Comparative Evaluation of Primary Stability of Two Different Types of Orthodontic Mini-Implants.

BACKGROUND: The mini-implants introduced new possibilities of adequate anchorage in orthodontics. Furthermore, due to its small size, it can even be placed at relatively difficult sites with ease. Removal torque should be high to prevent implant unscrewing.
OBJECTIVE: This prospective clinical trial was aimed to evaluate the insertion torque and removal torque of single-threaded and double-threaded cylindrical orthodontic mini-implants.
MATERIALS AND METHODS: A total of 36 cases were randomly divided into two groups, with an equal number of patients in each group (n = 18). In Group 1 single-threaded cylindrical mini-implant was placed, and in the other group, cylindrical implants with double-threaded were placed. Maximum insertion torque (MIT) and maximum removal torques (MRTs) were recorded for both groups. Data collected were subjected to statistical analysis.
RESULTS: MIT was found to be significantly higher than MRT for both the groups and between the groups. Intergroup comparison in the present study showed significantly higher values for MIT than MRT. Intergroup comparison of MIT showed more values for Group 2 as compared to Group 1. Similar statistically significant values were seen in terms with MRT, where double-threaded cylindrical mini-implants had more torque value than the other group.
CONCLUSIONS: Orthodontic mini screws represent effective temporary anchorage devices. Double-threaded cylindrical mini-implants have significantly higher insertion and removal torque than single-threaded mini-implants and hence better stability. Copyright:

INTRODUCTION

Anchorage is the resistance to undesired movement of the teeth, and it is essential for the treatment of dental and skeletal malocclusions.[1] The term temporary anchorage devices (TAD) are any sort of implant, screw, pin, or implant that is placed to enhance skeletal anchorage and then removed after the completion of the treatment. Mini-implants proved to be effective in establishing absolute orthodontic anchorage.[2] The miniscrews belong to the (TAD) and the modern ones are usually made of bioinert titanium compounds (Ti6 Al4 V), their diameter range between 1.2 mm to 2.3 mm, and are between 4 mm and 15 mm in length.[3] Primary stability is necessary for the mini screws, because of immediate loading on them, and differs according to the various patient, the design of the mini screws, and clinical technique factors, also it is considered as the clinical condition of mini-implant immobility and ability to resist loads in different directions.[4] In the in vitro studies, using the double-layer artificial bone is mandatory to simulate the human cortical and cancellous bone with appropriate thickness and mechanical properties corresponding to the specific areas of the jaws.[5] The insertion of the orthodontic mini screws can be done either manually or motorized, and the manual insertion method is usually more straightforward, it can achieve better tactile sensation than the motorized one.[6] It is recommended that the mini-implant should be inserted at a slow speed, with low and continuous forces, and hence that the load on both the mini-implant and the surrounding bone is kept low.[7] Primary stability is achieved by mechanical retention between bone and mini-implant. Secondary stability for orthodontic mini-implants is achieved by osseointegration through continued bone remodeling.[8]

Periotest (device to measure the initial stability of dental implants) and resonance frequency analysis have all been used to measure primary stability.[5] Since the mini screws exist to improve the anchorage during the orthodontic treatment, and there are many commercially available brands of mini screws, so that there is a need for a method to assess the primary stability of various mini screws from different manufactures and compare among them, and there are no previous Iraqi studies presented to assess the primary stability.[6] All factors lead to an increase in the interlocking surface area between implant and bone. These modifications in implant design can lead to microfractures in the bone while placing implants and can cause discomfort to the patient.[7] Recently, to improve stability, different thread designs for mini-implants have been developed. However, only a few studies have compared different thread designs and their role in improving implant stability. The present study was designed to compare the stability of two different thread types of orthodontic mini-implants using torque test.

MATERIALS AND METHODS

This present prospective clinical trial was performed on the patients within the age group of 18–26 years and with sound mental and physical health, registered for orthodontic treatment in the department of orthodontics. Stratified randomization was done to prevent the allocation bias while enrolling the patients in the study. However, a sample of forty patients (females: 24 males: 16) was used for this study. The Frankfort-Mandibular Plane Angle of 24°–30° depicting growth patterns as average. All the included cases were Type-A anchorage cases with Angle's Class I bimaxillary protrusion with anterior crowding <2–3 mm. Patients with an underlying bony disease such as arthritis were excluded from the study. The informed consent was obtained from all the study patients individually.

Titanium (Ti-6Al-4V) alloy mini-implants, based on thread design and shape were divided into the following two groups: cylindrical single-threaded mini-implants, and cylindrical double-threaded mini-implants. The standard MBT technique was used in all the study participants with sliding mechanics. All the mini-implants were placed between the second premolar and first permanent molar in the maxilla with the help of the implant guide. Patients were divided into two groups having an equal number of patients (n = 18). In one group single-threaded cylindrical mini-implant was placed while the double-threaded cylindrical was placed in the other group. All the mini-implants (n =20) were inserted as well as removed using a contra-angle handpiece and surgical engine with a speed of 30 rpm. Maximum removal torque (MRT) and MIT were then measured. Postsurgery, the patients were prescribed 2% chlorhexidine mouth wash and antibiotics for 3 days.

The results were assembled and compared between the two groups using one-way ANOVA (analysis of variance). P < 0.05 was taken as statistically significant in the one-way analysis of variance. Means and standard deviations were reported.

