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The Procedures, Limitations and Indications for Small Diameter Implants and a Case Report

August 1, 2004
by Carl E. Misch, DDS, MDS and Hom-Lay Wang, DDS, MSD


The most common congenital missing maxillary anterior tooth is a lateral incisor.1 The restoration of one maxillary anterior crown is one of the more difficult prosthetic treatments in a general practice. A single tooth implant is often the treatment of choice to replace a congenitally missing lateral incisor. The implant and crown have the highest success rate of any treatment option, the adjacent teeth are usually unaffected, and the aesthetic result is often ideal.2

The following article presents the indications for a small diameter implant and presents a case report of a patient missing one maxillary lateral incisor.

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The average lateral incisor is 6.5mm in width, 1mm less than the canine and 2mm less than the central incisor.3 When a permanent lateral incisor is missing, the deciduous tooth is retained longer then ideal, and the collapse of the permanent incisor mesio-distal space occurs as the canine positions itself in a more mesial position. In addition, when a lateral incisor is congenitally missing, the contralateral incisor is often peg or deficient in mesio-distal size.1

This condition further reduces the space dimension. As a consequence, the missing lateral incisor space is often less than average and approaches 5mm. Mandibular central and lateral incisors are missing less often, but also have a smaller mesial-distal dimension, and average 5.4mm.3 Therefore, single tooth replacement may on occasion require implants of small diameter to restore the missing tooth situation. For example, a 20-year-old male presented to the office with a congenitally missing lateral incisor. All the remaining teeth were healthy. The inter-tooth space was only 5.5mm. The available bone was greater than 5mm in width and 16mm in height. The soft tissues associated with the central incisor and edentulous site were within normal limits. The maxillary canines demonstrated delayed passive eruption and short clinical crowns (Fig. 1).

Most manufacturers fabricate their smallest two-piece implants in a dimension of 3.5mm or larger at the crest module, although the implant body may be smaller (i.e. 3.2mm). A two-piece implant should have the microgap of the abutment at least 1.5mm from the adjacent tooth.4 As a result, the ideal minimum inter-tooth space for a dental implant with this dimension should be 6.5mm (1.5mm from each tooth and 3.5mm for the top of the implant). This dimension is sufficient for the average dimension of a maxillary lateral incisor.

However, mandibular incisors are usually 5.2 to 5.4mm in width, and congenitally missing maxillary lateral incisors are often less than 6.0mm. As a result, most current implant designs are too large for smaller intra-tooth spaces.

A one-piece implant with a 3.0mm diameter was developed to overcome the challenge of small edentulous spaces in the anterior region of the mouth (Biohorizons Maximus Dental Implants) (Fig. 2). A one-piece implant does not have a microgap between the implant body and abutment connection, and therefore initial crestal bone loss over time may be reduced.5,6

When the implant is not expected to lose proximal bone when positioned at the height of the crestal bone, the implant may be placed as close as 1mm to the adjacent tooth root (Fig. 3). Therefore, mesio-distal spaces for a 3.0mm diameter implant employed here may be as little as 5.0mm (Fig. 4). The primary indications for an implant of this dimension is for single maxillary lateral incisors and mandibular anterior lateral and/or central incisors.

SURGICAL PROCEDURE

The surgical procedure for the 3mm diameter implant follows a similar protocol as other implants. A mucoperiosteal flap may be reflected and direct observation of the bone can be made when the available bone is in question. However, when abundant keratinized tissue and bone are present, a tissue punch and implant osteotomy without tissue reflection is often the surgical protocol of choice.2 This was the surgical method used in this case report. An esthetic crown lengthening procedure was also performed on the maxillary canines.

A 3mm diameter trephine bur was used to penetrate the soft tissue in the missing lateral incisor region (Fig.5). The soft tissue emergence was then contoured with a high-speed handpiece and coarse diamond, so the soft tissue profile was similar to the contralateral tooth.

