INTRODUCTION

In general, the talocalcaneal joint, a major synovial subtalar joint, is functionally important for ankle movements such as in version and eversion of the foot. This joint id formed by articulating of superior facets of calcaneal bone to calcaneal articular facets of the talus. The incidence and variations of types of subtalar joint facets have been reported in many populations (^{Arora et al., 1979}; ^{Bilodi, 2006}; ^{Kaur et al., 2011}; ^{Garg et al., 2013}; ^{Jung et al., 2015}). In addition, parts of the tarsal bones are also interested to be good candidate for sex and age determinations in the forensic research fields (^{Bidmos & Asala, 2004}; ^{Tuller & Duric, 2006}; ^{Harris & Case, 2012}; ^{Mahakkanukrauh et al., 2014}). In the literature reviews, the calcaneal articular facets have been classified into five to six subtypes (Arora et al.; Bilodi & Agrawal, 2003; Bilodi; Kaur et al.; Jung et al.). It has been found that the types of subtalar joint facets are different in many races. In this issue, the morphological classification and morphometry of tali on the basis of calcaneal articular facets in Thais especially in Northeastern population have never been documented. Although we have previously reported the patterns of superior articulating facet of dried calcanei in our population already (^{Iamsaard et al., 2015}), their facets on tali need to be further observed. Therefore, we attempted to investigate both types and possible lengths of individual facets on dried tali from our identified bone collection.

MATERIAL AND METHOD

The identified 372 dried tali (204 males, 168 females) from Khon Kaen University Bone Collection were observed for variant types and measured for their facet lengths. The age ranges of skeletal samples were approximately 20-80 year olds. The osteoporosis, osteophytes, or fractures of tali was excluded. The complete bone facets were recorded as classified for the types and incidence as shown in Figure 1.

The morphometric study: the digital caliper vernier gauge micrometer was used to measure the individual length of facets as followings (also shown in Fig. 1):

AP is the maximal length from anterior (A) to posterior talar process (P).

ML1 (superior facet aspect) is the length from medial (M) to lateral talar process (L1).

ML2 (inferior facet aspect) is the length from medial (M) to lateral talar process (L2) MP is the length of groove between middle (M) and posterior facets (P).

The measurements were performed in triplicates by internal and external examiners. This study was approved for human ethic (no. HE591164) from the office of the Khon Kaen University ethics committee in human research.

Fig 1 Showing types of calcaneal articular facets of talus and morphometric measurements on superior and inferior talar facets. Type I, tree facet are separated; Type II (A), the anterior and middle facets are partially connected with predominant ridge; Type II (B), the anterior and middle facets are partially connected with slight ridge; type III, the anterior and middle facets are fully fused to form a single facet; type IV, the anterior and middle facets are partially separated by a ridge and partly by a groove; type V, all facets are continuous fused to form a single facet. A, M, and P; anterior, middle, and posterior facets, respectively. AP; maximal length from anterior to posterior talar process, ML1; length from medial to lateral talar process (superior facet view), ML2; length from medial to lateral talar process (inferior facet view), MP; length of groove between middle and posterior facets. 

RESULTS

The calcaneal articulating facets observed in Thai population were classified into 6 types as shown in Figure 1: Type I, tree facet are separated ; Type II (A), the anterior and middle facets are partially connected with predominant ridge; Type II (B), the anterior and middle facets are partially connected with slight ridge; Type III, the anterior and middle facets are fully fused to form a single facet; Type IV, the anterior and middle facets are partially separated by a ridge and partly by a groove; Type V, all facets are continuous fused to form a single facet.

In Table I, the results showed that the incidence of type IIA was highest (34.68 %), followed by type II B (32.53 %), type IV (27.42 %), type III (2.96 %), type I (1.88 %), and type V (0.54 %), respectively. However, it was found that highest incidence in male was type II (B), approximately 32.84 % whereas in female was type II (B), approximately 42.89 % (Table I).

Table I Incidence of calcaneal articular facets on Thai- dried tali. 

^{Age data are expressed as means ± standard deviations}.

In comparison, Table II showed significant average lengths of AP, ML1, and ML2 in male than that of female. The AP, ML, and ML2 of male are 56.71±0.16, 41.63±0.18, and 37.85±0.36 mm, while of female are 51.21±0.12, 37.74±0.16, and 33.85±0.28 mm, respectively. There is no statistical difference between left & right sides of individual parameter in both male and female (Table II). It was found that MP length of both sexes is not significantly different (of male, 5.44±0.17 mm and of female, 5.35±0.15 mm) shown in Table II.

Table II Comparison of talar parameter lengths between left and right side of male and female Thai population. 

^{Data are expressed as means ± standard deviations. a p<0.05, compared between left tali of male and female; b p<0.05, compared between right tali of male and female; c p<0.05, compared between left & right-averaged lengths of male and female. AP; maximal length from anterior to posterior talar process, ML1; length from medial to lateral talar process (superior facet view), ML2; length from medial to lateral talar process (inferior facet view), MP; length of groove between middle and posterior facets}.

DISCUSSION

It is known that the subtalar region of different population is a common site of anatomical variation because of individual ligamentous structures, lesser extent, and osseous morphology. The variations of subtalar joints are clinically concerned in ankle surgeries including talo-calcaneal coalition. Previously, the types and incidence of superior articular facets of Isan-Thai dried calcanei has been documented for the first time (Iamsaard et al.). Present study has also demonstrated that the calcaneal articulating facets of dried tali observed in our population could be classified into 6 types (I, II [A], II [B], III, IV, and V, respectively) with morphometry of their facets as also shown in Figure 1. This finding is similar to that of previous investigations (Arora et al.; Bilodi & Agrawal; Bilodi; Kaur et al.; Garg et al.; Jung et al.). However, the types of single calcaneal facet (continuous with posterior calcaneal facet; 15 % in India) and two calcaneal facets (continuous between middle and posterior calcaneal facets; 3 % in India), observed by Garg et al., were not found in resent study. Compared to Indian populations, it was found that type II (A) in our classification has highest incidence (34.68 %), similar to previous studies that this types of faces were predominantly observed approximately 43-78 % (Kaur et al.; Garg et al.; ^{Rehman, 2014}). Moreover, this study attempted to perform the morphometric measurements on superior and inferior talar facets (Fig. 1 and Table II). Interestingly, it was found that the average lengths of AP, ML1, and ML2, but not MP in male were significantly longer than that of female. We assumed that males might bear the skeletal weights more than female resulting in subtalar bones including morphometric tali expanded in males. In conclusion, the incidence of articular surface of the talus and significant morphometric data can be used as basic information for ankle surgery consideration and an anthropological marker for sex dimorphism of unknown tali.