Based on the shape, nature of the coalitions, and the articular facets that coalitions involved, we devised a new classification system through analyzing the large sample of cases and found the coalition with both talus and calcaneus overgrew, involving the posterior and middle subtalar joint facets, was most common.
Type II-MP was the most common type, comprising 50.9% of the coalitions, manifesting with both overgrown talus and calcaneus. We found only 8 coalitions (7.8%) of type I (two I-P types and 6 I-MP types). It was lower than 23 coalitions (33%) in Sanghyeok Lim et al’s study. Type I and II showed the coalition orientation (sloping up or down, or horizontal) which was helpful to find the fibrocartilage or fibrous line between the talar part and calcaneal part to guide to operative resection, particularly during arthroscopic resection. Seong Jong Yun and his colleagues reported that 15 of 54 (27.8%) feet showed talocalcaneal coalitions with an accessory ossicle. They regarded the accessory bone as os sustentaculum, forming when the accessory ossification center ossified, at the medial and posterosuperior aspects of the sustentaculum tali and they believed that the accessory bone may be a cause of bone marrow edema and pain in osteoarthritis. Sanghyeok Lim et al. regarded this accessory ossicle as a fracture fragment and they found a coalition with a “fracture fragment” in 17 of 70 feet (24%). We found 7 coalitions (6.5%) with an accessory ossicle (Type III) in this study. We also thought the coalition might be an accessory ossicle but not a fracture fragment, because the sclerosis of nonunion was not found in the ossicle by CT.
A complete osseous coalition may be difficult for resection for it’s hard to identify the borderline of the coalition, particularly in arthroscopic surgery. There were 18 feet (16.7%) identified as complete osseous coalitions (Type IV) in this study. This was in line with the study of Rozansky et al.. However, Wael Aldahshan et al. reported 8 complete bony coalitions (40%) while Amir Khoshbin et al. also found 5 complete osseous coalitions(38.3%). But in Sanghyeok Lim et al.’s study, there were only 2 complete synostosis coalitions (3%)18. The difference may lie in the different sample sizes.
The subtalar middle facet was most commonly involved while the posterior facet coalition was rare as reported in some studies[14, 16, 23]. Soon Hyuck Lee et al. reported recently that the prevalence of the talocalcaneal coalition in the middle and posterior subtalar facets was 27%, while 68% of coalitions involved the posterior facet only. Seong Jong Yun et al. reported that the prevalence of subtalar posterior facet coalition (34.6%) was higher than the middle facet coalition (9.9%) in 81 patients. Scranton, P. E. et al. reported 10 posterior coalitions (55.6%) in 18 feet. These studies indicated that coalition in the posterior facet was not as rare as long believed and took up a great part of the talocalcaneal coalition. In the current study, we found that all coalitions (100%) involved the posterior articular facet, while 73.2% of coalitions involved both the middle and posterior articular facets. The finding that the coalition involving the posterior facets was more than the coalition involving the middle facets, was consistent with the studies of Soon Hyuck Lee et al. and Seong Jong Yun et al.. However, the coalition only involved the middle facet was not found in our case series. The studies about the coalition involving the anterior facets were rare[16, 26], and we found only five coalitions (4.6%) that involved the anterior facets.
Rozansky et al. depicted the features of coalitions on the 3D construction image, however, they didn’t emphasize the open of the tarsal sinus. In the current study, the open of the tarsal sinus could be found in the subtype-P coalitions on a 3D construction image, while for subtype-MP and AMP, it could not be found. So, we can also distinguish the facets that the coalitions involve from a 3D construction image.
The first-line strategy for symptomatic talocalcaneal coalitions is conservative treatment[8, 11]. Coalition resection is recommended if non-operative treatment failed. Traditional open techniques may prolong hospitalization for wound management and pain control. An open technique[28, 29] is often performed with an incision over the sustentaculum tali, and then, identifying the bridge edge through the talonavicular joint anteriorly and the residual talocalcaneal joint. Finally, the coalition is resected until the articular cartilage is visible. Arthroscopy has gained popularity recently and several authors reported good results after endoscopic coalition resection[21, 30, 31].
For the subtype-P coalitions, the excision is enough until healthy cartilage of the posterior subtalar joint is visualized. While for subtype-MP coalitions, the excision should be extended anteriorly to the medial open of the tarsal sinus in an open technique. It is similar to arthroscopic surgery, in which the flexor hallucis longus (FHL) is an important landmark[32, 33]. The excision under the arthroscope should be extended medially, according to the non-osseous coalitions of types I-III. In type IV (osseous coalition), an important landmark that can help identify the location of the subtalar joint is the posterior talofibular ligament.