wider core diameter and deeper, cutting threads can engage the bone walls more effectively than a parallel-walled implant, often salvaging a site that would otherwise require healing and delayed placement. 

Surgical and Prosthetic Protocols for Optimized Stability 

As clinicians, achieving stability is not merely a product of implant design but the result of a meticulously executed surgical protocol that synergizes with that particular design.

Site Preparation and Osteotomy 

Conservative osteotomy is paramount in soft bone sites. The final drill diameter should often be 0.5 mm narrower than the implant's nominal diameter to allow for bone condensation rather than removal. The use of bone condensers or osteotomes can further compress the peri-implant bone, increasing local bone density and improving the engagement of the implant threads. It’s critical to avoid overheating the bone through rigorous irrigation and sharp drills, as thermal necrosis may compromise the very stability we seek to build.  

Insertion Torque and Stability Quotient Monitoring

Insertion torque (IT) and implant stability quotient (ISQ) are the two primary quantitative measures of primary stability.

• Insertion Torque (IT): It’s measured in Ncm, and it represents the reflection of the rotational resistance encountered during implant placement. In soft bone, IT of >30 Ncm is often desirable for immediate loading protocols, which is achievable through strategic thread design and osteotomy refinement. 
  
• Implant Stability Quotient (ISQ): It’s measured via Resonance Frequency Analysis (RFA), and it provides a non-invasive measure of lateral stiffness. ISQ values above 60-65 is generally considered indicative of good primary stability. Moreover, monitoring ISQ values intraoperatively can validate surgical technique and implant selection. 
  
  

Prosthetic Considerations and Load Management

The achievement of high primary stability must be protected by a thoughtful restorative plan. 

• In challenging bone conditions, even with optimized thread design, the biological foundation is inherently more vulnerable. Thus, a progressive loading protocol is often recommended, utilizing provisional restorations to gradually condition the bone-implant interface before delivering the definitive prosthesis. 
  
• For single crowns, occlusal schemes should be designed to minimize non-axial loads.
  
• Cantilevers should be avoided whenever possible. 
  
  

Material and Surface Technology Synergy with Thread Design 

Although macro-design dictates the initial mechanical stability, the micro- and nano-scale surface characteristics dictate the subsequent biological response. Therefore, a synergistic relationship exists between thread design and surface technology. 

The Role of Surface Topography 

A moderately rough surface, between 1.0 and 2.0 µm, has been conclusively shown to accelerate bone apposition and enhance secondary stability compared to machined surfaces. When combined with an optimized thread design that provides immediate mechanical engagement, this surface treatment accelerates the transition from a mechanical bond to a biological one. As a result, the bone cells respond not only to the chemical cues of the surface but also to the mechanical environment created by the threads. 

Hydrophilic Surface Innovations 

Recent advancements have focused on chemically modifying implant surfaces to be super-hydrophilic. These surfaces attract blood components and facilitate rapid coagulation, effectively locking the implant into a fibrin scaffold that is rich in osteoprogenitor cells and growth factors. These surface treatments include SLA and other nano-structured, calcium-embedded surfaces. 

In low-density bone, where bone-to-implant contact (BIC) percentage is naturally lower, a hydrophilic surface can significantly increase the speed and degree of BIC, giving the implant a biological safe net that compensates for less ideal mechanical conditions. 

Immediate vs. Delayed Loading in Compromised Bone 

The decision to load an implant immediately or delay loading is directly related to primary stability, which is itself a product of bone quality and implant design. 

Criteria for Immediate Loading

The literature reports that immediate non-occlusal loading can be considered when primary stability is high enough (IT > 30-35 Ncm and ISQ > 65-70). In challenging bone sites, achieving these metrics often requires the use of implants with enhanced thread design. The aggressive threads provide the necessary mechanical retention to withstand the initial micro-motions that occur during the healing phase, preventing the formation of fibrous encapsulation.

Criteria for Delayed Loading 

When primary stability is suboptimal, often with IT < 20 Ncm and ISQ < 60, a delayed protocol is the safer approach. In these cases, the selected implant designs must still be capable of maintaining the existing stability. Surface treatments that promote rapid osseointegration are critical to swiftly establish secondary stability. The role of the thread design is to maintain the implant’s position without subsidence during the healing period. 

Complications, Maintenance and Long-Term Outcomes

An understanding of the long-term implications of thread design is essential for comprehensive treatment planning. 

Biomechanical Overload and Bone Loss

Implants designed for maximum stability in soft bone may generate excessively high stress concentrations at the crestal bone. This can lead to localized bone resorption via the process of bone remodeling according to Frost’s mechanostat theory. As a result, the most aggressive thread design is not universally indicated but is indicated for specific problems. 

Peri-implantitis and Maintenance Access 

The macro-threads of an implant are typically located at the apex, while the crestal module is often smoother to facilitate soft tissue attachment and allow easier debridement. Yet, in cases of advanced peri-implantitis where bone loss exposes the threaded portion, the complex geometry can complicate mechanical debridement. This frequent complication underscores the importance of a lifelong, structured maintenance protocol for all implant patients, regardless of the initial bone quality or implant design used. 

Clinical Recommendations and Considerations 

In clinical practice, the management of challenging bone conditions demands a systematic approach. It begins with a CBCT, 3D scans and digital assessment to accurately classify bone density and volume. 

Decision-Making for Type III and IV bone 

Select an implant system that features a differentiated thread design, such as the GDT ABA, MOR, RMB, CON NP, and CON RP implants. Dual-thread, progressive thread or deep square-thread designs, paired with modern surface treatments, are valuable options.

• Execute a refined surgical protocol using undersized osteotomies.
• Quantify your outcome by measuring both insertion torque and ISQ value.
• Finally, tailor the restorative protocol to the achieved levels of stability.
  
  

References

1. Yamaguchi, Y., Shiota, M., Fujii, M., Shimogishi, M., & Munakata, M. (2020). Effects of implant thread design on primary stability-a comparison between single- and double-threaded implants in an artificial bone model. International journal of implant dentistry, 6(1), 42. https://doi.org/10.1186/s40729-020-00239-1
   
2. Menini, M., Bagnasco, F., Calimodio, I., Di Tullio, N., Delucchi, F., Baldi, D., & Pera, F. (2020). Influence of Implant Thread Morphology on Primary Stability: A Prospective Clinical Study. BioMed research international, 2020, 6974050. https://doi.org/10.1155/2020/6974050
   
3. Oue, H., Doi, K., Oki, Y., Makihara, Y., Kubo, T., Perrotti, V., Piattelli, A., Akagawa, Y., & Tsuga, K. (2015). Influence of implant surface topography on primary stability in a standardized osteoporosis rabbit model study. Journal of functional biomaterials, 6(1), 143–152. https://doi.org/10.3390/jfb6010143