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Avoiding Complications With Mini Implants

This case report of two mini implants used to replace a single mandibular first molar illustrates possible reasons for failure when used in posterior applications.

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PURCHASE COURSE
This course was published in the July 2021 issue and expires July 2024. The authors have no commercial conflicts of interest to disclose. This 2 credit hour self-study activity is electronically mediated.

 

EDUCATIONAL OBJECTIVES

After reading this course, the participant should be able to:

  1. Explain terminology, basic specifications, and reported survival rates associated with implants ≤ 3.5 mm in diameter.
  2. Discuss applications for these types of dental implants, as well as potential clinical advantages and disadvantages.
  3. Describe possible reasons for failure of implants ≤ 3.5 mm in diameter.

Advances in dental implants have increased the selection of implant designs, as well as scientific understanding of which design is best suited for a specific clinical condition. Practitioners and patients have also benefited from increased longevity of implant-supported prostheses, better esthetics, and a more sophisticated understanding of the pathogenesis of peri-implant disease. Yet there remains much to learn, including predictable protocols for treating peri-implantitis, risk factors, and a better understanding of the limitations of implants with smaller diameters and lengths, such as mini implants. 

The ease of placement of mini implants has increased their use for a variety of clinical situations, including replacement of load-bearing posterior teeth. The standard root form endosseous implant can range in diameter from 3.5 to 7 mm, and from 5 to 16 mm in length. With respect to implants of less than 3.5 mm diameter, there is potential for confusion regarding terminology; for example, mini versus narrow diameter versus small diameter versus reduced diameter. The terms mini, narrow and small diameter implant are commonly used by clinicians and the literature to describe implants with diameters ranging from 1.8 to 3.3 mm.1–4 The term reduced diameter is more general in that it can refer to any implant within the range of 1.8 mm to 3.5 mm diameter, which includes mini and/or narrow diameter implants. More recent literature has begun to use the term reduced diameter. 

The length of mini and standard implants is similar. The reduced diameter of the mini implant enables use of less complex surgical techniques, as they can be positioned in areas with decreased bone thickness and without surgical reflection of a mucoperiosteal flap and/or bone augmentation. The quantity and quality of bone available in the jaw typically defines the characteristics (diameter and length) and number of implants. Thus, mini implants are often used to replace mandibular incisors because the alveolar bone is generally thin in the facial-lingual dimension. Another common application is using mini implants for support of overdentures.5,6 However, one might question their use in replacement of posterior teeth — especially molars — that are subject to increased bite pressures during mastication. 

In this paper, the authors present a case report of the failure of two mini implants used to replace a single mandibular first molar. The failure resulted in surgical removal of both implants, osseous grafting and, eventually, replacement with a conventional sized implant. In addition to the case report, the discussion will focus on possible reasons for failure of mini implants when used for posterior tooth replacement.

FIGURE 1. Panographic radiograph from 2010 showing normal interproximal bone height and excellent bone levels at edentulous site #30.
FIGURE 1. Panographic radiograph from 2010 showing normal interproximal bone height and excellent bone levels at edentulous site #30.

CASE REPORT

The patient, a 35-year-old female, presented to a general dentist in April 2010 for routine examination and dental prophylaxis. Records from that office indicated no history of periodontal disease, but probing depths were not recorded. Missing teeth included all third molars and the mandibular right first molar (#30; Figure 1). It was noted that tooth #30 had been extracted 10 years prior due to an irreparable tooth fracture. Her medical history was insignificant: no medications, no allergies, no systemic disease, and no history of hospitalization except for childbirth. 

In January 2014, the general dentist suggested replacing edentulous site #30 with an implant. To avoid surgery, mini implants were recommended and placed without mucoperiosteal flap reflection. The implants and single molar restoration at six months post-placement (June 2014) are seen in Figure 2. 

FIGURE 2. Periapical radiograph of site #30 taken six months (June 2014) after placing the two mini-implants and restoration.
FIGURE 2. Periapical radiograph of site #30 taken six months (June 2014) after placing the two mini-implants and restoration.

The patient was seen on emergency having been referred to a periodontist (N.E.B.) in May 2015 with a chief complaint of “severe” jaw pain associated with the mini implant site. The examination revealed the implant fixture exhibited Grade II mobility, and the buccal marginal crown extension overlapped the attached gingiva, essentially creating an “engineered” Grade III furcation involvement. While there was little clinical inflammation (Figure 3 and Figure 4), the exam revealed a diminished width of keratinized and attached gingiva at the buccal aspect of the prosthesis; that is, the mucogingival junction was nearly even with the extended crown margin (Figure 3). Based on the patient’s complaint of pain and Grade II prosthesis mobility, a diagnosis of fractured implant(s) was made and it was decided to remove both implants. 

