The juvenile flexible flatfoot deformity is pathomorphologically characterized by an increased eversion of the peritalar complex and a destabilization of the first ray . For quantification of the deformity various radiological parameters are described [4,5,6, 8, 20, 24]. Surprisingly, the available evidence of certain thresholds or values is sparse. In the present study, we were able to calculate threshold ranges of established radiographic parameters based on data from surgically treated symptomatic juvenile flatfeet and normal feet. Four out of nine parameters (talocalcaneal angle (dorsoplantar-view), talocalcaneal angle (lateral-view), calcaneus-fifth-metatarsal angle, calcaneal pitch angle) were not statistically different between the groups and their ability to distinct between normal foot and flatfoot (AUC values, Fig. 4) was shown to be low. Moreover, definition of reference values for these measurements is inaccurate. Additionally, we found the talometatarsal index as a very reliable and valid combination of two measurements with a very small threshold range between − 31 to − 34°.
Additionally, the given values often differ considerably and the studies are highly subjected to some sort of bias. For example, Vanderwilde et al. mainly reports on younger children and only included nine children ≥9 years. In addition, the dorsoplantar X-ray image which is exemplary shown in the original paper appears to be taken not with the foot under full weight bearing but rather “knee flexed” . Another study on this topic was published by Davids et al., whereby the unaffected side of patients with hemiparesis served as a control . The dorsoplantar pictures shown in the paper do not allow for visualization of the hindfoot to define the longitudinal axes of the calcaneus or talus, making these measurements probably unreliable. Additionally, the setting of normal values is reported insufficiently due to the fact that children included in the study were between 5 to 17 years (on average 10 years). In this period of life, the foot undergoes relevant physiologic changes of shape and a normal value calculated is possibly affected by some sort of systematic error. Nevertheless, for talo-first-metatarsal angle (lateral-view), talocalcaneal angle (lateral-view) and talonaviculare coverage there is high congruency with our values with only talo-first-metatarsal angle (dorsoplantar-view) showing relevant difference from our measurements.
Five out of nine parameters were able to significantly differentiate normal from flat feet, whereas, the following four parameters missed: talocalcaneal angle (lateral-view), talocalcaneal angle (dorsoplantar-view), calcaneal pitch angle and calcaneus-fifth-metatarsal-base angle. It must be concluded that in the current study these four parameters failed as valid measurements to diagnose juvenile flatfoot deformity. A radiological parameter classifying the severity of a flatfoot deformity is suitable to the extent that it can be measured reliably on the one hand (see Table 2) and has a good selectivity between normal and pathologic feet on the other hand. In this context, Table 3 shows the parameters’ selectivity of the tested angles and it can be concluded: The lower the threshold range, the more accurate a reference value can be defined. Then again, if a parameter has a large threshold range, a large overlap area of measurements indicating normal foot or flatfoot must be expected. We believe, a parameter with a threshold range > 10° must be interpreted with conscious. In this context, only the talometatarsal index and its sub-parameters talo-first-metatarsal-base angle and talo-first-metatarsal angle (lateral-view) had acceptable values with ranges ≤10°. The following reference values for normal feet of adolescents between 10 and 14 years could therefore be defined: Talometatarsal index >(−)31°, talo-first-metatarsal-base angle >(−)7.5°, talo-first-metatarsal angle (lateral-view) > (−)13.5°.
Analysis of individual parameters
The reliability testing for the talocalcaneal angle (dorsoplantar-view) was 0.77 and 0.65, and therefore significantly lower in comparison to all other parameters. The main difficulty in the measurement of this angle is the fact, that the shape of the talus and calcaneus at the level of their bodies is not always easily definable. The talocalcaneal angle (dorsoplantar-view) shows a comparatively low ability to distinct between the two groups (AUC = 0,660). This is higher for the talocalcaneal angle (lateral-view) (AUC = 0,757). In the end it must be noted, that talocalcaneal angles could hardly distinguish between the groups, and therefore it was not possible to set a reference value. The sum of both talocalcaneal angles led to a slightly improved performance to distinct between the groups (talocalcaneal index with AUC = 0,799), but nonetheless, the usefulness of the parameters is rather low. This also corresponds to the experience of other authors who used the same parameter with idiopathic clubfoot decades ago .
