The Buccal Object Rule
Professor Emeritus of Dentistry University of Michigan School of Dentistry Ann Arbor, Michigan
Localization technics for pathoses, supernumerary teeth, fractures, and foreign bodies are important uses of the x-ray examination. Professor Richards's Buccal Object Rule can be a very useful diagnostic tool in these radiographic procedures. This article should be of great interest to all who interpret radiographs.
The Buccal Object Rule is a method for determining the relative location of objects hidden in the oral region. The rule is: When two different radiographs are made of a pair of objects, the image of the buccal object moves, relative to the image of the lingual object, in the same direction that the x-ray beam is directed. The initial concepts of the procedure were reported in 1952 and 1953,1,2 and since that time the procedure has been developed to its present state of refinement and usefulness.
In 1909, Clark3 reported a radiographic procedure for the localization of impacted teeth. He wrote as follows:
...if we wish to ascertain [where] a buried canine is lying, three radiographs are taken: the first directly over the suspected tooth, and which we will call the central position, and another mesial to this position, while the third is taken distal of the first or central position. Each of the three radiographs must be marked immediately [after] they are taken, or confusion will arise. With the three radiographs placed in their relative position[s], we carefully note the position of each tooth shown in the central radiograph, and by comparing it with the other two the position of the buried tooth, whether it is on the palate or whether it is situated labially, can be ascertained readily with a little practice.
The number of films used and the vagueness of the instructions make Clark's rule unsatisfactory. Others have developed localization technics employing two films, but each required firsthand knowledge of how the two films were exposed before the hidden object could be located.
Application of the Buccal Object Rule (BOR) also requires two different radiographs of the suspect area, but firsthand knowledge of how the two radio- graphs were made is not necessary. Instead, a series of clues is provided which makes it possible to deduce how the two radiographs were made. From this deduction, the location of the hidden object relative to an object of known position can be ascertained.
The head of the modern dental x-ray machine can be rotated about a vertical axis so that the x-ray beam can be directed to the left or right. With this motion, all parts of the x-ray head move in horizontal planes and the horizontal direction or horizontal angulation (HA) of the central ray of the x-ray beam is changed. The head of the x-ray machine can also be rotated about a horizontal axis so that the x-ray beam can be directed up or down. With this motion, all parts of the x-ray head move in vertical planes, and the vertical direction or vertical angulation (VA) of the central ray of the x-ray beam is changed. The angle formed by the path of the central ray and the horizon is indicated by the VA scales located at either end of the x-ray head.
The BOR implies that two different radiographs have been made of the hidden object and its surroundings. These radiographs display the necessary clues to indicate how the films were exposed. The difference between the two radiographs will have been caused by HA or VA changes in directing the beam. All of the clues that will be discussed and illustrated can be applied to radiographs produced when the film packet was placed in the mouth at an angle to the long axis of the tooth. The clues that will also apply to radiographs produced when the film packet was placed parallel to the long axis of the tooth will be marked with an asterisk (*).
The successful application of the BOR requires that the two radiographs display clear images of the hidden object and of a second object, such as a part of an erupted tooth, the position of which is known. When choosing the hidden and known objects for making the BOR determination, it is best to choose points (e.g., cuspid tips, root apices) or lines (e.g., root canals) rather than surfaces of teeth.
The HA should be changed when attempting to localize a point relative to a vertical line, such as a root canal. The VA should be changed when attempting to localize a point relative to a horizontal line, such as the mandibular canal.
Radiographs can be viewed from the lingual surface, as though the observer were inside the mouth, or from the buccal surface, as though the observer were outside the mouth looking at the buccal or labial surfaces of the teeth. When the convex side of the identification dot on an intraoral radiograph faces the observer, the buccal surface of the film is being viewed and the observer's eye has assumed the same location that the focal spot of the x-ray tube occupied when the film was exposed.
All simulated radiographs that follow will be viewed from the buccal side, although the BOR is equally applicable with both methods of viewing films. Illustrations are viewed with the roots of maxillary teeth extending upward and the roots of mandibular teeth extending downward. To decide whether the teeth portrayed in the illustration are on the left or right side of the patient, simply note the location of the mesial surfaces of the teeth. If they are at the left side of the images of the teeth, the teeth are on the patient's left side. Teeth on the patient's right side display the mesial surfaces on the right side of the images.
