US2193993A - Method of and apparatus for making dental radiographs - Google Patents

Description

March 19, 1940. DE w 2,193,993

METHOD OF AND APPARATUS FOR MAKING DENTAL RADIOGRAPHS Filed May 13, 1938 I 2 Sheets-Sheet 1 March 19, 1940. 5 E L 2,193,993

' METHOD OF AND APPARATUS FOR MAKING DENTAL RADIOGRAPHS Filed May 15, 1938 2 Sheets-Sheet 2 f5 .16 I I 1 J7 19 m 1 l 21% R '7 g Patented Mar. 19, 1940 UNITED STATES PATENT OFFICE.

METHOD OF AND APPARATUS FOR. MAKING DENTAL RADIOGRAPHS 20 Claims.

My invention relates to a new and improved method of producing substantially correct or true dental radiographs and to novel and improved apparatus by the use of which the method may be practiced. Considered from the standpoint of the apparatus, my invention may be said to be an improvement upon the apparatus described and claimed in my copending application Serial No 98,257,.fi1ed August 2'7, 1936, which matured into Patent No. 2,127,502, dated August 23, 1938.

For many years, text book writers and dental radiologists have taught the art that to make an accurate radiograph of a tooth the central X-ray beam must be directed at right angles to the plane which bisects the angle between the film and the longitudinal axis of the tooth as determined from the visible portion of the tooth. Devices made as described in my said copending application and operating in accordance with the aforesaid bisection theory have been extensively used. While the films produced by the use of such devices were considered good, they were not as perfect or satisfactory as desired. For example, if the apparatus were adjusted to give perfect projection of the lower molars, the reproductions of the upper centrals would be elongated; and, if adjusted to give perfect projection of the upper centrals, the reproductions of the lower molars would appear short. These two groups represent the extremes. In either of said cases, the reproductions of other teeth between those extremes were found to be more or less faulty and inaccurate.

Many adjustments were made in the devices. A great number of radiographs were made and studied. A thorough examination of the literature which deals with the anatomy of the maxilla, mandible and teeth was made. Numerous clinical tests and skull measurements were also made. As a result, it was noted that practically all teeth in the maxilla and the lower six anterior teeth usually tilted outwardly and that their length axes, as determined by their crown portions, were by no means at right angles to the occlusal plane. The hatter the vault of the max illa the greater was the tendency for the roots of the teeth to be tilted. On the other hand, the lower bicuspids and molars either were positioned vertically or were tilted inwardly. It was also noted that the axis of the crown portion of a tooth generally was different from the axis of its root portion. I observed that a line drawn through the vertical axis of the visible part of a tooth and one drawn through that part normally covered by tissue and bone formed an angle which together with the inclination of the crown placed the length axis of the root at an angle which generally varied from an average of about 5 to about 16 from perpendicular to a bite blockshaped to adapt itself to the occlusal plane.

These observations led me to an understanding of the existence of a definite relationship between crown-root angles, the position of the teeth in the maxilla and mandible, and the shape of the mouth. I discovered that the apparatus had to be modified to give a relatively. higher and higher pointer inclination, speaking of the upper teeth, as the film is more and more depressed if true reproductions were to be obtained.

It is an object of my invention, therefore, to provide an improved method of making dental radiographs and to provide an improved apparatus by means of which the method may be practiced whereby an accurate, true-sized reproduction of any tooth may be readily obtained by any operator of average skill.

It is another object of my inventionto provide an improved apparatus for indicating mechanically outside the mouth the paths along which the X-rays should be directed in order to produce accurate reproductions of teeth on the films.

More specifically it is an object of my invention to provide a method of and means for directing the X-rays on a film positioned behind a tooth along a path which deviates, from the path perpendicular to the plane bisecting the angle between the plane of the film and the long axis of the tooth as indicated by the crown of the tooth, by the addition of an angle the size of which is determined by the extent the long axis of the root deviates from the long axis of the crown of the tooth.

