Fiber reinforced composites in prosthodontics - A systematic review.

Fiber-reinforced composite (FRC), prostheses offer the potential advantages of optimized esthetics, low wear of the opposing dentition and the ability to bond the prosthesis to the abutment teeth, thereby compensating for less-than-optimal abutment tooth retention and resistance form. These prostheses are composed of two types of composite materials: Fiber-composites to build the substructure and hybrid or micro fill particulate composites to create the external veneer surface. This article reviews the various types of FRCs and its mechanical properties.

The practice of restorative dentistry has been revolutionized by composites and these systems largely dominate the market of cosmetic and restorative materials. Over the past few years, their physical-chemical properties have improved considerably. In clinical practice, failure with composites has been mainly attributed to fatigue fracture. The fatigue on dental restorations of the composite resins is also influenced by water absorption by resin matrix and occlusion cyclical force. Fiber-reinforcement has been introduced to increase both flexural strength and modulus of these materials. However, reinforcement component can act as a stress concentrator due to fiber-composite interface.

Review of Literature

Some of the earliest of these experimental preimpregnated fiber-reinforced composites (FRCs) designed for dental applications were based on glass-reinforced thermoplastics by Goldberg et al.[1] in the year 1994. He studied the flexural property, stress relaxation and hydrolytic stability of FRC based on thermoplastic matrices, types of fibers, and fiber volume fractions. He concluded that Polycarbonate was the preferred matrix material. The flexural modulus and strength was improved when polycarbonate was reinforced with 42 volume percent of glass fibers. The apparent flexural modulus of all composites decreased with span length in the range of clinical interest. The prevalent mode of failure for all FRC investigated was brittle failure under flexure loading. Although researches have been revolving around improvisation of the mechanical properties of FRC, most of the clinical failures were the result of debonding of the retainers from the tooth surface.[1]

A subsequent clinical trial by Altieri et al. in 1994 evaluated the use of preimpregnated glass-reinforced polycarbonate as the framework for acid-etched fixed partial dentures (FPDs).[2] Fourteen 3 U restorations were placed both in anterior and posterior locations using adhesive techniques and no tooth preparation. Review after 9 years showed that, three restorations were still in service. All 11 failures were associated with separation in the region of the tooth restoration interface indicating the need for adequate mechanical properties of FRCs for use in prosthodontic applications. These problems were resolved by switching to a bisphenol glycidyl methacrylate based resin as the matrix for the FRCs.

In 1997, Samadzadeh et al.[3] studied the effect of the addition of chopped length of high modulus polyethylene fibers on the fracture resistance of FRC. He concluded that the fracture resistance improved with the addition of polyethylene fibers.

In 2002, Freilich et al.[4] evaluated of 39 light and heat polymerized fixed partial bridges made with a substructure of preimpregnated, unidirectional FRC, veneered with a hybrid particulate composite. Each of the prosthesis was assessed for surface integrity, anatomical contour, marginal integrity, and structural integrity. The results showed that survival was associated primarily with substructure design volume. The survival rate was 95% for prostheses made with a high-volume substructure, when patients with severe parafunctional habits were excluded. This study shows that a unidirectional, preimpregnated FRC can be used successfully to make bridges of variable retainer designs that last up to 4 or more years when a high-volume substructure is used.

In 2003, Li[5] et al. studied the failure modes and failure locations of the direct FRC dental bridge structures with and without adjacent teeth experimentally. The experimental results show a good agreement with the clinical observations. It is found that the bonded interface is indeed the weakest region in the composite bridges. Also, it is suggested that the composite resin reinforced with high modulus polymer fibers and the presence of adjacent teeth could significantly increase the structural strength and stiffness of the bridge and therefore improve its clinical performance.

In 2003, Li et al.[6] did a finite element (FE) study on FRC bridges. The FE model adopted was constructed from computer tomography images of a physical bridge specimen. The peak stresses and their variations with the different bridge designs were evaluated. The analysis showed stress concentration at the pontic-abutment interface, which results in failure at the interface. The numerical analysis of the bridge structure reveals that a high stress concentration occurs around the incisal portion of the adhesive interfaces between the pontic and abutment.

