ABCs of fiber-reinforced bridges

Fiber-reinforced bridges, or FRCFPD (Fiber Resin Composites Fixed Partial Dentures), have shown good clinical performance, capable of withstanding the functional loads of mastication with a more biomimetic behavior than rigid bridges.

The adhesive capacity of its materials allows these prostheses to have a lower biological cost than conventional alternatives since minimal or no preparation of the abutment teeth is necessary.

FRCFPDs can be prepared in the laboratory or on site ; their manufacture consists mainly of two parts: The structure made with the reinforcing fiber chosen by the dentist and the fiber coating material, usually composite resin.

The design of the structure should be oriented to resist and dissipate chewing forces; the pontic can be manufactured with CAD/CAM materials such as zirconia or ceramic, and teeth for total prostheses, natural teeth or composite resin can also be used.

The design of the cavities in the abutment teeth must allow the accommodation of the fiber and the lining material; normally these preparations are based on the design for inlays. In this respect, the dentist can take advantage of the cavities of old restorations or make new ones.

These treatments are typically less expensive than a conventional bridge, largely because they can be done directly in the patient's mouth using common dental office materials. Using composite resin to coat the fiber and construct the pontic simplifies the repair and maintenance process.

Additionally, as is well known, aesthetic perception evolves over time; what is considered beautiful today will not necessarily be perceived the same way in 10 years. If anterior teeth are involved in the bridges, the dentist can make aesthetic modifications during their lifespan. This is what modern dentistry seeks; modern dentistry loves to repair and rebuild. Modern dentistry is not in favor of changing restorations just for the sake of changing them (it is understood that when it has to be done, it should be done 😉).

CAVITY PREPARATION

The fracture resistance of materials depends, among other things, on the thickness of the restoration and the design of the preparation.

With the aim of allowing adequate space for the fiber and lining material in the abutments and connectors, the dentist can prepare the pieces or take advantage of existing cavities; the design is based on that of inlays with a minimum of 2mm in height in the abutment and 4mm in the connector area.

The preparations that respond best biomechanically are the box-shaped and tub-shaped (Fig 1). The tub-shaped preparation exhibits better biomechanical behavior because it allows for the fabrication of connectors with adequate thickness and proper transmission of masticatory forces along the remaining tooth structure.

If the prosthesis is to be used as a long-term provisional, this preparation is not necessary as it requires removing additional tooth structure vs. the box-shaped one.

Figure 3: Side view of the Box-shaped design and the Tub-shaped design with their proximal drawers.

FIBER SELECTION

If you want a more complete overview of the reinforcing fibers used in dentistry, I recommend you read my blog post titled “Introduction to Reinforcing Fibers 🤓”.

In this case I will refer only to glass fibers and ultra-high molecular weight polyethylene (UHMWPE) , I will mention some qualities that these present especially when used for the construction of structures in reinforced bridges.

I want to start by reminding everyone that the cornerstone of modern dentistry is adhesion, period.

Of these two fibers, fiberglass exhibits the best adhesion to composite resin. Due to its composition, primarily glass and silica, it can be silanized to achieve adhesion with composite resins. Silanization is performed during fiber manufacturing, and these fibers are typically already embedded in high-filler flowable resin to facilitate clinical application (Fig. 2) and reduce the possibility of gaps between the fibers that remain unmatrixed.

In the case of UHMWPE, due to the inert nature of the material, it does not exhibit an adhesive capacity comparable to glass fibers. However, the interweaving of the fibers allows for the entrapment of the composite resin matrix, and it is this interlocking that produces retention. Some manufacturers apply a high-energy plasma treatment, which generates free radicals on the surface of the fibers, enabling adhesion. This adhesive capacity is high at the time of plasma application and then decreases significantly over time. For this reason, the literature considers it prudent to use UHMWPE only for temporary and long-term temporary bridges, while glass fibers are recommended for the fabrication of permanent bridges.

Figure 2: Schematic drawing showing how the fibers are surrounded by the veneering material.

Fiberglass is transparent, which can be an advantage in the case of previous bridges.

When a dentist performs a treatment, they must consider that sooner or later that treatment will fail and/or require repair. Doctors, never forget this (it bears repeating): modern dentistry loves repairs, maintenance, and remodeling. With this in mind, we must consider which fiber performs best when exposed to the fluids of the oral environment and, more importantly, which fiber allows for proper adhesion with the resin used for the repair.

In this respect, fiberglass offers a clear advantage: its behavior in the presence of liquids is superior, due in part to the material's less porous nature. Furthermore, it allows for better decontamination, drying, and adhesive conditioning, resulting in a better substrate for bonding the repair resin. UHMWPE, being a woven fabric, has millions of spaces between the fibers that facilitate the trapping of liquids and the retention of contaminants. This hinders decontamination and does not provide a suitable substrate for re-adhering the repair material.

HANDLING

Although handling considerations have not been thoroughly studied, it is known that this factor affects the strength and clinical performance of reinforcing fibers. UHMWPE fibers cannot be cut with ordinary scissors; the need for a special tool can present a handling challenge. Additionally, their weave pattern can be easily altered when the fiber is cut, causing gaps and disorganization in the weave pattern, resulting in an inconsistent reinforcement effect.

The impregnation of the resin matrix into the fibers is governed by the wetting properties of the fiber surface. Factors such as the distance between individual fibers and the viscosity of the resin can produce an imbalance that could affect the fiber-matrix bond and leave voids that may compromise the material's mechanical properties. Generally speaking, woven fibers are more difficult to fully impregnate with resin than unidirectional fibers.

The rigidity of fibers can result in difficult manipulation, as is the case with unidirectional glass fibers. This can lead to thicker restorations, which could cause discomfort for the patient. In addition to rigidity, unidirectional glass fibers exhibit position memory, meaning that when bent, they tend to return to their original shape. This can lead to difficulties in adaptation. When the fiber is light-cured, it will maintain the shape of the matrix in which it is embedded.

This is not the case with UHMWPE because they do not have position memory, which allows them to be intimately adapted to uneven surfaces. This close contact with the substrate(s) is key for the correct transmission of masticatory loads to the abutment teeth.

STRUCTURE DESIGN

This parameter is fascinating; this is where the dentist puts into practice all the knowledge he has of the fibers he uses and how he arranges them to be mechanically efficient.

Glass fibers exhibit greater resistance than UHMWPE as long as the direction of the forces is perpendicular to the direction of the fibers; conversely, UHMWPE performs better than glass fibers when subjected to lateral and oblique loads.

As we know, chewing forces are present in different directions and their intensity varies according to the location of the teeth in the arch; for this reason, the dentist must know how each type of fiber reacts mechanically when a force is applied in a certain direction.

Figure 3: Photograph of the structure of a bridge to replace an upper canine illustrating the occlusal, palatal and vestibular arrangement of the unidirectional glass fibers in such a way as to resist lateral forces.

The way in which the fibers are arranged when forming the structure or framework will directly affect the strength of the bridge, its transmission capacity, dissipation of chewing loads and as a result will have a direct impact on the longevity of the prosthesis.

As you may have noticed, fiber-reinforced bridges are a whole world in themselves 🧠 and I'm happy to introduce you to the topic in an ethical and scientific way so that you can succeed in your cases when you put them into practice.

I will bring you more important information on the subject very soon.

I hope you find this information useful, best regards!

Dr. Marvin