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A Winning Combination: Stainless-Steel Mini-Screws and CBCT Simulations by John Pobanz, DDS, MS

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The envelope of dentoalveolar movement to compensate for skeletal maxillomandibular discrepancies is not as small as once thought.

Treatment planning of these types of cases is changing. The use of skeletal anchorage to create en-masse movements of dental arches is an exciting development in clinical orthodontics. Out of necessity, our colleagues in other countries, such as Taiwan, have embraced this for Class III treatment because of the volume of malocclusions that occur in the population.

Dr. John Jin-Jong Lin of Taipei, Taiwan, is a pioneer in mini-screw-facilitated orthodontics. His work of seeking nonsurgical corrections of Class III malocclusion through en-masse distalization of the mandibular dentition spans more than a decade. His treatment protocols are well documented.1, 2 One of his followers, Dr. Chris Chang, has emerged as a popular speaker at continuing-education venues all over the world. Chang shares his protocols, as well as those originated by Dr. Lin. The combined work of these two extraordinary clinicians can be studied in the e-book, "Orthodontics," available on iTunes. It is a must for the library of any orthodontic specialist.

I became completely engrossed in the case presentations contained in the volume. I intently studied them and began trying to apply the mini-screw protocols to cases that matched the cases shown in the textbook, but doing so with titanium mini-screws.

During those experiences, I faced the challenges of mini-screw fracture, and needing to make pilot holes prior to insertion with titanium mini-screws. A previous case report in Orthotown details my attempts at duplicating Dr. Lin's work.3

In order to distalize the mandibular dentition en-masse, a mini-screw can be placed in the buccal shelf of the mandible. In some anatomical references, this would be called the oblique ridge of the mandible. The horizontal shelf of bone that originates at the first molar becomes progressively wider and more horizontal lateral to the second molar, retromolar pad and then connects with the ascending ramus of the mandible (Fig. 1). The density of the cortical plate in this area is as dense as anywhere in the human skeleton, and much denser than any other area of the maxillomandibular complex.

Dr. Lin's work has shown that the most ideal location for placement in this area is mesiobuccal to the second molar. This insertion location usually offers enough horizontal surface area for vertical insertion of the 2x12 stainless-steel mini-screw. The screw can be inserted without a pilot hole, due to the sharpness of its stainless-steel, self-drilling tip. The stainless-steel properties allow for a fracture resistance triple that of titanium-alloy mini-screws.

The density of the bone in this location requires these features for successful and predictable placement without fracture and without the extra procedure of pilot-hole drilling.

In 2004, Dr. Lin and his colleagues developed the A-1 Bio-Ray stainless-steel mini-screw system in Taiwan. It became FDA-approved in 2010 and is now available for purchase in the U.S. under the trade name SuperTAD.

The advent of 3D imaging is very exciting. We now have the ability to view root structure and the location of neurovascular bundles with ease.

When first implementing buccal shelf mini-screw placement in practice, it is very helpful to perform an insertion simulation prior to the clinical procedure.

The Carestream 9300 allows for collimated views of the buccal shelf of mandibular dentition at very low radiation dosage. The DICOM data can be imported into Anatomage treatment studio software for simulation of placement of a 2x12 mini-screw at the location of the mandibular second molars (Fig. 2a). Clinically, the buccal contour of the second molar can create the illusion that vertical placement of a mini-screw lateral to this landmark could cause a root collision. However, a simulation beforehand will give the clinician confidence that this is not the case. In fact, the angle of the buccal root surface relative to the visible facial surface of the second molar occlusal to the height of contour is about 60 degrees.

The clinician can disregard the buccal surface of the molar, and instead visualize the buccal surface of the root of the second molar for absolute confidence during insertion, and even refer to the Anatomage simulation during insertion. The hounds-field density gradient also allows for assessment of anticipated resistance to insertion with the blue demarcated area. This indicates very dense bone that would otherwise dramatically increase the risk of mini-screw fracture with a titanium alloy screw versus a stainless-steel screw (Fig. 2b).4 The following case report shows correction of a Class III skeletal/dental malocclusion with maxillary dentoalveolar protraction using anterior palate mini-screw anchorage, as well as mandibular dentoalveolar retraction using miniscrew anchorage bilaterally in the mandibular buccal shelf.

