Shells for temporary and provisional crowns with an hourglass shaped occlusal profile

A set of fiber-polycarbonate shells for making bicuspid and molar crowns are formed with hourglass shaped occlusal profiles. Mesio-distal sidewalls of the shells have a concave shape matching the concavity of the occlusal profile. The shells are sized to fit loosely between adjacent teeth and relative to opposed teeth. The concavity of the mesio-distal sides and the occlusal surface of each shell allows a good fit between the shell and the convex sides of adjacent teeth. A good fit provides a substantially uniform-width proximal gap between mesio-distal sidewalls of the shell and the adjacent teeth. Each shell further has small mesio-distal windows in the mesio-distal sidewalls. The uniform-width gap and the mesio-distal windows control mesio-distal resin flow to adjacent teeth. A new method of forming crowns using such shells provides a good fit to a prepared tooth and relative to adjacent and opposed teeth by filling the shell with a glass-filled acrylic resin filler and then positioning the shell gingivally and occlusally. Resin extrudes through the window to form good proximal contacts, with the shell rocking about the facial gingival margin to a comfortable fit with an opposed tooth. The excess resin and shell are shaped to form the final crown. The crown, formed by Ti particle reinforced resin, is durable enough for long-term wear.

BACKGROUND OF THE INVENTION
 This invention relates generally to temporary and provisional dental
 crowns, and more particularly to flexible dimension crown shells and
 methods of making temporary and long-term provisional dental crowns using
 such shells.
 Presently, there are three primary methods for fabricating temporary and
 provisional crowns. In a first technique, conventional prefabricated crown
 forms or shells, made of a metal such as aluminum or stainless steel, or
 of a polycarbonate such as the Ion crown forms sold by 3M Corporation, are
 trimmed and shaped to fit a prepared tooth. Examples of temporary crowns
 of this type are disclosed in U.S. Pat. No. 4,015,332 (Manne), U.S. Pat.
 No. 4,678,435 (Long), U.S. Pat. No. 4,778,386 (Spiry), and U.S. Pat. No.
 5,458,489 (Tennyson).
 A second technique calls for making an impression of the tooth before the
 tooth is prepared for a crown. After the impression is made, the tooth is
 prepared and the impression, filled with a bis-acryl material, is placed
 over the prepared tooth. After the bis-acryl material sets, it is removed
 from the dental impression, and then trimmed, polished, and seated in the
 mouth.
 A third primary technique, which is used and recommended by Gordon
 Christenson, is also popular. According to the third technique, a
 putty-like ball of polymethyl-methacrylate is applied over a prepared
 tooth. The patient then bites down and the material begins to set. Before
 it completely sets, the putty-like material is removed from the tooth,
 trimmed and placed back on the tooth. Once the material sets, it is then
 trimmed again and the bite adjusted. Finally, the temporary crown is
 cemented to the tooth. In a variation of this technique, as disclosed in
 U.S. Pat. No. 5,385,469, a tubular dental form for forming a universal
 crown in situ is used.
 Each of these techniques has various advantages and disadvantages. Using
 prefabricated forms, as in the first technique, for example, is fast and
 simple, but the fit of the conventional shell is not very good. The
 margins, in particular, do not fit well. Specifically, it is hard to get
 good proximal contact to adjacent teeth, and the contours and occlusion
 are not always good. Some manufacturers try to overcome these drawbacks by
 proliferating sizes and shapes of shells, with some selections providing
 as many as 80 different sizes and shapes of molars and bicuspids.
 Unfortunately, this attempted solution is expensive in terms of materials
 and also in terms of the time required for the dentist to pick the right
 shell.
 Manne adds a degree of freedom to the first technique by providing an
 incisor shell that has slits in the mesio-distal sides to permit the shell
 to flex in the labio-lingual direction about a hinge axis at the occlusal
 surface. Long also provides a degree of freedom in this technique by
 having the mesio-distal sides of a temporary molar crown open to permit
 the acrylic resin filler material to protrude proximally to contact
 adjacent teeth. These shells, like others used in this technique, require
 trimming the free edges of their buccal and lingual sidewalls to ensure a
 good fit along the gingival margins, as well as a good occlusion. The
 shells in Long also appear to require substantial trimming of the filler
 material due to their open mesio-distal sides. Such trimming and fitting
 is time-consuming for the dentist and the patient.
 The second technique gives good contours and bite accuracy, but making an
 impression is time-consuming. Furthermore, neither the strength nor the
 durability of temporary crowns produced by this technique are very good.
 Furthermore, the impression cannot be made if the patient's tooth is
 already broken when initially treated. The third technique, namely,
 free-forming a temporary crown of putty-like material, can be accurate and
 fairly fast compared to the other techniques, but only if performed by a
 skilled dentist or technician. It, too, however, is more time-consuming
 than desirable. Another main problem with this approach is that it is very
 technique-sensitive. A dental technician must be highly skilled in order
 to accurately carve the tooth anatomy. Another problem with this technique
 is that special care must be taken to ensure that the patient's mouth is
 not injured by the exothermal reaction involved in curing the crown
 material.
