implant survival: biological and mechanical … introductory remarks have implications for dental...

TAGETE - ARCHIVES OF LEGAL MEDICINE AND DENTISTRY

TAGETE 2-2009 Year XV

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IMPLANT SURVIVAL: BIOLOGICAL AND MECHANICAL INFLUENCES

ON IMPLANTS' LIFE SPAN

Dr. Maria Sofia Rini* , Prof. Giorgio Borea**, Dr. Emilio Nuzzolese***,

Dr. Deborah Meleo****, Prof. Dario Betti*****

ABSTRACT Both in daily and Forensic Dentistry questions frequently arise about the average duration of fixtures in implant based rehabilitation, especially in relation to their actual renewal possibility. It is important to review previous opinions, clinical facts and bio-mechanical aspects. The Authors conclude that - as general agreement - a correctly positioned implant will not have shorter lifespan than the root of the natural tooth being substituted. In any treatment planning, anyhow, a failure is always to be considered, but it must be evaluated by a specific risk analysis. KEYWORDS Dental implant Life expectancy Mechanical failure Renewals

* Odontoiatra – Odontologo Forense - Prof. a c. Dip. di Scienze Odontostomatologiche dell'Università di Bologna – Referente per la specialistica Distr. Pianura Est Ausl di Bologna – Master Post-Universitario in "La responsabilità del Medico nelle prestazioni specialistiche ambulatoriali"- Corso di perfezionamento in “Odontologia forense”] ** Medico-chirurgo - Spec. in Odontoiatria e Medicina Legale - già Ordinario Ist. Disc.Odontostom. Univ. di Bologna *** Odontoiatra- Odontologo Forense **** Odontoiatra- Dottorato di ric. in Malat. Odontostomatol - Univ. La Sapienza - Roma ***** Medico-chirurgo - Spec. in Odontoiatria e Medicina Legale - Univ. di Padova



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Si ringrazia per la collaborazione:

Enrico Sandrini

Polit.di Milano- Dip.di Chimica, Materiali ed Ingegneria Chimica “G.Natta”

1. Introduction and literature review

Since 1909, implant-based prosthesis introduced a true revolution in the world of

dentistry. It has currently become a standard in oral prosthetic rehabilitation, especially

among young patients (1). Ever since the 1980’s, endo-osseous techniques have been

increasing their clinical applications (2), and since the 1990’s (1988-1999) they have

being widely used, not only for the rehabilitative possibilities they offer in general, but

also in cases of patients with significant mandibular or jaw reconstruction due to severe

bone and dental loss following both traumatic and neoplastic pathologies (3) (4) (5).

These promising results, supported by biomedical research and by important

technological advances, have led to optimistic expectations and projections on implants'

life span, although the artificial intrinsic mechanical nature of the prosthesis still elicited

some doubts.

An implant is artificially (that is mechanically) assembled from artificial materials in the

aim of restoring - even if only partially - a lost or impaired dental function. Clinical trials



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and experiments reported in the literature have shown that various factors can affect the

duration (meant as “useful survival”) of the implant fixtures. These factors include a

correct diagnosis, planning and clinical execution, the localization, quality and quantity

of the bone sites and soft tissues, immune response and general clinical conditions of the

subject, extra-gingival profiles, functional (and dysfunctional) loads, prosthetic

components and structures, patients' hygienic conditions and habits, the design and

characteristics of the fixture, etc. (6) (7). In addition to these biological, technical, and

methodological elements, there are also mechanical factors related to the make and

material of the device. The implant's life expectancy is affected not only by the chosen

bone site or by the age and physical conditions of the patient, but also by the

biotechnical and biomechanical characteristics of the material used (2). The devices are

inevitably “not alive” material. They are artificial and thus unable to reshape or to adjust

as live tissues do (8).

Like all “objects,” the prosthesis may malfunction, degrade and prone to a short life

expectancy. Biomedical research has studied several ways to limit this inevitable

phenomenon, and there has been constant improvement in the quality of the implant

device components (morphology, material properties, interface, etc.). As a result,

application methods and testing have also improved. However, these studies, as well as



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the constant new developments, have the side effect of making the individual

components to become rapidly obsolete, thus affecting their follow-up evaluation.

