Abstract:
An anchoring pin for retention of a dental restoration onto a tooth stub, the pin including an upper retention portion on which a restoration is to be formed, a lower anchoring threaded portion self-threadable in a bore pre-drilled in the tooth stub and a stop collar interconnected between the retention portion and the anchoring portion. A stepped dental drill having two coaxial drill bits of different diameters is provided, which in addition to forming the bore makes a counterbore at the mount of the bore at the exposed excavated surface of the tooth. The counterbore receives the stop collar of the anchoring pin as the latter is driven into the tooth bore thereby preventing bottoming out of the pin so as to avoid concentrated stress at the tip of the pin and also providing distribution of stress as the wider collar abuts the bottom of the counterbore.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a dental anchor or pin for aiding in the retention of a dental restoration and, more particularly to a dental anchor provided with means for reducing stress both upon a tooth stub in which it is inserted as well as reducing cracks in the restoration which is built up over the anchor. 
     Dental anchors or pins for retaining dental restorations are well-known in the art of dentistry, particularly anchors or pins having threads thereon for securement in the tooth structure. The tooth stub is typically prepared by cutting it down and making a bore in the tooth stub, in which a dental post is inserted. Additional bores are formed in the tooth structure, into which pins or anchors are threaded. These pins have portions extended upwardly of the tooth surface for aiding in the retention of the dental restoration as well as avoiding rotation of the restoration on the tooth stub. Such anchoring pins are very small, i.e. in order of 0.03 inches in diameter and 0.2 inches in length. These pins are typically first loaded into a dental power tool or holder and inserted into prepared bores or channels made in the body of the tooth. Such bores or channels are formed in the body of the tooth by means of dental drills. 
     Anchors or pins for anchoring and supporting a dental restoration are typically comprised of two portions, one of which is an anchoring portion threaded in the bore formed in the body of the tooth and another is a retention portion for embedding into the restoration. Typically the retention portion is coupled to a manipulating head extending from the retention portion and connected by at least one reduced diameter portion. In this way when the pin is threaded into the bore formed in the tooth the pin severs automatically upon bottoming out the lower end of the bore. The anchoring portion remains in the tooth with the retention portion extending upwardly from the tooth surface. 
     Dental anchors or pins having a buttress-type thread has been disclosed in assignee&#39;s co-pending patent application Ser. No. 191,347. They have proven to have a most satisfactory retention capability inasmuch as buttress-type threads, particularly the reverse buttress-type threads described, provide improved holding power in the dentition and reduce the amount of dental damage during insertion. 
     The aforementioned co-pending application discloses a dental anchor having a buttress-type thread, where the buttress-type thread includes a substantially planar lower surface facing toward the insertion end of the dental anchor, a beveled upper surface which tapers inwardly towards the retention portion of the dental anchor, and a sharp crest portion disposed between the planar lower surface and angled upper surface. A top member or collar is provided between the anchoring portion and the retention portion. The stop member limits the depth of insertion of the dental anchor into the bore. 
     It should be appreciated, however, that even with highly efficient dental anchors or pins having buttress-type threads there is a continuous interest in further reducing or minimizing any stress on existing dentin, especially at the bottom or seat portion of the bore which receives the dental anchor. 
     Accordingly, while the use of dental anchors having a buttress-type threads thereon has been proven quite useful in the dental art, further improvements in such dental anchors and means for anchoring the same in the tooth body are still of importance in order to yet further reduce any stress on dentin material and to facilitate anchoring or such pins. It is also of interest to facilitate insertion at surfaces of teeth which are inclined, curved or have any other non-planar exposed surfaces. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a dental anchor which avoids the aforementioned problems of prior art dental anchors. 
     Another object of this invention is to provide a drill for making channels or bores receiving the dental anchors to substantially reduce stress on the dentin material of the tooth. 
     Yet another object of the present invention is to facilitate preparation of the tooth stub for building thereon a dental restoration. 
     It is a further object of the present invention to provide means for and a method of installing dental pins which would substantially minimize stress on the tooth stub as well as on the dental restoration to be built thereon. 
     Briefly, the objects of the present invention are attained by a dental anchor for the retention of a superstructure on a tooth stub and insertable into a bore formed in the tooth stub. The dental anchor comprises an upper retention portion, on which the superstructure or a dental restoration is to be formed, an anchoring portion having a threaded portion thereon and being self-threadable into the bore in the tooth stub to secure therein the dental anchor, and a stop provided between the retention portion and the anchoring portion for limiting the depth of insertion of the dental anchor into the channel. The retention portion is of a smooth continuous configuration having no sharp corners thereby substantially reducing stress on the superstructure, avoiding the occurrence of cracks therein and providing a smooth distribution of the restoration material over the retention portion. 
