Introduction
Apical periodontitis (AP) is an acute or chronic inflammatory condition occurring around the root of a tooth. It is caused by microbial infection of the root canal space and is characterised by destruction of the periradicular bone (Huumonen and Ørstavik, 2002). Prevalence studies on AP show that, depending on the age and location of the population, up to 80% of individuals may be affected with AP when conventional radiography is used to assess the periapical status of their teeth (Kabak and Abbott, 2005). While acute AP is commonly diagnosed from its clinical presentation, the diagnosis of chronic AP is usually dependent on the presence of radiographic signs of the disease. The ability of radiographs to accurately detect signs of AP is essential for diagnosis, treatment planning, assessment of outcome, and epidemiological studies. Currently, the accepted reference standard for the radiological detection of AP is periapical radiography (European Society of Endodontology, quality guidelines, 2006). However, several studies have highlighted the limitations of conventional radiography for detecting AP (Bender and Seltzer, 1961a; Patel et al, 2009; Tsai et al, 2013).
Limitations of conventional periapical radiography
In a series of ex vivo investigations, Bender and Seltzer (1961a, b) concluded that simulated AP lesions confined to the cancellous bone could not be readily identified on radiographs (Fig 7-1). This was due to the lesions being masked by the overlying denser cortical bone (i.e. anatomical noise); other research groups reported similar results (Pauls and Trott, 1966; Schwartz and Foster, 1971). However, in a post-mortem study using human specimens, Brynolf (1967) found that AP confined to the cancellous bone in the anterior maxilla region could be detected using periapical radiographs. In some instances, it may be possible to detect destruction within the cancellous bone without associated loss of cortical or junctional bone (Shoha et al, 1974).
For AP to be detected radiographically, the bone loss has to reach a ‘critical threshold’ in relation to the surrounding bone. If the ratio of healthy (mineralised) bone to demineralised bone (i.e. AP) reaches this critical level, then AP will be detected. The ratio will depend on several factors, including: the density of the bone; the nature of cancellous and cortical bone; the X-ray beam angulation; the exposure parameters; and the nature of the lesion (i.e. size and degree of demineralisation). These factors not only vary depending on the position of the lesion within the jaw, but also between the maxilla and mandible, as well as between individuals. For example, the mineral density of the posterior mandible region is higher than the anterior maxilla region. Therefore, a small volume of demineralised bone may be readily identifiable in the anterior maxilla, but not in the more radiodense posterior mandible.
A second angled (parallax) radiograph has been suggested to improve the ability to diagnose periapical lesions (European Society of Endodontology, quality guidelines, 2006; Vertucci and Haddix, 2010). However, there is limited evidence that parallax radiographs do improve the detection of periapical lesions (Soğur et al, 2012). Recently, Davies et al (2015a) compared the ability of single radiographs, two parallax radiographs, and cone beam computed tomography (CBCT) to detect AP in vivo, and revealed periapical lesions in 41%, 38% and 68% of radiographic systems, respectively. Using CBCT as a reference standard, these results suggest that there is no increased accuracy in detecting AP with parallax radiographs compared to a single view. However, Kangasingam et al (2016a) found that the combination of two additional (parallax) images, with mesial and distal horizontal angulations, did improve the detection of AP lesions when compared to a single view. In this study, block dissection and histopathological analysis of the periapical tissues in relatively fresh (unpreserved) human cadavers was used as the reference standard (Figs 7-1 and 7-2).
With digital periapical radiographic systems, the image produced is dynamic and can therefore be enhanced (contrast/brightness) to potentially improve its diagnostic yield (Kullendorff et al, 1996). Several well-designed ex vivo studies have shown that there is no difference in the ability to detect artificially created periapical lesions using conventional radiographic films and digital sensors (Kullendorff and Nilsson, 1996; Stavropoulos and Wenzel, 2007; Özen et al, 2009). In the abovementioned autopsy study using human cadavers, Kangasingam et al (2016a) also found no statistical difference between the accuracy of parallax digital and conventional (film) periapical radiographs, for assessing AP. However, single digital periapical radiographs were found to be more accurate than single conventional radiographs.
‘Enhancing’ radiographic images (e.g. colourising and inverting) with software also does not appear to improve the detection of periapical lesions (Barbat and Messer, 1998).