It is another object of my invention to provide improved means for holding the apparatus in a fixed position with respect to the teeth of a patient.

The invention consists in the novel constructions, arrangements and devices to be hereinafter described and claimed for carrying out the above stated objects and such other objects as will appear from the following specification, and it consists further in the improved method of making dental radiographs to be hereinafter described and claimed.

The invention is illustrated in a certain preferred embodiment in the accompanying drawings, wherein- Fig". I is a side elevational view of an apparatus embodying the principles of my invention;

Fig. 2 is a plan view of the apparatus;

Fig. 3 is a longitudinal sectional view taken on the line 3-3 of Fig. 2;

Fig. 4 is a perspective View on an enlarged scale of my preferred form of bite block;

Fig. 5 is a sectional view on an enlarged scale taken on the line 55 of Fig. 1;

Fig. 6 is a view on an enlarged scale taken on the line $6 of Fig. 1;

Fig. '7 is a diagrammatic view illustrating the variation in angular movements of the pointer with respect to angular movements of the film holder and giving certain dimensions that should be followed in making one form of the apparatus; and

Fig. 8 is a diagrammatic view of a tooth illustrating the relation of the long axis of the root to the long axis of its crown and the relation of said axes to a plane perpendicular to the occlusal plane or bite surface of the tooth.

Like characters of reference designate like parts in the several views.

Referring now to Figs. 1 to 6 of the drawings, it will be seen that my improved apparatus comprises an elongated frame made up of two channel-shaped side members I0 and II within which a link I2 is slidably positioned. The frame members It and II terminate in downwardly extending grip portions Ito and Ila, the space between which is bridged by webportions Illb and Ho (Fig. 2).

The film holder includes a supporting post I3 the lower end of which is pivotally connected by a pin I to the adjacent end of the link I2. A tubular member I5 is adapted to fit snugly over the post I3. The post I3 is provided with a recess I6 adapted to receive the lower edge of a film I7, the upper end of the member I5 being provided with an overhanging portion I8 under which the upper edge of the film may be positioned. The arrangement is such that a film may be readily positioned as shown in Figs. 1 to 3 by curving it slightly and moving it transversely to permit its lower and upper edges to be centrally positioned in the recess I6 and under the portion I8, respectively. A link I9 is provided at its lower end with a hub 28 which is disposed between the side members It and II on a shaft 2! in the form of a two-part tubestud screw. The other end of the link I 9 extends through a slot in the member I5 (Fig. 3) and into a slot in the post I3 and is pivotally connected to the post by a pin 22.

The lower end of a pointer 23 is positioned between side arms 24 which in turn are positioned between the side frame members II] and H. The pointer 23 and the side arms 24 are pivoted on a shaft 25 in the form of a tubestud screw which is carried by the side members It and I I. Depending portions 23a of the pointer are connected by a rivet 26 to the side arms 24 thereby preventing any relative movement between said side arms and the pointer which in effect constitute a bell crank. One end of a coiled spring 2'! is hooked through an opening in the lower end of the pointer 23, the other end of said spring being secured on a pin 28 carried by the side frame members I 0 and I I, said spring tending to pull the pointer in a clockwise direction on the shaft 25. The side arms 24 are positioned between spaced finger members 29 which are pivotally connected at their upper ends to the link I2 by a pin 30. The side arms 24 are pivotally connected by a pin 3| to the members 29.

A bite block 32 is positioned on the frame between the film holder and the pointer. In the arrangement shown the bite block is provided with downwardly extending spaced legs 32a which fit into boxes 33, formed as shown in Fig. 5, and secured to the outer side faces of the frame members In and II by fastening means such as rivets 34. The bite block is provided with a surface 32b against which the biting surface of a tooth may bear. The bite block is provided with a vertical surface 320 adapted to bear against the outer surface of the crown of a tooth. On the frame opposite the bite block is mounted a saddle-like member 35 which is secured to one of the frame members, for example the member I0, by means of rivets 36 (Fig. 6). The member 35 is provided with a rib 35a which in cross section is rounded, as shown in Fig. 6, the rib being of arcuate form, as best shown in Figs. 1 and 3.

