Review Reinforcement in removable prosthodontics: a literature review

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1 J o u r n a l o f Oral Rehabilitation Journal of Oral Rehabilitation ; Review Reinforcement in removable prosthodontics: a literature review T. TAKAHASHI, T. GONDA, Y. MIZUNO, Y. FUJINAMI & Y. MAEDA Department of Prosthodontics, Gerodontology and Oral Rehabilitation, Osaka University Graduate School of Dentistry, Suita, Japan SUMMARY Removable prosthodontics are often associated with mechanical troubles in daily use, such as fracture or deformation. These troubles render prostheses unusable and reduce wearers QOL. Various reinforcements are used to prevent such problems, but consensus on reinforcement has not been reached. This review aimed to summarise the effects of reinforcement and to propose favourable reinforcement based on material, design and position in the prostheses. Initially, 139 articles were selected by electronic and manual searches. After exclusion of 99 articles based on the exclusion criteria, 40 articles were finally included in the review. Electronic searches were performed for articles published from 2005 to 2015 in PubMed, EMBASE, MEDLINE and Cochrane Library, and manual searches were performed in 10 journals relevant to the topic of removable prosthodontics. For in vitro studies, certain dental alloys and fibres were mainly used. Their forms were different, including complicated forms in dental alloys and various forms in fibres. The materials were examined for mechanical properties like fracture strength, flexural strength and elastic modulus and compared with one another or without reinforcement. There were a few clinical studies and one longitudinal study. Cast metal reinforcement seemed to be most favourable in terms of fracture toughness and stiffness. The most favourable forms differed depending on the prostheses, but placement around thin and deformable areas was effective. However, randomised or longitudinal clinical reports and comparative clinical studies on the use of reinforcement were still lacking and such studies are necessary in the future. KEYWORDS: review, reinforcement, removable denture, glass fibre, strength, stiffness Accepted for publication 23 November 2016 Introduction In clinical practice, removable dental prostheses have been used for oral rehabilitation and are essential for patients to improve and maintain their quality of life. However, these requisite prostheses for daily life often become unusable because of deformation or fracture, and wearers become disadvantaged. The reasons for these complications were reported to include improper usage by wearers, accidents like dropping or hitting, and insufficient strength (1), and various reinforcements have been embedded into prostheses to increase their strength and prevent these problems. The reinforcements used in removable dental prostheses, especially complete dentures, partial dentures and implant or root overdentures, have frequently been reported regarding their materials, designs and positions among other factors, but their variations were different and their reported effects were almost sufficiently high. Furthermore, the necessity for reinforcement was advocated in clinical studies (2). However, the most effective reinforcement is not apparent, and clinicians are confused about designing such reinforcement. Meanwhile, the materials for prostheses doi: /joor.12464

2 134 T. TAKAHASHI et al. have been improved in their mechanical properties to prevent complications, and thus, reinforcement may become unnecessary. Moreover, some authors reported that reinforcement involved the addition of a foreign material to prostheses and may thus be a risk factor for fracture development, rather than fracture prevention (3). Currently, reinforcement has two important purposes in a prosthesis. The initial purpose is to improve the strength and prevent fractures, and most previous studies were conducted with a focus on this purpose. The second purpose is to improve the stiffness and prevent residual ridge resorption and overloading to residual teeth or structures. Regarding the effect on the residual ridge, Maeda and Wood (4) reported that deformation of the denture base leads not only to denture base fractures, but also to ridge resorption by compressive stress transmitted to the underlying bone, while Gonda et al. (5) reported a finite element study on mandibular overdentures and showed that rigid metal reinforcement reduced the stress beneath the denture base, and could distribute the stress of the residual alveolar ridge area more widely and evenly. For overload to residual teeth, it was reported that denture deformation causes stress on the abutment teeth that can lead to loss of these teeth and that a rigid major connector or reinforcement was