A Rationale for Fixed Restorations Supported by Mini Dental Implants: Case Reports and Practical Case Selection Guidelines

Despite a substantial increase in the number of dental implants being placed in recent years, many patients still are left functionally and esthetically debilitated. Lack of adequate bone volume and financial resources as well as compromised health, often accompanied by aging, are predominant factors that prevent patients from receiving treatment using conventional dental implants (CDIs) (≥3 mm in diameter). However, more patients may have clinical situations that are better suited for mini dental implants (MDIs) (≤2.9 mm in diameter) than CDIs in terms of bone volume.¹

The small diameter of MDIs allows for minimally invasive surgical placement often without the need to raise a flap. Flapless implant insertion preserves more blood supply and leaves the periosteum intact, resulting in faster healing, greater patient comfort, and significant reduction in surgical complexity, treatment duration, and cost.^2,3 Because of the relatively simplified and minimally invasive surgical protocol with MDIs, as well as reduced cost, patients with financial limitations, compromised health, and deficiency in interdental space and/or ridge width may benefit from treatments using them. Furthermore, with an increasingly aging population, many elderly patients who may be unable to tolerate the rigors of complex bone grafting and conventional implant surgical procedures could benefit from mini-implant-supported fixed restorations (MISFRs).

MISFRs, in short, potentially reduce the need for bone grafting and the complications associated with such procedures, decrease patient morbidity related to invasive surgery, lessen treatment duration and costs, and minimize the need for cantilevered pontics. Additionally, their usage reduces the need for orthodontic intervention that might be required to gain interdental space to accommodate CDIs. These restorations may also improve patient satisfaction and comfort due to the avoidance of removable appliances and the potential for better mastication.

The use of MDIs began when 1.8 mm pure titanium, smooth-surfaced transitional small-diameter implants were placed to support fixed provisional prostheses during the bone grafting and osseointegration period for CDIs.^4,5 In the 1970s a provisional implant was introduced in the form of a Lew screw.⁶ Some years later, primarily surgical specialists started placing these 1.8 mm transitional mini implants to help restorative dentists provide more stable interim prostheses for their patients. These implants were often used without any specific guidelines or protocols. MDIs were frequently placed between CDIs and immediately loaded with fixed provisional prostheses with minimal regard for occlusal load considerations and adequate initial stability of transitional small-diameter implants. These transitional implants would be removed once their temporary purpose was fully served. This treatment modality has led some clinicians to believe that all small-diameter implants are for transitional use only. Although MDIs may be used for provisional purposes, in recent years they have been used mostly in long-term applications.¹

Since receiving US Food and Drug Administration approval in 2003 for long-term removable and fixed applications (510K by IMTEC Corp), MDIs have been used to stabilize removable prostheses successfully at a relatively economical cost. Surgical placement cost of a MDI, estimated to be US$760, can be as much as 43% lower when compared to that of a CDI, estimated at $1,756. Furthermore, most clinicians use a one-piece implant/abutment rather than a laboratory-made custom abutment.¹ Often, MDIs are surgically placed in a less-invasive flapless manner and coupled with immediate load, offering a high level of satisfaction for many edentulous patients.^7-9 In addition, MDIs are often used to successfully support fixed restorations.^10-12

Today, MDIs are made of titanium alloys for increased strength and with roughened surfaces to promote osseointegration similar to CDIs. Orthodontists also have been using 1.6-mm diameter implants, known as temporary anchorage devices, on short-term bases to create anchorage for a variety of teeth movements to lessen the need for orthognathic surgeries and reduce overall treatment duration and complexity. Studies have documented the clinical success of this application.^13,14

A number of studies have reported successful osseointegration of surface-treated (roughened surfaced) MDIs at histological and clinical levels under immediate load.^15-18 Until the adjustable torque-measuring wrench became available for use with MDIs in 2003, clinicians were unable to accurately determine the level of initial stability of these implants at the time of placement. Unstable implants, whether conventional or mini, that are immediately loaded do not osseointegrate, and, thus, fibrous encapsulation results, followed by subsequent implant failure. With an adjustable torque-measuring wrench, initial stability of MDIs can be more accurately assessed to determine feasibility for immediate load for more predictable and successful outcomes.

Currently, a method of determining the implant stability quotient of a one-piece MDI is not yet available. Although not highly predictable, the author has been using Periotest (Medizintechnik Gulden, med-gulden.com) to measure relative stability of MDIs immediately after surgical insertion and during follow-up maintenance visits. In the author's clinical experience, MDIs with initial stability of 30 Ncm to 35 Ncm at the time of surgical placement that are loaded immediately under controlled protocol for complete mandibular dentures appear to function and achieve osseointegration successfully.

