More people are keeping their natural dentition for life, and even those already wearing a prosthesis are starting with these at an older age. This later start has one major disadvantage: it is much harder and takes longer for the patient to become accustomed to this foreign body than for someone who started with dentures at a younger age.
For the patient, the difference between 1 or 2 mandibular teeth stabilizing a lower prosthesis versus a full lower denture is recognized as enormous. Not everyone can accustom themselves to this change. Dentists are well aware that the preservation of those remaining teeth is extremely important. After losing them, life for some patients is never the same again.
Implants have opened new horizons, and denture stabilizers are particularly important. However, not everyone can afford implants, and of those who can, not all have the necessary amount of bone to enable a denture stabilizing implant to be used. The question is, what can be done to improve the survival of those remaining teeth that make the difference between the patient being able to cope or being miserable with dentures?
FIRST PRINCIPLES: HELP TO SAVE
Frequently, patients own neglect of their dentition has been a contributory factor to tooth loss, and an attempt to improve this (preferably prior to any other treatment) is important. Checking the articulation is another prerequisite.
Next, consideration should be given to what can be done to prolong the life of the remaining teeth while still allowing them to stabilize the prosthesis.
There are several factors leading to premature loss of teeth that support prostheses:
(1) Occlusal overloading. As most patients wearing a partial prosthesis are aware and can demonstrate, when they wish to bite something hard, they will choose to locate it between their remaining natural teeth and the opposing denture. They can apply much more force and have better proprioception than in areas without natural teeth. The inevitable overloading of such a unit or units leads to bone resorption. Bone resorption results in pocketing and loosening. With a lower bone level creating a longer anatomical crown, the leverage on the remaining supporting structure perpetuates the cycle of bone resorption and premature loss of another unit, avoidance of which is vital.
(2) The leverage, both by natural opposing teeth and by the prosthesis on the tooth, is often considerable. The free-end saddle resting on soft tissue that moves during mastication and the mouth closing when the prosthesis is not being worn (as at night) are just 2 examples of causing leverage.
(3) The presence of clasps and occlusal rests necessary and legitimate adjuncts can have detrimental effects on the unit, the survival of which is now so very important.
WHAT CAN BE DONE?
Consideration must be given to what can be done to prolong the survival of such a tooth (unit) while still keeping it as a denture stabilizer. All routine considerations must be observed regarding oral hygiene, articulation of the teeth, the fit of the prosthesis, and bone resorption, and the question of a reline should be reviewed. It should be borne in mind that with bone resorption, additional stress is transmitted to the crown of the remaining tooth or teeth. The goal is to lessen the load on the unit but still keep it as a denture stabilizer.
If the tooth has had endodontic treatment, removal of the crown is recommended. The crown should be removed not only for single-rooted teeth, but for any tooth, including molars, provided the root is suitable. Removal of the crown reduces excessive occlusal forces, decreases the leverage on the root and the pressure the root applies to the bone, and can increase the survival time of such a unit in the jaw.
The clinical decision whether to endodontically treat a vital tooth so that the crown can be removed is of course a more complex decision. The relevant factors should be considered, and when they indicate that the procedure will increase the survival time of the tooth, the procedure should be followed.
FACTORS TO BE CONSIDERED
The following factors have to be taken into consideration: (1) which tooth is involved; (2) the amount of restoration of the crown; (3) the anatomical shape and length of the root; and (4) the length of the crown.
Which Tooth is involved
A molar can be made into the best denture stabilizing unit because of its root structure. It can survive heavy occlusal loads and leverage forces better than single-rooted teeth. However, one would not suggest routine endodontic treatment on the vital molar to make it into a stabilizer. Certainly, a vitality test should be carried out, and if negative, then endodontic treatment should be considered and followed by adding an overdenture unit.
If a single-rooted tooth is nonvital, endodontic treatment should be carried out. Factors against devitalizing would be a crown without any restoration or a substantial root with an unusually short crown.
