Discussion.
This presentation of six difficult disagreement and borderline cases from a radiographic study of endodontically treated teeth aims at adding to an open discussion of diagnostic procedures and problems in endodontology and the classification of periapical findings. A diagnostic procedure should ensure that the classification is valid. Thus, cases diagnosed and classified as ‘successes’ or ‘failures’ should, even when reexamined after an extended period of time, remain in the same categories. The knowledge base might originate from scientific studies supported by histology (Brynolf1967, Green et al.1997) or systematical follow-up as validation (Kaffe & Gratt 1988) or it might be that of experienced clinicians, for example, using the Delphi technique (Stheeman et al.1995).

Anatomical landmarks.
Even in the early literature on the radiological interpretation of the periapical structures relative to endodontic problems, emphasis was put upon alteration in the fine skeletal structures (Simpson1927, Coolidge1937).Eroded versus continuous lamina dura around the apex was considered essential for the diagnosis of initial periapical disease, together with widening of the periodontal space in the same area. These anatomical landmarks are still regarded as fundamental features when identifying early periapical changes. They are pointed out in textbooks both in radiology and endodontics (Gibilisco 1985, Brocklebank1997, Glickman1998, Orstavik1998), and the normal periapical situation forms a natural, basic part of diagnostics groupings (Reit & Hollender 1983, Halse & Molven 1986, Sjogren et al. 1990, Chugal et al. 2001). In the PAI (Orstavik et al. 1986, Trope et al. 1999) however, based on Brynolf’s (1967) extensive study on autopsy material, the score 1 refers to Brynolf’s marginal, nonendodontic cases, grades 1 and 2, and not to illustrations of normal, unaffected periapical situations (Brynolf1979).
The importance of changes in the continuity and shape of the lamina dura and the width and shape of the periodontal ligament space for the identification of non vital teeth has been stressed by Kaffe & Gratt (1988), who evaluated the influence of 18 radiographic features on the consistency and reliability of the radiographic diagnosis of the periapical area. Their advice is that the two features, the lamina dura and the periodontal ligament space, should be emphasized in order to decrease interobserver variations and add to correct radiographic diagnosis.

Periapical lesions.
Amore extensive inflammatory process in the periapical area will in turn create a radiolucency, as viewed from the images caused by destruction and removal of a certain amount of bony mineral. The classical studies of Bender & Seltzer (1961a, 1961b) showed that experimentally created lesions are visible in radiographs only if the junction of cortex and cancellous bone is eroded. As reviewed by Bianchi et al. (1991), several later investigators have come to the same conclusions in similar  experiments. On the other hand, Lee & Messer (1986) have emphasized the importance of alterations in structural details for a correct diagnosis. Periapical lesions confined to cancellous bone were detected in 80% of their cases. These authors point out that the anatomical features of the periapical area appear to promote the visualization of lesions that may not be detected in other locations. They also convincingly argue that the mandibular canalis seen, even though located with in cancellous bone.

Difficult borderline cases.
The fact that minor inflammatory processes present themselves through subtle structural alterations in the periapical area necessitates a thorough observer competency. In general, the transfer of knowledge, skill and experience may be ascertained by current discussion of cases or more systematic calibration. Although such methods are efficient, the exchange of competency is restricted to the same group of clinicians and the procedure and evaluation behind the findings and figures are not openly presented to others. Especially for more complicated cases, the presentation for and discussion by a broader group of specialists might be important. Such cases, like the ones selected from the 32 cases (12% of the total material) subjected to joint discussion, are often identified as difficult borderline cases and must be taken seriously in follow-up studies and epidemiological investigations. If not taken seriously, the validity of the findings might be questioned and their importance reduced.
An approach to solve the problem of a valid grouping of cases has been to accept as ‘abnormal’ only those where the observer feels ‘certain’ (Molander et al. 1998, Kvist & Reit 1999). This will not remove the problem and distinctions must be made between ‘certain’ cases and ‘uncertain’ or borderline cases, a well-known dilemma in endodontology where clinical decisions are frequently based solely on the evaluation of findings from radiographs.

Clinical implications.
The results from research and advice given by the researchers strongly influence treatment decisions made by clinicians. The importance therefore of guiding principles regarding periapical evaluations and findings formulated from a clinician’s perspective has recently been pointed out by Kvist (2001) in the summary of his studies of endodontic retreatment. The identification of periapical lesions forms the basis for the suggested two principles, which is not unexpected. Thus, both researchers and clinicians must decide upon disease/no disease for root-filled teeth and, in practical terms, also establish a success/failure grouping with diagnostic guidelines. As expected, the latter grouping is not only found within older studies (Grahne. n & Hansson 1961, Engstrom et al. 1964) but also presented in recent investigations based on currently used strategies - the three category classification (Tronstad et al. 2000, Chugal et al. 2001, Molven et al. 2002b ,) the PAI index (Sidaravicius et al. 1999, Kirkevang et al. 2001), and the strict definition of periapical disease (Kvist&Reit 1999).
It has been maintained that the PAI de-emphasizes the importance of the observers and may be - or at least has the potential of being - ‘objective’ (Orstavik et al. 1986, Trope et al. 1999, Kirkevang et al. 2001). In our opinion the radiographic evaluation of the apical area that comprises several structural components must always be subjective, and as such is a complex decision-making process not only influenced by scientific factors but is also present on sociological and psychological interacting levels (Reit 1987). One could speculate that computerized image analysis may have the capability of rendering more objective results (White 1996). However, comprehensive studies by Mol (1992) do not support a view that such methods are superior to conventional reading of films. At present this leaves us with the traditional approaches facing difficult borderline cases like the ones presented in this paper. As diagnosticians, we will profit from discussing our problems openly, accepting the limitations inherent in clinical decisions, and the comments and criticism from others.

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