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The TAD Axioms Sebastian Baumgaertel, DMD, MSD, FRCD(C)

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By Sebastian Baumgaertel, DMD, MSD, FRCD(C)

1985 Max Gunther wrote the "The Zurich Axioms"; twelve general rules that were devised to keep you out of trouble when investing.¹ The premise was with every time you put your money at risk in a speculative venture, no matter how conservative, you take a gamble with uncertain outcome. The Zurich Axioms are rules used by generations of Swiss bankers that could minimize your potential losses and tip the odds of the gamble in your favor. Anyone who has even the slightest amount of money invested can benefit from this book and should consider reading this short volume. How does this relate to temporary anchorage devices (TADs) in orthodontics? When treating a patient in orthodontics, a practitioner also engages in a venture with unknown outcome. In the broadest sense one could call orthodontic treatment a gamble in which the prudent clinician uses his experience and knowledge to choose the "best" treatment for an individual patient and thus tries to tip the odds for a desirable outcome in his favor.

Orthodontic treatment planning is a very complex process and should be based on individual patient-related factors, the experience of the clinician, as well as the current evidence available in the dental literature. When it comes to orthodontic mini-implants however, it appears as though today many treatment decisions are still arbitrarily chosen, almost at random, based on one or two criteria brought home from an afternoon continuing education seminar or a "hunch" the clinician might have. When playing a game of poker one might win a few hands, but surely lose on average if a similar approach was chosen and hands were played at random. The same applies to temporary anchorage devices. One would require luck to be successful in an individual patient and over the long term. When luck has less of an impact, failure rates would probably be excessively high.

Even though a success rate of 100 percent is difficult to attain, it is possible to achieve success rates in the 90 percent range.2,3 Evidence-based practice takes the uncertainty out of the equation and provides the basis for TAD use in orthodontic private practice where high failure rates are unacceptable.

The TAD Axioms are a group of general guidelines that, just like Max Gunther's Zurich Axioms for your finances, will help you tip the odds for success with TADs in your favor. Any clinician using TADs, or planning to use TADs in the future should be aware of them.

The First Axiom: On Success and Failure
When one tries to review the dental literature for articles on TAD success rates one soon realizes that this is a challenging task. Each study seems to use a different mini-implant insertion protocol and every author appears to use a different definition for success and failure. This makes it very difficult to compare studies and evaluate outcomes. For scientific purposes, a popular definition for success is absence of mobility. This makes sense because TADs are aimed at achieving absolute anchorage and even the slightest mobility of the anchorage device means that this ideal was not achieved. It is therefore a question with a categorical yes/no answer which can be answered easily and accurately.

Some studies speak of success if the mini-implant was in place until the treatment goal was achieved; no mention is made of the implant mobility. This definition should be regarded as a clinical measure of success, because even slight mobility of the TAD can be clinically acceptable and still provide near absolute anchorage. Studies that use this definition should be interpreted critically since they provide less information on the actual TAD stability.

Keeping this in mind, I suggest the term "success" for absolute rigidity of the mini-implant and the term "clinical success" for an implant that preserved the anchorage until the treatment goal was achieved. No distinction is made here between absolutely stable TADs and slightly mobile TADs. The former definition is more scientifically sound and should be used in studies that focus on mini implant survival rates and studies that are primarily concerned with anchorage quality. The latter definition is a more clinical definition and should be avoided in scientific studies that are focused on anchorage quality and implant rigidity. This definition is more outcome-oriented and thus can be used in case reports and publications with clinical focus.

Similar to the definition of success, failure can be defined as presence of implant mobility. This is a clearly defined state which leaves very little leeway for interpretation. Therefore this is the definition that should be used in scientific publications.

However, even if an implant is completely stable, treatment might not be carried out successfully. If for example, the implant interferes with the required tooth movement it might be completely stable and still not lead to the desired treatment outcome (Fig. 1). In this case the implant would be a success in the sense that it is absolutely rigid, but still a clinical failure since it prevented attainment of the treatment goal.

