The Dawn of Injection Molded Composite Dentistry

Dr. Jihyon Kim
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Dentistry Today’s editor-in-chief, Damon Adams, DDS, leads a discussion with Jihyon Kim, DDS, on why techniques for placing direct composites need to change.

What is injection molded composite?
Dr. Kim: Historically, many advances in science and technology have been spurred by perspectives gained from the cross-pollination of ideas across industries. A recent example might be the conceptualization of 3-D printing and its crossover from additive manufacturing to current uses in medicine and dentistry that has led to commercially successful products, such as TrueTooth from Dental Education Laboratories. Similarly, the concept of injection molding originates from a long-standing manufacturing process of injecting molten material into a mold to produce parts. Bioclear employed this concept as a solution to composite placement for direct restorations.

Figure 1. Four-year postoperative view of a midline diastema closure. Note the structural failure of the large direct posterior composite.
Figure 2. Two-year post-op view of tooth No. 7 peg lateral treatment. Clinical presentation does not reveal the subgingival ledges contributing to poor tissue health.
Figure 3. Tooth No. 7 peg lateral treatment. The preoperative and 8-year post-op views show the excellent color stability and integrity.
Figure 5. A Go/No-Go Probe (Bioclear) is used to measure depth of cure limits for the Filtek One Bulk Fill Restorative composite (3M). The green zone is within 4 mm. The yellow zone is within 5 mm. The red zone exceeds 5 mm. (Photo courtesy of Brandon Walker, third-year dental student, University of Washington.)

Legacy methods for direct composite resin restorations involve incremental placement techniques into traditional G.V. Black mechanically retentive cavity preparations. Composite resin is layered and hand manipulated for cavity adaptation, mitigation of C-factor concerns, and depth of cure. Layering techniques are also used in efforts to create polychromatic aesthetics in restorations. However, layering and hand manipulation of composite resins creates seams, gaps, and voids that may contribute to material weaknesses and early aesthetic failures (Figure 1). The use of a bulk-fill composite resin reduces the need for layering, but single incremental placement is still mired with issues from hand manipulation.

With indirect restorations, our profession is leaning toward the use of milled, monolithic restorations. Monolithic ceramic restorations have the advantage of increased strength over layered porcelain restorations. It would seem reasonable to infer that monolithic direct composite resin restorations might also be stronger than layered direct composite. What if we are able to create monolithic composite resin restorations directly in the mouth? That is the goal with injection molding of direct composite resin.

The injection molding method is a 3-step process following clinical total etching and immediate dentin sealing. Step one is the use of an adhesive (Scotchbond Universal [3M]) as a wetting agent, followed by aggressive air thinning. Scotchbond Universal, with its one-bottle system and self-etch/rinse-etch flexibility, is ideal for injection molding. Step 2 is placement of heated flowable composite. Step 3 is placement of heated regular composite resin. The 3 steps are done without curing in between. Injection molding of composite resin maximizes the material’s propensity to flow once heated, then captures and pressurizes it within a containment. The pressurization of composite within a containment creates shear thinning (thixotropic flow) that further encourages composite resin to flow like a liquid. Little to no hand manipulation minimizes the chance for internal defects that would otherwise contribute to structural weakness of the restoration. When done properly, injection molding and the Bioclear Method open up a third avenue of treatment possibilities. The options are no longer limited between temporary composite bonding vs irreversible, resective indirect restorations such as crowns. Injection molding offers advantages of both modalities: additive dentistry and preservation of tooth structure with strength, durability, and aesthetics.

What is the Bioclear Method, and how does injection molding fit in?
Dr. Kim: There are 4 cornerstones to the Bioclear Method: (1) biofilm disclosing and removal with aluminum trihydroxide to allow intentional excess of composite onto clean uncut enamel; (2) clear, anatomic Bioclear matrices capture and mold heated composite resin into monolithic shapes with mylar finishes in the inaccessible gingival and interproximal areas; (3) multi-viscosity composite resins are heated for better flow and adaptation; and (4) Bioclear’s trademark Rock Star polish—a simplified polishing method to create mylar finishes of the final restoration.

Figure 6a. Tooth No. 9 with deep caries, labial and palatal views. (Case courtesy of Dr. Reza Moezi, Vacaville, Calif.)
Figure 6b. Tooth No. 9 with caries removal. An A101 matrix (Bioclear) on the mesial and an A102 matrix (Bioclear) on the distal, ready for injection molding.
Figure 6c. Tooth No. 9 after injection molding and polishing, labial and palatal views.

