Mini-dental implants: an overview

Types of FDA-approved MDIs

The treatment protocol usually includes drilling to half of the MDI length followed by manual torqueing to achieve good primary stability of the implant. Immediate loading is commonly prescribed after MDI placement. The diameter of MDIs commonly ranges between 1.8 mm and 3 mm, while their lengths range between 7 mm and 18 mm. The characteristics of common MDIs are listed in Table 1.

Indications for the use of MDIs

Mini dental implants are suggested as a permanent replacement treatment option in areas where placing conventional dental implants may not be possible owing to deficient width, height or mesio-distal dimensions of an alveolar ridge. Their use as a definitive replacement option for single-tooth restorations and for the stabilization of overdentures, in either the maxilla or the mandible, is also well documented.^14,15

Single-tooth restoration

Maxillary lateral incisors, with an average mesio-distal width of 7 mm are the second most common developmentally missing tooth, second only to third molars. The prevalence of developmentally missing maxillary lateral incisors is 0.8–2%.^16,17 This prevalence varies between populations of different ethnic backgrounds; however, it does have a strong genetic association.¹⁷ This condition often presents a challenging problem for an orthodontic practitioner to obtain or redistribute the ideal amount of space to allow for its replacement. Inadequate mesio-distal space to allow the placement of a conventional implant is more often the norm, rather than the exception. Placing an implant in areas of inadequate interdental space can cause proximal bone loss, which will negatively affect the aesthetic outcomes with poor soft tissues contour, such as the final position of the papillae and supracrestal soft tissue.⁴ The placement of MDIs in such cases is strongly favoured over conventionally sized dental implants. MDIs in such cases can produce a pleasant aesthetic outcome without the need invasive restorative solutions or lengthy orthodontic treatment requiring complex tooth movement.

An additional area that is challenging to restore prosthodontically is the mandibular labial segment. The cause of missing teeth in this area can be developmental or due to previous dental extractions. The lower incisors' naturally occurring narrow root morphology in both a mesio-distal and bucco-lingual direction results in a significant amount of alveolar resorption following dental extractions. The same region is similarly affected in hypodontia. Agenesis in the anterior mandible region is rarely reported, but appears to be more common in Japanese and Chinese populations.¹⁸ MDIs use can negate the need for tooth-supported prostheses, which can in turn reduce the risk of accelerated bone resorption.¹⁹

Complete overdentures

Edentulous patients who have been fitted with complete dentures for a long time will typically develop thin ridges caused by deficiency in bone volume. The process of bone resorption, due to decreased stimulation from natural teeth is a continuous process that occurs more rapidly in the mandible.^20,21 Conventional mucosa-borne acrylic dentures can cause widespread patient dissatisfaction owing to poor denture retention, speech and mastication difficulties, and may even lead to psychological impairment.¹³

The use of dental implants to enhance retention of acrylic dentures has been advocated since their early days of adoption. The McGill consensus²² and the York consensus²³ both declared a mandibular two-implant-retained overdenture as the first-choice standard care for edentulous patients; however, an excessively resorbed ridge would not be compatible with conventional dental implants.²⁴ Satisfactory restoration using conventional dental implants requires additional complex procedures, such as bone augmentation, which can result in profound post-operative pain, discomfort, oedema and increase the risk of nerve injury.²⁵ Patients can also be unwilling to go ahead with such procedures due to the fear of complex surgery, anxiety related to implant treatment or due to limited finances.⁹

In such situations, insertion of MDIs would enable dental practitioners to prosthetically rehabilitate patients in a less-invasive, time-saving and cost-effective manner.

Mini-dental implants are placed in the intra-foraminal area of the mandible when aiding in the retention of removable overdentures. Several studies have compared the placement of two MDIs versus four MDIs. No significant differences were found in terms of marginal bone level changes, prosthodontic complications and patient satisfaction levels.^26–28

Figure 1 provides radiographic evidence of four, 3M ESPE (Seefeld, Germany) mini-implants placed in the intra-foraminal area of the mandible to aid in the retention of a lower partial denture. Figure 2 is a clinical photograph of the mini-implants 2 weeks post-insertion. A summary of the indications for MDIs is given in Table 2.

Advantages of using mini dental implants

The use of mini-dental implants allows the use of simpler surgical techniques and avoidance of bone augmentations procedures.⁸ The majority of MDI's are placed with a less invasive flapless technique,¹⁴ which the reduces post-operative pain and discomfort associated with conventional flap procedures and bone augmentation techniques.^4,29 The use of MDIs also negates the need for long and costly treatment plans which may be rendered less cost-effective when orthodontic treatment is indicated.^9,25 Commonly, MDIs are immediately loaded,³⁰ which in contrast to conventional dental implants, needing a minimum of three to six months before osseointegration is completed and hence functional loading indicated. The latter stage of dental implant treatment could be surpassed due to more recent protocols involving immediate loading for conventional implants.³¹ A summary of the advantages is given in Table 3.

Contraindications for using mini dental implants

Although considered a feasible and viable treatment option for patients who may not be amenable to conventional dental implants, MDIs share the same contraindications, particularly in regard to medical comorbidities. MDIs are contraindicated for patients who have uncontrolled diabetes, metabolic bone disease, clotting disorders, are undergoing chemotherapy or radiotherapy, use pharmaceuticals that inhibit or alter natural bone remodelling and are heavy smokers (Table 4).

Disadvantages of using mini dental implants

The main disadvantages of mini-dental implants use are two-fold: the limited available scientific evidence regarding their long-term survival; and the possible need for multiple implants (Table 5).³² Owing to the one-piece design of most MDIs, the lack of parallelism between implants is less forgiving, rendering MDIs more technique sensitive. Other disadvantages are attributable to the nature of flapless surgery, which as in conventional surgery, precludes the surgeon from visualizing the surgical site directly, resulting in the inability to directly visualize important anatomical landmarks and vital structures. It could also impair alveolar bone cooling by irrigation and rule out alveoloplasty if additional prosthetic space is required.³² The load transferred to the bone implant interface by a horizontal force is greater in narrow implants than in conventional implants, which can lead to an increased risk of fracture due to overload, or failure due to overload.³³ MDIs are also contraindicated in immediate extraction sites as the large socket diameter can preclude adequate implant–bone interface, resulting in a suboptimal primary implant stability.^8,10

Conclusion

The use of mini-dental implants in the replacement of missing teeth and in the retention of complete overdentures is a viable treatment option in patients with reduced mesio-distal space and bucco-lingual dimensions that would not be suitable for conventional-sized dental implants without additional bone augmentation procedures. They are less invasive, cause less post-operative comorbidity and are more cost effective. They have been found to be successful in the short and medium term, with a follow-up period of up to 7 years, but little evidence is available with regard to their long-term success.