Case report
The patient is 14 years old with no significant medical history. She is congenitally missing upper second molars. Her chief complaint is, "I would like my underbite to go away . . . can you do that without jaw surgery?"

Diagnostic parameters (Fig. 3)
  • Antero/posterior: Class III skeletal relationship. 8mm Class III molar and canine discrepancies, negative overjet of 5mm; there is no functional shift present
  • Vertical: 50 percent overbite, brachyfacial pattern
  • Perimeter: Moderate upper crowding and no lower crowding
  • Transverse: Bilateral posterior crossbite
Treatment objectives
  • En-masse protraction of the maxillary arch with mini-screw anchorage
  • En-masse retraction of the mandibular arch with mini-screw anchorage
  • Achieve Class I molars and canines
  • Posterior crossbite correction with disarticulation and early elastic wear
  • Resolution of upper crowding
  • Ensure contact of the mesial marginal ridge of the lower second molars with the distal marginal ridge of the upper first molars to prevent super eruption
Mechanics (Figs. 4a, 4b)
Passive self-ligating brackets were placed in both arches, including molar bands on the upper arch to support a transpalatal arch with distal facing hooks for power-chain placement. Posterior disarticulation was placed on the upper molars and 2.5-ounce elastics were worn to correct the posterior crossbite while progressing through a traditional archwire sequence to 19x25ss in seven months.

Anatomage mini-screw placement simulations were performed, and 1.5mm x 7mm stainless SuperTAD mini-screws were placed in the anterior palate. Some 2x12 stainless-steel mini-screws were placed in both buccal shelves of the mandible at the mesiobuccal of the second molars.

Two hundred and fifty grams of force was applied with elastomeric power chain from the anterior palatal mini-screws to the distal-facing hooks of the TPA, every five weeks. Three hundred and fifty grams of force was applied from the buccal shelf mini-screws to the lower archwire with nickel titanium closing coil springs. Positive overjet, Class I molar and canine relationship were achieved after eight months. Some 19x25 beta-titanium wires were inserted for finishing and detailing. Bonded wire retainers were placed and a removable interocclusal splint was prescribed for nighttime wear for the first year after bracket removal. Lifetime individual Essix retainers were delivered for nighttime wear.

Outcome assessment (Figs. 5a, 5b)
All treatment objectives were achieved. Aesthetic parameters of smile arc, buccal corridor fullness and enamel display were optimal. Patient and parent satisfaction was extraordinary. Cephalometric superimposition shows dramatic en-masse movements of both arches.

Conclusion
The following conclusions can be drawn from this discussion and case presentation:
  • En-masse movements of dental arches with mini-screw anchorage should be considered in modern orthodontic treatment planning.
  • Stainless-steel mini-screws offer advantages over titanium-alloy screws when the buccal shelf of the mandible is the insertion site.
  • 3D simulations of buccal-shelf mini-screw insertions can help the clinician visualize optimal placement with confidence.
References
  1. "Creative Orthodontics Blending the Damon System and TADS to Manage Difficult Malocclusions", 2009 Dr. John Jin-Jong Lin
  2. "Orthodontics" eBook iTunes 2013, Dr. Chris Chang, Dr. John Lin, Dr. Johnny Liaw, Dr. Eugene Roberts.
  3. "Non-Surgical Correction of a Class III Asymmetric Discrepancy with a Combined Approach of Mini-Screw-Facilitated Maxillary Dentoalveolar Protraction and Asymmetric Retraction of the Mandibular Dentition" Orthotown June 2012.
  4. Insertion Torque and Fracture Characteristics of Orthodontic Miniscrew Implants.2011 Dr. Nathan Bartschi. Master's Thesis, St. Louis University.
Dr. John Pobanz has operated Pobanz Orthodontics in his hometown of Ogden, Utah, for 16 years. He is a diplomate of the American Board of Orthodontics. He is an associate clinical professor at the University of the Pacific Orthodontic Residency Program. He speaks regularly to international audiences about the placement of stainless-steel mini-screws with CBCT imaging.
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April 2024
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