Images of adjacent teeth appear overlapped on the simulated radiograph (Figure 1A) when x-rays that have passed through parts of both teeth record the two teeth on one area of the film (Figure 1B). The overlapped area appears as a radiopaque image between the teeth, frequently pointed at the top and bottom and thicker in the center. Changing the direction of the x-ray beam, so that the image of each tooth records on separate areas of the film (Figure 1C), involves changing the HA (Figure 1D). Neither changing the position of the film packet in the mouth nor changing the VA will cause overlapping. The sole cause of overlapping is incorrect HA. Therefore, when viewing a pair of radiographs such as Figures 1A and 1C, the presence of overlapping images on one and not on the other indicates that the HA has been changed. It does not indicate how the HA was changed, only that it was the HA, and not the VA, that was changed.
Changing the HA causes the images of roots to tilt in the same direction in which the x-ray beam is directed. This concept is applicable in all regions of the mouth.
When a film packet is positioned in the mouth at an angle to the long axis of a tooth, it may actually touch the lingual cusp (Figure 2A) or the incisal edge (Figure 2B) of the tooth, while the root apex of the tooth lies at some distance from the film.
Directing an x-ray beam perpendicular to the maxillary right central incisor and the film plane (in the horizontal relationship) will cause the image of the apex to record directly above the crown of the tooth (Figure 3A). However, if the HA is then changed by directing the beam more toward the operator's left, the image of the incisal edge will remain fixed on the film where the tooth touches the packet, while the image of the root and crown will tilt or slope toward the left on the film (Figure 3B). Directing the x-ray beam toward the distal side will cause the images of roots to tilt distally, while directing the x-ray beam mesially will cause mesial tilting (Figure 4).
When comparing one radiograph with another of the same region, the two radiographs must display a common orientation or baseline if the tilting of the roots is to be used as a means for determining how the HA has been changed. Compared with Figure 5A, the roots in Figure 5B apparently tilt toward the right, or do they? A line drawn through the incisal edges of the teeth is horizontal in Figure 5A but not in Figure 5B. The lines through the incisal edges are not similarly oriented, so no changes in the HA can be inferred from the apparent tilt of the roots in Figure 5B. Actually, the images are identical. The film packet was merely rotated a bit when Figure 5B was recorded.
Figure 6 illustrates how the BOR can be used to determine the location of object X relative to the root apex of tooth R. Figure 6A depicts two maxillary central incisors with no overlapping present. The tooth labeled R is the maxillary right central incisor, because its mesial surface is at its right side. Overlapping is present at the contact point in Figure 6B. It appears as a radiopaque area, pointed at both ends and widest at the center. The fact that the contact area is overlapped in one radiograph and not on the other indicates that a change in the HA has occurred. At this stage in the application of the BOR, it is not known how the HA was changed, only that it was changed between the two exposures.
Lines through the incisal edges of the teeth in each radiograph would be horizontal; therefore, the images of the teeth have a common baseline and are oriented similarly for viewing purposes. The direction in which the roots are tilted can now be used with confidence to determine in which direction the HA has been changed. In Figure 6A, the root images are erect, while in Figure 6B the roots tilt toward the left of the simulated radiograph. Hence, after Figure 6A was exposed, the HA was changed by directing the beam toward the operator's left to expose Figure 6B. It is now known how the HA was changed to produce Figure 6B.
Object X could be a point on an impacted tooth, a supernumerary tooth, a tumor, a foreign object, an area of bone loss, or an anatomic structure.
In Figure 6A, the root apex of tooth R is seen directly below object X. In Figure 6B, the apex has moved to the left of the object when the x-rays were directed to the operator's left. Since the apex moved, relative to object X, in the same direction in which the x-ray beam was directed, according to the BOR, the apex is the buccal (labial) object and object X is located lingually to it. Because we are dealing with the anterior teeth, the word labial should be substituted for the word buccal.
Since some confusion may have arisen because the movement of the apex was determined relative to the position of object X, we will reverse the order and apply the BOR to the problem a second time. In Figure 6A, object X is located directly above the root apex of tooth R. In Figure 6B, object X has moved to the right of the apex, while the x-rays were directed to the operator's left. Since object X has not moved, relative to the root apex, in the same direction in which the x-ray beam was directed, it is not the buccal object; therefore, the root apex must be the buccal (labial) object and object X lies lingually to it. The same relationship between the root apex and object X has been determined by the two applications of the BOR described above.
