Why Mini Implants Fail: Lessons from a Recent Case Study

In last week’s Perio Update, a case report of two failed mini implants was presented. While the reason why the two mini implants were unable to support a single-unit fixed prosthesis replacing a mandibular first molar 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:

• Smoking^1,2
• Use of smaller diameter implants^2-7
• Posterior localization^1,5,9
• Prosthetic complications when using narrow diameter implants¹⁵
• Placement of implants in the posterior maxilla and atrophic bone^1,8
• Risk of implant fracture^10,11
• History of periodontal disease^12-15

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.

While a search of the literature regarding clinical trials in which mini implants were used to support posterior fixed restorations reveals relatively few papers,^1,5,8,17-25 the number is likely inflated due to problematic terminology (eg, mini vs narrow vs 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 are a mixture of mini, narrow and small diameter implant designs.²

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.^2-7,26,27 Implants with wider diameters achieve better long-term survival rates than those with narrower diameters and corresponding length.^2,28 In spite of this conclusion, numerous papers state there is no difference in survival rates of smaller diameter implants versus standard diameters.^{1,16,19-22,24,29,30} 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 al³¹ noted a significant occurrence of selective outcome reporting. And Klein et al³³ in their review of success rates of narrow diameter implants reported that studies were of low quality, and with a high risk of bias.

Also relevant to the current case are the issues of biomechanics and bite force resistance of the mini implant design. Hasan et al³ 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 al¹¹ 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.¹⁰ 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,^33,34 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.

Lastly, in a review of bite force and its impact on dental implants, Flanagan¹⁰ 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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This information originally appeared in Beaini NE, Cobb CM. Avoiding complications with mini implants. Decisions in Dentistry. 2021;7(7):31–34.