The SLOB Rule

 The SLOB rule is a radiographic rule for determining the location of certain objects intraorally with 2D xrays. I forgot about this rule until very recently when I saw a youtube video on it. There are many helpful youtube videos that will be much more helpful than this post as it is good to see the relationship of the objects in motion and with actual examples of xrays taken. 

The SLOB theory is essentially grounded in geometric principles. Use of this rule allows the clinician to determine the buccolingual relationship of two intraoral objects with 2 plain films. SLOB is broken down to "Same Lingual, Opposite Buccal" meaning that if the objects shift to the same side as the xray tube head is moved, the object is on the lingual and if the object in question moves to the opposite side, it is on the buccal.

The SLOB rule is useful in endodontics as it can be used to determine which of the canals or roots is which on the plain film. Once a 2D xray is taken, all spatial relationships are lost, use of 2 films with a tube shift allows the user to gain some perspective for multirooted teeth. This could also be useful in oral surgery if there are 2 retained roots left. During the working length phase, this may not be needed if you use two different types of files for different canals e.g Hedstrom file in the palatal and K file in the buccal as these can be distinguished on the xray and a second tube shift xray may not be needed. During the obturation phase, if the canals are prepared to different sizes and tapers, a single tube shifted xray may be all that is needed to differentiate the canals.

In figure 1, a straight on xray is taken showing the root canals in different colours. The MB and ML canals are overlapping and it is not possible to differentiate them apart e.g if there are endodontic files or GP points in each.

Figure 1: Straight on film in an endodontic case. Teeth are represented as the black rectangles with the xray film or sensor at the top of the picture. The red rectangle is the xray tube with light blue representing the xray beams. The root canals are noted on the lower molar with the ML canal in orange, MB in pink and D canal in blue

In Figure 2, The operator has taken a second radiograph with a mesial and a distal tube shift. It doesn't matter which one you do as long as there is a significant shift ~approx 30 degrees to allow enough movement of the structures on the xray to tell them apart. When using an xray holder with metal in it, it may be a good idea to move the tube to whichever direction will not have the metal obstructing the image (usually the distal). As much as is possible, the patient needs to remain still and the xray sensor needs to remain in place so that the only change in variable on the image is the tube position. This is more difficult to standardise with analogue films and PSPs but the use of a film holder can help. On the left side of figure 1 there has been a mesial tube shift and the mesiolingual canal (orange) has moved to the mesial relative to the mesiobuccal canal (pink) on the film. Using the SLOB rule, we note that the tube has moved mesially as well as the orange canal therefore the orange canal on the film is anatomically lingual relative to the pink canal. On the right side of figure 1 there is a distal shift of the same situation. The pink canal has moved to the mesial  relative to the orange canal therefore we can note that it has moved the opposite direction of the tube therefore the pink canal must be the mesiobuccal canal.

 

 

 

 

Figure 2: Mesial and distal tube shift.

 

The SLOB rules help determine the buccolingual relationship of one object to another. This doesn't have to be 2 canals relative to each other but is also useful in locating soft or hard tissue lesions relative to the teeth. In figure 3 there is a brown lesion on the buccal of the teeth e.g exostoses and a blue lesion on the lingual e.g salivary stone.

Figure 3: Parallel xray of lesions.

In figure 4, there has been a mesial and distal tube shift to localise the lesion. If the radioopacity doesn't change relative to the teeth we can assume it is in bone around the root apices, if there is a definite shift away from the teeth we can assume it is likely in soft tissue e.g in buccal mucosa or sublingual tissues. You will note that there is horizontal elongation of the structures when the technique departs from a parallel xray. There is also overlap of the teeth which can make interpretation difficult. In this case, the movement of structures is of the lesion relative to the teeth compared to canals relative to each other in the first two pictures. This is the general principle of the SLOB rule that the relative movement of any two objects can be compared as it is the geometrical basis of the rule that is important.

Figure 4: Mesial and distal tube shift

 

In figure 4, movement of the blue lesion is less apparent than movement of the brown lesion as it is closer to the teeth as well as closer to the film. Being closer to the teeth means that there is less relative movement between the tooth and the lesion and being closer to the film means that the xray beam has less distance to travel between the object and the film so any movement is less exaggerated. The brown lesion appears to move distal relative to the premolar in the mesial tube shift so it can be deduced it is placed to the buccal of the premolar and vice versa for the distal tube shift.

With the widespread use of 3D xrays the SLOB rule may become less utilised but 3D xrays aren't always feasible, affordable or appropriate for this use. Therefore the SLOB rule being utilised in 2D images is still a useful modality for problem solving in clinical situations.