Not every tapered connection is Morse Taper!

Author: Prof. Rodrigo Reis

Introduction

Among the different types of prosthetic connections in oral implantology, the so-called “Morse Taper” ones have gained popularity due to their biological and mechanical advantages. However, by analyzing the conicity, depth and locking mechanism of the several different taper connections, compared to the concepts and definitions of the “Morse effect” mechanical locking, it is possible to note that few tapered systems are truly Morse Taper, despite being commercialized as such. This brief review of the literature aims to provide a quick discussion on the peculiarities of the different types of taper connections.

Taper Connections

By the mid 1980´s, aiming at a greater mechanical stability of prosthetic components, the market introduced taper connection implants with internal friction between the walls of the implant and the component, generating locking. The first systems of that kind were the Bicon, in 1985, and the Ankylos, introduced in 1987. That was the beginning of the so-called Morse Taper systems. From then on, several versions of that type of internally tapered connections with different conicity angles, varied connection depth and the presence or not of a screw to guide the frictional locking between the walls of the component and the implant have been created. In a few systems, due to the presence of incongruent internal angles or due to excessive expulsive angles, the screw is associated to an internal index (hexagon or octagon) that promotes the retention of the prosthetic component, without any locking and with limited friction of the tapered portion of the connection. Mistakenly, and to fulfill a market demand, those systems have been commercialized as having the features of Morse Taper (self-locking).

 “Morse Effect” Taper Connections

The angulation of the real Morse Taper internal wall must be 1.5° on each wall, totalizing a conicity of 3°. That way, connections with those characteristics would feature self-locking for pure friction, without the need of a screw to generate torque and friction, or even an internal hexagonal or octagonal index as a locking mechanism. Examples of that type of prosthetic connection which, in implantology, would more faithfully reproduce what in Engineering is known as the original Morse Taper would be the Bicon implants, and, more recently, in 2016, the Arcsys System by FGM. In those systems, the prosthetic component obtains its stability through some light hits on the “head” of the prosthetic component with a tool similar to a hammer in the direction of the long axle along which the implant has been installed. With versatility, the Arcsys System offers, in a pioneering way, the possibility of customizing the angulation of the prosthetic component if necessary by means of a system-exclusive component. In order to generate a high friction coefficient, the Arcsys connection presents, besides a small angulation (3°), a depth that is 3.5mm larger than its 2.5mm diameter. That fact provides, from a mechanical point of view, a self-retentive system, whose compressive strengths along the axle of the implant-prosthetic component assembly favor, with time, retention and a greater stability to oblique forces. The success of that type of connection, both for fixation as well as hermetic sealing, has been described in clinical studies with implants using that type of connection1,2.

The possibility of reversing that type of connection happens by means of attractive shear forces through the help of a needle holder device or a similar tool3.

Taper-Screw Systems

Those systems are currently commercialized by several companies with very distinct features among themselves in terms of depth of the cone and angulation of the walls. The needed torque for the fixation of the prosthetic components may vary from 15Ncm to 40Ncm. That shows a mechanical design that is very distinct from one system to the other in that category, which may have an influence on the mechanical behavior during its function.

A few systems may even be called hybrid, since they comprise a very short initial tapered part (in some systems, 0.8mm) and expulsive angulation (for example, 30°) and in the apical limit of the short tapered connection of the system, an internal retention system such as an internal octagon or hexagon. Systems with those features do not have a mechanical behavior in which an internal cone is responsible for the locking of the prosthetic component. The tapered portion would have no mechanical function other than sealing due to its short depth and excessive expulsion characteristic of the conic projection lock. During masticatory forces, due to the fact that the locking device is an hexagonal or octagonal index, allowing for parallel surfaces between the component male and the implant female coupling, there are micro spaces between them, and, that way, the same micro-movements of the HE and HI connections may happen, breaking the sealing of the timid cone of that connection. Depending on the precision of the machining process and consequent adaptation between abutment and implant, that micro-movement may lead to an overload of the retaining screw, with possible complications like a screw loose and screw fractures, besides the formation of a gap viable enough for reabsorption, liquid pumping and bacterial proliferation4.

