The influence of functional forces on the biomechanics of implant-supported prostheses—a review

Journal of Dentistry 30 (2002) 271–282 www.elsevier.com/locate/jdent

Review

The influence of functional forces on the biomechanics of implant-supported prostheses—a review Saime S¸ahin, Murat C. C¸ehreli*, Emine Yalc¸ın Department of Prosthodontics, Faculty of Dentistry, Hacettepe University, Ankara, Turkey Revised 3 October 2002; accepted 16 October 2002

Abstract Objectives: To evaluate published evidence related to the influence of functional forces on the biomechanics of implant-supported prostheses. Data and sources. The literature was searched for original research articles relating control of loads on dental implants, effects of early and late occlusal loads, the influence of bone quality, prosthesis type, prosthesis material, number of supporting implants, and engineering techniques employed for evaluating mechanical and biomechanical behavior of implants using MEDLINEw and manual tracing of references cited in key papers otherwise not elicited. Study selection. Current literature on implant biomechanics as main focus and pertinent to key aspects of the review. Conclusions. The outcome of implant treatment is often maximized when implants are placed in dense bone, number of supporting implants are increased, implant placement configuration reduces the effects of bending moments, and when a fixed prosthesis is delivered to the patient. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Biomechanics; Dental implants; Occlusal force; Fixed prosthesis; Overdentures

Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Biological effects of location and magnitude of applied force. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Occlusal forces following implant treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Effects of prosthesis type, prosthesis material and implant support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. The influence of bone quality and properties of bone-implant interface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. Immediate or early implant loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7. Comparison of engineering methods used to evaluate the biomechanics of implants . . . . . . . . . . . . . . . . . . . . . . . . 8. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Since the preliminary studies on osseointegration, dental implants have been extensively used for the rehabilitation of completely and partially edentulous patients over the last three decades [1 –6]. Despite the high success rates reported by a vast number of clinical studies, early or late implant failures are still unavoidable [7]. Late implant failures are * Corresponding author. Tel.: þ 90-312-229-9669; fax: þ 90-3123113741. E-mail address: [email protected] (M.C. C ¸ ehreli).

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observed after prosthesis delivery and are mainly related to biomechanical complications. Yet, the mechanisms responsible for biomechanical implant failures are not fully understood and the literature concerning the influences of several biomechanical factors are inconclusive [8]. There is a consensus that, the location and magnitude of occlusal forces affect the quality and quantity of induced strains and stresses in all components of the bone-implantprosthesis complex [9 – 18]. When evaluating the biological effects of an applied load, it is essential to determine its source. An implant-supported prosthesis may be under the influence of external (functional or parafunctional forces)

0300-5712/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 3 0 0 - 5 7 1 2 ( 0 2 ) 0 0 0 6 5 - 9

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and/or internal (internal or external preload) forces [11,19, 20]. Qualification and quantification of these forces on implants and in bone is required to understand the in vivo behavior of these devices. So far, in vivo forces on implants have been measured only at the abutment level [9]. Since intraosseous strains in the vicinity of implants have not been measured by means of biosensors, strain gradients that guide bone modeling and remodeling processes around implants are unknown. Currently, strain measurements in bone around implants are undertaken by theoretical models implemented with in vivo data or experimental in vitro models [16,20]. Yet, it is not truly known whether the results of many studies really mirror the in vivo biomechanical characterization of implants. Because correct evaluation of forces is often a perplexing problem and a challenge to resolve due to several accompanying parameters involved in experiments, correct in vivo isolation of forces in the vicinity of implants are always avoided. As a result, obtaining an undisputed scientific proof becomes virtually impossible. In all incidences of clinical loading, occlusal forces are first introduced to the prosthesis and then reach the boneimplant interface via the implant. So far, many researchers have, therefore, focused on each of these steps of force transfer to gain insight into the biomechanical effect of several factors such as † † † † † † † †