 As noted, all of the foregoing techniques are undesirably slow. Even the
 fastest of these techniques generally takes half an hour or more of work
 for the dentist to fit a temporary or provisional crown to a patient.
 Additionally, the crowns resulting from the second and third techniques
 are typically not very durable and are therefore not well-suited for
 long-term wear. Although the stainless steel shells of the first technique
 are very durable, it is more difficult to fit stainless steel shells to
 the patient and to grind the shells' occlusal surfaces to get a
 comfortable bite.
 Accordingly, a need remains in the profession for a way of making temporary
 and provisional crowns that is quick and accurate, that provides a good
 fit without substantial trimming, that is durable enough for long-term
 use, and that is inexpensive.
 SUMMARY OF THE INVENTION
 It is, therefore, an object of the invention to simplify the fabrication,
 fitting, and installation of temporary and provisional crowns.
 Another object of the invention is to make temporary and provisional crowns
 that fit well and are durable enough for long-term use.
 A further object is to make crowns that fit well but are inexpensive both
 in terms of materials and time taken to fit the crowns.
 In general, the invention combines two of the conventional techniques,
 namely, the free-form and specially-designed shell techniques, in a way
 that takes the advantages of, yet avoids the major disadvantages of, both
 of these techniques as used individually. Specifically, the shells of the
 invention have a generally U-shaped window, unbounded along a gingival
 margin, in at least one of the mesio-distal sidewalls. A putty-like
 material or resin is used, similar to that used in the free-form
 technique, but in this case it is shape-controlled by the shell and the
 windows provided therein. This shape control saves the dentist valuable
 time in shaping the temporary or long-term provisional crown. The
 mesio-distal sidewalls each include a recessed area or indentation
 adjoining the windows for receiving and retaining protruding resin in
 proximal contact with adjacent teeth. The shells are further designed with
 short labial and lingual side walls so that no trimming of the shells
 themselves is needed. This, too, saves the dentist a great deal of time.
 The shells also give the dentist several degrees of freedom, so that most
 molars and bicuspids can be fitted with only a limited range of sizes and
 shapes of symmetric shells.
 A particular shell for making a temporary or long-term provisional crown on
 a prepared tooth includes a top wall, a buccal sidewall; a lingual
 sidewall, and opposite mesio-distal sidewalls. The top wall defines an
 occlusal surface of the shell. The mesio-distal sidewalls are each
 connected to the top wall and to the buccal and lingual sidewalls, and are
 spaced apart from each other to define a central cavity. The central
 cavity is configured to receive resin and to fit over a prepared tooth. A
 window is formed in at least one mesio-distal sidewall to provide a
 partial opening that allows a portion of the resin to protrude
 mesio-distally from the cavity to an adjacent tooth. Furthermore, at least
 one of the mesio-distal sidewalls is shaped concavely to interfit with a
 convex mesio-distal surface of the adjacent tooth.
 Preferably, the top wall of the shell is shaped concavely along
 mesio-distal edges to form an approximate hourglass shape conforming to a
 convex shape of mesio-distal surfaces of adjacent teeth, with both of the
 mesio-distal sidewalls shaped to align with the concavity of the top wall.
 In this way, the concave mesio-distal sidewall can provide an
 approximately uniform-width gap between the shell and the convex adjacent
 tooth and control a proximal flow of the resin.
 A method of fabricating temporary or long-term provisional crowns for
 molars and bicuspids is also provided. The method includes filling a
 central cavity of a shell with a quantity of resin. The shell has a top
 wall defining an occlusal surface, opposite buccal and lingual sidewalls,
 and opposite mesio-distal sidewalls spaced apart to define the central
 cavity. The resin-filled shell is positioned on a prepared tooth and a
 portion of the resin is extruded mesio-distally through a window forming a
 partial opening in at least one of the mesio-distal sidewalls. While the
 resin sets, the shell and resin are repeatedly put on and pulled off of
 the prepared tooth until the resin is set. After the resin has completely
 set, the shell and extruded resin are shaped to contour an external
 surface thereof to fit occlusally and proximally into the patient'mouth.
 Extrusion of the resin is controlled by sizing the windows to encompass
 only a limited portion of the mesio-distal side area. Further control is
 provided by the concavity of the mesio-distal sidewall relative to the
 convexity of the adjacent tooth. This arrangement provides a uniform
 mesio-distal gap to receive and retain the resin.
 Shaping the crown preferably proceeds by marking the mesial and distal
 contacts and margins of the crown after it has been removed from the
 prepared tooth. Resin that has extruded through the window in the
 mesio-distal sidewall is removed beyond the marked contacts and margins.
 Following shaping, the shell and resin are repositioned on the prepared
 tooth.
 Further advantages can be obtained by this invention if the resin contains
 Ti particles to improve the strength and durability of the crown. Also,
 the shell and resin can be made to contain radio-opaque substances, so
 they will appear on x-rays.
