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In the intricate world of full arch dentistry, achieving a passive fit for implant-supported restorations is crucial for the long-term success of the prosthesis and the overall satisfaction of your patients. However, several factors can significantly impact the accuracy of your work, leading to potential complications and failures. Consider the fact that full-arch dentists have a margin of only 100 – 150 microns to achieve
passivity. That’s about the size of a human hair! Unfortunately, each stage of the prosthetic process can introduce small errors, which “stack up” and may lead to a poor fit. Even a discrepancy of just a few microns can affect the passivity of a full-arch restoration. Misfits can lead to complications that result in costly remakes and ultimately impact patient satisfaction.
This blog explores several “hidden enemies of passive fit” and the dangers they present in the success of your full-arch cases. These are widely accepted and understood principles in manufacturing are largely unknown in the dental industry. Unfortunately, each “hidden enemy” will have a significant impact on the fabrication of your final full-arch prosthesis. Being aware of these principles will help ensure a passive fit that minimizes stress and increases the likelihood of long-term success for your cases.
Topics explored:
One often overlooked factor that can significantly impact the accuracy of a case is mandibular flexure. Mandibular flexure refers to the deformation of the jaw in at least three directions due to non-masticatory physiological movements. This deformation can range from a few microns to 1 mm, with an average value of approximately 0.073 mm. In some cases, patients can experience more than 4 mm of elastic displacement of the mandibular condyles during mandibular movement.
The complex biomechanical behavior of the mandible is influenced by several factors,
including the attachment of masticatory muscles, the patient’s age, bone density, and
individual anatomical variations. During wide opening, protrusion, and lateral excursion movements, the mandible undergoes both mediolateral compression and dorsoventral shear, resulting in a three-dimensional deformation that can significantly impact the accuracy of impressions and the fit of prosthetic restorations.
In full arch implant cases, mandibular flexure becomes particularly problematic because dental implants are rigidly anchored to bone, unlike natural teeth which have a periodontal ligament that allows for minor movement. This rigid connection means that any flexure in the mandible will transfer stress directly to the implant-prosthesis interface, potentially compromising the integrity of the restoration and the health of the surrounding bone. Research studies have demonstrated that the greatest amount of mandibular flexure occurs during maximum opening of the mouth, precisely when impressions are typically taken. This timing creates a significant challenge for achieving accurate impressions and subsequently passive-fitting prostheses.
Mandibular flexure can cause misalignments in the final restoration, leading to a poor fit and increased stress on the implant abutments. When impressions are taken with the mouth widely open, the mandible is in a deformed state. As the mouth closes, the mandible returns to its resting position, creating a discrepancy between the impression and the actual position of the implants.
This discrepancy can result in prosthetic misfits that place undue stress on the implant components and surrounding bone, ultimately increasing the risk of biological and mechanical complications.
When different materials are heated from room temperature (20°C) to body temperature (37°C), they expand at varying rates, creating critical challenges for full arch restorations.
Zirconium is the most thermally stable material, with a coefficient of just 5.7 μm/(m·K) and only 0.8 μm of expansion. Its minimal dimensional change under temperature fluctuations.
Titanium and steel have minimal thermal expansion, with 1.2 μm and 1.8 μm of expansion, respectively. Titanium’s modest expansion, combined with its excellent biocompatibility, explains why it is the preferred choice in full-arch dentistry.
Aluminum has the highest coefficient of linear thermal expansion at 23.1 μm/(m·K), leading to a 3.1 μm expansion over the 17°C temperature change. This significant expansion explains why aluminum components used in impression copings or healing abutments can introduce errors when moving from room temperature to oral
temperature.
When designing surgical guides or provisional restorations with aluminum components, clinicians must account for this expansion to prevent fit discrepancies that could compromise implant positioning or prosthetic outcomes.
Plastics and PEEK polymer have the highest thermal expansion at 50 μm/(m·K), leading to a 6.8 μm increase in size. This significant expansion rate makes plastic materials particularly susceptible to temperature-induced dimensional changes. Temporary abutments or scan bodies made from PEEK can introduce substantial errors. While PEEK offers advantages like shock absorption and weight reduction, its high thermal sensitivity presents dire consequences for precision applications.
The thermal expansion properties of materials directly affect outcomes. When designing full-arch restorations, the cumulative effect of expansion across components can greatly influence passive fit. Poor material choice may compromise prosthetic accuracy.
Error propagation refers to how uncertainty in the measurement process affects the result of calculations. Small measurement errors can accumulate and lead to significant deviations in the final product. In full arch dentistry, where precision is measured in microns, these propagated errors can be the difference between a successful restoration and clinical failure. Error propagation follows mathematical principles where errors compound through each step of the process. For example, a small deviation in the impression stage combines with minor inaccuracies in the scanning process, which further combine with manufacturing tolerances. The result is a cascade of errors that amplifies throughout the workflow.
