WO1999039651A2

WO1999039651A2 - Separation tools for abutment window and method

Info

Publication number: WO1999039651A2
Application number: PCT/US1999/002356
Authority: WO
Inventors: Inc. Implant Systems
Original assignee: Gittleman, Neal, B.
Priority date: 1998-02-05
Filing date: 1999-02-04
Publication date: 1999-08-12
Also published as: AU3063099A

Abstract

Tooling is described to aid in the removal of a cemented retrievable dental prosthesis from an underlying abutment. Methods of forming a gapped space in the abutment under the prosthesis and methods and tools for removing the cemented prosthesis by application of opposing forces to the surfaces forming the gapped space are detailed. A hand held prying tool and a jointed handle pliers are featured. Mechanically assisted tools using a hydraulic jack and an expanding hydrophilic polymer are described. Methods of forming negative draft windows with a dovetail milling cutter are shown. Electric discharge machining of an abutment window with hydrostatic relief grooves is featured.

Description

SEPARATION TOOLS FOR ABUTMENT WINDOW AND METHOD

Present dental practices tend toward the replacement of lost teeth with cylindrical or plate metal alloy implants embedded in the bone of the mandible or maxilla to support the artificial tooth restoration. If extensive replacement of several teeth is needed, several implants, alone or in conjunction with existing teeth prepared as abutments, are used to anchor the replacement prosthetic teeth. As the number and complexity of support abutments are increased, the difficulty in aligning and fastening the prosthetic restoration increases. The use of several mechanically connected parts at each post or plate implant site adds to the possibility of misalignment or biomechanical failure.

The last two decades have led to a revolution in implant prosthodontics. Titanium alloy implant cylinders or plates are intimately installed in holes or slots drilled in the underlying bone. It is the practice to allow several months to pass while the underlying bone bonds to the surface of the implant. For this reason, implants are provided with at least one threaded hole on the crestal surface or edge. The holes are temporarily capped with a healing screw to prevent the downgrowth of soft tissue and bone into the internal threads. The soft tissue is sutured over the implant until the intimate metal-bone bond is effected.

At the next surgical encounter, the soft tissue is resected and the healing screw is replaced with a metal alloy perimucosal extension of selectable height and emergence profile and the soft tissue is sutured around the base of this extension. This extension is usually bolted in place and prevented from rotating by means of locating pins and holes or internal and external matching hexagonal (or other regular polygon shaped) projections. These perimucosal extensions form the support for artificial abutments used to support the final prosthetic restoration. The final prosthodontic restoration requires a close mechanical mating between the abutments and the matching internal aspect or underside of the prosthesis. These closely matched parts often consist of telescoped, tapered cylindrical surfaces requiring a tight, non-binding, "passive" fit. This requirement for close tolerances places inordinate requirements on the precision and technical skills of the dentist and the laboratory technician. Parallel alignments of the axes of each abutment to prevent binding of tapered fits cannot be easily guaranteed.

Much of the current discussion in the field of dental implantology centers around the durability and maintainability of the various methods of attaching the final restoration to the underlying abutments. Bolting with threaded fasteners through the occlusal surface of the restoration and back filling with composite materials complicates the cosmetics and the retrievability of the prosthesis. Bolting through the non-cosmetic, lingual side of the prosthesis has the additional requirement for a greater thickness of metal to provide mechanical support, thus reducing room for the tongue and potentially affecting speech, and the periodontal health of the abutment.

Excessive inline or rocking pressure transmitted to an individual implant from the overlying restoration may lead to frank implant failure. Failures may occur from the loosening of a screw caused by thread walking or the backing out of a screw by micro-movements. The shifting of an abutment from repetitive stresses exceeding the elastic limits between the screw thread and the internal thread of the implanted post or plate may cause the flexure or excessive loading of a single implant. Long term changes in the underlying bone structure in response to uneven stresses may lead to the loss of an individual dental implant. For each additional mechanically attached connection, alignment errors accumulate and reduce the likelihood of a good non- binding, stress free "passive" fit.

In the event of alignment error, poor fit, failure of the prosthesis, infection, or other complication, it may be necessary to remove the prosthesis from the abutment. However, prior methods of removing cemented restorations involve hammering movements, which are characterized by a loss of control. It is, therefore, an object of the present invention to provide a prosthodontic apparatus which, in combination with an appropriate dental cement, yields a predictable technique for securing, yet retrieving the final restoration.