 Long-term provisional crowns made according to this invention provide an
 alternative to high cost crowns, offering benefits to patients, dentists,
 and insurance companies. Both temporary and long-term provisional crowns
 made according to this invention are accurate, can be made very fast, and
 are more durable than most of the prior art. Crowns made using the
 long-term provisional shells should last 5-7 years or longer. Crowns made
 using the temporary shells (having slightly thinner walls than in
 provisional shells) last from 3-4 months up to about 1 year. Crowns made
 according to this invention are also simpler and easier to install than
 molded crowns and are much more accurate than pre-fab crowns. The crowns
 of this invention offer good margins, good contacts, and good occlusions
 to prevent tooth shift while waiting for permanent crown. They are also
 more comfortable for the patient and are aesthetically attractive.
 The present invention enables temporary or provisional crowns to be fitted
 to a patient quickly, i.e., in half the time (or less) required by prior
 art techniques; to provide a good fit proximally, gingivally, and
 occlusally; and to provide long-term durability.
 The foregoing and other objects, features and advantages of the invention
 will become more readily apparent from the following detailed description
 of a preferred embodiment of the invention which proceeds with reference
 to the accompanying drawings.

DETAILED DESCRIPTION
 FIGS. 1-5 illustrate the basic concept of the present invention in a first
 embodiment with respect to a bicuspid crown. Although these figures are
 specifically directed toward bicuspid crowns, the following description
 applies equally to crowns for molars. Molar crowns are more specifically
 described in connection with subsequent figures.
 FIG. 1 is a side elevation view of a shell 20 for making a temporary or
 long-term provisional bicuspid crown. The shell 20 is preferably
 integrally molded of polycarbonate material but can be made of other
 polymeric materials and can be machined rather than molded. The shell 20
 has a top wall 22 that defines an occlusal surface; a buccal sidewall 24;
 and a lingual sidewall 26 spaced from the buccal sidewall. The buccal
 sidewall 24 can include a detachable tab (not shown) for handling the
 shell during making of the crown. Opposite mesio-distal sidewalls 28, 30,
 are connected to the top wall 22 and the buccal and lingual sidewalls 24,
 26, and are spaced apart from each other to define a central cavity. The
 central cavity is shaped to receive an acrylic resin and to fit over a
 prepared tooth.
 The lingual sidewall 26 is shorter than the buccal sidewall 24 for ease of
 fitting the gingival margins 25, 27, as further discussed below. The
 mesio-distal sidewalls 28, 30 are shorter occluso-gingivally than the
 buccal and lingual sidewalls 24, 26. Shells of various sizes can be
 provided, including shells having two or more different occluso-gingival
 lengths of the lingual and buccal sidewalls, to better serve a wide range
 of tooth lengths.
 Each of the mesio-distal sidewalls 28, 30 includes a mesio-distal window
 32, 34 that forms a partial opening in its respective sidewall. The
 mesio-distal windows 32, 34 allow the acrylic resin to protrude proximally
 from the cavity to adjacent teeth when the resin-filled shell 20 is fitted
 on a prepared tooth 60 (see FIG. 6). Although the shell 20 can be made and
 used with a mesio-distal window in only one sidewall, it preferably has
 windows in both mesio-distal sidewalls 28, 30. Each mesio-distal window
 32, 34 is sized to allow resin to protrude therefrom in an amount
 sufficient to form a good proximal contact 36A, 38A (see FIG. 2) with an
 adjacent tooth; but is sized sufficiently smaller than an overall size of
 the mesio-distal sidewall 28, 30 in order to control the flow of resin
 from the central cavity. The mesio-distal windows 32, 34, for example,
 have an area of about half the overall area of the mesio-distal sidewall
 of the shell.
 The preferred material for making the shells 20 of the invention is a 20%
 fine fiber-glass filled polycarbonate. The material forming shell 20 may
 also include a radio-opaque substance, such as barium sulfate (BaS), so
 that it will show up on x-rays. The resin used in the invention is
 preferably Super-T glass-filled acrylic resin and also preferably contains
 BaS or some other radio-opaque substance so that it will also appear on
 x-rays. Furthermore, fine size titanium (Ti) particles or powder can be
 added to the resin to make the resulting crown more durable and thereby
 increase its longevity.
 FIGS. 2-5 illustrate a method for fabricating a temporary or long-term
 provisional bicuspid crown using the shell 20 shown in FIG. 1. FIG. 2 is a
 perspective view of a portion of a patient's mouth showing a temporary or
 long-term provisional bicuspid crown 40A. Referring to FIGS. 1 and 2, a
 temporary or long-term provisional crown 40A is made by filling the
 central cavity of the polycarbonate shell 20 with a quantity of Super-T
 acrylic resin and by positioning the resin-filled shell on the prepared
 tooth. The patient then bites down, clamping the resin-filled shell 40A
 against an opposing tooth 66 (see FIG. 8) and thereby establishing an
 occlusal contact of the occlusal surface 22 of the shell 20 with the
 opposing tooth 66. This step also shapes the resin within the cavity to
 mate with the prepared tooth 60 (see FIG. 8).
 Furthermore, when the patient bites down on the shell 20, a portion of the
 resin is extruded from the cavity through the mesio-distal windows 32, 34.
