Effect of mismatch on maintaining preload in implant prosthesis retention screws: An in-depth analysis

Text by Francesco Biaggini

Link to PFD file

Implant-supported denture retention screws are the silent heroes of modern implantology, ensuring the stability and functionality of implant structures.

However, even the smallest mismatch between components can compromise their ability to maintain preload, a critical force for the stability of the entire system?

This study, based on the article "Effect of Misfit on Preload Maintenance" by Wirley Gon?alves Assun??o et al., explores how different levels of misfit affect the ability of screws to maintain preload over time, showing that even minimal misalignments can cause a significant reduction in preload, leading to the risk of screw loosening and prosthesis failure.?

Reducing mismatch is not just a technical challenge, but an essential investment in patient health and the longevity of implant rehabilitations.

The clinical implications are clear: every detail counts. Through accurate application of torque, the use of advanced materials, and the practice of retightening, we can ensure greater resilience of the implant system.

The ultimate goal is to offer patients long-lasting solutions, restoring their confidence in a stable and confident smile, because true innovation lies in precision and attention to detail.

Objective of the study and methodology

The primary objective of this study was to evaluate the effect of different levels of unilateral mismatch on maintaining the preload of retention screws in implant prostheses.

?Preload is a crucial factor in ensuring the stability of the connection between implant and abutment, and the initial hypothesis was that even small misfits could significantly compromise the long-term stability of the prosthesis.

To achieve this, 48 metal crowns were created using UCLA co-Cr alloy abutments. The crowns were divided into four groups of 12 units each, with mismatches designed to represent different levels of deviation:

?·?????? Group A: No mismatch (control)

·?????? Group B: 50 μm mismatch

·?????? Group C: 100 μm mismatch

·?????? Group D: 200 μm mismatch

Each crown was attached to external hexagon implants using titanium retention screws. The initial preload was applied using a torque wrench, setting a value of 30 N/cm to ensure uniform tightening in all configurations. After applying the preload, each sample was allowed to rest for 2 minutes to allow the materials to relax, a critical step to assess the immediate loss of preload due to elastic and plastic deformation.

?Next, the detorque force (i.e. the force required to loosen the screw) was measured using an analog dynamometer.

The detorque measurement was repeated for three screws on each implant, for a total of 72 detorque measurements per group.

This procedure was implemented to ensure the reproducibility of the data and to minimize the experimental error.

Statistical analysis of the data was conducted using ANOVA (Analysis of Variance) to determine the significance of differences between groups, followed by Tukey's test to identify significant differences between levels of maladaptation.

?These techniques allowed to better understand the impact of mismatch on preload values and to highlight which levels of mismatch were most critical for maintaining the long-term stability of the retaining screws.

The statistical analysis conducted highlighted some fundamental results that provide an in-depth understanding of the effects of mismatch on the preload of retaining screws:

?? All groups showed a significant reduction in preload compared to the initial value of 30 N/cm. This confirms that, regardless of the level of mismatch, there is an inevitable loss of preload due to the relaxation of the materials and elastic and plastic deformation.

?? Group A (no mismatch) kept the preload value closer to the initial one, with an average of 25.18 N/cm and a preload reduction of 16.1%. This group represents the ideal condition, in which the implant-prosthesis connection is optimal and the risk of screw loosening is minimal.

?? The maladjustment groups (B, C and D) showed a significantly greater reduction in preload than Group A. In particular, Group C (100 μm) showed the greatest reduction in preload, with an average of 22.47 N/cm, corresponding to a reduction of 25.1%.

This indicates that the 100 μm level of mismatch represents a critical point beyond which the stability of the system can be seriously compromised.

?? Group D (200 μm) recorded a 23.0% reduction in preload, similar to that of Group C, but without a statistically significant difference compared to Group B (50 μm).

This suggests that beyond a certain level of malmatch, the effect on preload tends to stabilize, probably due to an already significant deformation of the components involved. These results clearly indicate that even small misfits, such as that of 50 μm, can significantly compromise the long-term stability of implant-supported prostheses.

?It is important to note that the drop in preload becomes more pronounced as mismatch increases, highlighting the need to minimize any coupling error between crown and abutment.

Group A has shown that a well-fitting connection is crucial to reduce the risk of loosening and ensure a longer duration of implant treatment.

In summary, the analysis of the results emphasizes the importance of accurate and precise fitting of implant components to preserve screw preload and prevent future complications.

The reduction in preload observed in all malmatch groups demonstrates that even minor misalignments can have a significant impact, making rigorous control during the design and assembly phase of the prosthesis essential.

Clinical implications?

These results underscore the importance of minimizing mismatch between implant components to maintain the mechanical stability of the screw and prevent its loosening.

Dentures that do not adhere snugly to the abutment, even with misalignments as small as 50 μm, have shown a significant drop in preload, leading to a higher risk of screw loosening and potential denture failure.

?Preload loss ranged from 16.1% to 25.1% depending on the degree of mismatch.

?These data align with other studies that highlight how mismatch creates mechanical stresses in the components, increasing the flexibility of the screw and predisposing it to loosening under cyclic chewing loads.

In particular, studies such as those by Binon and McHugh confirm that a well-fitted implant-prosthesis connection offers greater resistance to unscrewing than those with mismatch.?

In addition, the article "Misfit simulation on implant prostheses" highlights that the use of non-engaging abutments is more tolerant of mismatches than engaged abutments, suggesting that selecting the correct abutment can significantly affect implant stability.

