In vitro wear of various orthotic device materials

Jeffery Casey, DMD,a William J. Dunn, DDS,b and Edward Wright, DMDc

United States Air Force Academy, Colorado Springs, Colo, and Wilford Hall Medical Center,Lackland Air Force Base, Tex

Statement of problem. Orthotic devices are advocated to decrease occlusal attrition caused by bruxism buttend to wear with time.Purpose. This study investigated the wear rate of various materials used to fabricate orthotic devices.Material and methods. Five experimental groups (n�8) were studied: Splint Biocryl autopolymerized (SBA),Splint Biocryl autopolymerized plus additional heat and pressure (SBHP), Forestacryl autopolymerized (FA),Forestacryl autopolymerized plus additional heat and pressure (FHP), and Quick Splint 15-minute (QS), light-polymerized composite. Specimens were mounted to the base of a universal testing machine. A wear device usingsteatite balls and a load of 9.1 kg was positioned against the specimens, submerged in a 37°C water bath andsubjected to 2500 reciprocal cycles. Wear, in micrometers, was calculated as the maximum peak to valleymeasurement (Ry) using profilometry. Data were subjected to analysis of variance (ANOVA) and Tukey’s HSD(��.05).Results. Mean acrylic wear in micrometers was as follows: FA 6.8 �3.0; FHP 7.1 � 1.8; SBA 20.4 � 5.6; SBHP23.7 � 7.8; and QS 23.8 � 6.9. One-way ANOVA detected significant differences between groups (P�.001);the Tukey honestly significant difference test determined that FA and FHP specimens were significantly moreresistant to wear than all other specimens (P�.007).Conclusion. Differences in in vitro wear resistance among various orthotic device materials exist. The in vitrowear resistance among other autopolymerizing materials appears to be related to proprietary differences.(J Prosthet Dent 2003;90:498-502.)

CLINICAL IMPLICATIONS

In this in vitro study, a significant difference in mean wear was demonstrated between differentgroups of materials used to fabricate interocclusal orthotic splints. Differences in the wear oforthotic device materials appear to be proprietary.

The use of orthotic devices to protect the dentitionfrom parafunction is common. Parafunctional habitscontribute to dental attrition, the wear of tooth struc-ture from the teeth sliding against each other. Interoc-clusal orthotic devices have been advocated as a means ofdecreasing or preventing occlusal attrition caused by di-urnal or nocturnal bruxism.1-3 The orthotic deviceshould fit comfortably, yet retentively, over the occlusalsurface of either the maxillary or mandibular teeth, pre-venting the teeth of 1 arch from contacting the opposingarch. A clinical concern is that the orthotic device tendsto wear with time, yet it is unknown whether the toothsurface continues to be abraded by the orthotic device.4

A variety of materials are currently used for interoc-clusal orthotic devices. Most manufacturers report that

these materials are based on methacrylate chemistry.The ideal material for an orthotic device should do thefollowing: (1) allow the orthotic device to be comfort-able for the patient to use, (2) be easily and preciselyadjusted so that contacts with the opposing teeth may beobtained, (3) be relatively inexpensive and easy to fabri-cate, and (4) be resistant to wear, yet not cause damageto the teeth.

Wear is a complex, poorly understood process. Wearis defined as a loss of material resulting from removal andrelocation of materials through the contact of 2 or morematerials.5 Many mechanisms of wear have been de-scribed, but in general, 4 predominate the dental litera-ture:6 (1) sliding or adhesive wear, (2) abrasive wear,which includes 2-body and 3-body wear, (3) fatiguewear, and (4) corrosive wear. Sliding wear requires con-tact between occluding surfaces. In this mechanism,submicroscopic fragments are torn from contact pointson the restorative material during sliding occlusal move-ments. During sliding, the restorative material is com-pressed and forms a ridge in front of the sliding cusp. As

The views expressed in this article are those of the authors and do notreflect the official policy of the Department of Defense or otherDepartments of the United States Government.

aDeputy Director, AEGD-1, United States Air Force Academy.bDirector of Research and Biomaterials, Wilford Hall Medical Center.cDirector, Orofacial Pain Clinic, Wilford Hall Medical Center.

