📄 medicine

Dependency of dental implants’ primary stability on rotational speed of insertion: In-vitro investigations in synthetic and natural bone

This in-vitro study utilizing CNC-controlled insertion in synthetic and natural bone demonstrates that increasing the rotational speed from 5 to 25 rpm significantly improves several primary stability indicators, whereas further increasing the speed to 50 rpm yields no additional benefit.

Original authors: Mahdi Farrokhi Kashtiban, Tahereh Mirzaie, Gholamreza Rouhi

Published 2026-10-22

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Mahdi Farrokhi Kashtiban, Tahereh Mirzaie, Gholamreza Rouhi

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

When a tooth is lost, a titanium screw can be planted into the jawbone to hold a new crown in place. For this artificial root to become a permanent part of the body, it must first survive a critical waiting period where it relies entirely on a tight mechanical grip within the bone. This initial grip, known as primary stability, is the foundation for the biological process that eventually fuses the metal to the living tissue. If the screw wobbles even slightly during those early weeks, the healing process fails. While surgeons know that the shape of the screw and the quality of the bone matter greatly, one variable has remained a mystery: does the speed at which the screw is twisted into the bone change how well it holds?

To answer this, researchers at Amirkabir University of Technology in Iran designed a precise experiment to test how the rotation speed of a dental implant affects its initial stability. They used sixty identical screws, each four millimeters wide and ten millimeters long, and inserted them into two different types of material. One material was a block of rigid polyurethane foam, a synthetic substance often used in labs because its properties are perfectly uniform and repeatable. The other was fresh bone from a cow's rib, which mimics the complex, uneven structure of human bone. To ensure that human hands or fatigue did not influence the results, a computer-controlled machine performed every single insertion. The machine twisted the screws into the materials at three specific speeds: a slow five rotations per minute, a medium twenty-five rotations per minute, and a fast fifty rotations per minute.

The team measured the stability of each screw using several different methods. They recorded the maximum force required to twist the screw in, the total energy used during the twisting process, and a resonance score that indicates how firmly the screw sits in its hole. They also performed a destructive test where they pushed the screw straight down until it failed, measuring how much force and energy the bone-screw combination could withstand before breaking. The results revealed a clear pattern in the synthetic foam: as the speed increased from five to twenty-five rotations per minute, the stability indicators improved significantly. The screws held tighter, required more energy to push out, and showed higher resonance scores. However, when the speed increased further from twenty-five to fifty rotations per minute, the stability did not improve any further. The extra speed simply did not add any extra grip.

The story was slightly different in the natural cow bone. Here, the speed of insertion did not significantly change the force required to twist the screw in, nor did it change the total energy used during the process. Yet, the other stability measures told a similar story to the foam. The resonance score and the energy required to push the screw out both increased significantly when the speed went from five to twenty-five rotations per minute, but again, increasing the speed to fifty rotations per minute offered no additional benefit. The researchers noted that the synthetic foam and the natural bone behaved differently regarding the force needed to twist the screw in, suggesting that the foam does not perfectly copy how real bone reacts to speed. This difference highlights that while synthetic materials are useful for controlled testing, they cannot fully replicate the complex, rate-dependent behavior of living bone.

Ultimately, the study suggests that for the materials and conditions tested, twisting the implant faster than twenty-five rotations per minute provides no extra advantage in terms of initial stability. The biggest gain in holding power occurs when moving from a very slow speed to a moderate one. Going even faster does not make the implant more secure, though it does finish the job in less time. The findings indicate that surgeons might not need to push for higher speeds to achieve better results, and that the choice of twenty-five rotations per minute could be a sweet spot that balances efficiency with maximum initial stability.

Technical Summary: Dependency of Dental Implants' Primary Stability on Rotational Speed of Insertion

Problem Statement
Primary stability is a critical prerequisite for the osseointegration and long-term success of dental implants. While it is well-established that primary stability depends on bone quality, implant macro-geometry, and surgical technique, the specific influence of the insertion rotational (IR) speed remains unclear. Existing literature presents conflicting findings: some in-silico studies suggest IR speed has no significant effect on maximum insertion torque (MIT), while experimental studies in synthetic foam indicate that increasing IR speed can reduce MIT and alter the Implant Stability Quotient (ISQ). Furthermore, previous experimental evidence has been largely confined to synthetic substrates (PU-foam) and non-destructive stability measures, leaving a gap in understanding how IR speed affects the mechanical strength of the implant-bone interface in natural bone. This study aims to determine if and how IR speed affects primary stability across both synthetic and natural bone substrates using a comprehensive set of stability indicators.