Also, even if we were able to eliminate all interface problems between live tissue and

artificial material, or all the problems pertaining to tissue integration and physiological

response, we still would not be able to prevent the strictly mechanical problem of the

“break down.” All objects can break. They all have a physical, structural limit and they

can fracture when too much pressure is applied. Since the early 19th century, structural

engineering has tried to explain ruptures that occur even without an excessive force on

an object, in cases where structural failure was not predictable. This has lead to studies

on the “fatigue” of metals (9), and - to our purposes - of titanium in particular.

2. Medical-legal implications and literature review.

Our introductory remarks have implications for dental practice and forensic dentistry. The

key issue is to be able to estimate most accurately the average duration of rehabilitations

and of prosthetic and implant renewals. In civil litigation, a medical and legal interest

emerges in relation to the principle of compensability for future damages. Since only truly

predictable damage is compensable, in the hypothesis of mere possibility no

compensation ensues. So, when we come to dental impairment compensation, it

becomes crucial to consider not only the expenses related to immediate prosthetic



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restoration, but also those expenses related to inevitable substitution of devices at the

end of their presumable optimal duration (1) (8) (10) (11).

Implant techniques have provided a great contribution to the resolution of dental

damage, especially in cases of multiple dental loss (whereby earlier standards would

have relied exclusively on partially or fully removable devices, usually poorly efficient and

often badly tolerated by patients) and in cases of single dental loss (1) in young

individuals.

The positive and promising results (1) (12) (13) (14) (15) that were achieved since

1909 (9) (16) have led to careful considerations, from a medical and legal perspective,

about their possible use in compensating and reducing damages to patients, and about

dentists’ responsibility in cases of failure (17), with distinction between accident and

error. The crucial issue, however, is the ability to establish a certain and accurate

quantification of the life expectancy of a single fixture. Clinically, the scientific literature

review does not come to a consent. The available data do not always match with each

other and are often based on different time frames and come from different assumptions.

Very few longitudinal studies are available for periods longer than 10 years: many of

them simply consider follow-ups of 3, 5, or 6 years maximum. In part, this is due to the

fact that some of the devices studied have become quickly obsolete, as the field itself has

rapidly progressed (7) (17) (18) (19) (20) (21) (22) (23) (24). Nonetheless, the literature



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shows “unsuccessful” cases (24) together with both premature and late “failures.” A

precise distinction seems to be difficult between “unsuccessful” cases due to “technically

wrong treatment” and cases of “functional and stability loss” after a reasonably long

period of functional time (2) (3) (4) (5) (7) (25). The literature also documents both early

and late instances of peri-implant disease (25) (26), with consequent fixture loss.

Excluding cases of obvious technical errors, clinical and scientific evidence has led

to support the hypothesis that over time a percentage of loss of the implant bone support

can occur anyway. This, however, tends to occur mainly in the first twelve months of the

fixture life. Later on the prevalence of the phenomenon is not statistically significant (2)

(3) (4) (5) (6) (7) (8) (9) (17) (25) (26), with a “reabsorbing” factor less than 0.1 mm per

year since the second year. Thus, the loss of 4 mm of support should theoretically occur

over the course of 36 years (8) (17) (18) (23) (24), time usually exceeding most of

clinical follow-ups.

Clinical reports on early failure in terms of loss of stability and function are

referred after four to six months after surgery in cases of fibrous tissue ensuing in place of

bone tissue in missed osteo-integration. According to some Authors (26) a new

procedure may be possible in the same location after 3 – 6 months. Implant failures are

reported as a later occurrence after functional loading (2) (3) (4) (5) (7) (25), especially

with the so called "immediate loading" techniques (27) (28) (29).



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Recent statistics seem to confirm the highest occurrence of implants removal in the

same year of insertion (5-10% of cases). (2) (3) (4) (5) (6) (7) (8) (9) (17) (18) (19) (20)

(21) (22) (23) (25) (26) (30). The percentages of failure would thus be diminished

significantly with time. Vice versa, if we consider the issue from the perspective of “so-

called successes,” the literature shows clinical successes after 5-10 years for cylindrical

osteointegrated implants, at a rate of 90-95% in the maxilla and 95-98% in the lower

jaw. Swedish researchers (1) (2) (3) (4) (5) (7) (8) (9) (12) (17) (26) have reported a

success rate higher than 80% in the maxilla and 90% in the mandible after 15 years.

