Evaluating the effects of consolidation on intrusion and retraction using temporary anchorage devices—a FEM study

The goal of the present study is to compare and evaluate the effects of consolidation on intrusion and retraction in two implant and three implant scenarios.

In this study, 12 finite element models were created with variations in number of mini-implants, height of placement of bilateral mini-implants and different force levels from the mid-implant and with and without consolidation.

The bilateral implants will be angulated to 30° to the long axis of the occlusal plane as per the studies conducted by Kyung HM, Park HS et al. [2], Ju-Eun Lim [35], and Fathima Jasmine [36] where they stated that if the mini-implant is angulated, the cortical bone and implant interface area will be increased as the most of the implant will be in the cortical bone, and thus, the stability will be increased and also the proper angle of insertion is important for cortical anchorage, patient’s safety, and biomechanical control and mid-implant is placed perpendicular to the occlusal plane. According to Madhupura et al. [28] and Masahiro Iijima [30], the implant of 8 mm?×?1.3 mm is placed because longer miniscrew implants were associated with a greater incidence of sinus and bi-cortical perforations. According to Tina Chugh et al. [37], the highest cortical bone density was observed between the second premolar and first molar. So, the implant was placed at the second premolar and first molar regions. In Gjessing’s [30] rule of thumb, the position of the center of resistance of the anterior segment will be at 9–10 mm apically, so the anterior retraction hook is placed constant at 9 mm from the archwire oriented gingivally and a force of 120 g was given from bilateral implants for retraction and a force of 60 g is given from mid-implant for intrusion (Figs. 1 and 2). The stress distribution patterns in teeth, PDL, and bone along with anterio-posterior and vertical displacements are observed in consolidation, non-consolidation in two implant and three implant systems at low pull (7 mm), medium pull (10 mm), and high pull (13 mm) bilateral (Fig. 3) implant heights from the archwire, and in three implant models, the mid-implant is placed constant at 12-mm height from the archwire. The models were broadly divided into two groups.

• Group I: Bilateral implants placed at different heights (7, 10, and 13 mm) in between the second premolar and first molar (Fig. 1).

• Group II: Along with bilateral implants (7, 10, and 13 mm), an additional mid-implant is placed in between the two central incisors at 12-mm height from the archwire (Fig. 2).

Comparing the consolidation and non-consolidation, the consolidated arches in group I showed more palatal tipping whereas non-consolidated models showed labial flaring of the teeth as crown moves labially and apex moves palatally, which is unfavorable. Whereas in group II, bodily movements are observed in consolidation and in non-consolidation system, the labial flaring of centrals and laterals are observed. The center of resistance for the present study model might be located at 10-mm height as the bodily movement is seen in 10-mm implant and 9-mm anterior retraction hook with consolidation in group II. The force levels passing away from the center of resistance will cause tipping and at the center of resistance causes bodily movement which is proved correct in the present study. Abhishek Parashar et al. [38] conducted a study and inferred that the bodily movement with very minimal torque loss was observed with 8-mm implant and concluded the same.

In vertical displacements, it is concluded that intrusion is seen in three implant system as expected. Intrusive movement was more in the high pull implant than in the low pull and medium pull implant in all cases. The force levels passing from the high pull implant to the retraction hook will cause intrusion and from the medium pull implants shows bodily movement and low pull implant showed tipping forces. A study conducted by Shrinivas S Ashekar et al. [39] suggest that the force levels passing from the high pull implant to the retraction hook will cause intrusion and from the medium pull implant shows bodily movement and the low pull implant showed tipping forces which supports the present study. So, the high pull implants show high intrusive effect, and the canines showed extrusion in all cases as the retraction force is more towards the canine as the tipping of the canines is observed. In consolidation and non-consolidation systems, the intrusion effect is high in non-consolidated system than in the consolidated system as the force are distributed among the teeth in consolidated arches, so less intrusive effect is seen in consolidation.

The present study showed highest stresses on lateral incisors, and this may be due to the short roots of the lateral incisors [40]. Burstone and Viecilli [41] stated that it is a natural concept that larger teeth have more PDL and root support than smaller teeth, and hence, when the same load is applied, the stress magnitudes in the PDL for larger teeth are smaller and larger for smaller teeth. Consequently, the resistance to tooth movement of larger teeth is larger compared to smaller teeth. The maxillary molars suggested that widely divergent roots will require higher loads (causing rotation about the vertical axis of the root) to achieve similar levels of stress, even if the surface area of the root is similar to other less divergent tooth roots and this supports the present study [42].

Stresses on PDL are high in laterals and canines compared to other teeth in all cases, and in 7 mm with mid-implant model (group II), the stresses are more in the posterior teeth than in the anterior teeth and consolidated system showed less stresses when compared with the non-consolidated system. Unfavorable compressive stresses are seen in non-consolidated group, which concluded that consolidation is better than non-consolidation as the stresses are distributed in consolidation system.

Stresses on the hard bone at the implant area in consolidation and non-consolidation in group I and group II inferred that the stresses on the implant area are more in non-consolidated models than in consolidated models in all groups, and the von Mises stresses are equal in group II 7-mm implant model. Stresses on the soft bone around the implant area in consolidation and non-consolidation at different implant heights are equal in all models. In 13-mm three implant model (group II), von Mises stresses are more in non-consolidated model than in consolidated models. Seven-millimeter implants showed highest stresses in the hard and soft bone in all cases. This concludes that low pull implants with high anterior hook produces more stresses in the bone around the implant area (Table 9).

Group I: Von Mises stresses are high in 7-mm implant height, and principle stresses are more in 10-mm implant height in consolidation and non-consolidation

Group II: Von Mises stresses and principle stresses are high in 7-mm implant height in consolidation and non-consolidation

Stresses on the teeth showed more stresses on laterals. This is supported by the study conducted by S. Reimann [30] as the study showed the similar results. But in group II, 7-mm implant height model showed highest stresses on the posterior teeth. In consolidation and non-consolidation in group I and group II, it is inferred that the stresses are more in the non-consolidated models than in the consolidated models in all cases. This suggests that consolidation is important in the case of en-masse retraction with anterior retraction hook (Table 10).

Inference: (laterals recorded the highest stresses in all models)

Group I: In consolidation, 10-mm implant height and non-consolidation 13-mm implant height showed highest von Mises stresses. Principle tensile stresses are high in 10-mm consolidation implant model and 13-mm non consolidation implant model

Group II: In consolidation and non-consolidation, von Mises stresses are high in 13-mm implant height, and principle stresses are more in 10-mm in consolidation and 13-mm in non-consolidation implant heights

So, the present study concludes that consolidation is better than non-consolidation in retraction using implant with anterior retraction hook, and intrusion is effective in three implants than bilateral implants.