Platform Switching

INTRODUCTION

In aesthetic areas, the use of dental implants as replacements for lost permanent teeth remains an important challenge due to the difficulty of restoring the natural sulcus and papillary anatomical appearance around the implants. Despite the available technology, in some cases dental implants are unable to achieve optimum esthetic results. Correct location of the soft tissues in dental implant restorations depends on the preservation of bone crestal height. Consequently, hard tissues are the principal determinant of aesthetic outcome¹.The optimal position is at the center of the tooth to be replaced, 1.5 to 2.0 mm to palatal which, over time, ensures the maintenance of the buccal bone margin and mucosa margin stability².

Some studies suggest countersinking procedure during implant placement, implant loading and stress concentration on the cortical plate could be responsible for bone remodeling³.According to Lazzara, bone loss is considered a normal loss–a loss of between approximately 1.5 and 2 mm down to the first thread during the first year of loading⁴.Others studies indicate that such remodeling is the result of localized inflammation of soft tissue at the implant- abutment junction (microgap), as a biological seal becomes established^5,6. Once the biologic width has been established, the soft tissue takes on a protective function of the crestal bone.

Currently, dental implants with platform switching are considered to represent the newest concepts in avoiding crestal bone remodeling. it refers to the use of an abutment of smaller diameter connected to a implant neck of larger diameter; this connection shifts the perimeter of the implant-abutment junction inwards towards the central axis (the middle of the implant) improving the distribution of forces^4,7,8.The use of abutment with reduced diameter in relation to the implant platform can greatly reduce crestal bone loss to far less than 1.5 mm, a reference point of successful implant treatment after 1 year of loading, proposed by Albrektsson et al.⁹

BIOMECHANICS

The relationship between bone and implant has been the essence of osseointegration.^10,11 In this regard, marginal bone level changes have often been followed to interpret the outcomes of experimental and clinical studies.^12,3 It is perceived that initial bone turnover around an implant after establishment of biological contact with bone results in a certain amount of bone loss. The underlying key factors relate mostly to the implant abutment complex, and its meeting point with the restoration, namely the microgap. Scientific evidence supports the fact that bone loss is due to combined and sustained activation of inflammatory cells that appear with the microgap at the bone level.¹³ Mechanical properties of the implant abutment connection have been frequently addressed to understand the relationship between crestal bone level and periimplant load transfer.14Early peri-implant bone loss was commonly observed in many implant systems and in different surgical approaches.^15-17 In particular, the leading sign of implant failure is crestal bone loss in which osseointegration has been achieved before failure. Most crestal bone loss occurs during the first year after implant placement, which can be up to 1.2-mm inferior superiorly. This is still considered to be clinically acceptable.¹⁸

Biomechanical studies of dental implants using finite elements analysis software are increasingly common ^20-23. In 2009, Hsu et al. analyzed the behavior of reduced platform restorations, using finite element analysis in three dimension, which showed a 10% decrease in all the prosthetic loading forces transmitted to the bone-implant interface²⁴. In two dimensions, such kind of studies show great variability in the results obtained. Some studies report a decrease in force to the cortical bone of less than 10% 21, other authors such as Tabata et al. have reported a decrease of 80%²². Rodriguez-Ciurana et al.²⁴, in a two-dimensional biomechanical study involving platform switching integrated into the implant design, failed to obtain peri-implant bone force attenuation values as high as those reported in earlier studies, when comparing platform expansion with a traditional restoration model. In addition, the authors concluded that force dissipation in the platform switching restoration is slightly more favorable in an internal than in an external junction, since it improves distribution of the loads applied to the occlusal surface of the prosthesis along the axis of the implant. On the other hand, this concentration of forces along the axis of the implant, transmitted through the retention screw, increases the possibility of abutment fracture, and thus may lead to failure of the global restoration

All studies contrasting platform switching versus continuity of the platform with the body of the implant agree that force to bone diffusion is improved by expanding the platform. According to Canay and Akça¹⁹, in a three dimensional finite elements analysis involving different implant-free expanded platform dimensions and a range of abutment designs, claimed that the effect of platform expansion is not attributable to the distribution of loads to the peri-implant bone but rather simply to redistribution of the new biological space. The most appropriate reduced platform abutment design for securing lesser implant abutment material fatigue is represented by conical emergence abutments with a variable height of 1.5-2mm, freeing extension of the implant platform between 0.5-0.75mm ¹⁹. Such platform switching is not advisable in mandibular implant-mucosal support prostheses, since reduction of the diameter of the junction lessens the abutment resistance in response to occlusal loading applied in the posterior area of the overdentures – fundamentally compromising the connecting abutment closest to the area where loading is applied²⁵.