Table 1 Factors influencing load distribution on implants Geometry, number, length, diameter and angulation of implants Location of implant(s) in the arch Type and geometry of the prosthesis Prosthesis material Superstructure fit Location, direction and magnitude of applied occlusal forces on the prosthesis Condition of the opposing arch (prosthesis versus natural dentition) Mandibular deformation Bone density Age and sex of the patient Stiffness of food

bone which are theoretically the same in magnitude, but in opposite directions. During clinical loading of an implant, the direction of forces almost never coincides along its central long axis, providing an absolute axial loading. On the contrary, the occlusal force is applied at different locations and frequently, in a direction that creates a lever-arm, which causes reacting forces and bending moments in the bone [19,23] (Fig 1). This

force directions, force magnitudes, prosthesis type, prosthesis material, implant design, number and distribution of supporting implants, bone density, and the mechanical properties of the bone-implant interface.

The aim of the review is to take these key elements and review the current knowledge about the influences of functional forces on the biomechanics of dental implants. Areas where further research is needed will be highlighted.

2. Biological effects of location and magnitude of applied force There are several factors that affect force magnitudes in peri-implant bone (Table 1). The application of functional forces induces stresses and strains within the implantprosthesis complex and affect the bone remodeling process around implants [21,22]. Yet, the physiologic tolerance thresholds of human jawbones are not known and some reported implant failures may be related to unfavorable stress magnitudes. The application of an external load on an implantsupported prosthesis induces stresses within the entire load-bearing system and stress reactions in the supporting

Fig. 1. Absolute axial loading (AL) provides even loading of implants. Laterally positioned axial loading (LL) and oblique loading (OL), however, create bending moments that cause unfavorable stresses in the gold screw (g), the abutment screw (a), and in and around the implant (i).

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bending moment is the force times the orthogonal distance between the force direction line and the counter-acting support. The longer the distance, the greater will be the bending moment [24]. Accordingly, the fraction of force transmitted to implants and the induced stresses are dependent particularly on where the load is applied on the prosthesis [14,17]. For instance, considering that two vertically placed implants supporting a fixed prosthesis is axially loaded from the middle, equal load partitioning is expected between implants. If the load is applied only on one implant, it will bear the entire load with a potential apical movement. Cantilever loading will result in a dramatic increase in load transferred to the implant neighbouring the cantilever [14,16,18,24 –27]. Hence, it is imperative to establish an equilibrium between acting and counter-acting forces. During functional loading, however, implants may not always reach this vital requirement and may fail. Studies on bone biology suggest that implant overloading may lead to implant failure. When overloaded, high deformations (above 2000 –3000 microstrain) occur in bone surrounding implants [28]. When pathologic overloading occurs (over 4000 microstrain), stress and strain gradients exceed the physiologic tolerance threshold of bone and cause micro-fractures at the bone-implant interface [25,29]. While overloading may be manifested by the application of repeated single loads, which causes micro-fractures within the bone tissue, continuous application of low loads may also lead to failure, namely, fatigue fracture. Excessive dynamic loading may also decrease bone density around the neck of implants and lead to crater-like defects [30]. Accordingly, overload-associated implant failures have been reported following the first year of prosthodontic treatment [31]. In experimental animal studies, similar findings have been reported. For instance, Hoshaw and co-workers [32] reported that overloading of implants resulted in an increased bone resorbtion around the implant collar, and a decreased percentage of mineralized bone tissue in the cortex within 350 mm of the implant was evident after 12 weeks of load application. In other studies, early signs (1 – 4 weeks) of implant overload in Macaca fascicularis monkeys resulted as an absence of gross bone loss [33], but loss of osseointegration was observed 4.5 – 15.5 months after occlusal overload was commenced [34]. Marginal bone resorption may also be related to the lack of mechanical coupling between the machined coronal region of the implant and the bone, which avoids effective transfer of occlusal forces from the implant to the cortical bone. The extremely low intraosseous strains ( ø below 100 microstrain) thus cause bone resorption due to disuse atrophy [35 – 39]. In this context, implant surface has a crucial role; increased surface roughness balances bone apposition and remodeling at the bone-implant interface. Indeed, implant surface topography controls stress and strain magnitudes at the interface [37,39]. If the surface is

