A Different Type of Talocalcaneal Coalition With Os Sustentaculum: The Continued Necessity of Revision of Classification

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1 Musculoskeletal Imaging Original Research Yun et al. Talocalcaneal Coalition With Os Sustentaculum Musculoskeletal Imaging Original Research Seong Jong Yun 1,2 Wook Jin 2 Gou Young Kim 3 Jae Hoon Lee 4 Kyung Nam Ryu 5 Ji Seon Park 5 So Young Park 2 Yun SJ, Jin W, Kim GY, et al. Keywords: ankle, CT, MRI, os sustentaculum, talocalcaneal coalition DOI: /AJR Received November 9, 2014; accepted after revision May 6, Based on a presentation at the Radiological Society of North America 2014 annual meeting, Chicago, IL. 1 Department of Medical Service, Republic of Korea Air Force, Gyeonggi-do, Korea. 2 Department of Radiology, Kyung Hee University Hospital at Gangdong, Kyung Hee University School of Medicine, 149 Sangil-dong, Gangdong-gu, Seoul , Korea. Address correspondence to W. Jin (jinooki@daum.net). 3 Department of Pathology, Kyung Hee University Hospital at Gangdong, Seoul, Korea. 4 Department of Orthopedic Surgery, Kyung Hee University Hospital at Gangdong, Seoul, Korea. 5 Department of Radiology, Kyung Hee University Medical Center, Seoul, Korea. WEB This is a web exclusive article. AJR 2015; 205:W612 W X/15/2056 W612 American Roentgen Ray Society A Different Type of Talocalcaneal Coalition With Os Sustentaculum: The Continued Necessity of Revision of Classification OBJECTIVE. The objective of our study was to retrospectively determine the prevalence and image findings of extraarticular talocalcaneal coalition with os sustentaculum, a type of talocalcaneal coalition that does not appear in current classification systems, in patients with an imaging diagnosis of foot coalition. MATERIALS AND METHODS. This study was performed using a database query of radiology reports of ankle and foot CT or MRI examinations performed from August 2001 to November Eighty-one patients were identified through a keyword search of the database for talocalcaneal coalition, tarsal coalition, coalition, or os sustentaculum. Imaging features of CT or MRI findings were evaluated. Chart review was used to identify demographic information. RESULTS. Extraarticular talocalcaneal coalition with os sustentaculum was diagnosed in 13 patients (nine men, four women), which represents a prevalence of 16.0% (13/81) in all foot coalitions and 24.1% (13/54) in all talocalcaneal coalitions. Four of 13 patients underwent surgical resection, and histology was obtained in three patients. Nine patients who had no history of trauma were symptomatic and all patients with bone marrow edema at the coalition sites on MRI (n = 5) were also symptomatic. Coexisting extraarticular talocalcaneal coalition with os sustentaculum and intraarticular talocalcaneal coalition were observed in 11 of 13 patients. CONCLUSION. The os sustentaculum is a component of extraarticular talocalcaneal coalitions and as such is usually related to the presence of symptoms. If a patient with an os sustentaculum has symptoms in the medial talocalcaneal joint area, an extraarticular talocalcaneal coalition related to the os sustentaculum should be considered. T he os sustentaculum was first described by Pfitzner [1] in 1896 as a small accessory bone, lodged at the medial and posterosuperior aspects of the sustentaculum tali and inferomedial aspect of the medial talar tubercle [2]. Of all the accessory ossicles reported in the foot, the os sustentaculum is relatively rare, with an estimated incidence of % [2]. Since the initial anatomic description, a few case reports have appeared dealing with findings on radiography, CT, and MRI [2 4]. As with many accessory bones, the os sustentaculum generally is believed to remain asymptomatic, although symptoms have occasionally been reported [3 5]. Talocalcaneal coalition, first described by Zuckerkandl [6] in 1877, is defined as a congenital union between the talus and calcaneus, usually associated with abnormal hypertrophy of the medial aspect of the talus and sustentaculum tali [7]. It is generally estimated to affect up to 48% of patients with coali- tions and traditionally presents in the second decade of life [7, 8]. Talocalcaneal coalitions may be osseous (synostosis), cartilaginous (synchondrosis), or