The results of this study show that the patient’s age at the time of lateral luxation injury should be taken into account when deciding on treatment and seems to question the present guidelines (Bourguignon et al. 2020). In fact, IADT writes in the guidelines that “every effort should be made to preserve the pulp, in both mature and immature teeth”, which may contradict the current recommendation of early endodontic treatment (Bourguignon et al. 2020). Based on the results from this study, it is worth considering whether early endodontic treatment of a lateral luxated mature tooth should be considered only when the patient is older than 25 years of age. The findings suggest that children and adolescents may have better healing potential than adults and that root canal treatment should only be performed if there are signs of PN for patients younger than 25 years of age.

According to a recent article by Abbott (Abbott 2023), ‘preventive’ endodontic management of teeth with lateral luxation injuries should only be provided if there is a concomitant fracture. Hence, endodontic management of a lateral luxation injury with no concomitant fracture should only be considered and commenced when there are definitive signs of pulp necrosis or/and infection of the root canal system (Abbott 2023). A clinical study on this topic would be interesting. Similarly, a study that investigated the healing potential of the pulp based on the patient’s age at the time of lateral luxation injury with concomitant fractures is needed.

Children and adolescents may have better healing potential than adults because the apical foramen is not completely constricted even though the root appears fully developed (with a closed apical foramen) on a periapical radiograph (Andreasen et al. 1986). A future study that compares the size of the apical foramen of lateral luxated teeth in two and three dimensions would be of great interest. A study such as this could establish whether there is any discrepancy regarding the size of the apical foramen between what we see in conventional periapical radiographs and, for example, a cone beam CT scan.

The risk of PCO seems very similar in the two cohorts. More teeth developed PCO in the age group younger than 15 years than in the 16–20 years age group. None of the teeth developed PCO in the 21–25-year-old age group. These findings were similar in both cohorts.

The results show that there is a possibility for developing PCO in patients younger than 20 years of age. Accordingly, there is potential for pulp healing after lateral luxation injury for mature teeth with closed apices in children and adolescents younger than 20 years of age.

The frequency of PN was greater in the cohort from 1972 to 1980. This can be explained by the fact that the observation period before initialling endodontic management of the tooth in the data from 2012 to 2020 was longer than that in the data from 1972 to 1980. For 12 of 49 patients from the 1972–1980 cohort, we currently recommend a longer observation period before endodontic treatment is initiated. In these patients, PN was diagnosed shortly after the injury (within 4 months) based on greyish discoloration of the crown and no response to pulp sensibility tests. Temporary loss of pulp sensibility is a well-known finding during posttraumatic pulp healing (Alghaithy & Qualtrough 2017; Andreasen & Kahler 2015b; Andreasen et al. 2019; Bastos et al. 2014). Regardless of this limitation, pulp sensibility testing should be performed at each appointment to determine if changes occur over time. A combination of cold and electric pulp sensibility tests is preferable due to the limitations of both techniques when used alone. However, this regime for pulp testing was not used in the cohort from 1972 to 1980.

In addition, the discoloration of the tooth can also be transient. Regarding colour changes, it is important to look at when changes in colour occur (Andreasen & Andreasen 1985; Andreasen et al. 2019; Bourguignon et al. 2020; Clark & Levin 2019).

Hence, an accurate diagnosis of the condition of the pulp after TDI is essential to make the correct treatment decision. A correct pulp diagnosis is only possible by combining and assimilating findings (clinical and radiological) and analysing the injury history and injury pattern over time. Regular follow-ups are, therefore, essential (Abbott 2023; Andreasen & Andreasen 1985; Andreasen et al. 2019; Bourguignon et al. 2020; Clark & Levin 2019; Krastl et al. 2022; Lee et al. 2012).

However, it is important to state that even though we would recommend a longer observation time in these 12 patients, the results/outcomes of PN could have been the same. Interestingly, in one patient in whom the dentist diagnosed PN 1.5 months after the injury based on the two clinical signs mentioned above, the pulp actually became vital again. The patient did not receive the recommended endodontic treatment. At the 1-year follow-up, the discoloration had disappeared, and the pulp sensibility test showed a positive response.

As mentioned earlier, the patients treated from 1972 to 1980 received both initial treatment and follow-up at the University Hospital by two specialised dentist in dental trauma. The patients treated from 2012 to 2020 received initial treatment at the Regional Emergency Clinic, Denmark. However, the dentists were trained by the specialists at University Hospital in management of traumatic dental injuries annually. The follow-up examination was performed at University Hospital by three specialised dentist in dental trauma.

The phenomenon of TAB can easily be misdiagnosed as external apical inflammatory resorption. Andreasen et al. first described the process of TAB in 1986 (Andreasen 1986). The exact mechanism of TAB is still not fully understood, but it seems to be linked to the repair process involving the ingrowth of neurovascular supply and elimination of necrotic and damaged pulp tissue after moderate traumatic dental injuries (such as lateral luxation and extrusion injuries) of mature teeth (Andreasen 1986; Andreasen et al. 2019; Cohenca et al. 2003).

TAB, which was misdiagnosed as an apical inflammatory infection, may also be part of the reason why the frequency of PN was higher in the cohort from 1972 to 1980.