The present invention acts to equitably distribute the loading forces with a retrievable dental cement between the matching faces of the abutments and the internal aspect of the final restoration. Each abutment is made with at least one step or shelf on the lingual face to act as a bearing surface for a removal instrument. The final prosthesis is equipped with a substantially flat-topped window ledge on the lingual side. There is a matching shelf on the implant abutment, with enough space between the surface of the shelf and the flat top of the window for the introduction of an instrument for removing the prosthesis from the abutment. This instrument is used to apply force between each abutment and the mating ledge in the underside of the final restoration. The instrument applies an even, opposing force between the overstructure and the abutment, eliminating the potential damage to both structures.

The present invention is also directed to a problem which results from the ingress of food particles into the space, or window, between the ledge and the shelf of the abutment. A conformal, removable and replaceable plug comprised of resilient, durable polymer, such as a high durometer rubber, flexible silicone rubber compound or urethane may be secured within the window to provide a continuous smooth lingual aspect to the cemented restoration and the adjacent prosthesis. In addition to the smooth non-irritating surface presented to the tongue, the plug may be made with such precision that a liquid- and gas-tight seal between the plug and the mating surfaces is achieved. In this way, a second objective of preventing the ingress of food particles, which can lead to bacterial fermentation and noxious odors is accomplished. Another objective is the use of a prying tool since the resilient, rubbery nature of the plug allows for forced removal. In combination with grooves, retaining recesses, or a shallow reverse (negative) draft in the window, the plug may also be securely held in place. The plug can also be slightly oversized to offer a compression seal against the window surfaces. The plug can be "hard surfaced" on the lingual aspect with a polymer of higher durometer by chemical, thermal or other means known to those skilled in the art. Alternately, a thin, hard metal or porcelain cover bonded to the resilient plus may provide a lingual surface with a longer useful service life.

Another embodiment of the invention places the window in the abutment near or at the soft tissue margin. The upper ledge of the window is supplied by the substantially flat surface on the bottom lingual side of the prosthesis, while the lower ledge of the window is formed by an inset undercut shelf in the abutment. An instrument is introduced into the window to apply opposing forces against the upper ledge and the undercut shelf to carefully separate the retrievably cemented prosthesis from the abutment.

In another aspect, the present invention utilizes the fact that intra-osseous implants are fabricated from pure titanium or titanium alloys for bio-compatibility with living tissue. All mating metal parts must be made from the same metal to avoid electrolytic corrosion effects, including the generation of nerve disturbing electrical currents. Thus, the abutment must be made from the same titanium as the implant post. Drawn titanium rods are machined to produce the finished product because cast titanium results in a more easily fractured grain structure that compromises the prosthesis.

Present methods of manufacturing artificial abutments for dental prostheses from titanium or its alloys require machining operations which include turning and milling in preference over more exotic techniques like electrical discharge machining (EDM). An easy method to produce a shelf on the abutment is to mill out a curved or circular sector with a milling cutter resulting in a cupped or concave curved shelf. The use of a dovetailed cutter to make a negative draft to aid in the retention of the resilient conformable plug is recommended resulting in the gap widening progressively toward the central axis of the abutment.

Alternately, EDM is used to form the specially shaped window in the abutment to accommodate variously shaped, drop-in resilient window plugs. In another embodiment of the invention the abutment gap can be manufactured at an upward angle which allows for easier introduction of an instrument for applying opposing forces to the abutment and prosthesis.

The separation of the prosthesis from the underlying abutment can be accomplished by application of a force between the abutment and the overstructure. In the presently preferred embodiment, this force is applied by introducing an instrument for applying equal and opposite force into the window formed by the ledge of the prosthesis and the shelf formed on the abutment. In one embodiment, the forces are applied by means of a flat bladed, wedge shaped tool offering a broad purchase against the upper surface of the abutment window outer edge and the undersurface of the prosthesis with a levering motion. The tool has an angled and offset neck connecting the grasping handle to the prying wedge end for lingual clearance. A left and right handed model is described for either side of the mouth. A broad even bearing tool tip edge has the advantage of avoiding point stresses and metal deformation at the bearing points. In another embodiment, the forces are applied by a tool which is fashioned as a jointed pliers with opposing jaws. The lingual-side jaw has a wedge shaped tine that enters the window between the abutment and the prosthesis. The opposing jaw has a curved, cushioned face to bear against the buccal side of the abutment and prosthesis. By closing the jaws of the jointed pliers with hand pressure, the wedge-shaped tine is introduced into the window with equal and opposite opposing forces applied to the prosthesis and abutment shelves until the cement holding the abutment and prosthesis separates.