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Preload management and torque application?

Preload management and torque application are key aspects to ensure the long-term stability of implant-supported prostheses. Accurate application of the preload helps keep the implant-abutment connection stable and reduce the risk of screw loosening.

The main preload and torque management strategies are explored below, based both on the article under analysis and on other previously studied clinical evidence.?

Using Calibrated Torque Wrenches Applying the correct torque is essential to ensure the right preload in the retention screws. The use of calibrated torque wrenches allows precise torque to be applied, reducing variability related to human error. An inadequate torque, in fact, can lead to both insufficient preload, with the consequent risk of loosening the screw, and excessive preload, which can damage the thread or cause plastic deformation of the screw.

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Ritightening After initial tightening, the retaining screws experience a loss of preload due to the elastic and plastic relaxation of the materials. To compensate for this loss, it is advisable to practice retightening after a short period (generally after 10-15 minutes from the initial application). This re-tightening allows to restore the desired preload and minimize the micro-movements of the prosthetic components, improving the stability of the entire system.

?Influence of Materials and Surface Treatments The choice of material of retention screws plays a crucial role in their ability to maintain preload. Titanium screws are commonly used for their biocompatibility and corrosion resistance, but treatments such as Titanium Nitride (TiN) coating can further increase surface hardness and reduce wear.

This type of coating helps to improve the reliability of the screws, reducing the risk of loosening and improving resistance to mechanical stress.?

Abutment Engaging vs Non-Engaging The article "Misfit simulation on implant prostheses" highlights that the use of non-engaging abutments can better tolerate misadaptations than engaging abutments.

Non-engaging abutments have an increased ability to absorb small misalignments, thereby reducing unwanted forces that can contribute to screw loosening. Choosing the right abutment for your specific clinical situation can make all the difference in ensuring a stable and long-lasting connection.

?Masticatory Loads and Elastoplasty Deformation During the masticatory function, implant prostheses are subjected to cyclic loads that can negatively affect the ability of the screws to maintain preload. The elastoplastic relaxation of the components can lead to a progressive reduction in preload, making the screws more susceptible to loosening. Adopting a prosthetic design that distributes loads evenly and minimizes stress spikes is essential to preserve preload over the long term.

?Use of MUAs (Multi-Unit Abutments) The use of MUAs (Multi-Unit Abutments) represents an additional tool to improve preload management in implant-supported prostheses. MUAs are designed to facilitate the connection between implants and final dentures, better distributing chewing loads and reducing mechanical stress on retention screws. Their ability to tolerate small angular discrepancies helps minimize the risk of mismatch and improves the long-term stability of dentures, especially in cases of multiple implants.

Validatable Fingerprint Detection Systems: ScanTransfer Using advanced fingerprint detection systems, such as ScanTransfer, allows for accurate recording of implant position and reduced coupling errors. The ScanTransfer allows the precise transfer of the implant position from the patient to the laboratory, ensuring better fit of the prosthetic components and reducing the risk of mismatch. The accuracy of the impression transfer process is crucial to preserve the preload of the retention screws and prevent future complications.

?Advanced Tightening Techniques It has been shown that the application of progressive torque, in which the torque is applied in several stages rather than in a single operation, can contribute to a better distribution of forces between components and reduce the risk of local deformations. This technique allows the desired preload to be gradually achieved, reducing the effects of immediate relaxation after tightening.?

Regular Monitoring and Maintenance Finally, a key element for preload management is the regular monitoring of implant prostheses. Periodic maintenance, with specific checks of the torque of the screws, allows timely intervention in the event of loosening or loss of preload.

This proactive approach is essential to prevent long-term complications and ensure successful implant treatment. In summary, preload management and torque application require meticulous attention to detail and the adoption of advanced techniques and materials to ensure the stability of the prostheses over time.

Each component of the implant system must be carefully selected and managed to minimize the risk of mismatch and to ensure a solid and durable implant-prosthesis connection.

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Conclusions and recommendations

Prosthetic connection screws are true silent heroes of implantology, ensuring the stability and functionality of implant-supported prostheses. However, as highlighted by this study, the main problem that compromises the maintenance of preload and, consequently, the long-term success of the implants, is the mismatch between the components. Even the slightest mismatch can cause significant loss of preload, leading to unscrewing and potential clinical failure.

Preload is the invisible force that holds the prosthetic structure together, and maintaining it is essential to ensure the stability and durability of the prosthesis. Reducing maladjustment is not just a technical choice, but a fundamental investment in the patient's health and quality of life. Misfits are the main cause of mechanical problems in implant restorations, as they create stress that is reflected in all components of the system, from screws to abutments.

Every detail, from component selection to applying the correct torque with calibrated torque wrenches, can make the difference between a treatment that lasts years and one that fails prematurely. The use of non-engaging abutments, the practice of retightening and the adoption of innovative materials, such as TiN coating, are all tools that the clinician has at his disposal to increase the resilience of the implant system and minimize the risk of mismatching.

?Ultimately, the goal is to guarantee the patient a reliable and lasting solution, which allows them to regain confidence in their smile and quality of life. Each screw tightened correctly represents a step towards a future where patients can live without worries, confident that their rehabilitation is solid and stable. As professionals, we have a responsibility to leave nothing to chance: every little detail counts, and every careful choice can turn a treatment into lasting success.