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rough, the total area used to transfer occlusal forces to the bone increases. Eventually, lower stresses and strains can be achieved in the vicinity of the implant. Roughsurface implants also provide better mechanical interlock with the bone in comparison with machined-surface implants [40,41]. Hence, implants with smooth surfaces have an inherent potential of experiencing debonding with bone, which leads to bone resorption due to stress-shielding [39]. Since greater amount of bone loss around total hip prostheses were observed within the first 2 years [37], stressshielding may be an important factor leading to marginal bone loss around implants, particularly within the first year of oral function. Overall, it is evident that force magnitudes around implants affect bone reactions. Although there have been some attempts to explore bone differentiation around implants so far, one can only understand the influence of load factors on bone when its reactions are examined with regard to tissue strains induced in the vicinity of loadcarrying implants. Prostheses supported by one or two implants replacing missing posterior teeth are subjected to an increased risk of bending overload [42]. There are a number of safety measures that may be employed during treatment such as increasing implant support [43] or using staggered implant placement. The philosophy of so-called tripodization (or staggered implant placement) was based on the aim of reducing bending moments when utilization of more than two implants is provided within a prosthesis [44,45]. Indeed, the rationale for staggered implant placement appears to be beneficial over in-line placement and has garnered wide-acceptance. However, staggered implant placement does not always compensate for the tensile forces at the fixation (prosthetic) screw [46]. Yet, this subject is also not understood in detail and needs further evaluation. Perhaps, strain-gauge analysis and finite element stress analysis may be helpful to enlighten the effects of these clinically relevant parameters. However, we should consider that this treatment option was initially created for Bra˚nemarkw implants, which have a butt-joint implant-abutment connection (Fig. 1). In this design, the abutment screw is the only element that keeps the implant and the abutment assembled. This property makes the design inherently weak to bending moments. In internalcone implants, i.e. ITIw and Astra Techw implants, however, friction plays a crucial role in the maintenance of screw-joint integrity in addition to the torque (preload) applied during abutment tightening. These fundamental differences in design affect the mechanical behaviors of implants. Tripodization has never been considered as a treatment option for rehabilitation of missing teeth with ITIw implants. Two ITIw implants can carry a three-unit fixed partial denture for several years without any significant episodes of biomechanical complications. Therefore, before accepting tripodization as a ‘must’ for the treatment of partially edentulous archs, one should explore whether it is really essential for all implant systems.

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3. Occlusal forces following implant treatment For dentate humans, the maximum biting force varies between individuals and different regions of the dental arch [47,48]. Maximum bite forces depend on the capacity of supporting tissues to tolerate force and the mental condition of the patient during force measurements [49]. The greatest maximum biting force reported to date is 443 kg N [50]. Dentate patients have 5 –6 times higher bite force than complete denture wearers [51]. Present evidence based principally on static force measurements indicates that, the average biting force is 100 –150 N in adult males, and males have higher biting force than females [47]. Raadsheer [52] reported maximal voluntary bite forces as 545.7 N in men (n ¼ 58) and 383.6 N in women (n ¼ 61), and the maximum biting force measured was 888 N in men and 576 N in women. Patients with implant-supported fixed prosthesis have a masticatory muscle function equal to or approaching to that of patients with natural teeth, or with tooth-supported fixed partial dentures [53]. Placement of a mandibular fixed implant-supported prosthesis in complete denture wearers improves masticatory function and the magnitude of bite force [54 – 56]. Haraldson and Carlsson [56] measured 15.7 N for gentle biting, 50.1 N for biting as when chewing, and 144.4 N for maximal biting for 19 patients who had been treated with implants for 3.5 years. In another study, Carr and Laney [57] reported maximum bite forces between 4.5 and 25.3 N before and 10.2 – 57.5 N after three months of treatment with implant-supported prosthesis, and emphasized that, the amount of increase was dependent on the duration of being edentulous. Forces on implants are also dependent on the location of the implant in the dental arch. Mericske-Stern and Zarb [58] investigated occlusal forces in a group of partially edentulous patients restored with ITIw implants supporting fixed partial prostheses and measured an average value of maximum occlusal force lower than 200 N for first premolars and molars and 300 N in second premolars. These data suggest that implants placed in the posterior region of the mouth are at greater risk for overloading. Therefore, the use of wider and longer implants may be recommended for implant treatment in the posterior region [59 –62]. Nevertheless, in most situations, occlusal forces are somewhat decreased due to age-related deterioration of the dentition [47]. However, marginal bone resorption occurs regardless of the force magnitudes applied on implants, location of implants in the dental arch, and implant design [63 –65]. Because the biological effects of maximum bite forces on implants is unknown, current data dealing with bite forces do not help to understand factors leading to marginal bone loss. The loading history of implants and the time required for accommodation of bone cells to implants may be the influencing factors [66,67]. These parameters need to be studied by quantifying