In another embodiment, the invention comprises the introduction of a hydraulic jack comprising two opposing surfaces into the window and application of the opposing forces by the two opposing surfaces of the jack that are driven apart with the introduction of a non-compressible hydraulic fluid forced between the hydraulic jack surfaces with a hydraulic pump. The two opposing surfaces are the face of a ram and the base of the jack housing respectively. Large pressures bearing against broad matching surfaces within the window gap with the least distortion of metal are accomplished in this way. The hydraulic pump can be hand or machine driven.

Another driving mechanism for enlarging the window gap between the prosthesis and the abutment uses a hydrophilic polymer hydrogel to force the two metal surfaces apart until the cement holding the prosthesis and the abutment separates. The polymer hydrogel swells with the introduction of activating water with a force of approximately 800 pounds per square inch. Although the window gap is small, the force can be increased by using a set of hinged plates with a dehydrated hydrogel pellet sandwiched in between the plates. Each plate has a projection jaw inserted to bear against the opposing surfaces of the window and will act to separate the prosthesis from the abutment with the application of water to the hydrogel. Referring now to the figures, Figure 1 shows a cross sectional elevated view of a final restoration with various means of support; Figures 2a and 2b display a cross sectional front and side view of a single implant with abutment "steps" and secondary restoration with "window;"

Figure 2c is a top view of the abutment;

Figure 3 details a cross sectional view of the abutment having an instrument for applying opposing forces between the abutment and the prosthesis introduced into the window of Figure 2;

Figure 4a shows an elevated, exploded view of an abutment with matching prosthesis and polymer plug constructed in accordance with the present invention;

Figure 4b shows a perspective view of a preferred embodiment of a tool for applying opposing forces for removing the prosthesis from the abutment of the present invention in the form of a prying tool;

Figure 4c shows a cross-sectional view of the end of the prying tool of Fig. lb;

Figures 4d and 4e show cross sections through the ends of various alternative embodiments of prying tools constructed in accordance with the present invention; Figure 5a shows a perspective view of a second embodiment of the instrument for applying opposing forces between abutment and prosthesis in the form of jointed pliers;

Figure 5b details the manner in which the jaws of the jointed pliers of Fig. 5a clamp about an abutment and prosthesis to introduce the instrument into the window to apply the force couple between abutment and prosthesis;

Figure 6 details an elevated view of an abutment and prosthesis having an instrument introduced into the window therebetween for application of a force couple in the form of a hydraulic jack and pump;

Figure 7 details an elevated view of an abutment and prosthesis having yet another alternative embodiment of an instrument introduced into the window therebetween for application of a force couple in the form of a hydrogel activated spreading mechanism;

Figures 8a and 8b detail a method of making an abutment shelf with negative draft using a dovetail milling cutter; Figures 9a and 9b detail a method of forming a substantially flat abutment shelf with negative draft using a dovetail milling cutter; and Figures 10a and 10b show a method of forming a shaped window in an abutment with a shaped electrode using electric discharge machining, or EDM.

The invention will now be described in more detail by referring first to the partially edentulous lower left jaw, shown schematically in elevated view from the lingual side in Figure 1 , which illustrates the combination of methods used to secure the secondary prosthesis 1. Natural tooth 2 is not modified, nor is its nearest neighbor

26. The next five dental positions are replaced with the prosthesis 1, with each artificial tooth bridged by the underlying structural member 15. The bone surface 4 is shown with its overlying soft tissue margin 3. Typical plate implant 5 and post implant 7 are firmly set within the bone 4 with extension penetrating through the soft tissue 3. The plate extension 8 has an internal thread and coupling means to retain tapered and stepped abutment 6. Post implant 7 rises through the soft tissue margin 3 and terminates in a similar tapered and stepped abutment. This abutment has a step or shelf 19 shown extending for a distance circumferentially around the long axis of the post implant and opposing the substantially parallel, flat-topped ledge 20 of a lingual side window in the prosthetic appliance. The gap or window 22 between these two substantially parallel surfaces allows for the introduction of a tool for applying opposing forces to part the overlying prosthesis from the abutment underneath. Those skilled in the art who have the benefit of this disclosure will recognize that the surfaces 19 and 20 forming window 22 need not be parallel to function for their intended purpose (described below).