In that same taper-screw category, there are systems with cone angulations of around 11.4°. Some prosthetic components may be solid, with an integrated screw, which guides the turning of the prosthetic component in the apical direction until friction is generated between the walls and locking is achieved3. On the other hand, in other components, there are passing screws that, when tightened, track the component in the apical direction without turning, which leads to a potential for lower friction between the pieces3. For some professionals used to working with HE and HI systems, the absence of an internal anti-rotational index is a disadvantage when transfer moldings of the position of the implant are made, because that would make it difficult to position the prosthetic component in the mouth in the same position of the model, with the need to make a jig positioner in acrylic. More recently, some of those systems developed an internal index for the use in connection to abutments with passing screws, with the purpose of making it easier to position the component in the mouth in the same position as the model, and, with that, fulfill a market demand. However, for that, the height of the cone was reduced, for example, in 20% (from 2.5 to 2.0mm), reducing the potential frictional retention area or taper locking.

Discussion

In spite of the different configuration designs of the taper connections (angulation and height of the cone, presence of an internal index for retention and positioning or just positioning of the prosthetic component), the taper connection systems, in general, show interesting characteristics such as the switching platform, the possibility of an infra-osseous positioning of the implant and possible biological advantages over traditional prosthetic connection implants. However, in some cases, the saucerization process may occur due to factors other than the prosthetic connection such as: bone heating during drilling, presence of a thin bone wall, grafting chosen and membranes, type of graft and reopening approach, besides the presence of excessive cement mistakenly left in the cemented prosthesis (frequently the cementing lines are in deep areas). Or rather, in those systems, a determining factor for saucerization (even incipient) has been eliminated (bone level gap). The remaining risk factors are still present, which demands attention from the professionals.

The several types of tapered connections have been conquering larger market shares and may be clinically evidenced for showing some common advantages. From the mechanical point of view, the behavior and the function method differ considerably in that category and the systems with a real Morse Taper connection are rare. The generalization and massive nomination “Morse Taper” for any taper connection confuses professionals and places different systems in the same category just following a commercial criterion. The difficulty degree, the precision and the manufacturing rigor to produce a real Morse effect adaptation and self-locking (internal angulation of the cone of 3°) are greater than those for systems with greater angulations and shorter cones. From a practical point of view, the Morse effect taper connections are simpler to work with, offer unlimited freedom for the positioning of the prosthetic component and easiness to obtain frictional self-locking, reproducing more faithfully the mechanical concepts of the original Morse Taper connection.

Conclusions

  • Not every tapered connection offers the real “Morse effect” of mechanical locking.
  • It is necessary to have a technical opinion to correctly inform the market of the real prosthetic connection instead of a purely market-oriented generalization.
  • The tapered systems with “Morse effect” have proven to be simpler and more friendly, prosthetically speaking, than the other tapered systems, and both show significant advantages over external and internal hexagon connections.

References

1- Mangano C, Mangano F, Piatelli A, Iezzi G, Mangano A, La Colla L. Prospective clinical evaluation of 307 single-tooth morse taper connection implants: a multicenter study. Int J Oral Maxillofac Implants 2010;25(2):394-400.

2- Canullo L, Fedele GR, Iannello G, Jepsen S. Platform switching and marginal bone-level alterations: the results of a randomized-controlled trial. Clin Oral Implants Res. 2010 Jan;21(1):115-21.

3- Tunes FSM, Pegoraro LF, Almeida ALPF, Bonfante EA, Coelho PG, Hirata R, Fardin VP. Prótese sobre implante: implicações clínicas do tipo de retenção e forma de fixação em 50 anos de osseointegração: reflexoes e perspectivas. São Paulo, SP: VM Cultural2015. P 101-111.

4- Zipprich H, Weigl P,  Lange B, Lauer HC. Micromovements at the Implant-Abutment Interface: Measurement, Causes, and Consequences. Implantologie 2007;15:31-46.

5- Lazzara RJ, Porter SS. Platform switching: a new concept in implant dentistry for controlling postrestorative crestal bone levels. Int J Periodontics Restorative Dent. 2006 Feb;26(1):9-17. 20(3):254-61.

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