Other studies report rates of success - or “survival” - ranging from 75% to 99.8% in a

time interval between 3 and 5-6 years (1) (7) (18) (19) (20) (21) (22) (23). It is easy to

note the lack of shared evaluation standards and, therefore, of definitive conclusions.

The implant “survival” rate does not appear uniform, and rather changes in

relation to the different time frames of different studies, to the functional charge, and to

the statistical analysis methods used (2). There is a reasonable doubt that the positive

results found in the short-term could be falsified by a longer term approach and by a

higher percentage of success among implants situated more recently (2). To conclude,

on average, according to the statistical data in variable intervals between 3 and 10



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years, the implant survival rates range from 70% and 98% (2) (3) (4) (5) (6) (7) (8) (9)

(17) (26) (28) (29) (30).

Such results, without absolving the need for the clinician to operate correctly

(correct diagnostic choices, non-traumatic operative techniques, correct functional

loading, absence of abnormal mechanical stress, careful selection of the implant site and

correct actualization of the implant-supported prosthetic device), have led some

researchers (8) (17) to suggest – at least theoretically - a virtually unlimited survival of the

implant after the first year.

3. Further Considerations

In light of the aforementioned considerations and of the clinical and bibliographic data

available, it is only natural to wonder whether the assumptions above can still be agreed

upon. To date, “implant success” has been evaluated on the basis of data coming from

statistically “short-term” observations for each methodology. The data reported in the

scientific literature normally come from samples selected among ideal cases, and

performed by highly specialized clinicians. It might be necessary, then, to adjust the

findings to take into account the setting of typical office practice (to avoid the risk of

overly deterministic evaluations).



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First of all, some authors have related their evaluation of implant success to a medical

evaluation table (8) (26), where specific elements were considered. Generally speaking,

it is appropriate to claim a clinical success if:

1. The implant remains immobile during clinical tests.

2. radiology show no signs of peri-implant transparencies.

3. patients report no pain, and there are no infections or neurological lesions.

4. bone loss is less than .2 mm per year, after the first year (during which it could be up

to 1-2 mm).

According to these criteria, the percentage of success could result on average around

85% after 5 yeas and 80% after 10 years.

Certain clinical approaches, based on merely intuitive factors, unsupported by published

longitudinal studies, but only by personal experience, have led to believe that the

removal of a fixture, at the end of its hypothetical duration of activity, cannot be followed

by a second implant rehabilitation in the same site, because of alleged bone loss. This

would exclude the possibility of renewals in the same site. On the contrary, others cite the

validity of better techniques of bone guided regeneration and the fact that more recent

implant techniques cause minimal bone sacrifice. These two positions are clearly in

contrast with one another, and the former is clearly reductive and penalizing.



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All these considerations, however, do not help answer questions about where to draw the

line between a so-called “natural loss” of an implant insert and a “failure,” or between a

“complication” and a “mistake” (in case of professional responsibility) or about the

duration of an implant.

5. From a mechanical perspective

A dental implant, like an orthopedic prosthesis, is an artificial and heterogeneous

element to the organism. Even when it is «state of the art», it is nonetheless a foreign

body. Several precautions are therefore necessary surgically (e.g., correct planning and

insertion, adequate load, etc.) and in terms of management (prophylactic care, dental

hygiene, etc.). Such precautions, which are undoubtedly more critical than in the case of

natural elements, are related to the peculiar variety of biological reactions to the foreign

body. Clinical data suggest that complications might ensue even after the «one year

mark» (2) (3) (4) (5) (6) (7) (8) (9) (15) (17) (18) (19) (26) (31).

The use of a prosthetic implant is founded upon the principle of a potential “intimate”

connection arising between bone structure and the artificial object, thus leading to the so

called “osteointegration” (11). Several implant structures and systems are nowadays

available to the dental profession, which are different from each other in terms of shape,



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structure, material, surface characteristics and properties, diameter and length (1) (2) (3)

(4) (5) (6) (7) (8) (9) (17) (18) (19) (20) (21) (22) (23) (26) (32) (33) (34), all of which are

supposed to interact “intimately” with bone tissues.