fibrous (syndesmosis) and may restrict normal functions of the subtalar joint, including motion, rotation, and gliding [9 11]. CT and MRI are useful examinations in the diagnosis of talocalcaneal coalitions, and a number of useful imaging findings of this condition have been reported on various imaging modalities [11 18]. In contrast to many reports about radiologic findings, the radiologic classification of talocalcaneal coalitions has been described in only a few reports [19 22]. Commonly, a talocalcaneal coalition are classified as intraarticular (anterior facet, middle facet, posterior facet) or extraarticular (posteromedial) according to its location [19, 22] (Figs. 1A 1D). Rarely have anterolateral extraarticular talocalcaneal coalitions been reported [23]. Recently, another new radiologic classification system for talocalcaneal coalition was re- W612 AJR:205, December 2015

2 Talocalcaneal Coalition With Os Sustentaculum ported using multiplanar CT or MRI [20, 21]. We sometimes observed that extraarticular talocalcaneal coalitions occurred with the os sustentaculum in patients presenting with medial ankle pain. Consequently, we hypothesized that the os sustentaculum was a component of the talocalcaneal coalition (unlike the simple symptomatic os sustentaculum), that the os sustentaculum related to the talocalcaneal coalition could be symptomatic, and that os sustentaculum may occur in conjunction with talocalcaneal coalitions more often than previously thought. Distinction between the extraarticular talocalcaneal coalition with os sustentaculum and the simple os sustentaculum is important because surgical resection A C E may be required in a patient with a foot coalition [24]. However, to our knowledge, no imaging and histologic correlation of extraarticular talocalcaneal coalitions with os sustentaculum has been reported. In addition, this condition is not included in any classification system for talocalcaneal coalition. Therefore, in this study, we present a retrospective review of extraarticular talocalcaneal coalitions with os sustentaculum using various image modalities with or without histologic manifestations. D Fig. 1 Talocalcaneal coalition classification according to location. T = talus, C = calcaneus, ANT = anterior, POST = posterior, LAT = lateral, MED = medial, dotted line on sagittal view means coronal section level for coronal view. (Drawings by Jin W) A D, Schematic drawings show sagittal (left) and coronal (right) views of right ankle with anterior facet type (A), middle facet type (B), posterior facet type (C), and extraarticular type without os sustentaculum (D). Coalitions (arrows) are seen between talus and calcaneus. E, Schematic drawing shows sagittal (left) and coronal (right) views of right ankle having extraarticular talocalcaneal coalition with os sustentaculum. Coalitions (arrows) are seen between sustentaculum tali and os sustentaculum (asterisks) and between os sustentaculum and talus. Materials and Methods Study Population This retrospective two-center study was approved by each institutional review board, and written informed consent was waived for adult patients. One pediatric patient gave written informed consent. Patients were identified through a database query of radiology reports of ankle and foot CT or MRI examinations performed between August 2001 and November 2013 using any or all of the following keywords on the PACS (Piview Star, Infinitt Healthcare) of each institution: talocalcaneal coalition, tarsal coalition, coalition, or os sustenaculum. All examinations included the entire hindfoot. In radiology reports, the clinical history of or examination indication for talocalcaneal coalition, tarsal coalition, coalition, or os sustentaculum was not included. Between both institutions, foot coalition was diagnosed in 81 patients based on CT or MRI findings, and talocalcaneal coalition was diagnosed in B AJR:205, December 2015 W613