The Amount of Restoration of the Crown
If a tooth has had a considerable amount of restoration and now needs additional restorative work, then it should be considered for removal of the crown. If a crown restoration requires replacing, removal of the crown should be considered.
The Anatomical Shape and Length of the Root
A short and more tapered root with narrow cross section has a poor prognosis, while a broader, longer, more cylindrical root has a good prognosis. The more the root is like a wedge, the more pressure is conducted onto the alveolar bone when loaded, which is then more likely to resorb, causing the root to loosen.
The Length of the Crown (Crown-Root Proportion)
The proportion of crown to root is an important factor. If the anatomical root is less than 60% of the single tooth length, then endodontic treatment and removal of the crown is likely to increase the life of the remaining root so that it can act as a denture stabilizer.
The above factors can only be considered if the root itself and the bone are healthy and indicate a good prognosis for such a unit.
THE ROOT AS DENTURE STABILIZER: THE CONNECTOR BETWEEN ROOT AND DENTURE
The aim of the treatment described in this article is to increase the number of years the root will survive in the jaw while giving support to the prosthesis so the patient feels secure, can speak and eat without fear of losing the prosthesis, and can remove, clean, and reinsert it with comfort. It has to be borne in mind that the patient is likely to be in advancing years. This approach permits sharing of the occlusal load between the root and the soft tissues while permitting normal movement of a free-end saddle without causing leverage on the unit.
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| Figure 1. Kurer Denture Anchor. Male: ball, neck, collar, and threaded shank. Female: cap. |
It has been shown over many years with attachments and also with implant attachments that it is the ball and socket that not only provides stabilization but also allows movement of the prosthesis without causing leverage on the root. The technique described in this article utilizes the Kurer Denture Anchor, which has a threaded shank with the ball at the coronal end (Figure 1) and is fitted into the root canal. A cap, which fits on to the ball, is secured into the denture by a simple method. The ball and socket permits movement of the prosthesis without leverage on the root and allows the occlusal load to be shared between root and supporting tissue. The threaded anchor shank optimizes post retention and stress distribution.1-7
A TYPICAL CASE
When a crown adjacent to a prosthesis fractures and becomes detached, it is not uncommon for the root to be removed, a tooth added to the denture, and perhaps a clasp added to the next tooth. When the patient is already wearing a partial, why go to the expense and trouble to save one tooth? This is exactly the case where the important question should be asked: Could this root be saved and become a denture stabilizing unit rather than an extraction?
If possible, the existing denture should be used and perhaps 6 months later consideration given to a new prosthesis; the original one then becomes a spare. By then, the patient has grasped the benefit of having saved the root. It is such a case that is now shown to illustrate the technique and how simple it is.
MAKING THE ROOT INTO A DENTURE STABILIZER
It is assumed that a satisfactory root filling has been completed, during which some widening of the canal and filling of the root canal system has taken place.
Some of the root filling material must now be removed, leaving 4 mm of apical filling material undisturbed. The instruments recommended for the removal of gutta-percha are Gates Glidden drills, used in a regular or speed-reducing contra-angle handpiece rotating at approximately 1,000 rpm. It is advised to avoid size 1 and commence with size 2 Gates Glidden. Then sizes 3 and 4 are used. The use of an endodontic stop is recommended.
The crown is now removed. The aim in removing the crown is to leave a slightly curved root face (the shape the gingiva would have if the root were absent). In the middle of this surface around the root canal, a small, flat surface is ideal, on which the collar or flange of the attachment will seat. The ideal level of the periphery of the root face would be just below the gingival crest yet above the periodontal ligament. There may be hollows left by previous fillings that must be restored. The root face should be left as smooth as possible to assist in keeping it clean.
The root canal is now prepared to receive the post in a simple 4-step preparation.
PREPARATION
The Denture Anchor Kit of required size is used.