SOFT TISSUE CHANGES

Ericsson et al,²⁶ detected inflammatory cell infiltrate in the connective tissue zone, contacting the implant-abutment interface of 2-piece implants with a regular connection and suggested that formation of infiltrate is a defensive action of the host from a micro-gap contaminated with oral bacteria. Because in implants with platform switching the micro-gap is shifted away from bone, inflammatory cell infiltrate does not form in close proximity to bone; therefore, crestal bone loss is reduced. This hypothesis was described by Lazzarra et al⁴ in an article based on a summary of radiographic observations of implants with platform switching from 5 to 13 years. The authors suggested that platform switching repositions inflammatory infiltrate within approximately a 90-degree-confined area of exposure, instead of a 180-degree surface of regular connection implants; thus the infiltrate is smaller around platform switched implants, which results in less bone loss.

According to Tomas L et al²⁷, platform switching modification does not prevent crestal bone loss if mucosal tissues are 2 mm or less at the top of the edentulous ridge before implant placement. This conclusion is contrary to prevailing opinion that platform switching is efficient in limiting bone remodeling. For example, Vela-Neblot et al⁵ have compared regular connection implants with platform switched implants. Implants in both groups were positioned equally with bone crest and later restored with single crowns. After 12 months of loading, the mean value of bone loss observed in the medial and distal measurements for the control group were 2.53 and 2.56 mm and for the test sample were 0.76 and 0.77 mm, respectively. This difference was found to be statistically significant and supported the recommendation to use implants with platform switching for crestal bone preservation.

Another clinical study, which retrospectively evaluated Ankylos (Fiadent, Manheim, Germany) implants revealed that bone loss around implants with platform switching was only 0.2 mm within 3 years of loading, although bone resorption after implants uncovering ranged from 0.5 to 2 mm on average.28 As initial tissue thickness before implant placement was not registered or reported, it can be assumed that bone loss up to 2 mm could be a consequence of thin tissue. Studies by Cappiello et al,²⁹ Canullo and Rasperini,30 and Calvo-Guilardo et al31 also did not describe measurements of mucosal tissue thickness before implant placement.

BONE CRESTAL LEVEL CHANGES

The following factors are proposed to be the causes of early implant bone loss: microgap if placed at or below the bone crest, implant crest module, occlusal overload, and the reformation of biologic width around dental implants.³² The crest module of the implant body refers to the transosteal region of the implant that receives the crestal stress to the implant after loading.³³ Once loading is done, bone loss is generally observed down to the first thread in many submerged implant systems which have different distances from the implant platform to the first thread.^34,35 various proposals have been put forward such as the relation of bone loss, in part, to crest module design instead of relating to a specific length and that the bone loss may slow down at the first thread due to the changes in the shear force of the crest module to a component of compressive force caused by the thread itself.³²

Force from mastication must be translated from a restorative prosthesis through the dental implant to the surrounding bony structures. The functionality and longevity of this system depends on the mechanical integrity of the prosthesis and implant,³⁶ and the ability of peri-implant structures to withstand and positively adapt to the forces.³⁹ Application of too much stress can cause bone resorption or failure of the implant-bone interface whereas the lack of stress to the bone may lead to atrophy and to subsequent bone loss.⁴⁰ Thus, the possibility of mechanical rearrangement leading to an increase or decrease of peri-implant structure quality is highly related to the magnitude and frequency of force applied to it. Therefore, it is essential for the design of an implant to have features that distribute stress effectively and evenly throughout peri-implant bone to best avoid such pathological conditions.

Gardner states that the main advantage of platform switcing is that it is an effective way to control circumferential bone loss around dental implants, however, concludes that platform switching needs further investigation as several potential disadvantages exists such as the need for components that have similar designs (the screw access hole must be uniform) and the need for enough space to develop a proper emergence profile.² According to Baumgarten et al., in clinical conditions where shorter implants must be used, where implants are placed in aesthetic zones, and where a larger implant is desirable but prosthetic space is limited, a sufficient tissue depth (approximately 3 mm or more) is necessary to accommodate an adequate biologic width and platform switching helps prevent the anticipated bone loss and also preserves crestal bone.⁶

Degidi et al. evaluated the histology and histomorphology of three morse cone connection implants in a real case report and he explains that when there is zero microgap and no micromovement, platform switching shows no resorption along with better aesthetic results³⁹.In their prospective study, Hürzeler et al. revealed that although bone remodelling is encountered during the first year after the final restoration with platform switching implants, there are significant differences compared to non-platform switching implants.⁴⁰

CONCLUSION

Having reviewed the available literature, it could be concluded that platform switching is capable of reducing or eliminating crestal bone loss. It also contributes to maintaining the width and height of crestal bone and the crestal peak between adjacent implants and it also limits the circumferential bone loss.

The implant design modifications involved in platform switching offer multiple advantages and potential applications, which include situations where a larger implant is desirable but the prosthetic space is limited and in the anterior zone where preservation of the crestal bone can lead to improved aesthetics.

REFERENCES