The only similarity between conventional bonding techniques and the Bioclear Method is the use of composite resin. Typical outcomes of conventional bonding techniques that engage subgingival areas can appear clinically acceptable. The dirty little secret of a less-than-ideal subgingival outcome is revealed with a radiograph (Figure 2). Bioclear’s patented method and anatomic matrices for restoratively driven papilla regeneration and injection molding of composite are built for favorable subgingival adaptation and tissue responses. The resulting monolithic injection molded composite restoration with Rock Star polish has excellent color stability and stain and abrasion resistance (Figure 3).

There has been a steady increase in patients seeking treatment with the Bioclear Method. Both patients and well-meaning clinicians have been dissatisfied with the typical outcomes from traditional composite bonding techniques. Patients are also realizing the physiologic costs of indirect restorations and are no longer willing to sacrifice sound tooth structure for treatment. They are seeking a viable alternative, and Bioclear is satisfying the demand for porcelain-esque outcomes with minimal to no physiologic cost.

Why do you favor Filtek composites by 3M?
Dr. Kim: For more than a decade, we preferred using 3M Filtek composites because of the handling and optical qualities of the “body” shade that perfectly matches the corresponding flowable shade. The intimate and void-free adaptation of injection molded composite resin, along with the mirror finish achieved with the Rock Star polish, reveals the true optical and structural integrity of the composite. The resulting chameleon effect and enhanced photon penetration allows us to achieve a non-layered polychromatic effect by varying the facial thickness of composite.

Experimentation with heating of composite resins is not new. Numerous studies exist noting many advantages of heating (Figure 4). Although many composite resins are available, each resin may respond differently to heating. We are familiar with the handling of 3M composites and their responses to repeated cycles of prolonged heating times. 3M has been committed to testing the performance of their composites and documented that the heating of their composite resins does not affect safety or efficacy. They are the first in the industry to receive an FDA clearance for a heating claim.

The growing industry trend with bulk-fill composites also fits naturally with injection molding. The greater depth of cure with modern curing lights means that direct posterior composite restorations can be injected instead of layered. This simplifies the placement technique in difficult-to-access posterior areas. The typical 1.5- to 2.0-mm depth of cure requirement for most composite resins is challenging to consistently or predictably adhere to. The 4.0- to 5.0-mm depth of cure of bulk-fill composite resins facilitates single-increment monolithic injection molding of small- to moderately sized posterior restorations. Eliminating the technical challenge of layering may mitigate one of the most common failure points with Class II restorations, which is the structural failure of marginal ridges. According to the Department of Polymer Technology at the Royal Institute of Technology in Stockholm, knit lines in polymers (similar to seams in hand-manipulated composites) should be avoided as they can result in up to a 70% reduction in strength. This is analogous to structural concerns our profession has with layered cold composite resin restorations.

Figure 7a. Teeth Nos. 4 and 5 preoperatively with disclosing solution to reveal biofilm. (Case courtesy of Brandon Walker.)
Figure 7b. Selective caries removal and disease-driven cavity preparations. Preparations were cleaned and biofilm was removed with aluminum trihydroxide in a Bioclear Bioblaster. Bicuspids were matrixed with Bioclear Biofit HD Bicuspid matrices and wedged with a Bioclear Large Diamond Wedge (tooth No. 4, distal) and Medium Diamond Wedge (tooth No. 4, mesial). A Bioclear Molar Twin Ring Separator was placed on the distal of tooth No. 4 in preparation for injection molding of tooth No. 4.
Figure 7c. Post-op radiograph and photograph of injection molded teeth Nos. 4 and 5 with Rock Star (Bioclear) polish.

Despite their advantages, numerous biases exist against bulk-fill composite resins, including concerns with depth of cure, polymerization shrinkage stress, and aesthetics. Clear matrices reduce depth of cure concerns. After occlusal curing, metal matrices must be peeled back to complete buccal and lingual curing in order to achieve the 5-mm depth of cure. Clear matrices avoid the potential hazards of disrupting the resin prior to full curing. A greater concern is confirming the cavity preparation does not exceed the manufacturer-recommended depth for single-increment bulk filling. This can be overcome with the use of a depth gauge, such as Go/No-Go Probe (Bioclear) (Figure 5). The use of a high-quality curing light is critical and often underappreciated. All composite resins have volumetric polymerization shrinkage. The concern lies with the associated shrinkage stress, which can be mitigated with a good dentin bonding system and proper dentin surface cleaning.7 3M’s proprietary monomers AUDMA and AFM help reduce the stiffness of Filtek One Bulk Fill (3M) while moderating volumetric shrinkage stress. Finally, Filtek One is as aesthetic as many aesthetic composite resins on the market.