Because of their reduced diameter, it is logical to question the ability of MDIs to withstand occlusal load. A finite element analysis was done to assess the fatigue life of 2-mm diameter implants and found that, mounted in rigid support and under a cyclic horizontal force of 200 N, the implants fractured after more than a million cycles.¹⁹ In another study, 2.4-mm diameter implants embedded in acrylic resin that were subjected to horizontal force at a 45-degree angle fractured at 462 N.¹ Song et al studied the effect of implant diameter on fatigue strength and found that the ultimate failure load and fatigue cycle decreased as the implant diameter became smaller, posing more potential risks on cyclic load.²⁰ Based on these findings, it appears that controlling cyclic horizontal forces is paramount for the long-term clinical success of MISFRs.

Although there have been concerns regarding the use of MDIs to support fixed restorations, such as the potential for implant fracture, their ability to withstand functional and parafunctional load, the degree of osseointegration achievable around the mini implant, as well as the need for clear clinical and laboratory protocols and more long-term studies, many clinicians have been using them for fixed applications successfully.^21-28 In a survey of 677 dentists experienced in implant dentistry, of which 95% were general dentists, 40% of respondents were using MDIs for single tooth replacement.¹ Additionally, 23% were placing mini-implant-supported splinted fixed partial dentures, and 14% were doing tooth/teeth and mini-implant-supported fixed partial dentures. Fixed MDI restorations in the posterior mandible opposing a removable prosthesis have shown 95% survival rate in 5 years.¹¹ Vigolo et al documented high survival rates of single-tooth MDI restorations ranging from anterior teeth to first molars.¹²

Some clinicians have found satisfactory clinical success in splinting MDIs with either CDIs or other MDIs.^10,18 Others have used MDIs to replace a single tooth in deficient interdental space and/or a compromised ridge width with a high success rate.^22,25 Degidi et al showed clinical success in replacing maxillary lateral incisors with MDIs.²³ Some clinicians who used transitional mini implants with a smooth surface for provisional purposes found an adequate degree of osseointegration and believed that MDIs could possibly be used under more definitive and long-term prostheses.^29,30

Risks associated with MDIs are similar to those of CDIs. Because of the smaller diameter of MDIs, clinicians need to focus on reducing occlusal overloads with proper occlusal strategies and case selection.

Five case reports using MDIs (3M ESPE MDI, 3m.com) are presented. (Author's note: Because not all laboratories may be familiar with fabricating MISFRs, clinicians should check with the lab as to whether or not it has experience with these restorations before selecting a lab.)

A female patient in her forties with an unremarkable medical history and flaccid masticatory muscles presented with a desire to replace her missing lower left posterior teeth with fixed implant restorations. Upon clinical and cone-beam computed tomography (CBCT) examination, buccolingual bone width on the lower left quadrant was found to be less than 5 mm and, thus, inadequate for CDIs (Figure 1). The patient declined bone grafting and subsequent conventional implant-supported fixed restorations due to the need for additional surgery, the morbidity associated with the procedure, and additional cost. An alternative treatment option using mini implants was presented, along with the risks and benefits.

Evaluation of the opposing dentition found that the patient was missing her upper left posterior teeth (Nos. 13 through 16), and she was wearing a removable partial denture. After the patient accepted the treatment involving the MISFR, two 2.4 mm x 10 mm mini implants were surgically placed according to the manufacturer's guideline in a flapless manner (Figure 2).

Four months of waiting time were allowed for osseointegration of the mini implants. A standard fixed crown-and-bridge protocol was followed for manufacturing splinted porcelain-fused-to-metal (PFM) crowns. The restoration was cemented with non-water-soluble resin-modified glass-ionomer cement.

Figure 3 shows a panoramic radiograph 3.5 years after cementation of the final restoration. The MISFR has been in function successfully for more than 6 years without any complications.

A graduating high school senior female student in unremarkable health presented with congenitally missing right and left maxillary lateral incisors and a desire to replace these two missing teeth with fixed implant restorations (Figure 4). The patient and her guardian stated that she had stopped growing for the past 2 years. The patient had recently finished orthodontic treatment that lasted 2.5 years and was fitted with Hawley orthodontic retainers with pontics.

Upon examination, mesiodistal space for both missing teeth was found to be 5.5 mm. Further orthodontic treatment was recommended to increase the mesiodistal dimension for two-piece conventional implant therapy. Both the guardian and patient immediately declined the recommendation and sought alternative treatment options. Additionally, the patient wanted treatment to be completed before leaving for college in a few months. The clinician informed and discussed with the patient and guardian the risks and benefits of treatment for a MISFR, and they quickly accepted this option.