3 Yun et al. 57 of 81 (70.4%) patients. Table 1 describes the location of foot coalitions in the 81 patients. To obtain a clinical history for patients who had extraarticular talocalcaneal coalition with os sustentaculum, an independent radiologist who was not involved in the image review reviewed the electronic medical records, including outpatient charts, discharge summaries, or operative notes. Patient age, sex, symptoms, trauma history, surgical history, and histologic examinations were assessed. D Fig year-old man with extraarticular talocalcaneal coalition with os sustentaculum. A, Anteroposterior (left) and lateral (right) conventional radiographs of right ankle reveal accessory ossicle (arrows), which is located posterosuperomedial to sustentaculum tali. This ossicle is os sustentaculum. B, Coronal CT (left) and volume-rendered 3D images (right) show pseudoarticulations between sustentaculum tali and os sustentaculum (arrows) and between os sustentaculum and medial talar tubercle. These pseudoarticulations, which show marginal irregularity and reactive sclerosis, suggest degenerative change. C, Axial (left) and coronal (right) 3D fat-suppressed proton density weighted MR images show narrow and hyperintense interfaces (arrowheads) between sustentaculum tali and os sustentaculum (asterisks) and between os sustentaculum and medial talar tubercle. D, Photomicrograph (H and E, 1) shows fibrocartilaginous coalition (F) of os sustentaculum (O) with calcaneus (C) and talus (T). B A Image Evaluation All radiologic examinations of patients with reports indicating a foot coalition in each institution were reviewed based on the consensus of two musculoskeletal radiologists using a PACS (Piview Star). First, two observers classified all cases of foot coalitions by location, such as talocalcaneal, calcaneonavicular, and naviculo-medial cuneiform. Talocalcaneal coalition was then classified by location and presence of os sustentaculum: intraarticular (anterior facet type, middle facet type, posterior facet type), extraarticular without os sustentaculum, and extraarticular with os sustentaculum. In cases of extraarticular talocalcaneal coalition with os sustentaculum in which intraarticular talocalcaneal coalition was also seen, we considered the cases as extraarticular talocalcaneal coalition with os sustentaculum. In addition, the side of the coalition was noted (left, right, or bilateral). Conventional radiographs of the con- C TABLE 1: Foot Coalitions of 81 Patients Type of Coalition No. of Patients (%) Talocalcaneal 54 (66.7%) Anterior facet type 0 (0%) Middle facet type 8 (9.9%) Posterior facet type 28 (34.6%) Middle and posterior facet types 3 (3.7%) Extraarticular type without OS 2 (2.5%) Extraarticular type with OS 13 (16.0%) Naviculo-medial cuneiform 13 (16.0%) Naviculo-medial cuneiform and 3 (3.7%) intermediate cuneiform Calcaneonavicular 6 (7.4%) Lateral cuneiform-cuboid 1 (1.2%) Lateral cuneiform-3rd metatarsal 4 (5.0%) Total 81 (100%) Note OS = os sustentaculum. W614 AJR:205, December 2015

4 Talocalcaneal Coalition With Os Sustentaculum TABLE 2: Clinical Data, Image Findings, and Imaging Studies of Extraarticular Talocalcaneal Coalition (TCC) With Os Sustentaculum (OS) Case No. Age (y) Sex Side tralateral feet were evaluated if available. Patients having extraarticular talocalcaneal coalition with os sustentaculum were assessed with regard to the size of the os sustentaculum, bone marrow edema on MRI, and bony fusion on MRI or CT. Diagnosis of Extraarticular Talocalcaneal Coalition With Os Sustentaculum The diagnosis of extraarticular talocalcaneal coalition with os sustentaculum was established with criteria from previous studies regarding talocalcaneal coalition or os sustentaculum [11 17, 25]. These studies [11 17, 25] reported that a dysmorphic sustentaculum tali, a talar beak, a bony continuity across the subtalar facet, an irregular articular surface, subchondral sclerosis, subchondral cysts, beaklike spurs, and a narrowing of the talocalcaneal junction indicated talocalcaneal coalition. The os sustentaculum was defined as an accessory bone located at the posterior end of the sustentaculum tali on radiography and CT and an ossicle separated from the sustentaculum tali by a low signal intensity line on MRI [3]. Therefore, we defined extraarticular talocalcaneal coalition with os sustentaculum as a marginal irregularity, subchondral sclerosis, subchondral cyst, or a narrowing of the two junctions (between the os sustentaculum and sustentaculum tali and between the os sustentaculum and talus), including bony fusion (Fig. 1E). Combined Other Type TCCs OS Size b (cm) Pain Trauma