The operation of the apparatus thus far described will be understood best by reference to Fig. 3. When the finger member 29 is drawn toward the grip portions Ifla, I Ia, the arms 24 and the pointer 23 are swung counter-clockwise on the shaft 25 against the tension of the spring 27, while at the same time the link I2 is pulled to the right thus pulling the lower end of the film holder to the right and causing it to swing on its pivot 22 in the same direction as the pointer. Upon release of the finger member 29, the film holder and the pointer swing clockwise under the influence of the spring 27.

In preparing to make a radiograph of a tooth, a film I! is inserted in the film holder, 2. new or sterilized bite block 32 is slipped into position on the frame, the apparatus is held by the operator by gripping the finger members I Go, lie and 29, the thumb of the operator resting on the web portions iIJb--I Ib, and the film holder is tilted backwardly to the limit of its motion. The forward end of the apparatus is then inserted in the patients mouth and the bite block 32 and the rib 35a of the saddle member are carefully positioned between the upper and lower teeth of the patient with the surface 320 of the bite block bearing against the outer surface of the crown of the tooth to be radiographed, the engagement of the bite block between the teeth being effective for holding the apparatus as a whole in position when the operator releases the finger pieces. The spring 27 then causes the film holder and the film to swing in clockwise direction as far as the conformation of the mouth will permit so as to bring the film into as nearly as possible parallel position with respect to the tooth to be radiographed.

In all prior apparatus, including that described in my said copending application, the parts operated on the bisection theory, the pointer being parallel to the frame when the film in its holder was positioned at right angles to the frame, the pointer swinging through an angle one-half as great as that of the film holder. Relative positions were obtained, therefore, as follows:

Film Pointer Degrees Degrees 0 0 correction angle should be no less than 5 and no more than 12, and that the maximum superimposed correction should be no less than 5" and no more than 10. I have also found that there is a definite relation between the surface 32b of the bite block and the fixed correction angle imposed on the pointer. When the latter is within the 5-12 range noted, the surface 32b of the bite block should incline at an angle of 2 to 5 with respect to a plane passing through the "intersection of the surfaces 32b-32c and parallel to the elongated frame. I have obtained highly satisfactory results by inclining the bite surface What an angle as aforesaid of 2, using a fixed correction angle of 9 and a superimposed correction angle proportionately increasing to 7.

In order to teach those skilled in the art how to construct one form of apparatus embodying the principles of my invention, I have given in Fig. '7 the necessary relative dimensions, angles and locations of pivot centers. The link I2 is not shown. In this view I have-used a bite surface inclined at 2, a fixed correction angle of 9 is set at a correction angle of 9 when the film is in 0 position, a line passing through the axis of shaft 25 and parallel to the base plane being the 0 position of the pointer. The angular positions of the pointer with respect to the five positions of the film noted are also shown. The extent to which the pointer moves as the film holder moves is arrived at by adding the'bisec'tor' or one-half of the angular movement of the film holder, the fixed correction angle and the proportionately increasing superimposed correction angle, and is apparent from the following table:

' The relation of the long axis of the root of a tooth to the long axis of its crown and the relation of those axes to a plane perpendicular to the occlusal plane is illustrated in Fig. 8,'

wherein the line a-a represents a plane perpendicular to the occlusal plane represented by the line bb. The line 0-0 represents the long axis of the crown and the line d-d the long axis of the root of the tooth. The fixed corrective angle of from 5 to 12, stated in the third column of the table as 9 is that angle between the lines aa and c-c. The increasing corrective angle of from 0 to 10, stated in the fourth column of the table as proportionately increasing from 0 to 7, is that angle between the lines cc and dd.

