JAD)A RESEARCH
R E P O R T S
Synthetic materials fo r surgical implant demces have evolved from the early metallic systems to a variety of material combinations and composites. Current biomaterial and biomechanical proper ties provide relatively optimal stable bone and soft tissue interfaces and simplified restorative treat ments. Further improvements in existing systems require a continuation of the multidisciplinary approach to laboratory, experimental animal, and human clinical research.
Dental implant biomaterials Jack E. Lem ons, PhD
aturally o c c u rrin g m in erals a n d m e ta ls su c h as g e m s to n e s a n d gold w ere first used fo r surgical im p la n t a n d to o th r o o t r e p la c e m e n t devices.1 Relative purity and strength con sid e ra tio n s eventually caused these sub stances to be rep la ce d by alloys o f iron, cobalt, o r tita n iu m .2 In 1937, polym eric b io m a te ria ls w ere in itia te d w ith p o ly m eth y lm ethacrylate (acrylic resin ), with m ost high m olecular w eight biopolym ers in tro d u c ed since 1950.s T he bioceram ics based on metallic oxides were introduced in the 1960s, and the carbon-based dental biom aterials were provided in the 1970s.4 D u rin g th e 1960s, th e r e la tio n s h i p between the inertness o f surgical im plants a n d th e associated tissue resp o n se s was em phasized.2 H igh purity, fired ceram ics an d carbons were investigated to provide chem ical and biochem ical environm ental stabilities. In th e 1970s, surgical m eth o d ologies th a t p ro d u ce d m inim al m echani cal, chemical, and therm al traum a to the tissue w ere in troduced to the dental pro fession. T he relative interactions between th e available b io m a te ria ls an d carefully p re p a re d tissue sites su b seq u en tly w ere elu cid ated.5-6 C oncepts for tissue integra tion followed, an d m u ltice n te r investiga tio n s show ed th a t stag ed tre a tm e n ts to
N
716 ■ JADA, Vol. 121, December 1990
p ro v id e p e rio d s fo r p r o te c te d h e a lin g co u ld be directly c o rre la te d w ith tissue interface conditions.7 In the 1980s, new an d m odified biom a terials were in tro d u ced an d were specifi cally designed, constituted, and fabricated for anticipated tissue interface responses,5 resulting in m echanically and chem ically anisotropic biomaterials. Force transfer is intended to be within the norm al limits of the tissues, an d b iom aterial surfaces are provided to bond chemically along the tis sue c o n ta c t zo n es.8 E x p erie n ce suggests further im provem ents in both biom aterial an d b io m e c h a n ic a l p r o p e r tie s will e n hance clinical longevities a n d an ex p an sion to broader patient populations in the 1990s.
Standards and classifications Standards and recom m ended practices are available for m ost dental an d m edical bio m a te ria ls. T h e A m e ric a n D e n ta l A sso ciation originally in itiated sta n d ard s for dental materials with am algam alloy.9 T he A m e ric a n S o cie ty f o r T e s tin g a n d M aterials (ASTM) co m m ittee F-4 in tro duced surgical im plant standards an d rec o m m e n d ed p ractices.10 T h e in fo rm atio n c o n ta in e d w ith in th e su rg ic a l im p la n t m aterial standards norm ally includes the chem ical analysis, m echanical properties, and surface finish.1» Com pliance of m anu facturers has partly provided a controlled an d re lia b le so u rce o f b io m a te ria ls fo r im p la n t devices. In a d d itio n , new sub-
Table 1 ■ Summary of synthetic biomaterials for dental implants. Metals an d alloys
Ceram ics an d carbon
Polymers
Ti an d Ti-Ai-V Co-Cr-Mo Fe-Cr-Ni
a i 2o , C a,„(P 0 4) 6 (O H )2 HA Ca3( P 0 4)j TCP C and C-Si
PMMA, PTFE, PE, PSF
T itanium (Ti), alu m in u m (Al), vanadium (V), cobalt (Co), ch ro m iu m (C r), m olybdenum (M o), iro n (Fe), nickel (Ni), a lu m in u m o x id e (A120 3), h y d ro x y la p a tite (H A ), tric alc iu m p h o s p h a te (T C P), c a rb o n (C ), silico n (S i), polym ethyl m ethacrylate (PMMA), polytetrafluoroethylene (PTFE), polyethylene (PE), polysulfone (PSF).
R E S E A R C H
Table 2
■ Classification of biomaterials using elastic moduli (lowest to highest magnitudes).
B iom aterial Polymers PE, PTFE PTFE, PMMA, PSF Ceram ics C aP 04 C arbons C a n d C-Si M etals an d alloys Ti an d Ti-Al-V Fe-Cr-Ni Co-Cr-Mo Ceram ics a i 2o
3
M odulus o f elasticity ratio (b io m ate ria l/b o n e ) *
Electrical o r th erm al co n d u cto r
0.01 -0.5x
No
Cream-white to am ber
0.5 - 5.0x
No
W hite
l.Ox
Yes
Black
5 .0 - 5.7x 8.0x ll.O x
Yes
Metallic
20. Ox
No
Cream-white
C olor
Table 3 ■ Classification of biomaterials using mechanical tensile strengths (lowest to highest magnitudes).
