The main objective of this study was to compare panoramic and peri-apical radiography with CBCT for detecting FI. Since panoramic and peri-apical radiographs are still taken regularly in daily practice, data on their accuracy is of pivotal relevance(26). Proper assessment of FI with a high degree of accuracy is also important for adequate treatment management of periodontitis patients(11).
FI is considered a locus minoris resistentiae in periodontal disease progression with high prevalence in periodontitis patients. Svärdström and Wennstrom(27) studied the distribution of furcation lesions and found that these lesions were most prevalent in the maxilla and more specific at distal sites of upper first molars. Furcation lesions could be found in more than 50% of chronic periodontitis patients older than 30 years. Every second molar was involved in patients older than 40 years.
Measuring clinical attachment level by horizontal probing is a standard tool in clinical periodontology. However, access to furcations is not always easy and furcations may be difficult to predict in architecture, number of walls and extent. An ideal method to identify FI is by reflecting a mucoperiosteal flap. Evidently, this is impossible for ethical reasons when there is no pathology or clear clinical indication for surgery. Intra-surgical registrations have been often used as gold standard in the diagnosis of periodontal defects with an indication for surgery(23)(26)(28). CBCT was used as gold standard in the present study since these were available in the context of planning implant surgery. CBCT has been shown to be a good gold standard in multiple studies describing high levels of agreement between pre-CBCT FIs and intra-surgical findings(29)(30). In addition, CBCT enables to visualize root characteristics such as fusions and proximities.
In the present study, only slight agreement was found between panoramic radiography and CBCT and between peri-apical radiography and CBCT. On the other hand, all diagnostic parameters need to be evaluated in detail before clinical recommendations can be made. Diagnostic parameters such as SENS, SPEC, PPV and NPV can only be calculated for dichotomous variables. For this purpose, FI Grades were recategorized into ‘no to limited FI’ (FI Grade 0 and I) and ‘advanced FI’ (FI Grade II and III). This recategorization makes sense from a clinical point of view since FI Grade 0 and I require no or limited non-surgical therapy, whereas FI Grade II and III need advanced surgical intervention. Panoramic and peri-apical radiography showed low SENS (0.550 and 0.441, respectively), yet high SPEC (0.791 and 0.790, respectively) for identifying advanced FI. These findings imply relatively high false negative ratings and low false positive ratings. In other words, advanced FI is frequently overlooked, but when it is identified on het basis of panoramic radiography or peri-apical radiography it is most likely present. This can be explained by the fact that panoramic and peri-apical radiographs are two-dimensional images of a three-dimensional anatomy. A certain overlap is to be expected. Even advanced lesions might be masked due to superimposition of bone, roots and restoration materials. On the other hand, peri-apical radiographs score superiorly for image quality (brightness, contrast) and bone details (quality of bone, contour of lamina dura)(31)(32). Panoramic radiographs provide a good overview, yet image distortion is an important limitation (6)(33). Interestingly however, is that panoramic radiography was not inferior to peri-apical radiography for detecting FI in this study. This may be explained by the fact that the quality of digital panoramic radiographs has been drastically enhanced in recent years. ROC-curves showed similar results with an even slightly higher area under the curve for panoramic radiography when compared to peri-apical radiography (0.79 versus 0.69).
The results on PPV and NPV are more difficult to interpret than SENS and SPEC since both are affected by the prevalence of advanced FI within the study sample. In this study, 16/60 furcations were Grade II or III. Only when this proportion resembles the proportion of advanced FI in the population, the data on PPV and NPV are valid. Clearly, the present study was based on a convenience sample of patients seeking implant therapy, which may not properly represent the population. In addition, substantial regional differences may exist in the prevalence of advanced FI among periodontitis patients.
In a retrospective study of Darby et al.(11), the diagnostic accuracy of furcation probing for detecting FI was investigated using CBCT as gold standard. Only 22% of the furcation sites were clinically accurate in grading compared with CBCT and 58% were overestimated. These findings indicate high SENS and low SPEC of furcation probing. Accuracy of furcation probing is dependent on factors like inclination and angulation of the probe, variability in operator’s technique/ inherent probing error, amount of force used when probing, tooth position, presence of adjacent teeth, restricted visualization of the probe due to limited mouth opening and difficult access to the entrance of the furcation. Also, the clinician can rather score the furcation concavity than the furcation itself. Deep root concavities may be confused with FI. All these factors may explain the high overestimation of clinical FI measurements. On the basis of the results of Darby et al.(11) and the results of the present study, it is clear that neither furcation probing, nor panoramic/peri-apical radiography are excellent examination methods on their own for the detection of FI. However combined, more furcations may be accurately assessed given the fact that furcation probing demonstrates high SENS whereas panoramic/peri-apical radiography show high SPEC. This is in line with a study of Gusmao et al.(34) and Greatz et al.(35) indicating that both furcation probing and radiographical assessment should be used in situations of suspected FI.
Especially when it comes to surgical decision making, it seems attractive to take a three-dimensional radiograph(8). Indeed, when teeth require complex periodontal therapy and also restorative treatment is necessary, CBCT might be a good additional tool for an accurate assessment and prognosis of multi-rooted teeth. Inaccurate diagnosis can lead to irreversible treatment planning decisions (11). Still, the diagnostic benefits of CBCT must be carefully balanced against a higher radiation dose(35). The radiation dose of a low-dose small-field CBCT is slightly higher when compared to a digital panoramic radiograph, yet substantially higher when compared to a peri-apical radiograph(11). When translated to clinical practice, a low-dose small-field CBCT might be considered when the architecture of the defect has a clear impact on the treatment strategy(36). This may apply to mandibular furcations with FI Grade II since these qualify for regenerative periodontal surgery.
An interesting finding was that diagnostic parameters for panoramic and peri-apical radiography were not affected by the experience of the clinician. Hence, gaining clinical experience does not seem to improve the accuracy of a radiological diagnosis of furcation sites.
When interpreting the results of the present study, a number of limitations should be taken in to account. First, this is a retrospective study based on a convenience sample. Second, we used the most common classification on FI because of its simplicity. On the other hand, FI also has a vertical component(37), which was not considered here. By classifying the vertical component, it could clinically influence the treatment strategy. Essentially, treatment of FI may involve periodontal regeneration, root resection, amputation, tunneling techniques and tooth extraction(38)(39)(40)(41)(42). Prognostically, the vertical component increases the risk of tooth loss significantly(43) and needs to be regarded as a risk factor in personalized maintenance programs(44). Third, CBCT was used as gold standard because of ethical restrictions. Assessing FI by means of intra-surgical examination remains the most accurate method(11).