necessary to make the denture more rigid and prevent tooth loss. Therefore, in this review, we focused on reinforcement in terms of improving the mechanical properties of prostheses, not only for their strength, but also their stiffness. The purposes of this review were to summarise (i) what is already revealed and what is not yet revealed about reinforcement, and (ii) what is the most favourable reinforcement to keep the initial status in the long term and maintain wearers quality of life from recent reports. Methods Search strategy Searches were performed both electronically and manually in articles published from 2005 to Electronic searches were performed in the PubMed, EMBASE, MEDLINE and Cochrane Library databases, and manual searches were performed in the following 10 journals: Acta Odontologica Scandinavica; Dental Materials; Dental Materials Journal; Gerodontology; International Journal of Prosthodontics; Journal of Dental Research; Journal of Oral Rehabilitation; Journal of Prosthetic Dentistry; The International Journal of Oral and Maxillofacial Implants; and Quintessence International. The key words for the searches were as follows: reinforcement denture ; reinforcement dental prosthesis ; reinforce denture ; reinforce dental prosthesis ; strengthened denture ; and strengthened dental prosthesis (Table 1). Inclusion and exclusion criteria The inclusion criteria were as follows: (i) type of study (clinical study, case report or in vitro study); (ii) availability of full text; and (iii) written in English. The exclusion criteria were as follows: (i) reports about fixed dental prostheses, core treatments after root canal treatment or orthodontic treatments; (ii) types of studies other than the abovementioned types; and (iii) reports about characteristics other than mechanical properties, such as discolouring, water resorption or surface morphology. Screening and selection The titles and abstracts of all reports were screened by one author (TT), and the full-text articles were reviewed by four authors (TT, GT, MY and FY). After screening of the reports, more detailed searches were conducted according to the inclusion and exclusion criteria described above. Data extraction After completion of the search strategies, the following information about reinforcement was extracted from the selected reports: reinforcement material and form; position in prosthesis; prosthesis material; and reinforcing effect compared with other reinforcements or without reinforcement. Results Searches and selection Initially, 139 articles were selected by both electronic and manual searches. Subsequently, 90, four and five articles were excluded based on the exclusion criteria

3 REINFORCEMENT IN REMOVABLE PROSTHODONTICS 135 Table 1. Studies on reinforcement in the decade from 2005 to 2015 Author and year Type of study Material of reinforcement Form of reinforcement Experimental situation Material of prosthesis Investigation item and method Brief of the results Takahashi et al (34) Murthy et al (10) Takahashi et al (36) In vitro Cast metal (Co-Cr) Framework Maxillary implant overdenture (strain gauge analysis) In vitro Stainless steel glass fibre polyethylene fibre Mesh woven In vitro Cast metal (Co-Cr) Framework Maxillary complete denture (strain gauge analysis) Auto-polymerised resin Strain gauge Four designs of cast reinforcement were compared Cast reinforcement over the residual ridge and the top of copings was most favourable design Specimen Heat-polymerised resin Impact strength Polyethylene fibres exhibit better impact strength followed by glass fibres and stainless steel mesh. Auto-polymerised resin Strain Reinforcement with a palatal bar or metal-based palate could reduce the strain. Yu et al (35) In vitro Glass fibre metal Mesh Complete denture Light-polymerised resin Fracture resistance The content (4%) of the glass fibre mesh seemed more important than the structures. Yu et al (11) In vitro ZrO2 Flake Specimen Heat-polymerised resin Flexural strength The flexural strength was maximised when 20wt% ZrO2 nanotubes were added. de Cruz Perez et al (12) Gonda et al (5) Finite element analysis In vitro Glass fibre Flake Specimen complete denture Auto-polymerised resin Impact strength Bar-shaped specimens can be a reliable method for evaluating influencing factors of resistance to impact of denture base. Wire (Co-Cr) cast metal (Co-Cr) Framework Mandibular overdenture (finite element analysis) Reinforcement adjacent to the top of the coping and the medial part reduces the stress beneath dentures and widely and evenly distributes the stress of the residual alveolar ridge. Xu et al (13) In vitro Ramie fibre Chopped Specimen Heat-polymerised resin Flexural strength Short ramie fibre-reinforced denture base had higher flexural modulus. Mansour et al (14) Venkat et al (15) Balch et al (7) Ozcelik et al (8) Takahashi et al (37) In vitro Mica Flake Specimen Heat-polymerised resin Flexural strength 20% mica contains showed highest mica microhardness. In vitro Wire polyethylene fibre Wire mesh Specimen mandibular complete denture Heat-polymerised resin auto-polymerised resin Fracture load deflection flexural strength elastic modulus All the denture specimens repaired with materials demonstrated higher fracture load values. Case report Cast metal (Co-Cr) Framework Mandibular complete denture Auto-polymerised resin Case report A method for fabricating and internally a metal framework in the denture base of a mandibular complete denture. Case report Cast metal (Co-Cr) Framework Implant overdenture Case report The technique presented describes not only the reinforcement of the denture base with a metal framework but also the inclusion of the attachment metal housing in the framework design to prevent fractures that could occur at the sites close to the implant abutments. In vitro Cast metal (Co-Cr) glass fibre metal wire Continuous Maxillary complete denture (strain gauge analysis) Auto-polymerised resin Strain Cast cobalt chromium reinforcement could mostly reduce the strain of a maxillary complete denture. (continued)

4 136 T. TAKAHASHI et al. Table 1. (continued) Author and year Type of study Material of reinforcement Form of reinforcement Experimental situation Material of prosthesis Investigation item and method Brief of the results Mowade et al (16) Farina et al (17) Goguta et al (6) Zhang et al (18) Rached et al (19) Yoshida et al (38) In vitro Glass fibre polyethylene fibre polypropylene fibre Continuous Specimen Heat-polymerised resin Impact strength Reinforcement with the fibre is an effective method to increase the impact strength of denture base. And the surface treatment further increases the impact strength. In vitro Glass fibre Continuous Specimen Heat-polymerised resin auto-polymerised resin Clinical study E-glass fibre Woven unidirectional In vitro Aluminium borate whisker In vitro Stainless steel glass fibre polyethylene fibre polyaramid fibre Vickers hardness Glass fibre reinforcement increased the Vickers hardness of both types of resin. Maxillary complete denture Clinical study 30 dentures with E-glass fibre reinforcement follow-up period: 5 years four partial fractures Flake Specimen Auto-polymerised resin Flexural strength surface hardness thermal stability Mesh woven Specimen Light-polymerised resin Fatigue failure flexural strength Silanised aluminium borate whiskers improved the flexural strength, surface hardness and thermal stability of PMMA. And optimal amount was 5 wt%. Glass fibres, woven polyethylene braids and polyaramid fibres withstood the fatigue regime and increased the flexural strength of implant overdenture. In vitro Wire (Co-Cr) Wire Maxillary complete denture Heat-polymerised resin Flexural load Location of the metal reinforcement affected the fracture resistance of the maxillary complete dentures. Ladha et al (20) In vitro Glass fibre nylon fibre Takahashi et al (39) Takahashi et al (40) Cheng et al (44) Orenstein et al (9) Hatamleh et al (21) Woven unidirectional Specimen Heat-polymerised resin Flexural strength Reinforcement of denture base resin with pre-impregnated glass fibres may be a useful means of strengthening denture bases. In vitro E-glass fibre Continuous Maxillary complete denture Heat-polymerised resin Flexural strength All reinforced dentures had higher flexural load at the proportional limit and lower flexural deflection than the denture without reinforcement. In vitro Cast metal (Co-Cr) Framework Maxillary complete denture (strain gauge analysis) Finite element analysis Polyethylene fibre Lamella Maxillary complete denture (finite element analysis) Auto-polymerised resin Strain Cast cobalt chromium reinforcement reduces strain and could contribute to fracture avoidance deformation in maxillary complete dentures. 3D FEA exhibits the effectiveness of high-performance polyethylene reinforcement together with fibre positions on enhancement of denture strength. Case report Cast metal (Co-Cr) Framework Implant overdenture Heat-polymerised resin Case report Using this technique existing denture can be converted into cast metal-reinforced implant-retained overdenture prosthesis, with only one additional office visit required. In vitro Glass fibre Woven net Specimen Heat-polymerised resin Shear bond strength Soft liner exhibited stronger bond to net fibre-reinforced surfaces when compared to smooth and rough acrylic interfaces after thermocycling. (continued)

