Temporary skeletal anchorage devices: The case for miniplates

The desire to have complete control over anchorage is no doubt universal among orthodontists. About 100 years after orthodontists first started using tooth-borne anchorage for orthodontic treatment, temporary skeletal anchorage devices appeared. It was clear that orthodontics would be a completely new ball game; soon temporary skeletal anchorage devices had become indispensable modalities in modern orthodontic practices for adults. Beyond that, temporary skeletal anchorage devices are at the center of innovations of surgical orthodontics for jaw deformities and the orthopedic treatment of growing patients with skeletal disharmonies.

As temporary skeletal anchorage devices were being developed in the 1990s, 2 types were widely put into use. There were great expectations for those that could osseointegrate with bone. This type included retromolar implants, palatal implants, and mini-implants. The other type, developing in parallel, was the mechanical retention type and included miniplates and miniscrews. Extensive clinical experience for a wide range of orthodontic problems and detailed evaluations of these modalities over the years have brought us to where we are now: the temporary skeletal anchorage devices in use are miniplates and miniscrews, and both offer mechanical retention.

These 2 types of devices actually function best when they are working in collaboration with each other. They function differently, but both are indispensible in cutting-edge orthodontic treatment. Although the focus of this Counterpoint article is on miniplates, miniscrews also have a valuable role in modern orthodontics.

Structure of miniplates

Miniplates are made of titanium or titanium alloys and come in various shapes and sizes. All miniplates have 3 parts: head, arm, and body. The head portion is intraorally exposed and positioned outside the dental arches. The head comes in a variety of shapes: circular, hooked, and tubular. Some are like bendable sticks that can be manipulated into the desired shape. The arm portion is transgingival or transmucosal and tends to be rectangular or round. The body portion is positioned subperiosteally, and its surface is attached to the bone. The body portions are classified into 4 basic shapes: T, L, Y, and I (straight). The body portion is fixed on the bone surface of the zygomatic buttress or the mandibular body with 2 or 3 miniscrews. Although there are many variations in miniplate heads, there are fewer variations in the body portions.

Success rates and stability

Perhaps the greatest advantage of miniplates is their high success rate. In a systematic review of temporary skeletal anchorage devices by Schätzle et al, the average failure rates of various devices were 7.3% for miniplates, 10.5% for palatal implants, and 16.4% for miniscrews. The authors concluded that based on the available evidence in the literature, miniplates provided reliable absolute orthodontic anchorage. In another report, Nagasaka et al reported that just 3 of 107 miniplates had to be replaced; this is equivalent to a failure rate of 2.8%. In another report by Choi et al, an average failure rate of 7% was reported for miniplates. The failure rates of miniplates were 6% according to Takaki et al and just 3% in a study by De Clerck and Swennen when miniplates were used as bone-anchored maxillary protraction for growing Class III patients. Clearly, although the numbers vary, all of these reports indicate the overwhelming success of miniplates, whether used in the maxilla or the mandible.

Since miniplates are made of pure titanium or titanium alloy, they exhibit onplant effects on the bone surface, and the screws inserted into the cortical bone exhibit implant effects in addition to the mechanical retention effects. This means that in addition to their excellent mechanical properties, miniplates have more benefits because of this osseointegration. This double effect undoubtedly contributes to the much higher anchorage value and better stability of miniplates than other temporary skeletal anchorage devices, and this is what makes them the best choice among temporary skeletal anchorage devices.

Success rates and stability

Perhaps the greatest advantage of miniplates is their high success rate. In a systematic review of temporary skeletal anchorage devices by Schätzle et al, the average failure rates of various devices were 7.3% for miniplates, 10.5% for palatal implants, and 16.4% for miniscrews. The authors concluded that based on the available evidence in the literature, miniplates provided reliable absolute orthodontic anchorage. In another report, Nagasaka et al reported that just 3 of 107 miniplates had to be replaced; this is equivalent to a failure rate of 2.8%. In another report by Choi et al, an average failure rate of 7% was reported for miniplates. The failure rates of miniplates were 6% according to Takaki et al and just 3% in a study by De Clerck and Swennen when miniplates were used as bone-anchored maxillary protraction for growing Class III patients. Clearly, although the numbers vary, all of these reports indicate the overwhelming success of miniplates, whether used in the maxilla or the mandible.