To date, clinicians can choose among more than 1,300 types of implants (1), even if

most of them follow the principles of osteointegration (a.k.a. osseointegration) according

to P. I. Brånemark (11) or its conceptual outcomes, thus ensuring - at least theoretically -

a certain methodological flexibility and the possibility of «tailored» choices, appropriate

to individual cases.

Despite its specific typology, each inanimate implant structure comprises of two

elements: the implant itself (submerged partially or totally under the mucous and/or bone

level), a mesostructure (comparable to the natural abutement) and a suprastructure,

either screwed or cemented, comparable to classical fixed prosthetic rehabilitations

(crowns, bars, o-ring, etc.). The guidelines regarding the average duration of fixed

prostheses in metal fused porcelain or in metal acrylic, or those regarding connecting

bars on natural teeth, are applicable to the implant suprastructure just as well (8) (17). In

the case of legal compensation, the average renewals are awarded every 5-8 years -

depending on the author - for most of removable appliances and metal-acrylic crowns,

and every 12 years for metal-fused porcelain or plain ceramic structures (8) (11) (17).



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The object of contention, however, is the issue of “duration” or useful time of service of

the endosseous insert (35). Taking for granted the need to operate correctly and the

improvement in techniques and materials, many considerations are still based on a

practical evaluation of the results and on clinical experiences, even if clinical data should

be scientifically compared to statistically significant data (1) (2) (3) (4) (5) (6) (7) (8) (9)

(17) (26). However, a longitudinal comparison of the failures and/or of the functional

end of service of the implants lacks of strict statistical evidence (because of the lack of

longitudinal data), even when we exclude those challenges related to biocompatibility,

individual response, and cases in which professional negligence is involved.

It is therefore very difficult to differentiate between problems of biocompatibility

(biological), defective materials or dynamic/functional problems (mechanical) (1) (2) (3)

(4) (5) (6) (7) (8) (9) (17) (26).

Theoretically speaking, any prosthetic element substituting an anatomical unit (as is the

case of teeth and related tissues) should have the same functional and dynamic qualities

of the element it replaces. Functional stimuli on the natural tooth, however, are

amortized by periodontal tissues, which dissipate the pressure. Even though implants are

osteointegrated (and precisely because of it), they have a “rigid” relation with the tissue

surrounding them; as a result, pressure distribution is obtained differently from how it

occurs around a natural tooth (1) (9). The amortizing elements, which are present in



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certain implant systems, certainly help improve the distribution of forces, but once the

implant receive a functional force it responds to the stimulus in a manner similar to the

natural tooth, but not identical (hence the importance of correctly positioning the implant

to minimize anomalous stimuli and excessive forces) (1) (36) (37).

Bianchi (38) in 1999 argued that it is impossible to know the maximum strain limit to an

implant, even though important researchers (39 (40) claim that loss of contact between

osseous lining and implant surface depends on the nature and amount of functional

force beyond physiological range (overloading).

This would favor bone resorption and the creation of a preferential path for resident

microbial flora. These elements would suggest that more research is needed on the

parallelism among inserts, elimination of parafunctional (non-physiological timing of

functional stress) or their control through bite plane appliances, the use of double

crowns, bar or button type connections (39) to remedy unfavorable anatomical

conditions. These theoretical considerations must be related to the dimensions, local

morphology, bone quality and – last but not least — the type of patient to receive a

prosthesis (hygienic and eating habits, smoking, medical conditions, immune system,

parafunctions, occlusion, etc.) (37).

Moreover, clinical experience does not exclude even cases of macroscopic factory

construction defects of the implant inserts, which can be observed through simple



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microscopic studies1. Therefore, problems arising from material defects, even apparently

unjustified, cannot be excluded. Manufacturers (9) (39) (40) do not exclude such

possibility, and pursue further research themselves.

6. “Fracture Mechanics” and intrinsic material limits

Generally speaking, we are interested in certain considerations from an engineering

standpoint emerging within the field of “fracture mechanics” (9). In general, the duration

of a mechanical device, regardless of its purpose, depends “intimately”2 on the “strain”3

to which it is exposed.

