New perspectives on miniscrews: improving stability

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Article received: 21-02-2016. Accepted for publication: 30-05-2016. Address for correspondence: Damien Brézulier 2, Avenue du Professeur Léon Bernard – 35000 Rennes – France E-mail: damien.brezulier@univ-rennes1.fr DOI: 10.1051/odfen/2016020

In 2002, there was considerable variability in screw design.Melsen's work has paved the way for numerous studies aimed at improving the stability of miniscrews in the short to medium term [14][15][16]28 . Thelatest developments are summarized below.
The morphology of the screws has been a topic of discussion for approximately 10 years 27 .The characteristics include the following: the design and length of the threaded part, the diameter of the screw (Fig. 1), the design of the gingival neck, and the shape of the head.These technical characteristics seem to be well defined for transgingival screws implanted in the cortical bone.There are two types of screws: self-drilling and self-tapping.There was no initial difference in grip nor in shape or strength 45,49 , be it conical or cylindrical.There is a recommended length of >8 mm at the maxilla and 6 mm at the mandible 42

SUMMARY
With the appearance of bone anchorages, orthodontists thought that they would finally be able to overcome the thorny problem of patients' cooperation.The first considerations were about the safety, the holding time, and the reliability of these devices.Both theoretical and clinical studies have attempted to answer to these questions, and several studies have been published on the same.What have we learned about the reliability of miniscrews 20 years after their introduction in our daily practice?Are they only a way to solve the problem of patient noncompliance?And finally, in which way are they going to evolve?These are questions we aim to answers in the light of the past and the current literature.

Short-term stability
The stability of the screws, defined as their stability over the entire duration of the active phase of treatment 43 , is linked to three types of parameters: screw morphology itself, the patients, and finally the practitioners.
The bone-related criteria play an important role in the short-term success rate 30,31,51 .The literature rarely differentiates between the density of the spongy bone and the thickness of the cortex, which are two different elements.However, a 1-mm cortical thickness ensures correct primary stability 37 .The density of the spongy bone is a secondary parameter 41 .Lastly, with regard to the installation of the screw itself, an insertion torque of 7.5 N/cm is recommended to strengthen the grip of the screw 29,45,48 .Loading, whether immediate or delayed, does not affect the screw's grip. 8,27,38rews made of steel, pure titanium, or titanium alloy-aluminum-vanadium Ti 6 Al 4 V are available on the market. 1The surgical steel and the titanium alloy have mechanical properties which decrease the risk of fracturing the screw during its installation or removal.Different surface treatments have been tested and are proposed: microsandblasting, chemical etching, and mechanical machining.
Nevertheless, none of these treatments have proven to be superior in medium-term behavior in comparison with the others 37 .The type of orthodontic mechanisms used does not influence the stability of the screws, whether it is molar distalization by anchoring in the alveolar or palatal bone, mass recession of the arch, molar intrusion, or correction of incisal overbite by intrusion 35,37 .Long-term stability is also dependent on the control of inflammation and the sprouting of soft tissue

Medium-Term Stability
The challenge here is to maximize the screw grip without having attained osseointegration 13 .Surgical NEW PERSPECTIVES ON MINISCREWS: IMPROVING STABILITY around the screw.Keratinized gingival implantation is strongly recommended in the literature 2,4,12,21,25,32,46 .However, implantation in the keratinized zone does not guarantee the absence of inflammation (Fig. 2).The implantation zone is a space bordered by the crestal apex and the mucogingival line.The placement of screws in these areas is hampered by the challenge of root morphology.Avoiding root proximities is crucial to the stability of the screws.
To do this, it is possible to vary their insertion angles 8,39 (Fig. 3).However, it should be noted that if a screw placed in contact with a root, only minimal damage occurs if it is quickly removed and if the cement repair is performed quickly and efficiently 3,7,18,20,22,23,40,57 .
Today, the stability rate of the screws is >92%.As a result, the use of miniscrews has increased considerably in recent years.For example, a recent study shows that 62% of German orthodontists use bone anchors, >50% of whom use them on more than two new patients per quarter 5 .