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the cusp slides, tensile stresses are induced behind thesliding cusp that can induce microcracks in the restor-ative material surface.4 Abrasive wear occurs when par-ticles are forced into the surface undergoing wear. Two-body wear occurs when the abrasive particles areattached to 1 of the surfaces, whereas 3-body wear oc-curs when a freely moving abrasive medium is placedbetween 2 independently moving surfaces.6-10 Abrasivewear is sometimes referred to as contact-free wear, ageneralized process based on the protection theory pro-posed by Jorgensen et al.11

Although abrasion appears to be the primary mecha-nism of wear in contact-free areas, fatigue wear is usuallyassociated with occlusal contact wear.12 Cyclical loadingcauses fatigue wear.4 High local stresses are induced atthe site of cusp tip loading, forming microscopic defectsin the stressed material. Leinfelder et al13 proposed themicrofracture mechanism of wear, suggesting that thefiller particles in a material have a higher modulus ofelasticity than the resin matrix. During function, theparticles are compressed and cause microfractures in theresin matrix.

Corrosive, or chemical wear, occurs when exogenouschemicals degrade the resin matrix, the filler, or thefiller-particle interface. It is well known that alcohol canplasticize resins, water can cause filler leaching, and thatcertain enzymes produced by microorganisms can causeresin degradation.7 Because chemical wear is dependenton many variables such as saliva composition, food anddrink intake, and their interactions, its effect on overallwear is difficult to establish.

Because clinical wear involves several mechanisms in-teracting at once, no in vitro device has been able toaccurately predict clinical wear. Studies have not beenable to establish a correlation between in vivo and invitro wear.14,15 Another important factor with many2-bodied wear tests is the selection of the material forthe abrader. If enamel is used, the inherent variability ofthe structure and physical characteristics of each individ-ual tooth is a confounding factor.16 The shape of theabrader is another important consideration. An abraderwith an edge can plow through the surface of the spec-imen, accelerating wear and yielding erroneous results.A spherical abrader may avoid these problems.16

In an attempt to standardize the laboratory proce-dure, this study used steatite ball abraders to cyclicallyload orthotic device specimens. This technique has beenpreviously shown to be a reliable means of assessing2-body wear.16 The purpose of this study was to com-pare the wear of different materials used to fabricateinterocclusal orthotic devices under simulated cyclicloading and to test the hypothesis that there would beno difference in wear between any of the experimentalgroups.

MATERIAL AND METHODS

Five experimental groups (n � 8) consisting of 3commercially available methacrylate-based resins werestudied. The orthotic device material composition andmanufacturer information is provided in Table I. TheSplint Biocryl and Forestadent specimens (SBA and FA)were prepared with a “salt and pepper” technique andallowed to autopolymerize for 24 hours following man-ufacturer’s recommendations. The other Splint Biocryland Forestadent groups were also prepared with a “saltand pepper” technique but were additionally polymer-ized under pressure at 15 psi and 118°F for 20 minutes,immersed in water. Quick-Splint resin-based compositespecimens were photopolymerized in a light-polymeriz-ing unit (Triad 2000; Dentsply Trubyte, York, Pa) for15 minutes.

All specimens were fabricated to a thickness of 3 mmand embedded in type IV dental stone (Silky-Rock;Whip Mix Corp, Louisville, Ky) to allow mounting onthe static base of the universal testing machine (model1521S; Instron Corp, Canton, Mass). Each specimenwas placed in a holding device fabricated following thedesign of Wassel et al13 (Fig. 1). The specimens andholding device were immersed in a distilled water bathmaintained at 37°C.