Surgery Pathology Results Clinical Data Table 2 summarizes the clinical results of the 13 patients who had extraarticular talocalcaneal coalition with os sustentaculum. These patients represented a prevalence of 16.0% (13/81) in all foot coalitions and 24.1% (13/54) in all talocalcaneal coalitions. The mean patient age ± SD was 27.8 ± 12.5 years (age range, years), with 10 of 13 patients (76.9%) younger than 30 years old. Extraarticular talocalcaneal coalition with os sustentaculum was more frequently found in male patients than in female patients at a ratio of 9:4 (69.2% male). Nine of 13 patients (69.2%) had no history of trauma but had symptoms. Four patients (30.8%) with extraarticular talocalcaneal coalition with os sustentaculum underwent surgical resection of the coalition, resulting in reduced ankle pain. Imaging Performed Bony Fusion Bone Marrow Edema 1 44 Male Right Yes 1.6 Yes Yes No NA CT Incomplete NA 2 25 Male Left a No 1.6 Yes No Yes Yes CT No NA 3 22 Male Right Yes 1.7 Yes Yes Yes Yes CT and MRI No Yes 4 19 Male Bilateral a Yes 1.2 Yes Yes No NA CT No NA 5 21 Female Right a Yes 0.8 Yes No No NA MRI No Yes 6 18 Male Left Yes 1.2 Yes No Yes No CT No NA 7 58 Male Left Yes 1.5 Yes No No NA MRI No Yes 8 29 Female Left No 1.6 Yes No No NA CT No NA 9 40 Male Left a Yes 1.1 Yes No No NA CT No NA Female Left a Yes 0.7 Yes No No NA CT Incomplete NA Male Left Yes 0.5 Yes No No NA MRI No Yes Male Right a Yes 0.4 No Yes No NA MRI No No Female Right Yes 0.4 Yes No Yes Yes MRI No Yes Note NA = not applicable. a Contralateral feet evaluated on conventional radiographs. b Maximum diameter. Radiologic Analysis The imaging studies and features of the 13 patients are also summarized in Table 2. The mean size of os sustentaculum was 1.2 ± 0.49 cm (range, cm). Incomplete bony fusion between the os sustentaculum and sustentaculum tali was seen in two of 13 patients (15.4%); however, complete bony fusion was not seen. Of the six patients who underwent MRI, bone marrow edema on MRI was seen in five (83.3%), all of whom displayed symptoms. The remaining patient, who underwent MRI but did not have bone marrow edema, showed no symptoms. Other types of talocalcaneal coalition coexisted in 11 patients (84.6%) as follows: posterior facet type only (n = 8), both middle and posterior facet types (n = 2), and middle facet type only (n = 1). Discussion The os sustentaculum develops by extension of the fibrocartilaginous talocalcaneal bridge from the posterior aspect of the sustentaculum tali [2 5, 26]. It forms when the accessory ossification center ossifies [3]. Talocalcaneal coalition results from failed normal joint formation and is thought to be caused by the absence of mesenchymal differentiation and segmentation of the fibrocartilaginous talocalcaneal bridge. The talocalcaneal bridge, which is formed by the fibrous and cartilaginous portion during the embryonic period, eventually results in the absorption of either the fibrous or the cartilaginous portion, the absorption of both portions, or ossification. In normal fetal ankle development, these fibrous and cartilaginous portions are all absorbed by 8.5 weeks gestational age and form a noncartilaginous and nonfibrous interval between the sustentaculum tali and the calcaneus. The talocalcaneal coalition forms from the remainder of either the cartilaginous or the fibrous portion [22, 26 29]. Given this embryologic background, the mechanism of the formation of the extraarticular talocalcaneal coalition with the os sustentaculum can be derived from the formation of the accessory navicular bone. In the case of an accessory navicular bone, a fi- AJR:205, December 2015 W615