 A portion of the mesio-distal sidewalls 28, 30 along the buccal and
 lingual margins of each mesio-distal window 32, 34 directs the extrusion
 of resin proximally toward the adjacent teeth 62, 64. The resin extruded
 through these windows thereby forms mesio-distal protrusions 36A, 38A that
 contact adjacent teeth on proximal sides of the temporary crown.
 The shells 20 are also preferably formed with the lingual sidewall 26
 slightly shorter than the buccal sidewall 24. This sizing permits a degree
 of freedom in positioning the occlusal or top wall 22, as the patient
 bites down, without the gingival margin 27 of the lingual sidewall 26
 engaging the patient's gingival 70. This feature is explained in further
 detail below.
 FIG. 3 shows the temporary crown 40A of FIG. 2 removed from the patient's
 tooth and held in a dentist's fingers. Referring now to FIG. 3, the
 protrusions 36A, 38A of the temporary crown 40A are trimmed and shaped
 using an acrylic burr 42 to contour the crown's external surface to fit
 into the patient's dentition both occlusally and proximally. Trimming the
 gingival margins of shell 20 is generally unnecessary.
 FIG. 4 shows the temporary crown 40B, with shaped extrusions 36B, 38B,
 placed back on the patient's prepared tooth. Shading 44 along an edge of
 the occlusal surface 22 indicates a buccal cusp of the temporary crown to
 be ground for bite adjustment. FIG. 5 shows the temporary crown 40C after
 shaping for bite adjustment. The dashed lines in FIGS. 4 and 5 indicate
 the portion of the temporary crown formed by the extruded and shaped
 resin, which protrudes through the windows on the mesio-distal sides of
 the crown 40C. The resulting crown is composed of polycarbonate-
 reinforced acrylic resin which is very durable. The crown is therefore
 suitable for use as a long-term provisional crown as well as a temporary
 crown.
 Once the crown is finished, the border between the shell and filler
 material is substantially smooth and nearly invisible except upon close
 inspection. The finished crown 40C is cemented and tightly sealed to the
 patient's prepared tooth 60 (see FIG. 8) along the gingival margins 25, 27
 to secure it within the patient's dentition. The resulting proximal fit to
 adjacent teeth 62, 64 closely replicates that of a natural tooth, as does
 the occlusion with opposed teeth 66 (see FIG. 8).
 According to this invention, the entire procedure for providing a temporary
 or long-term provisional crown, i.e., from picking the properly sized
 shell through filling, shaping, and cementing the crown onto the prepared
 tooth, takes about half the time required by the prior art. Remarkably,
 the entire procedure can typically be completed in under 15 minutes.
 FIGS. 6-8 show various views of a portion of a patient's dentition to
 illustrate an example of spacings used to select a shell of a proper
 mesio-distal and occluso-gingival size. A properly sized shell should fit
 loosely between adjacent teeth 62,64 on a prepared tooth 60 with the
 patient's bite in a closed position. FIGS. 6 and 8 show sizing parameters
 of the shell 20. FIG. 6 illustrates mesio-distal sizing parameters A, B
 for positioning the shell. Mesio-distal sidewalls of the shell are
 positioned approximately halfway between mesio-distal surfaces of the
 adjacent teeth 62, 64 and mesio-distal surfaces of the prepared tooth 60.
 In other words, the shell is centered between the prepared tooth's
 adjacent teeth with a width midway between width A and width B. The
 resulting position of shell 20 is shown by dashed lines in FIGS. 7 and 8.
 Using the embodiments of shells shown in FIGS. 9-16 or FIGS. 25-32, having
 recessed or concave mesio-distal sides, the proximal spacing is preferably
 closer, as described below with reference to FIGS. 33-35.
 It is preferable to provide a set of shells that includes shells for
 bicuspids and molars of at least two mesio-distal widths within the usual
 range of spacings for such teeth in permanent dentition. These sizing
 options enable the dentist to select a shell for a given prepared tooth
 which has a mesio-distal width such that the shell passively fits between
 the adjacent teeth with a clearance in the range of one-half to one
 millimeter. The remaining proximal gap is filled by the mesio-distal resin
 protrusions 36A, 38A. This arrangement makes proximally fitting the crown
 very easy compared to prior art shells.
 Referring to FIG. 8, the shell 20 is also sized having a length in an
 occluso-gingival direction that allows it to fit passively against an
 opposed tooth 66 when the patient's bite is in a closed position over the
 unfilled shell 20. Similar to above, a set of shells can include two
 different occluso-gingival lengths of a given bicuspid or molar, to
 accommodate the usual range of variations in tooth length that occurs
 among humans. For a given prepared tooth length, the shell is sized
 occluso-gingivally to provide a clearance C that is approximately half of
 the width D of the space between the occlusal wall of the shell 20 and the
 opposed tooth 66. The shell 20, selected for a particular tooth,
 preferably has an occluso-gingival length such that a gingival margin of
 the shell approximately fits a gingival margin of the prepared tooth and
 such that the occlusal surface of the shell has a clearance from the
 opposed tooth 66 of one-half to one millimeter.