The surgery was accomplished in late May 2015 and consisted of removing both implants and superstructure as a single unit by gentle use of a #217 extraction forceps. Once removed, resorption of the alveolar ridge to the lingual was noted (Figure 4). A full-thickness mucoperiosteal flap was reflected, allowing visualization of the remaining apical portions of both implants at roughly 5 and 6 mm (mesial and distal implants, respectively) below the bony crest (Figure 5). The apical portion of each implant was osseointegrated, necessitating removal by use of a trephine bur (Figures 6A through 6D). Upon retrieval of the fractured implants, it was determined both had dimensions of 2.4×12 mm. The osseous defects were grafted with particulate cancellous bone allograft and covered with pericardium membrane. Next, the surgical site was closed with 4.0 vicryl sutures.

FIGURE 3. Mirror image buccal view of implant restoration taken May 2015 showing significant overlap of restoration margin onto soft tissue and a minimal band of keratinized tissue due to resorption of edentulous ridge to the lingual.
FIGURE 3. Mirror image buccal view of implant restoration taken May 2015 showing significant overlap of restoration margin onto soft tissue and a minimal band of keratinized tissue due to resorption of edentulous ridge to the lingual.

In November 2015, at six months post-osseous grafting, the surgical site was reentered, revealing a well-formed alveolar ridge, with little evidence of resorption in the buccal-lingual dimension (Figure 7). Subsequently, a 4.8×10 mm tissue level, wide neck endosseous implant was positioned, a healing cap placed, and the flap closed with interrupted vicryl sutures. 

After nine months of healing, the implant was ready for restoration and the patient was referred to her general dentist. A single crown was cemented in August 2016. During a routine follow-up appointment in September 2020, a periapical radiograph showed good bone stabilization and no discernable changes in osseous contour or crestal level since 2016 (Figure 8 and Figure 9).

FIGURE 4. Immediate post-extraction occlusal view showing ridge resorption to lingual and mild inflammation of tissue between the two implant insertion wounds.
FIGURE 4. Immediate post-extraction occlusal view showing ridge resorption to lingual and mild inflammation of tissue between the two implant insertion wounds.

DISCUSSION 

As noted, this is a case report of the failure of two mini implants used to support a single-unit fixed prosthesis replacing a mandibular first molar. While the reason for failure can only be presumed in this case, a logical possibility would be implant fixture fatigue leading to fracture.

Multiple published reports have presented various reasons for failure of smaller diameter implants, including: 

  • Smoking7,8 
  • Use of smaller diameter implants8–13 
  • Posterior localization1,7,11,14 
  • Prosthetic complications when using narrow diameter implants15 
  • Placement of implants in the posterior maxilla and atrophic bone1,8 
  • Risk of implant fracture16,17 
  • History of periodontal disease18–21
FIGURE 5. Occlusal view of surgical site showing apical portion of the distal implant still within bone prior to removal using a 3-mm-diameter trephine bur.
FIGURE 5. Occlusal view of surgical site showing
apical portion of the distal implant still within
bone prior to removal using a 3-mm-diameter
trephine bur.

In this case, the patient’s initial complaint that “everything felt loose since they were placed” appears to support the possibility of implant fracture or lack of osseointegration. However, both implants were of equal length and diameter (2.4×12 mm) and, indeed, were fractured at approximately the same level within the supporting alveolar bone. The patient was a nonsmoker and radiographs taken before implant placement showed excellent bone volume and density. Thus, a small implant diameter, the possibility of inordinate torquing force during placement, and a posterior prosthesis with a heavy occlusal load may have interacted to cause failure.

FIGURES 6A through 6D. View of extracted implants and restoration as an intact unit showing fracture of both implants (A,B) at third and fourth thread (mesial and distal implants, respectively). Note the presence of biofilm/calculus on the gingival surface of the prosthesis (C) and evidence that both implants were osseointegrated (D).
FIGURES 6A through 6D. View of extracted implants and restoration as an intact unit showing fracture of both implants (A,B) at third and fourth thread (mesial and distal implants, respectively). Note the presence of biofilm/calculus on the gingival surface of the prosthesis (C) and evidence that both implants were osseointegrated (D).