What is the scope of treatment possibilities with injection molding and the Bioclear Method?
Dr. Kim: The patented method of injection molding composite resin around a tooth is the same regardless of the tooth. The scope of treatment can range from a single anterior or posterior tooth to the entire mouth (Figures 6 and 7). The additive nature of the method, combined with a lack of constraints for uniform thickness of restorative material, means ultimate flexibility. With indirect restorations, the tooth must be prepared to satisfy path of draw, path of insertion, and material thickness requirements. This often results in the sacrifice of sound tooth structure to accommodate material and technique requirements. With injection-molded restorations, the material is instead adapted to the tooth. Thus, sound tooth structure can be preserved while missing, diseased, or weak tooth structure is replaced or reinforced.

Many aesthetic concerns can be addressed with additive, instead of resective, methods (Figure 8). Teeth that were once thought hopeless can be restored to buy valuable time for the patient (Figure 9). Dentistry as a profession has made incredible strides in the last 60 years. Despite advances with composite resins, we still adhere to century-old cavity principles and designs that do not reflect sound engineering principles. With advances in implant options, we prematurely sacrifice existing tooth structure for replacement. Since the time of G.V. Black, the average lifespan of our patients has doubled. That fact needs to be an important decision factor in treatment planning. Injection molding as a third modality of treatment options opens up possibilities and impacts the algorithm for treatment decisions. We need to educate and involve patients regarding treatment options and let go of our professional biases for outcomes. The decision should be about patient-centered outcomes.

What do you mean by doctor/patient-centered outcomes?
Dr. Kim: Doctors have professional biases regarding aesthetics, acceptable physiologic cost, and treatment predictability. Our biases do not necessarily match our patients’ desires and are generally not evidence-based. While we strive for perfect replication of natural tooth contours and colors and orthodontic correction, there is a common patient aesthetic complaint in regard to incisal translucency and polychromaticity. Many patients prefer color-uniform teeth that are bright but believable. They are often seeking an improvement of their perception of nature, not our ideas about replication of nature. Clinicians pursue marks of good artisanship with no evidence that this is desired by patients. A simple example of this disconnect is the placement of occlusal stains on direct and indirect restorations. How often have we asked patients if they desire this or had patients ask if an occlusal stain was caries? We simply need to be aware that our professional aesthetic biases may not match that of our patients.

Figure 8. Pre- and post-op views of a severe periodontally compromised case. Black triangle treatment was done to improve smile aesthetics and reduce mobility. (Case courtesy of Dr. Charles Regalado, Spokane, Wash.)

There has been a significant increase in adult orthodontic treatment. However, orthodontic tooth movement in adults has a high probability of leading to open gingival embrasures, or “black triangles,” which are, ironically, a greater aesthetic dilemma for patients than crowded teeth.8 Clinicians were guilty of dismissing patients’ concerns regarding the aesthetic dilemma of black triangles, especially in the absence of disease. Before Bioclear, we had no conservative solutions for treating black triangles. In today’s youth culture, patients have strong negative reactions to presentations of “unfamiliar” negative space. High-contrast areas are more readily noticed than soft-color contrast areas. Thus, issues such as black triangles, uneven or harsh incisal embrasure spaces, dark restorative margins, dark caries, or occlusal staining are real aesthetic concerns for patients. We may be inadvertently heading into an informed consent crisis with orthodontic treatment by creating greater aesthetic concerns for the patient.

What advantages/disadvantages does injection molding and the Bioclear Method offer?
Dr. Kim: Significant advantages are the conservation of tooth structure, possibility of treatment reversal, and ability to maintain all treatment options with additive injection molding. With the increased lifespan, preservation of tooth structure should be a priority. Since everything has a limited lifespan, reliance on medical devices too early should be avoided.

Figure 9a. Pre-op photograph of rampant caries treatment. (Case courtesy of Dr. Les Miller, Lawrence, Kan.) Figure 9b. Post-op view of injected molded teeth Nos. 6 to 9 and 22 to 27.

We cannot heal and “restore” diseased teeth to their virgin states. We can only “repair” teeth when treating caries or structural compromises. The manner in which we prepare teeth to retain a restoration will have a significant impact on the tooth’s longevity. The advent and improvement of composite resins relying on adhesion should reduce our dependence on G.V. Black cavity prep designs of mortice and tenon joints for mechanical retention with their specific internal prep designs, dimensions, and line angles. Instead, we should shift our focus to the removal of diseased tooth structure and preservation of sound tooth structure. Our first touch on a virgin tooth can impact how rapidly that tooth progresses through its life cycle. The disadvantage is a lack of training and the status quo maintaining our professional biases. Adoption of a new concept and achieving proficiency in it is always challenging. This can be ever more difficult with concepts that are yet in the early stages of general acceptance. We continue to work hard at the Bioclear Learning Center to share our philosophy of preservation and patient-centered care.