 FIGS. 9-13 show various views of a second embodiment of a bicuspid shell
 120 according to the invention. The general arrangement, materials, and
 procedure for making temporary and provisional crowns using shell 120 can
 be the same as those described above for shell 20 (see FIG. 1). The
 preferred sizing and method of fitting are described below with reference
 to FIGS. 33-44. The parts of shell 120 that correspond to parts in shell
 20 are denoted by the same reference numerals, incremented by 100.
 Accordingly, shell 120 has an occlusal (or top) wall 122, buccal and
 lingual sidewalls 124, 126 with gingival margins 125,127, respectively,
 and proximal (mesio-distal) sidewalls 128,130.
 Unlike the mesio-distal windows 32, 34 of the shell 20 of the first
 embodiment, however, which are bounded along the gingival margins of the
 mesio-distal sidewalls, the mesio-distal windows 132,134 of the shell 120
 in this embodiment have a generally U-shape and unbounded along the
 gingival margins of the proximal sidewalls 132,134. The mesio-distal
 windows 132, 134 of this embodiment are also narrower than mesio-distal
 windows 32, 34 of the first embodiment, and preferably occupy an area that
 is about one-third of the overall area of the sidewall in which the window
 is formed.
 Also unlike the first embodiment, at least one occlusal window can be
 provided in the occlusal wall 122. The occlusal wall 122 preferably
 includes a pair of occlusal windows 150,152, in the form of elongate ovals
 spaced about a central bridge 154. The occlusal windows 150,152 are
 configured to allow a controlled portion of resin from the central cavity
 of the filled shell 120 to be extruded onto the upper side of the occlusal
 wall 122 when the patient bites down on the shell 120 during the
 fabrication step described previously with reference to FIG. 2.
 Referring specifically to FIG. 11, the top wall 122 of the shell 120
 further includes a first occlusal surface 122A formed along the top of the
 central bridge 154 and on buccal and lingual sides of the top wall 122. A
 second occlusal surface 122B is also part of the top wall 122 but is
 formed recessed from the first surface 122A, surrounding mesio-distal,
 buccal, and lingual sides of the occlusal windows 150, 152. The second
 occlusal surface 122B thereby forms a recessed margin around the occlusal
 windows 150, 152 for receiving the resin extruded through those windows
 150, 152 and for retaining the resin around them so that the resin can be
 shaped by contact with the occlusal surface of an opposed tooth 66 (see
 FIG. 8).
 Similarly, the proximal surfaces 128, 130 of the shell 120 preferably
 include recessed surface areas 128B, 130B between the vertical sides of
 the windows 132, 134 and the shell's outermost proximal surfaces 128A,
 130A. The recessed areas 128B, 130B form a recessed margin around the
 mesio-distal windows 132, 134 for receiving the resin extruded through
 those windows 132, 134 and for retaining the resin in proximal contact
 with adjacent teeth 62, 64 (see FIG. 8). This recessed or indented area
 provides a concavity that permits a closer fit to the convex proximal
 faces of adjacent teeth.
 As best seen in FIG. 12, the gingival margins 125, 127 of the shell 120 of
 this embodiment can be formed with a stair-shaped cross-sectional profile.
 This profile, which can also be tapered, aids in receiving and retaining
 resin around the gingival margins 125, 127 to form a superior seal with
 the prepared tooth 60 (see FIG. 8). Another unique feature of this
 embodiment is that the lingual sidewall 126 is shorter than the buccal
 sidewall 124 by a ratio of about 3:4.
 FIGS. 14-16 show a shell 220 for a molar having essentially the same design
 as the second bicuspid embodiment described above with reference to FIGS.
 9-13. The structural elements and features of this embodiment that
 correspond to those shown in the previous embodiments are indicated by the
 same reference numerals, incremented by 200, and need not be further
 described. General differences in the size and shape of the bicuspid and
 molar shells 120, 220, respectively, are due to their respective
 applications in bicuspid and molar crowns. In addition to their general
 size and shape differences, another main difference between the molar
 shell 220 and the bicuspid shell 120 is that the molar lingual sidewall
 226 is shorter than the molar buccal sidewall 224 by an even greater
 proportion than in the bicuspid shell 120. Specifically, the molar lingual
 sidewall 226 is shorter than the molar buccal sidewall 224 by a ratio of
 about 2:3, as compared to the 3:4 lingual-buccal sidewall ratio of the
 bicuspid shell 120.
 FIGS. 17-24 show a bicuspid shell 320 and a molar shell 420 according to a
 third embodiment of the invention. Structural elements and features in
 common with the previously-described embodiments are indicated by like
 reference numerals incremented by 300 and 400 for the bicuspid and molar
 shells, respectively. The proximal sidewalls can also be recessed or
 indented (not shown) as shown and described with reference to FIGS. 9-16
 and FIGS. 25-32.