The Costa-Bartani angle shows a particularly high reliability. In contrast, the distinction between the groups was only moderate with only half of the feet being correctly allocated to either groups. This can be explained by the large threshold range of 17 °. Therefore, the definition of a reference value is not reasonable. The Costa-B angle correlates poorly with the angular dimensions that characterize the talonavicular alignment in the transverse plane (Pearson-correlation to talonaviculare coverage = − 0.30, to talo-first-metatarsal-base angle = − 0.34), but better with those parameters reflecting the alignment in the sagittal plane (Pearson-correlation to talo-first-metatarsal angle (lateral-view) = − 0.63). One major drawback of the parameter in our experience is the difficulty to clearly determine the distal point on the sesamoid, at least in childhood.
All talometatarsal angles showed good intra- and interobserver reliability, some of which is well above 0.9. Establishing the longitudinal axis of the talus in the sagittal plane with the help of two incircles has proven effective. In the transversal plane it is noticeable, that the talo-first-metatarsal angle (dorsoplantar-view) and the talo-first-metatarsal-base angle differ from one another in the two groups, but this is not statistically significant. It is therefore important whether one uses the talo-first-metatarsal-base angle (which measures the conditions at the talonavicular joint more specifically) or the talo-first-metatarsal angle (dorsoplantar-view) (which is more susceptible to concomitant forefoot deformities, e.g. Metatarsus primus varus, skew-foot) to calculate the talometatarsal index.
The talometatarsal index showed the highest discrimination rate (AUC = 0.998) of all parameters. It was also possible to define a reference value between the two groups on an overlap area of only 3 °. 20 of 22 ft of the normal group and 21 of 22 flatfeet could be assigned to the appropriate group by the talometatarsal index alone. Due to the small threshold range a definition of reference values was also possible for the talo-first-metatarsal-base angle and the talo-first-metatarsal angle (lateral-view).
In the flatfoot group, the percentual fraction of the talo-first-metatarsal-base angle as part of the talometatarsal index was 41% on average and that of the talo-first-metatarsal angle (lateral-view) 59%, respectively. It is noteworthy, however, that the proportion of the talo-first-metatarsal-base angle ranges from 22 to 61%. This shows, that there is a wide range of individual fluctuations with regard to the involvement of the sagittal plane and transverse plane in the flatfoot deformity. It is also a confirmation of the very different planar dominance of the talocalcaneo-navicular complex in every single flatfoot . This observation can also be used to conclude, that both planes must be taken into account (as ensured by the talometatarsal index) to evaluate the flatfoot deformity correctly and in its entire form.
Measuring talonaviculare coverage showed a very high reliability, although it is sometimes not easy to define the boundaries of the articular surface, especially at the level of the talar head. In addition, talonaviculare coverage showed approximately the same AUC value (0.943) as compared to the talo-first-metatarsal-base angle (0.947). However, a reference value was not easily definable with a threshold range of 12.5°.
Calcaneal pitch angle, calcaneus-fifth-metatarsal angle
In contrast to all other parameters, these two radiographically well reproducible angles do not allow any statement about the positional relationship between the talus and the subtalar footplate. For basic considerations, these parameters alone are less suitable to characterize the flatfoot deformity. Additionally, the threshold ranges are particularly high and AUC values are far under 0.9. Definition of reference values for these parameters was not helpful.
A limitation of the present study may be the fact, that in the control group all patients had some kind of foot disorders and the term “normal” must be interpreted with conscious. Nevertheless, none of the patients had pathologies influencing the foot arch or causing flatfoot-like deformities. Additionally, all patients in this group had clinically a normal foot shape in the area of interest, namely the tarsal region. We are well aware, that classifying a flatfoot is sometimes subjective, especially if the deformity is not too pronounced. This could influence the measurements conducted in the study group and making the borders between the groups blur. On the other hand, surgery was only performed on patients with relevant pathology (clinically and radiographically), and therefore it can be assumed, that our study group consisted only of clear diagnoses regarding a flatfoot deformity. Another weakness of the current study is the small sample size. Even though, the most relevant studies with similar questions do have very similar patient numbers, studies with higher numbers are needed to confirm our findings [6,7,8].