5 REINFORCEMENT IN REMOVABLE PROSTHODONTICS 137 Table 1. (continued) Author and year Type of study Material of reinforcement Form of reinforcement Experimental situation Material of prosthesis Investigation item and method Brief of the results Gharehchahi et al (22) Nagata et al (43) Shimizu et al (45) Dogan et al (26) Kostoulas et al (24) Bertassoni et al (27) Kostoulas et al (25) Gonda et al (42) Dogan et al (29) Cokeliler et al (30) Nakamura et al (28) Hedzelek et al (41) In vitro Glass fibre Continuous Specimen Heat-polymerised resin Flexural strength The use of glass fibre improved the flexural strength in acrylic bases. Finite element analysis Glass fibre Continuous Telescope denture (finite element analysis) In vitro Wire (Co-Cr) Wire Specimen Auto-polymerised resin Transverse strength (after repairing) In vitro Glass fibre rayon fibre polyester fibre nylon 6 fibre nylon 6,6 fibre In vitro Glass fibre metal wire Continuous Specimen Heat-polymerised resin Flexural strength fracture strength Heat-polymerised resin Change in the fracture pass by the fibreglass increased the fracture resistance. The metal strengtheners with sufficient length may provide a preventive denture design against the acrylic fractures. Young s modulus and maximum load suggests that reinforcement makes resin resistant to fracture. Woven, wire Specimen Heat-polymerised resin Fracture strength The group reinforced with full lengths of metal wire offered the best potential for reinforcement. In vitro Glass fibre Continuous Specimen Heat-polymerised resin auto-polymerised resin Flexural strength elastic modulus In vitro Glass fibre Woven Specimen Heat-polymerised resin Fracture strength (after repairing) In vitro Wire cast metal (Co-Cr) In vitro Glass fibre rayon fibre polyester fibre nylon 6 fibre nylon 6,6 fibre Framework Mandibular overdenture (strain gauge analysis) Pre-polymerised fibres improved the overall mechanical properties of reinforced auto-polymerised acrylic resins more than post-polymerised fibres. Most effective repair method was the use of auto-polymerised resin reinforced with glass fibres. Auto-polymerised resin Strain Cast metal reinforcement that covers both the midline and the coping top significantly reduced the strain on the overdenture. Continuous Specimen Heat-polymerised resin Impact strength E-glass fibre reinforcement produced relatively stable, high values for each length In vitro E-glass fibre Continuous Specimen Auto-polymerised resin Flexural strength Plasma treatment with ethylenediamine monomer was an effective alternative method of increasing the flexural strength of denture base polymers through fibre reinforcement In vitro Glass fibre Short rod Specimen Heat-polymerised resin Flexural strength Flexural moduli of acrylic resin at fibre contents exceeding 20% were significantly greater than those without short-rod glass fibres. In vitro Glass fibre Bundle mesh Maxillary complete denture Heat-polymerised resin Fracture strength The applied glass fibre reinforcement increased the mechanical strength of the acrylic resin palatal denture bases. (continued)

6 138 T. TAKAHASHI et al. Table 1. (continued) Investigation item and method Brief of the results Form of reinforcement Experimental situation Material of prosthesis Material of reinforcement Author and year Type of study Specimen Heat-polymerised resin Fracture strength Specimens reinforced with stainless steel wires or Co-Cr-Ni wires resulted in significantly higher loads to fracture as compared to specimens without reinforcement. Woven wire In vitro Glass fibre metal wire In vitro Glass fibre Flake Specimen Heat-polymerised resin Fracture strength The significant improvement in fracture toughness of a denture base using glass flake is an extremely promising result. Flexural strength was significantly increased by the reinforcement. Flexural strength impact strength In vitro Glass fibre Sheet Specimen Auto-polymerised resin heat-polymerised resin light-polymerised resin Minami et al (31) Franklin et al (33) Kanie et al (32) 1, 2 and 3, respectively. Finally, a total of 40 articles were included this review (Fig. 1). The included articles consisted of one clinical report (6), three case reports (7 9) and 36 in vitro reports. Within the included in vitro articles, 24 reports examined standardised specimens (10 33), 11 reports examined prosthesis-shaped specimens (12, 15, 34 42) like complete and partial removable dentures, two reports examined both standardised and prosthesis-shaped specimens (12, 15), and three reports used a finite element method (5, 43, 44). In terms of the investigated items, 13 reports (11, 13, 14, 18, 20, 22, 26 28, 30, 32, 38, 39), four reports (10, 16, 26, 32), one report (23), nine reports (15, 17, 24 26, 31, 33, 35, 41) and one report (21) examined flexural strength, impact strength, transverse strength, fracture strength and shear bond strength, respectively. In the only clinical report (6), the 5-year prognosis after repair of fractured dentures was examined. The examined materials for prostheses were mainly auto-polymerised (7, 12, 15, 17, 18, 27, 30, 32, 34, 36, 37, 40, 42, 45), heat-polymerised (9 11, 13 17, 20 22, 24 29, 31 33, 38, 39, 41, 43) and light-polymerised (19, 32, 35) acrylic resin. These kinds of acrylic resin are usually used in clinical practice as removable denture bases. When included reports were categorised by evidence level according to GRADE and Shekelle s system (46), most of them were placed in category III and a few were in IV GRADE. Therefore, all of them were placed in category D recommendation level that is based on Shekelle s system. Regarding the types of prostheses, four reports examined overdentures including implant overdentures (5, 8, 9, 34), eight reports examined maxillary complete dentures (6, 36 41, 44), and four reports examined mandibular complete dentures (7, 15, 35, 42). The two other reports examined telescope dentures (43) and removable partial dentures (12). Materials and forms The materials for reinforcement reported in the previous articles were mainly metals (5, 7 10, 15, 19, 23, 24, 31, 34 37, 40, 42) and fibres (6, 10, 12, 15, 17, 19 22, 24 33, 35, 37, 39, 41, 43, 44), plus a small number of others (11, 13, 14, 18). Metal and fibre reinforcements appeared to already be applied in clinical practice. The methods for metal reinforcement of prostheses were cast metal and metal wire, and the materials

7 REINFORCEMENT IN REMOVABLE PROSTHODONTICS 139 Fig. 1. Flow chart of the systematic review. were mainly Co-Cr and Ni-Cr. With respect to their forms, various designs were made by wax-up techniques for cast metal reinforcement and bending techniques for wire reinforcement. The former can be made into more complex designs than the latter, but their procedures are more complicated and time-consuming. Although both types of reinforcements were more effective than no reinforcement in terms of improving various strength and stiffness properties, cast metal was more effective in direct comparisons of these two metal reinforcements (37). The reinforcing fibres were glass fibres (6, 10, 12, 16, 17, 19 22, 24 33, 35, 37, 39, 41), polyethylene fibres (10, 15, 16, 19, 44), rayon fibres (26, 29), polyester fibres (26, 29) and nylon fibres (26, 29) in order of numbers of articles, and a predominant number of reports were about glass fibres. Their forms were various, such as unidirectional or continuous (6, 16, 17, 20, 22, 26, 27, 29, 30, 37, 39, 43), chopped or flaked (11 14, 18, 28, 33), and mesh or woven (6, 10, 15, 19, 20, 24, 25, 31 33, 41). In these studies, unidirectional and mesh fibres were embedded into the denture base when polymerising the denture and chopped or flaked fibres were mixed with the resin polymer in advance and then mixed with the resin monomer. In this manner, the reinforcing methods differed among the forms, but their reinforcing effects were all reported to be beneficial compared with no reinforcement. However, when using chopped or flaked fibres, attention should be paid to the ratio of fibres and it should be not allowed to exceed 20% (28, 33). Reports on differences in forms for the same kind of fibre were few (6, 20, 41), and most reports compared prostheses with and without fibre reinforcement. From the results in a limited number of reports comparing the reinforcing effects between the forms or materials of fibres, mesh form (6, 20) type and polyethylene fibres (16, 19) seemed to be the most effective. There was also a report on the effect of their surface treatments, in which silane-treated glass fibres were more effective than non-treated glass fibres (16). In clinical practice, glass fibres have already been used for reinforcement in various materials and are no longer experimental materials (6). From the selected reports, most previous reports examined the various strengths of prostheses with reinforcement, being focused on improving only the strength and not the stiffness, and only two reports (15, 27) examined the stiffness by measuring the elastic modulus. Reports on other reinforcing materials were few, but ZrO 2 (11), mica (14) and aluminium borate whiskers (18) were used. These materials were all powders and mixed with resin polymers in varying proportions before polymerisation. They were reported to have some efficacy, but were not sufficient to be applied in clinical settings, and were no more than new attempts at the time. When comparing the effects among reinforcing materials in terms of various strengths, one study reported that metal reinforcement showed more improvement than fibre reinforcement (37), while others reported that fibre reinforcement was superior to metal reinforcement (10, 15, 19). Thus, their effects differed depending on experimental conditions like the size, morphology and material of the specimens. On the contrary, in comparisons of the effects in terms of elastic modulus, metal