What is a Toronto prosthesis, and how does it differ from a denture and a zirconia bridge?

This prosthesis goes by several names in clinics: the Toronto bridge, the hybrid prosthesis, the metal-acrylic fixed full-arch prosthesis. They all describe the same thing. Abutments are first fitted onto the implants, and a metal framework running the length of the jaw sits on top of them. The framework is either produced by casting or milled from a titanium block with computer assistance. Ready-made acrylic teeth are set on it, and pink acrylic is added to stand in for the gum and bone that have been lost. The patient cannot take this piece out; the screw holes are covered with composite, and the dentist removes the prosthesis by undoing the screws when needed. Two things define it: the framework is metal, the chewing surface is acrylic.

What separates it from a removable denture is not the material but where the support is taken from. A conventional complete denture rests on the gum and the palate; the Toronto prosthesis transfers the load to the bone through the implants, does not cover the palate in the upper jaw, and removes the problem of a lower denture moving about. In a systematic review examining patients with no teeth at all, implant-supported prostheses, whether fixed or removable, are reported to improve quality of life markedly compared with a conventional complete denture. The same review also writes the trade-off: fixed prostheses are superior on chewing and on the bite, but their maintenance is more complicated; with removable ones cleaning is easy and wear is more of a problem.

The difference from a zirconia bridge, on the other hand, is directly in the material. The chewing surface of a Toronto prosthesis is acrylic: it can wear and it can break, but in return it can be repaired. Monolithic zirconia is a single piece of ceramic; it wears very little, but when its body fractures it is hard to repair. The umbrella review combining seven systematic reviews shows the distinction in figures: with veneered zirconia frameworks, chipping of the veneer is reported at between 15% and 35%, framework fractures stay under 5%, and monolithic zirconia and screw-retained frameworks show a lower rate of technical complications.

Metal-acrylic has been compared against its own kind as well. In a systematic review and meta-analysis gathering five studies, no significant difference is found between metal-acrylic and metal-ceramic fixed full-arch prostheses in the risk of implant loss or prosthesis loss. The difference lies elsewhere: in the metal-acrylic group, the risk of peri-implantitis developing at implant level comes out significantly higher. The absolute difference between them is 6.9 percentage points; so a picture seen in, say, five out of a hundred implants with metal-ceramic is seen in roughly twelve with metal-acrylic. That figure counts implants, not patients. Peri-implant mucositis, gum recession, wear and chipping of the veneer also appear more often with metal-acrylic. This result comes from a single meta-analysis with five studies included, and the abstract does not say how many of those studies went into the peri-implantitis calculation; on top of that it comes from pooling comparative studies, not randomised trials. The direction is clear, the certainty is limited.

On the question of how many implants, the literature has measured the difference between four and six. In a systematic review and meta-analysis examining the upper jaw and covering 53 studies, there is no significant difference between the two groups in implant survival, prosthesis survival, or mechanical and biological complications. The one significant difference is in marginal bone loss: bone loss comes out higher in the four-implant group. The same study reports that using a milled framework, and the implants being spread out in the front-to-back direction, are associated with higher implant survival in the six-implant group. So the number on its own is not a measure of quality.

The most concrete thing about being screw-retained is retrievability. Because the prosthesis can be taken off, the surface underneath can be cleaned, a loose screw can be tightened, broken acrylic can be repaired, and a problem such as cement residue being left under the gum does not arise. The umbrella review writes that screw-retained frameworks show a lower rate of technical complications. The literature comparing retention types in general, though, looks the other way: in a meta-analysis gathering 20 studies and 8,989 implants, marginal bone loss, prosthetic complications and implant survival all three come out in favour of cement-retained restorations. Before setting these two findings side by side, one thing needs knowing: that meta-analysis draws mainly on single crowns and short bridges, not on full-arch prostheses. The difference in bone loss is also 0.19 millimetres, and the authors themselves write that a difference this small may have no counterpart in the clinic. A Toronto prosthesis is screw-retained by definition; the reviews that look at it directly support that choice. As for whether the retention type changes the risk of peri-implantitis, the critical review that searched the literature finds the result unclear.

Survival in its most collected form sits in the umbrella review: prosthesis survival at 5 to 15 years between 89.5% and 96.8%, implant survival between 95.2% and 99.2%, screw loosening between 5% and 15%, and the frequency of peri-implantitis between 4% and 18%. The ranges being this wide is no accident; the reviews differ from one another in their follow-up periods and in how they define outcomes. The real picture is shown by the review that looks at how complications accumulate over time. In 281 fixed full-arch prostheses that had been in the mouth for a mean of 9.5 years, 653 complication events were counted; the rate is 24.6% per 100 restoration years. The proportion of prostheses that had had no complication at all was 29.3% at five years and 8.6% at ten. Chipping of the veneer was 33.3% at five years and 66.6% at ten; screw fracture 10.4% and 20.8%; tissue growth 13% and 26%; bone loss of more than two millimetres 20.1% and 40.3%. The authors' emphasis is this: these events mostly do not lead to the loss of the implant or the prosthesis; they come back as a burden of repair and maintenance.