The various parts of the frame, film holder and pointer of my improved apparatus may be made of any suitable materials. For economy in production the fiat parts may be stamped of metal of suitable gauge and plated or otherwise treated to give them a finished appearance and make them sanitary by permitting sterilization through boiling, chemical or other means. The bite block may be made of wood, pressed pulp or any other suitable material which does not interfere with the passage of the X-rays.

As a result of thearrangement described, the pointer 23 is a true guide from a practical standpoint as to the position in which the X-ray tube is'to be placed. When the X-ray tube is placed in position as indicated by the pointer, the operator is assured that the rays from the tube to the film will be at the proper angle vertically and also horizontally to produce an accurate, true-sized image of the tooth on the film. With the film positioned edgewise into the transverse space under the ledge carrying the surface 32!) of the bite block, the operator is assured that the image of the tooth will be located completely within the margins of the film and in substantially centered position on the film with respect to the vertical. The bite block when made of wood or the like offers little or no interference to the passage of the X-rays through it. The provision of the ledge surface 320 on the bite block is an important feature. It forms a definite stop which comes to rest against the anterior tooth surface. It facilitates positioning of the apparatus in a patients mouth and makes for much greater uniformity of resuite with special reference to a high degree of uniformity of the location of the edge of the crown in relation to the edge of the film. The spring pressure holding the film against the lingual'surface of the teeth cooperates with the pressure of the vertical ledge of the bite block to locate the latter, and through it the entire apparatus, in proper position even with slight bite pressure. The bite block and the saddle ridge35a cooperate to locate the apparatus correctly when one or more teeth are missing or when all the teeth are missing above or below or both. Even in the latter case X-rays are sometimes made to determine the presence and shapeof root fragments.

My improved apparatus is capable of being sterilized quickly and. easily. .The bite blockv 32 can be thoroughly and satisfactorily sterilized if the operator desires to subject it to further use. In practice, however, the bite block may be discarded after use on one person and a fresh block inserted. The apparatus is readily inserted into the mouth with a minimum of interference and a minimum of contact with the walls of the mouth. I have found particularly that the contact with the palate is very limited and I am able to get much more satisfactory results including a noticeable reduction or elimination of the common tendency to gagging that can be obtained when the patient is xpected to hold the film in position by the pressure of one or more fingers.

My improved apparatus is shown in the drawings in position for use in making an X-ray exposure of an upper tooth. It will be understood that for an exposure of a lower tooth the apparatus as a whole will be inverted without affecting in any way the operation of the device. It will be understood also that if desired the film may be placed upon the holder out of center position transversely thereon if such expedient seems necessary or advisable for the proper positioning of the film with respect to the tooth to be radiographed.

The advantages of my improved apparatus may be summarized as follows:

1. It is simple to use, requires no unusual skill and offers definite economy in time and money because retakes due to improper direction of the X-rays are rare.

2. The consistency or uniformity of angulation is of marked value not only to the dentist who has difficulty in producing good films but becomes of special importance when an area is to be radiographed at intervals, as in orthodontia, in checking on regeneration of bone and similar procedures. In such cases, it is important to know that successive films were made at the same angle without appreciable longitudinal or lateral distortion in the normal mouth. Indeed, if because of abnormal formation or location a tooth does appear distorted, it is a dependable index of the direction and degree of its departure from normal. The films are diagnostic to a degree far beyond that of films made in the usual manner by expert dental radiographers. In this connection it may be mentioned that on different occasions by means of my improved apparatus several exposures were made of the same area of the same patient but by different operators and the films were found to be uniform enough that they could be superimposed one on the other and the image clearly read through them all.

3. As my improved apparatus comes into general use, the consultant who examines referred films will quite definitely know the angle at which they were made and duplicate films may be made for further diagnostic study if required.

4. In some areas, particularly the canine region and the anteriors, the film as heretofore employed was appreciably curved resulting in much distortion. With my improved apparatus the film is held practically fiat and distortion is thereby avoided.