     In an embodiment, smoothly shaped undercuts are formed along the retention portion adjacent the anchoring portion. The restorative material is packed in these undercuts to provide improved retention onto the tooth stub. 
     The objects of the invention are also attained by a stepped dental drill which not only drills the anchor receiving bore in the tooth stub but also forms a counterbore at the exposed excavated surface of the tooth. The drill is matched to the anchor so that the bore will be longer than the threaded portions of the anchor. When the dental anchor engaged in a holder is rotatably inserted into the bore the stop collar seats into the counterbore to stop further insertion of the anchor and prevent bottoming out of the anchor. The collar also serves to distribute the stress over a flat wide surface rather than concentrating stress on the bore edges having the threading. By making the collar wider than the crest of the threads, the collar will have a chance to sit on supported dentition which also distributes and reduces stress. 
     In an embodiment, the drill is gripped in a holder so that its outer stepped portion axially extends a limited amount from the holder corresponding to the depth of the counterbore desired. Thus the holder forms a shoulder which constitutes a stop for drilling the channel with the counterbore into the tooth stub. 
     The aforementioned objects, features and advantages of the invention will, in part, be pointed out with particularity, and will, in part, become obvious from the following more detailed description of the invention, taken in conjunction with the accompanying drawing, which form an integral part thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an elevational view of the dental anchor, in accordance with the present invention; 
     FIG. 2 is an elevational view, partly in section, of the manipulating tool for insertion of the dental anchor of FIG. 1; 
     FIG. 3 is an elevational view, partly in cross-section, and on an enlarged scale of a drill for making the bores into a tooth stub body, and engaged in a drill holder; 
     FIG. 4 is a side view of the two-step drill for making the bores for the dental anchors according to the invention; 
     FIG. 5 is an elevational view, partly in cross-section, showing bores being formed in a tooth stub by means of a drill according to the invention; 
     FIG. 6 is an elevational view, partly in cross-section, showing dental anchors of FIG. 1 being inserted into the bores found in FIG. 5; 
     FIG. 7 is a sectional elevation view, on an enlarged scale, of the dental anchors, in partly and fully inserted conditions; and 
     FIG. 8 is an elevational view, partly in cross-section, showing dental anchors of FIG. 1 in the final restored tooth. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings in detail, FIG. 1 illustrates a dental anchoror pin 10 according to an embodiment of the present invention. Pin 10 includes an upper manipulating portion 12 which is typically held in a shank portion as shown in FIG. 2 and described hereinbelow, for insertion into a manual driver or an automatic drill. The pin 10 includes a retention portion 14, an intermediate stop member 16 and a lower anchoringportion 18. The latter is inserted in a channel or bore formed in the body of the tooth stub to anchor the pin into the tooth body. The stop member 16 is intended to limit the depth of insertion of pin 10 into the bore so that the retention portion 14 will extend upwardly from the exposed excavated surface of the tooth stub to form a retention means for a dentalrestoration formed on the tooth stub. The stop member 16 also serves to distribute stress, as will hereinafter be explained. 
     The anchoring portion 18 is formed as an elongated cylindrical body provided with a thread, shown as a helical reverse buttress-type thread 20as is described in the aforementioned copending patent application. The buttress-type thread 20 has a substantially planar lower surface 22 which faces towards an insertion end 29 of the pin, and an angled or beveled upper surface 26 which tapers inwardly towards the retention portion 14 ofpin 10. A knife-like cutting edge 28 is provided in the thread 20 between the planar surface. The threads on anchoring portion 18 are of a thread forming type which deform the dentin rather than cutting into the dentin. This also reduce stress on the tooth material as compared to thread-cutting pins. 
