The practice of osteointegrated implants for installing dental prothesis has become wide spread.

According to a consolidated method, a metallic insert, commonly of titanium, optionally provided with a coatings of an osteointegration promoter substance, is permanently implanted in the mandibolar or mascellar bone.

On the implanted metal insert a so-called mesostructure that may be of titanium or alternatively of a gold alloy is mounted, the solid coupling between the two parts is fixed by at least a primary set screw, introduced through an axial hole in the mesostructure or trunk to be tightened in a threaded hole purposely realized in the metallic implant.

On to the so solidly fixed mesostructure is set a metallic core top structure or capsule onto which a crown of ceramic or resinous materials has been constructed.

The coupling between the mesostructure or trunk and the top structure or briefly the crown is realized with a polygonal section or alike section to prevent any relative rotation between the two parts. Moreover, a light conicity of the trunk and of the matching cavity of the crown piece ensure a solid coupling with substantial self-locking characteristics. In any case, in order to prevent that in time the crown piece may accidentally lift off the trunk piece, the joint between the two parts is usually cemented.

Such a practice has the disadvantage of making practically impossible or extremely difficult to remove the cemented crown piece without breaking or ruining it should need arise of intervening to tighten the primary set. screw or for any other reason. In order to prevent the need of reconstructing anew the crown piece, which is notoriously a costly operation, an alternative technique of fixing the crown piece to the mesostructure or trunk that is in turn fixed onto a osteointegrated implant has recently been proposed and developed.

According to such a technique, the crown piece, instead of being permanently cemented to the mesostructure is fixed by a grub screw or by a secondary set screw acting along an axis substantially radial although not necessarily orthogonal to the longitudinal axis of the primary set screw at fixes the mesostructure onto the osteointegrated implant.

In the crown piece, which if necessary may have a local thickening of the wall of the metallic core or capsule or even a radially extending protrusion, preferably oriented toward the inner part of the oral cavity, a receptacle-hole is formed for receiving a socket end grub screw or a socket head set screw suitable to be engaged by the tip of a screw driver.

According to a first embodiment, the generally radially oriented hole through the inner metallic wall of the crown piece is threaded and upon driving the grub screw or set screw, the tip forcibly engages into a niche or cavity purposely formed on the outer lateral surface of the mesostructure or trunk piece.

According to an alternative embodiment, instead of a niche or cavity, the mesostructure is provided with a threaded hole in which the grub screw or socket head set screw introduced through a geometrically aligned hole formed through the composite wall of the crown piece may be tightened.

According to a third embodiment, the crown piece and the mesostructure or trunk piece coupled one to other and eventually held in a casting may be simultaneously drilled and the hole through the wall of the crown piece and into the mesostructure of the trunk piece may be threaded in a single operation. The two pieces, once sequentially mounted on the implant, may receive a "bi-passing" grub screw λvhen finishing the installation of the dental prothesis into the oral cavity of the patient. Eventually, the niche or radial hole in the mesostructure of trunk piece may extend in depth as far as to reach (cross) the axial hole through which the primary set screw is installed, such that the tip of the grub screw or secondary set screw is forced against the primary set screw, thus enhancing its stability by preventing its loosening.

These techniques, ensuring an outstanding freedom of intervening for correcting and/or reestablishing a perfect functionality of even complex dental prothesis, have peculiar drawbacks and an underlying risk of damaging the components of the prothesis during the installation of the prothesis in the oral cavity of the patient.

Handling and maneuvering difficulties arise whenever the introduction and turning of the secondary fixing screw or grub screw of the top structure or crown piece can not be canned out by maneuvering a screw driver with a sufficiently long stem from outside the oral cavity, because the geometrical axis of the grub screw or secondary socket head fixing screw or better its geometrical extension, interferes with the teeth of the patient or is otherwise confined within the oral cavity; a situation that is very frequent (for example in case of front teeth prothesis) or in consideration of the fact that the opening of the receptacle for the secondary socket head screw or grub screw is, for obvious reasons, realized in a location such that it may be oriented toward the inner part of the oral cavity, in order to remain less visible.

In these cases, the maneuvering must be carried out within the oral cavity and it is normally performed by using special small and relatively short (stub like) screwdrivers to be held and maneuvered between the thumb and index fingers of the operator.

As immediately perceived, this type of maneuvering is not free of risks beside being extremely awkward.

Moreover, the enhanced difficulty of maneuvering such a stubby screwdrivers reduces the ability of the operator to control the maximum torque he is actually applying to the grub screw upon tightening it.

Being the diameter of the screws used for this purpose, the smallest possible for obvious reasons of encumbrance and being the metallic materials with which are generally made the mesostructure and of the inner metallic wall of the crown piece titanium or gold alloy, it is present and hardly avoidable the risk of ruining the threaded parts by inadvertently exerting an excessive tightening torque upon driving and tightening the grub screw or secondary socket head screw.

The ruining of the threads may in some cases oblige to reconstruct anew the mesostructure or trunk piece or the crown piece.

These problems and difficulties complicate the establishment, acceptance and consolidation of a precise operating protocol for these types of interventions, which limit the diffusion of these alternative fixing techniques, notwithstanding their potentially great advantages.