Errors can originate from various sources throughout the digital and conventional workflow:
Small errors in each step of the measurement process can accumulate, leading to significant deviations in the final restoration. This can result in a poor fit, increased stress on the implant abutments, and potential implant failure. A deviation of just 30 microns at the impression stage can multiply to more than 100 microns by the time the final prosthesis is fabricated, enough to compromise passivity and long-term success.
Error propagation can result in less predictable outcomes, compromising the overall accuracy and reliability of the restoration. This can lead to patient dissatisfaction and the need for additional corrective procedures. The unpredictability increases with the complexity of the case and the number of implants involved, as each additional step or component introduces new opportunities for error to accumulate.
Tolerance stacking refers to the accumulation of individual tolerances within an assembly, resulting in an overall error in the final product.
Individual tolerances can “stack” together, leading to a significant overall error in the final restoration. This can result in misalignments, poor fit, and increased stress on the implant abutments.
Tolerance stacking can cause prosthetic failures, such as fractures of implant screws or porcelain crowns, leading to increased rates of remakes and adjustments.
Imagine constructing a tower with building blocks. If each block is slightly uneven, the tower becomes unstable and may topple over. This analogy shows how cumulative errors can lead to instability and failure.
Heat transfer refers to the movement of thermal energy from one object to another. When ScanBodies are subjected to temperature changes, they undergo thermal expansion, which can affect the precision of the measurements.
Heat transfer can cause ScanBodies to expand or contract, leading to misalignments in the final restoration. This can result in a poor fit and increased stress on the implant
abutments.
Different materials expand at different rates, which can lead to inaccuracies in the measurements. This can compromise the overall accuracy of the restoration and lead to increased rates of remakes and adjustments.
Visualize heat transfer as a road trip with multiple stops. If each stop is slightly off course, the overall journey will deviate significantly from the planned route. This illustration shows how small deviations at each step can lead to a large overall error.
ICam’s precision measurements provide the most predictable and reliable results on the
market, giving dentists the best chance at achieving a passive-fit restoration. Here are some key reasons why ICam stands out:
ICam’s measurements are both precise and true, ensuring that each restoration fits perfectly with minimal stress on the implant abutments.
ICam minimizes the impact of error propagation, leading to more reliable and consistent outcomes.
ICam effectively addresses tolerance stacking, helping achieve passivity in restorations.
With ICam, manufacturers can identify and mitigate variability, reduce defect rates, and enhance process reliability.
When comparing ICam to other photogrammetry systems, several key differences become apparent. Systems like MicronMapper, Tupel, PIC, Shining 3D Aoral Elite, and IOS methods have been investigated, and the results highlight significant shortcomings:
These systems often struggle with achieving the same level of precision and trueness as ICam. The lack of four moving cameras results in less accurate triangulation, reduced redundancy, and a higher susceptibility to error propagation. As a result, they are less reliable in complex full-arch cases where even the smallest deviation can compromise the passive fit of the final prosthesis.
Unvetted and preliminary data shows that Shining 3D Aoral Elite is not as reliable, especially in the presence of blood, saliva, and other intraoral challenges. These clinical conditions can interfere with accuracy, resulting in inconsistent data capture and reduced confidence for full-arch implant cases.
Intraoral scanners (IOS) are generally less accurate than dedicated photogrammetry systems like ICam. This method is highly technique-sensitive, not only depending on the operator’s skill but also influenced by patient movement, saliva, blood, and limited intraoral access. While IOS may be sufficient for single-unit or short-span restorations, it often struggles to maintain the accuracy and trueness required for full-arch implant cases, leading to potential misfits and increased need for adjustments.
For full arch dentists, choosing the right photogrammetry solution is crucial for the success of implant-supported restorations. ICam’s superior predictability, precision, and reliability make it the best choice for achieving passivity and ensuring long-term success. By leveraging the principles of Six Sigma and maintaining maximum accuracy at each stage of the restorative process, ICam provides dentists with the confidence and peace of mind they need to deliver exceptional results for their patients.
With only 100 microns of allowable error, precision is an essential requirement for full-arch dentistry. The hidden dangers we’ve discussed can have profound effects on long-term outcomes. By carefully choosing your method for capturing implant positions, you’re ensuring a foundation of accuracy that will prevent costly failures and complications down the road.
Saving a few dollars on inferior methods for capturing implant position can lead to
significant long-term costs in failures and complications. The precision of ICam isn’t just a luxury; it’s an investment in your practice’s reputation and your patients’ satisfaction.
Copyright 2024 Imetric4D
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