Implant Screw Mechanics and the Settling Effect: An Overview

IMPLANT SCREW MECHANICS AND THESETTLING EFFECT: AN OVERVIEWSPECIAL REPORTSheldon Winkler, DDSKarla Ring, DMDJamie D. Ring, DMDKenneth G. Boberick, DMDKEY WORDSImplantScrew mechanicsPreloadTorqueSettling effectSheldon Winkler, DDS, is a professor ofRestorative Dentistry at Temple UniversitySchool of Dentistry, 3223 North Broad Street,Philadelphia, PA 19140-5096. He formerlyserved as professor and chairman of theDepartment of Prosthodontics at TempleUniversity. Karla Ring, DMD, is a recentgraduate of Temple University School ofDentistry, Philadelphia, PA 19140; Jamie D.Ring, DMD, is a recent graduate of TempleUniversity School of Dentistry, Philadelphia,PA 19140; and Kenneth G. Boberick, DMD,is an associate professor of RestorativeDentistry at Temple University School ofDentistry, Philadelphia, PA 19140. Addresscorrespondence to Dr Winkler [email protected] Vol. XXIX/No. Five/2003One of the most serious and prevalent problems associated with the restorativeaspect of dental implants is loosening and fracturing of screws. Implant screwsshould be retightened 10 minutes after the initial torque application as a routineclinical procedure to help compensate for the settling effect. Mechanical torquegauges should be used instead of hand drivers to ensure consistent tighteningof implant components to torque values recommended by implant manufacturers.d y e Oentistry. Seashell fragments were joint as 2 parts tighted e n t n are pteethlantswith imoplants.ewImtoMcGlumpS missing f o t emen epl r fessiacon has to offer is the with the screhCREWywtjoint.aMl1ECHANICnefindedtogethStheev in cs c e m t d e d un p d e th ces rvi seINTRODUCTIONne of the mostentaldramaroticof looserstanscrews,hethe clinihanician mustrscreolbfirstvedwaused to replace 3 missing mandibularincisors as early as 600 AD, and humantransplants were used in many diverseearly cultures. In recent years, oral implantologyhas undergone a well-deservedrebirth or rediscovery, and implantsare now considered the treatmentof choice in an increasing numberof carefully selected cases. Today, implantprosthodontics has reached thepoint at which it can be successfullyperformed by both general dentistsand specialists.One of the most serious and prevalentproblems associated with the restorativeaspect of dental implants isloosening and fracturing of the screwsthat attach the prosthesis to the implant.Screw loosening may be an earlywarning of inadequate biomechanicaldesign and/or occlusal overloading. Inorder to attempt to solve the problemscrew, such as an abutment and implantbeing held together by a screw.A screw is tightened by applyingtorque. The applied torque develops aforce within the screw called the preload.As a screw is tightened, it elongates,producing tension. Elastic recoveryof the screw pulls the 2 parts together,creating a clamping force.2 Thepreload in the screw, from elongationand elastic recovery, is equal in magnitudeto the clamping force.1Opposing the clamping force is ajoint-separating force, which attemptsto separate the screw joint. Screw looseningoccurs when the joint-separatingforces acting on the screw joint aregreater than the clamping forces holdingthe screw unit together.1 Excessiveforces cause slippage between threadsof the screw and threads of the bore,resulting in a loss of preload.3It is not necessary to eliminate separatingforces, only to minimize them.1Minimizing separating forces andmaximizing clamping forces will act toprevent screw loosening.When the clinician applies a torqueto a screw to tighten its componentstogether, the tightening torque createsa preload in the screw. The preload isdetermined by the applied torque andother factors, such as the screw alloy,screw head design, and abutment surface.The established preload is proportionalto the applied torque. Thetorque value can be controlled by theclinician and can be reproduced fromimplant prosthesis to implant prosthesis.Too little torque may allow separationof the joint and result in screwfatigue, loosening, and failure. Toolarge a torque may strip the screwthreads.1Increasing the torque will increasethe preload. Increasing the preloadmaximizes the stability of the screwjoint by increasing the clampingthreshold that separating forces mustovercome to cause screw loosening.The amount of torque than can be appliedis limited by the ultimatestrength of the screw. McGlumphy etal1 have stated that the optimal torquevalue is 75% of the torque needed tocause screw failure.Another variable in the amount oftorque that can be applied is how thetorque is produced