Cement applied within the bonding space 18 between the prosthesis and the abutment acts to firmly retain the prosthesis to the abutment with an even distribution of forces. This even distribution of forces prevents any undue stresses on any one abutment.

Figure 2a presents an elevated view of a typical single post implant 7 with a perforation 9 to allow for ingress of bone growth for additional reinforcement. Prosthesis 1 may be removed by application of opposing forces between surface 20 of the prosthesis and shelf 19 of the post abutment 6 together forming the top and bottom of window 22. Since the forces are in opposition, minimal pulling or twisting forces are transmitted to implant post 7 with less chance of loosening the implant or breaking the prosthesis. Multiple shelves 19 and 29 accommodate different elevations for the proper design and access to the window 22 of the prosthesis. Figure 2c shows the plan view of the top of the abutment post with the shelves or steps 19 and 29 extending for some distance around the circumference of the post abutment 6. Figure 2b shows the elevated sectional view through the post implant 7. Shelf

19 and opposing substantially flat-topped window ledge 20 show the area in which opposing forces are applied. Figure 3 illustrates the introduction of a tool for applying the opposing forces between shelf 19 and ledge 20 in the form of a prying tool described in more detail below. The choice of cement joining the surface of abutment 6 and the internal aspect of prosthesis 1 determines the maximum force needed for removal.

Figure 3 gives an elevated sectional view of one member of the prosthesis 1 and a post implant 7 with its attached abutment 6. The step or shelf 19 of the abutment 6 and the opposing substantially flat-topped ledge 20 of the window 22 in prosthesis 1 form the gapped, substantially parallel mating surfaces against which the opposing forces are applied. The instrument for applying the opposing forces applies force to the opposing surfaces of shelf 19 and ledge 20 and shears the cement bond in the bonding space 18 to release the fixed retrievable prosthesis 1.

Figure 4a shows a lateral exploded and elevated view of a typical abutment 34, prosthesis 33 and food ingress shielding plug 44. Opposing surfaces 37 and 36 form the upper and lower window prying surfaces for the removal of the prosthesis. Dental cement in space 18 holds the prosthesis in place until opposing forces are applied within the window. Profiles 46, 47, and 48 closely match the emergence profile through the gums of the natural tooth being replaced. Screw attachment hold 45 allows the abutment 34 to be mechanically mounted to the underlying dental implant (not shown). The food ingress prevention plug 44, made of a durable resilient polymer, is held captive while the prosthesis is in place but is removed with a dental pick.

Referring now to Figures 4-7, several alternative embodiments of the instrument for applying opposing forces between the abutment and the prosthesis are shown. Each of the instruments (or a portion of each instrument) shown in Figs. 4-7 is introduced into the window formed by the opposing surfaces 36 and 37 on abutment 34 and prosthesis 33, respectively. Once introduced between the surfaces 36 and 37, opposing forces are then applied, preferably simultaneously, against the surfaces 36 and 37 by the instrument. Those skilled in the art who have the benefit of this disclosure will recognize that, although it is preferred that the opposing forces applied to the surfaces 36 and 37 are normal to those surfaces, the opposing forces need not be applied in a direction parallel to the longitudinal axis of abutment 34. All that is required for use of the method of the present invention is that the opposing forces have a component which is normal to the surfaces 36 and 37. However, in the preferred embodiment, the lines of action of the opposing forces are substantially co- linear and parallel and they are substantially parallel to the longitudinal axis of the prosthesis-abutment assembly.