Steatite balls (9.5 mm MgO-SiO2 spheres; InsacoInc, Quakerstown, Pa) were used to simulate toothenamel and wear the prepared methacrylate-based resinspecimens. Steatite is a ceramic material with a Mohshardness value of 7.5.17 Tooth enamel is reported tohave a Mohs hardness of 5.0.18 The steatite abrader wasmounted to the movable platform of the universal test-ing machine. The abrader was placed against the speci-

Table I. Orthotic device materials tested

Product Manufacturer Lot no. Composition

Forestacryl Forestadent USASt. Louis, Mo

0791106243

Methyl methacrylatepowder monomer-based methylmethacylate

Splint Biocryl Great LakesOrthodonticsTonawanda, NY

2470121699

Methyl methacylatepowder monomer-based methylmethacylate

Quick Splint Moore TechnologiesDearborne, Mich

112890 Urethanedimethacrylateresin matrix,triethylene glycoldimethacrylatesilicon dioxidemicrofiller (0.04�m) radiopaqueglass fillers (Al2O3)

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men with a load of 9.1 kg. The abrader traveled in a1-cm linear path using a reciprocal motion at a speed of1 cm/sec. Each specimen was abraded for 2500 cycles.Previous studies have subjected specimens to wear rang-ing from 1000 cycles19 to as many as 400,000 cycles.20

Steatite balls were replaced after each specimen wasabraded.

After each specimen was removed from the cyclicwear device, it was analyzed by a profilometer (Surfana-lyzer System 4000; Mahr Federal, Providence, RI) todetermine the maximum peak-to-valley measurement(Ry) in micrometers for the groove worn in the resinspecimen block. Traverse speed was set at 0.25 mm/secwith a cut off control of 0.3 mm. Length of traverse was1 cm. Measurements were repeated 3 times for eachspecimen. The average of the 3 measurements was re-corded as the Ry for that specimen. Data were analyzedusing a 1-way analysis of variance (ANOVA) to test forany difference between the 5 experimental groups. TheTukey honestly significant difference (HSD) post-hoctest revealed where significant differences occurred(��.05)

RESULTS

Mean wear of the methacrylate-based resins is pre-sented in Table II. One-way ANOVA detected signifi-cant differences between groups (P�.001). The TukeyHSD determined that both Forestadent groups (au-topolymerized only and with additional heat and pres-sure) displayed significantly less 2-bodied wear than the

rest of the experimental groups (P�.007). Both SplintBiocryl groups (autopolymerized only and with addi-tional heat and pressure) and Quick Splint resin-basedcomposite material displayed significantly more wearthan FA and FHP.

DISCUSSION

Any laboratory experiment designed to study wearwill be subject to scrutiny as the actual clinical wearprocess is multifactorial and complex. This project at-tempted to standardize certain variables. Steatite ce-ramic balls were used in place of enamel, which inher-ently differs from 1 specimen to the next. Sphericalsteatite abraders were used to avoid any digging or plow-ing of the abrader into the resin. A reciprocating sliding-wear test without impact was selected to simulate theclinical situation. It has been demonstrated that impactdoes not significantly contribute to the wear processintraorally.7 The mandible undergoes a significant de-celeration immediately prior to impact.21 Gibbs et al22

demonstrated that the average force on posterior teethat closure was 8.3 kg. That study also reported a maxi-mum occlusal force of 74 kg. Clark et al23 stated that theforces placed on the molars during mastication and dur-ing parafunctional movements are greater than theforces on anterior teeth. A load of 9.1 kg was selected forthis study because it would generate a force within thenormal range of masticatory forces and would provide ameasurable trough after a reasonable number of cy-cles. It should be noted that wear devices, applicationand magnitude of forces, and method of wear deter-mination differ in most studies, making any compari-son difficult.8-10,19,20 Tribology, the study of wear, is acomplex phenomenon and researchers will continue toexperiment to find a method and device that will mostclosely resemble the clinical situation. Testing was per-formed while the specimens and wear apparatus wereimmersed in water held at 37°C. Therefore, some of theabraded particles contributed to 3-body wear during theabrasive movement.

Several components of the wear process occurred si-multaneously in this project. Initially, sliding, or adhe-

Fig. 1. Wear device and steatite ceramic abrader mounted touniversal testing machine. A, Mounted steatite abrader;B, mounted specimen; C, adjustable plate; D, heater/thermo-stat; E, chuck arm; F, cross rod; G, 9.1 kg weight; H, swivelbearing; I, cross head; J, 37°C water bath.