If object X and the root apex were touching, there would be no bucco-lingual difference in their positions, and there would be no apparent movement between the images of the two structures on the two radiographs. If the two structures were not touching, but were close together, a small change in the distance between their images would record. If the two structures were quite remote from one another, a large movement of their images would be seen on the radiographs. Thus, the BOR provides not only the buccolingual relationship between two structures, but also provides a qualitative measurement for the proximity of the two structures under examination.
Changing the HA causes the images of buccal cusps to move horizontally, relative to the lingual cusps, in the same direction in which the x-ray beam is directed. This clue applies only to multicusped teeth.
When a film packet is positioned in the mouth, either parallel with the long axis of the tooth (Figure 7A) or at an angle to it (Figure 7B), the image of the buccal cusp will record closer to the occlusal border of the film than will the lingual cusp. This applies to both maxillary and mandibular teeth.
From a view of the occlusal surfaces of the teeth in Figure 8A, it can be seen that when the x-ray beam is directed so that no overlapping of images of adjacent teeth occurs, the images of the buccal and lingual cusps will be superimposed. Changing the HA so that the beam is directed distally (Figure 8B) results in overlapping of adjacent teeth and causes the image of the buccal cusp of a tooth to record distally to the lingual cusp's image.
Figure 9 illustrates how the BOR can be used to determine the location of object X relative to the root apex of the first bicuspid. Figure 9A depicts a maxillary right cuspid and two bicuspids with no overlapping present. Since cusps 2 have recorded closer to the occlusal border of the film than cusps 1, they are the buccal cusps and cusps 1 are the lingual cusps. In Figure 9A, the buccal cusps (2) are situated directly below the lingual cusps (1). When the HA was changed to produce Figure 9B (evidence: overlapping), the buccal cusps (2) moved distally; therefore, according to Clue No. 2, the HA was changed in a distal direction. According to Clue No. 1, the distal sloping of the roots in Figure 9B would also indicate that the HA was changed in a distal direction.
Object X is located directly above the root apex of the first bicuspid in Figure 9A. In Figure 9B, object X has moved to the distal side of the root apex when the rays were directed distally. Since object X has moved, relative to the root apex, in the same direction in which the x-ray beam was directed, according to the BOR, it is the buccal object and the root apex is located lingually to it.
Changing the HA causes the images of buccal root apices to move horizontally, relative to the lingual root apices, in the same direction in which the x-ray beam is directed. This clue applies to the maxillary molar region.
Figure 10A portrays a maxillary left second bicuspid and first and second molars. The images of the crowns display no overlapping. Root 1 appears longer and wider than the other two roots of the first molar; also, it is centrally located relative to the crown, and its root canal is much wider than the others. These characteristics identify root 1 as the lingual root and the others as buccal roots. Root 2 is the mesiobuccal root and 3 is the distobuccal root.
In Figure 10A, the images of the buccal root apices lie one on each side of the lingual root. In Figure 10B, both buccal root apices have moved distally relative to the lingual root; therefore, according to Clue No. 3, the x-ray beam was directed more distally to produce Figure 10B According to Clues Nos. 1 and 2, the distal sloping of the roots and the distal movement of the buccal cusps relative to the lingual cusps also indicate that the x-ray beam was directed more distally to produce Figure 10B
The image of object X has recorded directly below the apex of the lingual root of the second molar and midway between the two buccal apices in Figure 10A. In Figure 10B the image of object X moved distally, relative to the lingual root, when the x-rays were directed distally; therefore, according to the BOR, object X is the buccal object and the broad root is the lingual object. In Figure 10B, object X is still midway between the two buccal apices. Since there has been no relative movement between object X and the buccal root apices, the three details lie in the same vertical plane.
Changing the HA causes the U-shaped image of the zygomatic process of the maxilla to move horizontally, relative to the maxillary molar roots, in the same direction in which the x-ray beam is directed. This clue applies to the maxillary molar region.
