The main finding of this study was the high agreement between the OF classification based on MRI compared to MRI + CT, particularly under consideration of the treatment recommendation using the OF score. Thus, the classification based on MRI and radiographs only seems to be sufficient for the purpose of generating a correct treatment recommendation. Notwithstanding, the inter-rater RR was higher using MRI + CT compared to MRI alone.
These results appear to be controversial and need to be discussed in more detail.
Fractures of types OF 1 and OF 5 were excluded for this analysis, as OF 1 fractures are only visible in the MRI by definition  and as the concomitant tension band injuries OF 5 fractures are ideally visualized by MRI . Thus, no beneficial effect of the CT for the identification of both OF 1 and OF 5 type fractures can be expected. CT might be particularly valuable to differentiate between OF type 2, 3, and 4 fractures [11, 12]. Interestingly, the agreement was still high and particularly for diagnosis of type 4 fractures with a substantial or almost perfect agreement for both MRI and MRI + CT, respectively. In contrast, the inter-rater RR of OF 2 fractures was only fair for both, MRI and MRI + CT. Notwithstanding, the agreement of the classification using MRI + CT was superior for all subtypes. These differences appear to be particularly relevant in the following two situations: First of all, fractures with only slight traumatic defect of the posterior wall in which a differentiation between a type 2 or type 3 fracture is difficult. Secondly, fractures with potential mild affection of the second endplate in which a differentiation between type 3 or type 4 fracture is hard. This can be particularly difficult in cases with central defects and a coronal split component without posterior wall affection. In these, the differentiation between type 2 und type 4 fractures can be difficult. This was visible in 3 of the patients in our collective. Based on the superior visualization of the CT to define fracture extent a superior inter-rater agreement was visible in these cases. However, these differences seem not to play a major role in the general treatment recommendation based of the OF score. On the one side, OF 2 or OF 3 fractures with little comminution are typically a domain of non-operative treatment without associated high instabilities and generally rather mild clinical courses. In contrast, severe OF 3 fractures with substantial lesions of the posterior cortex as well as type 4 fractures are commonly candidates for an operative treatment based on extended rates of fracture progress and the associated longer pain duration and limited mobilization. In contrast, fractures without severe posterior wall affection (type OF 2 or 3), but central vertebral body defects and potential affection of the second endplate (OF 4) might affect the treatment recommendation.
Thus, conventional radiographs with MRI alone seem to be sufficient for generating a treatment recommendation. However, an additional CT is useful in fractures in which the differentiation between OF 2 or 3 as well as between OF 3 and 4 cannot be done accurately particularly in those patients in which this difference would lead to a change in the treatment recommendation based on the OF score. Patients with OVBFs of type OF 2 or OF 3 and only minor pain and minor limitations as well as those with OVBFs of type OF 3 and OF 4 with major pain and limitations despite accurate treatment do not benefit from and additional CT examination with respect of the decision making. Additionally, CT evaluation is useful in those patients in which a differentiation between OF 2 and OF 4 fractures cannot be sufficiently performed. Hereby, an effect for the treatement recommendation based on the OF score is highly likely. Additionally, a CT is important if surgical treatment is indicated and preoperative planning cannot be done sufficiently or the treatment strategy may be unclear based on the conventional radiographs and the MRI findings.
Generally, the inter-rater reliability for MRI + CT (0.63) was identical to the inter-rater reliability reported by Schnake et al. . It can be expected, that the inter-rater reliabilities might be even higher if type OF 1 and/or type OF 5 fractures were included based on the clear definition that can be identified by MRI. This needs to be proven by future studies.
Altogether, this study offers several limitations. First, the exclusion of OF 1 and 5 fractures is a limitation. However, the authors believe that this rather strengthens our conclusions based on the above mentioned reasons. Next, no second round of ratings of the same raters to evaluate the intra-rater reliability was performed. However, the almost identical inter-rater reliability reported by Schnake et al.  seems to confirm the external validity of this classification. Unfortunately, so far no global validation of the OF classification has been published.
In contrast, the rather high number of patients that were prospectively collected including the collections of all parameters that are necessary to calculate the OF score are the strengths of this study.