Detection of apical periodontitis
Numerous ex vivo studies with reference standards in both animal (Stavropoulos and Wenzel, 2007) and human (Özen et al, 2009; Patel et al, 2009; Tsai et al, 2013) models have conclusively demonstrated that CBCT is a significantly more accurate imaging system than periapical radiography for detecting the presence (sensitivity) of artificially created bone lesions. All ex vivo studies have the disadvantage of not truly mimicking the ‘real-life’ clinical situation. However, the advantage of such studies is that standardised periapical defects have been intentionally created, thus giving a reference standard that allows imaging techniques to be assessed with more confidence (Fig 7-3).
In a well-designed animal study using histology as a reference standard, Paula-Silva et al (2009a) reaffirmed that CBCT was a more accurate diagnostic tool than conventional radiography for diagnosing chronic AP. In this study, 83 block dissections of periapical tissues and root tips were histologically assessed in teeth with and without radiographic signs of AP. The specificity and positive predictive value was 1 for radiographs and CBCT, i.e. both imaging systems were accurate in determining when no disease was present. However, the sensitivity for CBCT (<0.91) was much higher than for radiographs (0.77) for detecting existing disease.
Using similar methodology to Brynolf (1967), Kanagasingam et al (2016b) assessed the accuracy of single, parallax digital radiographs and CBCT for diagnosing AP in fresh human cadavers using histology as the reference standard. In total, 86 teeth were assessed (Fig 7-2). The specificity of all the imaging systems was excellent, i.e. all imaging techniques could correctly detect healthy periapical tissues. However, the sensitivity of digital radiographs varied from 0.27 to 0.38, depending on whether a single view or parallax views, respectively, were assessed. This compared to a sensitivity of 0.89 for CBCT. The overall accuracy of the digital radiographic techniques was 0.5 for a single view and 0.58 for parallax views, and there was a significantly higher accuracy (0.92) for CBCT.
Davies et al (2015b) reviewed the outcome of secondary (re-treatment) carried out in 98 teeth 1 year after re-treatment with conventional radiographs and CBCT. They found that there was a significantly different success rate between radiographs (93%) and CBCT (77%).
Radiographic appearance of apical periodontitis
Conventional radiography
While long-standing lesions of AP with significant bone destruction are generally readily discernible on conventional radiographs, incipient lesions of AP are often much more difficult to identify. Alterations to structures of the apical periodontium, such as the medullary bone trabeculae, the periodontal ligament (PDL) space, and the lamina dura may be early indicators of AP (Gröndahl and Huumonen, 2004). As such, an appreciation of the normal radiographic appearance of these structures is essential.
The conventional radiographic appearance of cancellous bone surrounding a healthy tooth varies between the maxilla and mandible. Typically, maxillary alveolar bone trabeculae have a fine, granular appearance, while mandibular alveolar bone trabeculae have a coarser, horizontally striated appearance and are interspersed with comparatively wider marrow spaces. Subtle structural changes in the cancellous bone are generally the earliest signs of AP recognisable on conventional radiographs. These changes include a disruption and disorganisation of the normal trabecular pattern around the apex (or other portal of exit) of the affected tooth. The disorganisation of the affected area may be well defined and easily differentiated from the surrounding bone or, alternatively, the margins of the disorganised area may blend with the surrounding bone such that it is difficult to delineate it from the surrounding healthy tissue, thus making interpretation more challenging (Fig 7-5a and b; Fig 7-6a and b).
Widening of the PDL space associated with the affected tooth may be an early indication of AP. However, endodontic infection is not the sole cause of a widened PDL space, which may also be a feature of tooth mobility, marginal periodontitis, or even neurogenic inflammation (Pope et al, 2014). Furthermore, the specific exposure angle of the radiograph may lead to the appearance of a widened PDL space (Bender et al, 1961a). This should be borne in mind when assessing a tooth for the early signs of AP (Fig 7-7a and b). A widened PDL space specifically associated with AP will be localised to the apex of the tooth (or the affected portal of exit) and the immediately adjacent areas. The PDL space coronal (and/or apical, if for example a lateral canal is involved) to this area will be unaffected, and there will be a marked transition between the affected and unaffected sites.