The hand tool shown in Figure 4b has a handle 13, offset bends at 14 and 15 with an offset neck 12 for lingual clearance, and a prying end 10 which may take several shapes. Figure 4c shows a cross section through the prying end (section C-C) of a hand tool maximizing tool strength for greatest prying force. The cross section in Figures 4d and 4e are alternate hand tool designs to allow for application of forces when a rocking or twisting motion is more easily accomplished. The prying tool is manufactured in a right and left handed version for use on opposite sides of the mouth. The handle 13 is wide and flat enough for the hand to grip the tool and easily impart a prying or rocking motion to the prying end. Those skilled in the art will recognize that the prying or rocking motion applies the opposing forces along lines of action which may or may not be co-linear and that they may not be parallel to the longitudinal axis of the abutment-prosthesis assembly. However, a substantial component of the opposing forces is exerted in a direction parallel to the longitudinal axis of the prosthesis-abutment assembly and normal to the surfaces 36 and 37.

A jointed pair of pliers with opposing jaws as shown in Figure 5b is designed to force a wedged tip 125 between window surface, or shelf, 36 on the abutment 34 and opposed window surface, or ledge, 37 on the prosthesis 33 to drive the abutment and prosthesis apart. An opposing soft resilient face 124 on opposing plier jaw 127 applies a backing force to oppose the force applied to introduce the wedged tip 125 into the window to prevent damage to the buccal side of the prosthesis and undue stress on the underlying implant (not shown). The practitioner squeezes handles 120 together to force jaws 126 and 127 together. In this manner, the tip 125 is introduced into the window and the angled surfaces forming tip 125 apply opposing forces against the opposed surfaces 36 and 37 forming the window and the lines of action of the opposing forces include a component which is normal to the surfaces 36 and 37. The two halves of the pliers are joined by a movable joint 121. Bends or offsets 122 and 123 in the arms of the pliers allow the jaws to reach over existing teeth or prostheses. In this way the cement hardened within gap 18 is parted. Figure 6 shows an exploded, elevated view of artificial abutment 34 and prosthesis 33 with a hydraulic jack comprising base 111 and moving ram 1 10 which has been introduced into the window formed by surfaces 36 and 37 by placing the base on window surface 36. The window surfaces 36 and 37 are forced apart by application of pressure on the hydraulic pressure source 113 by means of a plunger 115 pressed down by means of activator 114. Hydraulic fluid transmits the downward plunger pressure through tube 112 to the hydraulic jack forcing the base 111 and moving ram 110 apart. Application of opposing forces in this manner breaks the bond within the cemented gap 118 and frees the prosthesis. The hydraulic ram creates broad, even pressure with minimal stress to surfaces 36 and 37. Other methods of forcing an incompressible fluid into the hydraulic jack include a lever or motor to multiply the force applied to the plunger 115.

Figure 7 relies upon a pellet of hydrophilic polymer hydrogel 95 to absorb water, expand and exert a force between two hinged plates 93 and 94 introduced into the window. This hydrogel pellet forces jaws 91 and 92 apart within the window gap formed by abutment window surface 36 and prosthesis window surface 37 when contacted with water. Hinge 96 facilitates the motion of the two plates. Polymer hydrogels can exert a force of 800 pounds per square inch when soaked in water.

A convenient method for forming a window surface in the lingual side of an abutment relies on the use of a dovetail cutter 100 as detailed in Figures 8a and 8b. Figure 8a shows an elevated, lateral view of abutment 34 with rotating dovetail milling cutter teeth 102 being used to cut an abutment window surface 36 with an upwardly sloping lip 98.

Dovetail milling cutters have flat cutting teeth 101 and side cutting teeth 102 and a greater diameter at the farthest cutting end tapering to a smaller diameter toward the supporting shank 103. This creates a window with negative draft, i.e., a window space between the abutment surface 36 and the overlying prosthesis surface (not shown) that increases in depth as the center axis of the abutment is approached. The negative draft will prevent any drop-in window plug from falling out. Figure 8b details the elevated lingual side view of the rotating dovetail cutter forming the negative draft window surface in the abutment. Window lip 98 is closer than the central portion of surface 36 to the overlying prosthesis window surface (not shown).

Figure 9b shows a substantially flat-bottomed window surface formed in an artificial abutment 34 by means of a dovetail cutter 100. This is accomplished by moving the dovetail cutter from side to side. The negative draft of window surface 36 is still preserved to prevent the loss of a window plug.

