Assessing the relationship between mini-screw implant shape and primary stability: An experimental study in orthodontics

Abstract

OBJECTIVES:

Mini-screw implants (MSI) are widely used in orthodontics as temporary anchorage device. Implant geometry, including shape and diameter, is a major contributor to their mechanical retention, and osseointegration. This study aimed to evaluate the influence of MSI shape (cylindrical vs. tapered), and diameter (1.2 vs. 1.4 mm) on peak removal torque (PRT) over different subsequent periods with 7 mm length for all used MSI.

MATERIAL AND METHODS:

A total of 160 titanium mini-screws were inserted into the tibiae of three adult sheep. Implants were divided into four groups according to shape and diameter, further subdivided by different post-insertion removal intervals: immediate, 2 weeks, 4 weeks, and 6 weeks. As eight implants per animal multiple by three animals result in 24 implants per group per time. A standardized surgical protocol was used, and PRT was measured using a digital torque driver. Statistical analysis was conducted using three-way ANOVA, where P ≤ 0.05.

RESULTS:

PRT values increased significantly across all groups over time. Larger diameter implants (1.4 mm) exhibited significantly higher torque values compared to 1.2 mm implants at all time points. Cylindrical MSI generally outperformed tapered implants at 4 and 6 weeks. The highest torque values were recorded at 6 weeks in all groups.

CONCLUSION:

Implant diameter and shape significantly affect MSI stability. Larger diameters provide superior mechanical retention, while cylindrical shapes may offer advantages in later healing stages. These findings support selection of MSI geometry to optimize clinical outcomes in orthodontic anchorage.

Introduction

Temporary anchorage devices (TADs) are today an integral part of modern orthodontics, providing absolute anchorage by directly engaging the jaw bone. A plethora of terminologies for these devices in the literature can be seen, for example, mini-screws, micro-screws, micro-implants, and mini-implants, all meaning small, osseous anchorage devices used to facilitate orthodontic tooth movement.[1,2]

In terms of structure, the universal mini-screw implant (MSI) consists of four major components: the body, collar, neck, and head.[3] Each of these components has significant roles in the engagement of the implant with the bone and to its mechanical stability and performance.[4]

Osseointegration refer to the immediate structural and functional connection of living bone to the surface of a bearing implant that was originally defined by Brånemark et al. in the 1960s.[4,5] Extrapolations from their light microscopic observations of bone-implant contact set an era of revolution in implant dentistry and placed the stage for the development of titanium-based implants for use in skeletal anchorage.[6]

A key factor in the clinical success of TADs is primary stability, which refers to the mechanical stability of the implant immediately after insertion. Over time, as bone healing and remodeling occur, secondary stability develops through biological processes associated with osseointegration.[6,7] Together, these forms of stability determine the implant’s resistance to micro-motion that represent a crucial indicator of its success. The resistance of an implant to removal torque is a reliable indirect measure of implant–bone contact and, by extension, its stability. The removal torque test, first proposed by Roberts et al. and later refined by Johansson and Albrektsson, quantifies the rotational force required to disrupt the bone-implant interface, offering a functional evaluation of anchorage strength.[7,8,9]

Given that both implant shape and diameter are known to influence primary stability, the present study aims to evaluate, and compare the stability of MSIs with different shapes (cylindrical vs. tapered) and diameters (1.2 and 1.4 mm) over varying healing intervals on living animal model. This evaluation was performed in living being to find out a biological resonance, and simulate implant behavior during clinical application. Also, this study aimed to observed the mechanical effects of long-term duration extended to 6 weeks after initial insertion. Removal torque values are used as the primary outcome measure to assess how implant geometry impacts initial and late mechanical retention and the development of osseointegration over time.

Materials and Methods

Ethical approval: all the procedure of this research was evaluated and authorized by the ethics committee of the university of Mosul, collage of Dentistry with approval ID: UoM.Dent. 251095.

Research strategies

The experimental procedures of this research were carried out between March and May 2025 in a designated and fully equipped laboratory affiliated with the College of Veterinary Medicine, University of Mosul. The laboratory provided the necessary facilities and controlled environment to ensure the accuracy, consistency, and reliability of the experimental work.

Mini-screw implants

One hundred and sixty micro implants, type is of Absoanchor®, Dentos, Taegu, Korea. Made of Titanium alloy (Ti6Al4Va). Classified as 80 implants of tapered and cylindrical shape with identical length (7 mm). Two main diameters were precised for this research including (1.2 and 1.4 mm). These implants were sub-divide into four groups according to the date of unscrew including four precised time points (immediate, 2 weeks, 4 weeks, and 6 weeks interval) after insertion.