reinforcement, especially cast metal reinforcement, led to smaller moduli than various fibre reinforcements and wire reinforcement had almost equal or slightly lower moduli than fibre reinforcement. Nonetheless, their effects were size-dependent, with increasing reinforcement sizes being associated with increasing beneficial effects. However, there were no comparative studies directly comparing metal reinforcement to fibre reinforcement or if one type of fibre (woven or continuous) was clearly superior to another. Therefore, it might be difficult to know which approach is best. Positions in prostheses In the prosthesis-shaped specimen studies, the position of reinforcement was examined. The previous studies were about complete dentures (both maxillary

8 140 T. TAKAHASHI et al. and mandibular), partial dentures, and tooth or implant overdentures. For implant overdentures, one in vitro study (34) and one finite element study (5) examined the rigidity of prostheses and two case reports examined the strength of prostheses (8, 9). Considering these studies, the reinforcing effect seemed to differ depending on the position regardless of the materials. From these studies, to improve the strength and rigidity of prostheses, the reinforcements in addition to a metal framework in the denture base should run over the top of the residual ridge in complete dentures (36, 37) and run over the top of the coping or abutment in tooth and implant overdentures (5, 34, 42). Materials for prostheses The materials used in the studies were all acrylic resin and differed in their polymerisation methods, in particular heat-, auto- or light-polymerisation. The materials were usually used in clinical practice and selected in consideration of their advantages and disadvantages. In all studies, the reinforcements were reported to be effective regardless of both reinforcement and prosthesis materials. Only four articles (15, 17, 27, 32) compared the prosthesis materials, and other articles were conducted with only one material. From the results of the four studies (15, 17, 27, 32), there was no difference in reinforcing effects among the prosthesis materials regardless of the reinforcement material. Discussion In this literature review, the effects of reinforcement within different removable prostheses were summarised from previous reports published in the decade from 2005 to There have been many reports about such reinforcement, but they lack coherence and include only two reviews (3, 47), one of which was published more than 15 years ago. In the previous reviews (3, 47), the following information about reinforcement was already apparent: (i) cross-sectional forms of reinforcement should not be rectangular, but instead should be convex or semicircular; (ii) reinforcement should be placed in a direction perpendicular to the stress-concentrating line; and (iii) reinforcements should be positioned near the surface, especially the tensile surface. However, new organised information has not published for about 15 years since the last review was documented, while the materials or procedures have been further developed. Therefore, reports published after 2005 were focused upon in this review to clarify the current trends. There are two reasons why the reinforcement should be embedded into the prosthesis: one is to improve the strength and prevent fracture of the prosthesis, and the other is to make the prosthesis more rigid and prevent deformation of the prosthesis. The former was examined by measuring different strengths and using a finite element analysis, and the latter was examined by measuring the flexural modulus and using a strain gauge analysis, but the number of the former studies was much larger than the number of the latter studies. This seemed to arise because the effects on improvement in strength to prevent fracture were more frequently focused upon than improving rigidity to prevent deformation. From the results of the specimen and prosthesisshaped studies, various materials and designs of reinforcement were examined and the results showed that all were effective. The prerequisites for reinforcement to improve the mechanical properties should be considered the following points: (i) ease of making and placing into the prosthesis; (ii) sufficient rigidity to improve the mechanical properties; (iii) reasonable cost; (iv) stable characteristics; and (v) sensuousness. In the decade from 2005 to 2015, glass fibres were especially examined as reinforcement materials compared with other materials. Glass fibres met the first, fourth and fifth of the above conditions and were superior to metal in sensuousness. However, they were inferior in rigidity and cost. Furthermore, their effects differed depending on their position, shape and material, and the selection of these factors is important to produce the maximum reinforcing effect. Thinking about the reinforcement size, as the reinforcements increased in size, their beneficial effects increased. When selecting the material and form of a reinforcement, these points should be considered and ranked for their importance for each prosthesis and patient. Materials for reinforcement In terms of material, cast metal reinforcements, especially Co-Cr alloys, have the maximum effect in improving both strength and rigidity and can be made

