Much of the current discussion in the field of dental implantation 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 complicate 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 micromovements. 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.

It is an object of the present invention to provide a confortable and hygienic dental restoration with a secure, yet retrievable prosthesis. Accordingly, in one

aspect, the invention provides a dental restoration comprising a dental implant abutment having a substantially flat shelf; a retrievable prosthesis which mates with said abutment and is formed with a window, the prosthesis having a window ledge for location in proximity to said abutment shelf to define between substantially opposing surfaces of the window ledge and abutment shelf to form a gap for entrance of a prying instrument; and a conformal plug for insertion in said window to seal said gap, said plug having a smooth lingual surface which is substantially continuous and aligned with lingual surfaces of said prosthesis and abutment. In another embodiment of the invention, the abutment is formed 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 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 a wedge-tipped extraction instrument. This instrument is used to apply a prying force between each abutment and the mating ledge in the underside of the final restoration. The prying instrument applies an even opposing force between the overstructure and the abutment eliminating the potential damage to both structures. Prior methods of removing cemented restorations have involved hammering movements under much less control. Apparatus embodying the invention, in combination with an appropriate dental cement yields a predictable technique for securing, yet retrieving the final prosthetic restoration.

The plug provided in the window may be formed in situ or may be a molded drop-in plug and protects against the ingress of food into the window and provides a smooth continuous surface to the tongue. Furthermore, by sealing the access window with a shielding conformal plug, buildup of bacterial plaque that cause decay and bad breath are reduced substantially.

More specifically, the window plug forms a food ingress prevention shield with a smooth exterior. The conformal removable and replaceable plug 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. The conformal plug also allows the use of a prying tool, since the flexible nature of the plug allows for forced removal. In combination with grooves, retaining recesses, or a shallow reverse 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 plug may provide a lingual surface with a longer useful service life.

The plug of the present invention lends itself to production with standard plug and window dimensions to aid the dentist in fitting the prosthesis. Alternatively, caulking with photo-curable silicone or other polymers may be utilized as a method of casting this plug in place. This thixotropic bonding material, extruded into the window is hardened in place with a short exposure to ultraviolet (UV) or blue curing light.

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 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. A prying tool can be introduced to carefully separate the retrievably cemented prosthesis from the abutment. A molded plug made from a biocompatible and durable plastic compound is dropped into place within the abutment inset undercut just prior to bonding the prosthesis into place. Projections on the drop-in molded plug match recesses in the abutment inset undercut and act to prevent the plug from slipping out. The projections and matching recesses are of sufficient size and shape to retain the molded plug. yet allow for removal by the proper application of force with a dental pick.

Another object of the invention is to obviate build-up of hydrostatic pressure when mounting a cementable prosthesis on a dental abutment.

In accordance with this aspect of the invention, a method is provided to avoid build-up of hydrostatic pressure in dental cement in the interstices and bonding space between a dental abutment and a prosthesis mounted thereon, the steps of providing a continuous channel between an interior portion of said interstices and an exterior surface of said abutment; filling said interstices with dental cement; and venting excess dental cement from said interstices through said channel to said exterior surface of said abutment to avoid said build-up of hydrostatic pressure.

In this aspect the invention also provides a dental apparatus including a dental implant abutment having a flat shelf; at least one cementable, retrievable prosthesis, the prosthesis mating with said abutment and defining therewith interstices to be filled with dental cement: in which apparatus said abutment has a continuous channel leading from an internal portion of said interstices to an external surface of said prosthesis. said channel allowing venting of an excess of said dental cement from said interstices to relieve hydrostatic pressure for proper seating of said prosthesis upon said abutment.

The channel may include a groove in the surface of the implant abutment which acts to direct excess cement from within the intervening space between the abutment and the overlying prosthesis. This channel, by allowing excess cement to flow in a controlled manner to prearranged collection site, prevents the capture of an incompressible mass of cement which will hold the overlaying prosthesis in a lifted, malocclusive position. The channel described prevents the formation of blind pockets of dental cement that would exert an outward force and weaken the joint during solidification of the dental cement.

A plug of a dental restoration embodying the invention may be grooved, and together with the groove formed in the abutment, forms a continuous channel for the extension and egress of excess dental cement during the placement and alignment of the prosthesis.

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 prying instrument in use.

Figure 4a is a front view with abutment "steps" and conformal plug 30.

Figure 4b is a cross sectional, exploded view of a single implant conformal plug 30.

