Capsule for mixing and dispensing a dental material

A capsule for mixing and dispensing a dental material which has a capsule body and a cavity extending into the capsule body. The cavity is closed by a liquid container. The liquid container has a cartridge forming a channel that extends between a front opening and a rear opening of the cartridge. Further the cartridge has a front foil closing the front opening and a rear foil closing the rear opening. The front foil is attached at the cartridge along a closed path circumferentially around the front opening. The front foil and the cartridge in at least a partial area of the circumferential path are separably sealed with each other, thus defining an openable valve through that path. The capsule has further a piston received in the channel of the cartridge and arranged such that the liquid is enclosed in the cartridge between the front foil and the piston. The capsule facilitates the preparation of a two-component dental material.

FIELD OF THE INVENTION

The invention relates to a capsule for mixing and dispensing a dental material. In particular the invention relates to a capsule for mixing a powder and a liquid which are separately stored and which can be brought together for mixing in a shaker by activating the capsule.

BACKGROUND ART

Dental substances are often provided in devices allowing the substance to be dispensed directly to a desired location, for example on a dental pad or in a patient's mouth. Such dispensing devices typically have a chamber for holding the dental substance, an outlet, and a piston for extruding the substance from the chamber through the outlet.

A variety of dispensing devices are designed for dispensation of relatively high viscosity dental substances, like for example dental filling materials. Some of those dispensing devices are configured for use with an applicator providing an extrusion force that is sufficient for dispensation of high viscosity dental substances. In dentistry several types of manually operated applicators are available which provide leverage for increasing manual forces to provide sufficient extrusion forces. Many applicators are designed as a reusable tool which forms part of a dental practice's basic equipment.

A particular dispensing device often used to store, prepare and dispense a hardenable dental material mixed from a powder and a liquid is the so-called dental capsule in which the mixing of the powder and the liquid is performed within the capsule by shaking the capsule in a shaker.

U.S. Pat. No. 8,893,925 discloses container for mixing and dispensing material. The container comprises a body having a main chamber, a dispensing nozzle, a liquid receptacle and a plunger. The liquid receptacle has a front portion arranged to break away upon pressure being applied by the plunger so that the plunger can traverse the entire length of the body. This enables a charge of material in the main chamber to be entirely dispensed through a frangible wall into the nozzle. The container is particularly envisaged for use in mixing and dispensing of dental materials.

U.S. Pat. No. 8,968,000 discloses a mixing and application capsule for producing and discharging a dental preparation. To prevent a liquid loss, a mixing and application capsule for producing a dental preparation is proposed. The capsule has a capsule body with a mixing chamber for receiving a mixing component and for mixing the dental preparation from the mixing component and a fluid. The capsule further has an outlet opening for discharging the dental preparation. A first plunger body which can be displaced in the capsule body delimits the mixing chamber in the capsule body. The first plunger body has a channel to guide the fluid from a cavity into the mixing chamber and a projection. The capsule has further a second plunger body which can be displaced in the capsule body relative to the first plunger body. The second plunger body has the cavity to receive the fluid. The cavity is configured to receive the projection of the first plunger body.

Such capsules have a variety of advantages as, for example, a relatively long shelf life. However there is a still desire for a capsule with such a long shelf life, which is easy to use and which is nevertheless relatively inexpensive.

SUMMARY OF THE INVENTION

The invention relates to a capsule for mixing and dispensing a dental material. The capsule comprises:a capsule body having a front end and a rear end, anda cavity extending from the rear end into the capsule body.
The cavity merges adjacent the front end in an outlet through the capsule body. Further the cavity is closed adjacent the rear end of the capsule body by a liquid container.

The liquid container comprises a cartridge forming a channel therethrough. The channel extends between a front opening and a rear opening of the cartridge. In other words the channel preferably forms the front opening and the rear opening within the cartridge and is otherwise closed. Further the liquid container further has a front foil closing the front opening and a rear foil closing the rear opening. The front foil is attached at the cartridge along a closed path circumferentially around the front opening. The front foil and the cartridge in at least a partial area of the circumferential path are separably sealed with each other. The cartridge and the front foil thus define an openable valve through that circumferential path. The valve therefore may assume a closed position and an open position. In the closed position front foil is attached at the cartridge along a closed path circumferentially around the front opening. And the open position the valve preferably provides for an interruption of the circumferential attachment and a passageway extending across that circumferential path and between the front opening and an exterior of the cartridge.