Polymers Ceram ics C aP 04 Carbons C a n d C-Si Ceram ics a i 2o 3 Metals a n d alloys
cobalt-based alloy (Co-Cr-Mo). C urrently, some are being m ade from cast titanium . A n u m b e r o f the im p lan t designs now in co rp o rate coatings o f calcium phosphate ceram ics, carb o n s, o r polym ers th a t are p la c e d o n th e s u r fa c e s o f th e tis s u e im p la n t a re a s .14 T h ese co m b in atio n s o r com posites are in ten d ed to optim ize both b io m a te ria l a n d b io m e ch a n ic al in te ra c tions with the tissues. Classification by properties
* The modulus of elasticity of compact bone was taken to be 3 x 106 psi for these ratios.
Biomaterial
R E P O R T S
Tensile strength ratio (b io m ate ria l/b o n e )*
Ductility (% elongadon) ra d o (b io m ate ria l/b o n e ) f
0.1 -0.5x
1 - 300x
0.1 - 2.0x
0
1.0 - 5.0x
0
2.0 - 5.0x 1.5 - 7.0x
0 8 - 30x
* The tensile strength of compact bone was taken as 2 x 104 psi for these ratios. f The tensile elongation to fracture for compact bone was taken as 1% for these ratios.
stances can b e evaluated using sta n d ard ized b io m a te ria ls as a c o n tro l. D e m o n s tr a tio n s o f “e s s e n tia lly s im ila r ” o r “relativ ely im p ro v e d ” b io m a te ria l is an i m p o r ta n t c o n s id e r a tio n r e la te d to a rapidly evolving discipline as im plant den tistry.11 Biomaterials are classified according to th eir m aterial properties, interactions with tissues, o r prim ary area o f surgical applica tion, for exam ple, the dental, orthopedic, o r cardiovascular fields.5 In categories, the biom aterials are listed as metals and alloys, ceramics an d carbons, polymers, an d com binations an d com posites o f these m aterial types.10 T h e biom aterials with resistance to ch em ical o r b io lo g ica l d e g ra d a tio n are called in e rt o r passive, w hile th o se th a t interact slightly are surface active o r bioac tive.5 Biomaterials in ten d ed to be dissolved o r to b e a b s o r b e d in vivo a r e c a lle d biodegradable o r resorbable. Various bio m aterials can be reclassified into different c a te g o rie s, d e p e n d in g o n th e im p la n t application o r th e basic m aterial p ro p e r ties.
Implant designs
V arious synthetic substances co n stitu ted a n d fa b ric a te d fo r d e n ta l im p la n ts are sum m arized in Table 1. Biom aterials are categorized by basic m aterial type with the m o s t c o m m o n ly u s e d b io m a te r ia ls included.12 T hese biom aterials are used in a wide variety o f d en tal im p lan t designs. T h e d e sig n s in c lu d e th o se p la c e d in to b o n e (e n d o s s e o u s ), ro o t fo rm s, b lad es ( p la t e s ) , tr a n s o s te a ls , s ta p le s , ra m u s frames, and endodontic stabilizers.13 These have been fabricated from m ost of the bio m aterials listed in Table 1, although spe cific lim ita tio n s exist. For ex am p le, the ra m u s fra m e tra n s o s s e o u s , a n d sta b le designs are m ade from metallic biom ateri als (Ti, Ti-6A1-4V, Co-Cr-Mo, or Fe-Cr-Ni), while the endo d o n tic stabilizers are mostly m anufactured from titanium (Ti) and alu m inum oxide ceram ic (A120 3). T he other m ajo r im p la n t design categ o ry in clu d es th e devices placed o n to b o n e u n d e r the periosteum (the subperiosteals). Most subperiosteals have been fabricated from cast
T he various biom aterials can also be classi fied an d com pared using th e basic physi cal, m echanical, chem ical, an d biological p ro p ertie s o f th e synthetic substances.510 To provide this type o f classification within categories, th e biom aterials are from the lowest to h ig h est m ag n itu d es o f specific p ro p erties. For exam ple, com parisons of m aterial elastic m oduli or tensile strengths w ould result in d ifferen t rankings. These properties are used for m aking differential d ec isio n s, o r fo r e x p la n a tio n s o f tissue responses. These relative classifications by p roperties are provided in tabular form in Tables 2, 3, and 4. T h e elastic m oduli, strengths, an d duc tilities are used as m aterial p roperty con siderations for the design, fabrication, and p r o s th o d o n tic r e s to r a tio n o f im p la n t d evices.15 M oduli are d irectly associated with m icroscopic elastic strains along tis sue interfaces. T he m acroscopic stress and strain relationships are influenced m ost by the design’s size an d shape. Designs m ust co rp o rate configurations th at are specific n o t o nly to th e b io m a te ria l b u t also to an ato m ical, surgical, tech n ical, an d oral considerations. Relative cost, sterilization and resterilization, and availability o f com p le m e n ta r y su rg ic a l in s tr u m e n ts a n d restorative intraoral m aterials are also fac tors.
Table 4 ■ Classification of biomateri als using chemical inertness (lowest to highest magnitudes). B iom aterial
Relative ranking*
Ceram ics TCP HA Polymers M etals and alloys Ceram ics a n d carbons A12O s, C, C-Si
Biodegradable Bioactive PMMA to PTFE Fe to Ti alloys In e rt
* T h ese relative rankings are d e p e n d e n t o n the specific b io m a te r ia l p r o d u c t a n d th e c lin ic a l a p p lic a tio n . F o r exam ple, PMMA is p rese n ted as th e b o n e c e m e n t p ro d u c t u sed in o rth o p e d ic su rg ery a n d th e biochem ical inertness o f PTFE exceeds som e o f the m etallic m aterials.