Methodology
The study utilized an in-vitro experimental design involving 60 identical dental implants (4 mm diameter, 10 mm length) inserted into two distinct substrates:

Synthetic Bone: Rigid polyurethane (PU) foam with a density of 0.2 g/cm³.
Natural Bone: Bovine ribs from a single 2.5-year-old bull, selected via CT imaging to ensure uniform bone quality (average 500 Hounsfield Units, corresponding to Type 4 bone).

To eliminate human-related variability, all drilling and insertion procedures were performed using a Computer Numerical Control (CNC) machine. The implants were inserted at three rotational speeds: 5 rpm (low), 25 rpm (medium), and 50 rpm (high), representing the clinically relevant range. Ten implants were tested per speed per substrate.

Primary stability was assessed using six complementary metrics:

Insertion Phase: Maximum Insertion Torque (MIT) and Insertion Energy (IE).
Resonance Response: Implant Stability Quotient (ISQ) measured via Osstell.
Mechanical Strength (Destructive): Push-in test parameters, including push-in stiffness, push-in ultimate load, and push-in energy (PIE).

Statistical analysis was conducted using Welch's ANOVA and pairwise Welch's t-tests with Holm adjustment for multiple comparisons.

Key Results
The study revealed that the effect of IR speed on primary stability is dependent on both the substrate type and the specific stability indicator used:

Synthetic Bone (PU-foam):
- MIT: Showed a statistically significant overall dependence on IR speed ( $p=0.0466$ ), with mean torque increasing from 5 to 25 rpm. However, no pairwise comparisons remained significant after Holm adjustment.
- IE: Increased significantly from 5 to 25 rpm and from 5 to 50 rpm, but no significant difference existed between 25 and 50 rpm.
- ISQ & PIE: Both increased significantly when speed rose from 5 to 25 rpm and from 5 to 50 rpm. No significant difference was found between 25 and 50 rpm.
- Ultimate Load: Significantly increased from 5 to 25 rpm and 5 to 50 rpm, with no significant change between 25 and 50 rpm.
- Stiffness: No significant effect of IR speed was detected ( $p=0.078$ ).
Natural Bone (Bovine Rib):
- MIT & IE: Neither metric showed a significant dependence on IR speed ( $p=0.246$ and $p=0.949$ , respectively).
- ISQ: Increased significantly from 5 to 25 rpm ( $p=0.012$ ), but no significant difference was observed between 5 and 50 rpm or 25 and 50 rpm.
- Ultimate Load: Showed a significant increase from 5 to 50 rpm ( $p=0.024$ ), though the increase from 5 to 25 rpm was not statistically significant ( $p=0.055$ ).
- PIE: Increased significantly from 5 to 25 rpm and 5 to 50 rpm, with no significant change between 25 and 50 rpm.
- Stiffness: No significant effect of IR speed was detected ( $p=0.121$ ).

Key Contributions and Claims
The primary contribution of this work is the systematic comparison of IR speed effects across synthetic and natural bone using a controlled CNC environment and a multi-metric assessment strategy. The authors claim the following:

Optimal Speed Range: Increasing IR speed from 5 to 25 rpm improves several stability indicators, particularly ISQ and push-in energy, in both substrates. However, a further increase to 50 rpm does not yield significant additional improvements in these metrics.
Substrate Discrepancy: The study highlights a divergence between synthetic and natural bone regarding rate-dependent behavior. While PU-foam showed a dependence of MIT and IE on IR speed, natural bone did not. The authors suggest that PU-foam may not fully replicate the non-homogeneous, anisotropic, and rate-dependent properties of natural bone, particularly regarding torque and energy metrics.
Stiffness Independence: The elastic stiffness of the implant-substrate construct (measured via push-in stiffness) appears insensitive to IR speed in both substrates, suggesting that while the yield properties (ultimate load, energy) may be affected by insertion rate, the initial elastic response remains stable.
Mechanism of Improvement: The authors propose that the improved stability at higher speeds (5 to 25 rpm) may be due to reduced insertion time, which limits the accumulation of microdamage and microcracks in the material adjacent to the implant threads. Conversely, the lack of improvement beyond 25 rpm suggests a plateau in this benefit.

Significance
The paper concludes that while increasing IR speed from 5 to 25 rpm offers benefits for primary stability indicators like ISQ and push-in energy, increasing the speed further to 50 rpm provides limited additional advantage. The findings caution researchers against directly extrapolating results from synthetic bone (PU-foam) to natural bone, particularly when using MIT and IE as stability indicators. The study emphasizes the need for careful interpretation of stability metrics when differentiating between substrate types and suggests that the current range of clinical IR speeds (up to 50 rpm) may not require further optimization for the specific stability metrics measured, provided the speed is at least 25 rpm.

Technical Summary: Dependency of Dental Implants' Primary Stability on Rotational Speed of Insertion

More like this