Total arch intrusion with TADs: Short-lived heroics or a sensible means of limiting invasiveness

Introduction

The mechanical value of TADs in broadening the envelope of orthodontic tooth movement is undeniable. It is possible to apply forces to teeth in a much more predictable manner facilitating ‘compliance-lite’ correction of complex malocclusions in the vertical plane. However, what is less understood is the biomechanical implications of these changes. What are the limits to intrusion, for example? Furthermore, more importantly, how stable might these changes be?

Historically, treating total vertical maxillary excess was confined mainly to surgical impaction of the maxilla. However, the judicious use of TADs may facilitate intrusion both anteriorly and posteriorly, permitting a reduction in tooth and gingival display (and, by extension, lower anterior facial height) while allowing simultaneous control of the occlusion.

Nevertheless, how much intrusion is possible? What percentage of this remains 12 months after active treatment? Moreover, what factors influence these outcomes? This study carried out in South Korea and published in BMC Oral Health was undertaken to answer these questions

What did they do?

They conducted a retrospective follow-up of 30 patients who had undergone total arch intrusion using miniscrews as follows:

Participants: 

Patients with increased Frankfort-mandibular plane Angle (> 28.6˚) and skeletal 1 or 2 patterns. Their chief complaints included long face, gummy smile, or lip incompetence.

Intervention:

All patients had total arch intrusion using mini-implants. They placed 1.8mm diameter and 7mm long miniscrews buccally between the premolar and molar roots, directly applying elastomeric intrusive forces within 3-4 weeks. The clincians applied a force of 100–150 g  for each TAD on a 0.016 x 0.022-inch rectangular stainless steel archwire in an 0.018-inch slot. They also placed supplementary mandibular miniscrews to prevent compensatory extrusion in 20 patients. All patients had fixed retainers with circumferential removable retainers recommended for full-time wear for the first six months after treatment before being tapered to nights only.

Outcome assessment:

They took lateral cephalograms at the end of treatment and at least 12 months post-treatment. This allowed them to measure vertical movement of the anterior and posterior dentition by calculating the amount of change in U1-PP and U6-PP, respectively.

What did they find?

The results were relatively simple. As expected, the anterior and posterior teeth intruded significantly during treatment- 2.3mm posteriorly and 2.04mm anteriorly. The corresponding decrease in anterior facial height was 2.7mm, and SN-GoMe was 2.1 degrees. This change was accompanied by a decrease in ANB (0.95 degrees), reflecting some mandibular auto-rotation. 

During retention, the SN-GoMe rebounded somewhat (by 0.73°). U1-PP also reverted considerably (0.83mm), equating to approx—41%. The upper posteriors extruded less (0.22mm or 16%). 1.9mm of the 2.7mm (70%) reduction in lower anterior facial height was preserved at 12 months post-treatment. Importantly, they found a correlation  between the severity of the presentation in relation to molar and incisal intrusion.

What did I think?

I think that this was an interesting study. It was impressive that the authors evaluated a reasonable number of patients with this treatment modality. I also note that cases of TAD-based intrusion are commonly presented on social media (typically without supporting cephalometric data). It is, therefore, great to see that this treatment’s utility and stability have been assessed. 

There are some limitations inherent in the retrospective design. It is conceivable, for example, that only successful cases were selected. It is also possible that more stable cases were chosen. Notwithstanding this, it is likely that the relapse rates of 16% for maxillary molar intrusion and 41% for the maxillary incisors are fairly representative. In addition,  the lower relapse rate in the posterior region may reflect the effects of intermittent occlusal forces, which may help to retain the effects of posterior intrusion. 

Many of us have concerns about the appropriate selection of complex mechanics in modern-day orthodontics, with the phrase ‘To a man with a hammer, everything looks like a nail’ coming to mind. Appropriate case selection and sensible levels of invasiveness are undoubtedly important for TAD planning. The treatment protocol in this study was interesting, with forces being delivered solely from the buccal aspect. They did not mention the use of a palatal appliance or the use of palatal TADs. Transient torque-related issues are, therefore, likely to have been encountered during treatment.

It is notable, however, that the approach used was relatively simple. As such, while patient impacts and experiences were not measured, the amount of intervention was not excessive. Given that the only real active treatment alternative is likely to be surgical impaction, this seems realistic in less severe cases. However, some degree of post-treatment change is inevitable.

What can we conclude?

TAD-based intrusion may offer reasonable stability in the short term, although anterior intrusion appears less stable than posterior. While longer-term effects are certainly worth consideration, total arch intrusion seems to offer a reasonable less-invasive alternative to surgical maxillary impaction in selected instances.