A further, relatively early radiographic sign of AP is disruption of the lamina dura, the integrity of which may be breached and which may appear to lose density. Any changes will be localised to the affected portal of exit from which the microbes are egressing. However, in isolation, a break in the continuity of the lamina dura should be viewed with caution. Tiny perforations of the lamina dura, although not always seen radiographically, are necessarily present to accommodate vascular and neural supply from the adjacent medullary bone to the teeth. These perforations may manifest on some radiographs and not on others. Furthermore, a certain amount of variation in normal lamina dura radiodensity and thickness can be expected between individuals. These features may also be altered by the angle of the radiographic exposure. With the development of AP, the mineral content of the cancellous bone trabeculae becomes depleted, and the trabeculae become thinner and less dense, with a resultant increase in the size of the adjacent marrow spaces. The affected area sometimes develops what has been described as a ‘shotgun’ appearance. This represents an intermediate stage in the progression of the lesion, from the subtle structural changes of the bone described above to the development of a clear periapical radiolucency. However, it is not always identifiable.
When the bone demineralisation has reached a critical threshold, a radiolucency will develop. Diagnosis of AP at this stage is less complicated. Nevertheless, depending on the exposure angle of any given radiograph and the site of the subject tooth, adjacent anatomical features may impair interpretation of the radiograph, such that the radiolucency may not be readily identified, or conversely, may look larger. The margins of periapical radiolucencies may be well or poorly defined, and in some instances they will have a corticated appearance.
Another potential precursor of AP is ‘condensing osteitis’, which is a reactionary production of dense bone in the periapical area of the affected tooth in response to low-grade pulpal irritation. Condensing osteitis manifests radiographically as a localised radiopacity in the area around the apex (or other portal of exit) of the affected root. The nature of the areas of condensing osteitis is variable, and the border of the lesion may be well or poorly defined (Fig 7-8a). In some instances, this reactionary bone can completely obscure the anatomy of the affected root on conventional radiographs
Historically, it was thought that the histological nature of AP lesions could be determined by radiographic features of the associated periapical radiolucency, such as size and the presence or absence of a radiopaque, corticated rim (Bhaskar, 1966). These associations have since been disproved (Nair, 1998; Nair et al, 1999), and it is believed that the radiographic features of periapical radiolucencies, such as size and the presence of a corticated lamina dura (Ricucci et al, 2006), are in fact poor predictors of the true histological nature of AP lesions.
Due to the difficulties inherent in detecting AP on conventional radiographs, especially at an early stage, a scoring system for the registration of AP, called the ‘Periapical Index’ (PAI), was developed (Ørstavik et al, 1996). It is a five-point grading system that classifies AP according to some conventional radiographic features (previously described) associated with the development of the disease.
Cone beam computed tomography
The true extent of bone destruction associated with AP is underestimated on conventional radiographs (Paula-Silva et al, 2009c; Abella et al, 2014). Therefore, the absence of (conventional) radiographic signs of AP does not rule out its presence, while the presence of radiographic features of AP are inevitably associated with a true diagnosis of AP (Brynolf et al, 1970a, 1970b; Kanagasingam et al, 2016b). AP presents on a CBCT scan, even in its earliest stages, as a 2-mm, well-defined widening of PDL space, or as a clearly defined periapical radiolucency (Tsai et al, 2013; Abella et al, 2014) (Fig 7-7). These findings may have an impact on the treatment plan of a tooth provisionally diagnosed with reversible pulpitis. An AP lesion may be clearly detected with CBCT, even though conventional radiographs are within normal limits (Hashem et al, 2015), therefore root canal treatment rather than pulp preservation treatment (e.g. pulp capping) would be indicated. With the elimination of anatomical noise comes a more objective appreciation of the presence and extent of bone loss associated with the disease, when compared to conventional radiography (Figs 7-5 and 7-6). Condensing osteitis can also become more evident (Fig 7-8).
As such, AP can be reliably diagnosed, and the nature (i.e. dimensions and extent) of the lesion accurately determined using CBCT. Assessing the true nature of AP and its proximity to important anatomical structures can be especially relevant when considering periapical microsurgery (Bornstein et al, 2011; Patel et al, 2015). Any expansion and perforations of cortical bone can also be accurately identified and related to clinical findings.
The CBCT-PAI has been suggested for use with CBCT (Estrela et al, 2008). This index uses a six-point scale to quantify the maximum diameter of bone loss associated with AP. The scores range from 0 ‘intact periapical bone structures’ to 5 ‘diameter of periapical radiolucency >8 mm’. The variables +E (expansion of periapical cortical bone) and +D (destruction of periapical cortical bone) may be added to any score if they are detected on the CBCT analysis. Recent reports have suggested that this is a reproducible method of assessing the extent of AP (Esposito et al, 2011).
