In this study, the overall prevalence of tarsal coalition was 6.0%. As we expected, this result was appreciably higher than the prevalence of 1–2% from previous studies which assessed tarsal coalition using simple radiography [12–14, 44]. However, the prevalence was lower than that of recent cadaver and MRI studies [35, 45, 46]. We believed two factors may have influenced these results. First, considering that the proportion of symptomatic patients among the 50 tarsal coalition patients was high at 70%, asymptomatic patients were not sufficiently included. Second, CT is more vulnerable to the misdiagnosis of non-osseous coalition than MRI. Solomon et al.  reported nine non-osseous coalitions were diagnosed among 100 dissected feet, and only 55% of them were correctly diagnosed by prior CT. Guignand et al.  found four coalitions (2 cartilaginous and 2 fibrous forms) among 11 CN coalitions diagnosed during surgery were not diagnosed by CT, though diagnosis was possible by MRI. Therefore, the actual prevalence is expected to be higher. We found the proportion of tarsal coalition patients was significantly higher at 11% in the patient group under 20 years of age. Since tarsal coalition mostly causes symptoms in adolescence and early adulthood [14, 49], a relatively large number of symptomatic young patients may have visited the hospital. Therefore, it is expected that the probability of receiving a diagnosis by conducting an examination was relatively higher than that of the older group.
In terms of anatomical types, TC coalition was the most common followed by CN coalition. These two types accounted for most of the total tarsal coalition, which concurs with previous studies [15, 17]. In many Caucasian studies, either CN coalition dominated or the prevalence of both types were similar [5, 15–18], whereas in the present study, TC coalition was more common, which is in line with the results of recent East Asian studies [30, 31, 33, 50]. These results indicate the anatomical distribution of tarsal coalition is racially dependent. In addition, differences between the clinical presentation of TC and CN coalitions may explain reported TC coalition predominance. According Solomon et al. [15, 47], CN coalition appears to be unrelated to arthritic change, which means that limitations of subtalar joint motion are less likely for CN coalition. Since motion restriction causes symptoms of tarsal coalition and secondary arthritis, TC coalition likely accounts for a higher proportion of symptomatic patients than CN coalition. In fact, in this study, the proportion of symptomatic patients was significantly higher for TC coalition. Due to these differences in clinical presentation, TC coalition patients are more likely to visit the hospital and be diagnosed with tarsal coalition. Furthermore, coalition type may have influenced the results of this CT study. According to Nalaboff et al. , TC coalition involves osseous coalition in 33.3% of patients, whereas most of the CN coalition is composed of non-osseous coalition (56% cartilaginous and 44% fibrous union). In the present study, CT was the only imaging modality used, and thus, non-osseous forms of coalition may have been under diagnosed.
In current study, the bilateral occurrence of tarsal coalition was 56.5% among the 46 patients, which is in line with previous studies [19, 51, 52]. Regarding anatomical types, TC and CN coalition had bilateral prevalence of 76.0% and 26.7%, respectively. However, Caucasian studies have reported the bilateral occurrence of CN coalition is higher or similar to that of TC coalition [5, 17, 20, 26]. In addition, single posterior facet lesions accounted for the largest proportions (43.2%) of TC coalition subtypes, which concurs with other East Asian studies [30, 31, 33], whereas in Caucasian studies middle facet lesions accounted for more lesions than posterior facet lesions [20, 26–29]. Racial differences may explain these differences, but additional larger-scale Asian studies are needed to clarify this topic.
The proportions of patients with coalition-related symptoms were compared in TC and CN coalitions. TC coalition (88.0%) accounted for a significantly higher proportion of symptomatic patients than CN coalition (46.7%), which may be the result of less subtalar motion limitation in CN coalition .
Summarizing the characteristics of tarsal coalition in East Asians, TC coalition is the most common anatomical type and has a very high bilateral occurrence. In addition, TC coalition most often invades the posterior facet, and accompanies coalition-related symptoms at a high rate. This suggests that there are relatively more symptomatic tarsal coalition patients than Caucasians in East Asians. Therefore, it is necessary to consider the possibility of tarsal coalition in not only patients with rigid flat foot deformity but also all East Asian patients who visit the hospital for foot and ankle symptoms. If tarsal coalition is suspected, it is better to perform CT or MRI together than simple radiograph alone. Also, since patients with coalition-related symptoms are more likely to have TC coalition, bilateral examination is very important for diagnosis.
Our study has several strengths. First, it included a large number (839 subjects) of variously aged patients, and second, bilateral CT was performed on all patients. However, there is a possibility that the diagnosis of non-osseous coalition was insufficient due to non-availability of MRI findings. In addition, we depended on the medial records of the hospital to confirm the clinical features at the time of the examination. Therefore, the possibility of insufficient evaluation on other pathological conditions that may cause symptoms cannot be excluded.