ARE THE PATIENTS RELIEVED OF THEIR DUTY TO COOPERATE OR ARE THERE NEW RESPONSIBILITIES?
The use of the miniscrews implies that there is no need to request the patient's cooperation in terms of wearing extraoral appliances or interarch tractions.However, these bone anchoring devices have in fact D. BRÉZULIER, O. SOREL shifted the need for patient cooperation away from the device and toward maintaining hygiene in the area surrounding the screw.distalization, have been compared 24,34 .Among these are the traditional pendulum, distal jet, and jig.It seems that none of the devices is devoid of confounding effects (tipping, loss of vertical control, and rotation) or undesirable effects (pushing of the pellets in the palatal mucosa, breakage of the device).The desire to avoid having to constantly solicit our patients' cooperation has cost us in terms of efficiency.
To this end, it became mandatory to develop new techniques, which were both more efficient and less dependent on patient cooperation.The contributions of bone anchor screws facilitated the implementation of treatments, which were previously deemed complicated given the extent of the cooperation required

Problems with cooperation
The success of orthodontic treatments as well as their duration are dependent on different criteria among which three are the direct responsibility of the patients: poor oral hygiene, improper wearing of the interarch elastic, and the detachment of attachments 44 .Patient cooperation is, therefore, a key issue for us 6,11,17,33 .Most practitioners have encountered difficulties in motivating the most recalcitrant patients.Numerous patient compliance devices, e.g., in the area of upper molar

New perspectives
The bone anchors continue to improve, opening the way to new orthodontic mechanics.From the transgingival screws described by Melsen, which are inserted into the alveolar bone, other devices and insertion sites have been developed (Table I).This is the case for maxillary tuberosities.Screws ≤12 mm in length are implanted at a distance from the from patients.These treatments are both orthodontic 36 and orthopedic 19 .
However, unimaxillary bone anchor treatments do not facilitate a potentiating effect on mandibular growth in order to resolve class-II cases.It is, therefore, necessary to produce a detailed diagnosis and to define our objectives according to the extent to which a maxillary recoil and a mandibular advancement are desirable.The maxillary screws thus constitute an alternative, in the strictest sense, to the pendulum, distal jet, and other EOF devices used in the maxillary recoil, but they do not very often promote mandibular growth.
Although it is true that the patients' cooperation is no longer required because of the use of bone anchor screws, it is still necessary during the installation of the screws, a stage which can be tricky in children, adolescents, or even adults.
In addition, an impeccable hygiene regimen must be maintained by the patients throughout the treatment.If they fail to do this, they risk the manifestation of mucosal inflammation which will affect the smooth functioning of the mechanics and, in the worst case scenario, will compromise the stability of the screw.In the case of subperiosteal microscrews, the risk is the embedding of the connector, making it difficult to use and painful for patients.This is directly related to the patients' lack of hygiene.

Figure 6 Case of placement of a canine retained vestibular in an 11-year-old patient. A flap is lifted to stick a button on the canine. This is connected by a metallic ligature to the active end of the "CT8-1" connector (Cortical-TMA-8-1-arm termination) fixed in the same session against the cortical bone of the zygomatic process by two embedded microscrews for a 3D control, and which emerges at the bottom of the vestibule at the molar level. (Courtesy of
Dr. Daniel Chillès.) NEW PERSPECTIVES ON MINISCREWS: IMPROVING STABILITY treatment with aligners, may be performed 53,54,55 (Fig. 9).However, this type of device encounters the same challenges as more conventional transgingival screws, i.e., the embedding of the screws (Fig. 10) or of the connectors (Fig. 9).The distal faces of the last molars.They serve as a direct anchorage for a progressive recoil of the maxillary arch.
They offer an alternative to traditional anchor plates.Although they require an incision line, it is easier to insert them.There are multiple areas of implantation.In the maxilla, they are mainly infrazygomatic and subnasal.At the mandibular level, there are the symphyseal regions, partly posterior to the crown and the retromolar trigon.The indications are vast: direct or indirect anchorage, anterior retraction, incisal or molar intrusion, adjustment of the molar axis, and insertion of canines retained among others (Fig. 6) 9 .
An implantation in the medial raphe area is also possible because it is anatomically safe.Wilmes has developed a set of connectors that can be supported by two palatine screws 11-mm and 9-mm long by 2-mm wide.The most anterior one is placed on the third palatal papilla, and the second is placed approximately 0.9 mm behind (Fig. 7).The steel connectors allow symmetrical or nonsymmetrical molar distalization and mesialization, with vertical dimension control 26,50,52 (Fig. 8).
Other titanium alloy connectors can be used to position the retained canines or simply to intrude a sector.The advantage of these aids is to decrease the length of time spent wearing the multiattachment apparatus compared to other bone anchors, this allows both a better acceptance of the device and proportionally decreases the risk of problems like detachment, leucoma, or even gingivitis.This is only put in place after having established the desired molar ratios or after having engaged the canine on the arch.In addition, lingual or vestibular bonding, or even  analyze the system they set up and keep in mind that miniscrews do not remove all the confounding effects, as new mechanical constraints are produced.
depression of the anterior part of the connector may occur during anterior recoil in class-II treatment.It is noteworthy that there are other devices that rest on the palatal screws 47 .In the end, they all have a dual purpose: facilitating the orthodontic mechanics by decreasing the need for patient participation but also limiting any undesirable effects.To achieve this, the practitioner must carefully