The improved accuracy of the diagnosis of AP associated with CBCT has potential implications for the assessment of the outcome of endodontic treatment (Patel et al, 2012b; Fernández et al, 2013). Lesions will be identifiable on CBCT for longer periods post- treatment (when compared with conventional radiographs) due to the superior sensitivity of the system (Fig 7-4). It is therefore likely that the way the outcome of endodontic treatment (including prognostic factors) is determined will need to be reassessed in the future (Patel et al, 2011).
Dose reduction protocols should always be considered whenever possible to reduce the effective dose to which a patient is exposed. Recently, Al-Nuaimi et al (2016) demonstrated a good diagnostic accuracy for detecting simulated AP lesions with CBCT when the exposure settings were adjusted away from the manufacturer’s default settings. In this study, there was no significant reduction in diagnostic yield when the radiation was reduced by up to 74%.
Conclusion
The current evidence suggests that CBCT is more sensitive and accurate compared with periapical radiography in detecting AP. CBCT must not be used as a default imaging system to detect AP (Patel et al, 2015). Instead, it should be used in those instances where clinical and conventional radiographic assessment is equivocal for the diagnosis of odontogenic and/or non-odontogenic disease.
Apical periodontitis (AP) is an acute or chronic inflammatory condition occurring around the root of a tooth. It is caused by microbial infection of the root canal space and is characterised by destruction of the periradicular bone (Huumonen and Ørstavik, 2002). Prevalence studies on AP show that, depending on the age and location of the population, up to 80% of individuals may be affected with AP when conventional radiography is used to assess the periapical status of their teeth (Kabak and Abbott, 2005). While acute AP is commonly diagnosed from its clinical presentation, the diagnosis of chronic AP is usually dependent on the presence of radiographic signs of the disease. The ability of radiographs to accurately detect signs of AP is essential for diagnosis, treatment planning, assessment of outcome, and epidemiological studies. Currently, the accepted reference standard for the radiological detection of AP is periapical radiography (European Society of Endodontology, quality guidelines, 2006). However, several studies have highlighted the limitations of conventional radiography for detecting AP (Bender and Seltzer, 1961a; Patel et al, 2009; Tsai et al, 2013).
Limitations of conventional periapical radiography
In a series of ex vivo investigations, Bender and Seltzer (1961a, b) concluded that simulated AP lesions confined to the cancellous bone could not be readily identified on radiographs (Fig 7-1). This was due to the lesions being masked by the overlying denser cortical bone (i.e. anatomical noise); other research groups reported similar results (Pauls and Trott, 1966; Schwartz and Foster, 1971). However, in a post-mortem study using human specimens, Brynolf (1967) found that AP confined to the cancellous bone in the anterior maxilla region could be detected using periapical radiographs. In some instances, it may be possible to detect destruction within the cancellous bone without associated loss of cortical or junctional bone (Shoha et al, 1974).
For AP to be detected radiographically, the bone loss has to reach a ‘critical threshold’ in relation to the surrounding bone. If the ratio of healthy (mineralised) bone to demineralised bone (i.e. AP) reaches this critical level, then AP will be detected. The ratio will depend on several factors, including: the density of the bone; the nature of cancellous and cortical bone; the X-ray beam angulation; the exposure parameters; and the nature of the lesion (i.e. size and degree of demineralisation). These factors not only vary depending on the position of the lesion within the jaw, but also between the maxilla and mandible, as well as between individuals. For example, the mineral density of the posterior mandible region is higher than the anterior maxilla region. Therefore, a small volume of demineralised bone may be readily identifiable in the anterior maxilla, but not in the more radiodense posterior mandible.
A second angled (parallax) radiograph has been suggested to improve the ability to diagnose periapical lesions (European Society of Endodontology, quality guidelines, 2006; Vertucci and Haddix, 2010). However, there is limited evidence that parallax radiographs do improve the detection of periapical lesions (Soğur et al, 2012). Recently, Davies et al (2015a) compared the ability of single radiographs, two parallax radiographs, and cone beam computed tomography (CBCT) to detect AP in vivo, and revealed periapical lesions in 41%, 38% and 68% of radiographic systems, respectively. Using CBCT as a reference standard, these results suggest that there is no increased accuracy in detecting AP with parallax radiographs compared to a single view. However, Kangasingam et al (2016a) found that the combination of two additional (parallax) images, with mesial and distal horizontal angulations, did improve the detection of AP lesions when compared to a single view. In this study, block dissection and histopathological analysis of the periapical tissues in relatively fresh (unpreserved) human cadavers was used as the reference standard (Figs 7-1 and 7-2).