Currently the literature suggests that the average success rate (however it might be defined) lies at 75-85 percent. This also appears to be the threshold for successful implementation in clinical practice. Success rates below this mark make it difficult to achieve predictable results in a timely fashion.

Fig. 1: Canine substitution case with direct bilateral molar protraction. Right side: completed successfully; left side: TAD interferes with further protraction.
A 75 percent success rate, although at first glance it might seem high, is insufficient in private practice. It would mean that in a case that was treated with four bicuspid extractions and four TADs for space closure, one of these TADs would fail, on average. Hence it appears prudent to plan biomechanics that minimize the number of TADs and to plan far enough ahead to avoid interference of the TAD with the tooth movement.

The Second Axiom: On Patient Selection
As with regular orthodontic treatment, proper patient selection is important. Absolute and relative medical contra-indications should be respected to avoid complications and safeguard the patient's well-being. Contra-indications for orthodontic mini-implants have never been specifically listed and hence should, until noted otherwise, be mainly analogous to the recommendations prevailing in general implant dentistry (Table 1).



The impact of TAD placement in patients undergoing bisphosphonate therapy has not been investigated sufficiently. While patients that receive this medication IV appear to generally be at higher risk for osteomyelitis than patients receiving the medication PO, the general recommendations for implant dentistry should be followed until guidelines specifically for TAD use in such patients are developed.6,7

A specific relative contraindication is the need for endocarditis prophylaxis according to the revised guidelines of the American Heart Association (AHA) as adapted by the American Dental Association.8 Insertion of temporary anchorage devices might cause a detectable bacteremia. No official guidelines exist to date regarding the insertion of TADs and endocarditis risk. It appears advisable to prescribe antibiotic prophylaxis in high-risk patients according to the AHA. The other lower risk categories might not require any premedication, but the level of bacteremia associated with TAD insertion has not been investigated sufficiently to date. A consultation with the primary care physician or cardiologist will determine the proper protocol in the patient's individual situation.

While non-compliance might be a general contraindication for orthodontic treatment it is, within limits, an indication for the use of orthodontic mini implants. Lack of elastic wear for example, can result in anchorage loss which might be one of the most important factors leading to compromised treatment results. The use of mini-implants can help avoid anchorage loss or even offset already manifest loss of anchorage. However, unfortunately non-compliant patients oftentimes present with poor oral hygiene.

Placement of mini-implants in patients with poor oral hygiene should be weighed critically since this can lead to gingivitis, peri-implantits and ultimately contribute to loss of the anchorage device (Figs. 2 and 3).9 Before insertion of a mini-implant oral hygiene should be monitored and improved if necessary. Plaque indicators or indices such as the PBI aid in objectifying the oral hygiene state and in monitoring improvement. A potential solution to poor oral hygiene is the placement of TADs in habitually self-cleansing areas of the oral cavity, such as the palate. This would also apply to patients with physical or mental disabilities that are incapable of maintaining proper hygiene. Here hygiene considerations should be included in the planning of the implant site.

The Third Axiom: On Preparation
One of the most common mistakes made by inexperienced practitioners is the lack of preparation; not in the sense of the clinician being improperly prepared for the procedure, but the patient's dentition. Most interradicular TAD insertions require two fundamental rules to be fulfilled to reduce implant loss.

Rule number one that needs to be respected for the insertion of a TAD into the alveolar process is the presence of sufficient interraducular distance or root divergence. If it is lacking naturally, the roots should be positioned orthodontically to allow for a seamless insertion (Fig. 4 and 5). For that reason the majority of TADs are inserted after the leveling and alignment stage in my practice. By planning a preparatory orthodontic phase before placement of the TAD, I can control root positions and ensure near ideal parameters.

The second rule, oftentimes overlooked, is that the orthodontic treatment should have progressed so far that the patient will not require an archwire change during the TAD phase. For example, if a TAD-supported molar protraction is planned using a specific wire, then the TAD should not be inserted until that specific wire is in place. This rule will allow for immediate loading of the implant. It will also ensure that no unintended tooth movement around the implant takes place which could potentially affect the implant stability.