9 REINFORCEMENT IN REMOVABLE PROSTHODONTICS 141 into complex shapes, but require complicated laboratory work and equipment. When selecting the material for reinforcement, the mechanical character of the material itself is important, and it should be sufficiently strong and rigid to make the prosthesis more rigid. In this regard, cast metal reinforcements seemed to be superior to other materials. Meanwhile, the major drawbacks of cast metal reinforcements were their colour and adhesion to the prosthesis. A cast metal reinforcement can be seen through the prosthesis in some situations and is therefore not recommended for use in anterior areas, where clear fibre reinforcement is more appropriate for an aesthetic choice, although fibre reinforcement is more expensive. When a reinforcement was placed into a prosthesis regardless of the material, appropriate adhesive bonding procedures before placement, for example sandblasting and priming for metals and silane-coupling procedures for glass fibres, were necessary to produce the maximum effect of reinforcement (16, 23). If these procedures are not conducted sufficiently, the reinforcement will have the opposite effect and nothing will be gained by embedding. Positions and forms of reinforcement In terms of position and form, the reinforcement should be positioned in an area of stress and potential deformation where the prosthesis is subject to fracturing. (40). These areas are specifically over the residual ridge (1, 48), implants (49, 50) or roots (51), on the inflection point of the residual ridge. In conclusion, the reinforcement should run over the residual ridge in a complete denture (40), run over the top of the implants or roots in an overdenture (34, 42) and run across the residual ridge in a partial denture (45). Regarding the cross-sectional form, a reinforcement should be positioned near the tensile surface, as already clarified in previous reports (3, 47). This result seemed to be derived from the character of the denture base material, rather than that of the reinforcement material, given that the compressive strength was stronger than the tensile strength. To summarise this review, reinforcements within prostheses undoubtedly had some efficacy in improving the mechanical properties, but the levels varied by material, form and position of the reinforcement. Considering these results, clinicians had better placed some kind of reinforcement in all removable dental prostheses to prevent prosthetic and other complications. However, most previous studies were carried out in terms of improving the strength, rather than the rigidity, and studies aimed at improving rigidity remained insufficient even in experimental model and specimen studies. Furthermore, clinical studies about the effects of reinforcement on both prostheses and patients were very few, and in particular, there were no randomised long-term clinical studies comparing prostheses with and without reinforcement. Therefore, studies focused on the rigidity of prostheses with reinforcement and the effect on underlying structures such as the residual ridge or implant and longitudinal clinical studies are necessary to ensure the effect of reinforcement within dental prostheses. Conclusions From the results of this review on reports about reinforcement published in the decade from 2005 to 2015, the following conclusions were drawn. 1 There have been many in vitro studies, but most of them were focused on improving the strength, rather than the rigidity, of prostheses. 2 Cast metal reinforcement was most effective in improving the mechanical properties of prostheses, for both strength and rigidity. 3 Glass fibre reinforcements were examined more frequently than other materials in the decade from 2005 to 2015 and were increasingly applied in clinical practice. 4 The most favourable form of reinforcement differed depending on the prosthesis, but the reinforcement should be placed around the deformable and stressconcentrating area. 5 Longitudinal clinical reports about the effect of reinforcement on prostheses or patients were still lacking, and such future studies are required to ensure the effect of reinforcement. Ethical approval Ethical approval is not applicable for this study. Funding This research was conducted with no funding.

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