 There are several differences between the bicuspid and molar shells 320,
 420 of this embodiment and the shells of the previous embodiments. First,
 the occlusal walls 322, 422 each have a single cross-shaped occlusal
 window 353, 453, best seen in FIGS. 17 and 21. This window arrangement
 provides somewhat more window area for resin extrusion onto the top walls
 322, 422 as compared to the occlusal windows 150, 152 of the second
 embodiment, but still provides support for the filler in the finished
 crown. Second, the gingival margins of the shells are internally tapered,
 as shown by margins 325, 327 and 425, 427 in FIGS. 20 and 24,
 respectively. A third difference is that the lingual sidewalls 326, 426
 are shorter than the buccal sidewalls 324, 424 by a still greater
 proportion than either of the previous embodiments, this time having a
 lingual-buccal ratio of about 1:2. This ratio gives the greatest freedom
 for positioning the gingival margin of the lingual sidewall vertically
 along the lingual side of the prepared tooth. This freedom of positioning
 allows the buccal cusp of the shell to be moved buccal-lingually about a
 gingival margin of the buccal sidewall. Although an even shorter lingual
 sidewall could be used, it is not desirable because it would not provide
 much more freedom for positioning the lingual margin or the buccal cusp
 and because it would result in less control of lingual resin flow along
 the gingival margin.
 A further feature and advantage of the invention is best seen in the third
 embodiment. Specifically, this embodiment incorporates a generic design
 suited to fit all four quadrants of a patient's dentition easily. A kit
 consisting of four separately-sized molar shells and four separately-sized
 bicuspid shells (eight total), is provided. When sized as shown in FIGS. 7
 and 8, this kit suffices to fit 95% of all permanent posterior dentition
 without trimming. Furthermore, the kit of this embodiment does not require
 right or left mirror-image shells for right or left dentition, nor does it
 require maxilla- and mandibular-specific shells.
 This advantage is obtained because the shells 320, 420 are symmetrical
 about a buccal-lingual axis. Additionally, all of the sidewalls of the
 shells 320, 420 are short enough to fit a patient's dentition passively
 (i.e., without interference with the prepared tooth, adjacent teeth, or
 opposed teeth) when the patient's bite is in the closed position. The
 acrylic resin extrudes out of the mesio-distal sidewalls in a controlled
 way to form good proximal contacts with adjacent teeth. The resin also
 extrudes along the gingival margins of the shell to form an accurate
 marginal seal on all sides of the prepared tooth. Because the lingual
 sidewalls 326, 426 are substantially shorter than the buccal sidewalls
 324, 424, the shells 320, 420 are free to rotate about the facial gingival
 margins 325, 425 of the buccal sidewalls as the patient bites down on the
 resin-filled shell. Accordingly, this arrangement eliminates the need to
 trim the shell margins, facilitates proper positioning of the buccal cusp
 of the shell relative to opposed teeth, and reduces the need to trim
 excess resin and shell material from the facial (buccal) and occlusal
 surfaces of the crown.
 FIGS. 25-32 show a bicuspid shell 520 and a molar shell 620 according to a
 fourth embodiment of the invention. Structural elements and features in
 common with the previously-described embodiments are indicated by like
 reference numerals incremented by 500 for the bicuspid shell and 600 for
 the molar shell, respectively. There are several differences between this
 and the previous embodiments. First, the top wall 522, 622 does not have a
 window, as in the second and third embodiments, but is instead provided
 with a occlusal surface 522A, 622A that approximates the biting surface of
 a normal tooth, as in the first embodiment. Further, when viewed from the
 top, the occlusal wall 522, 622 has an approximate hourglass shape. This
 hourglass shape provides a profile that is roughly concave along the
 mesio-distal sides to allow the shell to conform to the convex shape of
 the mesio-distal sides of adjacent teeth. As in the second embodiment, the
 concave shape and spacing of the mesio-distal sides 528, 530, 628, 630
 allows the shell to fit with an approximately uniform-width proximal gap
 80 relative to the adjacent teeth (see FIG. 34).
 The buccal sidewalls 524, 624 and lingual sidewalls 526, 626 are
 substantially triangular in shape (see FIGS. 27 and 31), and wrap around
 to form part of the mesio-distal sidewalls 528, 530, 628, 630. The buccal
 and lingual sidewalls 524, 526, 624, 626 can also be of equal or
 near-equal length, although the lingual sidewall is preferably slightly
 shorter than the buccal sidewall. These features (the triangular shape and
 nearly equal length of the buccal and lingual sidewalls) allow the shell
 to cover the inter-dental papilla 72 adjacent to the prepared tooth, as
 will be described with reference to FIGS. 33-34. As in the third
 embodiment of the invention, the gingival margins 525, 527, 625, 627 can
 be internally tapered.
 The mesio-distal or proximal sidewalls 528, 530, 628, 630 are primarily
 formed as wrap-around portions of the buccal and lingual sidewalls 524,
 526, 624, 626 and from downwardly wrapping edges of the occlusal wall 522,
 622. The proximal sidewalls are shorter than the lingual sidewall. A
 portion of the mesio-distal sidewalls 528, 530, 628, 630 are open to
 provide mesio-distal windows 532, 534, 632, 634 through which resin can
 extrude into proximal gap 80. These mesio-distal windows 532, 534, 632,
 634 are substantially smaller in size than those of the previous
 embodiments, and generally comprise less than 1/4, and preferably between
 1/8 to 1/10, of the surface area of their respective mesio-distal
 sidewalls. Although the mesio-distal windows 532, 534, 632, 634 are shown
 substantially trapezoidal in FIGS. 26 and 30, they can be provided with a
 substantially semicircular shape or simply be provided with rounded
 corners.