What are your goals for teaching at the Bioclear Learning Center?
Dr. Kim: With any new method, there is a unique learning curve for every clinician to reach basic proficiency, then mastery. Our goal with Bioclear was to create a system and a method to enable committed clinicians to predictably and consistently deliver results superior to their current methods. We share the excitement from clinicians when they express how the method makes sense and that they are eager to develop their skills. There is a strong fellowship of clinicians who are excited about doing the right thing for their patients. Furthermore, at the Bioclear Learning Center, we focus on teaching the Bioclear Method in its purity. We have had attendees with all levels of experience, from dental students to seasoned clinicians with more than 50 years of experience. We strive to help every attendee reach a basic comfort level so he or she can conquer his or her first case Monday morning. If we fail in this goal, then adoption and continued progression of skill will falter. Most of our attendees do begin cases right away, which is gratifying. More clinicians trained in the Bioclear Method gives more patients access to this third avenue of conservative treatment.

What thoughts do you have about the future?
Dr. Kim: With the current economics of dentistry, there is enormous pressure to provide billable procedures dictated by codes in an insurance plan rather than the right service directed by a patient’s wants and needs. Financial pressures are pushing for the delivery of easier procedures at a faster rate and lower cost. The change in the economic landscape is devaluing our skill and judgment and trading it for our ability to deliver a commodity. This will negatively impact the doctor/patient relationship and patient care and undermine even the most well-meaning clinician.

Bioclear began with a desire to provide better and more predictable outcomes for our patients in our practices. Bioclear is continually striving to make its systems more prescriptive, and our teaching at the Learning Center is intensively hands on. We can never underestimate the importance of clinical judgment, experience, a love of problem solving, and striving for excellence. We hope to teach a better way while fostering a learning environment of mutual inspiration.

We believe that injection molding is a better method for direct composite dentistry. We have been sharing the concept of injection molding with educators and institutions. Currently, injection molding and the Bioclear Method have been fully incorporated at the Roseman University of Health Sciences College of Dental Medicine in South Jordan, Utah. Roseman was the first school to spearhead the conversion of their preclinical and clinical curriculum. The University of Alberta’s GPR program has also adopted the method. The Loma Linda School of Dentistry is working to incorporate the method in the near future.

With the beginning of the adoption of injection molding and the Bioclear Method in our nation’s schools, we are excited for what the future holds. We want to contribute a positive impact to our profession. We are excited for the recent educational partnership formed between Bioclear and 3M to help introduce the Bioclear Method globally. We are committed to facilitate and support the learning of practicing clinicians, dental students, and study clubs throughout the country.

Many thanks, Jihyon, for taking the time to thoroughly discuss this exciting and revolutionary approach to composite resin restorations! Your passion, expertise, and dedication to teaching others about these novel techniques are obvious. We wish you all the best for continued success!


References

  1. Lucey S, Lynch CD, Ray NJ, et al. Effect of pre-heating on the viscosity and microhardness of a resin composite. J Oral Rehabil. 2010;37:278-282.
  2. da Costa J, McPharlin R, Hilton T, et al. Effect of heat on the flow of commercial composites. Am J Dent. 2009;22:92-96.
  3. Mundim FM, Garcia Lda F, Cruvinel DR, et al. Color stability, opacity and degree of conversion of pre-heated composites. J Dent. 2011;39(suppl 1):e25-e29.
  4. Dunbar T, Abuelyaman A, Phillips C, et al. Does preheating a dental composite degrade its post-cure properties? J Dent Res. 2016;95(special issue A). Abstract 0952.
  5. Zimmerli B, Rickli C, Lussi A. Microhardness and marginal adaptation of pre-warmed composites. Poster presented at: Pan European Federation Meeting; September 15, 2006; Dublin, Ireland. Abstract 589.
  6. Daronch M, Rueggeberg FA, Hall G, et al. Effect of composite temperature on in vitro intrapulpal temperature rise. Dent Mater. 2007;23:1283-1288.
  7. Sherawat S, Tewari S, Duhan J, et al. Effect of rotary cutting instruments on the resin-tooth interfacial ultra structure: an in vivo study. J Clin Exp Dent. 2014;6:e467-e473.
  8. Cunliffe J, Pretty I. Patients’ ranking of interdental “black triangles” against other common aesthetic problems. Eur J Prosthodont Restor Dent. 2009;17:177-181.

Dr. Kim is a full-time clinician in Bellevue and Tacoma, Wash. She also teaches and develops curricula as full faculty and co-director of the Bioclear Learning Center. Dr. Kim is creator of the Smile Design Gauge, a multifunction tool for chairside smile design. She can be reached at drkim@jihyonkimdds.com.

Disclosure: Dr. Kim is faculty and co-director of the Bioclear Learning Center and a consultant for 3M and has a financial interest in the Smile Design Gauge.

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