Table II. Mean wear of specimens

Material

Ry (SD) [Maximum peakto valley depth in

microns]

FA (Forestacryl auto-polymerized) 6.8 (3.0)FHP (Forestacryl heat polymerized/pressure) 7.1 (1.8)SBA (Splint Biocryl auto-polymerized) 20.4 (5.6)SBHP (Splint Biocryl heat/pressure) 23.7 (7.8)QS (Quick Splint 15-min light-polymerized) 23.8 (6.9)

One-way ANOVA detected significant differences between groups (P � .001).Tukey HSD post-hoc test identified where differences occurred (P�.007). Linesconnect groups that are similar.

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sive wear, was present. In this type of wear, 2 surfacescontact each other only at the highest points. Duringsliding movements, microscopic fragments are shearedfrom the highest points of the softer material. The 2sliding surfaces will increase in contact with each otherover time because the highest points will wear down.Consequently, the initial wear rate may be much higherthan the final wear rate.6,7 Two-bodied abrasive wearwas also a factor in this experiment. Two-bodied wearoccurs when the particles causing wear are firmly at-tached to 1 of the sliding surfaces and a marked dissim-ilarity exists in the hardness of the 2 opposing surfaces.6

Such a hardness disparity existed in the current study:the Knoop hardness is reported as 343 kg/mm2 and 21kg/2mm2-17 for enamel and polymerized methacrylate-based resin, respectively.

Three-bodied wear occurs when a layer of freelymoving abrasive particles contributes to the abrasionof the softer material. This may be the most importantfactor in the overall clinical wear of resin-based pos-terior composite restorations, as mastication involvesthe generation of fine food particles that slide over thesurface of the composite.6 However, 3-bodied wearmay not be as important in the study of orthotic de-vice wear because most patients do not eat with theirorthosis in place. The importance of the resin matrixin 3-bodied wear is the most important consideration.Three-bodied wear will affect the soft polymeric ma-trix, whereas 2-bodied wear will involve both the fillerand the matrix.6,9 Indeed, some manufacturers ofnewer composites are replacing bis-GMA with ure-thane dimethacrylate in an attempt to strengthen theresin portion of their material.

Fatigue wear was another consideration in the me-chanics of this study. The methacrylate resin specimensin the present study were cyclically loaded. Cyclic load-ing causes fatigue wear.6 Fatigue failures occur in areasexposed to localized plastic deformation when the con-tact area is repeatedly stressed to its elastic limit. Fatiguewear occurs in areas of primary contact. This may be animportant factor in the wear of orthotic devices, but itsimportance as a mechanism in the clinical wear of pos-terior composites may be questionable.

Clinically, corrosive wear is unquestionably an impor-tant factor in the wear of orthotic devices. Alcohol plas-ticizes resins, water causes filler leaching, and certainmicroorganisms produce esterase enzymes that can de-grade resin.6 A limitation of this study was that thisavenue of wear was not pursued. Future studies shouldinvestigate the wear process of orthotic materials whenexposed to exogenous chemicals that are common to thediet.

Generally, the degree of polymerization in autopoly-merized resins is not as complete as that achieved usingheat-polymerized resins.5 The greater amount of unre-acted monomer can act as a plasticizer, decreasing the

strength of the resin. Methacrylate materials that areadditionally polymerized with heat are characterized byincreased crosslink density and tend to demonstrate lesswear.17 Polymerization with increased atmosphericpressure and heat should increase the degree of conver-sion and minimize porosity which could lead to in-creased wear.7 The results of this study did not identifyan improvement in wear resistance of the heat and pres-sure-polymerized specimens compared with specimensthat were not treated to heat and pressure. Instead, post-hoc statistical analyses identified differences in weargrouped according to manufacturers, suggesting thatproprietary elements played an important role in wearresistance.