time-dependent bone reactions around implants subjected to controlled loads.

4. Effects of prosthesis type, prosthesis material and implant support The type of prosthesis affects the mode of implant loading. In cement-retained implant restorations, the occlusal surface is devoid of screw holes and the occlusion can be developed that responds to the need for axial loading. Screw-retained fixed prosthesis or overdentures, however, are subjected to off-set loads that cause a substantial increase in bending moments [68 – 70]. Only a few studies appear on related literature and there are controversies. A comparative in vivo study on axial and bending moments on maxillary implants supporting a screw-retained fixed prosthesis or an overdenture revealed that, force application on an overdenture resulted in lower compressive force, but higher bending moments on abutments during function when compared to a fixed prosthesis [68]. Mericske-Stern and collaborators [13] also registered forces on implants supporting one-piece full-arch fixed prosthesis and barretained overdentures in the maxilla. They concluded that, the type of prosthesis did not have a determining effect on force pattern. However, in overdenture treatment, the resorption pattern of the maxilla affects positioning of the implants and the denture teeth. Since the positioning of denture teeth frequently creates an anterior or labial cantilever, which acts as a long lever-arm, high bending moments are created on maxillary implants. This situation may explain why implant survival rates are significantly lower in the maxilla, particularly with overdenture treatment [71 – 75]. Hence, from a biomechanical aspect, rehabilitation of the edentulous maxilla with implantsupported overdentures is probably one of the most challenging endeavors that faces the restorative dentist. In overdenture treatment, since a wide range of attachments are utilized, the detection of forces may also depend on the number of attachments that affect the number of rotational axis of the prosthesis. Factors that affect loading patterns also include incorporation of an internal metal frame (acrylic resin denture base versus chromium cobalt substructure), rheological properties of the foodstuff and framework fit [76,77]. Regardless of its design, an implant-prosthesis complex transmits occlusal forces to the peri-implant bone [78 –80]. The force absorption quotient of the prosthesis material has, therefore, been a topic of research interest. Skalak, envisaged that, the use of acrylic resin teeth would be useful for shock protection on implants [78] and Bra˚nemark and co-workers [79] have also recommended the use of acrylic resin as the material of choice for the occlusal surfaces of implant-retained prostheses. The resiliency of this material was suggested as a safeguard against the negative effects of impact forces and microfracture of