 The mesio-distal sidewalls 528, 530, 628, 630 generally follow the concave
 shape of the mesio-distal sides of the occlusal wall 522, 622 in order to
 achieve the desired conformity with the adjacent teeth 62, 64. Most
 preferably, the shells 520, 620 are shaped and positioned to have a
 relatively uniform-width proximal gap 80 between the concave mesio-distal
 sidewalls 532, 534, 632, 634 and the convex mesio-distal sides of the
 adjacent teeth 62, 64, as shown in FIG. 34. A significant benefit of this
 aspect of the invention is that the concavity of the mesio-distal
 sidewalls of the shells fits the convexity of adjacent teeth.
 The occlusal wall is preformed to approximate the anatomy of a natural
 human tooth. Referring specifically to FIG. 29, the molar shell 620 of the
 fourth embodiment has four protuberances 690, 692, 694, 696 on its
 occlusal surface 622, which are flattened on top, to approximate a biting
 surface of a tooth. The four protuberances are oriented in opposing pairs
 in a buccal-lingual direction, and are arranged to position one
 protuberance in each quadrant of the occlusal surface. Referring to FIG.
 25, the shell 520 designed for a bicuspid tooth has only two
 protuberances, which are similar to those of the molar shell 620.
 As with the other embodiments, the fourth embodiment incorporates a design
 that only requires a few shells to fit the dentition of a wide range of
 adults. A kit based on this embodiment can contain a set of either of two
 types of crowns--temporary and long-term provisional crowns. Eight sizes
 and shapes of temporaries are provided. Sixteen sizes and shapes of
 long-term provisionals are provided. Long-term provisionals have more
 sizes of shells to provide a more accurate fit, and are made of a more
 durable polycarbonate material. Temporary shells can be made with thinner
 walls, as they do not have to last as long.
 An adult kit having shells for temporary bicuspid and molar crowns of adult
 teeth according to the fourth embodiment, for example, consists of eight
 sizes and shapes of shells. Two sizes of shells are used for molars in the
 upper right portion of the jaw and two other sizes of shells are used for
 molars in the upper left portion of the jaw. Another two shells are used
 for molars in the lower jaw (interchangeably on either the right or left
 sides), and the final two shells are used interchangeably for bicuspids in
 either the upper or lower jaw, interchangeably on either the right or left
 side.
 Accordingly, a set of temporary shells in an adult kit includes two upper
 right molar shells, two upper left molar shells, two lower molar shells,
 and two bicuspid shells. A top view of each of the two upper right molar
 shells has an approximate parallelogram shape leaning to the right while a
 top view of each of the two upper left molar shells has an approximate
 parallelogram shape leaning to the left. The two lower molar shells have a
 top view with an approximate rectangular shape. And finally, a top view of
 the two bicuspid shells has an approximate oval shape. Adult kits for
 long-term provisional crowns contain sixteen sizes and shapes of shells to
 provide even better sizing options. Similar kits can be made for children.
 FIGS. 33-35 show sizing considerations for a molar shell 620 and FIGS.
 36-44 show a method for making a crown using the molar shell 620. The same
 technique is used for making crowns using both temporary and long-term
 provisional shells. FIG. 33 shows a prepared tooth 660 and the adjacent
 inter-dental papilla 72.
 A preferred method for installing the temporary or long-term provisional
 crowns of this invention proceeds as follows. First, the tooth 660 to be
 crowned is prepared. Referring now to FIGS. 34, 35, and 36, a shell of the
 proper mesio-distal and occluso-gingival sizes must then be selected. A
 properly sized shell 660 fits passively and comfortably within the space
 provided in the patient's dentition between adjacent teeth 662, 664 and
 opposite teeth (not shown), i.e., fitting both the margins and occlusions
 well. For instance, the shell can be sized so that the occlusal surface
 profile has a mesio-distal width adjacent each of the buccal and lingual
 sidewalls that approximates a medial mesio-distal spacing of teeth
 adjacent the prepared tooth. The concave mesio-distal sidewalls 632, 634
 further conform to the convex sidewalls of adjacent teeth 662, 664 to form
 a substantially uniform proximal gap 80 for receiving extruded resin 638A
 and 636A. A suitable proximal gap is typically less than 1 mm in width,
 with a width of about 0.5 mm being preferred.
 Next, as illustrated in FIG. 37, resin is prepared by slowly mixing Super-T
 acrylic resin powder 74 with a reactive liquid 76 until the liquid
 completely saturates the powder and the combination becomes a creamy
 consistency. This mixture is then allowed to set for approximately 20
 seconds. As shown in FIG. 38, the shell 620 then is filled with a quantity
 of the resin 78 up to the height of the buccal and lingual sidewalls 624,
 626.