Figure 11 is a transverse section through the maxillary molar region in which the maxillary sinus is located directly above the molar roots, while the zygomatic process is buccal to the roots. Therefore, relative to the maxillary molar roots, the zygomatic process is a buccal object.
The cortical plate of the zygomatic process is so formed that it frequently produces a U-shaped image in radiographs of the maxillary molar region (Figure 12). Maxillary right first and second molars are displayed in Figure 12, along with the U-shaped image of the zygomatic process and the image of the zygomatic bone.
Overlapping of the crowns is apparent in Figure 12B but not in Figure 12A. This indicates that a change in the HA has occurred. In Figure 12A, the image of the zygomatic process is recorded directly above the roots of the molars, while in Figure 12B it lies distally to the molar roots. The image of a known buccal object, the zygomatic process, has moved distally relative to the molar roots. According to Clue No. 4, this indicates that the beam has been directed more distally to produce Figure 12B. Recalling Clues Nos. 1, 2, and 3 (with regard to the slope of the roots and the buccal-lingual relationship of cusps as well as roots) also indicates that the x-ray beam was directed distally to produce Figure 12B.
The image of object X has recorded directly above the mesiobuccal root apex of the first molar in Figure 12A. In this root apex moved distally relative to object X when the rays were directed distally; therefore, the mesiobuccal root apex is the buccal object and object X is the lingual object. The lingual root apex also moved distally, but by a lesser amount than the mesiobuccal apex; therefore, the lingual apex is closer to object X, but still located buccally to it.
Changing the HA causes the image of the anterior border of the ramus to move horizontally, relative to the mandibular molars, in the same direction in which the x-ray beam is directed. This clue is applicable to the mandibular molar region.
The relationship between the anterior border of the ramus and the mandibular molars is displayed by lateral and occlusal photographs in Figures 13A and 13B. The presence of the anterior border of the ramus hides the distal portion of the third molar's crown in the lateral view, because it is located buccally to the tooth. Moving the camera to the left would have caused more of the crown to be hidden by the ramus in Figure 13A, while moving it to the right would have revealed more of the crown.
Three mandibular right molars and the anterior border of the ramus are shown in Figures 14A and 14B. The overlapping of crowns in one illustration (14B) and not in the other (14A) indicates that the HA has been changed, but it does not indicate how it was changed.
The anterior border of the ramus is shown aligned with the distolingual cusp of the third molar in Figure 14A and with the mesiolingual cusp in Figure 14B The image of this known buccal object - the anterior border of the ramus - has moved mesially relative to the teeth; therefore, according to Clue No. 5, the HA was changed by directing the beam more mesiaIly to produce Figure 14B. According to Clue No. 2, the mesial movement of the buccal cusps in Figure 14B, relative to the lingual cusps, would also indicate that the HA was changed in the mesial direction.
Now that the HA change used in Figure 14 has been determined, the relative position of object X to the second molar can be ascertained by applying the BOR. Object X appears distally to the second molar's distal root apex in Figure 14A and has moved to a position mesial to that root in Figure 14B. Since object X has moved in the same direction in which the x-ray beam was directed, it is the buccal object and the root apex is located lingually to it.
When a routine periapical x-ray examination is performed, the HA is changed mesially for each exposure to direct the x-ray beam through the interproximal surfaces of the teeth, starting with the central incisor region and progressing back to the molar region. This clue applies to all regions of the mouth, provided the teeth are positioned normally.
When radiographing the central incisor region, the tube head is located at position A in Figure 15 and the beam is directed posteriorly along the midsaggital plane. If the same HA were to be used in the lateral incisor and cuspid region, the tube head would be located at position A'. This direction of the beam would obviously produce undesirable overlapping of the lateral and cuspid crowns. To avoid this overlapping, the tube head is located instead at position B and the HA is changed mesially compared to that used in the central incisor region. Similar mesial changes in the HA of the beam occur when proceeding from the lateral incisor and cuspid region (B) to the bicuspid region (C), and from there to the molar region (D).
The two illustrations shown in Figure 16 have been removed from adjacent openings in a complete mouth survey. They display the maxillary central incisors, the left lateral incisor, and the left cuspid. According to Clue No. 6, after the maxillary central incisors were radiographed (Figure 16A), the HA was changed, with the beam directed more mesially, to radiograph the lateral incisor and cuspid area (Figure 16B).