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the bone-implant interface. The literature, however, is inconclusive on its effect on shock absorption [81 – 86]. In fact, acrylic resins are burdened with technical and subjective disadvantages. For example, due to their low wear resistances, premature contacts often occur after several months of prosthesis delivery. On the other hand, gold and porcelain surfaces are believed not to provide force absorption, but they are also frequently used. Although the choice of prosthesis material still remains as a topic of controversy and argument, there is a consensus that it does not have any influence on implant survival [87]. The number, length, diameter and positioning of implants also have an influence on force transfer and subsequent stress distribution around implants. The increase in number, length and diameter of implants improve the biomechanical behavior of implants, especially when subjected to bending forces [15,43,88 –90]. Duyck and coworkers [91] explored the distribution and magnitude of occlusal forces on implants carrying fixed prostheses when supported by 5 –6 and 3– 4 implants. Higher forces were observed with a decreasing number of implants. Bending moments were highest when three implants were used. Loading of the extension parts of the prostheses caused a hinging effect, which induced considerable compressive forces on the implants closest to the location of load application and lower compressive or tensile forces on other implants. The result of this in vivo study is not surprising, because the fraction of force that implants bear in similar situations was already calculated 10 years ago by Osier [27]. Nevertheless, its clinical relevance towards treatment outcome is questionable and requires further research. Since 10-year survival rates of fixed prosthesis supported by 4 or 6 implants [92], or three wide-diameter implants as introduced with the Bra˚nemark Novum Systemw (Nobel Biocare, Go¨teborg, Sweden), are quite high [93], the number of implant support may not have a remarkable effect on treatment outcome. However, we should also take into account that, a recent prospective clinical trial and in vivo force measurements on Novum Systemw implants revealed that, the amount of crestal bone loss around distal implants was not promising [94]. Overall, these clinical data suggest that the more the supporting implants, the safer the treatment may be. For three unit fixed partial dentures, the use of three implants in in-line configuration is believed to decrease stress concentrations in comparison to two terminal implant support [89]. On the other hand, it may not affect treatment outcome in the rehabilitation of partially edentulous jaws. The efficacy of staggered placement of three implants on reducing bending moments has also not been substantiated by clinical research and there is only a small pool of knowledge on this issue. Rangert et al. [42] reported the incidence of fractured Bra˚nemarkw implants as low. Of these, 90% occurred in the posterior region, the prostheses were supported by two implants, all patients with fractured implants were diagnosed to have parafunctional activities,

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and all implants were 3.75 mm in diameter. However, there is no report on ITIw standard 4.1 mm diameter solid screw implant fracture in literature. Therefore, the use of two wider implants for the treatment of three missing occlusal units may be an alternative to tripod design. Since fixed partial prosthesis in partially edentulous cases does not benefit from cross-arch stabilization, more bending moments are expected. However, conditions of opposing arch may also affect the magnitude and direction of bending forces such as a fixed partial denture opposing a complete denture [95]. The results of these studies suggest that, the mechanical characterization of implants have a great impact on treatment outcome. Comparative clinical trials are thus indicated to explore the effects of supporting implants, giving particular emphasis on the effects of implant design.

5. The influence of bone quality and properties of bone-implant interface Bone is the structural foundation for a load-carrying implant. Bone surrounding implants may be composed of woven, lamellar, bundle or composite bone, which depends on the age, functional status and systemic factors of the patient. When a commercially-pure titanium implant is installed in bone, a bridging callus which has minimal loadcarrying capability originates from the bone surrounding the implant, and a lattice of woven bone reaches the implant surface approximately in 6 weeks [96]. The woven bone is often not completely replaced by mature and load-bearing lamellar bone at 3 –6 months following implant surgery [97, 98]. A fibrous tissue interface exists at 1 month following implantation, an average of 50% bone-implant contact at 3 months, a 65% bone implant-surface at 6 months and an average of 85% bone-implant contact after 1 year following placement of a machined-surface implant [99]. Healing response subsequent to implant placement is characterized by an increase in interfacial bond strength and bone-implant contact, which improves the mechanical behavior of the interface [100]. The interface stiffness, which is accepted as a ruling factor for implant survival, has more than a doublefold increase in 3 months in dogs that correspond to a 4 –6 month healing period in human mandibles [101]. One of the most significant factors that affect the outcome of the implant treatment is the quality of the bone around implants. The increase in bone density improves the mechanical properties of the interface. Implants are demonstrated to have less micromovement, increased initial stability, and reduced stress concentrations in high density bone [102,103]. In addition, knowing the distribution of bone quality in various jaw regions assists the clinician in dental implant treatment planning. Bone quality types 1 and 4 are found much less frequently than types 2 and 3 [104]. Although variations in density exist in each region, quality 2 bone dominates the mandible, and quality 3 bone is more prevalent in the maxilla. Both anterior and