The capsule further comprises a piston received in the channel of the cartridge, particularly between the front and rear opening of the cartridge. The piston is further arranged such that the liquid is enclosed in the cartridge between the front foil and the piston. Thus the piston is preferably arranged for pressurizing the liquid toward the front opening. The piston is further preferably sealingly and movably fitting within the channel of the cartridge.

The invention is advantageous in that it provides a liquid container for storing a liquid in a tightly sealed configuration. Further the invention provides a capsule which can be activated for use relatively easily. In particular, although the capsule of the invention allows for tightly sealing a liquid, the seal can be opened for activation of the capsule by applying only relatively low forces to the capsule. In contrast to some prior art capsules which require high forces and therefore a separate tool for activation, the present invention provides a capsule that normally can be activated manually. Further the capsule of the invention allows for storing an amount of liquid which can be selected from a relatively wide range of different amounts, from very low amounts to relatively high amounts, without substantially modifying the configuration of the capsule. Further the capsule of the present invention provides a relatively precise mixing ratio of the dental material because the capsule allows for controlling the amount of liquid used for preparing the dental material relatively precisely. Additionally the capsule of the invention is relatively easy to use and relatively inexpensive in manufacturing.

The capsule body is preferably substantially cup-shaped with a front wall through which the outlet extends and a circumferential side wall extending rearwards to the rear end of the capsule body. The side wall may extend along a longitudinal axis of the capsule, and may be shaped essentially rotationally symmetric, for example circular, although other shapes (for example elliptical) is possible. The side wall and the front wall, in combination, preferably form the cavity. The cavity may extend along the longitudinal axis at a circular cross-section, although that circular cross-section may vary in diameter. The smallest cross-section of the outlet is preferably smaller than the smallest cross-section of the cavity. In one embodiment the diameter of the smallest cross-section of the outlet may be between 0.5 mm and 3 mm, and the smallest cross-section of the cavity may be between 5 mm and 9 mm.

The valve formed by the front foil and the cartridge is preferably irreversible. This means that the valve is preferably initially closed, can optionally be opened, but cannot be reclosed again. Thus the capsule is provided with a tamper-evident closure.

In one embodiment the capsule further comprises a plunger. The plunger may have a plug which cross-section is sized and shaped substantially corresponding to the size and shape of the rear opening of the cartridge. The plug may form a cutting edge for punching the rear foil of the liquid container. Thus by urging the plunger along the longitudinal axis toward the liquid container the rear foil can be punched through by the plug so that the plug is enabled to penetrate into the channel of the cartridge.

In a further embodiment the plunger has a guiding structure providing the plunger with an outer cross-section that is larger than the cross-section of the plug. Therefore the plunger may be guided for a movement along the longitudinal axis by the capsule body within the cavity. The guiding section is preferably sized and shaped to fit within the cavity in a guided fashion with the capsule body. For example the guiding section may be ring-shaped having an outer diameter which generally corresponds to the inner diameter of the cavity adjacent the rear end of the capsule body.

In one embodiment the plunger has a predetermined breaking point for separably connecting the guiding structure and the plug with each other. For example the guiding structure and the plug may form one monolithic piece and a weakened portion between them may provide the predetermined breaking point. In this regard the term “point” generally refers to an area or line which is prepared to break upon exposure to a threshold force.

In an embodiment the plug—separated from the guiding structure—and the cartridge each are sized and shaped to in combination complement to an outwardly substantial cylindrical stamp. Therefore the plug in combination with the cartridge may be used to dispense the dental material from the capsule.

In one embodiment the cartridge is shaped such that the channel has a stepped configuration. In particular the channel may have a rear section and a front section. The transition between the rear and front section preferably forms the step of the stepped configuration. The rear section preferably forms the rear opening and further preferably extends at a generally uniform cross-section, for example circular. Thus the piston may be movably received within the rear section, and may be movable along the dimension along which the rear section extends. Further the front section preferably forms the front opening. The front section is preferably narrower than the rear section at least at the front opening. The front section may be conical and may narrow from the step in the channel toward the front outlet. The largest diameter of the front section is preferably still smaller than the diameter of the rear section. The largest diameter of the front section may for example be smaller by about 1.5 mm than the diameter of the rear section. Preferred diameters of the smallest diameter of the front section are between 3 mm and 10 mm, preferred diameters of the greatest diameter of the front section are between 3.5 mm and 10.5 mm, and preferred diameters of the diameter of the rear section are between 5 mm and 12 mm.