     The buttress-type thread utilized in the dental anchor according to this invention is disclosed in detail in the aforementioned assignee&#39;s co-pending patent application. The buttress-type thread is of a reverse construction where the beveled surface 26 tapers inwardly towards the upper end of pin 10. In such a reverse construction, the force required topull pin 10 out from the bore formed in the tooth is increased as compared to dental anchors known in the dental art since, in addition to a verticalforce required for pulling out the pin an additional force normal to the plane of the buttress thread angle is required to pull out the pin. This substantially enhances the retention of the anchoring portion of the pin within the bore of the tooth over threads of other types. Distal end 24 ofthe anchoring portion 18 is chamfered at 30 and tapers inwardly towards theend surface of the anchoring portion 18. Anchoring portion 18 is integral with the substantially cylindrical stop member or collar 16. The external diameter of the collar is greater than the largest diameter of the thread crest 28. 
     Retention portion 14 is an elongated barrel-like body having curved side walls tapering towards the collar 16 and merging at the upper end thereof into the substantially cylindrical manipulating portion 12. The angle α of tapering of the retention portion 14 downwards may vary between5° and 10°. A transition zone or undercut 32 helps retain therestorative material in place. The elongated body of the retention portion 14 has no sharp corners or edges so that the whole retention portion of the pin 10, which receives thereon a dental restoration is extremely smooth. This significantly reduces stress in the restorative material thereby eliminating post curing (setting) cracks formed therein. Shaped undercut 32 permits packing of the restorative material to form a continuous band of restorative material under the upper part of the anchorto thereby help retain the dental restoration. The outer diameter of collar16 is greater than the major diameter of the threads by approximately 10-15/1000&#34;. 
     Cylindrical manipulating portion 12 terminates at a substantially frusto-conical end portion 36 which has a conical proximal end 38. A reduced diameter neck 34 forms a fracture groove between the manipulating portion 12 and the retention portion 14 to facilitate shearing of the anchor from the manipulating portion. The elongated cylindrical surface ofthe manipulating portion 12 constitutes guide means for the insertion of pin 10 in a standard shank driver 40 shown in FIG. 2. Shank driver 40 is provided with a standard locking arrangement 42 for insertion into a dental handpiece. In the alternative, shank 40 can also be connected to a manual drive tool. Shank driver 40 is used to drive the pin 10 into a boreformed in the tooth stub body. An indent 44 can be provided on the manipulating portion 12 of pin 10 to wedge the pin in the shank driver 40.Any other suitable means for firmly retaining pin 10 in the shank driver 40can be also used to allow pin 10 to be rotated and inserted in a corresponding bore in the tooth stub body. Pins 10 may be made of stainless steel, titanium or any other suitable material. 
     FIGS. 3 and 4, illustrate a spiral drill 50 for producing bores or channelsin the tooth body receiving the anchors previously described. Drill 50 is inserted in a shank driver 52 having an elongated cylindrical body provided at its proximate end with a standard coupling arrangement 54 for insertion into a dental handpiece. Drill 50 includes a substantially cylindrical guide portion 56 insertable and fixable in a cylindrical blindhole 58 formed in a drill receiving portion 60 of a reduced diameter front end 61 of the shank driver 52. The external cylindrical surface of the drill receiving portion 60 slightly tapers at 78 towards the distal end ofthe drill holder. 
     As best shown in FIG. 4, a recess 62 formed in the upper surface of the guide portion 56 of the drill bit extends in the direction of the axis of the drill. An inwardly protruding elongated portion 64 formed on the innersurface limiting the blind hole 58 is snapped or keyed in recess 62 to firmly grip drill 50 in the shank driver 52. Two coaxial stepped drilling portions or bits 70 and 72 of different diameters are provided in drill 50. Drill bit 72 of a greater diameter than that of drill bit portion 70 is provided to produce a counterbore at the mouth of the main bore formed by the drill portion 70, as will be explained below. 
     As shown in FIG. 3 guide portion 56 of the drill 50 is dimensioned so that it extends outwardly from the shank driver 52 to produce a protruding portion 76 of the larger diameter drill portion 72. The edge 78 of the shank forms a stop shoulder. This shoulder constitutes a stop for drillingand ensures a fixed length of the bore and a fixed depth of the counterbore. Holder portion 60 has its distal end curved at 78 to avoid sharp corners and prevent a striking effect on the exposed excavated surface of the tooth stub during the drilling process. 
     The ratio between the length of the larger stepped drilling portion 72 and that of the main thinner drilling portion 70 amounts to 1:4 to 1:5 and, with drills 0.300 in. long the length of portion 72 is between 0.015 and 0.20 in. 