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| Figure 2. Starter reamer. |
Step 1 (Figure 2): Place the starter reamer (depth reamer) in the contra-angle handpiece and drill to the required depth. Water should be used since it acts both as a lubricant and for cooling. The handpiece should be rotating at approximately 2,000 rpm. Two or 3 movements in and out of the canal will ensure that the preparation is to the correct depth and ready for further widening.
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| Figure 3. Width reamer. |
Step 2 (Figure 3): The finish reamer (width reamer) is next used to widen the preparation to the exact diameter so that the thread can be cut into the wall of the canal safely. It is recommended that the finish reamer is placed in the handle provided, and the preparation is reamed by hand to ensure that the correct width is obtained. A contra-angle handpiece may be used, but care should be taken to avoid too many in-and-out movements resulting in the preparation becoming too wide for the effective use of the thread cutter.
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| Figure 4. Thread cutter. |
Step 3 (Figure 4): The thread cutter is now used. It is a sharp, 3-bladed hand instrument, and when rotated into the root canal that has been widened to the correct dimension, gently cuts the thread into the wall of the root canal without damaging the root.8-10 As the thread cutter is a few microns wider than the matching threaded anchor shank, a circumferential vent is created. This vent is the ideal size for the optimal film thickness of cement, so that on cementation the space is filled with cement without any venting grooves being necessary in the shank.
The tip of the instrument is introduced into the prepared canal and with gentle apical pressure turned clockwise for approximately 2 full turns. For every further clockwise turn, a partial anticlockwise turn is made, which helps to free debris as the thread cutter progresses apically into the canal, cutting the thread into the canal wall.
When the thread cutter touches the apical end of the preparation, it feels clearly and distinctly tight, the sign that the thread-cutting procedure is complete. The thread cutter is then removed by unscrewing.
Sometimes, when the thread cutter is about halfway into the root canal, it may start to feel a little tight. When this occurs, it is unscrewed, the flutes cleaned with a bur brush, and the instrument reinserted. If, however, little or no resistance is felt, this may be the result of previous over-widening as already mentioned or the presence of remaining decayed dentin. In such cases, the next larger size thread cutter should be used.
Step 4: Using a small mounted flat diamond disc, a flat central portion with the root canal at its centre is made on which the shoulder of the attachment will seat.
ADVANTAGES OF CUTTING A THREAD
Cutting a thread into the canal wall provides 5 major advantages for any post system:
(1) Mechanical retention, due to the interlocking of the threads of the shank into the threads cut into the wall of the root canal.
(2) Because the threaded surface zigzags instead of being smooth, it has nearly double the surface area compared with a cylindrical post of the same length. This surface area provides a much greater surface for cement adhesion.
(3) The larger the surface area, the better the stress distribution.
(4) Because a precision cutting instrument is used to cut the thread and is made microns wider than the post to be used, there is ideal space between the post and the canal wall, which becomes the optimum film thickness of the cement lute. If a cement layer is too thick, it has poor shearing strength.
(5) A hand reamer leaves an accurate dimension. The thread cutter creates a precision preparation ready to receive the exact mating post.
FITTING AND CEMENTATION OF ANCHOR ATTACHMENT
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| Figure 5. Denture Anchor tried in. It is too long. |
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| Figure 6. Shorten using wire cutters. |
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| Figure 7. Bevel the cut end. |
The denture anchor of matching size is inserted into the driver and rotated into the prepared canal. It is usually too long (Figure 5), and the shank of the attachment has to be shortened. It can be shortened with a pair of wire cutters (Figure 6). At this stage, some dentists find a periodontal probe useful for measuring the canal depth. The cut end of the shank is smoothed off by rotating it on a revolving diamond disc (Figure 7). It is then retried and, if necessary, shortened a little more. The correct length will allow the shoulder of the attachment to seat on the flat area prepared on the root face when it is screwed into position. If the shank has not been shortened sufficiently, pressure could be created at the apical end when the post is tightened into position. If the post is slightly short, any space remaining will fill with cement.