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posterior jaw regions are often characterized by types 2 and 3 bone. The anterior mandible has the densest bone, followed by the posterior mandible, anterior maxilla, and posterior maxilla [105]. From a biomechanical point of view, although 70% bone appears to withstand functional forces [87], it is believed that implant survival rate is directly proportional to the bone density [106]. However, Truhlar and co-workers [107] reported that among 2,131 implants, quality 1 bone experienced the greatest failure rate, whereas quality 2 and 3 bone had the lowest incidences of implant failure. According to Bahat [108], the quality and quantity of bone do not have a significant effect on implant survival, but the surgical techniques are more important. Many clinical studies have focused on the success of endosseous implants with a variety of surface characteristics and to clarify the osseointegration process. In early 90’s, hydroxylapatite (HA) coated implants have been widely used to improve initial stabilization of implants and to increase bone-implant contact for treatments in low-density bone. Following immediate placement, HA implants have better bone-implant contact than titanium plasma-sprayed (TPS) implants after 2 months of healing [109], but their cumulative survival rates are relatively low when used for overdenture support [110 – 112]. This may depend on local and systemic factors. Although the HA coating does not need to stay for longer than 1 year [113], dissolution or mechanical failure of the HA coating has been reported, which was attributed to the crystallinity and thickness of the coating [114 – 116]. HA-coated implants may have better long-term prognosis in low-density bone and when placement of shorter implants are required [117,118]. Alterations in biomaterial surface morphology and roughness have been used to improve tissue response and the mechanical properties of the bone-implant interface. Although the results are encouraging, there is a large inconclusive literature on their clinical effects. In a recent study conducted by Carr and co-workers [119], commercially pure titanium, titanium alloy, and TPS implants placed in baboons after 6 months of healing demonstrated that bone-implant contact and percent bone area in maxilla (50.8, 43.6%) was lower than the mandibula (60.8, 52.6%). The biomaterial analyses, however, revealed no significant differences. In a comparative histometric analysis of boneimplant interface between a rough titanium surface and smooth implants in low-density human jawbone after 3, 6, and 12 months of submerged, undisturbed healing, the rough implant had significantly higher bone contact in comparison to the smooth implant [120]. Like-wise, sandblasted large grid acid-etched (SLA) titanium implants have also demonstrated greater bone-implant contact than TPS implants [121,122]. Overall, the earlier-mentioned studies suggest that implants with rough surfaces have more bone-implant contact, which increases interface stiffness. Indeed, this may improve implant survival. Nevertheless, the clinical relevance of these studies is also questionable. As mentioned previously in this paper, comparative clinical

studies between machined- and rough-surface implants reported similar marginal bone levels [63 – 65]. Hence, the very nature of implants does not appear to have any influence on marginal bone loss as well as the implant survival rate. The loading history [66,67] and the type of force (static versus dynamic) applied on implants [30,123] are probably more important. Despite a number of animal studies on the effects of non-passive superstructures on bone response [124,125], it is a well-known fact that static forces have little or no effect on bone tissue [123]. On the contrary, dynamic forces affect the form, mass and the internal structure of bone [35,36]. The biological effects of dynamic forces on bone reactions around oral implants have not been well-documented. Fundamental research is thus needed on biomechanics of peri-implant bone in well-controlled mechanical environments.

6. Immediate or early implant loading Osseointegration was based on a two-stage surgical protocol and it was considered crucial to avoid loading of the submerged implants during the healing period. However, the coincidental success of the first application of immediate (or early) loading [126] and consecutive research [127 –130] on fixed prosthesis have revealed that two-stage implants could be loaded in a relatively short period of time following placement only in the inter-foramina of the edentulous mandible to support a rigid permanent fixed cross-arch supraconstruction. Randow et al. [130] reported 100% success for immediately loaded implants after 18-month function and Horiuchi et al. [131] reported 97.2% success after a 8 –24-month follow-up period. Ten year survival rate decreases to 84.7% for immediately loaded implants [128]. This treatment option emphasized the fact that, the anterior mandible which is often composed of a highly dense bone had the inherent potential to provide adequate support and initial stability for early loading of implants. Accordingly, the ‘same-day treatment protocol’ followed for the Bra˚nemark Novum Systemw (Nobel Biocare, Go¨teborg, Sweden) comprised placement of majority of the implants (123 of 150) in bone quality 2 and provided immediate loading of implants in approximately 7.5 h [93]. The philosophy of this treatment was probably based on preventing micromotion of implants and distribution of functional loads with a rigid suprastructure. However, this treatment option does not offer many advantages. Recent experience with the Bra˚nemark Novum Systemw is not promising (personal communication of MC with Prof. Ignace Naert, Catholic University of Leuven, 2002). Failure of Novumw implants may be related to the timing of superstructure connection. In conventional immediate- or early-loading, the superstructure is usually connected within 3 weeks following implant placement. In the Novum Systemw, however, the prosthesis is delivered in the same day. Since the load-carrying ability and the micromotion resistance of the bone-implant interface