Now that the HA change used in Figure 16 has been determined, the relative position of object X to the root apex of the lateral incisor can be ascertained by applying the BOR. Object X lies distally to the root apex of the lateral incisor in Figure 16A and has moved mesially to it in Figure 16B. Since object X has moved in the same direction in which the x-ray beam was directed, object X is the buccal (labial) object and the root apex is located lingually to it.
The VA of the x-ray beam determines the length of a tooth's image when angular film positioning is utilized. Increasing the downward direction of the beam will shorten the images of the marillary teeth, while increasing the upward direction will shorten those of the mandibular teeth. This clue is applicable in all regions of the mouth.
Figure 17 illustrates how film packets can be positioned at an angle to the long axes of the teeth and how individual x-rays travel to record the opposite ends of the tooth on the film. The VA that will cause the x-rays to follow paths A will produce images with correct tooth length. The image of the root's apex will record at A' on the film. The location of the image of the other end of the tooth is determined by the position of the crown of the tooth on the film packet. When the VA is increased so that the x-rays follow paths B, the root apex records at B' and the length of the tooth's image is shortened by the distance A'B'
Figure 18 shows the maxillary left central and lateral incisors. The relationship between the two crowns is the same in Figures 18A and 18B therefore, the same HA was used for both. However, the length of the images of the teeth has changed, which suggests there has been a change in the VA. The teeth appear shorter in Figure 18B. According to Clue No. 7, after Figure 18A was produced, the images of these maxillary teeth were shortened by increasing the downward direction of the beam when Figure 18B was produced.
Now that the VA change involved in producing Figure 18B has been determined, the relative position of object X to the root apex of the lateral incisor can be ascertained by applying the BOR. Object X is just above the apex of the lateral incisor in Figure 18A. When the beam was directed downward to produce Figure 18B the image of the apex moved downward relative to object X; therefore, the apex is the buccal (labial) object and object X lies lingually to it.
Changing the VA causes the images of the buccal cusps to move vertically, relative to the lingual cusps, in the same direction in which the x-ray beam is directed. This clue is applicable only to multicusped teeth.
In Figure 19A, the correct VA was used to produce an accurate image of the tooth. The vertical distance or space between the images of the lingual (L) and buccal (B) cusps on the film is quite small. When a larger VA was used to produce Figure 19B, the beam was directed downward at a steeper angle, which caused the vertical distance between the images of the two cusps to increase. Also, as the VA was increased, the distance in the image from the apex (A') to the lingual (L') and buccal (B) cusps was decreased. This foreshortening of the tooth image is in accordance with Clue No. 7.
Figures 20A and 20B show the maxillary left first and second bicuspids and first molar. No overlapping is evident at the contact points in either illustration; therefore, the same HA was used for both. The vertical distance between the lingual and buccal cusps has changed, as has the length of the teeth. These changes indicate that there must have been a change in the VA.
The vertical distance between the lingual and buccal cusps is greater in Figure 20B. According to Clue No. 8, this increase resulted when the VA was increased so that the beam was directed downward at a steeper angle.
Now that the VA change involved in producing Figure 20B has been determined, the relative position of object X to the apex of the second bicuspid can be ascertained by applying the BOR. The apex of the second bicuspid was recorded about 2 mm below object X in Figure 20A. When the beam was directed downward to produce Figure 20B, the apex also moved downward to a position 5 mm. below object X; therefore, the apex is the buccal object and object X lies above and lingually to it.
Changing the VA causes the images of the buccal root apices to move vertically, relative to the lingual apices, in the same direction in which the x-ray beam is directed. This clue is applicable only to multirooted teeth.
In Figure 21A, the correct VA was used to produce an accurate image of the tooth. The vertical distance between the images of the lingual (L) and buccal (B) apices on the film is quite small. When a larger VA was used to produce Figure 21B, the beam was directed downward at a steeper angle, which caused the vertical separation of the two apices to increase. Also, as the VA was increased, the vertical separation of the lingual and buccal cusps increased, according to Clue No. 8, and the length of the tooth image decreased, as predicted by Clue No. 7.