Since miniplates are made of pure titanium or titanium alloy, they exhibit onplant effects on the bone surface, and the screws inserted into the cortical bone exhibit implant effects in addition to the mechanical retention effects. This means that in addition to their excellent mechanical properties, miniplates have more benefits because of this osseointegration. This double effect undoubtedly contributes to the much higher anchorage value and better stability of miniplates than other temporary skeletal anchorage devices, and this is what makes them the best choice among temporary skeletal anchorage devices.

Biomechanical features of miniplates

For molar distalization, whereas the development of miniplates made it possible to predictably move all molars 3 dimensionally, they have been most frequently applied for distalizing the maxillary molars. The distalization of the maxillary molars is necessary in patients with Class II malocclusion or anterior crowding. The common features of those malocclusions are a narrow maxillary arch and mesial rotation of the maxillary molars. In such cases, the most rational approach is to distalize the maxillary molars using absolute anchorage placed in the buccal side rather than the lingual side. This makes it possible to distalize the molars and rotate them distally simultaneously; this in turn expands the maxillary dentition. To distalize the maxillary molars in such a way, absolute anchorage is put in place at the thick cortical bone of the zygomatic buttress. The placement of miniplates at the zygomatic buttresses presents no difficulty because the practitioner has a clear view of the bone. If miniscrews are used, the practitioner is going in blind. If miniscrews are the temporary skeletal anchorage device of choice, then they should be installed in the palate to distalize the maxillary molars. However, because the distalization mechanics with palatally installed miniscrews tends to aggravate the mesial rotation of the maxillary molars, complicated orthodontic mechanics are necessary to offset these undesirable side effects. Miniplates are simply a better option in such cases.

The distalization of the mandibular molars is known to be much more difficult than that of the maxillary molars. Whereas traditional mechanotherapies gave practitioners no viable options, with orthodontic miniplates, the distalization of the mandibular molars is no longer a problem. All that is required is for patients to undergo minor surgery for the implantation of the miniplates. Since miniplates are put into place with short monocortical screws, there is no risk of injury to the roots or the inferior alveolar nerve. Thus, miniplates are frequently used to correct mandibular anterior crowding, an asymmetrical mandibular dentition, and an anterior crossbite with a Class III dentition, and for decompensation of the mandibular incisors in patients who need mandibular advancement.

For molar intrusion, the incredible difficulty of intruding molars has seemed to practitioners to be an insurmountable obstacle in meeting their goals. With traditional orthodontic mechanics, it was all but impossible. Until recently, in all patients when intrusion of the posterior dentition was required for open-bite correction, orthognathic surgery was the only successful treatment option. At the end of the 1990s, Umemori et al reported an open-bite patient who was successfully corrected with miniplates. This was a milestone in the history of orthodontics. Ever since, with miniplates offering absolute anchorage, there has been a nonsurgical treatment option for adults with a skeletal open bite. Today, miniplates and miniscrews are indispensable in the treatment of open-bite patients not characterized by a severe anteroposterior jaw relationship. It must be conceded here that a nonsurgical approach with temporary skeletal anchorage devices is by no means easy: in addition to the obvious vertical problems, most open-bite patients also have various anteroposterior and transverse problems, including maxillary protrusion, anterior crossbite, asymmetric dentition, crowding, and congenitally missing teeth. To effectively address the multiple problems we are typically confronted with, the orthodontic mechanics for open-bite correction must be flexible. Miniplates, without doubt, are the most reliable, predictable, and effective modality available to us. In the correction of open-bite patients with relatively less complex orthodontic problems, the use of miniscrews has become a standard option.

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Apr 6, 2017 | Posted by in Orthodontics | Comments Off on Temporary skeletal anchorage devices: The case for miniplates
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