Even if such preparation can take time – weeks or even months – it is an important step that can help avoid excessively high failure rates. Understanding these fundamental rules, having patience and foresight are the keys to proper preparation of a patient to receive a TAD.

The Fourth Axiom: On Implant Site Selection
It is now well-accepted that certain mini-implant insertion sites deliver higher success rates than other sites. This is most likely due to favorable anatomical relationships such as interradicular distance, proper soft-tissue, adequate cortical bone thickness and overall bone depth, etc.3,10 However, higher success rates at certain insertion sites might also be linked to better access for the clinician inserting the implant and thus a more precise placement. Another theory is that some insertion sites allow for better oral hygiene around the implant.9 A combination of all three is probably the reason that certain sites are superior to other sites in terms of success.

As a rule of thumb, a TAD should be placed in the most favorable site anatomically from which the treatment goal can be achieved. This allows highest possible clinical success rates – a prerequisite for successful application in private orthodontic practice. A mini-implant with some type of slot in the implant head allows for greater biomechanical versatility since it allows for both direct and indirect anchorage (Figs. 6 and 7). This greater biomechanical versatility allows for greater freedom of choice when it comes to the selection of implant sites. Mini-implants with a simple anchor head such as a ball or a hook will result in simple "pulling" mechanics since the force application is direct and teeth can only be pulled toward the implant (Fig. 6). This naturally will result in some implants being placed in less favorable sites which might increase failure rates.

The Fifth Axiom: On TAD Insertion
Once the proper implant site is selected under consideration of the local anatomy and the intended tooth movement, the next step is the insertion of the TAD. When it comes to the actual insertion, certain principles are the key to ensuring long-term success. Success of an orthodontic mini-implant, (i.e. lack of mobility until the conclusion of treatment) requires adequate primary and secondary stability.

Primary stability is proportionate to the initial mechanical retention of the implant. A measure of primary stability is the insertion torque.11 Therefore, the higher the insertion torque, the higher the primary stability. At first glance one could assume that the higher the primary stability the better. This would be true if insertion of the "screw" would take place in a non-vital structure such as drywall. Since TADs are inserted in humans though, we know that the situation must be different. TADs are inserted into living tissue that is capable of biological reactions in terms of healing and remodeling.


Fig. 2: Generally poor oral hygiene led to plaque
accumulation around the TAD head and collar
(at gingival level – yellow arrows) in a female smoker.


Fig. 3: Subsequent failure – poor hygiene was probably not the reason but a strong contributing factor to implant loss.


Fig. 4: Preparing the implant site by diverging the
adjacent roots. An optical plier is used to place a gable-
bend in the archwire.


Fig. 5: Follow-up to Fig. 4. Dental long axis of the adjacent teeth is now sufficiently divergent for TAD insertion (yellow lines indicate long axis).

Fig. 6: Direct anchorage schematic – the TAD is placed in the direction of intended tooth movement.

Fig. 7: Indirect anchorage schematic – the TAD can largely be placed independent of the intended tooth movement.

Secondary stability is the stability of the implant after the healing and remodeling processes have taken place. It is thus less a function of initial mechanical retention as it is a function of a favorable healing response of the peri-implant tissues (of course a requirement for proper healing is adequate primary stability – that's why initially loose implants usually fail and rarely gain stability). In order to obtain this favorable response the insertion procedure must be atraumatic and within the physiologic tolerance of the surrounding tissues. Excessively high insertion torques would damage the vital tissues surrounding the TAD by causing excessive compression and should therefore be avoided. An implant placed with such supra-physiologic torque levels would initially present with high primary stability but oftentimes fails a few weeks into treatment.12

One of the most important factors to influence insertion torque is the thickness of the cortical bone. Increased thickness of the cortical bone will increase the insertion torque.2 In order to achieve an atraumatic insertion it is important to be aware of the local anatomy and what cortical bone thickness to expect. In areas with increased cortical bone thickness it might be prudent to consider preparing the bone by pre-drilling before inserting the TAD. Also the use of a torque measuring or torque limiting insertion instrument is advisable; at least until the practitioner has achieved a certain proficiency level at which tactile feedback of the resistance when entering the bone is a sufficiently reliable indicator of insertion torque level.

