Mini implants vs conventional dental implants

Recently, I came across a newspaper advertisement on mini dental implants. I usually do not pay much attention to such advertisements but somehow, that day, I decided to read what people are saying about mini implants. They talked about how mini implants are less painful, less invasive, how it obviates the need for bone grafting, etc. They also claimed that mini implants are gaining popularity over conventional implants, etc. That reminded me of a comic strip I once read. It was about a man who dropped his keys in a dark corner but search them them under the lamp post because he can’t see in the dark. Doing mini implants where a bone graft and larger diameter implant is needed is like that man looking for his keys in a lit place instead of bringing a torch to the dark corner where the keys were dropped.

Dental implants have come a long way since it was first introduced fifty years ago. It is probably the most researched and studied branch of dentistry. Success rates of dental implants have consistently been in the range of 95-99%. Few other foreign objects placed in the human body can boast of such a high success rate. However, most of these studies showing such a tremendous success rate are done on implants that are made of commercially pure titanium and are of an average size of 4mm in diameter and 10mm in length. This is the so-called “conventional” implants that mini implant manufacturers talk about.

Even “conventional” implants come in a wide variety of diameters and lengths. At some point in the development of dental implants, it was believed that the longer and larger the diameter of the dental implant, the better it is. This is because the larger dimensions provide a greater surface area with which to bond with the bone, thereby improving the chances of success. With improvement in the surface of the implant, the dimensions are less important. The roughened surface of the implant increases the total surface area available to bone bonding without needing a larger implant. Nevertheless, “conventional” implants have sizes ranging from 3mm to 8mm in diameter and 7mm to 15mm in length.

So where do we draw the line? When does an implant becomes “mini” and when is it “conventional”? There is no official definition. The term mini is common applied to implants with a diameter smaller than 3mm. Most of the manufacturers of conventional implants do not make mini and vice versa. There is a certain philosophical reason for this.

As I mentioned earlier, conventional implants are mostly made of commercially pure titanium. However, titanium is a very soft metal and when machined into a screw as narrow as a mini implant which is about 2.5-2.8mm in diameter, it is not strong enough to withstand the biting forces over the long haul. As such, they need to be strengthened by alloying with another metal such as vanadium. The evidence for successful integration of titanium alloy with bone is not anywhere near as rigorous as commercially pure titanium. Furthermore, the narrow diameter presents a much reduced surface area for bone bonding. As such, the combination of a reduced bonding strength as well as less bone to implant contact significantly increase the risk of failure.

However, mini dental implants are a useful tool to have. In my practice, I use mini implants to replace single missing incisors in the lower jaw and sometimes in for the lateral incisors in the upper jaw. The use is not determined by whether there is sufficient bone to place a conventional implant but rather by the needs of the case. Incisors in the lower jaw are very narrow and to achieve a proper emergence profile of the crown as it arises through the gum, a narrow diameter implant is needed. Likewise if the upper lateral incisor that is to be replaced is narrower than usual. I also use mini implants to provide anchorage for a complete lower denture in patients who have lost all their teeth in the lower jaw for many years. These patients often have very severely shrunken jaws and placement of a conventional implant may cause a fracture. Due to the shrunken state, retention of the denture is often challenging, depending purely on the patient’s cheeks and tongue to keep it in place. Placing two mini implants into the jaw for the denture to clip onto improves the quality of life for these patients tremendously. In these situations, the forces applied to the mini implants are not great and hence do not overburden the reduced bonding ability.

When mini implants are used in other situations, there are more problems than just reduced bone bonding ability. Because of the narrow diameter, all mini implants are designed a one-piece implant ie the crown cements directly onto the implant, severely limiting the range of restorative options that conventional implants provide by having an abutment ie a connector that sits between the crown and the implant. For example, a molar crown is about 12mm wide. Imagine sticking a molar crown onto a mini implant which is 2.8mm wide. This is like a fishball on a chopstick. The connection being narrow, does not offer a lot of surface area for the cement to have the crown. Further more, the emergence from the bone through the gum will be very abrupt and that means more food trap which in turn compromises the long term gingival health. Biomechanically, when the force of biting in applied onto the edges of the molar crown, there will be tipping force on one end that pivots on the centre portion that is connected to the implant. In longer term, it leads to de-cementation more often.

Mini implants are not replacements for conventional implants. Implant diameters are in a continuum, ranging from the 2.5mm of the narrowest mini to the 8mm of the largest conventional implants. We do not place a large diameter implant to replace a solitary missing lower incisor. Likewise, a 2.8mm implant should not be use to replace a full size molar. If the patient needs a bone graft to have a larger diameter implant, than a bone graft should be done. In trained hands, bone grafting need not be any more invasive than dental implant surgery. One size cannot fit all.