5. Heretofore in taking radiographs of the lower incisors not much more than half the film was customarily used. With my improved apparatus all of the film is employed and parts of the osseous structure below the apices can be clearly seen.

6. Dental radiographs made according to prior methods have generally been made at a target film distance of a few inches because it is less diificult to position properly the apparatus and cone when they are close to the patients face. My improved apparatus permits positioning at much greater distances which results in marked increase of definition and lessening of distortion, the necessarily longer exposure time not being objectionable.

While I have illustrated in detail and have specifically described a certain preferred construction of apparatus, it is to be understood that such has been done merely for the purposes of illustrating the principles of my invention and teaching those skilled in the art how to practice the invention. I do not intend, therefore, to limit my invention to the details of construction shown and described, except only in so far as certain of the appended claims are specifically so limited, as it will be obvious that modifications may be made without departing from the principles of the invention.

I claim:

1. The method of making a dental radiograph which comprises the holding of a film adjacent the inner face of a tooth, and directing the X-rays on the film along a path deviating, from the path perpendicular to the plane bisecting the angle between the plane of the film and the long axis of the tooth as indicated by the crown of the tooth, by the addition of an angle the size of which is determined by the extent the long axis of the root deviates from the long axis of the crown of the tooth.

2. The method of making a dental radiograph which comprises the holding of a film adjacent the inner face of a tooth, and directing the X-rays on the film along a path deviating, from the path perpendicular to the plane bisecting the angle between the plane of the film and the long axis of the tooth as indicated by the crown of the tooth, by the addition of an angle the size of which is proportionate to the size of the angle between the long axis of the root and that of the crown of the tooth.

3. The method of making a dental radiograph which comprises the holding of a film adjacent the inner face of a tooth, and directing the X-rays on the film along a path deviating, from the path perpendicular to the plane bisecting the angle between the plane of the film and the long axis of the tooth as indicated by the crown of the tooth, by the addition of a corrective angle within the range 5 to 12 to which is added an angle of from 0 to 10 depending mainly on the inclination of the root of the tooth with respect to its crown.

4. The method of making a dental radiograph which comprises the holding of a film adjacent the inner face of a tooth, and directing the X-rays on the film along a path deviating, from the path perpendicular to the plane bisecting the angle between the plane of the film and the long axis of the tooth as indicated by the crown of the tooth, by the addition of a corrective angle of about 9 to which is added an angle of from 0 to 7 depending mainly upon the extent the root of the tooth is inclined with respect to its crown.

5. The method of making dental radiographs which comprises the holding of the films adjacent the inner faces of the differently positioned teeth, and directing the X-rays on the respective films along paths deviating, from the respective paths perpendicular to the respective planes bisecting the respective angles between the planes of the respective films and the long axes of the teeth as indicated by the crowns of the teeth, by the addition of a corrective angle within the range 5 to 12 to which is added an angle increasing from 0 to 10 as the deflection of the films increases from a position of 0 to a position of 60 with respect to a plane perpendicular to the occlusal plane.

6. The method of making dental radiographs which comprises the holding of the films adjacent the inner faces of the difierently positioned teeth, and directing the X-rays on the respective films along paths deviating, from the respective paths perpendicular to the respective planes bisecting the respective angles between the planes of the respective films and the long axes of the teeth as indicated by the crowns of the teeth, by the addition of a corrective angle of about 9 to which is added an angle which increases substantially proportionately to about 7 as the films are deflected from a position of 0 to a position of 60 with respect to a plane perpendicular to the occlusal plane.

. combination of a frame, a pivoted film holder mounted on said frame, a pivoted pointer mounted on said frame, and linkage means connecting said film holder and said pointer which upon being actuated by the pivoted movement of the film holder causes said pointer to swing in the same direction, said linkage means swinging the pointer through an angle one portion of which is one-half as great as the angular movement of the film holder and the remaining portion of which increases as the angular movement of the film holder increases.