Such pressure can lead to a [mechanical] failure over the course of time: how long,

depending on the intensity of the pressure itself. Environmental factors are decisive, and

product defects - even when microscopic - can intensify local “pressures” and reduce the

life of a component. These considerations can radically affect a designer’s approach in

response to all possible mechanical problems. The presence of mechanical [material]

defects can be tolerated if its outcomes can be predicted and/or controlled.

1 In one recent occasion, such evidences have made a well-known implant factory in Bologna to recognize a factory defect, ensuing a civil compensation to a local dentist for an implant neck rupture that occurred shortly after positioning, still without any reasonable technical explanation. 2 From a technical point of view it means that mechanical properties of an object are intrinsically determined by its "intimate" nature, actually based on its own atomic structures.

3 Il The term “strain” in building sciences means the relation occurring between a smallest surface and the force applied to its barycentre



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Therefore, implant components might also not have an infinite life (9). It is necessary to

accept a “damage tolerant” approach. Overall, we must recognize a priori that there are

intrinsic limitations to a mechanically inert material’s (regarding metals) ability to resist

stimuli, and we must recognize the possibility of delayed failure of components.

It is important to highlight the fact that when a structural element (implant) is

subjected to repeated stimulation, some “breaks” 4 might emerge after a certain amount

of exercise time, even when the stimulus level is much lower than the elasticity threshold

of the material under consideration. Critical areas, even if microscopic, allow for those

phenomena that lead eventually to a “break.” It is possible to see “microbreacks” on the

smooth or irregular surface (when broken through fatigue) even in the absence of pre-

existing defects (even though longer pressure and exercise times are necessary), and due

instead to “irreversible sledges” along metallographic crystal planes positively oriented in

the stimulation area.

The repetition of this phenomenon, over a certain number of pressure cycles, determines

superficial alterations that over time become the basis for a fracture. Hence the notion of

fatigue as a phenomenon statistically correlated to breakage, even in cases of

concentrated “pressures.”

4 Il The term “breack”, from a mechanical point of view, stands for the point where the fracture starts.



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Bioengineering is trying to increase implant’s resistance to fatigue (correct design,

improved materials with better intrinsic qualities of resistance to thermal treatments, of

surface hardening, plastic deformation, etc.). Despite this research having produced

undeniable results already, fracture is still possible. The physical laws discussed above

apply to titanium as well, which therefore presents a limited life under pressure (9). For

titanium in particular, no defined fatigue threshold has yet been found (9).

Labs have conducted experiments with function simulators (9). Such experiments,

however, have not allowed to evaluate and consider all variables that might affect a

mechanical object in a natural setting. The environment does not respond in a time-

depending manner in those experiments, but rather in a way that is completely

independent from time.

These engineering findings have influenced even legislatively design guidelines.

Nonetheless, the ISO international policy, applicable to the implant sector, defines a

geometry of pressure application but does not provide any indication regarding

minimum pressures that can and must be born by an implant and its abutment. It is also

impossible to predict which real life pressure scenarios might be verifiable.

Finally, we note that in vitro simulator tests always aim at leading the sample to the point

of fracture. This is to determine a threshold, certainly higher than in clinical practice.

Therefore for all mechanical components, characterization requires a standardization of



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risk analyses, specifying which values are acceptable, and which must decrease as the

application becomes increasingly more critical. Overall, experimental research “tests”

extreme conditions, hard to actualize in everyday practice.

7. Final Considerations: Implants are forever?

The average exercise time of an implant is subject to a variety of biological and

mechanical factors, which are influenced by the correct positioning of the implant, the

patient’s overall wellbeing, his or her habits and personal hygiene.

Scientific literature often shows contrasting positions, and clinical data do not always

support each other.

According to some authors (2) (3) (4) (5) (6) (7) (8) (9) (17) (26), for instance, short

implants (7-8mm) would show higher rates of failure.5 On this basis, they argue that

longer implants last longer. It is important to remember that, theoretically speaking, the

principles of “construction mechanics” suggest on the contrary that two implants of

different length have the same likelihood of “mechanical failure,” assuming that they

have the same exposed side, or a component not strongly integrated (9). It is nonetheless

logical to presume, however, that a longer implant might have a larger “integrated” side,

5 We do not mean mechanical failures, related to implant's length, but overall clinical failures, as fixture losses before or immediately after prosthetic functional loading.