Figure 22 shows the maxillary right first and second bicuspids and first molar. No overlapping is evident at the contact points in either illustration; therefore, the same HA was used to produce both Figures 22A and 22B. However, in Figure 22B the vertical distance between the lingual and buccal apices, the vertical spacing between the lingual and buccal cusps, and the length of the teeth have all changed, which suggests that there has been a change in the VA.
According to Clue No. 9, since the vertical distance between the lingual and buccal apices is greater in Figure 22B, the VA was increased by directing the beam downward at a steeper angle for this illustration.
Now that the VA change involved in producing Figure 22B has been deduced, the relative position of object X to the apex of the distobuccal root can be determined by applying the BOR. The images of object X and the apex are just touching in Figure 22A. The beam was directed downward to produce Figure 22B, and the two images are still touching. Since the image of object X has moved neither upward nor downward, relative to the apex, when the beam was directed downward, its location is neither lingual nor buccal to the apex, but, instead, it is touching the apex.
Changing the VA causes the U-shaped image of the zygomatic process of the maxilla to move vertically, relative to the maxillary molar root images, in the same direction in which the x-ray beam is directed. This clue applies only to the maxillary molar region.
Figure 23 is a drawing of a transverse section through the maxillary molar region. The maxillary sinus is located directly above the molar roots, and the zygomatic process is buccal to the roots. The cortical plate of the zygomatic process is so formed that it frequently produces a U-shaped image in radiographs of the maxillary molar region.
Figure 23 illustrates how the zygomatic process appears on film, relative to the maxillary molar root apices, when the VA of the beam is changed. When a small or low VA is used (solid lines), the image of the zygomatic process (Z) records above the lingual (L) and buccal (B) apices. Also, the lingual (L) and buccal (B) cusps appear close together and the tooth seems to be elongated. When a greater VA is used by directing the beam downward at a steeper angle (broken lines), the zygomatic process (Z') records below the lingual (L') and buccal (B') apices, the tooth's image becomes shorter, and the lingual and buccal (B') cusps record farther apart.
Three maxillary left molars are displayed in Figure 24, along with the U-shaped image of the zygomatic process and the image of the zygomatic bone.
No overlapping is evident at the contact points in either illustration; therefore, the same HA was used to produce Figures 24A and 24B. The VA must have been changed, as evidenced by the difference in vertical spacing between the zygomatic process and the root tips in the two illustrations. The image of a known buccal object, the zygomatic process, has moved downward in Figure 24B, relative to the root tips. This indicates, according to Clue No. 10, that the VA was changed, with the beam directed more steeply downward, to produce Figure 24B. Recalling Clues Nos. 7, 8, and 9 (with regard to the length of the tooth's image and the buccal-lingual relationship of cusps as well as roots) also indicates that the x-ray beam was directed more steeply downward to produce Figure 24B.
In Figure 24A, the image of object X is shown below the zygomatic bone and at the same level, but distally to the root apex of the third molar. With the beam directed downward in Figure 24B, the zygomatic bone has moved below object X; hence, it is buccal to object X. However, object X is no longer even with the third molar root apex, but has moved slightly downward when the beam was directed downward. Therefore, according to the BOR, object X lies distally and slightly buccally to the molar root tip.
Changing the VA causes the image of the anterior border of the ramus to move vertically, relative to the image of the mandibular molars, in the same direction in which the x-ray beam is directed. This clue is applicable only to the mandibular molar region.
The buccal location of the anterior border of the ramus, relative to the third molar, is pictured in Figure 25A. The vertical dotted line indicates the location of the transverse section shown in Figure 25B. This figure indicates that, on a radiograph made with the x-rays directed so as to superimpose the lingual (L) and buccal (B) cusp images (solid lines), the ramus (R) will record well below the cusps. Changing the VA so that the beam is directed upward (broken lines) causes the ramus (R) to record higher up on the crown (R') Also, as the VA increases, the vertical separation of the lingual (L') and buccal (B') cusp images increases, as predicted by Clue No. 8.
Three mandibular right molars and a bicuspid are displayed in Figures 26A and 26B, along with the anterior border of the ramus. No overlapping is evident at the contact points in either illustration; therefore, the HA has not been changed. However, the VA must have been changed for Figure 26B, as evidenced by the vertical movement of the anterior border of the ramus relative to the crown of the third molar. According to Clue No. 11, the VA was changed by directing the x-ray beam in a steeper upward direction for Figure 26B.