5 Yun et al. A C brocartilaginous precursor connects the two structures of the primary and accessory ossification centers [30]. The outcome of the ossification of the accessory center is the accessory navicular bone; the fibrocartilaginous precursor may be absorbed, left over, or ossified. These processes of fibrocartilaginous precursor absorption, nonabsorption, or ossification result in types 1, 2, or 3 of the accessory navicular bone, respectively [31]. Therefore, we speculated that the formation of extraarticular talocalcaneal coalition with os sustentaculum takes place in three steps: first, excessive extension of the fibrocartilaginous bridge to the medial margin of the extraarticular space past the medial margin of the junction between the medial talar process and the sustentaculum tali; second, ossification of the accessory center of the extended fibrocartilaginous bridge; and third, simultaneous overgrowth of the talus and medial margin of the sustentaculum tali as the os sustentaculum grows and the fibrocartilaginous bridges between the talus and os sustentaculum and between the os sustentaculum and the calcaneus fail to absorb. The assimilated os sustentaculum, a variant of the os sustentaculum, is the bony fusion of the sustentaculum tali and the os sustentaculum [5]. According to our hypothesis, the assimilated os sustentaculum may develop through ossification of the fibrocartilaginous bridge between the os sustentaculum and the sustentaculum tali. The remaining or ossified fibrocartilaginous bridge may cause symptoms due to shearing stress and osteoarthritic change, which several reports have termed symptomatic os sustentaculi [3 5]. In our study, all patients with bone marrow edema at the coalition site on MRI showed symptoms, whereas a patient without bone marrow edema at the coalition site was asymptomatic. Thus, we presume that bone marrow edema is related to the appearance of symptoms. Our conclusion is in accordance with previous studies [32, 33], which showed a relationship between bone marrow edema and pain in osteoarthritis or avascular necrosis. Mellado et al. [4] first reported that the os sustentaculum and talus may form an accessory joint. Their case is similar to those in our study. McNally [34] reported a case with similar CT findings, but that case was diagnosed by CT or MRI without histologic correlation. In our study, the two junctions (between the sustentaculum tali and the os sustentaculum and between the os sustentaculum and talus) presented as narrow and hyperintense interfaces on 3D fat-suppressed proton density weighted images and as irregular hypointense interfaces on T1- and T2-weighted images. Along with these presentations, the os sustentaculum, medial talar process, and sustentaculum tali also showed minimal cystic change and bone marrow edema. Therefore, we were able to diagnose interosseous cartilage and degeneration of the two junctions. The fibrocartilaginous bridges at the two junctions were also proven histo- Fig year-old man with extraarticular talocalcaneal coalition with os sustentaculum. A, Lateral conventional radiograph of left ankle shows os sustentaculum (arrows), which is located posterosuperior to sustentaculum tali. B, Sagittal (left) and coronal (right) CT scans show intercalated os sustentaculum (arrows) between medial talar process and sustentaculum tali. Marginal irregularity and subchondral sclerosis between medial talar process and os sustentaculum and between os sustentaculum and sustentaculum tali are seen. C, Photomicrograph (H and E, 1) shows os sustentaculum (O) has fibrocartilaginous coalitions (F) with calcaneus (C) and talus (T). B W616 AJR:205, December 2015

6 Talocalcaneal Coalition With Os Sustentaculum logically. Bony trabeculae lined with numerous osteoblasts and vascularized collagen, which would suggest healed fractures, were not seen. Using this radiologic and histologic evidence, we confirmed extraarticular talocalcaneal coalition with os sustentaculum (Fig. 2). In addition, a patient whose condition was diagnosed with CT but not MRI had similar histologic findings (Fig. 3). In addition to the radiologic and histologic differences, management is slightly different between simple symptomatic os sustentaculum and extraarticular talocalcaneal coalition with os sustentaculum, making distinction between the two conditions important. A simple accessory ossicle such as os sustentaculum can cause pain temporarily in response to overuse and irritation of the overlying soft tissues [35]. Correct diagnosis and proper treatment are crucial [36]. The treatment of extraarticular talocalcaneal coalition with os sustentaculum should begin with conservative measures, but surgical resection of the coalition may be required to restore subtalar kinematics, especially in pediatric patients [24]. Our study has several meaningful differences from previous studies. First, we obtained histologic findings related