Stability of immediately loaded 3 mm long miniscrew implants: a feasibility study

ABSTRACT Introduction: Shorter miniscrew implants (MSIs) are needed to make orthodontics more effective and efficient. Objective: To evaluate the stability, insertion torque, removal torque and pain associated with 3 mm long MSIs placed in humans by a novice clinician. Methods: 82 MSIs were placed in the buccal maxillae of 26 adults. Pairs of adjacent implants were immediately loaded with 100g. Subjects were recalled after 1, 3, 5, and 8 weeks to verify stability and complete questionnaires pertaining to MSI-related pain and discomfort. Results: The overall failure rate was 32.9%. The anterior and posterior MSIs failed 35.7% and 30.0% of the time, respectively. Excluding the 10 MSIs (12.2%) that were traumatically dislodged, the failure rates in the anterior and posterior sites were 30.1% and 15.2%, respectively; the overall primary failure rate was 23.6%. Failures were significantly (p= 0.010) greater (46.3% vs 19.5%) among the first 41 MSIs than the last 41 MSIs that were placed. Excluding the traumatically lost MSIs, the failures occurred on or before day 42. Subjects experienced very low pain (2.2% of maximum) and discomfort (5.5% of maximum) during the first week only. Conclusions: Shorter 3 mm MSIs placed by a novice operator are highly likely to fail. However, failure rates can be substantially decreased over time with the placement of more MSIs. Pain and discomfort experienced after placing 3 mm MSIs is minimal and temporary.


INTRODUCTION
Miniscrew implants (MSIs) have become popular due to their easy placement and removal, effectiveness as anchorage devices, the multiplicity of intraoral placement options, their affordability, minimal invasiveness, and patient acceptance. 1,2 Systematic reviews have reported failure rates ranging between 13.5 and 20% when mobile and displaced MSIs were included. 3,4 Approximately 85% of practicing orthodontists reported MSI failures of 25% or less. 5 Although root injuries caused during MSI placement usually heal unremarkably, they have been shown to cause localized bone loss, ankylosis, and pulpal damage, leading to devitalization of the tooth. [6][7][8] The risk of root contact during placement should be less for shorter 3 mm long MSIs than for traditional 6 to 8 mm long MSIs. Reports have shown that the soft tissue adjacent to the mucogingival junction is approximately 1-1.5 mm thick, 9,10 and that interradicular cortical bone is 0.8-3.1 mm thick in the mandible and 0.8-1.5 mm thick in the maxilla. 11,12 On that basis, no more than 1.5 mm of the screws' shanks would penetrate into medullary bone. That being the case, the risk of root contact in the buccal posterior region with 3 mm long MSIs should be minimal. For the same reasons, shorter screws should provide orthodontists with more MSI placement options, reduce the need for root separation, and could provide skeletal anchorage for dentofacial orthopedics in younger patients. Geshay D, Campbell P, Tadlock L, Schneiderman E, Kyung HM, Buschang P -Stability of immediately loaded 3 mm long miniscrew implants: a feasibility study 5 Experimental studies have shown that 3 mm long MSIs are stable in animal models. Excluding problematic MSIs (i.e. those whose tips broke off during insertion and those traumatically dislodged by animals chewing on their cages), an overall failure rate of 9.4% was reported for 3 mm screws placed in dog jaws and loaded with orthopedic level forces. 13 Failure rates under 10% have been reported in studies that placed 3 mm MSIs in the cranium of rabbits and loaded them with various orthodontic level forces. [14][15][16][17] The purpose of this study was to assess the stability of immediately loaded 3 mm miniscrew implants placed in human subjects by a novice operator. No studies to date have examined the feasibility of using 3 mm long, 1.7 mm wide, MSIs in humans. Moreover, little is known about the number of MSIs that novice operators have to place in order to attain acceptable failure rates.

SUBJECTS
After receiving approval by the Institutional Review Board, adults were recruited from the students and staff at Texas A&M University Geshay D, Campbell P, Tadlock L, Schneiderman E, Kyung HM, Buschang P -Stability of immediately loaded 3 mm long miniscrew implants: a feasibility study affect bone metabolism, 4) inadequate space between tooth roots and 5) buccal frenum in the placement site. Periapical radiographs were taken to ensure sufficient space between the MSIs and tooth roots (Fig 1).