Experimental animal

After ethical consideration, three adult sheep were selected for this experimental study with average weight near 50 kg per sheep, that had passed the age of puberty[10] that is between 5 and 8 months[9,10], with age between 12 and 19 months old. The sheep were subjected to clinical veterinarian examination for the respiration rate, heart rate, temperature, and activity. The normal physiological parameters include a body temperature of 38.9 ± 0.5°C, a heart rate ranging from 50 to 80 beats per minute (with an average of 75 bpm), and a resting respiratory rate between 15 and 40 breaths per minute.[11,12] All study samples (sheep) used in this study were maintained on a nutritionally balanced diet based on established recommendations for small ruminants used in biomedical research. Fresh water was provided This examination was proceed throughout the study, under a supervision of specialist in veterinary medicine.

Surgical procedure

All instruments were initially sterilized using a Class B autoclave (Steri24, CertoClav, Austria). Anesthesia was administered to the sheep by intramuscular injection of Xylazine base (0.15 mg per 10 kg of body weight) and Ketamine hydrochloride (500 mg) into the inner thigh. Within approximately 5–10 min, the animals exhibited signs of dizziness, followed by loss of consciousness. Subsequently the surgical procedure was performed. The surgical site over the tibia was prepared by shaving the external wool to expose the skin surface. A surgical scalpel and blade No. 11 were used to incise the skin and periosteum, thereby exposing the underlying bone. Continuous irrigation with normal saline was applied during the procedure. Drilling was performed using a hand piece from Kazan Medical Instruments (Kazan, Russia), with the speed set at 2000 RPM. This controlled drilling speed was selected to optimize osseointegration, while minimizing heat generation, thereby reducing the risk of creating a devitalized zone around the implant during placement.[13] the same experienced operator performed all surgical procedures to ensure consistency. The drilling protocol, insertion depth, and angulation were standardized for all implants.

To avoid site-related bias, implants from the four experimental groups (Cylindrical 1.4 mm, Cylindrical 1.2 mm, tapered 1.4 mm, tapered 1.2 mm) were randomly allocated to predefined insertion sites on the proximal, and distal positions of tibial surface of each animal. A randomization list generated by computer-generated randomization schedule before surgery ensured that each tibia received a balanced distribution of MSI types and diameters, as each tibia received eight implants, resulting in 16 implants per animal and 48 implants total per time point, with 160 implants overall.

For each implant, insertion torque was recorded using a digital torque driver (DID4, Cedar, Al-Berquque Industry, Michigan, USA). Torque values were monitored during placement to confirm consistent insertion forces across all groups and ensure appropriate primary stability by (Model XTD-20, Tohnichi Manufacturing Co., Tokyo, Japan). with an accuracy of ± 0.1 Ncm and was calibrated immediately prior each time interval according to manufacturer instructions. The mean insertion torque values were reported for Cylindrical 1.4 mm, 1.2 mm was 12.8 ± 0.6 Ncm, and 10.9 ± 0.5 Ncm respectively, while the Tapered 1.4 mm, 1.2 mm monitor as 13.2 ± 0.7 Ncm, and 11.4 ± 0.6 Ncm, respectively. Osteotomy preparation was carried out using Dentos drilling burs with diameters slightly smaller than those of the corresponding MISs, in accordance with the manufacturer’s guidelines. Specifically, a 1.2 mm diameter bur was employed for the insertion of 1.4 mm implants, as recommended in the Dentos MSI instruction manual. The cortical thickness of the tibial bone in adult sheep was reported to range between 6.35 and 7 mm, providing sufficient bone volume for secure implant placement and primary stability.[14,15] The spacing between the holes for the micro implants was set at 4 mm. This spacing will not affect the osseointegration process, as the minimum recommended distance between adjacent implants is 3 mm.[16] The mini-screws were then inserted into the bone using a digital insertion torque gauge screwdriver until they reached the neck of the implants. This procedure was repeated for 10 MISs in each tibia. To minimize experimental bias, randomization was used to assign different implant shapes and diameters to the tibial sites in each sheep. Additionally, the operator who recorded the removal torque values was blinded to the group allocation to prevent measurement bias. After all screws were inserted, the surgical site was thoroughly irrigated with sterile saline, cleansed using sterile gauze, and the wound margins were approximated and closed with 3-0 silk sutures. A sterile gauze dressing was applied over the incision site, followed by external bandaging with plaster for protection. Postoperatively, the animals received intramuscular Oxytetracycline base (200 mg; 1 ml/10 kg body weight) once daily, along with Diclofenac sodium (75 mg) administered every 24 h for three consecutive days to manage infection and inflammation. Veterinary monitoring was conducted daily to assess general health and postoperative recovery. Ten days after surgery, the bandages were removed, revealing complete wound healing without signs of suppuration. Subsequently, all sutures were removed under aseptic conditions.