JADA, Vol. 121, December 1990 ■ 717
R E S E A R C H
R E P O R T S
S tre n g th s are m a x im iz e d to p ro v id e resistance to cyclic-induced (fatigue) frac tures; and higher ductilities are critical to the b en ding o f abutm ents for parallelism o r to provide the best fit for an available anatom ical site. Metals are strong and duc tile, a n d th e ceram ic s a n d c a rb o n s are n o n d u c tile (b rittle ). T h e in e rt ceram ics an d metals have high elastic m oduli, and the polymers have high ductilities but low elastic m oduli. In g en eral, th e variety of p ro p e rtie s d ic ta te s th e a p p lic a tio n , fo r ex a m p le , p o ly m e rs fo r so ft tissues a n d metals o r ceramics for hard tissues. C lassification by chem ical in e rtn ess is com plicated because the relative listings
faces. Elem ents and electrochem ical fields are known to directly influence tissue reac tio n pathw ay s.14 T h u s, b io c o m p atib ility profiles provide o p p o rtu n itie s to investi gate and com pare the phenom enological d a ta g e n e ra te d from lab o rato ry , la b o ra tory animal, an d h um an clinical investiga tions. Surface and bulk properties
Device retrieval analyses
S y n th etic c o m p o u n d s su c h as m e tallic o x id e s , c a lc iu m p h o s p h a te c e ra m ic s, glasses, an d glass ceram ics have d em o n stra te d d ire c t b o n d in g to b o n e .414 T his b o n d in g , w h ich h a s te n s ile a n d sh e a r
Implant restorative treatments in dentistry provide a signifi cant opportunity to better understand the roles of biomaterial and biomechanical properties and their relationship to biocompatibility criteria for all types of surgical implants.
change d e p e n d in g on in te rfa ce bonding c o n d itio n (b o n e o r fib ro u s tissu e), th e specific tim e o f ev alu atio n , o r th e local en v iro n m ental co n d itio n s (fluid, soft tis sue, o r b o n e ). C o n d itio n s o f in terfacial m o tio n (slip) o r local in fectio n (altered pH ) would change the category. Generally, th e high ceram ics an d carb o n s are m ost inert, the metals are interm ediate, and the polym ers a re m o st subject to interfacial w ear o r the leach in g o f low er m olecular w eight o r plasticizer constituents.2-5 These various properties have been cor re la te d w ith b o th b io m a te ria l- an d b io m ech an ical-based co m p atib ility criteria. E la s tic m o d u li s im ila r to th e tis su e replaced, high strength and ductility, and chem ical inertn ess have b ee n co rrelated with biocom patibility profiles. T h ere are ex cep tio n s to th e se g en eralizatio n s, b u t these criteria have been broadly applied to m aterial and design selections for im plant devices. Tissue interfaces Biocom patibility profiles have been con sidered in term s o f the elem ents and the forces transferred across biomaterial-to-tissu e i n t e r f a c e s . 13-15 T h e p h y sic a l a n d m echanical properties o f biom aterials are directly c o m p a re d w ith biophysical and biom echanical requirem ents fo r function. S eparately, th e c h e m ic al a n d electrical p r o p e r tie s a re c o m p a r e d w ith th e b i chemical interaction along the tissue inter 718 ■ JADA, Vol. 121, D ecember 1990
cial relationships w ithout im posed force or m o tio n . T h e sta g in g o f tre a tm e n ts an d protected healing provide an optim al situ ation for b o n e an d soft tissue bonding to synthetic biom aterial surfaces. In contrast, if a stable soft tissue (p seu d o lig am e n t) were m ost functional over the long-term , early m echanical loading (one-stage) treat m ent is probably indicated.
strength m agnitudes w ithin th e range o f chem ical bo n d in g , su p p o rts an interface c o n d itio n th a t co u ld strongly in flu e n ce fu n ctio n al fo rce tran sfe r co n d itio n s.16 If these b io m a te ria l su rfaces are c o n tam i n a t e d w ith d ilu te im p u r itie s , th e n b io d ég rad atio n p ro d u cts could adversely influ en ce th e local tissue responses an d in terfacial b o n d in g . Also, w hen th e sub strate biodegrades, th e reaction products could result in adverse tissue responses.17 Design and force transfer
E n h a n c e m e n t o f b io m a te ria l-to -tis s u e interfaces, through com puter-based finite elem ent m odeling an d analysis, and com p u te r-a s s is te d d e s ig n (FEM , FEA, a n d CAD) should provide significant improve m en ts.18 However, basic selection criteria can be applied to any new design concept. Force transfer along interfaces loaded in m echanical shear strain could be signifi cantly influenced by biomaterial-to-tissue bonding. The p roduction o f localized n o n slip o r b o n d e d in terfaces th a t w ould be s ta b le in vivo c o u ld r e s u lt in a lte r e d im plant design concepts. This type of sta ble bonding may be possible with biom ate rials that are currently within clinical trials. T he tissue interface provides a healing stage u n d e r p ro te c te d (iso lated ) co n d i tio n s .7 T h e p r o c e d u re o f n o t ap p ly in g fu n ctio n al lo ad in g fo r e x ten d e d periods after surgical p lacem en t affords tim e for establishing biom aterial-to-tissue in te rfa