     Referring now to FIGS. 5 to 8, it is seen that the tooth to be prepared typically includes a dentin material 80, a pulp channel 90 and a layer of enamel 92. The stepped spiral drill 50 is urged into the dentin material of the tooth 80 to produce a plurality of channels and bores 82 extending into dentin 80 from an exposed excavated surface 84 from which decay was removed before drilling channels 82. The number of bores 82 varies with the area of exposed excavated surface 84 and the size of a dental restoration further to be built on the tooth. The diameter of the drill bit may be of a size depending on the pin size and tooth area. In one embodiment a diameter of 0.028 inches was used. As bore 82 is drilled by bit portion 70, the drill is further inserted into this bore until the enlarged drill bit portion 72 enters the dentin material at the proximal end or mouth of bore 82 and forms a counterbore 86. 
     As mentioned hereinabove shoulder 78 formed by the drill shank driver 52 limits the insertion of the enlarged portion 72 of the drill into the dentin material and thus the depth of the counterbore 86 substantially to the height of the collar 16 of the pin 10 which is to be received in the respective bore. After a desired number of bores 82 with counterbores 86 have been produced, anchoring pins 10 are inserted into each of bores 82. It will be apparent that the diameter of the anchoring portions 18 of eachpin is greater than the diameter &#34;a&#34; of the bore 82. Thus the anchoring portion 18 of each pin is tightly threaded into its respective bore 82 as the shank driver 40 which holds the manipulating portion 12 of the pin is rotated. As pin 10 is urged further into bore 82 collar 16 of pin 10 enters the counterbore 86 and seats in the counterbore thus limiting any further extension of the pin end 24 into the distal blind end 96 of the bore. The counterbore 86 may extend by 1/1000 to 15/1000&#34; below the surface 84 of the tooth. Upon insertion the pins shear at their reduced diameter portions and remain in place in the tooth stub. 
     In conventional anchoring pins, the pins are introduced into respective bores formed in the tooth stub. The blind end of each bore has a mating angled configuration similar to the distal end of the anchoring portion ofthe pin and forms a seat therefor. As the pin is inserted, it continues till the pin bottoms out at the bottom of the bore where stress is exertedon the body of the tooth. The stress is concentrated. Such stress not only exists at the time of insertion but even during loading. 
     In the present pin because collar 16 seats in the counterbore 86, it prevents the pin from bottoming out. This eliminates the stress at the lower end of the bore. Furthermore the collar distributes the stress over a much greater surface area than that of the distal end 24 of the pin. In addition, because the collar has a diameter greater than the crest of the threads, the collar sits on solid dentition that is not over a threaded undermined, edge, as shown at 95 in FIG. 7. Thus the stress is distributedin the region supported by solid dentition rather than in the region of thethread. The sizing of the drill with the mating size of the anchor is such that when the collar 16 is received in counterbore 86 a clearance of at least about one fourth of a mm remains between distal end 24 of the pin and the blind end 96. The collar and counterbore combination according to the present invention not only re-distributes stress on the tooth during the insertion of the pin but also significantly reduces stress during loading of the tooth. Collar 16 will constitute a positive stop preventingthe threads from crushing. As the lower surface of collar 16 reaches the bottom of counterbore 86, shank holder 40 shears off at the reduced diameter section to enable pin 10 to be anchored in the bore. 
     It will be also appreciated that due to the fact that the retention portionof pin 10 has no sharp corners and is anatomically shaped, stresses in the upper restorative material would also be reduced and post setting cracks in the amalgam or composite would be substantially prevent from the occurrence. 
     It should be also noted that at inclined portions of the excavated surface 84 the counterbore 86 produced in the tooth body 80 may be incomplete due to the tooth&#39;s anatomy as illustrated at the left-hand channel 82 of FIG. 5. However, by making even a part of the counterbore, a flat bottom surface is produced on which the collar can sit. Thus, even though the tooth surface is uneven or inclined, nevertheless the pin will sit along aflat even surface of the counterbore and distribute the stress. Also, the collar 16 will sit within such counterbore and still, limit the further insertion of the pin into the bore. 
     FIG. 8 shows a completed restoration with three pins 10 being employed and with the restorative material of choice 100 being secured over the pins. If necessary, a final outer cap 102 can be secured to the core. It should be appreciated that in some cases, a post could also be used with one or two of the pins and positioned in the tooth stub. 
     It should be understood, of course, that the foregoing disclosure relates to only a preferred embodiment of the invention and that it is intended tocover all changes and modifications of the example of the invention herein chosen for the purpose of the disclosure, which modifications do not constitute departures from the spirit and scope of the invention.