Unscrewing of mini-screw implants

Following flap reflection, the overlying soft tissue and callus surrounding the MSI were removed. A percussion test was then performed to evaluate the success rate and detect any signs of mobility prior to implant removal.

One MSI from the 6-week group (1.2 mm cylindrical type) fractured during the process and was excluded from the final analysis. Additionally, two MSI from the 2-week group (one cylindrical and one tapered) failed the percussion test and were classified as failures.

No fractures or failures were observed in the remaining implants, all of which were removed without any visible deformation of the screw head or long axis.

The overall success rate across all MSI was 98%.

The Digital insertion screw driver (DID4) (Cedar, Al-Berquque Industry, Michigan, USA) was used for evaluation of the torque required for microimplant removal, at four-time interval (20 MSI for each) including immediately after insertion, 2 weeks after, 4 weeks after and the last 20 implant were removed after 6 weeks period. All the MSI were removed with same operation by DID4 and the peak removal torque (PRT) was recorded for each micro implant independently. The range of the device measurements is between 5 and 50 N/Cm.

Statistical analyses

The collected output of PRT were uploaded and analyzed using Statistical Package for Social Sciences (SPSS) version 23. Including the Descriptive statistics (means, standard deviations. Shapiro–Wilk and Levene’s test were performed to confirm the normality and homogeneity of variance assumptions.

Because multiple MSI were placed within each animal, the data violate assumptions of independence. To address this, a mixed-effects model was used, with Shape, Diameter, and Time included as fixed factors and Animal ID included as a random intercept to account for within-animal clustering (pseudo-replication). The model was fitted using restricted maximum likelihood (REML). The mixed-effects results supported the findings from the three-way ANOVA, confirming that the effects of implant geometry and time on PRT were statistically robust. A threshold of P ≤ 0.05 was used to determine statistical significance.

Results

The descriptive statistic including the mean and standard deviation of PRT values for all MSI used in this study including different shapes and diameters, that were measured at post-insertion intervals (immediate, 2 weeks, 4 weeks, and 6 weeks) are summarized in Table 1 and Figure 1.

Table 1.

The mean (standard deviation) of peak removal torque values (Ncm) for different mini-screw implants types and diameters across time periods

Time period	Cylindrical 1.4 mm	Cylindrical 1.2 mm	Tapered 1.4 mm	Tapered 1.2 mm
2 weeks	13.72 (0.433)	10.54 (0.291)	13.38 (0.732)	12.26 (0.230)
Immediate	14.54 (0.258)	12.30 (0.305)	14.92 (0.414)	12.76 (0.320)
4 weeks	30.36 (0.626)	25.82 (0.785)	27.64 (0.614)	23.22 (0.576)
6 weeks	33.84 (0.896)	28.88 (0.825)	31.08 (1.202)	27.14 (0.740)

PRT=peak removal torque, MSI=mini-screw implants

Figure 1.

Illustrate the mean peak removal value across the three time points for the different MSI shapes. MSI = Mini-screw implantthe different MSI shapes. MSI = Mini-screw implant

PRT for all groups (regarding both tested shapes and diameters), increased progressively over time, indicating enhanced implant stability with bone healing. For the Cylindrical contour implant with 1.4 mm diameter group, the mean PRT increased from 14.54 ± 0.258 Ncm after immediate insertion to 33.84 ± 0.896 Ncm at 6 weeks which equal to 132.7% from initial torque value. While for the same shape with 1.2 mm diameter group, PRT values increased from 12.30 ± 0.305 Ncm after immediate insertion and progressed to 28.88 ± 0.825 Ncm 6 weeks after increased 134.6% from immediate torque value. For the Tapered MSI with 1.4 mm diameter group, the same scenario was observed, the table display an increase PRT value from 14.92 ± 0.414 Ncm at insertion to 31.08 ± 1.202 Ncm at 6 weeks equivalent to 108.4%. Similarly, the Tapered MSI with 1.2 mm diameter implants increased from 12.76 ± 0.320 Ncm to 27.14 ± 0.740 Ncm over the same period with increase in PRT approximately 112.6%. Notably, the peak torque was observed at 6 weeks in all groups, suggesting significant time-dependent enhancement in mechanical retention.