Figure 5 is a cross sectional view of a plug designed to mate more securely with a prosthesis.

Figure 6 details an elevated view of a polymer compound injector mechanism for filling in the window between an abutment and the prosthesis.

Figure 7 shows an elevated view of the cured polymer plug.

Figure 8a details a plan and figure 8b details an elevated view of a self-retaining. molded drop-in window plug for window located in the lingual aspect of a cementable. screw retained abutment. Figure 8c shows an alternate molded plug profile with snap-in projections.

Figure 9a details a plan and Figure 9b an elevated view of an alternate self-retaining. molded drop-in window plug located in the lingual aspect of a cementable, screw retained abutment. Figure 9c shows an alternate molded plug profile with snap-in projections.

Figure 10a shows a plan and Figure 10b an elevated exploded views of a self-retaining. molded drop-in window plug and corresponding abutment with an scalloped emergence profile and window with a cross section of a prosthesis shown in place over the abutment. Figure 10c details a plan view of a molded plug with snap-in projections.

Figure ii a displays a plan view of a grooved abutment and resilient plug with <BR> <BR> <BR> matching groove. Figure 1 lb shows an elevated cross-sectional view of the grooved abutment and matching resilient plug. Figure 1 le shows a plan view of a grooved molded drop-in plug with a differing means of retention.

Figure 12a displays a plan view of a grooved abutment with elongated lingual aspect. Figure 1 2b shows an elevated cross-sectional view of a grooved abutment with restoration in place. Figure 12c details an alternate grooved resilient plug with a different retention configuration.

The invention will now be described by reference to the attached figures. The partially edentulous lower left jaw, shown schematically in elevated view from the lingual side in Figure 1, 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 it's 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 22 between these two substantially parallel surfaces allows for the introduction of a prying or twisting tool 21 (see Figure 3) to part the overlying prosthesis from the abutment underneath.

Cement applied within the bonding space 18 between the prosthesis and the abutment acts to firmly retain the prosthesis to the abutments with an even distribution of forces. This 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 force between surface 20 of the prosthesis and surface 19 of the post abutment 6. 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 2g accommodate different elevations for the proper design of and access to the window 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 flat-topped window ledge 20 show the area of force application. Figure 3 illustrates the use of a tool 21 to pry between shelf 19 and ledge 20 to part the cement in the bonding space 18 by the application of levered force. The proper 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, opposed mating surfaces against which the prying instrument is worked. The flat blade of the prying instrument applies force to the opposing surfaces 1 9 and 20 and shears the cement bond in the bonding space 18 to release the fixed retrievable prosthesis 1.

The provision of a reverse draft 52 in Figure 5, rather than the radial sides 51 of the aperture in Figure 2c, enables two additional embodiments of value, one in apparatus and one in method of use.

In the embodiment shown in Figure 2c, a plug 50 can too easily come out of the aperture during mastication and tooth brushing. In the embodiment shown in Figure 5, the angle of reverse draft 52 makes it more difficult to remove the plug 50.

The inner diameter 53 of plug 50 is larger than the inner diameter 56 at the outer edge of the reverse draft 52, providing a "keystone like" effect to secure the plug 50 until removed by the dental practitioner.

As to methods, in Figure 6, in a first method a small mass 62 of silicone or other polymer is placed in the window 22, and with his finger tip or spatula, the practitioner rounds the exterior surface of the moldable material to blend with the adjacent prosthesis surface.

Alternately, in Figure 5, a plug 50 is molded to the dimensions of the window aperture, except that the plug is slightly larger. The plug is then be pressed into the aperture, where the larger end 53. expanding into the aperture, will secure it in the same keystone fashion.

In Figure 6, a syringe 63 filled with a paste-like, curable compound, is used to express some of the compound 62 into the window 22 between the abutment 6 and the prosthesis 1. This compound can be any of a number of biocompatible formulations known to the skilled dental practitioner. The compound can be smoothed with a spatula or the finger to form a smooth continuum on the lingual aspect of the restoration prior to curing into a hardened mass. In Figure 7, the compound 62 completely fills the window void 22 in a hardened state.

Figures 10a and 10b, in exploded views show the molded drop-in window plug 41 with interlocking projections 42 and recesses 43 providing an effective retention of the plug when overlay prosthesis 33 is cemented in place. This molded plug 41 is captive between substantially flat surface 37 on the bottom lingual margin of the overlay prosthesis 33 and the opposed surface 36 of the abutment 34 forming a window 22. The molded window plug projections 42 "snap" in or out of the recesses 43 between surfaces 36 and 37 and provide a seal against debris. Outer curved surface 32 of the molded plug provides a tight, continuous seal with the emergence profile of the abutment 34 and overlay 33. The emergence profile, on the lingual aspect. offers a wider shelf for improved purchase of the prying tool.