 The resin-filled shell 620 is then positioned on the prepared tooth 660 as
 shown in FIG. 39. A hand instrument 90 (i.e., Grade 4-5) is used to apply
 vertical pressure (represented by arrow 88) to the center of the shell 620
 to properly position the shell over the prepared tooth and between
 adjacent teeth 662, 664. The patient then bites down lightly to extrude
 resin from the windows in the mesio-distal sidewalls and out from the
 gingival margins and to align the occlusal surface 622 of the shell 620
 with the occlusal surfaces of adjacent teeth 662, 664. The dentist or
 technician could, alternatively, visually align the occlusal surface 622
 of the shell 620 with the occlusal surfaces of the adjacent teeth 662,
 664. The shell 620 and resin are left in place on the prepared tooth for
 approximately 10 seconds to allow the resin to partially set. After the
 resin becomes a little bit tacky, the hand instrument can then be used to
 remove some of the excess resin 78. Removing excess resin at this stage
 cuts down on the trimming time required later.
 Once the resin is putty-like, the resin-filled shell is repeatedly removed
 from and placed back on the prepared tooth until the resin has completely
 set. It is important during this step not to lift the crown too far off
 the prepared tooth (for example, do not lift more than a centimeter from
 the prepared tooth) and not to leave it off the prepared tooth for too
 long. This step further causes the resin to extrude through the windows of
 the mesio-distal sidewalls while preventing the shell 620 from locking
 onto the prepared tooth. After a few repetitions, the patient should then
 bite down again lightly. The shell 620 should then be lifted off and
 replaced yet again to prevent any undercuts. Even after the resin becomes
 stiff, some shrinkage will still occur. Therefore, the shell 620 should
 not be completely removed from the prepared tooth until the resin is
 completely set (about an extra minute or two). It should be noted that the
 reaction which causes the resin to set is slightly exothermic, but does
 not produce a significant amount of heat in this case because only a small
 amount of material is used.
 While waiting for the resin to finish setting, the occlusions can be
 adjusted, as represented in FIG. 40. Carbon paper can be used to mark
 occlusions on the crown for adjustment. To mark the occlusions, carbon
 paper is placed between opposing teeth. The patient then bites down and
 grinds the opposing teeth together. The occlusions marked by the carbon
 can then be adjusted using a diamond bit. This process is repeated until
 carbon appears on adjacent teeth, indicating that contact is being made
 between opposing and adjacent teeth, and therefore that a good occlusion
 has been obtained.
 Once the resin is completely set, the crown 620A can be removed from the
 prepared tooth 660, as shown in FIG. 41. The crown 620A is removed from
 the prepared tooth 660 by lifting it vertically using either a finger 92
 and thumb 94 or a crown remover (not shown). Margins 82 and contacts with
 adjacent teeth 84 are then marked with a pencil 96, as shown in FIG. 42,
 to enhance their visibility during the trimming operation. The adjacent
 contacts 84 should be kept wide for a good crown fit between the adjacent
 teeth. Once the margins and contacts are marked, the crown can be trimmed
 and finished, as illustrated in FIG. 43.
 Referring now to FIG. 43, it is important to perform the steps of the
 trimming process in the following order. The four corners are trimmed
 first to the marked margins using an acrylic bur lathe 42 or chair-side
 acrylic bur (not shown). The buccal, lingual, and mesio-distal walls are
 trimmed next to the marked margins. The acrylic bur is then used to trim
 underneath the marked contacts. The crown 620A is then finished and
 polished using a rubber wheel.
 Referring to FIG. 44, in the case of a long-term provisional crown 620B,
 the crown 620B should be relined if necessary. Using a medium-size round
 bur, the dentist or technician should grind inside of the shell right at
 the margins. More resin can be added if needed. The sulcus can also be
 packed with a retraction cord for better fit.
 Referring again to FIG. 35, when the crown 620C is finished, it is ready to
 be secured within the patient's mouth. The crown 620C is seated on the
 prepared tooth 660 and checked for fit. The dentist should make sure both
 the contours and margins are good. A final bite adjustment is therefore
 done to verify that the crown has tight-fitting adjacent contacts, good
 margins, and good occlusion.
 One of the major advantages of crowns made using the shells of FIGS. 25-32
 is that the occlusal anatomy is already built into the shells. This saves
 a significant amount of time which would otherwise be required to trim and
 form the occlusal surface. To ensure a good fit, some of the anatomy can
 still be taken away with occlusion adjustment to remove lateral and
 protrusive interferences. When the fit looks good, the crown 620C is then
 cemented onto prepared tooth 660 along the gingival margins.
 Having described and illustrated the principles of the invention in several
 preferred embodiments thereof, it should be apparent that the invention
 can be modified in arrangement and detail without departing from such
 principles. Various novel features described herein can be used in
 different combinations and can be modified in shape and dimension without
 exceeding the scope of the invention. I therefore claim all modifications
 and variations coming within the spirit and scope of the following claims.