In this study, only 3 teeth out of 93 had EIRR (2.79%). Hence, it is worth considering whether early endodontic treatment, which is the recommended guideline, is truly the right approach for all patients. EIRR can be arrested if it is diagnosed and treated in due time. There is no doubt that early detection and management of EIRR is crucial because tooth preservation is unpredictable if large parts of the root are affected/resported. Again, it must be emphasised how important these follow-up visits are (Abbott 2016, 2023; Andreasen et al. 2019; Heboyan et al. 2022). However, early initial EIRR can be difficult to detect, especially on 2D radiographs. Several studies have demonstrated that CBCT scans are more accurate than 2D radiographs at detecting the EIRR at the early stages. Nonetheless, CBCT scans should only be used for signs of EIRR and are not recommended for routine monitoring of teeth at risk of EIRR due to the increased dose of ionising radiation (Patel et al. 2015).

Endodontic treatment is not without consequences for a tooth. Posttreatment disease can occur, and the overall strength of the tooth can decrease, which may lead to a greater risk of subsequent fracture of the tooth (Andreasen et al. 2019; Lee et al. 2012; Ng et al. 2010; Petersson et al. 2016; Tang et al. 2010). The prognosis of a root canal treatment is measured in terms of periapical healing (posttraumatic disease) and tooth survival. This article focuses on tooth survival, which means that the tooth is still present in the mouth. A systematic review of tooth survival following non-surgical root canal treatment revealed that 86–93% of teeth survived more than 2–10 years (Ng et al. 2010). Only a few studies have examined tooth survival after a longer observation time. A study revealed that amongst 889 teeth in 889 patients, 46% of all treated teeth (with primary non-surgical root canal treatment) were still present in the mouth 25 years after trauma (Lee et al. 2012). Two Swedish studies with long observation periods reported that tooth survival of root canal treated teeth was 65% and 71.2% over 20 years (Eckerbom et al. 2007; Petersson et al. 2016). This finding is relevant to keep in mind, especially when treating children and adolescents, since it is expected that most young people will have their teeth for life. Therefore, the risk of EIRR must be weighed against the potential of pulp revascularization to avoid overtreatment (early endodontic treatment/preventive’ (Abbott 2023)) in patients younger than 25 years of age.

The long-term prognosis of traumatised teeth depends on emergency management and how quickly this treatment is provided (Andreasen & Kahler 2015a; Andreasen et al. 2019, 2002). In some parts of the world, patients may not seek treatment until days, weeks, or months after the injury. Lima et al. (2017) investigated the relationship between initial attendance after luxation injuries and the development of EIRR. The results showed that delayed treatment after a luxation injury significantly affects the prognosis and the risk of developing EIRR (Lima et al. 2017). Hence, Soares et al. (2015) reported a high prevalence of EIRR (80.2%) after lateral luxation. The patients in this study underwent initial examinations at 21–90 days, 3–12 months or > 12 months from the date of injury. It is considered very difficult to reposition the tooth correctly after 21 days since wound healing subsequent to the injury has begun, and the possibility of pulp healing must be reduced (Soares et al. 2015).

Likewise, it is important that all dentists maintain their knowledge about traumatic dental injuries so that the level of management is effective, efficient, and correct according to accepted guidelines (Andreasen & Andreasen 1985; Andreasen & Kahler 2015a; Bourguignon et al. 2020; Diangelis et al. 2012).

The recommended guidelines might make sense in areas where patients do not tend to seek immediate dental care after lateral luxation injuries because the risk of developing pulp necrosis and EIRR is significantly greater. However, in countries such as Denmark, where patients show high compliance, meaning that patients seek initial treatment quickly and show up to all the follow-up controls agreed upon, and where the dental health care system is well represented in all regions, we can perhaps be more restrained in our treatment approach. High compliance means that we can monitor the patient closely and, thus, quickly initiate treatment if necessary. In this study, the majority (69.9%) of the patients received initial treatment within 1–5 h after the injury. Only 4.3% of the patients received treatment ≥ 24 h after the injury (P value = 0.2956). In addition, all patients were seen at follow-up visits in accordance with applicable international guidelines.

Hence, parameters such as treatment delay, patient compliance, and patient age at the time of injury must be considered to make the best treatment decision for the individual patient.

The retrospective nature of the present study has several limitations. The data were collected during two periods of time (between 1972 and 1980 and between 2012 and 2020). Although the study material consisted of two cohorts, the management of mature teeth with lateral luxation injuries has not changed significantly over the years. Therefore, the inclusion of both cohorts seems to be justifiable. In addition, the data from each designated period were recorded prospectively according to predefined criteria for diagnosis and treatment. Furthermore, clinical and radiographic documentation was also collected. It was possible to verify the original diagnoses due to thorough registration along with clinical photographs and radiographs from the initial examinations. Another important detail that increases the quality of the material is that all radiographs were taken by individual filmholders. This ensured a uniform angle of the X-ray beam each time a radiograph was taken (Andreasen & Andreasen 1985; Andreasen & Kahler 2015a). Thus, the quality of the data can be considered satisfactory. The key differences and similarities between the two cohorts are listed in Table 4. Despite the quality of the data, a limitation is the limited number of patients included. Another limitation of this study is that we do not know the exact day that PCO or PN occurred because the patients were only checked at regular follow-up visits according to the accepted guidelines. Therefore, the onset date of PCO or PN was approximated by the middle of the calendar time interval, which ended at the diagnosis of the event and started at the date of the preceding control visit. To obtain a more accurate picture of the truth (a more exact approximation of when PCO or PN occurred), more frequent follow-up visits could have been carried out.