In Figure 26A, the image of object X has recorded at the apex of the third molar's mesial root. When the x-ray beam was directed upward to produce Figure 26B, the root apex moved upward relative to object X. According to the BOR, since the root apex has moved in the same direction as the beam, it is the buccal object and object X is located lingually to it. In this example, object X could have been the upper border of the mandibular canal, and the relative positions of the root apex and the canal would then have been ascertained.
A VA change occurs whenever a given region is radiographed using two different technics.
Figure 27A is a drawing of maxillary and mandibular posterior teeth being examined using the interproximal or bitewing technic. The VA is 8° downward. Periapical examinations of the same teeth are shown in Figures 27B and 27C. For the maxillary tooth, the VA value is 18° downward, and for the mandibular tooth it is 10° upward.
The BOR can be applied to the radiographs produced in Figures 27A and 27B to determine the buccal or lingual location of some object relative to the maxillary tooth, because the VA change is known. In Figure 27B, the beam has been directed more steeply downward than in Figure 27A. The same is true when comparing Figures 27C and 27A. The VA was changed in Figure 27C by directing the beam more steeply upward.
Maxillary right bicuspids and molars are depicted in Figures 28A and 28B. The VA used when making the interproximal examination (Figure 28A) was 8° downward. The VA used when making the periapical examination (Figure 28B) was 25° downward. The images of the lingual and buccal cusps of the maxillary first molar are superimposed in the interproximal examination, but the buccal cusps appear below the lingual cusps in the periapical radiograph (Figure 28B).
Object X is a small amalgam restoration located in either the lingual or buccal surface of the first molar. In Figure 28A, object X can be seen above the superimposed lingual and buccal cusps. In Figure 28B, the VA has been changed by directing the beam more steeply downward. Object X has moved downward compared to the lingual cusps; therefore, according to the BOR, it is buccally located relative to the lingual cusps. The distance between object X and the buccal cusps has remained constant in both radiographs; therefore, the buccal cusps and object X lie in the same vertical plane. Object X is located in the buccal surface of the first molar.
Many problems in dentistry can be solved radio- graphically by the application of the BOR. The following is a partial list of these problems.
1. Where does the supernumerary or the impacted tooth lie relative to the roots of adjacent erupted teeth?
2. In which direction is the occlusal surface of a horizontally impacted molar facing, when the long axis of the tooth lies in a buccolingual direction?
3. Where does the mandibular canal lie relative to the roots of the mandibular molar?
4. Which is the image of the lingual root apex and which is the buccal apex image on radiographs of multirooted teeth?
5. The image of a given root canal filling represents which root?
6. Where does the broken instrument or the metallic foreign object He relative to the roots of adjacent erupted teeth?
7. Does the hook at the end of a curved root extend mesially, distally, lingually, buccally, or somewhere between two of these directions?
8. Two cemento-enamel junctions are visible on a radiograph. Which is lingual and which is buccal (labial)?
9. In which surface of the tooth is a given restoration located?
10. Where is the overhang of the restoration located?
11. The area of bone loss is associated with which root?
12. Is the radiolucent area at the apex of the mandibular bicuspid an area of bone loss due to pathology, or is it the image of the buccally located mental foramen?
13. The image of the zygomatic process obscures the root tips of the molars. Which angulation must be changed, and in what direction, to eliminate this problem in a second radiograph?
14. The images of the teeth are badly overlapped. What changes in technic are necessary to remedy the problem?
15. The length of the tooth has been distorted on the initial radiograph. What changes in technic are necessary to remedy the problem?
16. The anterior teeth are so crowded that two of them overlap. From radiographic evidence alone, which tooth lies lingually and which lies buccally (labially)?
References
1. RICHARDS, A.G.: Roentgenographic Localization of the Mandibular Canal. Journal of Oral Surgery 10:325-329, October, 1952.
2. RICHARDS, A. G.: The Buccal Object Rule. Journal of the Tennessee State Dental Association, 33:263-268, October, 1953.
3. CLARK, C. A.: A Method of Ascertaining the Relative Position of Unerupted Teeth by Means of Film Radiographs. Royal Society of Medicine Transactions, 3:87-90, 1909-1910.