An alignment drill was then used to initially prepared the implant site and begin to develop the path of insertion for the implant drills (Fig. 6). This drill was also designed to level the crest of the ridge 3mm below the free gingival margin of the implant crown and allow the abutment head of the implant to be level with the bone. A radiograph was taken with the alignment drill in place to evaluate the path of insertion (Fig. 7).

A trial implant was then placed into the initial osteotomy site created by the alignment drill. The top of this device is the same size as the final implant abutment (Fig. 8). The aesthetic position and interocclusal clearance may be determined with this trial abutment (Fig. 9). A periapical radiograph may also be taken of the trial implant to confirm the mesio-distal position and angulation. The position and/or angulation of the initial site may be corrected with the side cutting feature of the alignment drill.

The depth of the osteotomy was established using a 2.0mm diameter depth drill of 12, 15 or 18mm. A 15mm was used in this patient. The longer the implant, the greater is the initial stability. In addition, if the opposing dense cortical plate can be engaged, a further benefit of rigid fixation occurs. A radiograph may be used to confirm the proper drill length and position (Fig. 10). The osteotomy can be widened to 2.5mm using the finishing drill when the bone is of a dense quality. Use of the final drill is not necessary in softer bone types, since the implant will condense the bone during insertion and provide greater fixation. A bone tap may be used when the bone is very dense (i.e., as is occasionally found in the anterior mandible).

The one piece implant body/ abutment was then inserted with a handpiece mount at 30rpm (a hand wrench insertion with a ratchet adapter may also be used) (Fig. 11). The implant was positioned so the threads were 1 to 2mm below the crest of the bone. A periapical radiograph confirmed the position.

A #702L preparation bur was used to modify the abutment as required, with consideration of the opposing teeth in occlusion (Fig. 12). A transitional crown, without any occlusal contact, was then fabricated for the implant.

TRANSITIONAL RESTORATION

There are two options for the transitional restoration for a one-piece, 3mm diameter implant. The first option is an acrylic crown (Fig. 13). However, this crown has no occlusal load for three to four months. In addition, the transitional crown may be splinted to a natural tooth, which has no clinical mobility (i.e. a canine). The diet should be restricted to only soft food and the patient is told to avoid this restoration as much as possible during the initial healing period. After the initial healing, the final restoration may be fabricated and the diet returned to normal.

The second option also uses a pre-made crown. The crown is modified to fit over the abutment, and places the gingival margin in close approximation to the tissue. The pre-made crown is not relined with acrylic. A hole is then made in the mesial and distal interproximal surfaces of the crown. The adjacent teeth are acid etched and composite resin is placed in the interproximal regions of the crown to lute it to the adjacent teeth. The occlusion was modified to eliminate occlusal contact.

This approach provides an aesthetic fixed replacement for the missing tooth, without the excess force on the implant. This approach will offer slightly less risk to the implant during initial healing, since the crown is not actually attached to the implant, and tooth contact will not overload the implant restoration.

The diet is also restricted with this technique to soft foods for the initial bone-implant healing period. After three to fo
ur months the transitional crown is removed and a final restoration was fabricated. Occlusal equilibration is performed to reduce the occlusal load (Fig. 14). A periapical radiograph was taken to confirm the position of the crown and implant (Fig. 15).

DISCUSSION

Limitations

Implants with smaller diameters have several limitations including less surface area, lower fatigue strength, and higher risk of screw loosening. Smaller diameter implants have a smaller surface area for bone-implant contact, and this could reduce the long-term survival of the fixture. The surface area of an implant is related to the amount of force the implant is able to resist when serving as a prosthetic abutment. The roots of posterior natural teeth have greater surface area than anterior teeth, and forces are greater on posterior teeth.

Likewise, an implant with greater surface area is less likely to be overloaded during function.6 A 1mm decrease in width of an implant may decrease the surface area of an implant by more than 40 percent7 (Fig. 16). Hence, a 3mm diameter implant may have almost one-third less surface area of contact with bone as compared to a 4mm diameter implant.