Residuals demonstrated normal distribution (Shapiro–Wilk P = 0.08, and variance equality was confirmed (Levene’s test P = 0.31). Thus, no transformation was necessary.

All three tested factors (Shape, Diameter, and Time) exerted statistically significant main effects as (all P < 0.001), as well as the Shape × Diameter and Diameter × Time interactions remained statistically significant. No interaction involving the random effect reached significance, confirming that intra-animal variability did not alter the primary conclusions as shown in Table 2.

Table 2.

Three-way analysis of variance results for peak removal torque (PRT, Ncm)

Source of variation	F	df	P	Partial η²	95% CI
Main effects
Shape (Cyl vs Tap)	18.5	1	<0.001	0.18	0.12–0.24
Diameter (1.4 vs 1.2)	42.3	1	<0.001	0.42	0.35–0.49
Time (Immediate → 6 w)	132.1	3	<0.001	0.78	0.70–0.85
Two-way interactions
Shape × Diameter	7.2	1	0.008	0.07	0.04–0.10
Shape × Time	8.5	3	0.002	0.08	0.05–0.12
Diameter × Time	28.9	3	<0.001	0.30	0.25–0.35
Three-way interaction
Shape × Diameter × Time	5.6	3	0.004	0.06	0.04–0.08

PRT=peak removal torque, ANOVA=analysis of variance

Table 2 display the MSIs with a 1.4 mm diameter demonstrated significantly higher PRT values compared to 1.2 mm implants across all tested time periods and in both cylindrical and tapered designs according to t statistical test.

At 6 weeks’ time point, the 1.4 mm cylindrical screws display a mean PRT of 33.84 Ncm, while the 1.2 mm screws reached 28.88 Ncm (P ≤ 0.05).

While the tapered shape group at the same time point, 1.4 mm MSI had a PRT of 31.08 Ncm, compared to 27.14 Ncm for 1.2 mm.

The comparison of MSI shape [Table 2] revealed the following; For the 1.4 mm diameter group, cylindrical screws exhibited significantly higher PRT values than tapered screws at 4 weeks (30.36 vs. 27.64 Ncm) and 6 weeks (33.84 vs. 31.08 Ncm) (P ≤ 0.05). No significant differences were observed at the immediate and 2-week time points in this group (P > 0.05), indicating that early-stage mechanical retention may not differ substantially between shapes at this diameter. While for the 1.2 mm diameter group, the tendency was more variable, as at 2 weeks, tapered screws demonstrated significantly higher torque than cylindrical ones (12.26 vs. 10.54 Ncm, P ≤ 0.05), suggesting that tapering may offer an advantage at smaller diameters in early stability. However, at 4 and 6 weeks, cylindrical implants again outperformed tapered implants (P ≤ 0.05). No significant difference was noted immediately post-insertion (P > 0.05).

Discussion

The significant anchorage control by TAD during active orthodontic treatment make the investigations regarding the suitable diameter and shape is important.[17,18] In this study the authors demonstrated a simulation for comparison between two main shaped of MSI performing two diameters, further they investigate the significant of MSI stability during four time point after insertion of MSI representing the primary and secondary stability. The primary stability representing the initial gripping of MIS in the bone, while the secondary one gives an idea about the effect of the biological response across the time.[14,15]

Although previous studies have examined the effects of MSI diameter and shape individually, their combined influence on primary stability has not been systematically evaluated in a single in-vivo model. By simultaneously assessing both parameters over multiple time points, our study provides new insight into the interaction between implant geometry and temporal changes in stability, offering quantitative evidence to guide the selection of temporary anchorage devices in clinical orthodontics.

Kumer et al. 2024 and Chen et al. in 2025 documented that the removal torque successfully used for detection both stability at early and long-term effect.[16,17,18,19,20,21]

Sheep were used as a model in this research for ethical consideration as their bone have a comparable composition and structure of the human jaws as well as mechanical behavior.[14,22,23] Thus they can give a valid insight for understanding the response could be expected in the human arch as reported by Nguyen et al. 2025 and Garcia et al. 2023. Although the number of animals used in this study was limited to three adult sheep, a high number of implants (160) was distributed across different tibial regions to maximize data collection, while adhering to ethical principles of animal research. The design followed the 3Rs principle (Replacement, Reduction, and Refinement) by minimizing the number of animals used while ensuring sufficient sample size through repeated measures. The implants were placed with a minimum inter-implant distance of 4 mm, based on previous studies suggesting this spacing does not compromise osseointegration or induce mechanical interference. Additionally, implants were evenly distributed to avoid overloading a specific area, and each implant was treated as an independent unit for statistical analysis, which is a commonly accepted approach in similar animal model studies.[15,17,23,24,25]