After CBCT scan evaluation was performed and study models were obtained for creating a restorative matrix for provisionalization, two 2.4 mm x 13 mm mini implants were placed in the No. 7 and 10 positions (Figure 5 through Figure 7). Both mini implants were fixated with initial torque value of 35 Ncm. Resin fixed provisional restorations were fabricated chairside using a restorative matrix and cemented to allow immediate esthetic replacement of the removable orthodontic retainer. The patient was very pleased with the immediate result.

After 4 months of osseointegration, a final impression was taken to fabricate two lithium-disilicate crowns. The crowns were cemented with non-water-soluble resin cement (Figure 8).

In function for more than 5 years, the MISFRs have been free of complications.

The patient was a 40-year-old woman with an unremarkable medical history. She presented with the intention of replacing missing teeth Nos. 19 through 21 with a fixed implant restoration. Bone width at Nos. 19 and 20 was sufficient for placement of 4-mm diameter conventional implants. However, bone width at the No. 21 area was inadequate to receive a conventional implant.

Two options were presented to the patient. One was to graft bone width at the area of tooth No. 21 with subsequent implant placement in that location. The other option was to manufacture a cantilever bridge with a pontic in the No. 21 position. Neither option was satisfactory for the patient. Therefore, as an alternative, placement of a mini implant in the No. 21 position to eliminate a cantilever pontic was discussed, and she agreed to proceed with this treatment.

A 2.4 mm x 10 mm mini implant was placed along with two 4 mm x 9 mm conventional implants on the same visit. The mini implant was left alone without any provisional restoration for 4 months along with the other two conventional implants for osseointegration before three-unit splinted PFM crowns were fabricated and cemented (Figure 9).

Figure 10 shows a panoramic radiograph taken 10 years after cementation of the final restoration. The MISFR has been in service for more than 11 years, and the patient has reported great satisfaction.

A healthy male patient who was graduating from high school presented with his guardian with the hope of having two congenitally missing mandibular first bicuspids replaced. When offered a treatment option of conventional implants, the guardian asked for a more affordable alternative. Treatment for a MISFR was presented, which included discussion of risks and benefits, and this option was readily accepted.

A study model was generated for fabrication of a restorative matrix. Two 2.4 mm x 13 mm mini implants were placed in teeth No. 21 and 28 positions with initial stability of 35 Ncm. Nonfunctional resin provisional restorations were made and cemented immediately after implant placement. Four months later, after allowing time for osseointegration of the implants (Figure 11), final lithium-disilicate crowns were fabricated using a conventional crown-and-bridge protocol and cemented with resin cement (Figure 12). Figure 13 shows a panoramic radiograph taken 4 months post placement.

The MISFR has been in function for 4 years without any complications.

A male patient with an unremarkable medical history presented with a failing Maryland bridge replacing tooth No. 25. The interdental space at No. 25, which was approximately 5 mm, was inadequate for a CDI (Figure 14). Non-implant treatment options were presented and discussed but declined by the patient, who insisted on an implant-supported fixed restoration. Due to the limited interdental space, the clinician determined that implant treatment would require the use of an MDI.

Before proceeding with the MISFR option, the clinician disclosed to and discussed with the patient the possible complication of adjacent roots being injured during implant placement, which may subsequently precipitate endodontic treatment or possible extraction. The patient decided to proceed with this option.

An MDI was carefully placed (Figure 15), with periapical radiographs taken to confirm the proper implant trajectory. The patient declined provisional restoration due to cost. A final PFM crown was fabricated and cemented with resin cement.

After 8 years of the MISFR being in function, the patient has reported no complications.

Achieving adequate initial implant stability and controlling occlusal load are critical factors in the clinical success of restorations using mini implants. In 2008, the author developed and has since been teaching at his Global Mini Implant Institute (GMI) (miniimplanteducation.com) guidelines to help clinicians properly select cases for MISFRs by assessing level of potential risks. Each factor in the proposed guidelines, as described in the following paragraphs, is important in evaluating the feasibility of achieving favorable occlusal load and/or adequate initial mini-implant stability. These guidelines can be used chairside to quickly formulate a tentative prognosis for MISFR.

In most cases, the mandible offers higher bone density in comparison to the maxilla. There is higher probability that the clinician can achieve better initial implant stability in the mandible and that the final restorations will have stronger support against occlusal load.

The anterior segments of both the mandible and maxilla offer higher bone density for better initial stability, and occlusal load is lighter in comparison to the posterior region.