The fatigue strength of an implant is affected by the diameter, the implant material and amount of force applied to the system.8 The formula for the fracture strength of a circular implant is */4(R4). This means that a unit decrease in width decreases the strength of the implant by a factor of 4. For example, a 2mm diameter implant is 16 times weaker than a 4mm diameter implant. Hence, clinicians may use a 2mm diameter transitional implant, but regular occlusal loads over an extended period of time would result in an unstable situation.

The fracture of the implant is also related to the metal of the implant. Regarding the materials from which implants are fabricated, the most common implant body is fabricated from titanium, since a direct bone to implant interface has been shown to develop.9 There are five grades of titanium used for implants.10 Grade one to four is 99 percent titanium and Grade five is titanium alloy (90 percent titanium, six percent vanadium and four percent aluminum). The strength of each of these materials is different. Grade one titanium is four times weaker than Grade five titanium, and although a few manufacturers have used this grade for 4mm diameter implants, it is inappropriate to use for a permanent small diameter implant (Fig. 17).

Some manufacturers of transitional implants select a lower grade titanium so the clinician can bend the abutment post for parallelism. Several implant companies use softer Grade three titanium for their implants, yet Grade three is two times weaker than Grade five. Further, Grade four titanium is 1.6 times weaker than Grade five. Therefore, when small diameter implants are used to permanently replace a natural tooth, a Grade five implant material should be used. The bone to implant contact is similar for all grades of titanium, because a similar oxide layer is formed regardless of the titanium grade.10

The fracture of an implant is also directly related to the amount of force placed on the implant component or body. Greater force is more likely to fracture an implant than lesser force. The maximum bite forces in the mouth are less in the anterior regions (25 to 50 lbs/in2) compared to the molar regions (200 to 250 lbs/in2).11 Therefore, smaller diameter implants should be limited to the anterior regions of the mouth to reduce the occurrence of fracture.

The prosthetic platform of a two-stage small diameter implant is more likely to have screw loosening.12 The narrower the abutment to implant attachment diameter, the more force applied to the abutment screw during occlusal loading. When the abutment screw becomes loose under a cemented crown, the crown may need to be cut off to gain access to the abutment screw. Abutment screw loosening is the most common prosthetic complication of single tooth implants, and has been reported to occur in seven to 40 percent of cases (depending upon patient factors and the implant system used).13 A one-piece implant body and abutment has a distinct advantage since abutment screw loosening does not occur.

The one-piece small diameter implant has several advantages when used to replace maxillary lateral incisors and mandibular incisors. The one-piece design eliminates the risk of abutment screw loosening. Since there is no micrograp between the abutment and implant, the amount of crestal bone loss may also be reduced.6 The abutment-implant connection of two-piece implants is often at or below the crestal bone. With traditional implant designs, bone loss of up to 3mm from the microgap has been reported.4,5

The primary disadvantage for a one-piece small diameter implant is the requirement of immediate restoration. Since the implant abutment is intraoral at the time of surgical placement (the implant body and abutment are a single component), an increased risk of overload is present during initial bone healing. Oral habits or activities such as gum chewing, tongue thrust, and playing some musical instruments (i.e. woodwinds) may overload the developing interface. The “open” transitional crown concept, whereby the pre-made crown is not relined with acrylic, reduces this risk.

Traditional implant sizes of 3.5mm and greater at the crest module are often too large to replace a missing tooth in the anterior regions of the jaws. On the other hand, temporary implants of less than 3.0mm diameter risk fatigue fracture. A fracture of an implant will also place the adjacent teeth at risk during implant removal. An integrated implant, which is fractured, must be removed from the bone using a bur at the expense of the bone and the adjacent teeth.