In one embodiment the piston is positioned in the liquid container such that a space is formed between the piston and the rear foil. In particular the piston may be positioned within the rear section of the channel of the cartridge enclosing the liquid between the front foil and the piston and enclosing air between the rear foil and the piston. Thus the liquid container combines the liquid and the air in one sealed area and the piston separates the liquid and the air. Therefore while the liquid and air are prevented from escaping from the liquid container very small portions of the air are enabled to permeate and/or diffuse toward the liquid. This is because the piston, although it may form a generally tight seal with the cartridge for the liquid, may still allow diffusion of air molecules through contact areas between the piston and the cartridge or permeation of atoms or molecules through the piston. In particular oxygen molecules may diffuse or permeate into the liquid and thus may prevent anaerobic polymerization of the liquid. Further due to the separation of the liquid and the air the liquid is kept generally free of air bubbles. Air bubbles may lead to inaccurate amounts of liquid in the preparation of the dental material. Therefore the capsule of the invention provides for a relatively reliable mixing ratio for preparing the dental material.

In one embodiment cartridge has at least one stopper for limiting a travel of the piston. The stopper is preferably arranged such that the piston is prevented from getting in contact with the step formed in the channel of the cartridge. Thus a pre-determined, though preferably small, amount of liquid is ensured to reside within the liquid container upon activation of the capsule. It has been found that particularly the last small amount of liquid can be extruded from the container at relatively high tolerances of that amount. Accordingly the capsule is adapted to capture such amount within the liquid container. Therefore a relatively reliable and precise amount of liquid can be transferred from the liquid container to the mixing chamber upon activation of the capsule.

In one embodiment the capsule body forms a resistance within the cavity, which impedes a movement of the cartridge or liquid container within the cavity toward the outlet. Thus the cartridge/liquid container is retained within the capsule body against movement toward the outlet, for example during the plunger is urged toward the liquid container for activation of the capsule. The resistance is preferably formed by a step in the cavity. In particular the step may be formed by a transition of a front section of the cavity extending along a generally uniform cross-section toward the outlet and a rear section of the cavity extending along a larger generally uniform cross-section toward the capsule body rear end.

In one embodiment the liquid container and thus the cartridge is received within the cavity, in particular at least partially within the front section. The cartridge is preferably sized and shaped to form a seal with the capsule body, in particular with the side wall of the capsule body in an area of the front section. Therefore the liquid container can be used to seal the front section of the cavity and enclose a powder component therein. Preferably the liquid container is positioned within the cavity such that the powder only partially fills the chamber formed between the liquid container and the capsule body. The chamber further forms a mixing chamber in which the powder may be mixed with the liquid transferred from the liquid container into the mixing chamber.

In an embodiment the capsule further has a nozzle for dispensing the dental material. The nozzle may be movable (for example pivotable) between a dispensing position and a storage position. In the dispensing position the nozzle is in fluid communication with the outlet and in the storage position the nozzle closes the outlet. The dispensing position may comprise several geometric positions between the nozzle and the body, or a range between two geometric extreme positions. For example the dispensing position of a nozzle that is pivotable with respect to the body may within the dispensing position be pivotable over an angular range whereas the storage position is outside that angular range.

In one embodiment the capsule comprises a powder within the cavity. The powder and the liquid are preferably adapted to form, in combination, a hardenable dental material. The powder may be a glass powder or resin modified glass powder. The liquid may be a water based polyacid or a monomer based polyacid with light initiator.

In one embodiment the capsule body comprises a catch for retaining the capsule in a dispensing gun. The catch may be formed as a circumferential bulge arranged adjacent the rear end of the capsule body and protruding radially outwardly therefrom.

In a further embodiment the front and rear foil each comprise a, preferably contiguous, metal layer and at least one polymer layer. Each the front and rear foil being arranged with the polymer layer toward the cartridge. The front and/or rear foil may have further layers, particularly polymeric layers. For example the front and/or rear foil may have two outer polymeric layers with the metal layer arranged between. A preferred configuration of the front and/or rear foil comprises a layer construction made up of PE (polyethylene), aluminum and PE in the order as listed.