Analyses of devices eventually retrieved for p sy ch o lo g ical, p ro s th e tic , o r tra u m a tic conditions provided clinical histories from which com parisons o f biomaterial-to-tissue interfaces an d the actual devices could be m ade.13-19 T itanium oxide (Tix Oy) surfaces f o r o n e -sta g e e n d o s s e o u s b la d e s have d e m o n s tra te d fib ro u s tissue in te rfa ce s. T his sam e su rface o x id e fo r ro o t form s and blades, restored after two-stage or pro tected healing restorative treatm ents, have d em o n strate d ad jac en t b o n e (osteointeg ra tio n ). T h e c h ro m iu m oxide surfaces (cobalt- o r iron-based alloys) for one-stage subperiosteals, blades, an d ram us fram es have shown fibrous tissue interfacial con ditions. In contrast, th e calcium phosphate and alum inum oxide ceram ic surfaces of ro o t form s an d the coated subperiosteals and blades have shown direct bone inter faces fo r two-stage o r p ro te c te d h ealing restorative treatm ents. C o m p a riso n s o f sim ila r d e sig n s a n d m aterials with an d w ith o u t surface coat ings may c o n trib u te significant in fo rm a tio n from fu tu re retrie v al analyses. For ex am p le, m any o f th e available designs with metallic oxide surfaces are now being coated with calcium p hosphate ceramics. T he constancy o f design and variability of surface chem istry could provide answers to questions ab o u t interfacial bo n d in g to bone, electrical conductivity, elasticity, and elem ental com positions. Summary Im plant restorative treatm ents in dentistry provide a significant opportunity to b etter u n d e rsta n d th e ro les o f biom aterial an d biom echanical p ro p ertie s an d th eir rela tionship to biocom patibility criteria for all types o f surgical im plants. Various biom a terials and designs are being used for long te rm tr e a t m e n t m o d a litie s . E x istin g science an d technology support the need f o r m e c h a n ic a lly a n d c h e m ic a lly a n iso tro p ic su b stan ce s to re p la c e fu n c tional, lo ad -b earin g tissues an d afford a m ost prom ising fu tu re for dental im plant r e s e a r c h a n d d e v e lo p m e n t. M u ltid is-
R E S E A R C H
ciplinary analyses should provide the basis for quantitative classifications of interfacial phenom ena, and, thereby, the directly associated clinical longevities. ---------------------- J!* O A ----------------------Dr. Lemons is professor and chairman, department o f b iom ateria ls, U niversity o f A labam a S c h o o l o f Dentistry, Birmingham 35294. 1. Lemons JE. General characteristics and classifica tions o f im plant materials. In: Lin O , Chao E, eds. P erspective on biom aterials. A m sterdam: Elsevier; 1986:1-15. 2. W illiam s DF, R o a f R. Im p la n ts in surgery. Philadelphia: Saunders; 1973. 3. Craig RG. Restorative dental materials. St. Louis: Mosby; 1985. 4. H ench LL, Ethridge EC. Biomaterials, an interfa cial approach. N e w a r k : Academic; 1982.
5. von Recum A. Handbook o f biomaterials evalua tion. New York: MacMillan; 1986. 6. Smith DC, Williams DF. Biocompatibility o f den tal materials 4. Boca Raton, FL: CRC Press; 1982. 7. B ra n em a rk PI. O s se o in te g r a te d im p la n ts. Chicago: Quintessence; 1989. 8. Rizzo A, ed. Proceedings, consensus development c o n fe r e n c e o n d e n ta l im p la n ts. J D e n t E duc 1988;52:678. 9. P h illip s RW. S c ie n c e o f d e n ta l m a ter ia ls. Philadelphia: Saunders; 1973. 10. Am Soc for Testing and Materials. Volume 13.01. In: Medical Devices. Philadelphia: ASTM Press; 1989. 11. L em ons JE, ed. Quantitative characterization and perform ance o f porous implants for hard tissue application, STP 953. Philadelphia: ASTM Press; 1987. 12. Lem ons JE, NatiellaJR . Biomaterials, biocom patibility and peri-im plant considerations. Guernsey L H , e d . In: D e n ta l C lin ic s N o r th A m e r ica . Philadelphia: Saunders; 1986:1, 3-23. 13. Lemons JE. Dental implant retrieval analyses. J Dent Educ 1988;52:748-57.
R E P O R T S
14. Ducheyne P, Lemons JE, eds. Bioceramics: mate rial characteristics versus in vivo behavior. New York:NY Acad o fS ci 1988:523. 15. Brunski JB. B iom ech an ics o f oral im plants: future research directions. J D ent Educ 1988;52:775-
88. 16. Cook SD, KayJF, Thomas KA, Jarco M. Interface mechanics and histology o f titanium and hydroxylapatite coated titanium for dental implant applications. IntJ Oral Maxillofac Implants 1987;2:15-22. 17. L ucas LC, B e a r d e n LF, L em o n s JE. Ultrastructural exam inations o f in vitro and in vivo cells exposed to solutions o f 316L stainless steel. In: Fraker A, Griffin C, eds. ASTM STP 859. Philadelphia: ASTM Press; 1985:208-21. 18. Bidez MW, Stephens BJ, Lemons JE. An investi gation into the effect o f blade dental implant length o f interfacial tissue stress profiles. Stiker RL, Simon BR, eds. Computational m ethods in bioengineering. Am Soc Mech Engr 1988:235-45. 19. Lem ons JE. Surface evaluations o f materials. J Oral Implantol 1986;3:396-406.