Before cementation, the canal is thoroughly washed and blown dry. It must not be assumed that the canal is dry just because air has been blown into it. A large paper point should be inserted into the canal to make sure that all moisture has been removed before air is blown in again. For additional security, it is recommended that the threaded shank be sand- blasted before cementation.
Any luting cement is suitable for the cementation procedure. It is worth recalling that zinc phosphate cement remains the standard against which other cements are compared.11 The threaded shank either can be dipped into the mixed cement or cement can be spun into the canal prior to the insertion of the anchor. With the latter cementation method, added care must be taken to be certain that the cement has not started to set prior to post insertion.
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| Figure 8. Cemented attachment. Shoulder resting on root face. | Figure 9. Radiograph of cemented denture anchor. |
Upon screwing the anchor into place, firm pressure can be exerted as the flat shoulder of the attachment is coming into contact with the flat root face. Two flats are coming into contact, ensuring that no apical pressure is created (Figures 8 and 9).
How to fit the Cap into the denture so that it properly seats on the Ball
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| Figure 10. Left spacer. Right anchor cap. |
The denture anchor consists of the threaded shank with the ball at its coronal end, onto which the cap in the prosthesis fits. Another component, the spacer (Figure 10), a little larger than the cap, is used to ensure that there is a simple and reliable way in which the cap is fitted into the denture. The use of the spacer is recommended whether a new prosthesis is being made or only one tooth is being added to the existing denture, as in the case illustrated.
ATTACHING THE ANCHOR CAP TO THE PROSTHESIS
Procedure, Part 1
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| Figure 11. Spacer in place prior to denture being inserted for impression. |
Place the spacer onto the ball (Figure 11) and secure it with temporary cement. Insert the denture into the mouth and take an impression using the material of choice. Remove the impression; the spacer will usually remain in the mouth and the denture will remain in the impression. Should the spacer have stayed in the impression, remove it from the impression and send the impression to your technician.
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| Figure 12. Lab model showing plaster replica of spacer. | Figure 13. Denture returned from lab ready to have cap processed into space. |
When the technician has cast the impression, he has a model where all he has to do is add a facing around the plaster replica of the spacer (Figure 12). No additional skill or experience is required by a technician beyond that of adding a facing. Because of the spacer, the recess inside the added tooth is located correctly above the ball attachment, ensuring that when the prosthesis is returned, the recess for the cap is in the right place and of correct dimension, ready for the cap to be fitted (Figure 13).
Procedure, Part 2
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| Figure 14. Cap on ball, with space between cap and shoulder blocked out. |
The technician has added the tooth containing the recess to the existing denture. The denture is ready to have the cap fitted. The anchor cap is placed onto the ball of the anchor and the denture is tried in the mouth to confirm the fit. The space between the lip of the cap and the shoulder of the attachment has to be blocked out. The self-curing resin must not flow onto the neck of the ball attachment! A simple way of blocking out this small space is to coat the lip of the cap, which will be facing the shoulder, with a calcium hydroxide lining material and then seating the cap onto the ball (Figure 14).
The denture is now ready to have the cap cured into the recess of the added tooth with the use of a self-polymerizing resin. Using a separating medium like Vaseline, coat everything around the cap recess, but stay clear of the recess. Mix the self-polymerizing resin and fill the cap recess. Insert the denture and have the patient bite together firmly, causing soft-tissue compression. Place your hand beneath the patients chin to ensure that this pressure between the upper and lower teeth is maintained. This biting together, giving mucocompression, ensures that the support for the denture is equalized between the denture stabilizing tooth and the soft tissues. If the patient were not to bite together when the attachment is fixed into the prosthesis, during mastication following treatment the soft tissues would yield and excessive pressure would load onto the root. The aim is to get equal support from both soft tissues and the stabilizing unit, and this is achieved at this stage of fitting the cap.