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depends only on the initial mechanical interlock between the implant and the bone, it is likely to have high micromotion [132] and stress gradients around the neck of implants. This may exceed the physiological tolerance threshold of bone particularly around distal implants. Indeed, excessive micromotion is directly implicated in the formation of fibrous encapsulation. The literature suggests that there is a critical threshold of micromotion above which fibrous encapsulation prevails over osseointegration. This critical level, however, is not zero micromotion as generally interpreted. Instead, the tolerated micromotion threshold was found to lie somewhere between 50 and 150 mm [132]. Lefkove and Beals [133] have applied early loads on four ITIw implants to support mandibular overdentures with bars and stated that a high level of predictability would be achieved when the technique was followed. Ledermann et al. [134] reported 60– 70% bone implant contact for 12-year functioning implants immediately loaded with bar-retained overdentures. This technique has over 95% success after 6.5 years of loading [135,136]. Recently, immediate loading of single-tooth implants has been reported [137]. Actually, it can be estimated that survival of single-tooth implants may also be high. Piatelli et al. [138] found 86.69% bone-implant contact in an immediately loaded single implant in man after 7 years of function and 60 – 70% for a TPS implant after 8 – 9 months of loading [139]. In an animal study, the boneimplant contact for early-loaded implants in the maxilla and mandible were 67.2% and 80.71%, respectively, [140]. As a sequel of immediate loading, a large part of the implant surface is covered by compact, mature lamellar bone with the presence of many Haversian systems and osteons. The bone at the interface with the implant is highly mineralized and connective tissue or inflammatory cells are not found [141]. These histological observations along with the results of clinical studies suggest that immediate loading of implants supporting full-arch one-piece fixed prosthesis, overdentures, and single-tooth restorations can be performed. There is an unavoidable evolution and rush for immediate loading of implants, which has an important impact on the psycho-social well-being of edentulous patients. To obtain high successes with immediately-loaded implants, it is essential to increase our knowledge on bone response around immediately loaded implants. Fundamental studies are, therefore, needed to elucidate mechanisms responsible for functional adaptation of bone to implants subjected to various loading regimens in order to control or avoid bone loss around conventionally loaded implants, and to provide predictable results for immediately loaded implants in man.

7. Comparison of engineering methods used to evaluate the biomechanics of implants When dealing with a complex stress analysis problem in which a complete theoretical solution may prove impractical with respect to time, cost or degree of difficulty,