With digital periapical radiographic systems, the image produced is dynamic and can therefore be enhanced (contrast/brightness) to potentially improve its diagnostic yield (Kullendorff et al, 1996). Several well-designed ex vivo studies have shown that there is no difference in the ability to detect artificially created periapical lesions using conventional radiographic films and digital sensors (Kullendorff and Nilsson, 1996; Stavropoulos and Wenzel, 2007; Özen et al, 2009). In the abovementioned autopsy study using human cadavers, Kangasingam et al (2016a) also found no statistical difference between the accuracy of parallax digital and conventional (film) periapical radiographs, for assessing AP. However, single digital periapical radiographs were found to be more accurate than single conventional radiographs.
‘Enhancing’ radiographic images (e.g. colourising and inverting) with software also does not appear to improve the detection of periapical lesions (Barbat and Messer, 1998).
Detection of apical periodontitis
Numerous ex vivo studies with reference standards in both animal (Stavropoulos and Wenzel, 2007) and human (Özen et al, 2009; Patel et al, 2009; Tsai et al, 2013) models have conclusively demonstrated that CBCT is a significantly more accurate imaging system than periapical radiography for detecting the presence (sensitivity) of artificially created bone lesions. All ex vivo studies have the disadvantage of not truly mimicking the ‘real-life’ clinical situation. However, the advantage of such studies is that standardised periapical defects have been intentionally created, thus giving a reference standard that allows imaging techniques to be assessed with more confidence (Fig 7-3).
In a well-designed animal study using histology as a reference standard, Paula-Silva et al (2009a) reaffirmed that CBCT was a more accurate diagnostic tool than conventional radiography for diagnosing chronic AP. In this study, 83 block dissections of periapical tissues and root tips were histologically assessed in teeth with and without radiographic signs of AP. The specificity and positive predictive value was 1 for radiographs and CBCT, i.e. both imaging systems were accurate in determining when no disease was present. However, the sensitivity for CBCT (<0.91) was much higher than for radiographs (0.77) for detecting existing disease.
Using similar methodology to Brynolf (1967), Kanagasingam et al (2016b) assessed the accuracy of single, parallax digital radiographs and CBCT for diagnosing AP in fresh human cadavers using histology as the reference standard. In total, 86 teeth were assessed (Fig 7-2). The specificity of all the imaging systems was excellent, i.e. all imaging techniques could correctly detect healthy periapical tissues. However, the sensitivity of digital radiographs varied from 0.27 to 0.38, depending on whether a single view or parallax views, respectively, were assessed. This compared to a sensitivity of 0.89 for CBCT. The overall accuracy of the digital radiographic techniques was 0.5 for a single view and 0.58 for parallax views, and there was a significantly higher accuracy (0.92) for CBCT.
Several clinical studies have concluded that the diagnostic accuracy of CBCT in the detection of AP is superior to that of periapical radiography (Low et al, 2008; Patel et al, 2012a). Lofthag-Hansen et al (2007) examined and compared the periapical status of teeth with suspected endodontic disease using CBCT and periapical radiography. The authors reported that 38% more periapical lesions were detected with CBCT. Subsequent studies have reported similar findings (Bornstein et al, 2011; Abella et al, 2012). Patel et al (2012a) compared the prevalence of AP lesions associated with the roots of teeth with primary endodontic disease. CBCT was able to identify lesions of AP in 28% more teeth than periapical radiographs (Fig 7-4). Similar findings have been reported for endodontically treated teeth (Davies et al, 2015a).