8. In apparatus of the type described, the combination of a frame, a pivoted film holder mounted on said frame, a pivoted pointer mounted on said frame, and linkage means connecting said film holder and said pointer which upon being actuated by the pivoted movement of the film .holder causes said pointer to swing in the same direction, said linkage means swinging the pointer through an angle one portion of which is onehalf as great as the angular movement of the film holder and the remaining portion of which increases substantially proportionately as the angles of inclination of the film holder increase.

9. In apparatus of the type described, the combination of a frame, a pivoted film holder mounted on said frame, a pivoted pointer mounted on said frame, and linkage means connecting said film holder and said pointer which upon being actuated by the pivoted movement of the film holder causes said pointer to swing in the same direction, said linkage means swinging the pointer through an angle one portion of which is onehalf as great as the angular movement of the film holder and the remaining portion of which is a corrective angle within the range 5 to 1 2 and which is increased from to 10 as the angles of inclination of the film holder increase from 0 to about 60 with respect to a plane perpendicular to the occlusal plane. I

10;In apparatus of the type described, the combination of a frame, a pivoted film holder mounted on said frame, a pivoted pointer mounted on said frame, and linkage means connecting said film holder and said pointer which upon be ing actuated by the pivoted movement of the film holder causes said pointer to swing in the same direction, said linkage means swinging the pointer through an angle one portion of which is one-half as great as the angular movement of the film holder and theremaining portion of which is a corrective angle of about 9 and which is increased substantially proportionately to about 16 as the angles of inclination of the film holder increase from 0 to about 60 with respect to a plane perpendicular to the occlusal plane.

11. In apparatus of the type described, the combination of an elongated frame, a pivoted film holder mounted on said frame at one end, a pivoted pointer mounted on said frame at the other end, a bite block mounted on said frame between the film holder and the pointer, the biting surface of the block being at such inclination that when a film in the film holder is positioned substantially perpendicular to the occlusal plane the angle between said surface and the film is from 2 to 5 less than 90, and linkage means connecting said film holder and said pointer which upon being actuated by the pivoted movement of said film holder causes said pointer to swing in the same direction, said linkage means swinging the pointer through an angle one portion of which is one-half as great asthe angular movement of the film holder and the remaining portion of whichincreases as the angular movement of the film holder increases.

12. In apparatus of the type described, the combination of an elongated frame, a pivoted film holder mounted on said frame at one end, a pivoted pointer mounted on said frame at the other end, a bite block mounted on said frame between the filmholder and the pointer, the biting surface of the block being at such inclination that when a film in the film holder is positioned substantially perpendicular to the occlusal plane the angle between said surface and the film is from 2 to 5 less than 90, and linkage means connecting said film holder and said pointer,

which upon being actuated by the pivoted movement of said film holder causes said pointer to swing in the same direction, said linkage means swinging the pointer through an angle one portion of which is one-half as great as the angular movement of I the film holder and the remaining portion of which increases substantially proportionately as the angles of inclination of the film holder increase.

13. In apparatus of the type described, the combination of an elongated frame, a pivoted film holder mounted on said frame at one end, a pivoted pointer mounted on said frame at the other end, a bite block mounted on said frame between the film holder and the pointer, the biting surface of the block being at such inclination that when a film in the film holder is positioned substantially perpendicular to the occlusal plane the angle between said surface and the film is from 2' to 5 less than 90, and linkage means connecting said film holder and said pointer which upon being actuated by the pivoted movement of said film holder causes said pointer to swing in the same direction, said linkage means swinging the pointer through an angle one portion of which is one-half as great as the angular movement of the film holder and the remaining portion of which is a corrective angle within the range 5 to 12 and which is increased from 0 to as the angles of inclination of the film holder increase from 0 to 1 about 60 with respect to a plane perpendicular to the occlusal plane.

