A Modified C-Implant for Simpler and More Efficient Treatment

The original C-orthodontic micro-­implant (C-implant^*), developed in 2000, gained wide usage because of a unique surface treatment that facilitates partially osseointegrated stability against rotational moments and heavy forces, as well as a removable head with various designs and lengths (Fig. 1A).^1,2

The C-implant directly refuted the academic argument that titanium miniscrews should not be osseointegrated. The primary concern was that a screw with a small diameter (1.2-2mm) could easily be fractured. This argument might have applied to miniscrews that were used as simple anchorage for conventional orthodontic treatment. When mini-implants serve not only as skeletal anchorage but also as orthodontic appliances, however, a stronger retention force is required. For that reason, the titanium C-implant was designed with a sandblasted, large-grit, acid-etched (SLA) surface treatment to enable osseointegration as the main means of retention. The partially osseo­integrated C-implant could thus serve as an alternative to the conventional large-diameter dental implant in a number of heavy-force applications (Fig. 2). The osseointegration potential of the C-implant has been proven through various animal experiments and clinical studies, and the nanostructure of the screw surface and bone attachment has been evaluated using atom-­probe tomography.^1,3-6

The C-implant head is available with neck lengths ranging from 1mm to 3mm, allowing for various traction applications. The .8mm-diameter hole can accommodate an orthodontic wire if needed. The longest screw (10.5mm) can also be used as a provisional implant for a congenitally missing anterior tooth.

The C-implant has three main design features to resist fracture. First, the modified cylindrical body promotes even stress distribution during insertion; since the pitch of the screw is less sharp than that of other mini-implants, it also avoids stress concentration during removal, even if it has become osseointegrated. Second, to compensate for this feature, the screw is self-tapping (after a pilot hole is drilled) instead of self-drilling. Third, the two-piece design, with the head and screw as separate parts, makes it possible to reduce stress on the neck during placement and removal. In a 2008 clinical study, while the average removal torque was 16.4Ncm, the C-implant was removed without fracture even with forces as high as 35.4Ncm.⁷

The hole in the MC-implant head is .8mm in diameter—the same as that of the C-implant, but .1mm larger than those of other miniscrews. The hole is slightly offset from the long axis of the screw to minimize the risk of loosening if a wire is inserted into the MC-implant and a counterclockwise rotation force is applied (Fig. 3).⁷ This eccentric position also avoids the tendency of an applied force to unscrew the implant, since the long axes of the screw and the hole are not coincident. Although finite element analysis has confirmed the rotational resistance of this screw and head-hole design, the actual clinical effects will need further study.

Case Report

Two months later, temporary crowns were placed over the MC-implants (Fig. 8). After 29 months of treatment, the brackets were debonded, and fixed retainers were delivered (Fig. 9). The upper and lower dental midlines were coincident.

Discussion

Lack of mini-implant stability can be a serious problem during orthodontic treatment, causing delays and even bone loss.^12,13 The SLA-­treated titanium mini-implant has proved to be more stable than machined-surface mini-implants, thanks to its capacity for partial osseointegration.^6,14,15 Kim and colleagues used atomic-force micro­scopy to demonstrate an increased space for bone attachment in the microstructure and rough macrostructure produced by the surface treatment of the C-implant, resulting in its enhanced stability.¹⁶ The versatile, two-piece C-implant can anchor challenging orthodontic movements such as en masse retraction of anterior teeth, distalization of posterior teeth, and intrusion of posterior teeth.¹⁴ It can also be used for maxillomandibular fixation after orthognathic surgery¹⁷ or as a provisional implant.²

Although the C-implant has many advantages as a partially osseointegrated mini-implant, there are some weaknesses in its design. A conventional orthodontic miniscrew is a narrower 1.6mm in diameter, with a one-piece design and a hexagonal driver adaptation, and is self-drilling for easy insertion and removal. The MC-implant was developed to incorporate these useful features while retaining the multidirectional force resistance of the C-­implant. Its stability is attributable to the offset hole in the screw head and to the notches in the screw spirals.^10,18 Inclined insertion is recommended to increase resistance to unscrewing forces and to avoid damage to the adjacent roots.

The MC-implant can be used for the same treatment applications as the C-implant. For anterior retraction, an archwire can slide with minimal friction through the .8mm hole, eliminating the need for posterior fixed appliances. Anterior torque is easily achieved by adding gable bends to an .016" × .022" or .017" × .025" stainless steel archwire.^19,20 Resistance to rotational moments permits many more biomechanical applications. In the case shown here, the MC-implant was used as a provisional crown-supporting implant for congenitally missing lower incisors in a young patient.² Heavy lateral or occlusal forces should be avoided in such cases; the main purposes of the provisional implants are space maintenance, prevention of alveolar bone loss, and improvement of esthetics until growth is complete.

Finite element analysis and in vivo debonding-­strength tests of MC-implants with various configurations are needed to assess the degree of structural osseointegration that can be achieved in comparison with the C-implant.