RESULTS

The final study included 36 patients (females = 20, males = 16) as four patients did not comply with the treatment and backed out during the study period. The maximum insertion torque (MIT) was calculated in N cm for both study groups individually.

On comparing MIT for both the groups, the values showed a statistically significant difference between the groups [Table 1].

Table 1

Intergroup comparison of maximum insertion torque using one-way analysis of variance

Groups	n	Mean	SD	SER	P
Group 1	18	7.2222	1.5551	0.3665	0.000642
Group 2	18	8.7222	0.6691	0.1577

SD: Standard deviation, SER: Standard error of the regression

The statistical calculations suggested that MRT value for double-threaded cylindrical mini screws, i.e., Group 2 were statistically higher as compared to Group 1 with a mean value and standard deviation [Table 2].

Table 2

Intergroup comparison of maximum removal torque using one-way analysis of variance

Groups	n	Mean	SD	SER	P
Group 1	18	1.2222	0.4278	0.1008	0.000436
Group 2	18	1.7778	0.4278	0.1008

SD: Standard deviation, SER: Standard error of the regression

While comparing MIT of Group 1, i.e, single-threaded cylindrical mini screws with MRT of Group 1 mini-implants, it was seen that MIT showed statistically significant higher values with the mean value and standard deviation [Table 3].

Table 3

Intragroup comparison of maximum insertion torque and maximum removal torque using one-way analysis of variance for Group 1

Groups	n	Mean	SD	SER	P
Group 1 MIT	18	7.2222	1.5551	0.3665	<0.00001
Group 1 MRT	18	1.2222	0.4278	0.1008

SD: Standard deviation, SER: Standard error of the regression, MIT: Maximum insertion torque, MRT: Maximum removal torque

The intragroup comparison of Group 2 showed a statistically significant difference between the values of MIT and MRT, which is consistent with Group 1 results. The difference between the two values was statistically significant with a P ˂ 0.00001 [Table 4].

Table 4

Intragroup comparison of maximum insertion torque and maximum removal torque using one-way analysis of variance for Group 2

Groups	n	Mean	SD	SER	P
Group 1 MIT	18	8.7222	0.6691	0.1577	<0.00001
Group 2 MRT	18	1.7778	0.4278	0.1008

SD: Standard deviation, SER: Standard error of the regression, MIT: Maximum insertion torque, MRT: Maximum removal torque

While comparing intergroup MIT value was significantly higher than MRT for both the groups, the values showed a statistically significant difference between the groups [Table 5].

Table 5

Intergroup comparison of maximum insertion torque and maximum removal torque

Groups	n	Mean	SD	SER	P
MRT	18	1.5	0.5071	0.0854	<0.00001
MIT	18	7.9722	1.4038	0.234

SD: Standard deviation, SER: Standard error of the regression, MIT: Maximum insertion torque, MRT: Maximum removal torque

DISCUSSION

Proper insertion and careful removal of orthodontic mini-implants is a necessary aspect for which torque plays an important role.

Attaining primary stability at the time of implant insertion depending upon the mini-implant characteristic, design, placement condition, and insertion site is crucial for success.[8] Few indirect measures for primary mini-implant stability are insertion torque,[9] and density of bone[10] are also necessary factors as inordinately low or high torque values can result in compromised implant stability.[11] Values of 5–10 Ncm for MIT are recommended by various authors for implant success.[12] A systematic review suggested a mean value of MIT as 13.28 Ncm with a standard deviation of 0.34 for self-tapering mandibular mini-implants.[13] Insertion torque is related to the length of implant and thickness of cortical bone positively,[14] in contrast, it relates to predrilling diameter negatively.[6] Thicker cortical bone, high thread angle, and large implant diameter are few factors that predict higher values of MIT in vitro with a value of 24.7%, 12.3%, and 10.7%, respectively.[15] Pull-out strength and MIT values were not affected by bone density, for a cortical bone thickness of 1 mm in an in vitro study.[16] These results were consistent with the present study.

A systematic review considered cadaver, animal, and clinical studies concluded that mini-implants placed near roots had higher values for insertion torque as compared to those without contact. Therefore, torque levels should be recorded during the entire process of mini-implant insertion.[17]

In cases where immediate implant loading is done, insertion torque is to be kept high. In cases with less insertion torque, delayed implant loading (6–8 weeks) should be attempted. Implants with a minimum insertion torque value of 15 Ncm survived immediate loading better.[10] Even in cases with longer treatment duration, the median survival time for implants was sufficient.[18] The risk of fracture during implant removal is a crucial factor. A systematic review reported a value of 10.01Ncm with a standard deviation of 0.17 for MRT in maxillary cylindrical mini-implants.[19] However, removal torque value is considerably lower than value for insertion torque for cylindrical as well as conical orthodontic mini-implants.20 This was in consent with present study where it had no significant correlation with insertion torque. Also, it did not relate significantly to placement period and thickness of cortical bone in vitro.[820]

CONCLUSIONS

The success of mini-implants depends on the proper selection of implant length, tapering, diameter, and insertion site. Furthermore, adequate insertion is assessed in terms of predrilling angle, insertion site, primary stability, proper loading, lack of inflammation at the placement site, absence of mobility, and no injury. Furthermore, proper insertion and removal torque are necessary for achieving the primary stability of orthodontic mini-implants. However study with more patients, more extended monitoring periods, and different designs of mini screws are required to reach the definitive conclusion.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.