CONCLUSION
Owing to the evolution of miniscrews over the past 20 years, as well as the experience gained by orthodontists, the use of these aids is becoming more and more frequent.However, pushing the limits of treatments in terms of the type and amplitude of movements begs two questions.The first concerns the safety of the treatments and their undesirable effects.For instance, can we assume that a 5-mm molar recoil is feasible and poses no threat of radicular resorption under the pretext that it is assisted by bone anchors?The second question concerns the stability of the complex treatments facilitated by these anchorages.For example, will the closure of an anterior open bite NEW PERSPECTIVES ON MINISCREWS: IMPROVING STABILITY be stable in the long-term without making any assumptions as to its etiology.The use of anchorage screws must always be well thought out and must under no circumstances be displaced by the rigorous control of the mechanical devices installed.
; regardless of the NEW PARAMETERS FOR IMPROVED STABILITY OVER TIME New perspectives on miniscrews: improving stability D. Brézulier 1 , O. Sorel 2 1 Resident, University of Rennes 1, CHU Rennes 2 Professor -hospital practitioner, Head of the departement of Orthodontics, University of Rennes 1, CHU Rennes

Figure 1
Figure 1 Diagram illustrating the relationship between the diameter and the internal force of the miniscrew.Note that the force is not inversely proportional to screw diameter.(Reproduced from Melsen B.) Skeletal anchorage indications in orthodontics.(Review of Dento-Facial Orthopedics, 2006, 40 (1): 41-61.)

Figure 2
Figure 2 These palatal screws were inserted into the intermaxillary suture.Note the edematous aspect of the mucosa 1 week after placement because of the lack of hygiene in this area.

Figure 3
Figure 3 In this patient, four transgingival screws were placed to allow for the ingression of the second maxillary molar.Corticotomy points were created in the same session.Note the angulation of the screws relative to the cortex.

•
No inflammation around the screws Disadvantages • Root proximity and noble elements • Sprouting and possible burial • Unimaxillary device • Custom-made connectors • Cost • Incision needed • Custom-made connectors J Dentofacial Anom Orthod 2016;19:406 5 NEW PERSPECTIVES ON MINISCREWS: IMPROVING STABILITY

Figure 4
Figure 4 Insertion of subperiosteal screws.A very thick flap was created, concomitant to whic the connection was made in steel.The two screws were inserted into the loops.The flap was replaced and sutured with a 3-0 suture material.

Figure 5
Figure 5  In the case of subperiosteal microscrews, the risk is the embedding of the connector, making it difficult to use and painful for patients.This is directly related to the patients' lack of hygiene.

Figure 7 The
Figure 7The Beneslider system is based on two screws implanted at the level of the medial raphe or on either side of the palatal suture.Note the proximity of the screws to the nasal cavity.

Figure 8
Figure 8 Wilmes device with its rigid connection allows a recoil intrusion of the molars.Tipping is limited to the play of the tube surrounding the connector.Depending on the divergence of the connection with the occlusal plane, an intrusion or extrusion may be imposed on the molars.

Figure 9
Figure 9After establishing molar class-I ratios, the maxillary arch is glued.The Beneslider® then constitutes a maximum anchorage on the molars.The multiattachment can be vestibular or lingual.Note the embedding of the device when the incisors are retracted.

Figure 10
Figure 10The central plate of the connector can be covered in case of insufficient hygiene.Disinfection can only be conducted under local anesthesia.

Table I :
Comparison of the devices studied or described by Melsen, Wilmes et Chillès.