Assessment of the outcome of endodontic treatment
It follows that the superior diagnostic accuracy of CBCT over conventional radiography in the detection of AP permits a more accurate and objective assessment of the outcome of endodontic treatment. Liang et al (2011) compared the outcome of endodontic treatment using periapical radiographs and CBCT 2 years after treatment. They found that a favourable outcome was reached in 87% of cases assessed with periapical radiographs, compared to only 74% of cases assessed with CBCT; the 13% difference being attributed to the superior sensitivity of CBCT in detecting AP. Patel et al (2012b) compared the outcome of primary endodontic treatment carried out on 132 teeth 1 year after treatment. The ‘healed’ rate (absence of radiolucency at review) of the treated teeth was 87% and 62.5% when assessed using periapical radiographs and CBCT, respectively. When more relaxed criteria (i.e. healing and healed) were used to assess outcome, the percentage of teeth demonstrating a reduction in the size of the associated apical radiolucency was 95.1% and 84.7% for conventional radiography and CBCT, respectively. Paula-Silva et al (2009b) assessed the outcome of root canal treatment in dogs and found a 44% lower success rate when the periapical tissues were assessed with CBCT (35%), compared with periapical radiographs (79%).Davies et al (2015b) reviewed the outcome of secondary (re-treatment) carried out in 98 teeth 1 year after re-treatment with conventional radiographs and CBCT. They found that there was a significantly different success rate between radiographs (93%) and CBCT (77%).
Radiographic appearance of apical periodontitis
Conventional radiography
While long-standing lesions of AP with significant bone destruction are generally readily discernible on conventional radiographs, incipient lesions of AP are often much more difficult to identify. Alterations to structures of the apical periodontium, such as the medullary bone trabeculae, the periodontal ligament (PDL) space, and the lamina dura may be early indicators of AP (Gröndahl and Huumonen, 2004). As such, an appreciation of the normal radiographic appearance of these structures is essential.
The conventional radiographic appearance of cancellous bone surrounding a healthy tooth varies between the maxilla and mandible. Typically, maxillary alveolar bone trabeculae have a fine, granular appearance, while mandibular alveolar bone trabeculae have a coarser, horizontally striated appearance and are interspersed with comparatively wider marrow spaces. Subtle structural changes in the cancellous bone are generally the earliest signs of AP recognisable on conventional radiographs. These changes include a disruption and disorganisation of the normal trabecular pattern around the apex (or other portal of exit) of the affected tooth. The disorganisation of the affected area may be well defined and easily differentiated from the surrounding bone or, alternatively, the margins of the disorganised area may blend with the surrounding bone such that it is difficult to delineate it from the surrounding healthy tissue, thus making interpretation more challenging (Fig 7-5a and b; Fig 7-6a and b).
Widening of the PDL space associated with the affected tooth may be an early indication of AP. However, endodontic infection is not the sole cause of a widened PDL space, which may also be a feature of tooth mobility, marginal periodontitis, or even neurogenic inflammation (Pope et al, 2014). Furthermore, the specific exposure angle of the radiograph may lead to the appearance of a widened PDL space (Bender et al, 1961a). This should be borne in mind when assessing a tooth for the early signs of AP (Fig 7-7a and b). A widened PDL space specifically associated with AP will be localised to the apex of the tooth (or the affected portal of exit) and the immediately adjacent areas. The PDL space coronal (and/or apical, if for example a lateral canal is involved) to this area will be unaffected, and there will be a marked transition between the affected and unaffected sites.
A further, relatively early radiographic sign of AP is disruption of the lamina dura, the integrity of which may be breached and which may appear to lose density. Any changes will be localised to the affected portal of exit from which the microbes are egressing. However, in isolation, a break in the continuity of the lamina dura should be viewed with caution. Tiny perforations of the lamina dura, although not always seen radiographically, are necessarily present to accommodate vascular and neural supply from the adjacent medullary bone to the teeth. These perforations may manifest on some radiographs and not on others. Furthermore, a certain amount of variation in normal lamina dura radiodensity and thickness can be expected between individuals. These features may also be altered by the angle of the radiographic exposure. With the development of AP, the mineral content of the cancellous bone trabeculae becomes depleted, and the trabeculae become thinner and less dense, with a resultant increase in the size of the adjacent marrow spaces. The affected area sometimes develops what has been described as a ‘shotgun’ appearance. This represents an intermediate stage in the progression of the lesion, from the subtle structural changes of the bone described above to the development of a clear periapical radiolucency. However, it is not always identifiable.
When the bone demineralisation has reached a critical threshold, a radiolucency will develop. Diagnosis of AP at this stage is less complicated. Nevertheless, depending on the exposure angle of any given radiograph and the site of the subject tooth, adjacent anatomical features may impair interpretation of the radiograph, such that the radiolucency may not be readily identified, or conversely, may look larger. The margins of periapical radiolucencies may be well or poorly defined, and in some instances they will have a corticated appearance.