Conclusion
Orthodontic mini-implants do not appear to be a fad, but rather a useful treatment supplement to our traditional mechanics that will allow us to treat cases in a new, more efficient manner, with more predictable outcomes and higher patient satisfaction. This seems to be the common consensus from orthodontic clinicians to the most critical minds in orthodontic academia.

If you have already gained some experience in the field of absolute anchorage I suspect that you might have already experienced the value, but also some of the limitations of TAD-supported orthodontics. Even if your initial failure rates might be higher than you would like them to be, do not get discouraged. The learning curve is steep and you will improve rapidly, but whatever you do, respect the five TAD axioms.

References
  1. Gunther M. The Zurich Axioms. New York: New American Library, 1985.
  2. Motoyoshi M, Hirabayashi M, Uemura M, Shimizu N. Recommended placement torque when tightening an orthodontic mini-implant. Clin Oral Implants Res. 2006 Feb;17(1):109-14.
  3. Tseng YC, Hsieh CH, Chen CH, Shen YS, Huang IY, Chen CM. The application of mini-implants for orthodontic anchorage. Int J Oral Maxillofac Surg. 2006 Aug;35(8):704-7.
  4. Hwang D, Wang HL. Medical contraindications to implant therapy: part I: absolute contraindications. Implant Dent. 2006 Dec;15(4):353-60.
  5. Hwang D, Wang HL. Medical contraindications to implant therapy: Part II: Relative contraindications. Implant Dent. 2007 Mar;16(1):13-23.
  6. Phillips G. Bisphosphonate therapy and dental treatment. J Indiana Dent Assoc. 2007-2008 Winter;86(4):4-8.
  7. Bell BM, Bell RE. Oral bisphosphonates and dental implants: a retrospective study. J Oral Maxillofac Surg. 2008 May;66(5):1022-4.
  8. Wilson W, Taubert KA, Gewitz M, Lockhart PB, Baddour LM, Levison M, Bolger A, Cabell CH, Takahashi M, Baltimore RS, Newburger JW, Strom BL, Tani LY, Gerber M, Bonow RO, Pallasch T, Shulman ST, Rowley AH, Burns JC, Ferrieri P, Gardner T, Goff D, Durack DT; American Heart Association. Prevention of infective endocarditis: guidelines from the American Heart Association: a guideline from the American Heart Association Rheumatic Fever, Endocarditis and Kawasaki Disease Committee, Council on Cardiovascular Disease in the Young, and the Council on Clinical Cardiology, Council on Cardiovascular Surgery and Anesthesia, and the Quality of Care and Outcomes Research Interdisciplinary Working Group. J Am Dent Assoc. 2008 Jan;139 Suppl:3S-24S. Review. Erratum in: J Am Dent Assoc. 2008 Mar;139(3):253.
  9. Ludwig B, Baumgaertel S, Bowman SJ, editors. Mini-Implants in Orthodontics-Innovative Anchorage Concepts. London: Quintessence Publishing, 2008.
  10. Poggio PM, Incorvati C, Velo S, Carano A."Safe zones": a guide for miniscrew positioning in the maxillary and mandibular arch. Angle Orthod. 2006 Mar;76(2):191-7.
  11. Wilmes B, Rademacher C, Olthoff G, Drescher D. Parameters affecting primary stability of orthodontic miniimplants. J Orofac Orthop. 2006 May;67(3):162-74.
  12. Baumgaertel S. Pre-drilling of the implant site-is it necessary for orthodontic mini-implants? Am J Orthod Dentofacial Orthop. 2010;137:825-9.
Author’s Bio
Sebastian Baumgaertel, DMD, MSD, FRCD(C), received his orthodontic education at Case Western Reserve University where he now holds the position of assistant clinical professor and is co-director of the sub-specialty clinic for skeletal anchorage. In addition, Dr. Baumgaertel maintains an active private practice in the Cleveland area. He is a diplomate of the American Board of Orthodontics, a fellow of the Royal College of Dentists of Canada and a certified orthodontic specialist in Germany.
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