Tube shift and parallax

The tube shift technique locates an object buccal or lingual to its neighbouring teeth by comparing two radiographs taken with the X-ray tube in two different positions [1, 2]. The object's image moves between them. Under the SLOB rule, an image that moves the same way as the tube lies lingual, and one that moves the opposite way lies buccal [3].

Synonyms: SLOB rule; same lingual opposite buccal; buccal object rule; Clark's rule; horizontal parallax; vertical parallax

What is the tube shift technique?

A single radiograph flattens the jaw onto one plane. When two objects sit side by side or overlap, it is difficult to tell whether they lie in the middle of the bone or buccal or lingual to each other [2].

Parallax is the apparent displacement of an object when the observer changes position. In radiography, that means two radiographs of the same object taken with the tube head in two different positions [1]. Compare the position of an unerupted tooth on each against the teeth around it, and you can tell its bucco-palatal position [4].

Charles Clark described the method in 1910 to localise unerupted teeth. Albert Richards simplified it and renamed it the buccal object rule [3, 5]. In 1952 Richards recognised that the tube could be shifted vertically as well as horizontally [6].

You'll meet the same method as the tube shift technique, the image shift method, Clark's rule, the buccal object rule and the SLOB rule [3, 7]. Its first use was placing unerupted and supernumerary teeth before orthodontic tooth movement or surgical removal, and its principles also apply to endodontic diagnosis and treatment [8]. In oral surgery, a periapical radiograph pair read with the SLOB rule supplements the panoramic radiograph when an impacted tooth overlaps its neighbours [9].

A note on words. In dental radiography the direction you name is the shift of the tube head. In medical radiography, the convention names the tilt of the beam [3]. This entry says tube shift throughout.

How does the SLOB rule tell you whether an object is buccal or lingual?

SLOB stands for same lingual, opposite buccal. Pick reference teeth, note which way the tube moved between the two exposures, then watch the object against those teeth [2].

• The image moves in the same direction as the tube: the object lies lingual, or palatal in the maxilla [3, 2].
• The image moves in the opposite direction to the tube: the object lies buccal [3, 2].

Picture a pair taken to plan removal of an impacted maxillary premolar. The first periapical is orthoradial, with the beam straight on. For the second, the tube moves mesially. On the second radiograph the premolar appears to have moved mesially. It moved with the tube, so it sits on the palatal side [3]. Had it moved distally, against the direction of the tube, it would sit buccal [3].

The answer is a relative one. The rule gives the position of the object compared with the reference teeth you read it against [2].

A 2025 narrative review of localisation techniques adds two habits. Check the crown and the root apex separately, since together they give the full position of the tooth. And expect the call to be easier the further the image moves for a given movement of the tube [2].

Should the tube shift be horizontal or vertical?

The shift can be made in the horizontal plane, called horizontal parallax, or in the vertical plane, called vertical parallax [1].

For a horizontal shift, the classic technique took three periapicals: one on the tooth of interest, one mesial and one distal. A 2025 narrative review reports that two periapicals are now routine, kept in the same horizontal plane with a tube shift of 20° to 30° between them [2].

Royal College of Surgeons of England guidance for the palatally ectopic maxillary canine lists these pairs [1]:

For an ectopic canine, British Orthodontic Society guidance asks for at least two conventional radiographs that permit parallax: two intraoral radiographs, or one intraoral radiograph and a panoramic radiograph [4].

The vertical pair with a panoramic radiograph is common for a practical reason. The panoramic radiograph is often taken first anyway, so the pair costs one additional exposure, the occlusal [10].

Studies disagree on which plane reads better, and they looked at different teeth. The Royal College of Surgeons of England guidance states that horizontal parallax is more reliable than vertical parallax for localising unerupted canines [1]. For supernumerary teeth in the anterior maxilla, one clinical study found the vertical shift more successful than the horizontal [11]. A dried-skull study with radiopaque markers found vertical parallax produced as many correct conclusions as other methods [12].

Outside localisation of unerupted teeth, endodontics uses the horizontal shift to pull overlapping canals apart. Two 2025 studies that measured beam angles on cone-beam CT scans found that a horizontal shift separates the buccal canals of maxillary molars from the palatal canal, and the two canals of a maxillary first premolar [13, 14].

What is the tube shift confused with?

The vertex occlusal has limitations, and a published case shows it can mislead [15]. A 2025 review notes it fell out of favour, probably over dose and poor image quality [2].

When does the tube shift give the wrong answer?

Run through these before you trust a pair.

The direction of the shift is misread. The reading rests on knowing which way the tube moved. A 2017 commentary gives the example of a dentist asking for a mesial shift and a radiographer with little dental radiography experience taking a mesial tilt, which is the opposite direction. The tooth then appears to move distally and gets read as buccal, which may lead to surgery on the wrong side [3]. The same commentary suggests pairing two standardised views, an occlusal and a panoramic radiograph, so that the direction of the beam is unlikely to be misunderstood [3].

The panoramic radiograph is read with the tube in the wrong place. For a panoramic radiograph the tube sits behind the patient's head and the receptor in front. The images keep their relationship to the reference teeth if you read the pair as if the tube were on the facial side of the arches, rather than lingual or buccal [2].

The shift is too small. A larger image movement for the same tube movement makes the position easier to call [2]. The Royal College of Surgeons of England guidance notes a suggestion that vertical parallax might be more reliable with more tube-head angulation [1].

The shift is too large. A 2025 study measuring beam angles on cone-beam CT scans of maxillary first premolars found that beyond a corridor of horizontal angles the neighbouring teeth begin to overlap the tooth of interest [13].

The object is buccal. Parallax localises buccally placed maxillary canines less reliably than palatal ones [16, 17]. One comparison of localisation methods for impacted maxillary canines concluded that if a canine looks buccal on the panoramic radiograph and cannot be palpated, further views are justified [17].

Readers disagree on the same pair. One study found wide variation between six examiners [17], and another found slight agreement between examiners [11]. Skill at reading parallax is associated with visual-spatial ability [18].

The question is the mandibular canal. For the relationship between a mandibular third molar root and the mandibular canal, studies against cone-beam CT disagree. One found the vertical tube shift inaccurate [19]; another found it valid [20]. The two used different vertical angulations, radiograph pairs and samples.

UK and European guidance suggests that small field of view cone-beam CT (CBCT) may be appropriate for an ectopic canine in selected cases where conventional radiographs fail to give enough information to plan treatment, and it is typically associated with a higher effective dose than conventional radiography [1]. There is no indication for its routine use [1]. British Orthodontic Society guidance sets the order: take at least two conventional radiographs that permit parallax, decide whether they are enough to plan, and consider localised CBCT only if they are not [4]. In most cases, conventional radiographs and CBCT agree on where the canine is [4]. In a typodont study, CBCT placed the canine more accurately than either horizontal or vertical parallax [21].