INTRODUCTION

The success of skeletal anchorage using miniscrews is related to their stability in the bone. Most of the failures occur immediately after mini-implant placement1 because the lack of primary stability may lead to progressive mobility of the device and to its subsequent loss.2 Factors that influence the immediate stability are related to the design of the device, the quantity and quality of bone, and the insertion technique.3

The term “bone quality” is not clearly defined in the literature. This includes physiological and structural aspects and the degree of bone tissue mineralization.4 Some bone properties, such as bone mineral density4–7 and cortical thickness,8–10 have been related to the stability of implants or mini-implants. Although the role of trabecular bone in the stability of dental implants11 has been investigated, there is still a lack of literature on miniscrews.

The aim of this study was to compare the primary stability of miniscrews inserted into bone blocks of different bone mineral densities (BMDs), as well as investigate whether there were any bone properties such as trabecular thickness, trabecular number, trabecular separation, cortical thickness, BMD, or bone volume density (BV/TV), that could influence the primary stability of miniscrews in the presence and absence of cortical bone.

MATERIALS AND METHODS

The sample consisted of 13 bovine pelvises (Bos taurus, Angus lineage) obtained from a slaughterhouse immediately after the animals were slaughtered. Four bone sections were taken from each pelvis—two from the pubic and two from the iliac region—with the use of a trephine bur (8 mm × 20 mm; Sin Implants, São Paulo, Brazil) adapted to a low-speed motor handpiece (Beltec LB100; Beltec, Araraquara, Brazil) under irrigation. In the samples taken from each bone region, the cortical bone was preserved in one and removed in the other using a diamond disc under irrigation. The final sample dimension was 8 mm × 10 mm. The material was immersed in saline solution and stored by freezing (−20°C).12 Division of the groups was based on thebone region and the presence of cortical bone (Table 1). The number of samples was calculated using the sample size data of a previous pilot study (α  =  5%, power of study  =  80%).

Table 1.  Division of the Groups Based on the Bone Region and the Presence and Thickness of the Cortical Bone

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Insertion Torque

Fifty-two conical, self-drilling miniscrews (Ti-6Al-4V alloy; INP, São Paulo, Brazil) 1.4 mm in diameter and 6 mm long were inserted into the bone blocks. The implant sites were predrilled to a depth of 2 mm with a pilot drill 1.0 mm in diameter (INP). The mini-implants were placed by a single operator with the use of a manual key connected to a digital torque meter (Lutron TQ-8800; Lutron, Taipei, Taiwan). To guarantee perpendicular insertion of the miniscrews into the bone, the torque meter and the bone blocks were adapted to a mechanical device16 (Figure 1). The peak insertion torque (IT) values were recorded in Newton centimeters (Ncm).

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Miniscrew Mobility

Immediately after miniscrew placement, their mobility was evaluated with the Periotest instrument (Medizintechnik Gulden, Modautal, Germany). A special acrylic device was used to fix the sample and Periotest handpiece and to standardize the distance between the sleeve and the mini-implant (Figure 2). The handpiece was calibrated before each screw was measured. Two recordings were collected for each mini-implant, and the average value was designated as the Periotest value (PTV), which is on a scale from −8 to +50. A small PTV indicates low mobility and high primary stability.

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Pull-Out Strength

This mechanical test was performed in a universal test machine (Emic DL 2000; Emic. São José dos Pinhais, Brazil) using a 500 kgF load cell at a crosshead speed of 0.05 mm/s to remove the miniscrew16 (Figure 3). The maximum pull-out strength (PS) was recorded (N).

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RESULTS

Means, standard deviations, and intergroup comparisons for the bone properties are described in Tables 2 and 3. Trabecular thickness, trabecular number, trabecular separation, and trabecular BMD presented lower values in the iliac than in the pubic groups (Figure 4). BV/TV and total BMD presented increasing values from GI0 to GI1to GP0 to GP1 (Table 2). For GI1 and GP1, cortical thickness and cortical BMD did not differ (Table 3).