JADA,VoL 121, December 1990 ■ 719
R E P O R T S
Synthetic materials fo r surgical implant demces have evolved from the early metallic systems to a variety of material combinations and composites. Current biomaterial and biomechanical proper ties provide relatively optimal stable bone and soft tissue interfaces and simplified restorative treat ments. Further improvements in existing systems require a continuation of the multidisciplinary approach to laboratory, experimental animal, and human clinical research.
Dental implant biomaterials Jack E. Lem ons, PhD
aturally o c c u rrin g m in erals a n d m e ta ls su c h as g e m s to n e s a n d gold w ere first used fo r surgical im p la n t a n d to o th r o o t r e p la c e m e n t devices.1 Relative purity and strength con sid e ra tio n s eventually caused these sub stances to be rep la ce d by alloys o f iron, cobalt, o r tita n iu m .2 In 1937, polym eric b io m a te ria ls w ere in itia te d w ith p o ly m eth y lm ethacrylate (acrylic resin ), with m ost high m olecular w eight biopolym ers in tro d u c ed since 1950.s T he bioceram ics based on metallic oxides were introduced in the 1960s, and the carbon-based dental biom aterials were provided in the 1970s.4 D u rin g th e 1960s, th e r e la tio n s h i p between the inertness o f surgical im plants a n d th e associated tissue resp o n se s was em phasized.2 H igh purity, fired ceram ics an d carbons were investigated to provide chem ical and biochem ical environm ental stabilities. In th e 1970s, surgical m eth o d ologies th a t p ro d u ce d m inim al m echani cal, chemical, and therm al traum a to the tissue w ere in troduced to the dental pro fession. T he relative interactions between th e available b io m a te ria ls an d carefully p re p a re d tissue sites su b seq u en tly w ere elu cid ated.5-6 C oncepts for tissue integra tion followed, an d m u ltice n te r investiga tio n s show ed th a t stag ed tre a tm e n ts to
N
716 ■ JADA, Vol. 121, December 1990
p ro v id e p e rio d s fo r p r o te c te d h e a lin g co u ld be directly c o rre la te d w ith tissue interface conditions.7 In the 1980s, new an d m odified biom a terials were in tro d u ced an d were specifi cally designed, constituted, and fabricated for anticipated tissue interface responses,5 resulting in m echanically and chem ically anisotropic biomaterials. Force transfer is intended to be within the norm al limits of the tissues, an d b iom aterial surfaces are provided to bond chemically along the tis sue c o n ta c t zo n es.8 E x p erie n ce suggests further im provem ents in both biom aterial an d b io m e c h a n ic a l p r o p e r tie s will e n hance clinical longevities a n d an ex p an sion to broader patient populations in the 1990s.
Standards and classifications Standards and recom m ended practices are available for m ost dental an d m edical bio m a te ria ls. T h e A m e ric a n D e n ta l A sso ciation originally in itiated sta n d ard s for dental materials with am algam alloy.9 T he A m e ric a n S o cie ty f o r T e s tin g a n d M aterials (ASTM) co m m ittee F-4 in tro duced surgical im plant standards an d rec o m m e n d ed p ractices.10 T h e in fo rm atio n c o n ta in e d w ith in th e su rg ic a l im p la n t m aterial standards norm ally includes the chem ical analysis, m echanical properties, and surface finish.1» Com pliance of m anu facturers has partly provided a controlled an d re lia b le so u rce o f b io m a te ria ls fo r im p la n t devices. In a d d itio n , new sub-
Table 1 ■ Summary of synthetic biomaterials for dental implants. Metals an d alloys
Ceram ics an d carbon
Polymers
Ti an d Ti-Ai-V Co-Cr-Mo Fe-Cr-Ni
a i 2o , C a,„(P 0 4) 6 (O H )2 HA Ca3( P 0 4)j TCP C and C-Si
PMMA, PTFE, PE, PSF
T itanium (Ti), alu m in u m (Al), vanadium (V), cobalt (Co), ch ro m iu m (C r), m olybdenum (M o), iro n (Fe), nickel (Ni), a lu m in u m o x id e (A120 3), h y d ro x y la p a tite (H A ), tric alc iu m p h o s p h a te (T C P), c a rb o n (C ), silico n (S i), polym ethyl m ethacrylate (PMMA), polytetrafluoroethylene (PTFE), polyethylene (PE), polysulfone (PSF).
R E S E A R C H
Table 2
■ Classification of biomaterials using elastic moduli (lowest to highest magnitudes).
B iom aterial Polymers PE, PTFE PTFE, PMMA, PSF Ceram ics C aP 04 C arbons C a n d C-Si M etals an d alloys Ti an d Ti-Al-V Fe-Cr-Ni Co-Cr-Mo Ceram ics a i 2o
3
M odulus o f elasticity ratio (b io m ate ria l/b o n e ) *
Electrical o r th erm al co n d u cto r
0.01 -0.5x
No
Cream-white to am ber
0.5 - 5.0x
No
W hite
l.Ox
Yes
Black
5 .0 - 5.7x 8.0x ll.O x
Yes
Metallic
20. Ox
No
Cream-white
C olor
Table 3 ■ Classification of biomaterials using mechanical tensile strengths (lowest to highest magnitudes).