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experimental techniques are often used. Current techniques employed to evaluate the biomechanical loads on implants comprises the use of mathematical calculations [46,142], photoelastic stress analysis [143], two- or three-dimensional finite element stress analysis [88,89] and strain-gauge analysis (SGA) [10,11]. Since an almost actual representation of stress behaviors can precisely be provided, threedimensional finite element stress analysis (3D FEA) has been introduced as a superior theoretical tool over twodimensional finite element stress analysis. 3D FEA and SGA have been extensively used to evaluate the biomechanical loads on implants for accurate clinical prediction. Generally, one of the major purposes of 3D FEA technique is to solve physical problems or to determine the effectiveness or behavior of an existing structure or structural component subjected to certain loads. The idealization of the physical problem to a mathematical model requires certain assumptions that lead to differential equations governing the mathematical model and, since the procedure is numerical, it is imperative to assess the solution accuracy. Additonally, the production of an appropriate and effective mathematical model is crucial to elucidate the physical phenomena, which requires the inclusion of comprehensive structural simulation [144 – 146] of dental implants, particularly for accurate quantification of induced stress or strain. The application of SGA on dental implants is based on the use of electrical resistance strain-gauges and its associated equipment, and provides both in vivo and in vitro measurement of strains under static or dynamic loads. Under an applied force, a strain gauge measures the mean dimensional change where it is bonded [9,10,14,147,148] or embedded [149]. The configurations of strain-gauges often used for implant biomechanics are uniaxial and/or rosette, and are usually bonded to implants, abutments and/or to rigid connectors of a prosthesis. [9,10,14]. Comparative studies have revealed that there are contradictions between data obtained from photoelastic stress analysis and in vitro SGA on the quantification of strains [143,149,150]. The application of 3D FEA and in vitro and in vivo SGA has provided mutual compatibility and agreement of obtained results [151,152]. However, in these studies, strain-gauges were bonded on the surfaces of solidlike structures and comprehensive finite element modeling was not included. Thus, it may be estimated that comparison of strains by both techniques may provide agreement on solid or undetailed structures, i.e. the surface of rigid prosthetic connectors, prosthetic retainers, cantilever extensions, and in or around bone surrounding implants [153]. However, the compatibility of these techniques are unknown when analyzing structures such as the internal hex or morse-taper of an implant body [153]. It is an undisputed fact that, onepiece finite element modeling is not the actual scenario for most commercially-available dental implants. Hence, for loading conditions i.e. lateral or oblique loading, specific parts of the implant – abutment interface will separate, or new

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parts which were initially not in contact will come in contact. Consequently, more deformation may be expected, especially at the neck of implants. In this regard, the pattern and magnitude of deformation will be influenced by the implant design [145,146]. In a three-dimensional finite element model (theoretical model), precise loading over predetermined points on the occlusal surface of a prosthesis can be accomplished. For in vivo or in vitro strain-gauge experimentation, however, this may not be provided due to several factors included in force transmission during load application by opposing teeth or by an apparatus. Placement of the gauges may have slight inaccuracies or the angulation of implants may not be as precise as in a theoretical model. Overall, the very nature of the physical experimental technique makes it inherently subject to random error. Currently, although SGA is the only technique that allows in vivo measurements during clinical loading, the results of in vivo and in vitro SGA do not agree on the quantification of bending moments [9,19]. Additionally, to determine the ‘actual’ amount of load on an implant complex in vivo, isolation of the strains on each implant abutment and/or component of the prosthesis prior and/or after the cementation or screw tightening and following clinical loading must be provided through several measurements. However, even with this approach, strains can only be recorded where gauges are bonded; measurements on abutment and gold screws cannot be provided. This can be measured only by 3D FEA which necessitates a comprehensive structural finite element simulation and non-linear contact analysis. Such finite element models offer the advantage of evaluating vital parameters like the effects of clamping force of the screws or the effect of the internal design of an implant collar [144 – 146], but since contact can also be defined between the bone and the implant, this technique offers several advantages for future biomechanical studies. For instance, dynamic time-dependent bone response to dental implants subjected to various loading conditions can be studied. As mentioned in the very beginning of this review, correct qualification and quantification of forces on implants are extremely crucial to understand the biomechanics of implants. Biomechanical studies should, therefore, be designed not only for descriptive purposes but also to offer reliable and accurate data that has clinical relevance. Contradictions between the results of many studies suggest that validation studies are indicated.

8. Conclusion A growing field of research is implant biomechanics due to the fact that many aspects of implant treatment are based on biomechanical principles. Some evidence exists on basic tenets of bone reactions to loaded implants, but information on the issue still remains scarse. Accordingly, the lack

fundamental studies on implant biomechanics coupled with bone biology has, in many ways, led to insufficient interpretation of the large pool of clinical data collected in the last three decades. Nevertheless, in the light of the current knowledge, it seems that treatment outcome is improved when implants do not bear excessive occlusal forces, implants are placed in dense bone, the number or diameter of supporting implants are increased, implant placement reduces bending moments, and when implants support fixed prostheses.

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