In one embodiment the capsule body, the nozzle, the cartridge and the piston each are made of a plastic material. For example the capsule body may be made of POM (polyoxymethylene) or PP, the nozzle may be made of PC (polycarbonate), and the cartridge and the piston may be made of PE or PP (polypropylene). The cartridge may comprise side walls made from two or more layers, for example a first layer providing a good barrier against permeation and/or diffusion of substances (for example oxygen, water or other solvents) and a second layer providing good mechanical strength. Such a multilayer wall may be obtained by two- or multi-component injection molding of the cartridge. Suitable polymers for a suitable layer combination may comprise LCP (liquid crystal polymer) and HDPE (high density polyethylene).

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1shows a capsule1for mixing and dispensing a dental material. The capsule1has a capsule body11that has a front end111and a rear end112. A cavity113extends from the rear end112into the capsule body11. The cavity113merges adjacent the front end111of the capsule body11in an outlet114through the capsule body11. The cavity113is closed by a liquid container12. The liquid container12is positioned adjacent the rear end112of the capsule body11and spaced from the outlet114. The space formed in the cavity113between the outlet114, the capsule body11and the liquid container12forms a mixing chamber for mixing the dental material. At an initial stage as shown the mixing chamber is preferably (partially) filled with a powder material and the liquid container12is preferably filled with a liquid. The capsule1is adapted to be activated by a user. Such an activation of the capsule1causes the liquid to be transferred into the mixing chamber, as further described in detail in the following. The liquid and then powder thus are received within the mixing chamber. At the activated stage the capsule1can be placed in an agitator (not shown) in which the capsule may be shaken to cause the powder and the liquid to mix with one another. The powder and the liquid are preferably adapted to form a hardenable composition in combination. An exemplary powder material comprises a glass powder or resin modified glass powder and an exemplary liquid comprises a water based polyacid or a monomer based polyacid with light initiator, and the mixture thereof comprises a glass-ionomer dental material. The mixture from the powder and the liquid typically forms a pasty material which can be dispensed through the outlet114of the capsule body as further described in the following. For dispensing the dental material the capsule1further comprises a nozzle14which is attached adjacent the front end111of the capsule body11. The nozzle14and the capsule body11are pivotable between a dispensing position (shown) and a storage position. In the dispensing position the nozzle14opens the outlet114of the capsule1for dispensing the dental material, whereas in the storage position the nozzle14closes the outlet114. In the example the nozzle14is retained at the capsule body11by a cap16. In another example the nozzle14and the capsule body11may however be molded into each other, for example by first molding the nozzle14and subsequently overmolding the nozzle14by the capsule body11. Further the capsule1has a catch118for retaining the capsule1in a dispensing gun (not shown). Such a dispensing gun is for example commercially available under the designation 3M™ ESPE™ Capsule Dispenser form the company 3M Deutschland GmbH, Germany. The catch118in the example is formed by a circumferential bulge at the rear end112of the capsule body11. A circumferential rim119is arranged spaced from the catch118further to the front end111of the capsule body. As indicated by the dashed line the rim119may form a finger plate. The finger plate can assist in retaining the capsule manually while activating the capsule1for bringing the powder and113the liquid in contact with each other.

FIG. 2is an enlarged view of a portion of the capsule1, showing the liquid container in more detail. The liquid container12comprises a cartridge121. The cartridge121in the example is generally ring-shaped and forms a channel122therethrough. The channel122extends between a front opening123and a rear opening124of the cartridge121. The cartridge121is preferably made of a plastic material, in particular may be made of a thermoplastic material, for example polyethyelene or polypropylene. The liquid container12further has a front foil125that closes the front opening123and a rear foil126that closes the rear opening124. The front foil125and the rear foil126each comprise a contiguous layer of metal, preferably an aluminum layer. In one example the aluminum layer has a thickness of between 0.008 mm and 0.1 mm, preferably about 0.03 mm. The metal layer is preferably generally adapted to block substances, in particular air and moisture from permeating through the front and/or rear foil125,126. Preferably each of the front and the rear foil125,126further comprise a polymeric layer, preferably a layer comprising or consisting of polyethylene. This allows for the front and the rear foil125,126to be heat sealed with the polymeric layer to the thermoplastic cartridge. Heat sealing may be performed using a heated tool or by ultrasonic welding, for example. Further the thermoplastic layer forms a protective layer for the metal layer so as to avoid undesired chemical or physical interaction between the liquid and the metal layer.