Figure 1: A)
Periapical radiograph taken to visualize bone between the teeth (planned MSI sites indicated by black arrows), B) insertion of MSI, C) periapical radiograph taken after MSI placement, D) pair of MSIs after immediate loading with a force of 100 g. Geshay D, Campbell P, Tadlock L, Schneiderman E, Kyung HM, Buschang P -Stability of immediately loaded 3 mm long miniscrew implants: a feasibility study 7 A power analysis indicated that 80 screws were needed to establish a 15% difference in stability, assuming a power of 95% and an alpha of 0.05. A total of 82 MSIs were placed in 26 subjects (10 males, 16 females) with a mean age of 27.4 years.
Each subject received $25 to compensate for their time and efforts. Seventeen subjects had MSIs placed on both sides (17 x 4 = 68), 6 had only one side that qualified (6 x 2 = 12), and 2 had only one (unloaded) MSI placed. The anterior MSI was placed between the maxillary canines and first premolars; the posterior MSI was placed between the second premolars and first molars. The protocol was approved by the Texas A&M University IRB. The purpose of the study and potential risks were explained, and an informed consent was obtained from each subject.

PLACEMENT PROTOCOL
All procedures were performed by one orthodontic resident using a standardized placement protocol. Subjects brushed their teeth and rinsed with Peridex chlorhexidine (3M ESPE, Irvine, CA) for 45 seconds. Topical anesthesia (20% lidocaine, 4% tetracaine, 2% phenylephrine) was applied at each MSI site for two minutes. Three subjects who requested additional anesthesia received anesthetic infiltration with 1/8 carpule of 2% lidocaine with 1:100,000 epinephrine. Gingival thickness was measured three times at the insertion site using a sharp explorer with an endodontic rubber stop. Geshay D, Campbell P, Tadlock L, Schneiderman E, Kyung HM, Buschang P -Stability of immediately loaded 3 mm long miniscrew implants: a feasibility study 8 Each 3 mm long, 1.7 mm wide MSI (Dentos, Seoul, Korea) was inserted perpendicularly into bone with a manual driver until the screw threads were no longer visible. Using a digital torque screwdriver (Imada, Northbrook, IL), each MSI was then rotated a quarter turn to measure insertion torque.
Periapical radiographs were taken after placement to ensure that there was space between the MSIs and the adjacent bone.
Pairs of adjacent implants were immediately loaded with one nickel-titanium closed-coil spring (Ormco, Orange, CA) delivering a force of 100g. Triad gel (Dentsply, York, PA) was applied to prevent wire abrasion of the cheeks or gingiva. Following placement, subjects rinsed with Peridex for 45 seconds, and were instructed to rinse each night for one week. Intraoral photographs were taken and each subject was given written oral hygiene and miniscrew care instructions. Orthodontic wax was given to each subject to prevent cheek irritation. The distance between each pair of MSIs was measured with a caliper on the day of placement, as well as 1, 3, 5 and 8 weeks after placement. The distance was measured three times and averaged at each occasion.
MSIs were considered as failures if they exhibited any degree of mobility upon examination. If a screw failed, it was replaced if there was sufficient space intraorally to relocate the MSI apically. Screws were replaced whenever possible to maintain the Geshay D, Campbell P, Tadlock L, Schneiderman E, Kyung HM, Buschang P -Stability of immediately loaded 3 mm long miniscrew implants: a feasibility study 9 100g force on the other MSI that did not fail. Importantly, MSIs that were replaced were counted as failures. If both screws on one side failed, they were removed and not replaced. Failures were classified as either primary or incidental. Incidental failures occurred when the MSIs were traumatically dislodged by the subjects. After eight weeks, removal torque was recorded based on the first counterclockwise turn of each MSI using the digital torque screwdriver.

FOLLOW-UP
Subjects were recalled after 1, 3, 5, and 8 weeks. The eight week duration was chosen because the majority of MSI failures occur within one month after placement. [18][19][20] At each appointment, miniscrew stability was verified and the distance between implants was measured. Subjects also completed a questionnaire asking them to rate the worst pain that they ever experienced and the pain they were currently feeling, using a 10-cm Visual Analog Scale (VAS) anchored with "No pain" and "Worst pain ever". Another question, anchored with "No discomfort" and "Worst discomfort ever", asked how much discomfort the subjects were currently experiencing.
The next two questions asked whether they took medications to relieve pain or discomfort associated with, and not associated with, the MSIs. The final question asked if the miniscrew implants caused any type of injury.