Polymers Ceram ics C aP 04 Carbons C a n d C-Si Ceram ics a i 2o 3 Metals a n d alloys
cobalt-based alloy (Co-Cr-Mo). C urrently, some are being m ade from cast titanium . A n u m b e r o f the im p lan t designs now in co rp o rate coatings o f calcium phosphate ceram ics, carb o n s, o r polym ers th a t are p la c e d o n th e s u r fa c e s o f th e tis s u e im p la n t a re a s .14 T h ese co m b in atio n s o r com posites are in ten d ed to optim ize both b io m a te ria l a n d b io m e ch a n ic al in te ra c tions with the tissues. Classification by properties
* The modulus of elasticity of compact bone was taken to be 3 x 106 psi for these ratios.
Biomaterial
R E P O R T S
Tensile strength ratio (b io m ate ria l/b o n e )*
Ductility (% elongadon) ra d o (b io m ate ria l/b o n e ) f
0.1 -0.5x
1 - 300x
0.1 - 2.0x
0
1.0 - 5.0x
0
2.0 - 5.0x 1.5 - 7.0x
0 8 - 30x
* The tensile strength of compact bone was taken as 2 x 104 psi for these ratios. f The tensile elongation to fracture for compact bone was taken as 1% for these ratios.
stances can b e evaluated using sta n d ard ized b io m a te ria ls as a c o n tro l. D e m o n s tr a tio n s o f “e s s e n tia lly s im ila r ” o r “relativ ely im p ro v e d ” b io m a te ria l is an i m p o r ta n t c o n s id e r a tio n r e la te d to a rapidly evolving discipline as im plant den tistry.11 Biomaterials are classified according to th eir m aterial properties, interactions with tissues, o r prim ary area o f surgical applica tion, for exam ple, the dental, orthopedic, o r cardiovascular fields.5 In categories, the biom aterials are listed as metals and alloys, ceramics an d carbons, polymers, an d com binations an d com posites o f these m aterial types.10 T h e biom aterials with resistance to ch em ical o r b io lo g ica l d e g ra d a tio n are called in e rt o r passive, w hile th o se th a t interact slightly are surface active o r bioac tive.5 Biomaterials in ten d ed to be dissolved o r to b e a b s o r b e d in vivo a r e c a lle d biodegradable o r resorbable. Various bio m aterials can be reclassified into different c a te g o rie s, d e p e n d in g o n th e im p la n t application o r th e basic m aterial p ro p e r ties.
Implant designs
V arious synthetic substances co n stitu ted a n d fa b ric a te d fo r d e n ta l im p la n ts are sum m arized in Table 1. Biom aterials are categorized by basic m aterial type with the m o s t c o m m o n ly u s e d b io m a te r ia ls included.12 T hese biom aterials are used in a wide variety o f d en tal im p lan t designs. T h e d e sig n s in c lu d e th o se p la c e d in to b o n e (e n d o s s e o u s ), ro o t fo rm s, b lad es ( p la t e s ) , tr a n s o s te a ls , s ta p le s , ra m u s frames, and endodontic stabilizers.13 These have been fabricated from m ost of the bio m aterials listed in Table 1, although spe cific lim ita tio n s exist. For ex am p le, the ra m u s fra m e tra n s o s s e o u s , a n d sta b le designs are m ade from metallic biom ateri als (Ti, Ti-6A1-4V, Co-Cr-Mo, or Fe-Cr-Ni), while the endo d o n tic stabilizers are mostly m anufactured from titanium (Ti) and alu m inum oxide ceram ic (A120 3). T he other m ajo r im p la n t design categ o ry in clu d es th e devices placed o n to b o n e u n d e r the periosteum (the subperiosteals). Most subperiosteals have been fabricated from cast
T he various biom aterials can also be classi fied an d com pared using th e basic physi cal, m echanical, chem ical, an d biological p ro p ertie s o f th e synthetic substances.510 To provide this type o f classification within categories, th e biom aterials are from the lowest to h ig h est m ag n itu d es o f specific p ro p erties. For exam ple, com parisons of m aterial elastic m oduli or tensile strengths w ould result in d ifferen t rankings. These properties are used for m aking differential d ec isio n s, o r fo r e x p la n a tio n s o f tissue responses. These relative classifications by p roperties are provided in tabular form in Tables 2, 3, and 4. T h e elastic m oduli, strengths, an d duc tilities are used as m aterial p roperty con siderations for the design, fabrication, and p r o s th o d o n tic r e s to r a tio n o f im p la n t d evices.15 M oduli are d irectly associated with m icroscopic elastic strains along tis sue interfaces. T he m acroscopic stress and strain relationships are influenced m ost by the design’s size an d shape. Designs m ust co rp o rate configurations th at are specific n o t o nly to th e b io m a te ria l b u t also to an ato m ical, surgical, tech n ical, an d oral considerations. Relative cost, sterilization and resterilization, and availability o f com p le m e n ta r y su rg ic a l in s tr u m e n ts a n d restorative intraoral m aterials are also fac tors.
Table 4 ■ Classification of biomateri als using chemical inertness (lowest to highest magnitudes). B iom aterial
Relative ranking*
Ceram ics TCP HA Polymers M etals and alloys Ceram ics a n d carbons A12O s, C, C-Si
Biodegradable Bioactive PMMA to PTFE Fe to Ti alloys In e rt
* T h ese relative rankings are d e p e n d e n t o n the specific b io m a te r ia l p r o d u c t a n d th e c lin ic a l a p p lic a tio n . F o r exam ple, PMMA is p rese n ted as th e b o n e c e m e n t p ro d u c t u sed in o rth o p e d ic su rg ery a n d th e biochem ical inertness o f PTFE exceeds som e o f the m etallic m aterials.