by the clinician.Torque can be applied manually orwith a mechanical device. Until the introductionof mechanical torquing devicesto the profession, implant componentswere tightened manually. Thenovice or inexperienced clinician oftenundertightened the screws in an implantsystem. Dellinges and Tebrock4found that the average torque appliedwith a screwdriver is only 10 N-cm.In a pilot study, Jaarda et al5 foundthat test subjects with little implant experiencewere not generally able toprovide the recommended torque toimplant prosthesis-retaining slottedgold screws. The investigators also reportedthat experienced test subjectsFIGURE 1. Tohnichi torque gauge used for research in Jaarda et al,5,7 Bakaeen et al,9 Siamoset al,10 and Winkler et al (unpublished data, 2003), (Tohnichi America Corporation, Northbrook,Ill).tended to generate more than the recommendedtorque, and none of the testsubjects were able to generate consistenttorque values (Figure 1).JOINT-SEPARATING FORCESClinically, the screw unit within an implantprosthesis is constantly subject toexternal joint-separating forces. Suchintraoral separating forces may includeoff-axis occlusal contacts, lateral excursivecontacts, interproximal contactsbetween natural teeth and implant restorations,protrusive contacts, parafunctionalforces, and nonpassiveframeworks that attach to the implants.Once external forces exceed the screwjoint preload, the joint becomes unstable.The external load rapidly erodesthe preload, resulting in vibration andmicromovement that lead to screwloosening. Once loosening occurs, thescrew joint ceases to perform the functionfor which it was intended and canbe considered as failed.3The clinician must recognize thepossible forces that will be acting onthe screw joint, so that screw looseningand other possible complications canbe minimized or avoided.Clinicians are urged to use sometype of mechanical torque-applying instrumentto ensure consistent tighteningof implant components to the spec-Sheldon Winkler et alified torque values recommended byimplant manufacturers.SETTLING EFFECTA significant mechanism that results inscrew loosening of implant-supportedrestorations is the settling effect. Thesettling effect (embedment relaxation),which plays a critical role in screw stability,is the result of no surface beingcompletely smooth. No matter howcarefully machined an implant surfaceis, it is slightly rough when viewed microscopically.Because of this microroughness,no two surfaces are completelyin contact with one another.Settling occurs as the rough spotsflatten under load, since they are theonly contacting surfaces when the initialtightening torque is applied. Whenthe screw interface is subjected to externalloads, micromovement occursbetween the surfaces. Wear of the contactareas brings the 2 surfaces closertogether. It has been reported that 2%to 10% of the initial preload is lost asa result of settling.6 As a result, thetorque necessary to remove a screw isless than the torque initially used toplace the screw.7The extent of settling depends onthe initial surface roughness, surfacehardness, and magnitude of the loadingforces. Rough surfaces and largeJournal of Oral Implantology 243OVERVIEW OF IMPLANT SCREWSexternal loads increase the settling.When the total settling effect is greaterthan the elastic elongation of the screw,the screw works loose because thereare no longer any contact forces to holdit in place.8Thread friction is higher for thefirst tightening and loosening of ascrew, and then decreases after repeatedtightening and loosening cycles.8 Anumber of authors have suggestedtightening of implant screw joints afterthe initial screw insertion and periodicallythereafter.2,9,10Bakaeen et al9 have reported thatwhen prosthetic gold screws weretightened to 10 N-cm according to themanufacturer's instructions, the untighteningtorque of the differentgroups tested was about 2 to 3 N lessthan the tightening torque. These observationscorrespond to the findingsof Sakaguchi and Borgersen,6 who reporteda 2% to 10% reduction in preloadwithin the first few seconds orminutes after tightening as a result ofthe settling effect.Siamos et al,10 as a result of an invitro investigation, also suggested thatretightening abutment screws 10 minutesafter initial torque applicationsshould be routinely performed. The investigatorsalso reported that increasingthe torque values for abutmentscrews above 30 N-cm can be beneficialfor abutment-implant stability and todecrease screw loosening.To reduce the settling effect, implantscrews should be retightened 10minutes after the initial torque application.2,9,10 