INDICATIONS

The indications for small diameter implants are primarily related to limited mesio-distal spaces in the anterior region of the mouth. In this way the occlusal forces are reduced to decrease the risk of fracture. The single tooth replacement for maxillary lateral incisors, mandibular central incisors and mandibular lateral incisors often require smaller implant dimensions.

In addition, when adjacent mandibular incisors are missing, splinting two smaller diameter implants together is a better option than cantilevers from one implant. Two small diameter implants have greater surface area than one traditional implant and the moment force is reduced when the cantilever is eliminated.

SUMMARY

Single tooth replacement with an implant and restoration is becoming common, especially in the anterior regions of the jaws. Maxillary lateral incisors are one of the most common congenitally missing teeth, especially in the female population.1 Traditional small diameter two-piece implants are often too large to insert into the mesio-distal space of a missing maxillary lateral incisor. In addition, two-piece implant designs have increased risk of screw loosening, fatigue fracture, and crestal bone loss. A one-piece small diameter implant abutment/body design has been developed by BioHorizons.

This implant is designed to reduce the risk of fatigue fracture, screw loosening and crestal bone loss. A two-year prospective clinical trial on the 22 BioHorizons Maximus 3.0 implants observed a 99.4 percent success rate.14 Future reports are necessary to evaluate the long-term risks of this procedure. Although too early to report, the benefits appear to be greater than the expected risks.

Dr. Carl E. Misch is Adjunct Clinical Professor, Dept. Periodontics, University of Michigan, School of Dentistry.

Dr. Hom-Lay Wang is a Professor and director Graduate Periodontics, University of Michigan, School of Dentistry.

Oral Health welcomes this original article.

REFERENCES

1.Graber JM. Anomalies in number of teeth. In Orthodontics Principles and Practice editor TM Graber, 2nd edition, WS Saunders, Philadelphia, 1966.

2.Misch CE, ed. Single Tooth Implants: Contemporary Implant Dentistry, 2nd ed. St Louis, MO: Mosby; 397-428, 1999.

3.Wheeler RC. A textbook of dental anatomy and physiology, ed 4, 185-283, Philadelphia, Lea
& Febiger, 1965.

4.Tarnow DP, Eskow RM. Preservation of implant esthetics, soft tissue and restorative consideration. J Esthet Dent 8(1):12-19, 1996.

5.Wallace SS. Significance of the biologic width with respect to root form implants. Dent Impl Update 5(6) 20-29, 1999.

6.Misch CE. Early crestal bone loss etiology and its effect on treatment planning for implants. Post grad Dent 2 (3):3-17, 1995.

7.Misch CE. Divisions of available bone in implant dentistry. Int J Oral Implant 7(1):9-17, 1990.

8.Bidez MW, Misch CE. Clinical biomechanics in implant Dentistry. In Contemporary Implant Dentistry, CE Misch (ed) 303-316, Mosby St. Louis MI, 1999.

9.Brown SA, Lemons JE. Medical applications of titanium and its alloys, ASTM STP 1272, 103, Philadelphia, 1996.

10.Lemons JE, Dietsh-Misch F. Biomaterials for Dental Implant. In Contemporary Implant Dentistry, CE Misch editor CV Mosby, St. Louis, 271-302, 1999.

11.Helkimo E, Carlsson GE, Helkimo M, Bite force and state of dentition. Acta Odentol Sand 35:297-303, 1977.

12.Boggan S, Strong JT, Misch CE, Bidez MW. Influence of hex geometry and prosthetic table width on static and fatigue strength of dental implants. J. Prosthe Dent 82 (4): 436-440, 1999.

13.Jorneus L, Jemt T, Carlsson L. Loads and designs of screw joints for single crowns supported by osseointegrated implants. Int J Oral Maxillofac Impl 7 (2): 353-359, 1992.

14.Misch CE, Wang H-L. Small diameter implants. A Prospective 2 year preliminary report. Implant Dentistry. (Submitted for publication).


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