JADA, Vol. 121, December 1990 ■ 717
R E S E A R C H
R E P O R T S
S tre n g th s are m a x im iz e d to p ro v id e resistance to cyclic-induced (fatigue) frac tures; and higher ductilities are critical to the b en ding o f abutm ents for parallelism o r to provide the best fit for an available anatom ical site. Metals are strong and duc tile, a n d th e ceram ic s a n d c a rb o n s are n o n d u c tile (b rittle ). T h e in e rt ceram ics an d metals have high elastic m oduli, and the polymers have high ductilities but low elastic m oduli. In g en eral, th e variety of p ro p e rtie s d ic ta te s th e a p p lic a tio n , fo r ex a m p le , p o ly m e rs fo r so ft tissues a n d metals o r ceramics for hard tissues. C lassification by chem ical in e rtn ess is com plicated because the relative listings
faces. Elem ents and electrochem ical fields are known to directly influence tissue reac tio n pathw ay s.14 T h u s, b io c o m p atib ility profiles provide o p p o rtu n itie s to investi gate and com pare the phenom enological d a ta g e n e ra te d from lab o rato ry , la b o ra tory animal, an d h um an clinical investiga tions. Surface and bulk properties
Device retrieval analyses
S y n th etic c o m p o u n d s su c h as m e tallic o x id e s , c a lc iu m p h o s p h a te c e ra m ic s, glasses, an d glass ceram ics have d em o n stra te d d ire c t b o n d in g to b o n e .414 T his b o n d in g , w h ich h a s te n s ile a n d sh e a r
Implant restorative treatments in dentistry provide a signifi cant opportunity to better understand the roles of biomaterial and biomechanical properties and their relationship to biocompatibility criteria for all types of surgical implants.
change d e p e n d in g on in te rfa ce bonding c o n d itio n (b o n e o r fib ro u s tissu e), th e specific tim e o f ev alu atio n , o r th e local en v iro n m ental co n d itio n s (fluid, soft tis sue, o r b o n e ). C o n d itio n s o f in terfacial m o tio n (slip) o r local in fectio n (altered pH ) would change the category. Generally, th e high ceram ics an d carb o n s are m ost inert, the metals are interm ediate, and the polym ers a re m o st subject to interfacial w ear o r the leach in g o f low er m olecular w eight o r plasticizer constituents.2-5 These various properties have been cor re la te d w ith b o th b io m a te ria l- an d b io m ech an ical-based co m p atib ility criteria. E la s tic m o d u li s im ila r to th e tis su e replaced, high strength and ductility, and chem ical inertn ess have b ee n co rrelated with biocom patibility profiles. T h ere are ex cep tio n s to th e se g en eralizatio n s, b u t these criteria have been broadly applied to m aterial and design selections for im plant devices. Tissue interfaces Biocom patibility profiles have been con sidered in term s o f the elem ents and the forces transferred across biomaterial-to-tissu e i n t e r f a c e s . 13-15 T h e p h y sic a l a n d m echanical properties o f biom aterials are directly c o m p a re d w ith biophysical and biom echanical requirem ents fo r function. S eparately, th e c h e m ic al a n d electrical p r o p e r tie s a re c o m p a r e d w ith th e b i chemical interaction along the tissue inter 718 ■ JADA, Vol. 121, D ecember 1990
cial relationships w ithout im posed force or m o tio n . T h e sta g in g o f tre a tm e n ts an d protected healing provide an optim al situ ation for b o n e an d soft tissue bonding to synthetic biom aterial surfaces. In contrast, if a stable soft tissue (p seu d o lig am e n t) were m ost functional over the long-term , early m echanical loading (one-stage) treat m ent is probably indicated.
strength m agnitudes w ithin th e range o f chem ical bo n d in g , su p p o rts an interface c o n d itio n th a t co u ld strongly in flu e n ce fu n ctio n al fo rce tran sfe r co n d itio n s.16 If these b io m a te ria l su rfaces are c o n tam i n a t e d w ith d ilu te im p u r itie s , th e n b io d ég rad atio n p ro d u cts could adversely influ en ce th e local tissue responses an d in terfacial b o n d in g . Also, w hen th e sub strate biodegrades, th e reaction products could result in adverse tissue responses.17 Design and force transfer
E n h a n c e m e n t o f b io m a te ria l-to -tis s u e interfaces, through com puter-based finite elem ent m odeling an d analysis, and com p u te r-a s s is te d d e s ig n (FEM , FEA, a n d CAD) should provide significant improve m en ts.18 However, basic selection criteria can be applied to any new design concept. Force transfer along interfaces loaded in m echanical shear strain could be signifi cantly influenced by biomaterial-to-tissue bonding. The p roduction o f localized n o n slip o r b o n d e d in terfaces th a t w ould be s ta b le in vivo c o u ld r e s u lt in a lte r e d im plant design concepts. This type of sta ble bonding may be possible with biom ate rials that are currently within clinical trials. T he tissue interface provides a healing stage u n d e r p ro te c te d (iso lated ) co n d i tio n s .7 T h e p r o c e d u re o f n o t ap p ly in g fu n ctio n al lo ad in g fo r e x ten d e d periods after surgical p lacem en t affords tim e for establishing biom aterial-to-tissue in te rfa