Quick Splint material is a composite with a matrix ofurethane dimethacrylate, microfine silica, and high-mo-lecular weight monomers. Acrylic resin beads are in-cluded as inorganic filler. The Quick Splint material dif-fers from the other materials in the study in that it isprepackaged in an arch form and exhibits a workableconsistency without mixing.

The in vitro study of composites has been exten-sively investigated and numerous wear-testing deviceshave been developed to simulate the clinical condi-tion.8-10,19,20 However, good correlation across thesestudies is lacking, as some wear devices deliver only2-bodied wear,8,10,19 and other studies use 3-bodiedwear.9,20 Further, the application of the load and thesize and shape of the stylus tip differ in all of the stud-ies.8-10,19,20 Hu, Marquis, and Shortall8 used a 2-bodiedwear system based on a stainless steel countersampleusing an alternating load to simulate in vivo conditions.The steel countersample may have confounded resultsbecause the physical properties of steel differ from hu-man enamel. Enamel cusps have been used to serve asthe antagonist, but the variability of enamel makes itunsuitable for standardized wear testing.9 Making directcomparisons of results from different in vitro investiga-tions and extrapolating results to the clinical condition isproblematic. Further study is needed to understand thecomplex science of masticatory wear on orthotic devicesand dental restorations.

CONCLUSION

Under the conditions of this in vitro study,Forestacryl autopolymerized and Forestacryl autopoly-merized with additional heat/pressure treatment speci-mens were significantly more resistant to wear thanSplint Biocryl autopolymerized, Splint Biocryl with ad-ditional heat/pressure treatment, and Quick Splintlight-polymerized specimens (P�.007). There was noevidence to suggest an improvement in wear resistancewhen specimens were additionally heat and pressuretreated. Quick Splint light-polymerized composite spec-

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imens exhibited wear resistance similar to Splint Biocrylpolymethyl methacrylate specimens.

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3. Sheikholeslam A, Holmgren K, Riise C. Therapeutic effects of the planeocclusal splint on signs and symptoms of craniomandibular disorders inpatients with nocturnal bruxism. J Oral Rehabil 1993;20:473-82.

4. al-Hiyasat AS, Saunders WP, Sharkey SW, Smith GM, Gilmour WH.Investigation of human enamel wear against four dental ceramics andgold. J Dent 1998;26:487-495.

5. Kohn DH. Mechanical properties. In: Craig RG, Powers JM, editors. Re-storative dental materials. 11th ed. St. Louis: Mosby; 2001. p. 109.

6. Soderholm KJ, Richards ND. Wear resistance of composites: a solvedproblem? Gen Dent 1998;46:256-63.

7. Rawls HR, Esquivel-Upshaw JF. Restorative resins. In: Anusavice KJ, edi-tor. Phillips’ science of dental materials. 11th ed. St Louis: Elsevier Sci-ence; 2003. p. 399-442.

8. Hu X, Marquis PM, Shortall AC. Two-body in vitro wear study of somecurrent dental composites and amalgams. J Prosthet Dent 1999;82:214-20.

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17. Callister WD Jr. Materials science and engineering. 6th ed. New York:John Wiley & Sons; 2002. p. 162-8.

18. O’Brien WJ. Dental materials and their selection. 3rd ed. Carol Stream (IL):Quintessence Publishing; 2002. p. 343.

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Noteworthy Abstractsof theCurrent Literature

Maxillary bone growth and implant positioning in a youngpatient: A case reportEnzo Rossi, Jens O. Andreasen. Int J Periodontics RestorativeDent 2003;23:113-9.

The literature supports the efficacy of osseointegrated implants for partially edentulous patients, butcare must be exercised in adolescents with incomplete bone formation. Implants do not follow thenormal growth of the jaws, and they behave like ankylosed teeth. They may also interfere with thenormal growth of the alveolar process and jeopardize the germs of the adjacent permanent teeth oralter eruption. This case report analyzes the unfavorable clinical and radiographic findings of asingle-tooth replacement in a young male over a 15-year period.—Reprinted with permission ofQuintessence Publishing

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