In 2005, Visser and Rensburg[7] did a study to review FRC as an alternative to tooth replacement in South Africa. Although the use of FRCs for this purpose is relatively new in South Africa, 5 years clinical results are very promising (Vallitu 2004). It is not necessary to prepare adjacent teeth, so the biological costs are low. In fact, it makes more sense to conserve as much as possible that part of the tooth which displays the best bonding surface in the oral cavity, that is, the enamel of the tooth. Additionally, as this technique is reversible it allows other restorative options to be evaluated at a later time. These restorations offer a viable alternative to more expensive fixed or removable prostheses.

Shinya et al.[8] in 2008 studied the stress distribution in anterior adhesive fixed dental prosthesis and at tooth/framework interface. The design of FPD consists of retainers in maxillary central and canine and pontic lateral incisor. Two different materials were compared: Isotropic Au-pd alloy and anisotropic continuous unidirectional E-glass FRC. A three-dimensional FE model of 3 U FPD with 154 N loading was analyzed to determine the stress distribution at FPD and adhesive interface. The general observation was that the FRC-FPD provided more even stress distribution from the occluding contact point to cement interface than did metal-FPD.

Matheus et al.[9] in 2010 used optical coherence tomography (OCT) compared to scanning electron microscopy and optical microscopy to evaluate qualitatively crack propagation and final fracture in restorative composite materials with fiber-reinforcement after cyclic loading. The failures were analyzed using the three methods described. OCT permitted good characterization of internal crack propagation. The results indicated that the deformation occurred in the dental composite and fiber in the direction of the force.

Monaco et al. in 2003[10] clinically evaluated the inlay FRC FPD's.

This clinical study evaluated the behavior of inlay fixed partial dentures (IFPD) with conventional and modified framework designs over a period of 12–48 months. Forty-one glass FRC IFPDs were made to replace one missing maxillary or mandibular tooth. The samples were divided into two groups. Group (19 samples) 1 had parallel fibers while Group 2 (22 samples) had parallel as well as woven fibers. All restorations were evaluated by color match, marginal discoloration, secondary caries, surface texture, marginal adaptation, fracture, and postoperative sensitivity. Group 1 showed a 16% fracture failure rate; Group 2 showed a 5% failure rate. However, the difference between the two groups was statistically insignificant.

Bell et al. in 2004[11] studied the bonding properties of two types of FRC posts cemented into root canals of molars. Serrated titanium posts served as a reference. Prefabricated carbon/graphite FRC posts with cross-linked polymer matrix and individually formed glass FRC posts with interpenetrating polymer network polymer matrix were compared. Push-out force was measured by pushing the post from one end. The push-out force increased with increased height of dentin disc in all groups. Unlike the other posts, there were no adhesive (postcement) failures with the individually formed glass FRC posts, suggestive of an improvised interfacial adhesion of cement to these posts.

Malferrari et al. in 2003[12] did a prospective clinical follow-up and evaluated the acceptability of quartz-fiber-reinforced epoxy posts used in endodontically treated teeth over a 30-month period. In 132 patients, 180 endodontically treated teeth were restored using quartz-fiber posts. Displacement, detachment, or fracture of posts; core or root fracture; and crown or prosthesis de-cementation were considered as the parameters of clinical failure. Patients were re-evaluated at 6, 12, 24, and 30 months. After 2 weeks, one cohesive failure was observed and recall after 2 months showed two adhesive failures. All three failures occurred during removal of the temporary crown at the cement dentin interface. Over a 30-month period, the success rate was 1.7% but however, it was possible to successfully replace the restoration in all three failed cases.

E-glass fiber (“E” stands for electric) is a recent advancement in FRC. E-glass fiber is made of aluminoborosilicate glass with <1 wt% alkali oxides. Recent studies by Zhang and Matinlinna[13] in 2011 have proved that E-fibers are able to maintain strength properties over a wide range of conditions, relatively insensitive to moisture and chemical-resistant.