Analyses of devices eventually retrieved for p sy ch o lo g ical, p ro s th e tic , o r tra u m a tic conditions provided clinical histories from which com parisons o f biomaterial-to-tissue interfaces an d the actual devices could be m ade.13-19 T itanium oxide (Tix Oy) surfaces f o r o n e -sta g e e n d o s s e o u s b la d e s have d e m o n s tra te d fib ro u s tissue in te rfa ce s. T his sam e su rface o x id e fo r ro o t form s and blades, restored after two-stage or pro tected healing restorative treatm ents, have d em o n strate d ad jac en t b o n e (osteointeg ra tio n ). T h e c h ro m iu m oxide surfaces (cobalt- o r iron-based alloys) for one-stage subperiosteals, blades, an d ram us fram es have shown fibrous tissue interfacial con ditions. In contrast, th e calcium phosphate and alum inum oxide ceram ic surfaces of ro o t form s an d the coated subperiosteals and blades have shown direct bone inter faces fo r two-stage o r p ro te c te d h ealing restorative treatm ents. C o m p a riso n s o f sim ila r d e sig n s a n d m aterials with an d w ith o u t surface coat ings may c o n trib u te significant in fo rm a tio n from fu tu re retrie v al analyses. For ex am p le, m any o f th e available designs with metallic oxide surfaces are now being coated with calcium p hosphate ceramics. T he constancy o f design and variability of surface chem istry could provide answers to questions ab o u t interfacial bo n d in g to bone, electrical conductivity, elasticity, and elem ental com positions. Summary Im plant restorative treatm ents in dentistry provide a significant opportunity to b etter u n d e rsta n d th e ro les o f biom aterial an d biom echanical p ro p ertie s an d th eir rela tionship to biocom patibility criteria for all types o f surgical im plants. Various biom a terials and designs are being used for long te rm tr e a t m e n t m o d a litie s . E x istin g science an d technology support the need f o r m e c h a n ic a lly a n d c h e m ic a lly a n iso tro p ic su b stan ce s to re p la c e fu n c tional, lo ad -b earin g tissues an d afford a m ost prom ising fu tu re for dental im plant r e s e a r c h a n d d e v e lo p m e n t. M u ltid is-
R E S E A R C H
ciplinary analyses should provide the basis for quantitative classifications of interfacial phenom ena, and, thereby, the directly associated clinical longevities. ---------------------- J!* O A ----------------------Dr. Lemons is professor and chairman, department o f b iom ateria ls, U niversity o f A labam a S c h o o l o f Dentistry, Birmingham 35294. 1. Lemons JE. General characteristics and classifica tions o f im plant materials. In: Lin O , Chao E, eds. P erspective on biom aterials. A m sterdam: Elsevier; 1986:1-15. 2. W illiam s DF, R o a f R. Im p la n ts in surgery. Philadelphia: Saunders; 1973. 3. Craig RG. Restorative dental materials. St. Louis: Mosby; 1985. 4. H ench LL, Ethridge EC. Biomaterials, an interfa cial approach. N e w a r k : Academic; 1982.
5. von Recum A. Handbook o f biomaterials evalua tion. New York: MacMillan; 1986. 6. Smith DC, Williams DF. Biocompatibility o f den tal materials 4. Boca Raton, FL: CRC Press; 1982. 7. B ra n em a rk PI. O s se o in te g r a te d im p la n ts. Chicago: Quintessence; 1989. 8. Rizzo A, ed. Proceedings, consensus development c o n fe r e n c e o n d e n ta l im p la n ts. J D e n t E duc 1988;52:678. 9. P h illip s RW. S c ie n c e o f d e n ta l m a ter ia ls. Philadelphia: Saunders; 1973. 10. Am Soc for Testing and Materials. Volume 13.01. In: Medical Devices. Philadelphia: ASTM Press; 1989. 11. L em ons JE, ed. Quantitative characterization and perform ance o f porous implants for hard tissue application, STP 953. Philadelphia: ASTM Press; 1987. 12. Lem ons JE, NatiellaJR . Biomaterials, biocom patibility and peri-im plant considerations. Guernsey L H , e d . In: D e n ta l C lin ic s N o r th A m e r ica . Philadelphia: Saunders; 1986:1, 3-23. 13. Lemons JE. Dental implant retrieval analyses. J Dent Educ 1988;52:748-57.
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14. Ducheyne P, Lemons JE, eds. Bioceramics: mate rial characteristics versus in vivo behavior. New York:NY Acad o fS ci 1988:523. 15. Brunski JB. B iom ech an ics o f oral im plants: future research directions. J D ent Educ 1988;52:775-
88. 16. Cook SD, KayJF, Thomas KA, Jarco M. Interface mechanics and histology o f titanium and hydroxylapatite coated titanium for dental implant applications. IntJ Oral Maxillofac Implants 1987;2:15-22. 17. L ucas LC, B e a r d e n LF, L em o n s JE. Ultrastructural exam inations o f in vitro and in vivo cells exposed to solutions o f 316L stainless steel. In: Fraker A, Griffin C, eds. ASTM STP 859. Philadelphia: ASTM Press; 1985:208-21. 18. Bidez MW, Stephens BJ, Lemons JE. An investi gation into the effect o f blade dental implant length o f interfacial tissue stress profiles. Stiker RL, Simon BR, eds. Computational m ethods in bioengineering. Am Soc Mech Engr 1988:235-45. 19. Lem ons JE. Surface evaluations o f materials. J Oral Implantol 1986;3:396-406.
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