The front foil125is attached at the cartridge121along a closed path circumferentially around the front opening123, and the rear foil126is attached at the cartridge121along a closed path circumferentially around the rear opening124. Thus the channel122is tightly sealed so that the front foil125, the rear foil126and the cartridge121in combination form a closed liquid chamber. In the example the liquid chamber contains a liquid forming one component of the dental material, as described.

The front foil125and the cartridge121in at least a partial area of the circumferential path are separably sealed with each other. In the example the front foil125and the cartridge121are overall separably sealed with each other. Such a separable seal can be provided by heat sealing the front foil125with its polymeric layer oriented to the cartridge121for a predetermined time, at a predetermined temperature and a predetermined sealing pressure.

Thus the separably sealed area defines an openable valve through the circumferential path. In particular if the liquid is pressurized toward the sealed front opening123at a certain predetermined minimum or threshold pressure the interconnection between the front foil125and the cartridge starts yielding and thus enables the liquid to creep between the front foil125and the cartridge121. It has been found that the front foil125does not entirely separate from the cartridge121. In contrast the liquid reproducibly creates only a passageway continuing from the front opening123toward an outer boundary of the front foil125. Therefore the front foil125remains attached at the cartridge although a portion of the front foil125separates by liquid pressure.

To build up a pressure on the liquid a piston127is received in the channel122of the cartridge121. The piston127is arranged between the front and rear opening123,124of the cartridge121. Therefore the piston127is sealed within the liquid chamber together with the liquid. In particular the piston127is sealingly received within the channel122, for example by a press fit, and is movable along the channel. The piston127is arranged in the cartridge121such that the liquid is enclosed in the cartridge121between the front foil125and the piston127. The piston127may be moved toward the front opening123for pressurizing the liquid toward the front opening, thus causing the valve to open and for transferring the liquid into the mixing chamber. This is further explained in detail below. In the example further the piston127is arranged in the cartridge121such that a space is formed between the rear foil126and the piston127. The space may be filled with air. Any air sealed within the liquid chamber may be used to avoid anaerobic polymerization of the liquid. On the other hand the air and the liquid are separated by the piston so that any liquid transferred in the mixing chamber substantially does not contain air or air bubbles. Therefore a reliable predetermined amount of liquid may be transferrable to the mixing chamber by movement of the piston127for a predetermined stroke. Further it has been found that in attempts to entirely empty the liquid chamber by moving the piston127in an end position adjacent the front opening123results in tolerances of the amount of liquid transferred. This is because as the piston127approaches the end position, the space between the piston127and the inside of the cartridge121gets smaller and the smaller the space the more is the liquid hindered in being displaced toward the front opening123. Thus, depending on the viscosity of the liquid and any force at which the piston127is moved, a non foreseeable residual amount of liquid may be captured in that space. The cartridge121of the invention therefore has at least one stopper128which stops the piston127at a position prior to the end position. In other words the piston127and the cartridge121are shaped such that in the closest position of the piston127relative to the front opening123still sufficient space for easy displacement of the liquid is available between the piston127and the cartridge121. Therefore the tolerances in the amount transferred to the mixing chamber can be minimized.

The cavity113has a stepped configuration provided by a front section115and a wider rear section116. Preferably the cavity113extends into the capsule body11at a generally circular cross-section. Further the front section115and the rear section116extend substantially concentrically and each at a generally uniform circular cross-section. Thus the transition between the front and rear section115,116forms a step117in the cavity113. The step117in forms a resistance for impeding a movement of the cartridge121within the front section115of the cavity113. Therefore the cartridge121further has a detent129, in the example a circumferential ridge or bulge, for cooperating with the resistance. In particular the detent provides the cartridge with a cross-section that is larger than the inner cross-section of the front section115of the cavity113. Thus the cartridge121and the capsule body11are adapted such that the cartridge121can be only press fit within the front section115of the cavity113.