Figure 10c details a thinned section 61 of the molded plug 41 designed to allow projections 42 to flex into the mating recesses 43 on the abutment 34. Similar thinned sections 61 can be molded into the plug 41 detailed in Figure 8c to allow the plug to be snapped in and out of the prying window 22.

In Figure 1 orb. the collar 46 of the abutment 34 extends through the margin of the gum with an emergence profile 47 that closely mimics that of the natural tooth.

The cylindrical projection 48 makes an intimate, non-rotating mechanical connection to the underlying post implant (not shown) by means of a threaded fastener (not shown) through interior hole 45. The tapered projection 44 of the abutment is retrievably cemented to the overlay restoration 33.

The overlay restoration 33 is shown as a cosmetic porcelain over metal but is not limited to this composite. The window 22 formed by surfaces 37 and 36, when freed of molded plug 41, provides a space for insertion of and action by a suitable prying tool to free the overlay prosthesis without damage.

Figures 8a and 8b show another embodiment of the locking mechanism for the molded drop-in plug 41. The plug can be made from a biologically acceptable plastic compound, such as one of the polymethacrylates. Projections 42 and recesses 43 are sized so that the plug 41 can be dropped into place just prior to cementing the prosthesis in place and yet allow for removal with moderate force. The molded drop-in plug can be forced out with an appropriate tool if the plug is carefully designed and manufactured, using a durable and flexible plastic for the plug, in combination with the correct size for projections 42 and recesses 43. In Figure 8c, projections 42 have a thinned cross section 61 to act as curved leaf springs to snap in and out of rounded recesses 43.

Figures 9a-9c show an alternate molded plug profile with snap-in projections.

Window 22 is filled by drop-in plug 32 with thinned sections 61 allowing projections 42 to act as latches that snap into recesses 43 on abutment 44.

In an embodiment detailed in Figures 12a and 12b, a continuous channel is formed in the abutment tapered projection 44 that is comprised of horizontal groove 73 and vertical groove 74. Mating groove 75 in the molded, drop-in window plug 41 provides an effective egress of dental cement from the curved outer surface 32 of the plug as the overlay prosthesis 33 is cemented in place. The molded plug 41 is captive between the substantially flat surface 37 on the bottom lingual margin of the overlay prosthesis 33 and the opposed surface 36 of the abutment 34. The molded window plug projections 42 "snap" in or out of the recesses 43 between flat surfaces 36 and 37 in the manner described above.

When prosthesis 33 is mated against the abutment tapered projection 44, excess dental cement is expressed from within gap 18 and forced along horizontal groove 73, vertical groove 74 and along horizontal groove 75 in the molded drop-in plug 41. The excess dental cement is easily removed from the surface of the lingual aspect of the seated prosthesis and plug after the cement has set.

Figure 1 c details alternate embodiments of molded drop-in plug designs with channel groove 75. Thinned sections 61 of the molded plug 41 are designed to allow projections 42 to flex into the mating recesses 43 on the abutment 34. Similar thinned sections 61 can be molded into the plug 41 detailed in Figure 12c to allow the plug to

be snapped in and out of the prying window. Groove 75 is formed during the plug 41 molding process.

In Figure 11 b, the collar 46 of the abutment 34 extends through the margin of the gum with an emergence profile 47 that closely mimics that of the natural tooth.

The cylindrical projection 48 makes an intimate, non-rotating mechanical connection to the underlying post implant (not shown) by means of a threaded fastener (not shown) through interior hole 45. The tapered projection 44 of the abutment is retrievably cemented to the overlay restoration 33. As an added protection against the loosening of the threaded fastener holding the abutment to the implant post, the threaded fastener is locked in place by the dental cement filling interior hole 45 and surrounding the head of the threaded fastener. Removable dental cement is used in this region. In figure l lc, projections 42 have a thinned cross section 61 to act as curved leaf springs to snap in and out of rounded recesses 43.

Another embodiment of the invention allows for the placement of a continuous channel groove in the abutment mating surface of the overlaying prosthesis with the same intention to reiieve the pressure of excess dental cement by expression along said channel to an external collection area.

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 site. 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 thereto, and, together with the description, serve to explain the principles of the invention.