While a search of the literature regarding clinical trials in which mini implants were used to support posterior fixed restorations reveals relatively few papers,1,7,11,22–31 the number is likely inflated due to problematic terminology (e.g., mini versus narrow versus small diameter implant). Further, as previously noted, among these designations, there is an overlap in diameters ranging from 1.8 to 3.3 mm. Consequently, data for implant success is a mixture of mini, narrow and small diameter implant designs.8

FIGURE 7. Occlusal view of exposed alveolus at six months post-osseous grafting (November 2015) showing well-formed alveolus, without evidence of buccal-lingual resorption.
FIGURE 7. Occlusal view of exposed alveolus at six months post-osseous grafting (November 2015) showing well-formed alveolus, without evidence of buccal-lingual resorption.

Of relevance to the present case is that several reviews addressing success rates of smaller diameter implants have concluded that implant diameter is a significant factor in implant survival.8–13,32,33 Implants with wider diameters achieve better long-term survival rates than those with narrower diameters and corresponding length.8,34 In spite of this conclusion, numerous papers state there is no difference in survival rates of smaller diameter implants versus standard diameters.7,15,23,25–28,30,35 It should be noted that among these latter studies, disparate measures were employed to assess implant performance. Further, in a recent evaluation of clinical trials that reported implant survival rates, Sendyk et al36 noted a significant occurrence of selective outcome reporting. And Klein et al3 in their review of success rates of narrow diameter implants reported that studies were of low quality, and with a high risk of bias.

FIGURE 8. Buccal view of restored implant (mirror image) taken August 2020 at the four-year follow-up appointment shows no evidence of peri-implant mucositis.
FIGURE 8. Buccal view of restored implant (mirror image) taken August 2020 at the four-year follow-up appointment shows no evidence of peri-implant mucositis.

Also relevant to the current case are the issues of biomechanics and bite force resistance of the mini implant design. Hasan et al9 reviewed the literature related to fatigue life of small diameter and mini implants under normal biting force, and their survival rates. The authors noted that while small diameter and mini implants are reported to have a survival rate over five years of 98.3% to 98.4%, experimental studies conclude that implants with diameters < 3 mm pose an increased risk of fracture. This statement appears to be supported by Flanagan et al17 and the results of their in vitro study of the fatigue life of mini implants. The authors used a fatigue-testing machine designed to apply a cyclic off-axis force. The cyclic load was 300 Newtons (N). The average bite force for an adult ranges between 720 N and 900 N, roughly two to three times that of the experimental load.16 Using this device, implant fracture occurred, on average, after 480,000 cycles. Given the Newton load was roughly one-half to one-third that of average and that humans make tooth contact an average of 700 to 1000 times per day,37,38 this would roughly calculate to 240 to 345 days before implant fracture might occur (using the lower average number of tooth contacts per day). Of course, this is a lab study with many assumptions. Even so, the study constitutes a proof-of-principle with considerations for the practicing clinician.

FIGURE 9. Periapical radiograph taken at the fouryear follow-up appointment. There is no evidence of crestal bone loss or change in architecture.
FIGURE 9. Periapical radiograph taken at the four-year follow-up appointment. There is no evidence of crestal bone loss or change in architecture.

Lastly, in a review of bite force and its impact on dental implants, Flanagan16 made several observations of importance: 

  • There is little consistency from patient to patient in the maximum bite force they can generate
  • Posterior occlusal bite force is roughly three times that of the anterior
  • Implants can be overloaded by a patient’s average bite force
  • Bite force should be an important parameter in implant treatment planning
  • Clinicians should use qualitative judgment when selecting implant diameter and length, as well as prosthetic design 

CONCLUSION 

Evolving clinical, biologic and mechanical science is continually challenging the concepts and tenets for successful implant therapy. In spite of innovations in dental implants, clinicians may sometimes exceed the mechanical and biological limits of mini implant design — especially when used for replacing an occlusal load-bearing molar. Caution is needed when considering use of mini implants in situations requiring replacement of a single tooth or multiple posterior teeth. Failure in such scenarios can result in extensive surgery and bone augmentation procedures, thereby negating one of the major reasons for originally choosing the mini implant in lieu of a standard implant. 

REFERENCES

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From Decisions in Dentistry. July 2021;7(7)31–34.

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