The capsule1further has a plunger13. The plunger13has a plug131which is receivable within the cartridge121through the rear opening124, for pushing the piston127toward the front opening123. The plug131has a cross-section that is sized and shaped substantially corresponding to the size and shape of the rear opening124of the cartridge121. In the example the plug131and the rear opening124have a generally circular cross-section. Further the plug131may form a cutting edge (not shown in this view) for punching through the rear foil126of the cartridge121. The plunger13further has a guiding structure132. The guiding structure132enables the plunger to be guided within the cavity113of the capsule1. The guiding structure132provides the plunger13with an outer cross-section that is larger than the cross-section of the plug. Further the guiding structure is sized and shaped to fit within the rear section116of the cavity113of the capsule body11in a guided fashion. Therefore the plunger13can pushed toward the cartridge121, and because the cartridge121is retained within the capsule body11, thereby with the plug131can penetrate through the rear foil126for moving the piston127toward the front opening123. The guiding structure132may have extensions132a. The extensions protrude rod-shaped from a front end134of the plunger13. The capsule body11further has corresponding recesses for receiving or absorbing the extensions132atherein. The extensions132aallow for the plunger13to be positioned substantially outside the capsule body11only held by the extensions132awithin the capsule body11. Thus the possible stroke of the plunger13can be enhanced without increasing the overall length of the capsule1.

FIGS. 3 to 6illustrate the activation of the capsule1in more detail. For convenience some of the Figures illustrate the cartridge121and the plunger13only.

FIG. 3shows the cartridge121and the plunger13in an initial position. In the initial position the liquid is sealed within the channel122by the cartridge121, and the front and rear foil125,126. The plunger13and the cartridge121are coaxially aligned with each other along longitudinal axis A.

FIG. 4shows cartridge121and the plunger13moved further toward each other along the longitudinal axis A. At the stage shown the plug131is penetrated in the channel122and abuts the piston127. Although not illustrated the rear foil is punched or ruptured in consequence of the plug131having penetrated into the channel122of the cartridge.

FIG. 5shows cartridge121and the plunger13moved even further toward each other such that the piston127is positioned at the closest possible position relative to the front opening123. At this stage a portion of the front foil125is separated from the cartridge121and provides a passageway123abetween the front opening123and the exterior of the cartridge121. Further the liquid is extruded from the liquid chamber through the passageway123a. Although not shown the liquid extruded from the cartridge121is received within the cavity of the capsule body. The plunger13has a predetermined breaking point or predetermined breaking area133which separably connects the guiding structure132and the plug131. The predetermined breaking point/area133allows the plug131and the guiding structure132to be separated for dispensing any dental material mixed within the capsule1as further described in the following.

FIG. 6shows the activated capsule1after mixing the liquid and the powder. A hardenable dental material15is present in the mixing chamber. Further at the stage shown a ram100of a dispensing gun (not shown in detail) is positioned within the cavity113. The predetermined breaking point/area133is broken and the guiding structure132and the plug131are separated from each other. This is because the guiding structure132fits into the rear section116of the capsule body11but not in the front section115. Thus upon urging the plunger13into the capsule body the guiding structure132by the step117is prevented from moving and by pushing the plunger13at the plug131a mechanical force is exerted to the predetermined breaking point/area133which breaks upon reaching a minimum force in consequence. The plug131and the container12in combination form an outwardly generally cylindrical stamp which can be used to extrude the dental material15from the capsule1.

In the embodiment in which the rim119forms the finger plate the capsule body11can be retained at the rim119while the plunger13is pushed toward the capsule body11. For example the capsule1may be placed with the plunger downwards on a table and the capsule body11, using the finger plate, may be manually pressed down toward the table to cause the plunger13to be pushed into the capsule body11.

FIG. 7shows the capsule1in an exterior view. The capsule1is shown at a stage at which the plunger13is positioned at its desired end position within the capsule body11. In this end position the desired amount of liquid stored in the liquid container (not visible in this view) is normally transferred into the mixing chamber. To indicate a user of the capsule1that the plunger13is positioned in its end position relative to the capsule body lithe capsule1is provided with an activation control7. The activation control7comprises one or more recesses extending into the capsule body11from the rear end112and protrusions at the plunger13which are sized and shaped to substantially exactly fill any structure recessed by the recesses in the capsule body. In the example the shape of the read end112of the capsule body11and the protrusions13complement to a circumferential ring-shaped structure. Therefore any malposition relative to the end position can be easily recognized by the user.