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| Figure 15. Cap processed into denture. Its location is 100% accurate. | Figure 16. Denture stabilized. |
When the polymerizing material has set, the prosthesis can be removed. The first time this is done, considerable force is needed to remove it. (Figure 15). Surplus resin is easily removed, provided Vaseline was applied as a separating medium. Care must be taken that no sharp spicules remain. The patient is invited to insert the denture and remove it, and the articulation is checked (Figure 16).
Oral hygiene instruction including brushing the root face with fluoride toothpaste and regular examination appointments are essential.
In the future, a new prosthesis is considered so that the original becomes the spare. The technique for the new prosthesis is identical to the one described for adding a tooth and attachment to the existing denture.
CONCLUSION
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| Figure 17. A molar, the ideal denture stabilizer. |
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| Figure 18. Radiograph of attachment in palatal canal of molar. |
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| Figure 19. Cap in saddle of a skeleton denture. |
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| Figure 20. Stabilized. |
Once a patient is wearing a partial denture, the importance of every tooth is paramount. A great deal can be done to increase the survival of a denture stabilizing unit by removing the crown. This procedure is very successful and has been in use worldwide for more than 30 years. It is a simple procedure as described, and it can be applied to a molar or a single-rooted tooth (Figures 17 through 20). It is an excellent alternative to an implant being used as a stabilizing unit; the patient benefits by a lower cost and a secure solution.
This procedure will be successful if, as previously mentioned, the root is sound and of adequate size.
References
1. Mizukawa M. Threaded post system. Japaneses Dent J. 1972;11:19-27.
2. Standlee JP, Caputo AA, Hanson EC. Retention of endodontic dowels: effects of cement, dowel length, diameter, and design. J Prosthet Dent. 1978;39:400-405.
3. Kirchhoff W. The axial and horizontal carrying capacity of various intraradicular pin anchorages (permador pin, radix anchor, kurer anchor) in natural teeth [in German]. Zahnarztliche Welt Reform. 1978;87(21):1014-1024.
4. Johnson JK, Sakumura JS. Dowel form and tensile force. J Prosthet Dent. 1978;40:645-649.
5. Ruemping DR, Lund MR, Schnell RJ. Retention of dowels subjected to tensile and torsional forces. J Prosthet Dent. 1979;41:159-162.
6. Kurer HG. An evaluation of the retentive properties of various permanent crown posts. J Prosthet Dent. 1983;49:633-635.
7. Standlee JP, Caputo AA. Endodontic dowel retention with resinous cements. J Prosthet Dent. 1992;68:913-917.
8. Zmener O. Adaption of threaded dowels to dentin. J Prosthet Dent. 1980;43:530-535.
9. Standlee JP, Caputo AA, Collard EW, et al. Analysis of stress distribution by endodontic posts. Oral Surg Oral Med Oral Pathol. 1972;33:952-960.
10. Felton DA, Webb EL, Kanoy BE, et al. Threaded endodontic dowels: effect of post design on incidence of root fracture. J Prosthet Dent. 1991;65:179-187.
11. Jacobi R, Shillingburg HT Jr. Pins, dowels, and other retentive devices in posterior teeth. Dent Clin North Am. 1993;37:367-390.
Dr. Kurer resides in Cheshire, England, and specializes in the restoration of endodontically treated teeth. He was the president of the British Society of Restorative Dentistry in 1984, a council member of the British Endodontic Society from 1983 to 1986, and president of the Newcastle Dental Graduate Association in 1988. He has lectured extensively in Japan and the United States. He was awarded the Fellowship of General Dental Practitioner UK in 2003 for his service to world dentistry. He can be reached at (44) 161-428-5080 or peter@kurer.co.uk.
Disclosure: Dr. Kurer is the inventor of the Kurer Anchor System, and is the author of the book The Kurer Anchor System (Quintessence). He receives no financial remuneration from the sale of the Kurer Anchor System.