To all these risks and difficulties has now been found an excellent solution based on the use of a special screwdriver or kit that, beside allowing to tighten a grub screw or secondary socket head screw for fixing a crown piece on a mesostructure or trunk piece by maneuvering the instrument essentially from outside the oral cavity even in case of grub screws or secondary socket head screws having an axis of introduction and tightening oriented toward the inner part of the oral cavity, ensures a tightening of the screw substantially free of the risk of ruining the threads through the inner metallic wall of the crown piece and/or in a hole of the mesostructure or trunk piece, formed during their fabrication in laboratory.

Substantially, the instrument is composed of a tip, the tip portion of which has a polygonal cross section and a sufficiently long holding stem that are joined together by a cardanic joint, and metallic a twist crown mounted at the end of the handling stem through a safety dynamometric limiting device. Such a dynamometric limiting device, preliminarily trimmed before using the special screwdriver, render substantially impossible to exceed in applying a proper tightening torque upon terminating the driving of the grub or socket head screw, thus preserving the integrity of the threaded parts during the final steps of installation of the dental prothesis within the oral cavity of the patient.

The invention for which protection is sought defined in the appended claims.

Figures 1, 2 and 3 schematically show the technique of installing a dental prothesis with a crown piece removably fixed by a grub screw or socket head screw.

Figure 4 is a view, partially in section, of a screwdriver provided with safety dynamometric limiting device of the present invention.

Figure 5 is a partial detail view of the safety dynamometric torque limiting device.

Figure 6 schematically shows a typical situation of utility of the screwdriver of the present invention.

Figures 1, 2 and 3 illustrate the various components and the manner in which a dental prothesis is installed.

A metal insert 1, commonly of titanium, is permanently implanted in the mandibular or mascellar bone.

A mesostructure or trunk piece 2, often of the same metallic material of the implant 1, is precisely conformed to be inserted with a dedicated tail portion 3, commonly having a truncated pyramidal cross section, in a cooperating cavity of complementary geometric shape formed on the implant 1. A primary axially driven set screw 4 is eventually tightened in a axial threaded hole present at the bottom of the cavity of the implant 1, thus solidly fixing the mesostructure 2 onto the implant 1. The top structure or crown piece 5, commonly including an inner metallic capsule, generally made of the same metallic material of mesostructure 2 and of the implant 1 , over which the outer crown of ceramic or resinous material is modeled, has a radially oriented hole 4 through it lateral wall. The outer opening and the radial axis of such a hole-receptacle 4 are preferably oriented toward the inner part of the oral cavity. Through the receptacle-hole 4, having previously foπned on the mesostructure • a geometrically coinciding niche or alternatively a radially extending coaxial hole 6, a grub screw or secondary socket head screw 7 is introduced and tightened.

The grub screw or secondary socket head screw 7 may be driven through either a threaded inner metallic portion of the hole 4 passing through the composite wall of the crown piece or capsule 5, or in a threaded hole 6 formed in the mesostructure 2 or in both coaxially aligned and threaded holes to complete the assembly of the dental prothesis. With the exception of the implanted metal body 1 , the other components of the prothesis may be disassembled for carrying out corrective and/or repair operations of the prothesis.

A preferred embodiment of the cardanic screwdriver with twist crown incorporating a safety torque limiting dynamometric device for driving the grub or secondary fixing screw 7 is shown in Figure 4 and 5.

The instrument as a tip piece 10 and a handling stem 11 joined by a cardanic joint 12.

The twist crown 13 presents at the extremely of the stem 11 to be turned by the operator with his finger tips, has a safety torque limiting dynamometric device that effectively prevents the application to the grub or secondary socket head fixing screw 7 of an excessive torque that could ruin the threads foπned either in the hole passing through the metallic inner wall of the crown piece 5 (capsule) and/or of threaded hole 6 formed into the mesostructure 2, parts that are generally made of titanium or of a gold alloy and which therefore have a relatively low mechanical strength. On the other hand, the grub or secondary socket head fixing screw 7 has essentially the function of stabilizing (preventing an accidental lifting/pull off) the substantially self-locking conical coupling that is established between the mesostructure and the crown piece and which is instrumental to providing a great rigidity of the coupling.

According to the preferred embodiment shown in the figures, the safety torque limiting dynamometric device is realized in an extremely simple and effective manner by a left-head threads screw coupling between the crown 13 and a threaded end 14 of the handling stem 11. The maneuver crown 13 is screwed on the left-hand threaded end 14 of the stem 11 as far as abutting against the step 15 present at the base of the left-hand threaded end 14 of the stem and thereafter it is tightened using a dynamometric spanner such to confer to the coupling a limit torque of tightening/releasing that may be generally comprised between 0,50 and 3,0 Ncm.

The operator, even when maneuvering the screwdriver under difficult handling conditions, is no longer subject to the risk of damaging the components of the prothesis upon tightening the grub or secondary socket head fixing screw 7 for concluding the installation steps of the prothesis in the oral cavity of the patient.

As depicted schematically in Figure 6, the cardanic joint 12 of the screwdriver allows to drive and tighten the grub or secondary screw 7 for fixing the crown piece 5 of the prothesis even at locations of difficult access to the grub or socket head screw, ensuring in any case the possibility of driving the screwdriver from outside the oral cavity of the patient.

The special screwdriver of this invention overcomes all the problems discussed in the preamble of the instruments normally used according to the prior art.

The whole cardanic screwdriver and in particular the tip 10 is preferably made of hardened stainless steel in order to confer to the tip a relatively high mechanical strength and hardness. A particularly preferred material is hardened stainless steel AISI 440, and the tip has a square cross section.