What is a mini implant, who is it suitable for, and what are its limits?

Implants are classified by diameter. A meta-analysis of narrow-diameter implants published in Clinical Oral Implants Research defines three groups: those under 3 millimetres in diameter are 'mini implants', those between 3 and 3.25 millimetres form the second group, and those between 3.3 and 3.5 millimetres the third. In other words, not everything referred to in the clinic as a 'thin implant' or 'narrow implant' is a mini implant. This distinction matters, because the results vary by group: in the same meta-analysis, narrow implants of 3 millimetres and above showed survival similar to standard implants, while mini implants gave significantly worse results.

In numbers: in the same study, mean survival was 94.7% for mini implants, 97.3% in the second group and 97.7% in the third. In the calculation comparing mini implants with standard-diameter implants, the odds ratio for failure comes out at about 4.5, but the confidence interval is very wide (between 1.5 and 13.7). The authors add two caveats: the included literature has a high risk of bias, and long-term data are lacking. This figure should not be read as 'one in five mini implants fails'; what it should be read as is that a mini implant does not have the same safety margin as a standard implant.

The design difference is as decisive as the size. A standard implant is usually in two parts: the body that goes into the bone and the connecting piece screwed on top of it. Mini implants, on the other hand, are often made in one piece; the end that stays in the mouth is itself an attachment, frequently in the shape of a small ball. A rubber or metal ring fitted inside the denture clips over this ball and the denture settles into place. This simplicity is an advantage, but it comes at a price: if the implant goes in at the wrong angle, it cannot be corrected later with an intermediate part, and if the attachment wears, the attachment is the implant itself.

The main use of mini implants is the lower removable denture. A lower denture lacks the broad palate surface that an upper denture holds on to, and it moves constantly with the movements of the tongue, cheeks and lips. The McGill consensus statement, published in 2002, defined a lower denture held by two implants as the first-choice standard for people with no teeth. A mini implant works like a version of this idea adapted to narrow bone: several thin implants instead of one thick one. In the review of mini implants used to retain lower dentures, most patients were given four implants.

In the lower jaw the results are good. In a systematic review pooling 17 studies, 475 patients and 1,715 mini implants, overall survival was 95.63%; follow-up ranged from 6 to 84 months, with a mean of about 28 months. In most of the studies the implants were placed without opening the gum. The authors regard mini implants as a reasonable option for holding a lower denture in the short to medium term. Here 'short to medium term' is not decoration: the mean follow-up is under two and a half years.

In the upper jaw the picture is different. A review of 24 studies and 2,494 mini implants found failure of 31.71% in the upper jaw and 4.89% in the lower jaw. In another review looking only at the upper jaw, survival was 77.1% with a mean follow-up of 1.79 years. In a five-year multicentre study of 2.4-millimetre mini implants in the upper jaw, cumulative failure at the end of five years was 23.2%; a large share of the losses happened in the first months, before the implants had been fully connected to the denture. In the same study, patients' quality-of-life scores nevertheless improved markedly. So in the upper jaw a mini implant can make a patient's life easier, but, as in this five-year study, you need to allow from the outset for the possibility of losing between a fifth and a quarter of the implants.

Long-term data are scarce, but not non-existent. A 2013 review of the use of mini implants for definitive prosthetic treatment reported that almost all losses occurred within the first year and that there was no evidence on long-term survival. Longer follow-ups have been published in the years since: in a retrospective study following 83 patients and 334 mini implants in the lower and upper jaws, cumulative survival at 15 years was 86.3%, and prosthesis survival 95.45%. This is a single-centre, retrospective study; it gives a direction, not certainty.

A point that is often confused: in orthodontics, small screws also called 'mini implants' or 'mini screws' are used. These are not implants that carry teeth; they are placed as temporary anchorage to move teeth during brace treatment and are removed once treatment ends. Everything below concerns mini implants placed to carry teeth or a denture.

A mini implant cannot be called good or bad on its own; what decides the outcome is which jaw it is used in, how many are used and for which prosthesis. The distinction below follows the picture that stands out in the reviews.

The sequence below is the general course of securing a lower removable denture with mini implants. The timing of loading and the number of implants vary with the clinic and the bone.

1. 1

   Assessing the denture and the bone

   First the fit of the current denture is checked: a denture with worn teeth and shortened edges will not work properly even with implants underneath it. The height and width of the bone are measured with a CBCT scan; the length of the mini implant matters too, because a thin body has to get its hold from its length.

2. 2

   Deciding the number and positions of the implants

   In the lower jaw, four mini implants are usually used; the review recommends at least four in the lower jaw and at least six in the upper jaw. It is important for the implants to stand parallel to each other, because with a one-piece design a difference in angle cannot be corrected later.

3. 3

   Placement, usually without opening the gum

   In most studies, mini implants were placed through a small hole in the gum, without an incision. This can reduce complaints after surgery; but because the bone is not seen directly, the accuracy of the plan becomes even more important. In hard bone, turning the implant with excessive force can cause the tip to break.

4. 4

   Immediate or delayed loading

   In some studies the denture was connected to the attachments on the same day; in others, the implants were left to fuse with the bone first. The reviews report large differences between studies in how loading was done. In the five-year study in the upper jaw, the denture was supported with a soft liner in the early period and the definitive connection was made at six months.

5. 5

   Fitting the retaining rings into the denture

   Retaining housings are fitted into the inner surface of the denture so that they sit over the implants. The rings wear over time and are replaced; this is not a fault but a routine maintenance item of the system.

6. 6

   Check-ups and relining

   The gum and bone under the denture change over time; if the denture is left with a gap underneath, the load falls on the implants. In the study carried out in dental practice, 16% of dentures were relined repeatedly. Regular check-ups are therefore part of the system.

When weighing up a mini implant, the comparison is not 'implant or no implant'; it is which prosthesis will be carried, on how many implants and of what thickness.

The problems with mini implants are mostly concentrated in the first year and in the upper jaw. The maintenance burden on the denture side also needs to be taken into account.

With placement done without opening the gum, healing is usually short; the period that really needs attention is the first few months, when the implants fuse with the bone.

1. The first few days

   Mild pain and tenderness are to be expected. If the denture was connected on the same day, you need to start with soft foods in the first few days and either not take the denture out for as long as your dentist says, or take it out exactly as instructed. This instruction varies with the type of loading.

2. The first three months

   The period in which losses are most common. The denture rocking on one implant, or increasing pain or bleeding around an implant, always needs checking during this time.

3. The first year

   At check-ups, the gum around the implants, the fit of the denture and the condition of the retaining rings are assessed. For mini implants that get past one year, losses fall markedly.

4. The years that follow

   The rings are replaced as they wear and the denture is relined when needed. When a gap opens under the denture, the load falls on the implants; putting off relining is riskier for thin implants than for thick ones.

You will not find figures here. These are the headings to look at when comparing two quotes:

Number of implants and which jaw

Usually four mini implants are discussed for the lower jaw and at least six for the upper jaw. A quote for the upper jaw should state in writing the likelihood of losses and how re-placement would be covered.

The denture: existing or new

Fitting attachments inside the existing denture and making a new denture are two different jobs. If the denture fits poorly, looking only at the implant price is misleading.

The implant's size and system

Ask whether the diameter is below or above 3 millimetres and which system is used. Even in the long-term study, differences in bone loss and complications were found between two different brands of mini implant.

Maintenance items

How ring replacement, relining and denture repair will be charged should be discussed from the start. With mini implant-retained dentures these are not rare; they are expected work.