Cleaning is a heading of its own with this type of prosthesis, because the place the brush has to reach is not between the teeth but the underside of the prosthesis, the surface facing the gum. If no gap is left between the prosthesis and the gum for an interdental brush and a flow of water to pass through, the layer that builds up there stays there every day. The critical review examining the tissue around the implant reports that the emergence angle and contour of the prosthesis, and restricted access for oral care, play a part in the picture of peri-implant mucositis and peri-implantitis, and that correcting the contour is even an effective addition to treating established mucositis. In a feedback study with patients wearing a fixed full-arch prosthesis, 16% of the participants say oral care takes an excessive amount of time and 14% say the bulk of the prosthesis bothers them; in the same study, satisfaction nevertheless comes out high with all the designs.

This prosthesis is discussed for a whole jaw. The distinction below shows who the subject concerns, and in which situations another design moves ahead.

The sequence below shows the general order of construction. When the implants are loaded and how many steps there are vary with the clinic and the protocol.

1. 1

   Planning the vertical space and the implant positions

   With a CBCT scan, the state of the bone, where the implants will go and how much height will be left for the prosthesis are planned together. The implants being spread out in the front-to-back direction is among the factors found to be associated with higher implant survival in the meta-analysis examining fixed prostheses in the upper jaw.

2. 2

   Placing the implants and choosing the abutments

   Abutments that will carry the prosthesis are fitted onto the implants; if the back implants have been placed at an angle, angled abutments are used. Which surface of the tooth the screw hole will come out of is decided at this step, and it directly affects the appearance.

3. 3

   The impression or digital scan, and producing the framework

   The position of the abutments is recorded with an impression or an intraoral scan, and the framework is cast or milled to that record. What is looked for when it is tried in the mouth is a passive fit: the framework should go into place without straining any of the screws, otherwise the tension is passed on to the screws and to the acrylic.

4. 4

   The tooth try-in and shaping the pink tissue

   The length of the teeth, the midline, lip support and the bite are adjusted by trying them in the mouth. At the same try-in the shape of the underside is settled: the surface facing the gum is shaped so as to leave a gap the brush and water can pass through.

5. 5

   Screwing it down, delivery and follow-up

   The prosthesis is screwed down to the torque the manufacturer specifies, the screw holes are covered, and it is polished so that the underside stays smooth. The most important part of delivery is being shown in the mouth which brush goes where. In the first months the torque is reviewed, and in the first year the radiographic bone level is recorded.

The decision comes down to two questions: where the support will be taken from, and which material the chewing surface will be made of.

What follows are the headings most often counted in the literature. Most of them do not lead to the loss of the prosthesis; they come up as a repair appointment and a burden of maintenance.

With this prosthesis, the period that decides the outcome begins after delivery. The sequence below shows the expected course and what the check-ups look at.

1. The first weeks

   Speech settles within a few days and the tongue gets used to the new volume. The real job in this period is establishing the cleaning routine: passing an interdental brush along the underside and using a water flosser have to become part of the daily routine.

2. Between the third month and the first year

   The torque on the screws is reviewed, the prosthesis is taken off and its underside cleaned if needed, and whether the gum is bleeding is checked. Bleeding is the earliest sign that there is a point the cleaning is not reaching. In the first year the radiographic bone level is recorded; that image is the line of comparison for the years that follow.

3. The fifth year and beyond

   Acrylic wear and the bite dropping come up in this period; in some prostheses renewing the teeth or the acrylic layer is discussed. In the review with long follow-up, the proportion of prostheses with no complication at all is 29.3% at five years and 8.6% at ten. That does not mean most prostheses are lost; it means most of them have had at least one intervention.

You will not find figures here. These are the headings that genuinely change the outcome when you compare two quotes:

The framework's material and how it is produced

Whether the framework is cast or milled changes both the cost and the outcome. Using a milled framework is among the factors found to be associated with higher implant survival in the meta-analysis examining fixed prostheses in the upper jaw.

The number and position of the implants, and the abutments

Although no difference in survival has been shown between four and six implants, marginal bone loss is found higher in the four-implant group. Alongside the number, how far the implants are spread out in the front-to-back direction comes into it too.

The material of the chewing surface

Acrylic, composite and ceramic behave differently in wear and fracture, and they are not equally easy to repair. Find out in writing which material is being offered and what route would be followed if it had to be renewed.

Whether follow-up and maintenance items are included

Ask from the start whether changing a screw, repairing the acrylic, taking the prosthesis off to clean it and the check-up appointments are within the scope. The data showing that complications accumulate over the years explain why this item matters.