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Table 2.  Descriptive Analysis and Analysis of Variance/Tukey Result for Bone Quality^a

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Table 3.  Descriptive Analysis and t-Test Result for Bone Quality^a

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Means, standard deviations, and intergroup comparisons for the variables used to evaluate miniscrew stability are shown in Table 4. IT values were lower for GI0, followed by GP0 and GI1 (without statistical difference between them), and the highest value was for GP1. PTV values in decreasing order were from GI0 to GI1 to GP0 to GP1. PS differed statistically among the groups, being higher in GP1, followed by GI1, then GP0, then GI0. Pearson correlation test results are presented in Table 5.

Table 4.  Descriptive Analysis and Analysis of Variance/Tukey Result for Primary Stability^a

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Table 5.  Pearson Correlation Test^a

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DISCUSSION

Practitioners have traditionally assessed primary stability by manual verification.17 Nevertheless, there are other less-subjective methods, such as IT; removal torque; PS, by which mechanical behavior can be assessed in the axial direction; and the mobility and lateral displacement test, by which mechanical behavior can be assessed laterally. However, there is still no gold standard for primary stability assessment.17 In this study, three methods were used for evaluating primary stability of mini-implants: IT, PTV, and PS.

The Periotest instrument allows a nondestructive and objective measurement of implant movement.18 It is probably the method that best reproduces the clinical verification of primary stability: manual verification of mobility. There is no consensus about the reliability of the Periotest for assessing implant stability. Some authors consider the Periotest a good tool for measuring stability in dental implants19–22 and mini-implants23,24; others disagree.25 In our study, there was correlation between the PTV value and the other two stability measures. IT presented a substantial negative correlation with PTV, as has previously been found.20,24,26 The correlation between PTV and PS was also negative. The two mechanical measures used to evaluate stability axially (IT and PS) presented strong positive correlation, in agreement with previous studies.16

It is known that a PTV of −8 to +9 corresponds to a mobility index of 0 with no distinguishable movement.27 In this study, the only group with PTV above +9 was GI0 (PTV  =  20.19), which indicated palpable mobility.28

Considering bone properties, values for trabecular thickness, trabecular number, and trabecular BMD were higher for pubic bone, and trabecular separation was lower. These findings corroborate the difference in the trabecular characteristics of iliac and pubic bones.29 When the trabecular number increases, its separation diminishes and its thickness tends to increase. It reflects on BV/TV and BMD, which also increase. Both bone density measurements of the total block—BV/TV and BMD—showed very strong positive correlation. As previously mentioned,16 the BMD of trabecular bone and the total bone block presented a very strong positive correlation.

Iliac and pubic bone presented similar cortical bone characteristics. No difference was observed for cortical thickness and cortical BMD in GI1 and GP1. The presence of cortical bone per se numerically increased the primary stability, corroborating the importance of the cortical effect on primary stability.3,6,8–10,26 However, when the cortical bone was absent, the importance of the trabecular bone became more apparent. IT, PTV, and PS values differed between GI0 and GP0, with higher stability in pubic bone. When GI1 and GP0 were compared, IT and PTV values presented no statistical difference. It seems that the cortical bone plays an important role when trabecular bone has lower BMD, lower trabecular number, is thinner, and has more separated trabeculae. When comparing the stability values for GI1 and GP1, groups with similar cortical coverage, IT, PTV, and PS were higher for the pubic bone, which has a higher level of BMD, is thicker, and has less separate trabeculae and a higher trabecular number.

A positive correlation was found between cortical thickness and IT3,10,26,30–33 and between cortical thickness and PS.8,34 Cortical thickness did not influence the PTV value. It is believed that cortical thickness had a greater influence on axial than on lateral measures. Cortical BMD presented no valid correlation with IT, PS, or PTV, in disagreement with previous studies.16,24,16 However, the methodology of BMD evaluation differed. While this study evaluated BMD three-dimensionally by means of micro-CT, a more accurate method,35 the others used CT slices.

Although the results of the present study cannot be directly extrapolated to clinical practice because of the animal model and ex vivo methodology used, it was found that cancellous bone plays an important role in the primary stability of miniscrews, redirecting the spotlight that was previously focused only on the cortical bone.

CONCLUSIONS

• As the BMD of the receptor site increases, the primary stability increases numerically as well;

• In the presence and in the absence of cortical bone, trabecular number, trabecular thickness, BV/TV, trabecular BMD, and total BMD are correlated with the primary stability of mini-implants, showing the importance of trabecular bone in the stability of miniscrews.