In the present study we evaluated the development of tarsal bones in the long term after treatment of clubfeet with the Ponseti method. Unsurprisingly, we found that tarsal bones in treated clubfeet were deformed compared to normal feet. Nevertheless, several radiological parameters describing the shape and interaction of tarsal bones showed satisfactory treatment success. Additionally, it was found that radiological outcome does not necessarily depend on the severity of deformity prior treatment initiation. We are not aware of any other comparable studies on tarsal shape development in adolescence or adulthood.
Ponseti was the first to compare X-rays of both feet in two planes of 32 patients with unilateral clubfoot after completion of growth and found qualitatively very similar changes to the patient group presented here [4]. In the quantitative analysis, however, Ponseti and Co-authors considered different parameters than in the present study, e.g. no ankle a.-p. view was performed. Additionally, the treatment regimen varied compared to current standards, e.g. the retention phase was considerably shorter than the 4 years recommended nowadays.
The shape of tarsal bones in severe idiopathic clubfeet treated with the Ponseti method differs from that of unaffected feet of the same age (Table 1). In comparison to the literature on results based on previous primary-operative treatment concepts, however, these differences appear to be significantly smaller. In particular, no cases of “small dome talus” or “flat top talus” were observed, as was the case in other studies reporting up to 22 % of these changes after dorsomedial release [13]. In the present study, talar length was only minimally reduced to an average of 95 % compared to the control group. On the other hand, talar height was highly reduced to 84 % compared to normal feet. All morphological differences of clubfeet compared to normal feet appeared to be more pronounced in the more severely affected patients of the present study. However, significant differences could only be found for the FTI. This might also be explainable by the small number of patients. Summarizing the aforementioned morphological data, the talus in clubfeet after Ponseti treatment is slightly shortened with only a very slight increase in the trochlear radius. This coincides with magnetic resonance imaging findings in small children after Ponseti primary therapy [23]. The height of the trochlea tali appears to be significantly reduced compared to healthy feet. The shape of the subtalar joint and the configuration of the lateral talar process (e.g. dysplasia) could influence this parameter as already reported [4]. However, this was not examined in detail in this study. The reason for the comparatively favorable formation of the trochlea tali after Ponseti primary therapy could be caused by the amount of free dorsiflexion after tenotomy, which was achieved very early in the course of treatment (around the age of 6–13 weeks). In the previously common primary surgical treatment concepts, this was usually achieved not before 6 months in conjunction with the surgical intervention.
The differences regarding the “anterior talar motion segment” were clearly detectable, corresponding to the limitation in dorsiflexion often observed in clubfoot patients. Numerous factors are included in this parameter, e.g. talar height, radius of the trochlea tali, shape of the talus neck (“off-set”), but also the talus-floor angle and others. Some authors stated the significantly reduced shape of the talus-neck transition to be the main cause for persistent restriction in dorsiflexion of the ankle in toddlers treated with the Ponseti method [23].
A common finding after Ponseti treatment is an incomplete talonavicular repositioning and mobility in the transverse plane [4]. As a sort of compensation mechanism a wedge-shaped navicular bone with lateral flattening and lateral deviation in the naviculo-cuneiform joint-line can be observed (Fig. 3). This tarsal configuration was often found in our patients and consequently led to significantly altered talonavicular position angles and a FTI that deviated significantly from healthy feet.
The TMT index [24] was used in the present study to describe the foot position in general. It showed clear differences between clubfeet and healthy feet. This demonstrates the reduced flexibility in eversion of the rear- and midfoot in idiopathic clubfoot.
Several authors described rates up to 66 % of supramalleolar valgus deformities after primary surgical treatment of clubfeet [12, 18]. The cause of this growth disorder is not fully understood; it is observed frequently in over-correction conditions [25]. In the patient group examined here, valgus deformities occurred only in 4 cases (18 %), of which one patient represents a special case within the study group due to persistent equinus foot position after tenotomy of the Achilles tendon as a result of an accessory soleus muscle found after open dorsal release. The low rates of supramalleolar valgus deformity after Ponseti treatment in our study shows a clear advantage of this treatment in comparison to primary operative therapy concepts.
The main limitations of the present study are the small number of patients which is mainly caused by the single-center study design, and the lack of clinical assessment. Nevertheless, the sample size is in accordance with other studies on this topic [2, 4]. The present investigation meant to focus primarily on radiological parameters after Ponseti treatment. Even though it is well accepted, that reduction of the deformity correlates with clinical outcome after clubfoot treatment, we did not acquire clinical outcome data in our cohort. Nevertheless, this has to do with form and function. If correction of anatomy is incomplete, limitations in foot function are likely to persist. Another weakness of the study is the fact that in most cases the development of the foot was not yet completed at final follow-up. The patients’ sex with the known temporally different maturation of the feet was neither considered in the clubfoot group nor in the normal group. Patients were included in the control group due to foot problems others than clear rear- or midfoot disorders. These feet were considered “normal” even though some foot problems were present. However, radiographic imaging of children can only be performed with strict ethical considerations making recruitment of control groups for study purposes without any foot disorders very difficult or rather impossible.