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Injection moulding is not what it used to be. Injectable composites were once criticised as too weak for use in restorative dentistry. However, advancements in injectable materials and injection moulding techniques have changed the game. As a result, injection moulding is becoming an increasingly popular alternative to indirect restorations like ceramic veneers.
That’s largely because injection moulding is cheaper, faster and, in some cases, more durable than ceramics. Injection moulding techniques are also less invasive for the patient, and injectable composites are fully repairable chair-side. It doesn’t require specialised tools and injection moulding techniques are simple enough to learn via training webinars or in hands-on short courses. That means no big investments are needed for dentists to start offering direct composite treatment options to patients.
Injection moulding is also now suitable for use in a larger range or restoration applications including direct composite veneers, post-orthodontic treatments, occlusal and interproximal posterior restorations, and full smile design.
And many dentists are achieving cosmetic results comparable to ceramic veneers, says Dr RL Castillo, Professional Development Manager at GC Australasia Dental. “We have the technology now to achieve a really nice gloss finish with shade matching. While injection moulding techniques have been around for some time, newer products are making the restorations look better, and it is becoming very popular.”
Well, injection moulding is pretty much what it sounds like. Avant Dental uses scans or impressions to create a smile design for the patient. Once this is approved, we 3D print a wax-up that we use to make a clear stent.
The stent essentially acts as a mould for the injectable material. The patient’s teeth are isolated, etched and a bonding compound is applied. Then the clear stent is placed over the patient’s existing dentition, and the dentist injects a flowable material directly into the stent mould. The flowable material is light cured, excess material is removed, and the tooth is polished to a ceramic-like finish.
We’ll provide a much more detailed step-by-step workflow later in this article, but that’s injection moulding in a nutshell.
There are three key materials used in the injection moulding process, and new advancements in these materials have vastly improved treatment outcomes.
Like all treatments, an injection moulding restoration should begin with a thorough patient assessment. Dr Castillo says injection moulding techniques are particularly effective for post-orthodontic treatments, management of tooth wear, smile makeovers and for transitional restorative work.
Of course, it’s important to identify any under-lying issues that might make the patient a particularly good candidate for an injection composite over a ceramic veneer.
“For example, if a patient is grinding their teeth and you put a ceramic in, it’s more likely to break as they are typically more brittle. Composites, on the other hand, have the ability to manage that situation better and are fully repairable in challenging clinical situations like this.”
Scans or impressions and photos of the patient’s dentition and facial structure are taken, and Avant begins the smile design process with input from the dentist. Determine your patient’s aesthetic goals, and discuss preferred shapes, contours, proportions and shades.
Avant will use this information to create a smile design proposal including a wax-up, so it’s critical to provide as much detailed information as possible to ensure the best treatment outcome.
Our smile design proposal goes back to the dentist for approval, and Avant technicians work on any changes that may be required. Some dentists may choose to include the patient in the design approval stage to help manage their expectations.
Once the design proposal and wax-up has been approved, Avant’s technicians will use the wax-up to fabricate the EXACLEAR stent. The stent will accurately transfer all details from the wax-up into the mouth, so it’s critical for close attention to be paid to the wax-up.
Depending on the dentist’s preference, Avant can put vent holes in the stent, or the dentist can do it themselves with the tip of the G-ænial Universal Injectable applicator.
“We usually recommend two vent holes per tooth, however dentists may opt for one vent hole,” Dr Castillo explains. “The vent holes eliminate air trapping and reduce the amount of excess the dentist will need to remove. If you’re using two vent holes for the interior, one hole should be situated on the mesial and the other one on the distal.”
The teeth must be properly prepared before injection moulding begins. First, check the stent for fit. It must be a perfect fit to ensure accuracy, and the silicone key on the stent should extend to at least two teeth distally from the teeth to be treated on both sides for stability.
“Clean the surfaces of the teeth either with air abrasion or pumice to remove plaque,” says Dr Castillo. “It’s critical to use the alternate tooth technique, so use teflon tape on every second tooth to isolate the teeth you’ll work on in the first round of moulding. You’ll repeat the process on the alternate teeth in the second round of moulding.”
With the teflon tape in place, etch the exposed teeth. To achieve maximum strength and reduce the risk of chipping, the composite should be bonded entirely on enamel with no more than 0.5mm if unsupported composite.
Then apply G-Premio Bond. It goes on yellow for better visibility, and dries invisible with max air pressure for five seconds. Then light cure for 10 seconds.
“A universal bonding agent is highly recommended in this case, to have more predictability in the adhesive interface, ensuring a reliable and durable adhesive performance,” says Dr Castillo. “We also recommend working under a split dam so it doesn’t interfere with the stent fit, and use retraction cords. It’s critical to keep the area dry. Moisture will interfere with the etching and bonding process.
With alternate teeth still isolated with teflon tape, it’s time to fit the stent and begin injection moulding. Dr Castillo explains that one vent hole provides access to insert the injectable, while the other is for excess overflow.
“You need a little bit of overflow to eliminate voids,” she says. “During injection, you want to compress the labial and palatal area of the clear stent under controlled pressure to reduce excess on the margins.
“It’s also important to keep introducing the material as you remove the injectable tip from the stent so that you don’t end up with a void,” she continues. “This will likely weaken the restoration and need to be repaired.”
Then light cure the composite through the clear stent, and move on to the next tooth. When the first round is complete, you can carefully remove the stent and inspect the results.
There will be excess material that needs to be removed. This is a straightforward process you can typically achieve with a scalpel blade #12 and metal finishing strips to clear the interproximal spaces.
“It’s really important not to stick teeth together as this will prevent the patient from cleaning properly,” says Dr Castillo. “Using metal finishing strips will help you ensure the patient will be able to floss between the teeth. If the restoration prevents the patient from practising proper dental hygiene, then you’re defeating the purpose.”
Dr Castillo adds that it’s critical to effectively remove all excess from the first set of alternating teeth before moving onto the second round.
“Any excess that is left after the first round might affect how the stent fits for the second round. And a poor stent fit will compromise the accuracy of the second round of injection moulding.”
With all excess removed, you can start the process for the second round of teeth – beginning with teflon tape isolation, then moving on to etching, bonding and finally injection moulding. Once cured, you’ll also need to remove excess.
Inspect the restorations for any sharp edges, and remove with discs. These can also be used for shape correction, and medium (pink) and fine (grey) grit diamond rubber wheels are useful for polishing. For a ceramic-like gloss finish, apply Diapolisher paste and polish with a cotton buff.
“It’s very easy to polish to a ceramic-like finish, and the polish retention is very good,” says Dr Castillo. “Since you will be working very closely to the gums during the treatment, the gums can be injured. Hence, it is important that this is communicated to the patient before starting the procedure.”
If the patient suffers bruxism or has a tendency to grind their teeth, you might choose to prescribe a dental mouthguard for nightly wear to prevent premature composite chipping.
The composite material can also be easily repaired in the chair, so letting the patient know that any chip repairs or reshaping they may require in the future can be accomplished in follow-up appointments. The patient can simply return to their normal oral hygiene regime, with no food or drink restrictions.
“It’s a very straightforward technique,” Dr Castillo concludes. “You need to understand what the patient wants, and you need a good treatment plan so you can communicate your requirements with the lab. Other than that, it’s just a matter of developing a feel for the technique.”
Watch our free dental webinar on-demand: Latest innovations in dental injection moulding
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