Occlusal load on a MISFR depends largely on the type of teeth that are on the opposing arch. The opposing arch may have no teeth, removable denture teeth, natural teeth, or implant-supported teeth. For example, fixed implant-supported restorations will not have any vertical resiliency as compared to natural dentition and will exert the strongest occlusal force on a MISFR. Further, a soft-tissue-supported conventional removable prosthesis in the opposing arch will vertically move the most due to resiliency of soft tissue and will exert the least amount of occlusal force on a MISFR.^31,32Depending on the type of teeth present on the opposing arch, the clinician can formulate an appropriate prognosis of a proposed MISFR.

The most posteriorly positioned tooth in an arch will be used most extensively for mastication. During swallowing and clenching, the same tooth will contact first before any other teeth come in contact. Therefore, the presence of an occlusal stop, whether a healthy natural or implant-supported tooth, posterior to the tooth/teeth being replaced will protect any teeth anterior to that tooth by absorbing the load. This will prevent overloading of a more anteriorly positioned MISFR and allow clinicians to create a desired level of centric contact on the MISFR for reduced occlusal load.

In general, younger patients can generate more masticatory and parafunctional forces.

Generally speaking, male patients can generate more masticatory and parafunctional forces than female patients, provided they are of similar age and physical stature.

Misch found that increase in crown height from 10 mm to 20 mm would correspond with an increase in the occlusal force applied on an implant by 100%.³³ Thus, shorter crown height would be beneficial for a MISFR considering the reduced implant surface area of a MDI. Greater available bone height can increase the probability of achieving better initial implant stability.

A recent 3-year study found no correlation between non-splinted short implants and crestal bone loss.³⁴ However, only the non-splinted crowns showed screw loosening, whereas splinted prostheses exhibited no abutment screw loosening. Because a MDI is a one-piece implant without any abutment screw, the same force that causes screw loosening with a conventional implant possibly may cause crestal bone loss and/or implant body fracture of MDIs. Therefore, a MDI should be splinted to other consecutive MDIs or CDIs whenever possible.

Some studies have suggested that carefully and properly selected tooth-implant-supported fixed prostheses can be a viable alternative to bone grafting and fixed prostheses supported by implant only.^35,36 Splinting reduces abutment screw loosening when restoring adjacent conventional implants. A MISFR will withstand occlusal load more favorably when splinted to other stable entities.

Brief intraoral examination can reveal the presence of parafunction. Soft-tissue ridging on the lateral border of the tongue and buccal mucosa as well as teeth wear can be indications of parafunction. Conventional diameter implants and bone augmentation would be more preferable for teeth replacement in patients with severe parafunction.

Controlling occlusal load can be vitally important for long-term survival of a MISFR. Cyclic occlusal forces with horizontal components on the restoration have been associated with implant complications.¹⁹ Because implants are vulnerable to forces with any degree of horizontal component, a monoplane occlusal design is typically most desirable for a MISFR to eliminate any deleterious lateral occlusal forces.

Misch and Bidez suggested reducing the size of the occlusal table (buccolingual dimension) for implant restorations.³⁷

In the presence of an occlusal stop posterior to teeth being replaced by a MISFR, there should be very light or no centric occlusal contact and no lateral occlusal interferences. For a multiple-unit MISFR, fabrication of a nightguard may be very helpful and appropriate to minimize and reduce any uncontrollable and excessive parafunctional forces during sleep.

Implants should be given 3 to 4 months to allow osseointegration when possible. If the patient desires immediate provisionalization or the clinician needs to provisionalize immediately in an esthetically critical region, the provisional restoration should be nonfunctional with no centric or lateral contacts.

Many patients today are unable to overcome financial barriers to implant treatment. By reducing the complexity of treatment through the use of MDIs, more patients may benefit from implant-supported fixed restorations, with improved mastication and comfort. Furthermore, more than 6 million people, or about 2% of the US population, have one or both maxillary lateral incisors missing.³⁸ Maxillary lateral and mandibular incisors with deficient interdental space and ridge width may be safely treated with MDIs with less risk of injuring adjacent teeth. Clinical success of fixed applications of MDIs can be significantly enhanced through proper case selection based on practical guidelines presented in this article that clinicians can use chairside.

MDIs are currently used successfully in a variety of clinical situations. As more well-controlled long-term studies and improved protocols become available, fixed applications of MDIs hold great promise and possibilities in broadening access to implant treatment for patients who otherwise may be unable to receive fixed implant restorations. Future research and development in stronger metal for implant bodies and more forgiving but strong restorative material may enhance performance of MISFRs.

Raymond Choi, DDS
Private General Practice with emphasis on Implant Dentistry, Tustin, California