to the os sustentaculum, which had previously not been reported. Previous studies [3 5, 21] predicted the relationship between the os sustentaculum and surrounding bone with radiologic evidence only and assumed the talocalcaneal coalition was simply an accessory bone or an old fracture of the sustentaculum tali. However, if these coalitions had been simply accessory bones, they would not have showed fibrocartilaginous connections. Similarly, if they had been fractures, they would not have shown fibrocartilaginous connections and would have manifested histologic findings according to prior trauma, which they did not. Second, we present data on extraarticular talocalcaneal coalitions with os sustentaculum, a different type of talocalcaneal coalition. Instead of the os sustentaculum being a simple accessory ossicle, pathologic correlation indicated that the os sustentaculum is rather a component of coalition between the talus and calcaneus, one that was not authenticated before our study. According to our study, the extraarticular talocalcaneal coalition with os sustentaculum formed in 16.0% of all foot coalitions and in 24.1% of all talocalcaneal coalitions. This percentage was substantially higher than we had expected. In light of the supporting evidence presented above, we conclude that the extraarticular talocalcaneal coalition with os sustentaculum is a different type of talocalcaneal coalition. Current talocalcaneal coalition classification systems [19 22], which do not describe this type of coalition, should be revised to include it. Additionally, previous reports [37, 38] stated that the middle facet type was the most common subtype among talocalcaneal coalitions and that the posterior facet type was rare; however, in our study, the posterior facet type was the most common. In two previous studies [21, 39], 13 of 19 feet (68.4%) in 14 patients and 63 of 70 feet (90.0%) in 59 patients showed middle facet types, whereas 4 of 19 feet (21.1%) and 2 of 70 feet (2.9%) showed posterior facet types, respectively. In our study dealing of 81 patients, the percentage of posterior facet type (34.6%) was higher than the middle facet type (9.9%). For a more accurate percentage of each talocalcaneal coalition type, further study with a larger sample size is needed. There are several limitations to our study. First, because our study depended on a retrospective review through a keyword search, it represents an estimate of the prevalence of extraarticular talocalcaneal coalition with os sustentaculum in patients with an imaging diagnosis of foot coalition. In particular, because we dealt with coalitions detected on CT and MRI only and did not include those identified initially on conventional radiography, our results may not represent the exact prevalence of coalitions. Certainly, some cases of extraarticular talocalcaneal coalition with os sustentaculum may have been missed or misdiagnosed as old fractures or simple os sustentaculum if CT was not performed or if the os sustentaculum had a small area of osseous fusion. Also, five musculoskeletal radiologists with various experience levels interpreted ankle and foot radiographs in our two institutions, so some cases may have been missed. Second, only three cases were surgically resected and proven histopathologically. Therefore, the presence of the os sustentaculum may not necessarily indicate an extraarticular talocalcaneal coalition. Third, there is a possibility of selection bias because talocalcaneal coalition formed a large fraction of all tarsal coalitions in our study. Also, other tarsal coalitions such as calcaneonavicular coalition might have been underestimated, because we did not evaluate all radiographs. However, our study focused on the prevalence of extraarticular talocalcaneal coalition with os sustentaculum out of all talocalcaneal coalitions as opposed to the prevalence of each tarsal coalition out of all foot coalitions. Last, consensus interpretation was used in this study, but intra- and interobserver reliability was not assessed. In conclusion, the os sustentaculum is not simply an accessory ossicle or old fracture but is a component of a type of extraarticular talocalcaneal coalition. This condition is usually symptomatic. 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