The target length of 7 mm of TAD used in this study related to the high primary stability rate of this specific length with suitable engagement to the cortical bone, which further suitable for orthodontic force.[26,27,28] Two different MSI diameters were tested in this study, the selected diameter consider the most common used in the clinical studies with the least faille rate.[26,27,28,29,30] Beside that these diameters and more suitable to the sheep bone with minimum destruction and less heat generation.[2,30]

According to the outcome of this study, a positive correlation was detected between torque values and different diameters of MSI used, shows that these values change with the change in diameter; whatever the type is, it had been noticed that as diameter increased, the removal torque value increased increase with the Torque value and this means higher stability than the smaller diameter. It has predominantly been found that the diameter has an impact on MSI stability according to both experimental and clinical studies. These results also agree with Michael et al. (2008), who use bovine bone to compare between primary stability of three diameter MSI.[17,18,24,28]

Lee and Park in 2023, also notice that as increase in diameter of implant in human jaw will result in increase in primary stability because of increase compression of the surrounding bone that has less modulus of elasticity than the Titanium.[18]

A recent study by Smith in 2024 comparing three diameters and three lengths of implants in artificial bone block. He noticed that implant stability increases as increasing diameter and length.[19]

Another study for primary stability in synthetic bone block by Holm concluded that as increase in diameter of MSI, results in increase stability.[20]

Miyawaki et al. found a significant association between the diameter of MSI and their stability. They reported that the 1-year success rate of implants with a diameter of 1.5 or 2.3 mm was significantly higher than that of implants with a 1 mm diameter.[21] Similarly, Berens et al. observed increased success rates for MSIs with a 2 mm diameter in the mandible and recommended a minimum micro implant diameter of 1.5 mm for the palate.[22]

It has also been suggested that mini-implants smaller than 1.3 mm should be avoided due to a decrease in stability.[23,30,31]

Cylindrical MSI represent a more uniform diameter across the length good have a more surface area of contact to the bone enable the parallel sides engaged evenly,[32] all these factors could significantly affect the torque value for the taper type used in this investigation, which has a conical shape, with high wedging effect to the sounding bone producing a lateral compression of the bone. This design enables increasing the friction resistance and improve bone mechanical lock.[33,34]

In this study the mean torque of MSI is significantly affected by the diameter of the Micro implant thus this study showed that the larger diameter (1.4 mm) was thus more stable than the smaller one (1.2 mm) one. This is probably may be explained as increase in the diameter this mean more surface area and thus more area of bone to implant contact.[22,25,33] Thus, offering more surface area for osseointegration around the implant in healing phases. Thus, more power required to break bone-implant contact.[20,24,34,35,36,37]

Extension of the durability of MSI insertion in the bone highly affected the removal torque value of all tested MSI in this study as this interferes with the biological process and implant bone integration and mechanical locking with the surrounding bone. Four time points were selected to give a clear image about the MSI behavior a cross a n extended period, which has an impact on the clinical implication id practical orthodontic treatments. With the first 2 weeks a remodeling process of the surrounding bone was active they effectively affect the torque removal.[20,38,39,40,41,42]

Over extended period to 4 weeks, a formation of the woven bone could highly affect biological stability around the MSI. Final 6 weeks, Lamellar bone started to form with significant bone maturation, producing a stronger mechanical interlocking.[42,43,44,45,46,47]

Main limitation of this study good missing of the histological section of the bone surrounded the different types and shape of MSI used nor X-ray image evaluation. Beside that only three sheep were used which could affect the impact of the biological variability.[20,34,35,36,37,47]

Conclusions

This experimental study demonstrates that the geometry of the mini screw implants including both the diameter and shape influence their primary and secondary stability, over the tested time points. regardless of mini-implant shape, Mini-implants with 1.4 mm diameter consistently exhibited higher stability than their smaller counterparts 1.2 mm across all tested time points. Also, Cylindrical implants generally outperformed tapered designs at later healing stages, particularly at 4 and 6 weeks, indicating more favorable long-term mechanical retention. Notably, all groups showed a progressive gain in torque values over tested time points.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.