Another potential precursor of AP is ‘condensing osteitis’, which is a reactionary production of dense bone in the periapical area of the affected tooth in response to low-grade pulpal irritation. Condensing osteitis manifests radiographically as a localised radiopacity in the area around the apex (or other portal of exit) of the affected root. The nature of the areas of condensing osteitis is variable, and the border of the lesion may be well or poorly defined (Fig 7-8a). In some instances, this reactionary bone can completely obscure the anatomy of the affected root on conventional radiographs
Historically, it was thought that the histological nature of AP lesions could be determined by radiographic features of the associated periapical radiolucency, such as size and the presence or absence of a radiopaque, corticated rim (Bhaskar, 1966). These associations have since been disproved (Nair, 1998; Nair et al, 1999), and it is believed that the radiographic features of periapical radiolucencies, such as size and the presence of a corticated lamina dura (Ricucci et al, 2006), are in fact poor predictors of the true histological nature of AP lesions.
Due to the difficulties inherent in detecting AP on conventional radiographs, especially at an early stage, a scoring system for the registration of AP, called the ‘Periapical Index’ (PAI), was developed (Ørstavik et al, 1996). It is a five-point grading system that classifies AP according to some conventional radiographic features (previously described) associated with the development of the disease.
Cone beam computed tomography
The true extent of bone destruction associated with AP is underestimated on conventional radiographs (Paula-Silva et al, 2009c; Abella et al, 2014). Therefore, the absence of (conventional) radiographic signs of AP does not rule out its presence, while the presence of radiographic features of AP are inevitably associated with a true diagnosis of AP (Brynolf et al, 1970a, 1970b; Kanagasingam et al, 2016b). AP presents on a CBCT scan, even in its earliest stages, as a 2-mm, well-defined widening of PDL space, or as a clearly defined periapical radiolucency (Tsai et al, 2013; Abella et al, 2014) (Fig 7-7). These findings may have an impact on the treatment plan of a tooth provisionally diagnosed with reversible pulpitis. An AP lesion may be clearly detected with CBCT, even though conventional radiographs are within normal limits (Hashem et al, 2015), therefore root canal treatment rather than pulp preservation treatment (e.g. pulp capping) would be indicated. With the elimination of anatomical noise comes a more objective appreciation of the presence and extent of bone loss associated with the disease, when compared to conventional radiography (Figs 7-5 and 7-6). Condensing osteitis can also become more evident (Fig 7-8).
As such, AP can be reliably diagnosed, and the nature (i.e. dimensions and extent) of the lesion accurately determined using CBCT. Assessing the true nature of AP and its proximity to important anatomical structures can be especially relevant when considering periapical microsurgery (Bornstein et al, 2011; Patel et al, 2015). Any expansion and perforations of cortical bone can also be accurately identified and related to clinical findings.
The CBCT-PAI has been suggested for use with CBCT (Estrela et al, 2008). This index uses a six-point scale to quantify the maximum diameter of bone loss associated with AP. The scores range from 0 ‘intact periapical bone structures’ to 5 ‘diameter of periapical radiolucency >8 mm’. The variables +E (expansion of periapical cortical bone) and +D (destruction of periapical cortical bone) may be added to any score if they are detected on the CBCT analysis. Recent reports have suggested that this is a reproducible method of assessing the extent of AP (Esposito et al, 2011).
The improved accuracy of the diagnosis of AP associated with CBCT has potential implications for the assessment of the outcome of endodontic treatment (Patel et al, 2012b; Fernández et al, 2013). Lesions will be identifiable on CBCT for longer periods post- treatment (when compared with conventional radiographs) due to the superior sensitivity of the system (Fig 7-4). It is therefore likely that the way the outcome of endodontic treatment (including prognostic factors) is determined will need to be reassessed in the future (Patel et al, 2011).
Dose reduction protocols should always be considered whenever possible to reduce the effective dose to which a patient is exposed. Recently, Al-Nuaimi et al (2016) demonstrated a good diagnostic accuracy for detecting simulated AP lesions with CBCT when the exposure settings were adjusted away from the manufacturer’s default settings. In this study, there was no significant reduction in diagnostic yield when the radiation was reduced by up to 74%.
Conclusion
The current evidence suggests that CBCT is more sensitive and accurate compared with periapical radiography in detecting AP. CBCT must not be used as a default imaging system to detect AP (Patel et al, 2015). Instead, it should be used in those instances where clinical and conventional radiographic assessment is equivocal for the diagnosis of odontogenic and/or non-odontogenic disease.
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