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and thus a higher likelihood to remain anchored in place enough to guarantee clinical

success (9).

Nasatzky et al (41) (42) argue instead that it is possible to use rather short implants (6-

8mm) successfully. In sum, at present, there seem to be no conclusive data and all the

hypotheses should be supported and confirmed by statistically significant mechanical and

experimental evidence.

It is not coincidental that manufacturers of implants or their distributors do not express an

opinion regarding the exercise time of the implant in the accompanying labels and

instructions, nor do they indicate a “presumed” generic time-frame.6

Everyday experience, however, attests of cases of old implant abutments being re-made

into prostheses even after three decades. Vice versa, there are also cases of removal of

recent inserts.

The qualitative improvement of materials and new and less traumatic implant and

prosthetic techniques (2) (3) (4) (5) (6) (7) (8) (9) (17) (26) might not allow us to

hypothesize that implants can last forever, but they certainly can assure us of longer

prospects for the exercise duration of the implants - so much so, that some researchers

6 As reported by net commercials (e.g.: www.dentalfind.com and www.plenitas.com).



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have seen their duration as comparable to that of the natural tooth’s root in the same

environmental and clinical patient conditions.

One of the co-authors (8) (11) (17), for instance, concluded some earlier works arguing

that there are no reasons that could explain why an insert, that has remained in place for

years without any bone loss, would suddenly and without obvious causes (such as a

change in pressure, degradation, trauma, loss of nearby bone structure, etc) start to lose

support and become mobilized.

The comparison to a natural tooth and its root apparatus is not random. The loss of the

implant insert would actualize through a loss of bone support and the event of mobility,

just as it happens in the case of traumatic or pathological loss in natural teeth.

It is important to remember, however, that in addition to microbial and traumatic

components, individual immune response also plays a role in periodontal disease.

Along these lines, we could hypothesize the existence of an analogous mechanism in the

case of implants (in the absence of obvious risk factors, such as changes in functional

occlusion and pressure, degradation, emergence of dysfunctional habits, changes in

hygiene practices) for the modality and prevalence of bone structure loss. This analogy to

a tooth root, however, does not allow us to express an evaluation regarding the life

expectancy of an implant insert. It is also not possible to establish a priori the “exercise

duration” or “biological life” of a natural tooth or its root.



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On the basis of these considerations, the interest of a medical examiner or forensic

dentist (31) in implant-prosthetic techniques appears justified. On the one hand such

techniques revolutionize our bases to estimate dental damage (43), but on the other hand

they present additional challenges of difficult resolution.

On the issue of allocating professional responsibility, we must cite the “technical rule”

active in the “historical” moment when the treatment has been administered.

Nonetheless, this rule does not clear out all questions, for different schools of thought and

technical practices often coexist, and it does not determine the likelihood of implant

survival (8) (26).

These clinical and mechanical considerations highlight the risk of error of judgment, and

they suggest that we should avoid too strict an approach or, on the contrary, too simple

absolutions.

While our claims do not offer a firm answer to the initial question, they create the bases

for more critical and reasoned further evaluations. Indeed, given these premises, the

problem of endosseous implant renewals acquires relevance. Even though an implant

duration cannot at present, given the state of the art, be assumed to be infinite (26), the

assumption reported in manufacturing companies’ product literature - of an average

duration of 20 years - also does not seem scientifically tenable. Incidentally, the claim of

a 20-year duration would add a notion of limited duration to the question of hypothetical



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damage. It follows logically that we cannot accept any prediction or estimate regarding

renewals, which should be performed only when needed and only in relation to the life

expectancy of the subject in question (44).

To conclude, while we acknowledge that implant material behave and are experienced as

foreign objects, we must first of all consider the fundamental principle of correct design

and application, and the notion that inserts share a common life path with the dental

structures they replace. We cannot assume that an insert “fails” or rather presents a “life

expectancy” shorter than the hypothetical life expectancy of the natural tooth it replaces.

At the same time, we must remember that loss is possible in any rehabilitation project,

and that the acceptability of such a loss is to be evaluated in relation to its risks.

Statistically, the possibility of fracture or breakage of any material (including prosthetic

materials) is never equal to zero, though with adequate precautions it can be minimized.



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implant survival: biological and mechanical … introductory remarks have implications for dental...

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