This technique should beused as a routine clinical procedure.In vitro studiesIn vitro studies examining the dynamicsof screw loosening using servohydraulictesting machines have limitationsdue to the difficulty of reproducingthe complex nature of the chewingcycle. Clinical variables, such as intermittenthigh impact loads, varying anglesof load application, and varyingpositions of load application in relationto the implant axis, may have signifi-244 Vol. XXIX/No. Five/2003cant damaging effects on the implantabutmentinterface leading to screwloosening and failure.11(p68)Ideally, load application and durationapplied during testing should simulatethese normal functional parameters.Cyclic fatigue studies frequentlyreport maximum load applications of100 to 200 N, which are at the lowrange of reported data for maximumbite forces (200-3500 N).11(p68) It is assumedthat an individual has 3 episodesof chewing per day, each 15 minutesin duration, at a chewing rate of60 cycles per minute. This produces anequivalent of 2700 chewing cycles perday or 106 cycles per year.12,13 Accuratelysimulating these normal functionalparameters is both time consumingand technically challenging.DISCUSSIONGenerally, simple tightening or replacementof loose retaining or abutmentscrews is all that is necessary,which is an inconvenience for both theclinician and the patient. Often extensiverepair is involved, especially ifabutment screws cannot be retrieved.This may necessitate abandonment ofthe involved implant and/or requiremodification or remake of the affectedprosthesis.Bakaeen et al9 found that screwloosening can be reduced by narrowingthe occlusal table of molar singletoothimplants when using 1 implantfor support.Ongoing research at Temple UniversitySchool of Dentistry (S. Winkleret al, unpublished data, 2003) suggeststhat the percentage of difference betweenthe applied torque and the countertorque increases significantly atlower initial torque values. The researchalso indicates that high initialtorque values recommended by somemanufacturers for their implant screwsare beyond the limitations of thescrews provided and may result instripping, breakage, and other problems.CONCLUSIONSTo reduce the settling effect, implantscrews should be retightened 10 minutesafter the initial torque applicationas a routine clinical procedure. Mechanicaltorque instruments should beused instead of hand drivers to ensureconsistent tightening of implant componentsto the specified torque valuesrecommended by implant manufacturers.ACKNOWLEDGMENTSThis research was partially supportedby Summer Research Fellowships andsponsored by a grant from Myersonthe Tooth Company, Chicago, Illinois.REFERENCES1. McGlumphy EA, Mendel DA,Holloway JA. Implant screw mechanics.Dent Clin N Am. 1998;42:71-89.2. Haack JE, Sakaguchi RL, CoeffyJP. Elongation and preload stress indental implant abutment screws. Int JOral Maxillofacial Implants. 1995;10:529-536.3. Lang LA, May KB, Wang RF.The effect of the use of a countertorquedevice on the abutment-implantcomplex. J Prosthet Dent. 1999;81:411-417.4. Dellinges MA, Tebrock OC. Ameasurement of torque values obtainedwith hand-held drivers in a simulatedclinical setting. Int J Prosthodont.1993;2:212-214.5. Jaarda MJ, Razzoog ME, GrattonDG. Providing optimum torque toimplant prostheses: a pilot study. ImplantDent. 1993;2:50-52.6. Sakaguchi RL, Borgersen SE.Nonlinear contact analysis of preloadin dental implant screws. Int J OralMaxillofac Implants. 1995;10:295-302.7. Jaarda M, Razzoog M, GrattonD. Effect of preload torque on the ultimatetensile strength of implant prostheticretaining screws. Implant Dent.1994;3:17-21.8. Jorneus L, Jemt T, Carlsson L.Loads and designs of screw joints forsingle crowns supported by osseoin-tegrated implants. Int J Oral MaxillofacImplants. 1992;7:353-359.9. Bakaeen LG, Winkler S, NeffPA. The effect of implant diameter, restorationdesign, and occlusal table variationson screw loosening of posteriorsingle-tooth implant restorations. JOral Implantol. 2001;27:63-72.10. Siamos G, Winkler S, Boberick 12. Binon PP, McHugh MJ. The ef-KG. The relationship between implant fect of eliminating implant/abutmentpreload and screw loosening on im- rotational misfit on screw joint stabiliplant-supported restorations. J Oral ty. Int J Prosthodont. 1996;9:511-519.Implantol. 2002;28:67-73. 13. Wiskott HW, Nicholas JL, BelserUC. Stress fatigue: basic principlesand prosthodontic implications. Int J11. Craig RG. Restorative DentalMaterials. 11th ed. St Louis, Mo: CVMosby; 2002.Sheldon Winkler et alProsthodont. 1995;8:105-116.Journal of Oral Implantology 245