14. In apparatus of the type described, the combination of an elongated frame, a pivoted film holder mounted on said, frame at one end, a pivoted pointer mounted on said frame at the other end, 'a bite block mounted on said frame between the film holder and the pointer, the biting surface of the block being at such inclination that when a film in the film holder is positioned substantially perpendicular to the occlusal plane the angle between said surface and the film is about 88, and linkage means for swinging the pointer through an angle one portion of which is one-half as great as the angular movement of the film holder and the remaining portion of which is a corrective angle of about 9 which is increased substantially proportionately to about 16 as the angles of inclination of the film holder increases from 0 to about 60 with respect to a plane perpendicular to the occlusal plane.

15. In apparatus of the class described, the

the frame with respect to a tooth being radiographed, said block having a. surface extending vertically and bearing against the outer side of the crown of the tooth and another surface slidable over and against which the bite surface of the tooth bears.

16. In apparatus of the class described, the combination of an elongated frame, a pivoted film holder mounted on said frame at one end, a pivoted pointer mounted on said frame at the other end, linkage means connecting said film holder and said pointer which upon being actuated by the pivoted movement of the film holder causes the pointer to swing, and a bite block mounted on the frame for fixing the position of the frame with respect to a tooth being radiographed, said block having a surface extending vertically and bearing against the outer side of the crown of the tooth and another surface at an angle of from 92 to 95 with respect to the first named surface slidable over and against which the bite surface of the tooth bears.

17. In apparatus of the class described, the combination of an elongated frame, a pivoted film holder mounted on said frame at one end, a pivoted pointer mounted on said frame at the other end, linkage means connecting said film holder and said pointer which upon being actuated by the pivoted movement of the film holder causes the pointer to swing, and a bite block mounted on the frame for fixing the position of the frame with respect to a tooth being radiographed, said block having an inclined surface slidable over and against which the bite surface of the tooth bears, and an arcuate substantially rigid saddle member on the frame opposite said bite block against which the tooth or gum opposite the first named tooth bears.

18. In apparatus of the class described, the combination of an elongated frame, a pivoted film holder mounted on said frame at one end, a pivoted pointer mounted on said frame at the other end, linkage means connecting said film holder and said pointer which upon being actuated by the pivoted movement of 'the film holder causes the pointer to swing, and a bite block mounted on the frame for fixing the position of the frame with respect to a tooth being radiographed, said block having one surface extending vertically and bearing against the outer side of the crown of the tooth and another surface at an angle of from 92 to 95 with respect to the first named surface slidable over and against which the bite surface of the tooth bears, and a saddle member on the frame opposite said bite block and having an arcuate substantially rigid rounded rib against which the tooth or gum opposite the first named tooth bears.

19. In apparatus of the class described, the combination of an elongated frame, a film holder, a link pivoted at one end to said frame and at the other end to said film holder, a link slidably carried by the frame and pivoted to the film holder, :5, bell crank pivoted to the frame one arm of which serves as a pointer, and a member pivoted at one end to said slidable link and at its other end to the other arm of said bell crank whereby swinging movement of the film holder causes the pointer to swing in the same direction, the centers of said pivotal connections being such that the pointer swings through an angle one portion of which is one-half as great as the angular movement of the film holder and the remaining portion of which increases substantially proportionately as the angles of inclination of the film holder increase.

20. In apparatus of the class described, the combination of an elongated frame, a film holder, a link pivoted at one end to said frame and at the other end to said film holder, a link slidably carried by the frame and pivoted to the fi m holder, a bell crank pivoted to the frame one arm of which serves as a pointer, and a member pivoted at one end to said slidable link and at its other end to the other arm of said bell crank whereby swinging movement of the film holder causes the pointer to swing in the same direction, the centers of said pivotal connections being such that the pointer swings through an angle one portion of which is one-half as great as the angular movement of the film holder and the remaining portion of which is a corrective angle within the range 5 to 12 which is increased from 0 to 10 as the angles of inclination of the film holder increase from 0 to about 60 with respect to a plane perpendicular to the occlusal plane.

ANDRE DE WEAL.

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