FAILURES
An implant was considered a failure if it exhibited any mobility upon examination ( Table 1). The overall failure rate was 32.9% Geshay D, Campbell P, Tadlock L, Schneiderman E, Kyung HM, Buschang P -Stability of immediately loaded 3 mm long miniscrew implants: a feasibility study 11 There was no significant difference in failure rate between MSIs placed on the right and left sides ( Table 2) or between anterior and posterior screws. There were significantly more failures among the first 41 screws placed by the investigator than among the last 41, when all screws were considered and when incidentally displaced screws were excluded (Fig 2).
All primary failures occurred on or before day 42 (Fig 3) (Table 3). There also was no statistically significant difference in removal torque between the anterior (1.7 ± 0.9 Ncm) and posterior (1.7 ± 0.7 Ncm) screws.
Insertion torque was significantly (p < 0.01) greater than removal torque. While insertion and removal torque were higher for the MSIs that failed than for those that did not fail, none of the differences were statistically significant differences. However, insertion and removal torque were significantly less for the first 41 MSIs than the last 41 MSIs that were placed (Table 4).

Anterior MSIs
Geshay D, Campbell P, Tadlock L, Schneiderman E, Kyung HM, Buschang P -Stability of immediately loaded 3 mm long miniscrew implants: a feasibility study

GINGIVAL THICKNESS & TIPPING
Mean gingival thickness was 1.1 ± 0.3 mm at the anterior insertion sites, and 1.1 ± 0.1 mm at the posterior insertion sites (Fig. 4), a difference that was not statistically significant (p = 0.745). There were no statistically significant differences in gingival thickness between MSIs that failed and did not fail, or between the first 41 MSIs placed and the last MSIs placed. The distances between pairs of adjacent MSIs decreased over time (Fig 5). Statistically significant decreases occurred between placement and week 1 (p < 0.01), as well as between weeks 1 and 3 (p = 0.027). Decreases thereafter were small and not statistically significant. While there were no statistically significant differences in the distances that the screws moved during the first three weeks between the first and last 41 MSIs placed, there were significantly greater movements of the MSIs that failed (Table 5).

DISCUSSION
In the present study, 10 screws were traumatically dislodged by the subjects. These were the "incidental failures" that mostly (70%) occurred when subjects bit into large or tough foods. While there are no comparative human studies using 3 mm MSIs, experimental animal studies have reported good success rates. The overall failure rate of loaded 3 mm MSIs placed in dog jaws was 9.4% after excluding MSIs whose tips broke off while being inserted and MSIs that were traumatically dislodged. 13 Studies that placed 3 mm MSIs in the cranium of rabbits and loaded them with various expansion forces reported failure rates of less than 10%. [14][15][16][17] The marked discrepancy between human and animal failure rates suggests that the stability of reported a failure rate of 6.6%, with no differences in stability between immediately-loaded 5, 6, and 7 mm MSIs. In contrast, a 24.8% failure rate was reported for 5 mm MSIs that were loaded after 2-3 weeks. 23 The difference between these two studies further supports the notion that factors other than length determine the stability of shorter MSIs.
The insertion site could have been one factor that explains the higher than expected failure rates. Although not statistically significant, the primary failure rate for the anterior screws was twice as high ( In the present study, the distances between pairs of implants decreased significantly during the first three weeks. Clinically, the change was minimal, with an average total decrease of only 0.5 mm. Longer 17 mm MSIs have been reported to tip after placement, 36,37 with the amount being related to the amount of force applied. 13 The majority of MSI tipping probably occurs during the first few weeks, before newly remodeled bone achieves intimate contact with the MSI threads. 30 Geshay D, Campbell P, Tadlock L, Schneiderman E, Kyung HM, Buschang P -Stability of immediately loaded 3 mm long miniscrew implants: a feasibility study 22 Pain and discomfort experienced after MSI insertion was min- imal. There was only slight pain and discomfort after the first week, and little or none thereafter. While patients expect MSIs to be moderately or very painful before placement, they report no pain or mild pain after treatment. 38 Longer (6-12 mm) MSIs produce less than half as much pain as traditional orthodontic appliances. 39,40 Pain could be even further reduced with the shorter 3 mm MSIs because they are less likely to contact the periodontal ligament, assuming that less than 1.5 mm of the screw's shank is expected to penetrate the medullary bone.
Pain for the subjects in the present study was due to soft tissue injuries caused by MSI placement, including cheek rubbing, ulceration, gingival sloughing from topical anesthetic, and gingival irritation from the coil springs. Pain and discomfort experienced after 3 mm MSI placement is minimal and temporary.