Several concurrent machining or forming operations can be accomplished by means of electric discharge machining, EDM, with a die-sink electrode. Figures 10a and 10b show the lateral and lingual elevated views respectively of a tungsten-copper EDM electrode 108. The electrode is connected to one terminal of a source of electrical current while the abutment 34 is connected to the return terminal. The electrode is positioned and slowly lowered in such a manner as to just sustain an arc or spark between the EDM electrode and the abutment workpiece. A non-conductive electrolyte, usually clean, deionized water, is forced between the EDM electrode and the abutment workpiece to cool and remove the abutment metal eroded by the arc or spark. The amount of electrical current is monitored and used to control the distance and thus the spark between the electrode and workpiece. Complicated shapes can be sunk in difficult to matching substances such as drawn titanium used for most abutments. As illustrated, EDM electrode 108 is machined with curves 105 and 106 and sloped surface 109 to form abutment window shelf 36 with negative draft. Additionally, protrusion 107 simultaneously forms the hydrostatic relief grooves 73 and 74 in the abutment. Several machining operations are accomplished simultaneously. The 70% tungsten/30% copper electrode holds its shape for many machining operations while eroding away unwanted metal to form windows in titanium abutments. The eroded surface of the titanium has a smooth mat finish and the underlying titanium metal is stress free. Those skilled in the art will readily recognize from Figures 8-10 that the surface 36 on abutment 34 having a negative draft, even though it is not flat, together with surface 37 on prosthesis 33 provides the same function as the surface 36 shown in Figures 4-7. Specifically, when the instrument for removing the prosthesis 33, is introduced into the window formed by surfaces 36 and 37, the surface 36 shown in Figs 8-10 is acted upon by the opposing forces applied by the instrument to remove the prosthesis from the abutment.

Whereas these drawings and descriptions shown herein for the purpose of illustrating the invention show one tooth being replaced, the method and apparatus described apply to a multiple tooth replacement sites. These and other variations of the present invention may be made which fall within the scope of the appended claims even though such variations were not related above. The accompanying drawings referred to herein are illustrative of the invention but not restrictive hereto, and, together with the description, serve to explain the principles of the invention.

Claims

WHAT IS CLAIMED IS:

1. A method for separating a dental prosthesis from a dental abutment comprising the steps of introducing an instrument for applying opposing forces into a window formed by opposing surfaces on the abutment and the prosthesis and applying the opposing forces against the opposing surfaces with the instrument.

2. The method of claim 1 wherein the instrument comprises a prying tool comprised of a prying end for introducing into the window and the opposing forces are applied by prying against the opposing surfaces on the abutment and the prosthesis.

3. The method of claim 1 wherein the instrument comprises a wedge and the opposing forces are applied by introducing the wedge into the window.

4. The method of claim 3 additionally comprising applying a backing force to oppose the force of introducing the wedge into the window.

5. The method of claim 4 wherein the backing force is applied on the side of the prosthesis opposite the window.

6. The method of claim 1 wherein the opposing forces are substantially co-linear and parallel to the longitudinal axis of the abutment.

7. An instrument for separating a dental prosthesis from a dental abutment comprising a forcing end for introduction into a window formed between a shelf formed on the abutment and a ledge formed on the prosthesis and a handle, said handle being connected to said forcing end by an offset and angled neck.

8. The instrument of claim 7 wherein said forcing end is formed in a wide, substantially rectangular cross section for maximum tool strength and durability.

9. The instrument of claim 7 wherein said forcing end is provided with curved surfaces for bearing against the shelf formed on the abutment and the ledge formed on the prosthesis for applying opposing forces therebetween by rocking and prying.

10. An instrument for separating a dental prosthesis from a dental abutment comprised of a jointed pliers having handles, arms, and opposing jaws on the ends of said arms, one said jaw having a wedge formed on the end thereof for introduction into a window formed by a shelf on the dental abutment and a ledge on the prosthesis, and the other said jaw having a resilient pad formed on the end thereof.

11. A method for forming a window surface in a dental abutment comprising the steps of: securing a dental abutment; rotating and lowering a dovetail milling cutter to cut a curved window surface with negative draft in the lingual aspect of the dental abutment; and removing the dovetail milling cutter from contact with the dental abutment.

12. The method of claim 11 additionally comprising moving the dovetail milling cutter laterally.

13. A method of forming a window surface in a dental abutment comprising the steps of: applying an electric current lead to a shaped electrode; positioning and lowering the shaped electrode into controlled arcing contact with the dental abutment; and eroding metal from the dental abutment to form a window surface in the abutment.